P2PSIP C. Jennings
Internet-Draft Cisco
Intended status: Standards Track B. Lowekamp
Expires: December 12, 2008 SIPeerior Technologies
E. Rescorla
Network Resonance
S. Baset
H. Schulzrinne
Columbia University
June 10, 2008
REsource LOcation And Discovery (RELOAD)
draft-bryan-p2psip-reload-04
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Abstract
This document defines REsource LOcation And Discovery (RELOAD), a
peer-to-peer (P2P) signaling protocol for use on the Internet. A P2P
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signaling protocol provides its clients with an abstract storage and
messaging service between a set of cooperating peers that form the
overlay network. RELOAD is designed to support a P2P Session
Initiation Protocol (P2PSIP) network, but can be utilized by other
applications with similar requirements by defining new usages that
specify the kinds of data that must be stored for a particular
application. RELOAD defines a security model based on a certificate
enrollment service that provides unique identities. NAT traversal is
a fundamental service of the protocol. RELOAD also allows access
from "client" nodes which do not need to route traffic or store data
for others.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Basic Setting . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Architecture . . . . . . . . . . . . . . . . . . . . . . 7
1.2.1. Usage Layer . . . . . . . . . . . . . . . . . . . . . 9
1.2.2. Routing Layer . . . . . . . . . . . . . . . . . . . . 9
1.2.3. Storage . . . . . . . . . . . . . . . . . . . . . . . 10
1.2.4. Topology Plugin . . . . . . . . . . . . . . . . . . . 10
1.2.5. Forwarding Layer . . . . . . . . . . . . . . . . . . 11
1.3. SIP Usage . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4. Security . . . . . . . . . . . . . . . . . . . . . . . . 12
1.5. Structure of This Document . . . . . . . . . . . . . . . 12
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 13
3. Overlay Management Overview . . . . . . . . . . . . . . . . . 15
3.1. Security and Identification . . . . . . . . . . . . . . . 15
3.1.1. Shared-Key Security . . . . . . . . . . . . . . . . . 16
3.2. Clients . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.1. Client Routing . . . . . . . . . . . . . . . . . . . 17
3.2.2. Client Behavior . . . . . . . . . . . . . . . . . . . 17
3.3. Routing . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.3.1. Routing Alternatives . . . . . . . . . . . . . . . . 21
3.4. Connectivity Management . . . . . . . . . . . . . . . . . 25
3.5. Overlay Algorithm Support . . . . . . . . . . . . . . . . 26
3.5.1. Support for Pluggable Overlay Algorithms . . . . . . 26
3.5.2. Joining, Leaving, and Maintenance Overview . . . . . 26
3.6. First-Time Setup . . . . . . . . . . . . . . . . . . . . 27
3.6.1. Initial Configuration . . . . . . . . . . . . . . . . 27
3.6.2. Enrollment . . . . . . . . . . . . . . . . . . . . . 27
4. Application Support Overview . . . . . . . . . . . . . . . . 28
4.1. Data Storage . . . . . . . . . . . . . . . . . . . . . . 28
4.1.1. Storage Permissions . . . . . . . . . . . . . . . . . 30
4.1.2. Usages . . . . . . . . . . . . . . . . . . . . . . . 30
4.1.3. Replication . . . . . . . . . . . . . . . . . . . . . 31
4.2. Service Discovery . . . . . . . . . . . . . . . . . . . . 32
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4.3. Application Connectivity . . . . . . . . . . . . . . . . 32
5. P2PSIP Integration Overview . . . . . . . . . . . . . . . . . 32
6. Overlay Management Protocol . . . . . . . . . . . . . . . . . 33
6.1. Message Routing . . . . . . . . . . . . . . . . . . . . . 34
6.1.1. Request Origination . . . . . . . . . . . . . . . . . 34
6.1.2. Message Receipt and Forwarding . . . . . . . . . . . 34
6.1.3. Response Origination . . . . . . . . . . . . . . . . 37
6.2. Message Structure . . . . . . . . . . . . . . . . . . . . 37
6.2.1. Presentation Language . . . . . . . . . . . . . . . . 38
6.2.2. Forwarding Header . . . . . . . . . . . . . . . . . . 41
6.2.3. Message Contents Format . . . . . . . . . . . . . . . 47
6.2.4. Signature . . . . . . . . . . . . . . . . . . . . . . 50
6.3. Overlay Topology . . . . . . . . . . . . . . . . . . . . 51
6.3.1. Topology Plugin Requirements . . . . . . . . . . . . 51
6.3.2. Methods and types for use by topology plugins . . . . 52
6.4. Forwarding Layer . . . . . . . . . . . . . . . . . . . . 54
6.4.1. Transports . . . . . . . . . . . . . . . . . . . . . 54
6.4.2. Connection Management Methods . . . . . . . . . . . . 57
7. Data Storage Protocol . . . . . . . . . . . . . . . . . . . . 67
7.1. Data Signature Computation . . . . . . . . . . . . . . . 68
7.2. Data Models . . . . . . . . . . . . . . . . . . . . . . . 69
7.2.1. Single Value . . . . . . . . . . . . . . . . . . . . 69
7.2.2. Array . . . . . . . . . . . . . . . . . . . . . . . . 70
7.2.3. Dictionary . . . . . . . . . . . . . . . . . . . . . 70
7.3. Data Storage Methods . . . . . . . . . . . . . . . . . . 71
7.3.1. Store . . . . . . . . . . . . . . . . . . . . . . . . 71
7.3.2. Fetch . . . . . . . . . . . . . . . . . . . . . . . . 76
7.3.3. Remove . . . . . . . . . . . . . . . . . . . . . . . 79
7.3.4. Find . . . . . . . . . . . . . . . . . . . . . . . . 80
8. Certificate Store Usage . . . . . . . . . . . . . . . . . . . 82
9. TURN Server Usage . . . . . . . . . . . . . . . . . . . . . . 83
10. SIP Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 84
10.1. Registering AORs . . . . . . . . . . . . . . . . . . . . 85
10.2. Looking up an AOR . . . . . . . . . . . . . . . . . . . . 87
10.3. Forming a Direct Connection . . . . . . . . . . . . . . . 88
10.4. GRUUs . . . . . . . . . . . . . . . . . . . . . . . . . . 88
10.5. SIP-REGISTRATION Kind Definition . . . . . . . . . . . . 88
11. Diagnostic Usage . . . . . . . . . . . . . . . . . . . . . . 89
11.1. Diagnostic Metrics for a P2PSIP Deployment . . . . . . . 91
12. Chord Algorithm . . . . . . . . . . . . . . . . . . . . . . . 91
12.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 91
12.2. Routing . . . . . . . . . . . . . . . . . . . . . . . . . 92
12.3. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 92
12.4. Joining . . . . . . . . . . . . . . . . . . . . . . . . . 92
12.5. Routing Connects . . . . . . . . . . . . . . . . . . . . 93
12.6. Updates . . . . . . . . . . . . . . . . . . . . . . . . . 93
12.6.1. Sending Updates . . . . . . . . . . . . . . . . . . . 95
12.6.2. Receiving Updates . . . . . . . . . . . . . . . . . . 95
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12.6.3. Stabilization . . . . . . . . . . . . . . . . . . . . 96
12.7. Route Query . . . . . . . . . . . . . . . . . . . . . . . 98
12.8. Leaving . . . . . . . . . . . . . . . . . . . . . . . . . 98
13. Enrollment and Bootstrap . . . . . . . . . . . . . . . . . . 98
13.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 99
13.2. Overlay Configuration . . . . . . . . . . . . . . . . . . 99
13.3. Credentials . . . . . . . . . . . . . . . . . . . . . . . 102
13.3.1. Self-Generated Credentials . . . . . . . . . . . . . 102
13.4. Joining the Overlay Peer . . . . . . . . . . . . . . . . 103
14. Message Flow Example . . . . . . . . . . . . . . . . . . . . 104
15. Security Considerations . . . . . . . . . . . . . . . . . . . 109
15.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 109
15.2. Attacks on P2P Overlays . . . . . . . . . . . . . . . . . 110
15.3. Certificate-based Security . . . . . . . . . . . . . . . 110
15.4. Shared-Secret Security . . . . . . . . . . . . . . . . . 111
15.5. Storage Security . . . . . . . . . . . . . . . . . . . . 112
15.5.1. Authorization . . . . . . . . . . . . . . . . . . . . 112
15.5.2. Distributed Quota . . . . . . . . . . . . . . . . . . 113
15.5.3. Correctness . . . . . . . . . . . . . . . . . . . . . 113
15.5.4. Residual Attacks . . . . . . . . . . . . . . . . . . 113
15.6. Routing Security . . . . . . . . . . . . . . . . . . . . 114
15.6.1. Background . . . . . . . . . . . . . . . . . . . . . 114
15.6.2. Admissions Control . . . . . . . . . . . . . . . . . 115
15.6.3. Peer Identification and Authentication . . . . . . . 115
15.6.4. Protecting the Signaling . . . . . . . . . . . . . . 116
15.6.5. Residual Attacks . . . . . . . . . . . . . . . . . . 116
15.7. SIP-Specific Issues . . . . . . . . . . . . . . . . . . . 116
15.7.1. Fork Explosion . . . . . . . . . . . . . . . . . . . 116
15.7.2. Malicious Retargeting . . . . . . . . . . . . . . . . 117
15.7.3. Privacy Issues . . . . . . . . . . . . . . . . . . . 117
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 117
16.1. Overlay Algorithm Types . . . . . . . . . . . . . . . . . 117
16.2. Data Kind-Id . . . . . . . . . . . . . . . . . . . . . . 117
16.3. Data Model . . . . . . . . . . . . . . . . . . . . . . . 118
16.4. Message Codes . . . . . . . . . . . . . . . . . . . . . . 118
16.5. Error Codes . . . . . . . . . . . . . . . . . . . . . . . 119
16.6. Route Log Extension Types . . . . . . . . . . . . . . . . 119
16.7. reload: URI Scheme . . . . . . . . . . . . . . . . . . . 119
16.7.1. URI Registration . . . . . . . . . . . . . . . . . . 120
17. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 121
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 121
18.1. Normative References . . . . . . . . . . . . . . . . . . 121
18.2. Informative References . . . . . . . . . . . . . . . . . 122
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 125
Intellectual Property and Copyright Statements . . . . . . . . . 127
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1. Introduction
This document defines REsource LOcation And Discovery (RELOAD), a
peer-to-peer (P2P) signaling protocol for use on the Internet. It
provides a generic, self-organizing overlay network service, allowing
nodes to efficiently route messages to other nodes and to efficiently
store and retrieve data in the overlay. RELOAD provides several
features that are critical for a successful P2P protocol for the
Internet:
Security Framework: A P2P network will often be established among a
set of peers that do not trust each other. RELOAD leverages a
central enrollment server to provide credentials for each peer
which can then be used to authenticate each operation. This
greatly reduces the possible attack surface.
Usage Model: RELOAD is designed to support a variety of
applications, including P2P multimedia communications with the
Session Initiation Protocol [I-D.ietf-p2psip-concepts]. RELOAD
allows the definition of new application usages, each of which can
define its own data types, along with the rules for their use.
This allows RELOAD to be used with new applications through a
simple documentation process that supplies the details for each
application.
NAT Traversal: RELOAD is designed to function in environments where
many if not most of the nodes are behind NATs or firewalls.
Operations for NAT traversal are part of the base design,
including using ICE to establish new RELOAD or application
protocol connections as well as tunneling application protocols
across the overlay.
High Performance Routing: The very nature of overlay algorithms
introduces a requirement that peers participating in the P2P
network route requests on behalf of other peers in the network.
This introduces a load on those other peers, in the form of
bandwidth and processing power. RELOAD has been defined with a
simple, lightweight forwarding header, thus minimizing the amount
of effort required by intermediate peers.
Pluggable overlay Algorithms: RELOAD has been designed with an
abstract interface to the overlay layer to simplify implementing a
variety of structured (DHT) and unstructured overlay algorithms.
This specification also defines how RELOAD is used with Chord,
which is mandatory to implement. Specifying a default "must
implement" overlay algorithm will allow interoperability, while
the extensibility allows selection of overlay algorithms optimized
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for a particular application.
These properties were designed specifically to meet the requirements
for a P2P protocol to support SIP, and this document defines a SIP
Usage of RELOAD. However, RELOAD is not limited to usage by SIP and
could serve as a tool for supporting other P2P applications with
similar needs. RELOAD is also based on the concepts introduced in
[I-D.ietf-p2psip-concepts].
1.1. Basic Setting
In this section, we provide a brief overview of the operational
setting for RELOAD. See the concepts document for more details. A
RELOAD Overlay Instance consists of a set of nodes arranged in a
partly connected graph. Each node in the overlay is assigned a
numeric Node-ID which, together with the specific overlay algorithm
in use, determines its position in the graph and the set of nodes it
connects to. The figure below shows a trivial example which isn't
drawn from any particular overlay algorithm, but was chosen for
convenience of representation.
+--------+ +--------+ +--------+
| Node 10|--------------| Node 20|--------------| Node 30|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 40|--------------| Node 50|--------------| Node 60|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 70|--------------| Node 80|--------------| Node 90|
+--------+ +--------+ +--------+
|
|
+--------+
| Node 85|
|(Client)|
+--------+
Because the graph is not fully connected, when a node wants to send a
message to another node, it may need to route it through the network.
For instance, Node 10 can talk directly to nodes 20 and 40, but not
to Node 70. In order to send a message to Node 70, it would first
send it to Node 40 with instructions to pass it along to Node 80.
Different overlay algorithms will have different connectivity graphs,
but the general idea behind all of them is to allow any node in the
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graph to efficiently reach every other node within a small number of
hops.
The RELOAD network is not only a messaging network. It is also a
storage network. Records are stored under numeric addresses which
occupy the same space as node identifiers. Nodes are responsible for
storing the data associated with some set of addresses as determined
by their Node-Id. For instance, we might say that every node is
responsible for storing any data value which has an address less than
or equal to its own Node-Id, but greater than the next lowest
Node-Id. Thus, Node-20 would be responsible for storing values
11-20.
RELOAD also supports clients. These are nodes which have Node-Ids
but do not participate in routing or storage. For instance, in the
figure above Node 85 is a client. It can route to the rest of the
RELOAD network via Node 80, but no other node will route through it
and Node 90 is still responsible for all addresses between 81-90. We
refer to non-client nodes as peers.
Other applications (for instance, SIP) can be defined on top of
RELOAD and use these two basic RELOAD services to provide their own
services.
1.2. Architecture
Architecturally RELOAD is divided into several layers, as shown in
the following figure.
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Application
+-------+ +-------+
| SIP | | XMPP | ...
| Usage | | Usage |
+-------+ +-------+
-------------------------------------- Message Routing API
+------------------+ +---------+
| |<->| Storage |
| | +---------+
| Routing | ^
| Layer | v
| | +---------+
| |<->|Topology |
| | | Plugin |
+------------------+ +---------+
^ ^
v |
+------------------+ <------+
| Forwarding |
| Layer |
+------------------+
-------------------------------------- Transport API
+-------+ +------+
|TLS | |DTLS | ...
+-------+ +------+
The major components of RELOAD are:
Usage Layer: Each application defines a RELOAD usage; a set of data
kinds and behaviors which describe how to use the services
provided by RELOAD. These usages all talk to RELOAD through a
common Message Routing API.
Routing Layer: The Routing Layer is responsible for routing messages
through the overlay. It also manages request state for the usages
and forwards Store and Fetch operations to the Storage component.
It talks directly to the Topology Plugin, which is responsible for
implementing the specific topology defined by the overlay
algorithm being used.
Storage: The Storage component is responsible for processing
messages relating to the storage and retrieval of data. It talks
directly to the Topology Plugin and the routing layer in order to
send and receive messages and manage data replication and
migration.
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Topology Plugin: The Topology Plugin is responsible for implementing
the specific overlay algorithm being used. It talks directly to
the Routing Layer to send and receive overlay management messages,
to the Storage component to manage data replication, and directly
to the Forwarding Layer to control hop-by-hop message forwarding.
Forwarding Layer: The Forwarding Layer provides packet forwarding
services between nodes. It also handles setting up connections
across NATs using ICE.
1.2.1. Usage Layer
The top layer, called the Usage Layer, has application usages---such
as the SIP Location Usage---that use the abstract Message Routing API
provided by RELOAD. The goal of this layer is to implement
application-specific usages of the generic overlay services provided
by RELOAD. The usage defines how a specific application maps its
data into something that can be stored in the overlay, where to store
the data, how to secure the data, and finally how applications can
retrieve and use the data.
The architecture diagram shows both a SIP usage and an XMPP usage. A
single application may require multiple usages, for example a SIP
application may also require a voicemail usage. A usage may define
multiple kinds of data that are stored in the overlay and may also
rely on kinds originally defined by other usages.
This draft also defines a Diagnostics Usage, which can be used to
obtain diagnostic information about a peer in the overlay. The
Diagnostics Usage is interesting both to administrators monitoring
the overlay as well as to some overlay algorithms that base their
decisions on capabilities and current load of nodes in the overlay.
1.2.2. Routing Layer
The Routing Layer provides a generic message routing service for the
overlay. Each peer is identified by its location in the overlay as
determined by its Node-ID. A component which is a client of the
Routing Layer can perform two basic functions:
o Send a message to a given peer, specified by Node-Id or
Resource-Id.
o Receive messages that other peers sent to a Node-Id or Resource-Id
for which this peer is responsible.
All usages are clients of the Routing Layer and use RELOAD's services
by sending and receiving messages from peers. For instance, when a
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usage wants to store data, it does so by sending Store requests.
Note that the Storage component and the Topology Plugin are
themselves clients of the Routing Layer, because they need to send
and receive messages from other peers.
The Routing Layer provides a fairly generic interface that allows the
topology plugin control the overlay and resource operations and
messages. Since each overlay algorithm is defined and functions
differently, we generically refer to the table of other peers that
the overlay algorithm maintains and uses to route requests
(neighbors) as a Routing Table. The Routing Layer component makes
queries to the overlay algorithm to determine the next hop, then
encodes and sends the message itself. Similarly, the overlay
algorithm issues periodic update requests through the logic component
to maintain and update its Routing Table.
1.2.3. Storage
One of the major functions of RELOAD is to allow nodes to store data
in the overlay and to retrieve data stored by other nodes or by
themselves. The Storage component is responsible for processing data
storage and retrieval messages from other peers. For instance, the
Storage component might receive a Store request for a given resource
from the Routing Layer. It would then store the data value(s) in its
local data store and sends a response to the Routing Layer for
delivery to the requesting peer.
The node's Node-ID determines the set of resources which it will be
responsible for storing. However, the exact mapping between these is
determined by the overlay algorithm used by the overlay, therefore
the Storage component always the queries the topology plugin to
determine where a particular resource should be stored.
1.2.4. Topology Plugin
RELOAD is explicitly designed to work with a variety of overlay
algorithms. In order to facilitate this, the overlay algorithm
implementation is provided by a Topology Plugin so that each overlay
can select an appropriate overlay algorithm that relies on the common
RELOAD core protocols and code.
The Topology Plugin is responsible for maintaining the overlay
algorithm Routing Table, which is consulted by the Routing Layer
before routing a message. When connections are made or broken, the
Forwarding Layer notifies the Topology Plugin, which adjusts the
routing table as appropriate. The Topology Plugin will also instruct
the Forwarding Layer to form new connections as dictated by the
requirements of the overlay algorithm Topology.
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As peers enter and leave, resources may be stored on different peers,
so the Topology Plugin also keeps track of which peers are
responsible for which resources. As peers join and leave, the
Topology Plugin issues resource migration requests as appropriate, in
order to ensure that other peers have whatever resources they are now
responsible for. The Topology Plugin is also responsible for
providing redundant data storage to protect against loss of
information in the event of a peer failure and to protect against
compromised or subversive peers.
1.2.5. Forwarding Layer
The Forwarding Layer is responsible for getting a packet to the next
peer, as determined by the Routing and Storage Layer. The Forwarding
Layer establishes and maintains the network connections as required
by the Topology Plugin. This layer is also responsible for setting
up connections to other peers through NATs and firewalls using ICE,
and it can elect to forward traffic using relays for NAT and firewall
traversal.
The Forwarding Layer sits on top of transport layer protocols which
carry the actual traffic. This specification defines how to use DTLS
and TLS to carry RELOAD messages.
1.3. SIP Usage
The SIP Usage of RELOAD allows SIP user agents to provide a peer-to-
peer telephony service without the requirement for permanent proxy or
registration servers. In such a network, the RELOAD overlay itself
performs the registration and rendezvous functions ordinarily
associated with such servers.
The SIP Usage involves two basic functions:
Registration: SIP UAs can use the RELOAD data storage
functionality to store a mapping from their AOR to their Node-Id
in the overlay, and to retrieve the Node-Id of other UAs.
Rendezvous: Once a SIP UA has identified the Node-Id for an AOR it
wishes to call, it can use the RELOAD message routing system to
set up a direct connection which can be used to exchange SIP
messages.
For instance, Bob could register his Node-Id, "1234", under his AOR,
"sip:bob@dht.example.com". When Alice wants to call Bob, she queries
the overlay for "sip:bob@dht.example.com" and gets back Node-Id 1234.
She then uses the overlay to establish a direct connection with Bob
and can use that direct connection to perform a standard SIP INVITE.
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1.4. Security
RELOAD's security model is based on each node having one or more
public key certificates. In general, these certificates will be
assigned by a central server which also assigns Node-Ids, although
self-signed certificates can be used in closed networks. These
credentials can be leveraged to provide communications security for
RELOAD messages. RELOAD provides communications security at three
levels:
Connection Level: Connections between peers are secured with TLS
or DTLS.
Message Level: Each RELOAD message must be signed.
Object Level: Stored objects must be signed by the storing peer.
These three levels of security work together to allow peers to verify
the origin and correctness of data they receive from other peers,
even in the face of malicious activity by other peers in the overlay.
RELOAD also provides access control built on top of these
communications security features. Because the peer responsible for
storing a piece of data can validate the signature on the data being
stored, the responsible peer can determine whether a given operation
is permitted or not.
RELOAD also provides a shared secret based admission control feature
using shared secrets and TLS-PSK. In order to form a TLS connection
to any node in the overlay, a new node needs to know the shared
overlay key, thus restricting access to authorized users.
1.5. Structure of This Document
The remainder of this document is structured as follows.
o Section 2 provides definitions of terms used in this document.
o Section 3 provides an overview of the mechanisms used to establish
and maintain the overlay.
o Section 4 provides an overview of the mechanism RELOAD provides to
support other applications.
o Section 5 provides an overview of the SIP usage for RELOAD.
o Section 6 defines the protocol messages that RELOAD uses to
establish and maintain the overlay.
o Section 7 defines the protocol messages that are used to store and
retrieve data using RELOAD.
o Sections 8-10 define three Usages of RELOAD that provide
certificate storage, SIP, and Diagnostics.
o Section 11 defines a specific Topology Plugin using Chord.
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o Section 12 defines the mechanisms that new RELOAD nodes use to
join the overlay for the first time.
o Section 13 provides an extended example.
o Sections 14 and 15 provide Security and IANA considerations.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
We use the terminology and definitions from the Concepts and
Terminology for Peer to Peer SIP [I-D.ietf-p2psip-concepts] draft
extensively in this document. Other terms used in this document are
defined inline when used and are also defined below for reference.
Terms which are new to this document (and perhaps should be added to
the concepts document) are marked with a (*).
DHT: A distributed hash table. A DHT is an abstract hash table
service realized by storing the contents of the hash table across
a set of peers.
Overlay Algorithm: An overlay algorithm defines the rules for
determining which peers in an overlay store a particular piece of
data and for determining a topology of interconnections amongst
peers in order to find a piece of data.
Overlay Instance: A specific overlay algorithm and the collection of
peers that are collaborating to provide read and write access to
it. There can be any number of overlay instances running in an IP
network at a time, and each operates in isolation of the others.
Peer: A host that is participating in the overlay. Peers are
responsible for holding some portion of the data that has been
stored in the overlay and also route messages on behalf of other
hosts as required by the Overlay Algorithm.
Client: A host that is able to store data in and retrieve data from
the overlay but which is not participating in routing or data
storage for the overlay.
Node: We use the term "Node" to refer to a host that may be either a
Peer or a Client. Because RELOAD uses the same protocol for both
clients and peers, much of the text applies equally to both.
Therefore we use "Node" when the text applies to both Clients and
Peers and the more specific term when the text applies only to
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Clients or only to Peers.
Node-ID: A 128-bit value that uniquely identifies a node. Node-IDs
0 and 2^128 - 1 are reserved and are invalid Node-IDs. A value of
zero is not used in the wire protocol but can be used to indicate
an invalid node in implementations and APIs. The Node-ID of
2^128-1 is used on the wire protocol as a wildcard. (*)
Resource: An object or group of objects associated with a string
identifier see "Resource Name" below.
Resource Name: The (potentially) human readable name by which a
resource is identified. In unstructured P2P networks, the
resource name is used directly as a Resource-Id. In structured
P2P networks the resource name can be mapped into a Resource-ID by
using the string as the input to hash function. A SIP resource,
for example, is often identified by its AOR (see Resource Name
below).(*)
Resource-ID: A value that identifies some resources and which is
used as a key for storing and retrieving the resource. Often this
is not human friendly/readable. One way to generate a Resource-ID
is by applying a mapping function to some other unique name (e.g.,
user name or service name) for the resource. The Resource-ID is
used by the distributed database algorithm to determine the peer
or peers that are responsible for storing the data for the
overlay. In structured P2P networks, resource-IDs are generally
fixed length and are formed by hashing the resource identifier.
In unstructured networks, resource identifiers may be used
directly as resource-IDs and may have variable length.
Connection Table: The set of peers to which a node is directly
connected. This includes nodes with which Connect handshakes have
been done but which have not sent any Updates. (*)
Routing Table: The set of peers which a node can use to route
overlay messages. In general, these peers will all be on the
connection table but not vice versa, because some peers will have
Connected but not sent updates. Peers may send messages directly
to peers which are on the connection table but may only route
messages to other peers through peers which are on the routing
table. (*)
Destination List: A list of IDs through which a message is to be
routed. A single Node-ID is a trivial form of destination list.
(*)
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Usage: A usage is an application that wishes to use the overlay for
some purpose. Each application wishing to use the overlay defines
a set of data kinds that it wishes to use. The SIP usage defines
the location, certificate, STUN server and TURN server data kinds.
(*)
3. Overlay Management Overview
The most basic function of RELOAD is as a generic overlay network.
Nodes need to be able to join the overlay, form connections to other
nodes, and route messages through the overlay to nodes to which they
are not directly connected. This section provides an overview of the
mechanisms that perform these functions.
3.1. Security and Identification
Every node in the RELOAD overlay is identified by one or more Node-
IDs. The Node-ID is used for three major purposes:
o To address the node itself.
o To determine its position in the overlay topology when the overlay
is structured.
o To determine the set of resources for which the node is
responsible.
Each node has a certificate [RFC3280] containing one or more Node-
IDs, which are globally unique.
The certificate serves multiple purposes:
o It entitles the user to store data at specific locations in the
Overlay Instance. Each data kind defines the specific rules for
determining which certificates can access each resource-ID/kind-id
pair. For instance, some kinds might allow anyone to write at a
given location, whereas others might restrict writes to the owner
of a single certificate.
o It entitles the user to operate a node that has a Node-ID found in
the certificate. When the node forms a connection to another
peer, it can use this certificate so that a node connecting to it
knows it is connected to the correct node. In addition, the node
can sign messages, thus providing integrity and authentication for
messages which are sent from the node.
o It entitles the user to use the user name found in the
certificate.
If a user has more than one device, typically they would get one
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certificate for each device. This allows each device to act as a
separate peer.
RELOAD supports two certificate issuance models. The first is based
on a central enrollment process which allocates a unique name and
Node-Id to the node a certificate for a public/private key pair for
the user. All peers in a particular Overlay Instance have the
enrollment server as a trust anchor and so can verify any other
peer's certificate.
In some settings, a group of users want to set up an overlay network
but are not concerned about attack by other users in the network.
For instance, users on a LAN might want to set up a short term ad hoc
network without going to the trouble of setting up an enrollment
server. RELOAD supports the use of self-generated and self-signed
certificates. When self-signed certificates are used, the node also
generates its own Node-Id and username. The Node-Id is computed as a
digest of the public key, to prevent Node-Id theft, however this
model is still subject to a number of known attacks (most notably
Sybil attacks [Sybil]) and can only be safely used in closed networks
where users are mutually trusting.
3.1.1. Shared-Key Security
RELOAD also provides an admission control system based on shared
keys. In this model, the peers all share a single key which is used
to authenticate the peer-to-peer connections via TLS-PSK/TLS-SRP.
3.2. Clients
RELOAD defines a single protocol that is used both as the peer
protocol and the client protocol for the overlay. This simplifies
implementation, particularly for devices that may act in either role,
and allows clients to inject messages directly into the overlay.
We use the term "peer" to identify a node in the overlay that routes
messages for nodes other than those to which it is directly
connected. Peers typically also have storage responsibilities. We
use the term "client" to refer to nodes that do not have routing or
storage responsibilities. When text applies to both peers and
clients, we will simply refer to such a device as a "node."
RELOAD's client support allows nodes that are not participating in
the overlay as peers to utilize the same implementation and to
benefit from the same security mechanisms as the peers. Clients
possess and use certificates that authorize the user to store data at
its locations in the overlay. The Node-ID in the certificate is used
to identify the particular client as a member of the overlay and to
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authenticate its messages.
The remainder of this section discusses how RELOAD supports clients
in terms of routing issues specific to clients, minimum functionality
requirements for clients, and alternatives for devices not capable of
meeting those requirements.
3.2.1. Client Routing
There are two routing options by which a client may be located in an
overlay.
o Establish a connection to the peer responsible for the client's
Node-ID in the overlay. Then requests may be sent from/to the
client using its Node-ID in the same manner as if it were a peer,
because the responsible peer in the overlay will handle the final
step of routing to the client.
o Establish a connection with an arbitrary peer in the overlay
(perhaps based on network proximity or an inability to establish a
direct connection with the responsible peer). In this case, the
client will rely on RELOAD's Destination List feature to ensure
reachability. The client can initiate requests, and any node in
the overlay that knows the Destination List to its current
location can reach it, but the client is not directly reachable
directly using only its Node-ID. The Destination List required to
reach it must be learnable via other mechanisms, such as being
stored in the overlay by a usage, if the client is to receive
incoming requests from other members of the overlay.
3.2.2. Client Behavior
There are a wide variety of reasons a node may act as a client rather
than as a peer [I-D.pascual-p2psip-clients]. This section outlines
some of those scenarios and how the client's behavior changes based
on its capabilities.
3.2.2.1. Why Not Only Peers?
For a number of reasons, a particular node may be forced to act as a
client even though it is willing to act as a peer. These include:
o The node does not have appropriate network connectivity---
typically because it is behind an overly restrictive NAT, or it
has a low-bandwidth network connection.
o The node may not have sufficient resources, such as computing
power, storage space, or battery power.
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o The overlay algorithm may dictate specific requirements for peer
selection. These may include participation in the overlay to
determine trustworthiness, control the number of peers in the
overlay to reduce overly-long routing paths, or ensure minimum
application uptime before a node can join as a peer.
The ultimate criteria for a node to become a peer are determined by
the overlay algorithm and specific deployment. A node acting as a
client that has a full implementation of RELOAD and the appropriate
overlay algorithm is capable of locating its responsible peer in the
overlay and using CONNECT to establish a direct connection to that
peer. In that way, it may elect to be reachable under either of the
routing approaches listed above. Particularly for overlay algorithms
that elect nodes to serve as peers based on trustworthiness or
population, the overlay algorithm may require such a client to locate
itself at a particular place in the overlay.
3.2.2.2. Minimum Functionality Requirements for Clients
A node may act as a client simply because it does not have the
resources or even an implementation of the topology plugin required
to acts as a peer in the overlay. In order to exchange RELOAD
messages with a peer, a client must meet a minimum level of
functionality. Such a client must:
o Implement RELOAD's connection-management connections that are used
to establish the connection with the peer.
o Implement RELOAD's data storage and retrieval methods (with client
functionality).
o Be able to calculate Resource-IDs used by the overlay.
o Possess security credentials required by the overlay it is
implementing.
A client speaks the same protocol as the peers, knows how to
calculate Resource-IDs, and signs its requests in the same manner as
peers. While a client does not necessarily require a full
implementation of the overlay algorithm, calculating the Resource-ID
requires an implementation of the appropriate algorithm for the
overlay.
RELOAD does not support a separate protocol for clients that do not
meet these functionality requirements. Any such extension would
either entail compromises on the features of RELOAD or require an
entirely new protocol to reimplement the core features of RELOAD.
Furthermore, for P2PSIP and many other applications, a native
application-level protocol already exists that is sufficient for such
a client, as described in the next section.
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3.2.2.3. Clients as Application-Level Agents
SIP defines an extensive protocol for registration and security
between a client and its registrar/proxy server(s). Any SIP device
can act as a client of a RELOAD-based P2PSIP overlay if it contacts a
peer that implements the server-side functionality required by the
SIP protocol. In this case, the peer would be acting as if it were
the user's peer, and would need the appropriate credentials for that
user.
Application-level support for clients is defined by a usage. A usage
offering support for application-level clients should specify how the
security of the system is maintained when the data is moved between
the application and RELOAD layers.
3.3. Routing
This section will discuss the requirements RELOAD's routing
capabilities must meet, then describe the routing features in the
protocol, and provide a brief overview of how they are used. The
section will conclude by discussing some alternative designs and the
tradeoffs that would be necessary to support them.
RELOAD's routing capabilities must meet the following requirements:
NAT Traversal: RELOAD must support establishing and using
connections between nodes separated by one or more NATs, including
locating peers behind NATs for those overlays allowing/requiring
it.
Clients: RELOAD must support requests from and to clients that do
not participate in overlay routing.
Client promotion: RELOAD must support clients that become peers at a
later point as determined by the overlay algorithm and deployment.
Low state: RELOAD's routing algorithms must not require
significant state to be stored on intermediate peers.
Return routability in unstable topologies: At some points in
times, different nodes may have inconsistent information about the
connectivity of the routing graph. In all cases, the response to
a request needs to delivered to the node that sent the request and
not to some other node.
To meet these requirements, RELOAD's routing relies on two basic
mechanisms:
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Via Lists: The forwarding header used by all RELOAD messages
contains both a Via List (built hop-by-hop as the message is
routed through the overlay) and a Destination List (providing
source-routing capabilities for requests and return-path routing
for responses).
Route_Query: The Route_Query method allows a node to query a peer
for the next hop it will use to route a message. This method is
useful for diagnostics and for iterative routing.
The basic routing mechanism used by RELOAD is Symmetric Recursive.
We will first describe symmetric routing and then discuss its
advantages in terms of the requirements discussed above.
Symmetric recursive routing requires a message follow the path
through the overlay to the destination without returning to the
originating node: each peer forwards the message closer to its
destination. The return path of the response is then the same path
followed in reverse. For example, a message following a route from A
to Z through B and X:
A B X Z
-------------------------------
---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Via=A, B
Dest=Z
<----------
Dest=X, B, A
<----------
Dest=B, A
<----------
Dest=A
Note that the preceding Figure does not indicate whether A is a
client or peer---A forwards its request to B and the response is
returned to A in the same manner regardless of A's role in the
overlay.
This figure shows use of full via-lists by intermediate peers B and
X. However, if B and/or X are willing to store state, then they may
elect to truncate the lists, save that information internally (keyed
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by the transaction id), and return the response message along the
path from which it was received when the response is received. This
option requires greater state on intermediate peers but saves a small
amount of bandwidth and reduces the need for modifying the message
enroute. Selection of this mode of operation is a choice for the
individual peer---the techniques are mutually interoperable even on a
single message. The Figure below shows B using full via lists but X
truncating them and saving the state internally.
A B X Z
-------------------------------
---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Dest=Z
<----------
Dest=X
<----------
Dest=B, A
<----------
Dest=A
For debugging purposes, a Route Log attribute is available that
stores information about each peer as the message is forwarded.
RELOAD also supports a basic Iterative routing mode (where the
intermediate peers merely return a response indicating the next hop,
but do not actually forward the message to that next hop themselves).
Iterative routing is implemented using the Route_Query method, which
requests this behavior. Note that iterative routing is selected only
by the initiating node. RELOAD does not support an intermediate peer
returning a response that it will not recursively route a normal
request---the willingness to perform that operation is implicit in
its role as a peer in the overlay.
3.3.1. Routing Alternatives
Significant discussion has been focused on the selection of a routing
algorithm for P2PSIP. This section discusses the motivations for
selection of symmetric recursive routing for RELOAD and describes the
extensions that would be required to support additional routing
algorithms.
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3.3.1.1. Iterative vs Recursive
Iterative routing has a number of advantages. It is easier to debug,
consumes fewer resources on intermediate peers, and allows the
querying peer to identify and route around misbehaving peers
[stoica-non-transitive-worlds05]. However, in the presence of NATs
iterative routing is intolerably expensive because a new connection
must be established for each hop (using ICE) [bryan-design-hotp2p08].
Iterative routing is supported through the Route_Query mechanism and
is primarily intended for debugging. It is also the most reliable
technique in the presence of network transitivity because the
querying peer can evaluate the routing decisions made by the peers at
each hop, consider alternatives, and detect at what point the
forwarding path fails. An algorithm to implement this approach is
beyond the scope of this draft.
3.3.1.2. Symmetric vs Forward response
An alternative to the symmetric recursive routing method used by
RELOAD is Forward-Only routing, where the response is routed to the
requester as if it is a new message initiating by the responder (in
the previous example, Z sends the response to A as if it were sending
a request). Forward-only routing requires no state in either the
message or intermediate peers.
The drawback of forward-only routing is that it does not work when
the overlay is unstable. For example, if A is in the process of
joining the overlay and is sending a Join request to Z, it is not yet
reachable via forward routing. Even if it is established in the
overlay, if network failures produce temporary instability, A may not
be reachable (and may be trying to stabilize its network connectivity
via Connect messages).
Furthermore, forward-only responses are less likely to reach the
querying peer than symmetric recursive because the forward path is
more likely to have a failed peer than the request path (which was
just tested to route the request) [stoica-non-transitive-worlds05].
An extension to RELOAD that supports forward-only routing but relies
on symmetric responses as a fallback would be possible, but due to
the complexities of determining when to use forward-only and when to
fallback to symmetric, we have chosen not to include it as an option
at this point.
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3.3.1.3. Direct Response
Another routing option is Direct Response routing, in which the
response is returned directly to the querying node. In the previous
example, if A encodes its IP address in the request, then Z can
simply deliver the response directly to A. In the absence of NATs or
other connectivity issues, this is the optimal routing technique.
The challenge of implementing direct response is the presence of
NATs. There are a number of complexities that must be addressed. In
this discussion, we will continue our assumption that A issued the
request and Z is generating the response.
o The IP address listed by A may be unreachable, either due to NAT
or firewall rules. Therefore, a direct response technique must
fallback to symmetric response [stoica-non-transitive-worlds05].
The hop-by-hop ACKs used by RELOAD allow Z to determine when A has
received the message (and the TLS negotiation will provide earlier
confirmation that A is reachable), but this fallback requires a
timeout that will increase the response latency whenever A is not
reachable from Z.
o Whenever A is behind a NAT it will have multiple candidate IP
addresses, each of which must be advertised to ensure
connectivity, therefore Z will need to attempt multiple
connections to deliver the response.
o One (or all) of A's candidate addresses may route from Z to a
different device on the Internet. In the worst case these nodes
may actually be running RELOAD on the same port. Therefore,
establishing a secure connection to authenticate A before
delivering the response is absolutely necessary. This step
diminishes the efficiency of direct response because multiple
roundtrips are required before the message can be delivered.
o If A is behind a NAT and does not have a connection already
established with Z, there are only two ways the direct response
will work. The first is that A and Z are both behind the same
NAT, in which case the NAT is not involved. In the more common
case, when Z is outside A's NAT, the response will only be
received if A's NAT implements endpoint-independent filtering. As
the choice of filtering mode conflates application transparency
with security [RFC4787], and no clear recommendation is available,
the prevalence of this feature in future devices remains unclear.
An extension to RELOAD that supports direct response routing but
relies on symmetric responses as a fallback would be possible, but
due to the complexities of determining when to use direct response
and when to fallback to symmetric, and the reduced performance for
responses to peers behind restrictive NATs, we have chosen not to
include it as an option at this point.
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3.3.1.4. Relay Peers
SEP [I-D.jiang-p2psip-sep] has proposed implementing a form of direct
response by having A identify a peer, Q, that will be directly
reachable by any other peer. A uses Connect to establish a
connection with Q and advertises Q's IP address in the request sent
to Z. Z sends the response to Q, which relays it to A. This then
reduces the latency to two hops, plus Z negotiating a secure
connection to Q.
This technique relies on the relative population of nodes such as A
that require relay peers and peers such as Q that are capable of
serving as a relay peer. It also requires nodes to be able to
identify which category they are in. This identification problem has
turned out to be hard to solve and is still an open area of
exploration.
An extension to RELOAD that supports relay peers is possible, but due
to the complexities of implementing such an alternative, we have not
added such a feature to RELOAD at this point.
A concept similar to relay peers, essentially choosing a relay peer
at random, has previously been suggested to solve problems of
pairwise non-transitivity [stoica-non-transitive-worlds05], but
deterministic filtering provided by NATs make random relay peers no
more likely to work than the responding peer.
3.3.1.5. Symmetric Route Stability
A common concern about symmetric recursive routing has been that one
or more peers along the request path may fail before the response is
received. The significance of this problem essentially depends on
the response latency of the overlay---an overlay that produces slow
responses will be vulnerable to churn, whereas responses that are
delivered very quickly are vulnerable only to failures that occur
over that small interval.
The other aspect of this issue is whether the request itself can be
successfully delivered. Assuming typical connection maintenance
intervals, the time period between the last maintenance and the
request being sent will be orders of magnitude greater than the delay
between the request being forwarded and the response being received.
Therefore, if the path was stable enough to be available to route the
request, it is almost certainly going to remain available to route
the response.
An overlay that is unstable enough to suffer this type of failure
frequently is unlikely to be able to support reliable functionality
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regardless of the routing mechanism. However, regardless of the
stability of the return path, studies show that in the event of high
churn, iterative routing is a better solution to ensure request
completion [ng-analytical-churn-ieeep2p06]
[stoica-non-transitive-worlds05]
Finally, because RELOAD retries the end-to-end request, that retry
will address the issues of churn that remain.
3.4. Connectivity Management
In order to provide efficient routing, a peer needs to maintain a set
of direct connections to other peers in the Overlay Instance. Due to
the presence of NATs, these connections often cannot be formed
directly. Instead, we use the Connect request to establish a
connection. Connect uses ICE [I-D.ietf-mmusic-ice-tcp] to establish
the connection. It is assumed that the reader is familiar with ICE.
Say that peer A wishes to form a direct connection to peer B. It
gathers ICE candidates and packages them up in a Connect request
which it sends to B through usual overlay routing procedures. B does
its own candidate gathering and sends back a response with its
candidates. A and B then do ICE connectivity checks on the candidate
pairs. The result is a connection between A and B. At this point, A
and B can add each other to their routing tables and send messages
directly between themselves without going through other overlay
peers.
There is one special case in which Connect cannot be used: when a
peer is joining the overlay and is not connected to any peers. In
order to support this case, some small number of "bootstrap nodes"
need to be publicly accessible so that new peers can directly connect
to them. Section 13 contains more detail on this.
In general, a peer needs to maintain connections to all of the peers
near it in the Overlay Instance and to enough other peers to have
efficient routing (the details depend on the specific overlay). If a
peer cannot form a connection to some other peer, this isn't
necessarily a disaster; overlays can route correctly even without
fully connected links. However, a peer should try to maintain the
specified link set and if it detects that it has fewer direct
connections, should form more as required. This also implies that
peers need to periodically verify that the connected peers are still
alive and if not try to reform the connection or form an alternate
one.
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3.5. Overlay Algorithm Support
The Topology Plugin allows RELOAD to support a variety of overlay
algorithms. This draft defines a DHT based on Chord [Chord], which
is mandatory to implement, but the base RELOAD protocol is designed
to support a variety of overlay algorithms.
3.5.1. Support for Pluggable Overlay Algorithms
RELOAD defines three methods for overlay maintenance: Join, Update,
and Leave. However, the contents of those messages, when they are
sent, and their precise semantics are specified by the actual overlay
algorithm; RELOAD merely provides a framework of commonly-needed
methods that provides uniformity of notation (and ease of debugging)
for a variety of overlay algorithms.
3.5.2. Joining, Leaving, and Maintenance Overview
When a new peer wishes to join the Overlay Instance, it must have a
Node-ID that it is allowed to use. It uses one of the Node-IDs in
the certificate it received from the enrollment server. The details
of the joining procedure are defined by the overlay algorithm, but
the general steps for joining an Overlay Instance are:
o Forming connections to some other peers.
o Acquiring the data values this peer is responsible for storing.
o Informing the other peers which were previously responsible for
that data that this peer has taken over responsibility.
The first thing the peer needs to do is form a connection to some
"bootstrap node". Because this is the first connection the peer
makes, these nodes must have public IP addresses and therefore can be
connected to directly. Once a peer has connected to one or more
bootstrap nodes, it can form connections in the usual way by routing
Connect messages through the overlay to other nodes. Once a peer has
connected to the overlay for the first time, it can cache the set of
nodes it has connected to with public IP addresses for use as future
bootstrap nodes.
Once the peer has connected to a bootstrap node, it then needs to
take up its appropriate place in the overlay. This requires two
major operations:
o Forming connections to other peers in the overlay to populate its
Routing Table.
o Getting a copy of the data it is now responsible for storing and
assuming responsibility for that data.
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The second operation is performed by contacting the Admitting Peer
(AP), the node which is currently responsible for that section of the
overlay.
The details of this operation depend mostly on the overlay algorithm
involved, but a typical case would be:
1. JP (Joining Peer) sends a Join request to AP (Admitting Peer)
announcing its intention to join.
2. AP sends a Join response.
3. AP does a sequence of Stores to JP to give it the data it will
need.
4. AP does Updates to JP and to other peers to tell it about its own
routing table. At this point, both JP and AP consider JP
responsible for some section of the Overlay Instance.
5. JP makes its own connections to the appropriate peers in the
Overlay Instance.
After this process is completed, JP is a full member of the Overlay
Instance and can process Store/Fetch requests.
3.6. First-Time Setup
Previous sections addressed how RELOAD works once a node has
connected. This section provides an overview of how users get
connected to the overlay for the first time. RELOAD is designed so
that users can start with the name of the overlay they wish to join
and perhaps a username and password, and leverage that into having a
working peer with minimal user intervention. This helps avoid the
problems that have been experienced with conventional SIP clients
where users are required to manually configure a large number of
settings.
3.6.1. Initial Configuration
In the first phase of the process, the user starts out with the name
of the overlay and uses this to download an initial set of overlay
configuration parameters. The user does a DNS SRV lookup on the
overlay name to get the address of a configuration server. It can
then connect to this server with HTTPS to download a configuration
document which contains the basic overlay configuration parameters as
well as a set of bootstrap nodes which can be used to join the
overlay. role.
3.6.2. Enrollment
If the overlay is using certificate enrollment, then a user needs to
acquire a certificate before joining the overlay. The certificate
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attests both to the user's name within the overlay and to the node-
ids which they are permitted to operate. In that case, the
configuration document will contain the address of an enrollment
server which can be used to obtain such a certificate. The
enrollment server may (and probably will) require some sort of
username and password before issuing the certificate. The enrollment
server's ability to restrict attackers' access to certificates in the
overlay is one of the cornerstones of RELOAD's security.
4. Application Support Overview
RELOAD is not intended to be used alone, but rather as a substrate
for other applications. These applications can use RELOAD for a
variety of purposes:
o To store data in the overlay and retrieve data stored by other
nodes.
o As a discovery mechanism for services such as TURN.
o To form direct connections which can be used to transmit
application-level messages.
This section provides an overview of these services.
4.1. Data Storage
RELOAD provides operations to Store, Fetch, and Remove data. Each
location in the Overlay Instance is referenced by a Resource-ID.
However, each location may contain data elements corresponding to
multiple kinds (e.g., certificate, SIP registration). Similarly,
there may be multiple elements of a given kind, as shown below:
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+--------------------------------+
| Resource-ID |
| |
| +------------+ +------------+ |
| | Kind 1 | | Kind 2 | |
| | | | | |
| | +--------+ | | +--------+ | |
| | | Value | | | | Value | | |
| | +--------+ | | +--------+ | |
| | | | | |
| | +--------+ | | +--------+ | |
| | | Value | | | | Value | | |
| | +--------+ | | +--------+ | |
| | | +------------+ |
| | +--------+ | |
| | | Value | | |
| | +--------+ | |
| +------------+ |
+--------------------------------+
Each kind is identified by a kind-id, which is a code point assigned
by IANA. As part of the kind definition, protocol designers may
define constraints, such as limits on size, on the values which may
be stored. For many kinds, the set may be restricted to a single
value; some sets may be allowed to contain multiple identical items
while others may only have unique items. Note that a kind may be
employed by multiple usages and new usages are encouraged to use
previously defined kinds where possible. We define the following
data models in this document, though other usages can define their
own structures:
single value: There can be at most one item in the set and any value
overwrites the previous item.
array: Many values can be stored and addressed by a numeric index.
dictionary: The values stored are indexed by a key. Often this key
is one of the values from the certificate of the peer sending the
Store request.
In order to protect stored data from tampering, by other nodes, each
stored value is digitally signed by the node which created it. When
a value is retrieved, the digital signature can be verified to detect
tampering.
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4.1.1. Storage Permissions
A major issue in peer-to-peer storage networks is minimizing the
burden of becoming a peer, and in particular minimizing the amount of
data which any peer is required to store for other nodes. RELOAD
addresses this issue by only allowing any given node to store data at
a small number of locations in the overlay, with those locations
being determined by the node's certificate. When a peer uses a Store
request to place data at a location authorized by its certificate, it
signs that data with the private key that corresponds to its
certificate. Then the peer storing the data is able to verify that
the peer issuing the request is authorized to make that request.
Each data kind defines the exact rules for determining what
certificate is appropriate.
The most natural rule is that a certificate authorizes a user to
store data keyed with their user name X. This rules is used for all
the kinds defined in this specification. Thus, only a user with a
certificate for "alice@example.org" could write to that location in
the overlay. However, other usages can define any rules they choose,
including publicly writable values.
The digital signature over the data serves two purposes. First, it
allows the peer responsible for storing the data to verify that this
Store is authorized. Second, it provides integrity for the data.
The signature is saved along with the data value (or values) so that
any reader can verify the integrity of the data. Of course, the
responsible peer can "lose" the value but it cannot undetectable
modify it.
The size requirements of the data being stored in the overlay are
variable. For instance, a SIP AoR and voicemail differ widely in the
storage size. RELOAD leaves it to the Usage to address the size
imbalance of various kinds.
4.1.2. Usages
By itself, the distributed storage layer just provides infrastructure
on which applications are built. In order to do anything useful, a
usage must be defined. Each Usage specifies several things:
o Registers kind-id code points for any kinds that the Usage
defines.
o Defines the data structure for each of the kinds.
o Defines access control rules for each kinds.
o Provides a size limit for each kinds.
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o Defines how the Resource Name is formed that is hashed to form the
Resource-ID where each kind is stored.
o Describes how values will be merged after a network partition.
Unless otherwise specified, the default merging rule is to act as
if all the values that need to be merged were stored and that the
order they were stored in corresponds to the stored time values
associated with (and carried in) their values. Because the stored
time values are those associated with the peer which did the
writing, clock skew is generally not an issue. If two nodes are
on different partitions, clocks, this can create merge conflicts.
However because RELOAD deliberately segregates storage so that
data from different users and peers is stored in different
locations, and a single peer will typically only be in a single
network partition, this case will generally not arise.
The kinds defined by a usage may also be applied to other usages.
However, a need for different parameters, such as different size
limits, would imply the need to create a new kind.
4.1.3. Replication
Replication in P2P overlays can be used to provide:
persistence: if the responsible peer crashes and/or if the storing
peer leaves the overlay
security: to guard against DoS attacks by the responsible peer or
routing attacks to that responsible peer
load balancing: to balance the load of queries for popular
resources.
A variety of schemes are used in P2P overlays to achieve some of
these goals. Common techniques include replicating on neighbors of
the responsible peer, randomly locating replicas around the overlay,
or replicating along the path to the responsible peer.
The core RELOAD specification does not specify a particular
replication strategy. Instead, the first level of replication
strategies are determined by the overlay algorithm, which can base
the replication strategy on the its particular topology. For
example, Chord places replicas on successor peers, which will take
over responsibility should the responsible peer fail [Chord].
If additional replication is needed, for example if data persistence
is particularly important for a particular usage, then that usage may
specify additional replication, such as implementing random
replications by inserting a different well known constant into the
Resource Name used to store each replicated copy of the resource.
Such replication strategies can be added independent of the
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underlying algorithm, and their usage can be determined based on the
needs of the particular usage.
4.2. Service Discovery
RELOAD does not currently define a generic service discovery
algorithm as part of the base protocol. A variety of service
discovery algorithm can be implemented as extensions to the base
protocol, such as ReDIR [opendht-sigcomm05].
4.3. Application Connectivity
There is no requirement that a RELOAD usage must use RELOAD's
primitives for establishing its own communication if it already
possesses its own means of establishing connections. For example,
one could design a RELOAD-based resource discovery protocol which
used HTTP to retrieve the actual data.
For more common situations, however, the overlay itself is used to
establish a connection rather than an external authority such as DNS,
RELOAD provides connectivity to applications using the same Connect
method as is used for the overlay maintenance. For example, if a
P2PSIP node wishes to establish a SIP dialog with another P2PSIP
node, it will use Connect to establish a direct connection with the
other node. This new connection is separate from the peer protocol
connection, it is a dedicated UDP or TCP flow used only for the SIP
dialog. Each usage specifies which types of connections can be
initiated using Connect.
5. P2PSIP Integration Overview
The SIP Usage of RELOAD allows SIP user agents to provide a peer-to-
peer telephony service without the requirement for permanent proxy or
registration servers. In such a network, the RELOAD overlay itself
performs the registration and rendezvous functions ordinarily
associated with such servers.
The basic function of the SIP usage is to allow Alice to start with a
SIP URI (e.g., "bob@dht.example.com") and end up with a connection
which Alice's SIP UA can use to pass SIP messages back and forth to
Bob's SIP UA. The way this works is as follows:
1. Bob, operating Node-ID 1234, stores a mapping from his URI to his
Node-ID in the overlay. I.e., "sip:bob@dht.example.com -> 1234".
2. Alice, operating Node-ID 5678, decides to call Bob. She looks up
"sip:bob@dht.example.com" in the overlay and retrieves "1234".
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3. Alice uses the overlay to route a Connect message to Bob's peer.
Bob responds with his own Connect and they set up a direct
connection, as shown below.
Alice Peer1 Overlay PeerN Bob
(5678) (1234)
-------------------------------------------------
Connect ->
Connect ->
Connect ->
Connect ->
<- Connect
<- Connect
<- Connect
<- Connect
<------------------ ICE Checks ----------------->
INVITE ----------------------------------------->
<--------------------------------------------- OK
ACK -------------------------------------------->
<------------ ICE Checks for media ------------->
<-------------------- RTP ---------------------->
It is important to note that RELOAD's only role here is to set up the
direct connection between Alice and Bob. As soon as the ICE checks
complete and the connection is established, then ordinary SIP is
used. In particular, the establishment of the media channel for the
phone call happens via the usual SIP mechanisms, and RELOAD is not
involved. Media never goes over the overlay. After the successful
exchange of SIP messages, call peers run ICE connectivity checks for
media.
As well as allowing mappings from AORs to Node-IDs, the SIP Usage
also allows mappings from AORs to other AORs. For instance, if Bob
wanted his phone calls temporarily forwarded to Charlie, he could
store the mapping "sip:bob@dht.example.com ->
sip:charlie@dht.example.com". When Alice wants to call Bob, she
retrieves this mapping and can then fetch Charlie's AOR to retrieve
his Node-ID.
6. Overlay Management Protocol
This section defines the basic protocols used to create, maintain,
and use the RELOAD overlay network. We start by defining how
messages are transmitted, received, and routed in an existing
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overlay, then define the message structure, and then finally define
the messages used to join and maintain the overlay.
6.1. Message Routing
This section describes procedures used by nodes to route messages
through the overlay.
6.1.1. Request Origination
In order to originate a message to a given Node-ID or resource-id, a
node constructs an appropriate destination list. The simplest such
destination list is a single entry containing the peer or
resource-id. The resulting message will use the normal overlay
routing mechanisms to forward the message to that destination. The
node can also construct a more complicated destination list for
source routing.
Once the message is constructed, the node sends the message to some
adjacent peer. If the first entry on the destination list is
directly connected, then the message MUST be routed down that
connection. Otherwise, the topology plugin MUST be consulted to
determine the appropriate next hop.
Parallel searches for the resource are a common solution to improve
reliability in the face of churn or of subversive peers. Parallel
searches for usage-specified replicas are managed by the usage layer.
However, a single request can also be routed through multiple
adjacent peers, even when known to be sub-optimal, to improve
reliability [vulnerabilities-acsac04]. Such parallel searches MAY BE
specified by the topology plugin.
Because messages may be lost in transit through the overlay, RELOAD
incorporates an end-to-end reliability mechanism. When an
originating node transmits a request it MUST set a 3 second timer.
If a response has not been received when the timer fires, the request
is retransmitted with the same transaction identifier. The request
MAY be retransmitted up to 4 times (for a total of 5 messages).
After the timer for the fifth transmission fires, the message SHALL
be considered to have failed. Note that this retransmission
procedure is not followed by intermediate nodes. They follow the
hop-by-hop reliability procedure described in Section 6.4.1.2.
6.1.2. Message Receipt and Forwarding
When a peer receives a message, it first examines the overlay,
version, and other header fields to determine whether the message is
one it can process. If any of these are incorrect (e.g., the message
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is for an overlay in which the peer does not participate) it is an
error. The peer SHOULD generate an appropriate error but if local
policy can override this in which case the messages is silently
dropped.
Once the peer has determined that the message is correctly formatted,
it examines the first entry on the destination list. There are three
possible cases here:
o The first entry on the destination list is an id for which the
peer is responsible.
o The first entry on the destination list is a an id for which
another peer is responsible.
o The first entry on the destination list is a private id which is
being used for destination list compression.
These cases are handled as discussed below.
6.1.2.1. Responsible ID
If the first entry on the destination list is a ID for which the node
is responsible, there are several sub-cases.
o If the entry is a Resource-Id, then it MUST be the only entry on
the destination list. If there are other entries, the message
MUST be silently dropped. Otherwise, the message is destined for
this node and it passes it up to the upper layers.
o If the entry is a Node-Id which belongs to this node, then the
message is destined for this node. If this is the only entry on
the destination list, the message is destined for this node and is
passed up to the upper layers. Otherwise the entry is removed
from the destination list and the message is passed it to the
routing layer. If the message is a response and there is state
for the transaction ID, the state is reinserted into the
destination list first.
o If the entry is a Node-Id which is not equal to this node, then
the node MUST drop the message silently unless the Node-Id
corresponds to a node which is directly connected to this node
(i.e., a client). In that case, it MUST forward the message to
the destination node as described in the next section.
Note that this implies that in order to address a message to "the
peer that controls region X", a sender sends to resource-id X, not
Node-ID X.
6.1.2.2. Other ID
If neither of the other two cases applies, then the peer MUST forward
the message towards the first entry on the destination list. This
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means that it MUST select one of the peers to which it is connected
and which is likely to be responsible for the first entry on the
destination list. If the first entry on the destination list is in
the peer's connection table, then it SHOULD forward the message to
that peer directly. Otherwise, it consult the routing table to
forward the message.
Any intermediate peer which forwards a RELOAD message MUST arrange
that if it receives a response to that message the response can be
routed back through the set of nodes through which the request
passed. This may be arranged in one of two ways:
o The peer MAY add an entry to the via list in the forwarding header
that will enable it to determine the correct node.
o The peer MAY keep per-transaction state which will allow it to
determine the correct node.
As an example of the first strategy, if node D receives a message
from node C with via list (A, B), then D would forward to the next
node (E) with via list (A, B, C). Now, if E wants to respond to the
message, it reverses the via list to produce the destination list,
resulting in (D, C, B, A). When D forwards the response to C, the
destination list will contain (C, B, A).
As an example of the second strategy, if node D receives a message
from node C with transaction ID X and via list (A, B), it could store
(X, C) in its state database and forward the message with the via
list unchanged. When D receives the response, it consults its state
database for transaction id X, determines that the request came from
C, and forwards the response to C.
Intermediate peer which modify the via list are not required to
simply add entries. The only requirement is that the peer be able to
reconstruct the correct destination list on the return route. RELOAD
provides explicit support for this functionality in the form of
private IDs, which can replace any number of via list entries. For
instance, in the above example, Node D might send E a via list
containing only the private ID (I). E would then use the destination
list (D, I) to send its return message. When D processes this
destination list, it would detect that I is a private ID, recover the
via list (A, B, C), and reverse that to produce the correct
destination list (C, B, A) before sending it to C. This feature is
called List Compression. I MAY either be a compressed version of the
original via list or an index into a state database containing the
original via list.
Note that if an intermediate peer exits the overlay, then on the
return trip the message cannot be forwarded and will be dropped. The
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ordinary timeout and retransmission mechanisms provide stability over
this type of failure.
6.1.2.3. Private ID
If the first entry on the destination list is a private id (e.g., a
compressed via list), the peer MUST that entry with the original via
list that it replaced indexes and then re-examine the destination
list to determine which case now applies.
6.1.3. Response Origination
When a peer sends a response to a request, it MUST construct the
destination list by reversing the order of the entries on the via
list. This has the result that the response traverses the same peers
as the request traversed, except in reverse order (symmetric
routing). Note that this rule will need to be relaxed if other
routing algorithms are supported.
6.2. Message Structure
RELOAD is a message-oriented request/response protocol. The messages
are encoded using binary fields. All integers are represented in
network byte order. The general philosophy behind the design was to
use Type, Length, Value fields to allow for extensibility. However,
for the parts of a structure that were required in all messages, we
just define these in a fixed position as adding a type and length for
them is unnecessary and would simply increase bandwidth and
introduces new potential for interoperability issues.
Each message has three parts, concatenated as shown below:
+-------------------------+
| Forwarding Header |
+-------------------------+
| Message Contents |
+-------------------------+
| Signature |
+-------------------------+
The contents of these parts are as follows:
Forwarding Header: Each message has a generic header which is used
to forward the message between peers and to its final destination.
This header is the only information that an intermediate peer
(i.e., one that is not the target of a message) needs to examine.
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Message Contents: The message being delivered between the peers.
From the perspective of the forwarding layer, the contents is
opaque, however, it is interpreted by the higher layers.
Signature: A digital signature over the message contents and parts
of the header of the message. Note that this signature can be
computed without parsing the message contents.
The following sections describe the format of each part of the
message.
6.2.1. Presentation Language
Most of the structures defined in this document (with the exception
of the forwarding header defined in the next section) are defined
using a C-like syntax based on the presentation language used to
define TLS. Advantages of this style include:
o It is easy to write and familiar enough looking that most readers
can grasp it quickly.
o The ability to define nested structures allows a separation
between high-level and low level message structures.
o It has a straightforward wire encoding that allows quick
implementation, but the structures can be comprehended without
knowing the encoding.
This presentation is to some extent a placeholder. We consider it an
open question what the final protocol definition method and encodings
use. We expect this to be a question for the WG to decide.
Several idiosyncrasies of this language are worth noting.
o All lengths are denoted in bytes, not objects.
o Variable length values are denoted like arrays with angle
brackets.
o "select" is used to indicate variant structures.
For instance, "uint16 array<0..2^8-2>;" represents up to 254 bytes
but only up to 127 values of two bytes (16 bits) each..
6.2.1.1. Common Definitions
The following definitions are used throughout RELOAD and so are
defined here. They also provide a convenient introduction to how to
read the presentation language.
An enum represents an enumerated type. The values associated with
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each possibility are represented in parentheses and the maximum value
is represented as a nameless value, for purposes of describing the
width of the containing integral type. For instance, Boolean
represents a true or false:
enum { false (0), true(1), (255)} Boolean;
A boolean value is either a 1 or a 0 and is represented as a single
byte on the wire.
The NodeId, shown below, represents a single Node-ID.
typedef opaque NodeId[16];
A NodeId is a fixed-length 128-bit structure represented as a series
of bytes, most significant byte first. Note: the use of "typedef"
here is an extension to the TLS language, but its meaning should be
relatively obvious.
A ResourceId, shown below, represents a single resource-id.
typedef opaque ResourceId<0..2^8-1>;
Like a NodeId, a resource-id is an opaque string of bytes, but unlike
Node-IDs, resource-ids are variable length, up to 255 bytes (2048
bits) in length. On the wire, each ResourceId is preceded by a
single length byte (allowing lengths up to 255). Thus, the 3-byte
value "Foo" would be encoded as: 03 46 4f 4f.
A more complicated example is IpAddressPort, which represents a
network address and can be used to carry either an IPv6 or IPv4
address:
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enum {reserved(0), ip4_address (1), ip6_address (2), (255)}
AddressType;
struct {
uint32 addr;
uint16 port;
} IPv4AddrPort;
struct {
uint128 addr;
uint16 port;
} IPv6AddrPort;
struct {
AddressType type;
uint8 length;
select (type) {
case ipv4_address:
IPv4AddrPort v4addr_port;
case ipv6_address:
IPv6AddrPort v6addr_port;
/* This structure can be extended */
} IpAddressPort;
The first two fields in the structure are the same no matter what
kind of address is being represented:
type
the type of address (v4 or v6).
length
the length of the rest of the structure.
By having the type and the length appear at the beginning of the
structure regardless of the kind of address being represented, an
implementation which does not understand new address type X can still
parse the IpAddressPort field and then discard it if it is not
needed.
The rest of the IpAddressPort structure is either an IPv4AddrPort or
an IPv6AddrPort. Both of these simply consist of an address
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represented as an integer and a 16-bit port. As an example, here is
the wire representation of the IPv4 address "192.0.2.1" with port
"6100".
01 ; type = IPv4
06 ; length = 6
c0 00 02 01 ; address = 192.0.2.1
17 d4 ; port = 6100
6.2.2. Forwarding Header
The layout of the forwarding header is shown below. We present this
as a bit diagram because it is mostly fixed and to show the
similarities with other packet headers.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| R | E | L | O |
4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Overlay |
8 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |F|L| |
| TTL | Reserved |R|F| Fragment Offset |
| | |A|R| |
| | |G|G| |
12 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
| Version | Length |
| | |
16 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transaction ID |
+ +
| |
24 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
| Flags | Via List Length |
| | |
28 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Destination List Length |
| |
30 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Via List //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Destination List //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Route Log //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first four bytes identify this message as a RELOAD message. The
message is easy to demultiplex from STUN messages by looking at the
first bit.
The Overlay field is the 32 bit checksum/hash of the overlay being
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used. The variable length string representing the overlay name is
hashed with SHA-1 and the low order 32 bits are used. The purpose of
this field is to allow nodes to participate in multiple overlays and
to detect accidental misconfiguration. This is not a security
critical function.
TTL (time-to-live) is an 8 bit field indicating the number of
iterations, or hops, a message can experience before it is discarded.
The TTL value MUST be decremented by one at every hop along the route
the message traverses. If the TTL is 0, the message MUST NOT be
propagated further and MUST be discarded. The initial value of the
TTL should be TBD.
FRAG is a 1 bit field used to specify if this message is a fragment.
NOT-FRAGMENT : 0x0
FRAGMENT : 0x1
LFRG is a 1 bit field used to specify whether this is the last
fragment in a complete message.
NOT-LAST-FRAGMENT : 0x0
LAST-FRAGMENT : 0x1
[[Open Issue: This is conceptually clear, but the details are still
lacking. Need to define the fragment offset and total length be
encoded in the header. Right now we have 14 bits reserved with the
intention that they be used for fragmenting, though additional bytes
in the header might be needed for fragmentation.]]
Version is a 7 bit field that indicates the version of the RELOAD
protocol being used.
Version0.1 : 0x1
The message Length is the count in bytes of the size of the message,
including the header.
The Transaction ID is a unique 64 bit number that identifies this
transaction and also serves as a salt to randomize the request and
the response. Responses use the same Transaction ID as the request
they correspond to. Transaction IDs are also used for fragment
reassembly.
The flags word contains control flags. There is one currently
defined flag.
ROUTE-LOG : 0x1
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The ROUTE-LOG flag indicates that the route log should be included
(see Section 6.2.2.2).
The Destination List Length and the Via List Length contain the
lengths of the route and via lists respectively, in bytes.
The Via List contains the sequence of destinations through which the
message has passed. The via list starts out empty and grows as the
message traverses each peer.
The Destination List contains a sequence of destinations which the
message should pass through. The destination list is constructed by
the message originator. The first element in the destination list is
where the message goes next. The list shrinks as the message
traverses each listed peer.
6.2.2.1. Destination and Via Lists
The destination list and via lists are sequences of Destination
values:
enum {reserved(0), peer(2), resource(2), compressed(3), (255) }
DestinationType;
select (destination_type) {
case peer:
NodeId node_id;
case resource:
ResourceId resource_id;
case compressed:
opaque compressed_id;
/* This structure may be extended with new types */
} DestinationData;
struct {
DestinationType type;
uint8 length;
DestinationData destination_data;
} Destination;
This is a TLV structure with the following contents:
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type
The type of the DestinationData PDU. This may be one of "peer",
"resource", or "compressed".
length
The length of the destination_data.
destination_value
The destination value itself, which is an encoded DestinationData
structure, depending on the value of "type".
Note: This structure encodes a type, length, value. The length
field specifies the length of the DestinationData values, which
allows the addition of new DestinationTypes. This allows an
implementation which does not understand a given DestinationType
to skip over it.
A DestinationData can be one of three types:
peer
A Node-ID.
compressed
A compressed list of Node-IDs and/or resources. Because this
value was compressed by one of the peers, it is only meaningful to
that peer and cannot be decoded by other peers. Thus, it is
represented as an opaque string.
resource
The Resource-ID of the resource which is desired. This type MUST
only appear in the final location of a destination list and MUST
NOT appear in a via list. It is meaningless to try to route
through a resource.
6.2.2.2. Route Logging
The route logging feature provides diagnostic information about the
path taken by the request so far and in this manner it is similar in
function to SIP's [RFC3261] Via header field. If the ROUTE-LOG flag
is set in the Flags word, at each hop peers MUST append a route log
entry to the route log element in the header or reject the request.
The order of the route log entry elements in the message is
determined by the order of the peers were traversed along the path.
The first route log entry corresponds to the peer at the first hop
along the path, and each subsequent entry corresponds to the peer at
the next hop along the path. If the ROUTE-LOG flag is set in a
request, the route log MUST be copied into the response and the
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ROUTE-LOG flag set so that the originator receives the ROUTE-LOG
data.
If the responder wishes to have a route log in the reverse direction,
it MAY set the ROUTE-LOG flag in its response as well. Note,
however, that this means that the response will grow on the return
path, which may potentially mean that it gets dropped due to becoming
too large for some intermediate hop. Thus, this option must be used
with care.
The route log is defined as follows:
enum { (255) } RouteLogExtensionType;
struct {
RouteLogExtensionType type;
uint16 length;
select (type){
/* Extension values go here */
} extension;
} RouteLogExtension;
enum { reserved(0), tcp_tls(1), udp_dtls(2), (255)} Transport;
struct {
opaque version<0..2^8-1>; /* A string */
Transport transport; /* TCP or UDP */
NodeId id;
uint32 uptime;
IpAddressPort address;
opaque certificate<0..2^16-1>;
RouteLogExtension extensions<0..2^16-1>;
} RouteLogEntry;
struct {
RouteLogEntry entries<0..2^16-1>;
} RouteLog;
The route log consists of an arbitrary number of RouteLogEntry
values, each representing one node through which the message has
passed.
Each RouteLogEntry consists of the following values:
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version
A textual representation of the software version
transport
The transport type, currently either "tcp_tls" or "udp_dtls".
id
The Node-ID of the peer.
uptime
The uptime of the peer in seconds.
address
The address and port of the peer.
certificate
The peer's certificate. Note that this may be omitted by setting
the length to zero.
extensions
Extensions, if any.
Extensions are defined using a RouteLogExtension structure. New
extensions are defined by defining a new code point for
RouteLogExtensionType and adding a new arm to the RouteLogExtension
structure. The contents of that structure are:
type
The type of the extension.
length
The length of the rest of the structure.
extension
The extension value.
6.2.3. Message Contents Format
The second major part of a RELOAD message is the contents part, which
is defined by MessageContents:
struct {
MessageCode message_code;
opaque payload<0..2^24-1>;
} MessageContents;
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The contents of this structure are as follows:
message_code
This indicates the message that is being sent. The code space is
broken up as follows.
0 Reserved
1 .. 0x7fff Requests and responses. These code points are always
paired, with requests being odd and the corresponding response
being the request code plus 1. Thus, "ping_request" (the Ping
request) has value 1 and "ping_answer" (the Ping response) has
value 2
0xffff Error
message_body
The message body itself, represented as a variable-length string
of bytes. The bytes themselves are dependent on the code value.
See the sections describing the various RELOAD methods (Join,
Update, Connect, Store, Fetch, etc.) for the definitions of the
payload contents.
6.2.3.1. Response Codes and Response Errors
A peer processing a request returns its status in the message_code
field. If the request was a success, then the message code is the
response code that matches the request (i.e., the next code up). The
response payload is then as defined in the request/response
descriptions.
If the request failed, then the message code is set to 0xffff (error)
and the payload MUST be an error_response PDU, as shown below.
When the message code is 0xffff, the payload MUST be an
ErrorResponse.
public struct {
uint16 error_code;
opaque reason_phrase<0..2^8-1>; /* String*/
opaque error_info<0..65000>;
} ErrorResponse;
The contents of this structure are as follows:
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error_code
A numeric error code indicating the error that occurred.
reason_phrase
A free form text string indicating the reason for the response.
The reason phrase SHOULD BE as indicated in the error code list
below (e.g., "Moved Temporarily"). [[Open Issue: These reason
phrases are pretty useless. Like the rest of this error system,
They're a holdover from SIP. Should we remove?]]
error_info
Payload specific error information. This MUST be empty (zero
length) except as specified below.
The following error code values are defined. [[TODO: These are
currently semi-aligned with SIP codes. that's probably bad and we
need to fix.]
302 (Moved Temporarily): The requesting peer SHOULD retry the
request at the new address specified in the 302 response message.
401 (Unauthorized): The requesting peer needs to sign and provide a
certificate. [[TODO: The semantics here don't seem quite
right.]]
403 (Forbidden): The requesting peer does not have permission to
make this request.
404 (Not Found): The resource or peer cannot be found or does not
exist.
408 (Request Timeout): A response to the request has not been
received in a suitable amount of time. The requesting peer MAY
resend the request at a later time.
412 (Precondition Failed): A request can't be completed because some
precondition was incorrect. For instance, the wrong generation
counter was provided
498 (Incompatible with Overlay) A peer receiving the request is
using a different overlay, overlay algorithm, or hash algorithm.
[[Open Issue: What is the best error number and reason phrase to
use?]]
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6.2.4. Signature
The third part of a RELOAD message is the signature, represented by a
Signature structure. The message signature is computed over the
payload and parts of forwarding header. The payload, in case of a
Store, may contain an additional signature computed over a StoreReq
structure. All signatures are formatted using the Signature element.
This element is also used in other contexts where signatures are
needed. The input structure to the signature computation varies
depending on the data element being signed.
enum {reserved(0), signer_identity_peer (1),
signer_identity_name (2), signer_identity_certificate (3),
(255)} SignerIdentityType;
select (identity_type) {
case signer_identity_peer:
NodeId id;
case signer_identity_name:
opaque name<0..2^16-1>;
case signer_identity_certificate:
opaque certificate<0..2^16-1>;
/* This structure may be extended with new types */
} SignerIdentityValue;
struct {
SignerIdentityType identity_type;
uint16 length;
SignerIdentityValue identity[SignerIdentity.length];
} SignerIdentity;
struct {
SignatureAndHashAlgorithm algorithm;
SignerIdentity identity;
opaque signature_value<0..2^16-1>;
} Signature;
The signature construct contains the following values:
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algorithm
The signature algorithm in use. The algorithm definitions are
found in the IANA TLS SignatureAlgorithm Registry.
identity
The identity or certificate used to form the signature
signature_value
The value of the signature
A number of possible identity formats are permitted. The current
possibilities are: a Node-ID, a user name, and a certificate.
For signatures over messages the input to the signature is computed
over:
overlay + transaction_id + MessageContents + SignerIdentity
Where overlay and transaction_id come from the forwarding header and
+ indicates concatenation.
[[TODO: Check the inputs to this carefully.]]
The input to signatures over data values is different, and is
described in Section 7.1.
6.3. Overlay Topology
As discussed in previous sections, RELOAD does not itself implement
any overlay topology. Rather, it relies on Topology Plugins, which
allow a variety of overlay algorithms to be used while maintaining
the same RELOAD core. This section describes the requirements for
new topology plugins and the methods that RELOAD provides for overlay
topology maintenance.
6.3.1. Topology Plugin Requirements
When specifying a new overlay algorithm, at least the following need
to be described:
o Joining procedures, including the contents of the Join message.
o Stabilization procedures, including the contents of the Update
message, the frequency of topology probes and keepalives, and the
mechanism used to detect when peers have disconnected.
o Exit procedures, including the contents of the Leave message.
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o The length of the Resource-IDs and Node-IDs. For DHTs, the hash
algorithm to compute the hash of an identifier.
o The procedures that peers use to route messages.
o The replication strategy used to ensure data redundancy.
6.3.2. Methods and types for use by topology plugins
This section describes the methods that topology plugins use to join,
leave, and maintain the overlay.
6.3.2.1. Join
A new peer (but which already has credentials) uses the JoinReq
message to join the overlay. The JoinReq is sent to the responsible
peer depending on the routing mechanism described in the topology
plugin. This notifies the responsible peer that the new peer is
taking over some of the overlay and it needs to synchronize its
state.
struct {
NodeId joining_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} JoinReq;
The minimal JoinReq contains only the Node-ID which the sending peer
wishes to assume. Overlay algorithms MAY specify other data to
appear in this request.
If the request succeeds, the responding peer responds with a JoinAns
message, as defined below:
struct {
opaque overlay_specific_data<0..2^16-1>;
} JoinAns;
If the request succeeds, the responding peer MUST follow up by
executing the right sequence of Stores and Updates to transfer the
appropriate section of the overlay space to the joining peer. In
addition, overlay algorithms MAY define data to appear in the
response payload that provides additional info.
6.3.2.2. Leave
The LeaveReq message is used to indicate that a node is exiting the
overlay. A node SHOULD send this message to each peer with which it
is directly connected prior to exiting the overlay.
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public struct {
NodeId leaving_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} LeaveReq;
The default LeaveReq contains only the Node-ID of the leaving peer.
Overlay algorithms MAY specify other data to appear in this request.
Upon receiving a Leave request, a peer MUST update its own routing
table, and send the appropriate Store/Update sequences to re-
stabilize the overlay.
6.3.2.3. Update
Update is the primary overlay-specific maintenance message. It is
used by the sender to notify the recipient of the sender's view of
the current state of the overlay (its routing state) and it is up to
the recipient to take whatever actions are appropriate to deal with
the state change.
The contents of the UpdateReq message are completely overlay-
specific. The UpdateAns response is expected to be either success or
an error.
6.3.2.4. Route_Query
The Route_Query request allows the sender to ask a peer where they
would route a message directed to a given destination. In other
words, a RouteQuery for a destination X requests the Node-ID where
the receiving peer would next route to get to X. A RouteQuery can
also request that the receiving peer initiate an Update request to
transfer his routing table.
One important use of the RouteQuery request is to support iterative
routing. The sender selects one of the peers in its routing table
and sends it a RouteQuery message with the destination_object set to
the Node-ID or Resource-ID it wishes to route to. The receiving peer
responds with information about the peers to which the request would
be routed. The sending peer MAY then Connects to that peer(s), and
repeats the RouteQuery. Eventually, the sender gets a response from
a peer that is closest to the identifier in the destination_object as
determined by the topology plugin. At that point, the sender can
send messages directly to that peer.
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6.3.2.4.1. Request Definition
A RouteQueryReq message indicates the peer or resource that the
requesting peer is interested in. It also contains a "send_update"
option allowing the requesting peer to request a full copy of the
other peer's routing table.
struct {
Boolean send_update;
Destination destination;
opaque overlay_specific_data<0..2^16-1>;
} RouteQueryReq;
The contents of the RouteQueryReq message are as follows:
send_update
A single byte. This may be set to "true" to indicate that the
requester wishes the responder to initiate an Update request
immediately. Otherwise, this value MUST be set to "false".
destination
The destination which the requester is interested in. This may be
any valid destination object, including a Node-ID, compressed ids,
or resource-id.
overlay_specific_data
Other data as appropriate for the overlay.
6.3.2.4.2. Response Definition
A response to a successful RouteQueryReq request is a RouteQueryAns
message. This is completely overlay specific.
6.4. Forwarding Layer
Each node maintains connections to a set of other nodes defined by
the topology plugin.
6.4.1. Transports
RELOAD can use multiple transports to send its messages. Because ICE
is used to establish connections (see Section 6.4.2.1.3), RELOAD
nodes are able to detect which transports are offered by other nodes
and establish connections between each other. Any transport protocol
needs to be able to establish a secure, authenticated connection, and
provide data origin authentication and message integrity for
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individual data elements. RELOAD currently supports two transport
protocols:
o TLS [REF] over TCP
o DTLS [RFC4347] over UDP
Note that although UDP does not properly have "connections", both TLS
and DTLS have a handshake which establishes a stateful association, a
similar stateful construct, and we simply refer to these as
"connections" for the purposes of this document.
6.4.1.1. Future Support for HIP
The P2PSIP Working Group has expressed interest in supporting a HIP-
based transport. Such support would require specifying such details
as:
o How to issue certificates which provided identities meaningful to
the HIP base exchange. We anticipate that this would require a
mapping between ORCHIDs and NodeIds.
o How to carry the HIP I1 and I2 messages. We anticipate that this
would require defining a HIP Tunnel usage.
o How to carry RELOAD messages over HIP.
We leave this work as a topic for another draft.
6.4.1.2. Reliability for Unreliable Transports
When RELOAD is carried over DTLS or another unreliable transport, it
needs to be used with a reliability and congestion control mechanism,
which is provided on a hop-by-hop basis, matching the semantics if
TCP were used. The basic principle is that each message, regardless
of if it carries a request or responses, will get an ACK and be
reliably retransmitted. The receiver's job is very simple, limited
to just sending ACKs. All the complexity is at the sender side.
This allows the sending implementation to trade off performance
versus implementation complexity without affecting the wire protocol.
In order to support unreliable transport, each message is wrapped in
a very simple framing layer (FramedMessage) which is only used for
each hop. This layer contains a sequence number which can then be
used for ACKs.
6.4.1.2.1. Framed Message Format
The definition of FramedMessage is:
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enum {data (128), ack (129), (255)} FramedMessageType;
struct {
FramedMessageType type;
select (type) {
case data:
uint24 sequence;
opaque message<0..2^24-1>;
case ack:
uint24 ack_sequence;
uint32 received;
};
} FramedMessage;
The type field of the PDU is set to indicate whether the message is
data or an acknowledgement. Note that these values have been set to
force the first bit to be high, thus allowing easy demultiplexing
with STUN. All FramedMessageType values must be > 128.
If the message is of type "data", then the remainder of the PDU is as
follows:
sequence
the sequence number
message
the original message that is being transmitted.
Each connection has it own sequence number. Initially the value is
zero and it increments by exactly one for each message sent over that
connection.
When the receiver receive a message, it SHOULD immediately send an
ACK message. The receiver MUST keep track of the 32 most recent
sequence numbers received on this association in order to generate
the appropriate ack.
If the PDU is of type "ack", the contents are as follows:
ack_sequence
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The sequence number of the message being acknowledged.
received
A bitmask indicating whether or not each of the previous 32
packets has been received before the sequence number in
ack_sequence. The high order bit represents the first packet in
the sequence space.
The received field bits in the ACK provide a very high degree of
redundancy for the sender to figure out which packets the receiver
received and can then estimate packet loss rates. If the sender also
keeps track of the time at which recent sequence numbers were sent,
the RTT can be estimated.
6.4.1.2.2. Retransmission and Flow Control
Because the receiver's role is limited to providing packet
acknowledgements, a wide variety of congestion control algorithms can
be implemented on the sender side while using the same basic wire
protocol. It is RECOMMENDED that senders implement TFRC-SP [RFC4828]
and use the received bitmask to allow the sender to compute packer
loss event rates. Senders MUST implement a retransmission and
congestion control scheme no more aggressive then TFRC-SP.
6.4.1.3. Fragmentation and Reassembly
In order to allow transport over datagram protocols, RELOAD messages
may be fragmented. If a message is too large for a peer to transmit
to the next peer it MUST fragment the message. Note that this
implies that intermediate peers may re-fragment messages if the
incoming and outgoing paths have different maximum datagram sizes.
Intermediate peers SHOULD NOT reassemble fragments.
Upon receipt of a fragmented message by the intended peer, the peer
holds the fragments in a holding buffer until the entire message has
been received. The message is then reassembled into a single
unfragmented message and processed. In order to mitigate denial of
service attacks, receivers SHOULD time out incomplete fragments.
[[TODO: Describe algorithm]]
6.4.2. Connection Management Methods
This section defines the methods RELOAD uses to form and maintain
connections between nodes in the overlay. Three methods are defined:
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Connect: used to form connections between nodes. When node A
wants to connect to node B, it sends a Connect message to node B
through the overlay. The Connect contains A's ICE parameters. B
responds with its ICE parameters and the two nodes perform ICE to
form connection.
Ping: is a simple request/response which is used to verify
connectivity (analogous to the UNIX ping command) along a path and
to gather a small amount of information about the resources held
by the target peer
Tunnel: in some cases, it will be too expensive for an application
layer protocol to set up a connection in order to send a small
number of messages. The Tunnel message allows applications to
route individual application layer protocol messages through the
overlay.
6.4.2.1. Connect
A node sends a Connect request when it wishes to establish a direct
TCP or UDP connection to another node for the purposes of sending
RELOAD messages or application layer protocol messages, such as SIP.
Detailed procedures for the Connect and its response are described in
Section 6.4.2.1.
A Connect in and of itself does not result in updating the routing
table of either node. That function is performed by Updates. If
node A has Connected to node B, but not received any Updates from B,
it MAY route messages which are directly addressed to B through that
channel but MUST NOT route messages through B to other peers via that
channel. The process of Connecting is separate from the process of
becoming a peer (using Update) to prevent half-open states where a
node has started to form connections but is not really ready to act
as a peer.
6.4.2.1.1. Request Definition
A ConnectReq message contains the requesting peer's ICE connection
parameters formatted into a binary structure.
typedef opaque IceCandidate<0..2^16-1>;
struct {
opaque ufrag<0..2^8-1>;
opaque password<0..2^8-1>;
uint16 application;
opaque role<0..2^8-1>;
IceCandidate candidates<0..2^16-1>;
} ConnectReqAns;
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The values contained in ConnectReq and ConnectAns are:
ufrag
The username fragment (from ICE)
password
The ICE password.
application
A 16-bit port number. This port number represents the IANA
registered port of the protocol that is going to be sent on this
connection. For SIP, this is 5060 or 5061, and for RELOAD is TBD.
By using the IANA registered port, we avoid the need for an
additional registry and allow RELOAD to be used to set up
connections for any existing or future application protocol.
role
An active/passive/actpass attribute from RFC 4145 [RFC4145].
candidates
One or more ICE candidate values. Each candidate has an IP
address, IP address family, port, transport protocol, priority,
foundation, component ID, STUN type and related address. The
candidate_list is a list of string candidate values from ICE.
These values should be generated using the procedures described in
Section 6.4.2.1.3.
6.4.2.1.2. Response Definition
If a peer receives a Connect request, it SHOULD follow the process
the request and generate its own response with a ConnectReqAns It
should then begin ICE checks. When a peer receives a Connect
response, it SHOULD parse the response and begin its own ICE checks.
6.4.2.1.3. Using ICE With RELOAD
This section describes the profile of ICE that is used with RELOAD.
RELOAD implementations MUST implement full ICE. Because RELOAD
always tries to use TCP and then UDP as a fallback, there will be
multiple candidates of the same IP version, which requires full ICE.
In ICE as defined by [I-D.ietf-mmusic-ice], SDP is used to carry the
ICE parameters. In RELOAD, this function is performed by a binary
encoding in the Connect method. This encoding is more restricted
than the SDP encoding because the RELOAD environment is simpler:
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o Only a single media stream is supported.
o In this case, the "stream" refers not to RTP or other types of
media, but rather to a connection for RELOAD itself or for SIP
signaling.
o RELOAD only allows for a single offer/answer exchange. Unlike the
usage of ICE within SIP, there is never a need to send a
subsequent offer to update the default candidates to match the
ones selected by ICE.
An agent follows the ICE specification as described in
[I-D.ietf-mmusic-ice] and [I-D.ietf-mmusic-ice-tcp] with the changes
and additional procedures described in the subsections below.
6.4.2.1.4. Collecting STUN Servers
ICE relies on the node having one or more STUN servers to use. In
conventional ICE, it is assumed that nodes are configured with one or
more STUN servers through some out-of-band mechanism. This is still
possible in RELOAD but RELOAD also learns STUN servers as it connects
to other peers. Because all RELOAD peers implement ICE and use STUN
keepalives, every peer is a STUN server [I-D.ietf-behave-rfc3489bis].
Accordingly, any peer a node knows will be willing to be a STUN
server -- though of course it may be behind a NAT.
A peer on a well-provisioned wide-area overlay will be configured
with one or more bootstrap peers. These peers make an initial list
of STUN servers. However, as the peer forms connections with
additional peers, it builds more peers it can use as STUN servers.
Because complicated NAT topologies are possible, a peer may need more
than one STUN server. Specifically, a peer that is behind a single
NAT will typically observe only two IP addresses in its STUN checks:
its local address and its server reflexive address from a STUN server
outside its NAT. However, if there are more NATs involved, it may
discover that it learns additional server reflexive addresses (which
vary based on where in the topology the STUN server is). To maximize
the chance of achieving a direct connection, a peer SHOULD group
other peers by the peer-reflexive addresses it discovers through
them. It SHOULD then select one peer from each group to use as a
STUN server for future connections.
Only peers to which the peer currently has connections may be used.
If the connection to that host is lost, it MUST be removed from the
list of stun servers and a new server from the same group SHOULD be
selected.
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6.4.2.1.5. Gathering Candidates
When a node wishes to establish a connection for the purposes of
RELOAD signaling or SIP signaling (or any other application protocol
for that matter), it follows the process of gathering candidates as
described in Section 4 of ICE [I-D.ietf-mmusic-ice]. RELOAD utilizes
a single component, as does SIP. Consequently, gathering for these
"streams" requires a single component.
An agent MUST implement ICE-tcp [I-D.ietf-mmusic-ice], and MUST
gather at least one UDP and one TCP host candidate for RELOAD and for
SIP.
The ICE specification assumes that an ICE agent is configured with,
or somehow knows of, TURN and STUN servers. RELOAD provides a way
for an agent to learn these by querying the overlay, as described in
Section 6.4.2.1.4 and Section 9.
The agent SHOULD prioritize its TCP-based candidates over its UDP-
based candidates in the prioritization described in Section 4.1.2 of
ICE [I-D.ietf-mmusic-ice].
The default candidate selection described in Section 4.1.3 of ICE is
ignored; defaults are not signaled or utilized by RELOAD.
6.4.2.1.6. Encoding the Connect Message
Section 4.3 of ICE describes procedures for encoding the SDP for
conveying RELOAD or SIP ICE candidates. Instead of actually encoding
an SDP, the candidate information (IP address and port and transport
protocol, priority, foundation, component ID, type and related
address) is carried within the attributes of the Connect request or
its response. Similarly, the username fragment and password are
carried in the Connect message or its response. Section 6.4.2.1
describes the detailed attribute encoding for Connect. The Connect
request and its response do not contain any default candidates or the
ice-lite attribute, as these features of ICE are not used by RELOAD.
The Connect request and its response also contain a application
attribute, with a value of SIP or RELOAD, which indicates what
protocol is to be run over the connection. The RELOAD Connect
request MUST only be utilized to set up connections for application
protocols that can be multiplexed with STUN.
Since the Connect request contains the candidate information and
short term credentials, it is considered as an offer for a single
media stream that happens to be encoded in a format different than
SDP, but is otherwise considered a valid offer for the purposes of
following the ICE specification. Similarly, the Connect response is
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considered a valid answer for the purposes of following the ICE
specification.
Similarly, the node MUST implement the active, passive, and actpass
attributes from RFC 4145 [RFC4145]. However, here they refer
strictly to the role of active or passive for the purposes of TLS
handshaking. The TCP connection directions are signaled as part of
the ICE candidate attribute.
6.4.2.1.7. Verifying ICE Support
An agent MUST skip the verification procedures in Section 5.1 and 6.1
of ICE. Since RELOAD requires full ICE from all agents, this check
is not required.
6.4.2.1.8. Role Determination
The roles of controlling and controlled as described in Section 5.2
of ICE are still utilized with RELOAD. However, the offerer (the
entity sending the Connect request) will always be controlling, and
the answerer (the entity sending the Connect response) will always be
controlled. The connectivity checks MUST still contain the ICE-
CONTROLLED and ICE-CONTROLLING attributes, however, even though the
role reversal capability for which they are defined will never be
needed with RELOAD. This is to allow for a common codebase between
ICE for RELOAD and ICE for SDP.
6.4.2.1.9. Connectivity Checks
The processes of forming check lists in Section 5.7 of ICE,
scheduling checks in Section 5.8, and checking connectivity checks in
Section 7 are used with RELOAD without change.
6.4.2.1.10. Concluding ICE
The controlling agent MUST utilize regular nomination. This is to
ensure consistent state on the final selected pairs without the need
for an updated offer, as RELOAD does not generate additional offer/
answer exchanges.
The procedures in Section 8 of ICE are followed to conclude ICE, with
the following exceptions:
o The controlling agent MUST NOT attempt to send an updated offer
once the state of its single media stream reaches Completed.
o Once the state of ICE reaches Completed, the agent can immediately
free all unused candidates. This is because RELOAD does not have
the concept of forking, and thus the three second delay in Section
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8.3 of ICE does not apply.
6.4.2.1.11. Subsequent Offers and Answers
An agent MUST NOT send a subsequent offer or answer. Thus, the
procedures in Section 9 of ICE MUST be ignored.
6.4.2.1.12. Media Keepalives
STUN MUST be utilized for the keepalives described in Section 10 of
ICE.
6.4.2.1.13. Sending Media
The procedures of Section 11 apply to RELOAD as well. However, in
this case, the "media" takes the form of application layer protocols
(RELOAD or SIP for example) over TLS or DTLS. Consequently, once ICE
processing completes, the agent will begin TLS or DTLS procedures to
establish a secure connection. The nodes MUST verify that the
certificate presented in the handshake matches the identity of the
other peer as found in the Connect message. Once the TLS or DTLS
signaling is complete, the application protocol is free to use the
connection.
The concept of a previous selected pair for a component does not
apply to RELOAD, since ICE restarts are not possible with RELOAD.
6.4.2.1.14. Receiving Media
An agent MUST be prepared to receive packets for the application
protocol (TLS or DTLS carrying RELOAD, SIP or anything else) at any
time. The jitter and RTP considerations in Section 11 of ICE do not
apply to RELOAD or SIP.
6.4.2.2. Ping
Ping is used to test connectivity along a path. A ping can be
addressed to a specific Node-ID, the peer controlling a given
location (by using a resource ID) or to the broadcast Node-ID (all
1s). In either case, the target Node-IDs respond with a simple
response containing some status information.
6.4.2.2.1. Request Definition
The PingReq message contains a list (potentially empty) of the pieces
of status information that the requester would like the responder to
provide.
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enum { responsible_set(1), num_resources(2), (255)}
PingInformationType;
struct {
PingInformationType requested_info<0..2^8-1>;
} PingReq
The two currently defined values for PingInformation are:
responsible_set
indicates that the peer should Respond with the fraction of the
overlay for which the responding peer is responsible.
num_resources
indicates that the peer should Respond with the number of
resources currently being stored by the peer.
6.4.2.2.2. Response Definition
A successful PingAns response contains the information elements
requested by the peer.
struct {
PingInformationType type;
select (type) {
case responsible_set:
uint32 responsible_ppb;
case num_resources:
uint32 num_resources;
/* This type may be extended */
};
} PingInformation;
struct {
uint64 response_id;
PingInformation ping_info<0..2^16-1>;
} PingAns;
A PingAns message contains the following elements:
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response_id
A randomly generated 64-bit response ID. This is used to
distinguish Ping responses in cases where the Ping request is
multicast.
ping_info
A sequence of PingInformation structures, as shown below.
Each of the current possible Ping information types is a 32-bit
unsigned integer. For type "responsible_ppb", it is the fraction of
the overlay for which the peer is responsible in parts per billion.
For type "num_resources", it is the number of resources the peer is
storing.
The responding peer SHOULD include any values that the requesting
peer requested and that it recognizes. They SHOULD be returned in
the requested order. Any other values MUST NOT be returned.
6.4.2.3. Tunnel
A node sends a Tunnel request when it wishes to exchange application-
layer protocol messages without the expense of establishing a direct
connection via Connect or when ICE is unable to establish a direct
connection via Connect and a TURN relay is not available. The
application-level protocols that are routed via the Tunnel request
are defined by that application's usage.
Note: The decision of whether to route application-level traffic
across the overlay or to open a direct connection requires careful
consideration of the overhead involved in each transaction.
Establishing a direct connection requires greater initial setup
costs, but after setup, communication is faster and imposes no
overhead on the overlay. For example, for the SIP usage, an
INVITE request to establish a voice call might be routed over the
overlay, a SUBSCRIBE with regular updates would be better used
with a Connect, and media would both impose too great a load on
the overlay and likely receive unacceptable performance. However,
there may be a tradeoff between locating TURN servers and relying
on Tunnel for packet routing.
When a usage requires the Tunnel method, it must specify the specific
application protocol(s) that will be Tunneled and for each protocol,
specify:
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o An application attribute that indicates the protocol being
tunneled. This the IANA-registered port of the application
protocol.
o The conditions under which the application will be Tunneled over
the overlay rather than using a direct Connect.
o A mechanism for moving future application-level communication from
Tunneling on the overlay to a direct Connection, or an explanation
why this is unnecessary.
o A means of associating messages together as required for dialog-
oriented or request/response-oriented protocols.
o How the Tunneled message (and associated responses) will be
delivered to the correct application. This is particularly
important if there might be multiple instances of the application
on or behind a single peer.
6.4.2.3.1. Request Definition
The TunnelReq message contains the application PDU that the
requesting peer wishes to transmit, along with some control
information identifying the handling of the PDU.
struct {
uint16 application;
opaque dialog_id<0..2^8-1>;
opaque application_pdu<0..2^24-1>;
} TunnelReq;
The values contained in the TunnelReq are:
application
A 16-bit port number. This port number represents the IANA
registered port of the protocol that is going to be sent on this
connection. For SIP, this is 5060 or 5061, and for RELOAD is TBD.
By using the IANA registered port, we avoid the need for an
additional registry and allow RELOAD to be used to set up
connections for any existing or future application protocol.
dialog_id
An arbitrary string providing an application-defined way of
associating related Tunneled messages. This attribute may also
encode sequence information as required by the application
protocol.
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application_pdu
An application PDU in the format specified by the application.
6.4.2.3.2. Response Definition
A TunnelAns message serves as confirmation that the message was
received by the destination peer. It implies nothing about the
processing of the application. If the application protocol specifies
an acknowledgement or confirmation, that must be sent with a separate
Tunnel request. The TunnelAns message is empty (has a zero length
payload)
7. Data Storage Protocol
RELOAD provides a set of generic mechanisms for storing and
retrieving data in the Overlay Instance. These mechanisms can be
used for new applications simply by defining new code points and a
small set of rules. No new protocol mechanisms are required.
The basic unit of stored data is a single StoredData structure:
struct {
uint32 length;
uint64 storage_time;
uint32 lifetime;
StoredDataValue value;
Signature signature;
} StoredData;
The contents of this structure are as follows:
length
The length of the rest of the structure in octets.
storage_time
The time when the data was stored in absolute time, represented in
seconds since the Unix epoch. Any attempt to store a data value
with a storage time before that of a value already stored at this
location MUST generate a 412 error. This prevents rollback
attacks. Note that this does not require synchronized clocks:
the receiving peer uses the storage time in the previous store,
not its own clock.
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lifetime
The validity period for the data, in seconds, starting from the
time of store.
value
The data value itself, as described in Section 7.2.
signature
A signature over the data value. Section 7.1 describes the
signature computation. The element is formatted as described in
Section 6.2.4
Each resource-id specifies a single location in the Overlay Instance.
However, each location may contain multiple StoredData values
distinguished by kind-id. The definition of a kind describes both
the data values which may be stored and the data model of the data.
Some data models allow multiple values to be stored under the same
kind-id. Section Section 7.2 describes the available data models.
Thus, for instance, a given resource-id might contain a single-value
element stored under kind-id X and an array containing multiple
values stored under kind-id Y.
7.1. Data Signature Computation
Each StoredData element is individually signed. However, the
signature also must be self-contained and cover the kind-id and
resource-id even though they are not present in the StoredData
structure. The input to the signature algorithm is:
resource_id + kind + StoredData
Where these values are:
resource
The resource ID where this data is stored.
kind
The kind-id for this data.
StoredData
The contents of the stored data value, as described in the
previous sections.
[OPEN ISSUE: Should we include the identity in the string that forms
the input to the signature algorithm?.]
Once the signature has been computed, the signature is represented
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using a signature element, as described in Section 6.2.4.
7.2. Data Models
The protocol currently defines the following data models:
o single value
o array
o dictionary
These are represented with the StoredDataValue structure:
enum { reserved(0), single_value(1), array(2),
dictionary(3), (255)} DataModel;
struct {
Boolean exists;
opaque value<0..2^32-1>;
} DataValue;
select (DataModel) {
case single_value:
DataValue single_value_entry;
case array:
ArrayEntry array_entry;
case DictionaryEntry:
DictionaryEntry dictionary_entry;
/* This structure may be extended */
} StoredDataValue;
We now discuss the properties of each data model in turn:
7.2.1. Single Value
A single-value element is a simple, opaque sequence of bytes. There
may be only one single-value element for each resource-id, kind-id
pair.
A single value element is represented as a DataValue, which contains
the following two values:
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exists
This value indicates whether the value exists at all. If it is
set to False, it means that no value is present. If it is True,
that means that a value is present. This gives the protocol a
mechanism for indicating nonexistence as opposed to emptiness.
value
The stored data.
7.2.2. Array
An array is a set of opaque values addressed by an integer index.
Arrays are zero based. Note that arrays can be sparse. For
instance, a Store of "X" at index 2 in an empty array produces an
array with the values [ NA, NA, "X"]. Future attempts to fetch
elements at index 0 or 1 will return values with "exists" set to
False.
A array element is represented as an ArrayEntry:
struct {
uint32 index;
DataValue value;
} ArrayEntry;
The contents of this structure are:
index
The index of the data element in the array.
value
The stored data.
7.2.3. Dictionary
A dictionary is a set of opaque values indexed by an opaque key with
one value for each key. single dictionary entry is represented as
follows
A dictionary element is represented as a DictionaryEntry:
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typedef opaque DictionaryKey<0..2^16-1>;
struct {
DictionaryKey key;
DataValue value;
} DictionaryEntry;
The contents of this structure are:
key
The dictionary key for this value.
value
The stored data.
7.3. Data Storage Methods
RELOAD provides several methods for storing and retrieving data:
o Store values in the overlay
o Fetch values from the overlay
o Remove values from the overlay
o Find the values stored at an individual peer
These methods are each described in the following sections.
7.3.1. Store
The Store method is used to store data in the overlay. The format of
the Store request depends on the data model which is determined by
the kind.
7.3.1.1. Request Definition
A StoreReq message is a sequence of StoreKindData values, each of
which represents a sequence of stored values for a given kind. The
same kind-id MUST NOT be used twice in a given store request. Each
value is then processed in turn. These operations MUST be atomic.
If any operation fails, the state MUST be rolled back to before the
request was received.
The store request is defined by the StoreReq structure:
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struct {
KindId kind;
DataModel data_model;
uint64 generation_counter;
StoredData values<0..2^32-1>;
} StoreKindData;
struct {
ResourceId resource;
uint8 replica_number;
StoreKindData kind_data<0..2^32-1>;
} StoreReq;
A single Store request stores data of a number of kinds to a single
resource location. The contents of the structure are:
resource
The resource to store at.
replica_number
The number of this replica. When a storing peer saves replicas to
other peers each peer is assigned a replica number starting from 1
and sent in the Store message. This field is set to 0 when a node
is storing its own data. This allows peers to distinguish replica
writes from original writes.
kind_data
A series of elements, one for each kind of data to be stored.
If the replica number is zero, then the peer MUST check that it is
responsible for the resource and if not reject the request. If the
replica number is nonzero, then the peer MUST check that it expects
to be a replica for the resource and if not reject the request.
Each StoreKindData element represents the data to be stored for a
single kind-id. The contents of the element are:
kind
The kind-id. Implementations SHOULD reject requests corresponding
to unknown kinds unless specifically configured otherwise.
data_model
The data model of the data. The kind defines what this has to be
so this is redundant in the case where the software interpreting
the messages understands the kind.
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generation
The expected current state of the generation counter
(approximately the number of times this object has been written,
see below for details).
values
The value or values to be stored. This may contain one or more
stored_data values depending on the data model associated with
each kind.
The peer MUST perform the following checks:
o The kind_id is known and supported.
o The data_model matches the kind_id.
o The signatures over each individual data element (if any) are
valid.
o Each element is signed by a credential which is authorized to
write this kind at this resource-id
o For original (non-replica) stores, the peer MUST check that if the
generation-counter is non-zero, it equals the current value of the
generation-counter for this kind. This feature allows the
generation counter to be used in a way similar to the HTTP Etag
feature.
o The storage time values are greater than that of any value which
would be replaced by this Store. [[OPEN ISSUE: do peers need to
save the storage time of Removes to prevent reinsertion?]]
If all these checks succeed, the peer MUST attempt to store the data
values. For non-replica stores, if the store succeeds and the data
is changed, then the peer must increase the generation counter by at
least one. If there are multiple stored values in a single
StoreKindData, it is permissible for the peer to increase the
generation counter by only 1 for the entire kind-id, or by 1 or more
than one for each value. Accordingly, all stored data values must
have a generation counter of 1 or greater. 0 is used by other nodes
to indicate that they are indifferent to the generation counter's
current value. For replica Stores, the peer MUST set the generation
counter to match the generation_counter in the message. Replica
Stores MUST NOT use a generation counter of 0.
The properties of stores for each data model are as follows:
Single-value:
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A store of a new single-value element creates the element if it
does not exist and overwrites any existing value. with the new
value.
Array:
A store of an array entry replaces (or inserts) the given value at
the location specified by the index. Because arrays are sparse, a
store past the end of the array extends it with nonexistent values
(exists=False) as required. A store at index 0xffffffff places
the new value at the end of the array regardless of the length of
the the array. The resulting StoredData has the correct index
value when it is subsequently fetched.
Dictionary:
A store of a dictionary entry replaces (or inserts) the given
value at the location specified by the dictionary key.
The following figure shows the relationship between these structures
for an example store which stores the following values at resource
"1234"
o The value "abc" in the single value slot for kind X
o The value "foo" at index 0 in the array for kind Y
o The value "bar" at index 1 in the array for kind Y
Store
resource=1234
/ \
/ \
StoreKindData StoreKindData
kind=X kind=Y
model=Single-Value model=Array
| /\
| / \
StoredData / \
| / \
| StoredData StoredData
StoredDataValue | |
value="abc" | |
| |
StoredDataValue StoredDataValue
index=0 index=1
value="foo" value="bar"
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7.3.1.2. Response Definition
In response to a successful Store request the peer MUST return a
StoreAns message containing a series of StoreKindResponse elements
containing the current value of the generation counter for each
kind-id, as well as a list of the peers where the data was
replicated.
struct {
KindId kind;
uint64 generation_counter;
NodeId replicas<0..2^16-1>;
} StoreKindResponse;
struct {
StoreKindResponse kind_responses<0..2^16-1>;
} StoreAns;
The contents of each StoreKindResponse are:
kind
The kind-id being represented.
generation
The current value of the generation counter for that kind-id.
replicas
The list of other peers at which the data was/will-be replicated.
In overlays and applications where the responsible peer is
intended to store redundant copies, this allows the storing peer
to independently verify that the replicas were in fact stored by
doing its own Fetch.
The response itself is just StoreKindResponse values packed end-to-
end.
If any of the generation counters in the request precede the
corresponding stored generation counter, then the peer MUST fail the
entire request and respond with a 412 error. The error_info in the
ErrorResponse MUST be a StoreAns response containing the correct
generation counter for each kind and empty replicas lists.
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7.3.2. Fetch
The Fetch request retrieves one or more data elements stored at a
given resource-id. A single Fetch request can retrieve multiple
different kinds.
7.3.2.1. Request Definition
struct {
int32 first;
int32 last;
} ArrayRange;
struct {
KindId kind;
DataModel model;
uint64 generation;
uint16 length;
select (model) {
case single_value: ; /* Empty */
case array:
ArrayRange indices<0..2^16-1>;
case dictionary:
DictionaryKey keys<0..2^16-1>;
/* This structure may be extended */
} model_specifier;
} StoredDataSpecifier;
struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} FetchReq;
The contents of the Fetch requests are as follows:
resource
The resource ID to fetch from.
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specifiers
A sequence of StoredDataSpecifier values, each specifying some of
the data values to retrieve.
Each StoredDataSpecifier specifies a single kind of data to retrieve
and (if appropriate) the subset of values that are to be retrieved.
The contents of the StoredDataSpecifier structure are as follows:
kind
The kind-id of the data being fetched. Implementations SHOULD
reject requests corresponding to unknown kinds unless specifically
configured otherwise.
model
The data model of the data.. This must be checked against the
kind-id.
generation
The last generation counter that the requesting peer saw. This
may be used to avoid unnecessary fetches or it may be set to zero.
length
The length of the rest of the structure, thus allowing
extensibility.
model_specifier
A reference to the data value being requested within the data
model specified for the kind. For instance, if the data model is
"array", it might specify some subset of the values.
The model_specifier is as follows:
o If the data is of data model single value, the specifier is empty.
o If the data is of data model array, the specifier contains of a
list of ArrayRange elements, each of which contains two integers.
two integers. The first integer is the beginning of the range and
the second is the end of the range. 0 is used to indicate the
first element and 0xffffffff is used to indicate the final
element. The beginning of the range MUST be earlier in the array
then the end. The ranges MUST be non-overlapping.
o If the data is of data model dictionary then the specifier
contains a list of the dictionary keys being requested. If no
keys are specified, than this is a wildcard fetch and all key-
value pairs are returned. [[TODO: We really need a way to return
only the keys. We'll need to modify this.]]
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The generation-counter is used to indicate the requester's expected
state of the storing peer. If the generation-counter in the request
matches the stored counter, then the storing peer returns a response
with no StoredData values.
Note that because the certificate for a user is typically stored at
the same location as any data stored for that user, a requesting peer
which does not already have the user's certificate should request the
certificate in the Fetch as an optimization.
7.3.2.2. Response Definition
The response to a successful Fetch request is a FetchAns message
containing the data requested by the requester.
struct {
KindId kind;
uint64 generation;
StoredData values<0..2^32-1>;
} FetchKindResponse;
struct {
FetchKindResponse kind_responses<0..2^32-1>;
} FetchAns;
The FetchAns structure contains a series of FetchKindResponse
structures. There MUST be one FetchKindResponse element for each
kind-id in the request.
The contents of the FetchKindResponse structure are as follows:
kind
the kind that this structure is for.
generation
the generation counter for this kind.
values
the relevant values. If the generation counter in the request
matches the generation-counter in the stored data, then no
StoredData values are returned. Otherwise, all relevant data
values MUST be returned. A nonexistent value is represented with
"exists" set to False.
There is one subtle point about signature computation on arrays. If
the storing node uses the append feature (where the
index=0xffffffff), then the index in the StoredData that is returned
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will not match that used by the storing node, which would break the
signature. In order to avoid this issue, the index value in array is
set to zero before the signature is computed. This implies that
malicious storing nodes can reorder array entries without being
detected. [[OPEN ISSUE: We've considered a number of alternate
designs here that would preserve security against this attack if the
storing node did not use the append feature. However, they are more
complicated for one or both sides. If this attack is considered
serious, we can introduce one of them.]]
7.3.3. Remove
The Remove request is used to remove a stored element or elements
from the storing peer. Any successful remove of an existing element
for a given kind MUST increment the generation counter by at least 1.
struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} RemoveReq;
A RemoveReq has exactly the same syntax as a Fetch request except
that each entry represents a set of values to be removed rather than
returned. The same kind-id MUST NOT be used twice in a given
RemoveReq. Each specifier is then processed in turn. These
operations MUST be atomic. If any operation fails, the state MUST be
rolled back to before the request was received.
Before processing the Remove request, the peer MUST perform the
following checks.
o The kind-id is known.
o The signature over the message is valid or (depending on overlay
policy) no signature is required.
o The signer of the message has permissions which permit him to
remove this kind of data. Although each kind defines its own
access control requirements, in general only the original signer
of the data should be allowed to remove it.
o If the generation-counter is non-zero, it must equal the current
value of the generation-counter for this kind. This feature
allows the generation counter to be used in a way similar to the
HTTP Etag feature.
Assuming that the request is permitted, the operations proceed as
follows.
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7.3.3.1. Single Value
A Remove of a single value element causes it not to exist. If no
such element exists, then this is a silent success.
7.3.3.2. Array
A Remove of an array element (or element range) replaces those
elements with null elements. Note that this does not cause the array
to be packed. An array which contains ["A", "B", "C"] and then has
element 0 removed produces an array containing [NA, "B", "C"]. Note,
however, that the removal of the final element of the array shortens
the array, so in the above case, the removal of element 2 makes the
array ["A", "B"].
7.3.3.3. Dictionary
A Remove of a dictionary element (or elements) replaces those
elements with null elements. If no such elements exist, then this is
a silent success.
7.3.3.4. Response Definition
The response to a successful Remove simply contains a list of the new
generation counters for each kind-id, using the same syntax as the
response to a Store request. Note that if the generation counter
does not change, that means that the requested items did not exist.
However, if the generation counter does change, that does not mean
that the items existed.
struct {
StoreKindResponse kind_responses<0..2^16-1>;
} RemoveAns;
7.3.4. Find
The Find request can be used to explore the Overlay Instance. A Find
request for a resource-id R and a kind-id T retrieves the resource-id
(if any) of the resource of kind T known to the target peer which is
closes to R. This method can be used to walk the Overlay Instance by
interactively fetching R_n+1=nearest(1 + R_n).
7.3.4.1. Request Definition
The FindReq message contains a series of resource-IDs and kind-ids
identifying the resource the peer is interested in.
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struct {
ResourceID resource;
KindId kinds<0..2^8-1>;
} FindReq;
The request contains a list of kind-ids which the Find is for, as
indicated below:
resource
The desired resource-id
kinds
The desired kind-ids. Each value MUST only appear once.
7.3.4.2. Response Definition
A response to a successful Find request is a FindAns message
containing the closest resource-id for each kind specified in the
request.
struct {
KindId kind;
ResourceID closest;
} FindKindData;
struct {
FindKindData results<0..2^16-1>;
} FindAns;
If the processing peer is not responsible for the specified
resource-id, it SHOULD return a 404 error.
For each kind-id in the request the response MUST contain a
FindKindData indicating the closest resource-id for that kind-id
unless the kind is not allowed to be used with Find in which case a
FindKindData for that kind-id MUST NOT be included in the response.
If a kind-id is not known, then the corresponding resource-id MUST be
0. Note that different kind-ids may have different closest resource-
ids.
The response is simply a series of FindKindData elements, one per
kind, concatenated end-to-end. The contents of each element are:
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kind
The kind-id.
closest
The closest resource ID to the specified resource ID. This is 0
if no resource ID is known.
Note that the response does not contain the contents of the data
stored at these resource-ids. If the requester wants this, it must
retrieve it using Fetch.
7.3.4.3. Defining New Kinds
A new kind MUST define:
o The meaning of the data to be stored.
o The kind-id.
o The data model (single value, array, dictionary, etc.)
o Access control rules for indicating what credentials are allowed
to read and write that kind-id at a given location.
While each kind MUST define what data model is used for its data,
that does not mean that it must define new data models. Where
practical, kinds SHOULD use the built-in data models. However, they
MAY define any new required data models. The intention is that the
basic data model set be sufficient for most applications/usages.
8. Certificate Store Usage
The Certificate Store usage allows a peer to store its certificate in
the overlay, thus avoiding the need to send a certificate in each
message - a reference may be sent instead.
A user/peer MUST store its certificate at resource-ids derived from
two Resource Names:
o The user names in the certificate.
o The Node-IDs in the certificate.
Note that in the second case the certificate is not stored at the
peer's Node-ID but rather at a hash of the peer's Node-ID. The
intention here (as is common throughout RELOAD) is to avoid making a
peer responsible for its own data.
A peer MUST ensure that the user's certificates are stored in the
Overlay Instance. New certificates are stored at the end of the
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list. This structure allows users to store and old and new
certificate the both have the same node-id which allows for migration
of certificates when they are renewed.
Kind IDs This usage defines the CERTIFICATE kind-id to store a peer
or user's certificate.
Data Model The data model for CERTIFICATE data is array.
Access Control The CERTIFICATE MUST contain a Node-ID or user name
which, when hashed, maps to the resource-id at which the value is
being stored.
9. TURN Server Usage
The TURN server usage allows a RELOAD peer to advertise that it is
prepared to be a TURN server. When a node starts up, it joins the
overlay network and forms several connection in the process. If the
ICE stage in any of these connection return a reflexive address that
is not the same as the peers perceived address, then the peers is
behind a NAT and not an candidate for a TURN server. Additionally,
if the peers IP address is in the private address space range, then
it is not a candidate for a TURN server. Otherwise, the peer SHOULD
assume it is a potential TURN server and follow the procedures below.
If the node is a candidate for a TURN server it will insert some
pointers in the overlay so that other peers can find it. The overlay
configuration file specifies a turnDensity parameter that indicates
how many times each TURN server should record itself in the overlay.
Typically this should be set to the reciprocal of the estimate of
what percentage of peers will act as TURN servers. For each value,
called d, between 1 and turnDensity, the peer forms a Resource Name
by concatenating its peer-ID and the value d. This Resource Name is
hashed to form a Resource-ID. The address of the peer is stored at
that Resource-ID using type TURN-SERVICE and the TurnServer object:
struct {
uint8 iteration;
IpAddressAndPort server_address;
} TurnServer;
The contents of this structure are as follows:
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iteration
the d value
server_address
the address at which the TURN server can be contacted.
Note: Correct functioning of this algorithm depends critically on
having turnDensity be an accurate estimate of the true density of
TURN servers. If turnDensity is too high, then the process of
finding TURN servers becomes extremely expensive as multiple
candidate resource-ids must be probed.
Peers peers that provide this service need to support the TURN
extensions to STUN for media relay of both UDP and TCP traffic as
defined in [I-D.ietf-behave-turn] and [I-D.ietf-behave-tcp].
[[OPEN ISSUE: This structure only works for TURN servers that have
public addresses. It may be possible to use TURN servers that are
behind well-behaved NATs by first ICE connecting to them. If we
decide we want to enable that, this structure will need to change to
either be a peer-id or include that as an option.]]
Kind IDs This usage defines the TURN-SERVICE kind-id to indicate
that a peer is willing to act as a TURN server. The Find command
MUST return results for the TURN-SERVICE kind-id.
Data Model The TURN-SERVICE stores a single value for each
resource-id.
Access Control If certificate-based access control is being used,
stored data of kind TURN-SERVICE MUST be authenticated by a
certificate which contains a peer-id which when hashed with the
iteration counter produces the resource-id being stored at.
Peers can find other servers by selecting a random Resource-ID and
then doing a Find request for the appropriate server type with that
Resource-ID. The Find request gets routed to a random peer based on
the Resource-ID. If that peer knows of any servers, they will be
returned. The returned response may be empty if the peer does not
know of any servers, in which case the process gets repeated with
some other random Resource-ID. As long as the ratio of servers
relative to peers is not too low, this approach will result in
finding a server relatively quickly.
10. SIP Usage
The SIP usage allows a RELOAD overlay to be used as a distributed SIP
registrar/proxy network. This entails three primary operations:
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o Registering one's own AOR with the overlay.
o Looking up a given AOR in the overlay.
o Forming a direct connection to a given peer.
10.1. Registering AORs
In ordinary SIP, a UA registers its AOR and location with a
registrar. In RELOAD, this registrar function is provided by the
overlay as a whole. To register its location, a RELOAD peer stores a
SipRegistration structure under its own AOR. This uses the SIP-
REGISTRATION kind-id, which is formally defined in Section 10.5.
Note: GRUUs are handled via a separate mechanism, as described in
Section 10.4.
As a simple example, if Alice's AOR were "sip:alice@dht.example.com"
and her Node-ID were "1234", she might store the mapping
"sip:alice@example.org -> 1234". This would tell anyone who wanted
to call Alice to contact node "1234".
RELOAD peers MAY store two kinds of SIP mappings:
o From AORs to destination lists (a single Node-ID is just a trivial
destination list.)
o From AORs to other AORs.
The meaning of the first kind of mapping is "in order to contact me,
form a connection with this peer." The meaning of the second kind of
mapping is "in order to contact me, dereference this AOR". This
allows for forwarding. For instance, if Alice wants calls to her to
be forwarded to her secretary, Sam, she might insert the following
mapping "sip:alice@dht.example.org -> sip:sam@dht.example.org".
The contents of a SipRegistration structure are as follows:
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enum {sip_registration_uri (1), sip_registration_route (2),
(255)} SipRegistrationType;
select (SipRegistration.type) {
case sip_registration_uri:
opaque uri<0..2^16-1>;
case sip_registration_route:
opaque contact_prefs<0..2^16-1>;
Destination destination_list<0..2^16-1>;
/* This type can be extended */
} SipRegistrationData;
struct {
SipRegistrationType type;
uint16 length;
SipRegistrationData data;
} SipRegistration;
The contents of the SipRegistration PDU are:
type
the type of the registration
length
the length of the rest of the PDU
data
the registration data
o If the registration is of type "sip_registration_uri", then the
contents are an opaque string containing the URI.
o If the registration is of type "sip_registration_route", then the
contents are an opaque string containing the callee's contact
preferences and a destination list for the peer.
RELOAD explicitly supports multiple registrations for a single AOR.
The registrations are stored in a Dictionary with the dictionary keys
being Node-IDs. Consider, for instance, the case where Alice has two
peers:
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o her desk phone (1234)
o her cell phone (5678)
Alice might store the following in the overlay at resource
"sip:alice@dht.example.com".
o A SipRegistration of type "sip_registration_route" with dictionary
key "1234" and value "1234".
o A SipRegistration of type "sip_registration_route" with dictionary
key "5678" and value "5678".
Note that this structure explicitly allows one Node-ID to forward to
another Node-ID. For instance, Alice could set calls to her desk
phone to ring at her cell phone. It's not clear that this is useful
in this case, but may be useful if Alice has two AORs.
In order to prevent hijacking, registrations are subject to access
control rules. Before a Store is permitted, the storing peer MUST
check that:
o The certificate contains a username that is a SIP AOR that hashes
to the resource-id being stored at.
o The certificate contains a Node-ID that is the same as the
dictionary key being stored at.
Note that these rules permit Alice to forward calls to Bob without
his permission. However, they do not permit Alice to forward Bob's
calls to her. See Section 15.7.2 for more on this point.
10.2. Looking up an AOR
When a RELOAD user wishes to call another user, starting with a non-
GRUU AOR, he follows the following procedure. (GRUUs are discussed
in Section 10.4).
1. Check to see if the domain part of the AOR matches the domain
name of an overlay of which he is a member. If not, then this is
an external AOR, and he MUST do one of the following:
* Fail the call.
* Use ordinary SIP procedures.
* Attempt to become a member of the overlay indicated by the
domain part (only possible if the enrollment procedure defined
in Section 13.1 indicates that this is a RELOAD overlay.)
2. Perform a Fetch for kind SIP-REGISTRATION at the resource-id
corresponding to the AOR. This Fetch SHOULD NOT indicate any
dictionary keys, which will result in fetching all the stored
values.
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3. If any of the results of the Fetch are non-GRUU AORs, then repeat
step 1 for that AOR.
4. Once only GRUUs and destination lists remain, the peer removes
duplicate destination lists and GRUUs from the list and forms a
SIP connection to the appropriate peers as described in the
following sections. If there are also external AORs, the peer
follows the appropriate procedure for contacting them as well.
10.3. Forming a Direct Connection
Once the peer has translated the AOR into a set of destination lists,
it then uses the overlay to route Connect messages to each of those
peers. The "application" field MUST be 5060 to indicate SIP. If
certificate-based authentication is in use, the responding peer MUST
present a certificate with a Node-ID matching the terminal entry in
the route list. Note that it is possible that the peers already have
a RELOAD connection between them. This MUST NOT be used for SIP
messages. However, if a SIP connection already exists, that MAY be
used. Once the Connect succeeds, the peer sends SIP messages over
the connection as in normal SIP.
10.4. GRUUs
GRUUs do not require storing data in the Overlay Instance. Rather,
they are constructed by embedding a base64-encoded destination list
in the gr URI parameter of the GRUU. The base64 encoding is done
with the alphabet specified in table 1 of RFC 4648 with the exception
that ~ is used in place of =. An example GRUU is
"sip:alice@example.com;gr=MDEyMzQ1Njc4OTAxMjM0NTY3ODk~". When a peer
needs to route a message to a GRUU in the same P2P network, it simply
uses the destination list and connects to that peer.
Because a GRUU contains a destination list, it MAY have the same
contents as a destination list stored elsewhere in the resource
dictionary.
Anonymous GRUUs are done in roughly the same way but require either
that the enrollment server issue a different Node-ID for each
anonymous GRUU required or that a destination list be used that
includes a peer that compresses the destination list to stop the
Node-ID from being revealed.
10.5. SIP-REGISTRATION Kind Definition
The first mapping is provided using the SIP-REGISTRATION kind-id:
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Kind IDs The Resource Name for the SIP-REGISTRATION kind-id is the
AOR of the user. The data stored is a SipRegistrationData, which
can contain either another URI or a destination list to the peer
which is acting for the user.
Data Model The data model for the SIP-REGISTRATION kind-id is
dictionary. The dictionary key is the Node-ID of the storing
peer. This allows each peer (presumably corresponding to a single
device) to store a single route mapping.
Access Control If certificate-based access control is being used,
stored data of kind-id SIP-REGISTRATION must be signed by a
certificate which (1) contains user name matching the storing URI
used as the Resource Name for the resource-id and (2) contains a
Node-ID matching the storing dictionary key.
Data stored under the SIP-REGISTRATION kind is of type
SipRegistration. This comes in two varieties:
sip_registration_uri
a URI which the user can be reached at.
sip_registration_route
a destination list which can be used to reach the user's peer.
11. Diagnostic Usage
The Diagnostic Usage allows a node to report various statistics about
itself that may be useful for diagnostics or performance management.
It can be used to discover information such as the software version,
uptime, routing table, stored resource-objects, and performance
statistics of a peer. The usage defines several new kinds which can
be retrieved to get the statistics and also allows to retrieve other
kinds that a node stores. In essence, the usage allows querying a
node's state such as storage and network to obtain the relevant
information.
The access control model for all kinds is a local policy defined by
the peer or the overlay policy. The peer may be configured with a
list of users that it is willing to return the information for and
restrict access to users with that name. Unless specific policy
overrides it, data SHOULD NOT be returned for users not on the list.
The access control can also be determined on a per kind basis - for
example, a node may be willing to return the software version to any
users while specific information about performance may not be
returned.
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The following kinds are defined:
ROUTING_TABLE_SIZE A single value element containing an unsigned 32-
bit integer representing the number of peers in the peer's routing
table.
SOFTWARE_VERSION A single value element containing a US-ASCII string
that identifies the manufacture, model, and version of the
software.
MACHINE_UPTIME A single value element containing an unsigned 64-bit
integer specifying the time the nodes has been up in seconds.
APP_UPTIME A single value element containing an unsigned 64-bit
integer specifying the time the p2p application has been up in
seconds.
MEMORY_FOOTPRINT A single value element containing an unsigned 32-
bit integer representing the memory footprint of the peer program
in kilo bytes.
Note: What's a kilo byte? 1000 or 1024? -- Cullen
Note: Good question. 1000 seems like not quite enough room but
1024 is too much? -- EKR
DATASIZE_StoreD An unsigned 64-bit integer representing the number
of bytes of data being stored by this node.
INSTANCES_StoreD An array element containing the number of instances
of each kind stored. The array is index by kind-id. Each entry
is an unsigned 64-bit integer.
MESSAGES_SENT_RCVD An array element containing the number of
messages sent and received. The array is indexed by method code.
Each entry in the array is a pair of unsigned 64-bit integers
(packed end to end) representing sent and received.
EWMA_BYTES_SENT A single value element containing an unsigned 32-bit
integer representing an exponential weighted average of bytes sent
per second by this peer.
sent = alpha x sent_present + (1 - alpha) x sent
where sent_present represents the bytes sent per second since the
last calculation and sent represents the last calculation of bytes
sent per second. A suitable value for alpha is 0.8. This value
is calculated every five seconds.
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EWMA_BYTES_RCVD A single value element containing an unsigned 32-bit
integer representing an exponential weighted average of bytes
received per second by this peer. Same calculation as above.
[[TODO: We would like some sort of bandwidth measurement, but we're
kind of unclear on the units and representation.]]
11.1. Diagnostic Metrics for a P2PSIP Deployment
(OPEN QUESTION: any other metrics?)
Below, we sketch how these metrics can be used. A peer can use
EWMA_BYTES_SENT and EWMA_BYTES_RCVD of another peer to infer whether
it is acting as a media relay. It may then choose not to forward any
requests for media relay to this peer. Similarly, among the various
candidates for filling up routing table, a peer may prefer a peer
with a large UPTIME value, small RTT, and small LAST_CONTACT value.
12. Chord Algorithm
This algorithm is assigned the name chord-128-2-16+ to indicate it is
based on Chord, uses SHA-1 then truncates that to 128 bit for the
hash function, stores 2 redundant copies of all data, and has finger
tables with at least 16 entries.
12.1. Overview
The algorithm described here is a modified version of the Chord
algorithm. Each peer keeps track of a finger table of 16 entries and
a neighborhood table of 6 entries. The neighborhood table contains
the 3 peers before this peer and the 3 peers after it in the DHT
ring. The first entry in the finger table contains the peer half-way
around the ring from this peer; the second entry contains the peer
that is 1/4 of the way around; the third entry contains the peer that
is 1/8th of the way around, and so on. Fundamentally, the chord data
structure can be thought of a doubly-linked list formed by knowing
the successors and predecessor peers in the neighborhood table,
sorted by the Node-ID. As long as the successor peers are correct,
the DHT will return the correct result. The pointers to the prior
peers are kept to enable inserting of new peers into the list
structure. Keeping multiple predecessor and successor pointers makes
it possible to maintain the integrity of the data structure even when
consecutive peers simultaneously fail. The finger table forms a skip
list, so that entries in the linked list can be found in O(log(N))
time instead of the typical O(N) time that a linked list would
provide.
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A peer, n, is responsible for a particular Resource-ID k if k is less
than or equal to n and k is greater than p, where p is the peer id of
the previous peer in the neighborhood table. Care must be taken when
computing to note that all math is modulo 2^128.
12.2. Routing
If a peer is not responsible for a Resource-ID k, but is directly
connected to a node with Node-Id k, then it routes the message to
that node. Otherwise, it routes the request to the peer in the
routing table that has the largest Node-ID that is in the interval
between the peer and k.
12.3. Redundancy
When a peer receives a Store request for Resource-ID k, and it is
responsible for Resource-ID k, it stores the data and returns a
success response. [[Open Issue: should it delay sending this
success until it has successfully stored the redundant copies?]]. It
then sends a Store request to its successor in the neighborhood table
and to that peers successor. Note that these Store requests are
addressed to those specific peers, even though the Resource-ID they
are being asked to store is outside the range that they are
responsible for. The peers receiving these check they came from an
appropriate predecessor in their neighborhood table and that they are
in a range that this predecessor is responsible for, and then they
store the data. They do not themselves perform further Stores
because they can determine that they are not responsible for the
resource-ID.
Note that a malicious node can return a success response but not
store the data locally or in the replica set. Requesting peers which
wish to ensure that the replication actually occurred SHOULD contact
each peer listed in the replicas field of the Store response and
retrieve a copy of the data. [[TODO: Do we want to have some
optimization in Fetch where they can retrieve just a digest instead
of the data values?]]
12.4. Joining
The join process for a joining party (JP) with Node-ID n is as
follows.
1. JP connects to its chosen bootstrap node.
2. JP uses a series of Pings to populate its routing table.
3. JP sends Connect requests to initiate connections to each of the
peers in the connection table as well as to the desired finger
table entries. Note that this does not populate their routing
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tables, but only their connection tables, so JP will not get
messages that it is expected to route to other nodes.
4. JP enters all the peers it contacted into its routing table.
5. JP sends a Join to its immediate successor, the admitting peer
(AP) for Node-ID n. The AP sends the response to the Join.
6. AP does a series of Store requests to JP to store the data that
JP will be responsible for.
7. AP sends JP an Update explicitly labeling JP as its predecessor.
At this point, JP is part of the ring and responsible for a
section of the overlay. AP can now forget any data which is
assigned to JP and not AP.
8. AP sends an Update to all of its neighbors with the new values of
its neighbor set (including JP).
9. JP sends UpdateS to all the peers in its routing table.
In order to populate its routing table, JP sends a Ping via the
bootstrap node directed at resource-id n+1 (directly after its own
resource-id). This allows it to discover its own successor. Call
that node p0. It then sends a ping to p0+1 to discover its successor
(p1). This process can be repeated to discover as many successors as
desired. The values for the two peers before p will be found at a
later stage when n receives an Update.
In order to set up its neighbor table entry for peer i, JP simply
sends a Connect to peer (n+2^(numBitsInNodeId-i). This will be
routed to a peer in approximately the right location around the ring.
12.5. Routing Connects
When a peer needs to Connect with a new peer in its neighborhood
table, it MUST source-route the Connect request through the peer from
which it learned the new peer's Node-ID. Source-routing these
requests allows the overlay to recover from instability.
All other Connect requests, such as those for new finger table
entries, are routed conventionally through the overlay.
If a peer is unable to successfully Connect with a peer that should
be in its neighborhood, it MUST locate either a TURN server or
another peer in the overlay, but not in its neighborhood, through
which it can exchange messages with its neighbor peer
12.6. Updates
A chord Update is defined as
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enum { reserved (0), peer_ready(1), neighbors(2), full(3), (255) }
ChordUpdateType;
struct {
ChordUpdateType type;
select(type){
case peer_ready: /* Empty */
;
case neighbors:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;
case full:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;
NodeId fingers<0..2^16-1>;
};
} ChordUpdate;
The "type" field contains the type of the update, which depends on
the reason the update was sent.
peer_ready: this peer is ready to receive messages. This message
is used to indicate that a node which has Connected is a peer and
can be routed through. It is also used as a connectivity check to
non-neighbor pers.
neighbors: this version is sent to members of the Chord neighbor
table.
full: this version is sent to peers which request an Update with a
RouteQueryReq.
If the message is of type "neighbors", then the contents of the
message will be:
predecessors
The predecessor set of the Updating peer.
successors
The successor set of the Updating peer.
If the message is of type "full", then the contents of the message
will be:
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predecessors
The predecessor set of the Updating peer.
successors
The successor set of the Updating peer.
fingers
The finger table if the Updating peer, in numerically ascending
order.
A peer MUST maintain an association (via Connect) to every member of
its neighbor set. A peer MUST attempt to maintain at least three
predecessors and three successors. However, it MUST send its entire
set in any Update message sent to neighbors.
12.6.1. Sending Updates
Every time a connection to a peer in the neighborhood set is lost (as
determined by connectivity pings or failure of some request), the
peer should remove the entry from its neighborhood table and replace
it with the best match it has from the other peers in its routing
table. It then sends an Update to all its remaining neighbors. The
update will contain all the Node-IDs of the current entries of the
table (after the failed one has been removed). Note that when
replacing a successor the peer SHOULD delay the creation of new
replicas for 30 seconds after removing the failed entry from its
neighborhood table in order to allow a triggered update to inform it
of a better match for its neighborhood table.
If connectivity is lost to all three of the peers that succeed this
peer in the ring, then this peer should behave as if it is joining
the network and use Pings to find a peer and send it a Join. If
connectivity is lost to all the peers in the finger table, this peer
should assume that it has been disconnected from the rest of the
network, and it should periodically try to join the DHT.
12.6.2. Receiving Updates
When a peer, N, receives an Update request, it examines the Node-IDs
in the UpdateReq and at its neighborhood table and decides if this
UpdateReq would change its neighborhood table. This is done by
taking the set of peers currently in the neighborhood table and
comparing them to the peers in the update request. There are three
major cases:
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o The UpdateReq contains peers that would not change the neighbor
set because they match the neighborhood table.
o The UpdateReq contains peers closer to N than those in its
neighborhood table.
o The UpdateReq defines peers that indicate a neighborhood table
further away from N than some of its neighborhood table. Note
that merely receiving peers further away does not demonstrate
this, since the update could be from a node far away from N.
Rather, the peers would need to bracket N.
In the first case, no change is needed.
In the second case, N MUST attempt to Connect to the new peers and if
it is successful it MUST adjust its neighbor set accordingly. Note
that it can maintain the now inferior peers as neighbors, but it MUST
remember the closer ones.
The third case implies that a neighbor has disappeared, most likely
because it has simply been disconnected but perhaps because of
overlay instability. N MUST Ping the questionable peers to discover
if they are indeed missing and if so, remove them from its
neighborhood table.
After any Pings and Connects are done, if the neighborhood table
changes, the peer sends an Update request to each of its neighbors
that was in either the old table or the new table. These Update
requests are what ends up filling in the predecessor/successor tables
of peers that this peer is a neighbor to. A peer MUST NOT enter
itself in its successor or predecessor table and instead should leave
the entries empty.
If peer N which is responsible for a resource-id R discovers that the
replica set for R (the next two nodes in its successor set) has
changed, it MUST send a Store for any data associated with R to any
new node in the replica set. It SHOULD not delete data from peers
which have left the replica set.
When a peer N detects that it is no longer in the replica set for a
resource R (i.e., there are three predecessors between N and R), it
SHOULD delete all data associated with R from its local store.
12.6.3. Stabilization
There are four components to stabilization:
1. exchange Updates with all peers in its routing table to exchange
state
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2. search for better peers to place in its finger table
3. search to determine if the current finger table size is
sufficiently large
4. search to determine if the overlay has partitioned and needs to
recover
A peer MUST periodically send an Update request to every peer in its
routing table. The purpose of this is to keep the predecessor and
successor lists up to date and to detect connection failures. The
default time is about every ten minutes, but the enrollment server
SHOULD set this in the configuration document using the "chord-128-2-
16+-update-frequency" element (denominated in seconds.) A peer
SHOULD randomly offset these Update requests so they do not occur all
at once. If an Update request fails or times out, the peer MUST mark
that entry in the neighbor table invalid and attempt to reestablish a
connection. If no connection can be established, the peer MUST
attempt to establish a new peer as its neighbor and do whatever
replica set adjustments are required.
Periodically a peer should select a random entry i from the finger
table and do a Ping to resource (n+2^(numBitsInNodeId-i). The
purpose of this is to find a more accurate finger table entry if
there is one. This is done less frequently than the connectivity
checks in the previous section because forming new connections is
somewhat expensive and the cost needs to be balanced against the cost
of not having the most optimal finger table entries. The default
time is about every hour, but the enrollment server SHOULD set this
in the configuration document using the "chord-128-2-16+-ping-
frequency" element (denominated in seconds). If this returns a
different peer than the one currently in this entry of the peer
table, then a new connection should be formed to this peer and it
should replace the old peer in the finger table.
As an overlay grows, more than 16 entries may be required in the
finger table for efficient routing. To determine if its finger table
is sufficiently large, one an hour the peer should perform a Ping to
determine whether growing its finger table by four entries would
result in it learning at least two peers that it does not already
have in its neighbor table. If so, then the finger table SHOULD be
grown by four entries. Similarly, if the peer observes that its
closest finger table entries are also in its neighbor table, it MAY
shrink its finger table to the minimum size of 16 entries. [[OPEN
ISSUE: there are a variety of algorithms to gauge the population of
the overlay and select an appropriate finger table size. Need to
consider which is the best combination of effectiveness and
simplicity.]]
To detect that a partitioning has occurred and to heal the overlay, a
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peer P MUST periodically repeat the discovery process used in the
initial join for the overlay to locate an appropriate bootstrap peer,
B. If an overlay has multiple mechanisms for discovery it should
randomly select a method to locate a bootstrap peer. P should then
send a Ping for its own Node-ID routed through B. If a response is
received from a peer S', which is not P's successor, then the overlay
is partitioned and P should send a Connect to S' routed through B,
followed by an Update sent to S'. (Note that S' may not be in P's
neighborhood table once the overlay is healed, but the connection
will allow S' to discover appropriate neighbor entries for itself via
its own stabilization.)
12.7. Route Query
For this topology plugin, the RouteQueryReq contains no additional
information. The RouteQueryAns contains the single node ID of the
next peer to which the responding peer would have routed the request
message in recursive routing:
struct {
NodeId next_id;
} ChordRouteQueryAns;
The contents of this structure are as follows:
next_peer
The peer to which the responding peer would route the message to
in order to deliver it to the destination listed in the request.
If the requester set the send_update flag, the responder SHOULD
initiate an Update immediately after sending the RouteQueryAns.
12.8. Leaving
Peers SHOULD send a Leave request prior to exiting the Overlay
Instance. Any peer which receives a Leave for a peer n in its
neighbor set must remove it from the neighbor set, update its replica
sets as appropriate (including Stores of data to new members of the
replica set) and send Updates containing its new predecessor and
successor tables.
13. Enrollment and Bootstrap
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13.1. Discovery
When a peer first joins a new overlay, it starts with a discovery
process to find an enrollment server. Related work to the approach
used here is described in [I-D.garcia-p2psip-dns-sd-bootstrapping]
and [I-D.matthews-p2psip-bootstrap-mechanisms]. The peer first
determines the overlay name. This value is provided by the user or
some other out of band provisioning mechanism. If the name is an IP
address, that is directly used otherwise the peer MUST do a DNS SRV
query using a Service name of "p2p_enroll" and a protocol of tcp to
find an enrollment server.
If the overlay name ends in .local, then a DNS SRV lookup using
implement [I-D.cheshire-dnsext-dns-sd] with a Service name of
"p2p_menroll" can also be tried to find an enrollment server. If
they implement this, the user name MAY be used as the Instance
Identifier label.
Once an address for the enrollment servers is determined, the peer
forms an HTTPS connection to that IP address. The certificate MUST
match the overlay name as described in [RFC2818]. The peer then
performs a GET to the URL formed by appending a path of "/p2psip/
enroll" to the overlay name. For example, if the overlay name was
example.com, the URL would be "https://example.com/p2psip/enroll".
The result is an XML configuration file with the syntax described in
the following section.
13.2. Overlay Configuration
This specification defines a new content type "application/
p2p-overlay+xml" for an MIME entity that contains overlay
information. This information is fetched from the enrollment server,
as described above. An example document is shown below.
<?xml version="1.0" encoding="UTF-8"?>
<overlay instance-name="chord.example.com" expiration="86400">
<toplogy-plugin algorithm-name="chord-128-2-16+"/>
<root-cert>[PEM encoded certificate here]</root-cert>
<required-kind name="SIP-REGISTRATION" max-values="10"
max-size="1000"/>
<credential-server url="https://www.example.com/csr"/>
<bootstrap-peer address="192.0.2.2" port="5678"/>
<bootstrap-peer address="192.0.2.3" port="5678"/>
<bootstrap-peer address="192.0.2.4" port="5678"/>
<multicast-bootstrap="192.0.2.99" port="5678"/>
</overlay>
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The file MUST be a well formed XML document and it SHOULD contain an
encoding declaration in the XML declaration. If the charset
parameter of the MIME content type declaration is present and it is
different from the encoding declaration, the charset parameter takes
precedence. Every application conferment to this specification MUST
accept the UTF-8 character encoding to ensure minimal
interoperability. The namespace for the elements defined in this
specification is urn:ietf:params:xml:ns:p2p:overlay.
The file can contain multiple "overlay" elements where each one
contains the configuration information for a different overlay. Each
"overlay" has the following attributes:
instance-name: name of the overlay
expiration: time in future at which this overlay configuration is
not longer valid and need to be retrieved again. This is
expressed in seconds from the current time.
Inside each overlay element, the following elements can occur:
topology-plugin
This element has an attribute called algorithm-name that describes
the overlay-algorithm being used.
root-cert
This element contains a PEM encoded X.509v3 certificate that is
the root trust store used to sign all certificates in this
overlay. There can be more than one of these.
required-kinds
This element indicates the kinds that members must support. It
has three attributes:
* name: a string representing the kind.
* max-count: the maximum number of values which members of the
overlay must support.
* max-size: the maximum size of individual values.
For instance, the example above indicates that members must
support SIP-REGISTRATION with a maximum of 10 values of up to 1000
bytes each. Multiple required-kinds elements MAY be present.
credential-server
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This element contains the URL at which the credential server can
be reached in a "url" element. This URL MUST be of type "https:".
More than one credential-server element may be present.
self-signed-permitted
This element indicates whether self-signed certificates are
permitted. If it is set to "TRUE", then self-signed certificates
are allowed, in which case the credential-server and root-cert
elements may be absent. Otherwise, it SHOULD be absent, but MAY
be set "FALSE". This element also contains an attribute "digest"
which indicates the digest to be used to compute the Node-ID.
Valid values for this parameter are "SHA-1" and "SHA-256".
bootstrap-peer
This elements represents the address of one of the bootstrap
peers. It has an attribute called "address" that represents the
IP address (either IPv4 or IPv6, since they can be distinguished)
and an attribute called "port" that represents the port. More
than one bootstrap-peer element may be present.
multicast-bootstrap
This element represents the address of a multicast address and
port that may be used for bootstrap and that peers SHOULD listen
on to enable bootstrap. It has an attributed called "address"
that represents the IP address and an attribute called "port" that
represents the port. More than one "multicast-bootstrap" element
may be present.
clients-permitted
This element represents whether clients are permitted or whether
all nodes must be peers. If it is set to "TRUE" or absent, this
indicates that clients are permitted. If it is set to "FALSE"
then nodes MUST join as peers.
chord-128-2-16+-update-frequency
The update frequency for the Chord-128-2-16+ topology plugin (see
Section 12).
chord-128-2-16+-ping-frequency
The ping frequency for the Chord-128-2-16+ topology plugin (see
Section 12).
credential-server
Base URL for credential server.
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shared-secret
If shared secret mode is used, this contains the shared secret.
[[TODO: Do a RelaxNG grammar.]]
13.3. Credentials
If the configuration document contains a credential-server element,
credentials are required to join the Overlay Instance. A peer which
does not yet have credentials MUST contact the credential server to
acquire them.
In order to acquire credentials, the peer generates an asymmetric key
pair and then generates a "Simple Enrollment Request" (as defined in
[I-D.ietf-pkix-2797-bis]) and sends this over HTTPS as defined in
[I-D.ietf-pkix-cmc-trans] to the URL in the credential-server
element. The subjectAltName in the request MUST contain the required
user name.
The credential server MUST authenticate the request using the
provided user name and password. If the authentication succeeds and
the requested user name is acceptable, the server and returns a
certificate. The SubjectAltName field in the certificate contains
the following values:
o One or more Node-IDs which MUST be cryptographically random
[RFC4086]. These MUST be chosen by the credential server in such
a way that they are unpredictable to the requesting user. These
are of type URI and MUST contain RELOAD URIs as described in
Section 16.7 and MUST contain a Destination list with a single
entry of type "node_id".
o The names this user is allowed to use in the overlay, using type
rfc822Name.
The certificate is returned in a "Simple Enrollment Response".
[[TODO: REF]]
The client MUST check that the certificate returned was signed by one
of the certificates received in the "root-cert" list of the overlay
configuration data. The peer then reads the certificate to find the
Node-IDs it can use.
13.3.1. Self-Generated Credentials
If the "self-signed-permitted" element is present and set to "TRUE",
then a node MUST generate its own self-signed certificate to join the
overlay. The self-signed certificate MAY contain any user name of
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the users choice. Users SHOULD make some attempt to make it unique
but this document does not specify any mechanisms for that.
The Node-Id MUST be computed by applying the digest specified in the
self-signed-permitted element to the DER representation of the user's
public key. When accepting a self-signed certificate, nodes MUST
check that the Node-ID and public keys match. This prevents Node-ID
theft.
Once the node has constructed a self-signed certificate, it MAY join
the overlay. Before storing its certificate in the overlay
(Section 8) it SHOULD look to see if the user name is already taken
and if so choose another user name. Note that this only provides
protection against accidental name collisions. Name theft is still
possible. If protection against name theft is desired, then the
enrollment service must be used.
13.4. Joining the Overlay Peer
In order to join the overlay, the peer MUST contact a peer.
Typically this means contacting the bootstrap peers, since they are
guaranteed to have public IP addresses (the system should not
advertise them as bootstrap peers otherwise). If the peer has cached
peers it SHOULD contact them first by sending a Ping request to the
known peer address with the destination Node-ID set to that peer's
Node-ID.
If no cached peers are available, then the peer SHOULD send a Ping
request to the address and port found in the broadcast-peers element
in the configuration document. This MAY be a multicast or anycast
address. The Ping should use the wildcard Node-ID as the destination
Node-ID.
The responder peer that receives the Ping request SHOULD check that
the overlay name is correct and that the requester peer sending the
request has appropriate credentials for the overlay before responding
to the Ping request even if the response is only an error.
When the requester peer finally does receive a response from some
responding peer, it can note the Node-ID in the response and use this
Node-ID to start sending requests to join the Overlay Instance as
described in Section 6.3.
After a peer has successfully joined the overlay network, it SHOULD
periodically look at any peers to which it has managed to form direct
connections. Some of these peers MAY be added to the cached-peers
list and used in future boots. Peers that are not directly connected
MUST NOT be cached. The RECOMMENDED number of peers to cache is 10.
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14. Message Flow Example
In the following example, we assume that JP has formed a connection
to one of the bootstrap peers. JP then sends a Connect through that
peer to the admitting peer (AP) to initiate a connection. When AP
responds, JP and AP use ICE to set up a connection and then set up
TLS.
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Connect Dest=JP | | | | |
|---------------------------------------------------------->|
| | | | | | |
| | | | | | |
| | |Connect Dest=JP | | |
| | |<--------------------------------------|
| | | | | | |
| | | | | | |
| | |Connect Dest=JP | | |
| | |-------->| | | |
| | | | | | |
| | | | | | |
| | |ConnectAns | | |
| | |<--------| | | |
| | | | | | |
| | | | | | |
| | |ConnectAns | | |
| | |-------------------------------------->|
| | | | | | |
| | | | | | |
|ConnectAns | | | | |
|<----------------------------------------------------------|
| | | | | | |
| | | | | | |
|TLS | | | | | |
|.............................| | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
Once JP has connected to AP, it needs to populate its Routing Table.
In Chord, this means that it needs to populate its neighbor table and
its finger table. To populate its neighbor table, it needs the
successor of AP, NP. It sends a Connect to the Resource-IP AP+1,
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which gets routed to NP. When NP responds, JP and NP use ICE and TLS
to set up a connection.
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Connect AP+1 | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | |Connect AP+1 | |
| | | |-------->| | |
| | | | | | |
| | | | | | |
| | | |ConnectAns | |
| | | |<--------| | |
| | | | | | |
| | | | | | |
|ConnectAns | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|Connect | | | | | |
|-------------------------------------->| | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|.......................................| | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
JP also needs to populate its finger table (for Chord). It issues a
Connect to a variety of locations around the overlay. The diagram
below shows it sending a Connect halfway around the Chord ring the JP
+ 2^127.
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JP NP XX TP
| | | |
| | | |
| | | |
|Connect JP+2<<126 | |
|-------->| | |
| | | |
| | | |
| |Connect JP+2<<126 |
| |-------->| |
| | | |
| | | |
| | |Connect JP+2<<126
| | |-------->|
| | | |
| | | |
| | |ConnectAns
| | |<--------|
| | | |
| | | |
| |ConnectAns |
| |<--------| |
| | | |
| | | |
|ConnectAns | |
|<--------| | |
| | | |
| | | |
|TLS | | |
|.............................|
| | | |
| | | |
| | | |
| | | |
Once JP has a reasonable set of connections he is ready to take his
place in the DHT. He does this by sending a Join to AP. AP does a
series of Store requests to JP to store the data that JP will be
responsible for. AP then sends JP an Update explicitly labeling JP
as its predecessor. At this point, JP is part of the ring and
responsible for a section of the overlay. AP can now forget any data
which is assigned to JP and not AP.
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JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|JoinReq | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|JoinAns | | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreReq Data A | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|StoreReq Data B | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
In Chord, JP's neighbor table needs to contain its own predecessors.
It couldn't connect to them previously because Chord has no way to
route immediately to your predecessors. However, now that it has
received an Update from AP, it has APs predecessors, which are also
its own, so it sends Connects to them. Below it is shown connecting
to its closest predecessor, PP.
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JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Connect Dest=PP | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | |Connect Dest=PP | | |
| | |<--------| | | |
| | | | | | |
| | | | | | |
| | |ConnectAns | | |
| | |-------->| | | |
| | | | | | |
| | | | | | |
|ConnectAns | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|...................| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|------------------>| | | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<------------------| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|-------------------------------------->| | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<--------------------------------------| | |
| | | | | | |
| | | | | | |
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Finally, now that JP has a copy of all the data and is ready to route
messages and receive requests, it sends Updates to everyone in its
Routing Table to tell them it is ready to go. Below, it is shown
sending such an update to TP.
JP NP XX TP
| | | |
| | | |
| | | |
|Update | | |
|---------------------------->|
| | | |
| | | |
|UpdateAns| | |
|<----------------------------|
| | | |
| | | |
| | | |
| | | |
15. Security Considerations
15.1. Overview
RELOAD provides a generic storage service, albeit one designed to be
useful for P2PSIP. In this section we discuss security issues that
are likely to be relevant to any usage of RELOAD. In Section 15.7 we
describe issues that are specific to SIP.
In any Overlay Instance, any given user depends on a number of peers
with which they have no well-defined relationship except that they
are fellow members of the Overlay Instance. In practice, these other
nodes may be friendly, lazy, curious, or outright malicious. No
security system can provide complete protection in an environment
where most nodes are malicious. The goal of security in RELOAD is to
provide strong security guarantees of some properties even in the
face of a large number of malicious nodes and to allow the overlay to
function correctly in the face of a modest number of malicious nodes.
P2PSIP deployments require the ability to authenticate both peers and
resources (users) without the active presence of a trusted entity in
the system. We describe two mechanisms. The first mechanism is
based on public key certificates and is suitable for general
deployments. The second is based on an overlay-wide shared symmetric
key and is suitable only for limited deployments in which the
relationship between admitted peers is not adversarial.
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15.2. Attacks on P2P Overlays
The two basic functions provided by overlay nodes are storage and
routing: some node is responsible for storing a peer's data and for
allowing a peer to fetch other peer's data. Some other set of nodes
are responsible for routing messages to and from the storing nodes.
Each of these issues is covered in the following sections.
P2P overlays are subject to attacks by subversive nodes that may
attempt to disrupt routing, corrupt or remove user registrations, or
eavesdrop on signaling. The certificate-based security algorithms we
describe in this draft are intended to protect overlay routing and
user registration information in RELOAD messages.
To protect the signaling from attackers pretending to be valid peers
(or peers other than themselves), the first requirement is to ensure
that all messages are received from authorized members of the
overlay. For this reason, RELOAD transports all messages over a
secure channel (TLS and DTLS are defined in this document) which
provides message integrity and authentication of the directly
communicating peer. In addition, when the certificate-based security
system is used, messages and data are digitally signed with the
sender's private key, providing end-to-end security for
communications.
15.3. Certificate-based Security
This specification stores users' registrations and possibly other
data in an overlay network. This requires a solution to securing
this data as well as securing, as well as possible, the routing in
the overlay. Both types of security are based on requiring that
every entity in the system (whether user or peer) authenticate
cryptographically using an asymmetric key pair tied to a certificate.
When a user enrolls in the Overlay Instance, they request or are
assigned a unique name, such as "alice@dht.example.net". These names
are unique and are meant to be chosen and used by humans much like a
SIP Address of Record (AOR) or an email address. The user is also
assigned one or more Node-IDs by the central enrollment authority.
Both the name and the peer ID are placed in the certificate, along
with the user's public key.
Each certificate enables an entity to act in two sorts of roles:
o As a user, storing data at specific Resource-IDs in the Overlay
Instance corresponding to the user name.
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o As a overlay peer with the peer ID(s) listed in the certificate.
Note that since only users of this Overlay Instance need to validate
a certificate, this usage does not require a global PKI. Instead,
certificates are signed by require a central enrollment authority
which acts as the certificate authority for the Overlay Instance.
This authority signs each peer's certificate. Because each peer
possesses the CA's certificate (which they receive on enrollment)
they can verify the certificates of the other entities in the overlay
without further communication. Because the certificates contain the
user/peer's public key, communications from the user/peer can be
verified in turn.
If self-signed certificates are used, then the security provided is
significantly decreased, since attackers can mount Sybil attacks. In
addition, attackers cannot trust the user names in certificates
(though they can trust the Node-Ids because they are
cryptographically verifiable). This scheme is only appropriate for
small deployments, such as a small office or ad hoc overlay set up
among participants in a meeting. Some additional security can be
provided by using the shared secret admission control scheme as well.
Because all stored data is signed by the owner of the data the
storing peer can verify that the storer is authorized to perform a
store at that resource-id and also allows any consumer of the data to
verify the provenance and integrity of the data when it retrieves it.
All implementations MUST implement certificate-based security.
15.4. Shared-Secret Security
RELOAD also supports a shared secret admission control scheme that
relies on a single key that is shared among all members of the
overlay. It is appropriate for small groups that wish to form a
private network without complexity. In shared secret mode, all the
peers share a single symmetric key which is used to key TLS-PSK
[RFC4279] or TLS-SRP [I-D.ietf-tls-srp] mode. A peer which does not
know the key cannot form TLS connections with any other peer and
therefore cannot join the overlay.
One natural approach to a shared-secret scheme is to use a user-
entered password as the key. The difficulty with this is that in
TLS-PSK mode, such keys are very susceptible to dictionary attacks.
If passwords are used as the source of shared-keys, then TLS-SRP is a
superior choice because it is not subject to dictionary attacks.
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15.5. Storage Security
When certificate-based security is used in RELOAD, any given
Resource-ID/kind-id pair (a slot) is bound to some small set of
certificates. In order to write data in a slot, the writer must
prove possession of the private key for one of those certificates.
Moreover, all data is stored signed by the certificate which
authorized its storage. This set of rules makes questions of
authorization and data integrity - which have historically been
thorny for overlays - relatively simple.
When shared-secret security is used, then all peers trust all other
peers, provided that they have demonstrated that they have the
credentials to join the overlay at all. The following text therefore
applies only to certificate-based security.
15.5.1. Authorization
When a client wants to store some value in a slot, it first digitally
signs the value with its own private key. It then sends a Store
request that contains both the value and the signature towards the
storing peer (which is defined by the Resource Name construction
algorithm for that particular kind of value).
When the storing peer receives the request, it must determine whether
the storing client is authorized to store in this slot. In order to
do so, it executes the Resource Name construction algorithm for the
specified kind based on the user's certificate information. It then
computes the Resource-ID from the Resource Name and verifies that it
matches the slot which the user is requesting to write to. If it
does, the user is authorized to write to this slot, pending quota
checks as described in the next section.
For example, consider the certificate with the following properties:
User name: alice@dht.example.com
Node-ID: 013456789abcdef
Serial: 1234
If Alice wishes to Store a value of the "SIP Location" kind, the
Resource Name will be the SIP AOR "sip:alice@dht.example.com". The
Resource-ID will be determined by hashing the Resource Name. When a
peer receives a request to store a record at Resource-ID X, it takes
the signing certificate and recomputes the Resource Name, in this
case "alice@dht.example.com". If H("alice@dht.example.com")=X then
the Store is authorized. Otherwise it is not. Note that the
Resource Name construction algorithm may be different for other
kinds.
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15.5.2. Distributed Quota
Being a peer in a Overlay Instance carries with it the responsibility
to store data for a given region of the Overlay Instance. However,
if clients were allowed to store unlimited amounts of data, this
would create unacceptable burdens on peers, as well as enabling
trivial denial of service attacks. RELOAD addresses this issue by
requiring each usage to define maximum sizes for each kind of stored
data. Attempts to store values exceeding this size MUST be rejected
(if peers are inconsistent about this, then strange artifacts will
happen when the zone of responsibility shifts and a different peer
becomes responsible for overlarge data). Because each slot is bound
to a small set of certificates, these size restrictions also create a
distributed quota mechanism, with the quotas administered by the
central enrollment server.
Allowing different kinds of data to have different size restrictions
allows new usages the flexibility to define limits that fit their
needs without requiring all usages to have expansive limits.
15.5.3. Correctness
Because each stored value is signed, it is trivial for any retrieving
peer to verify the integrity of the stored value. Some more care
needs to be taken to prevent version rollback attacks. Rollback
attacks on storage are prevented by the use of store times and
lifetime values in each store. A lifetime represents the latest time
at which the data is valid and thus limits (though does not
completely prevent) the ability of the storing node to perform a
rollback attack on retrievers. In order to prevent a rollback attack
at the time of the Store request, we require that storage times be
monotonically increasing. Storing peers MUST reject Store requests
with storage times smaller than or equal to those they are currently
storing. In addition, a fetching node which receives a data value
with a storage time older than the result of the previous fetch knows
a rollback has occurred.
15.5.4. Residual Attacks
The mechanisms described here provide a high degree of security, but
some attacks remain possible. Most simply, it is possible for
storing nodes to refuse to store a value (i.e., reject any request).
In addition, a storing node can deny knowledge of values which it
previously accepted. To some extent these attacks can be ameliorated
by attempting to store to/retrieve from replicas, but a retrieving
client does not know whether it should try this or not, since there
is a cost to doing so.
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Although the certificate-based authentication scheme prevents a
single peer from being able to forge data owned by other peers.
Furthermore, although a subversive peer can refuse to return data
resources for which it is responsible it cannot return forged data
because it cannot provide authentication for such registrations.
Therefore parallel searches for redundant registrations can mitigate
most of the affects of a compromised peer. The ultimate reliability
of such an overlay is a statistical question based on the replication
factor and the percentage of compromised peers.
In addition, when a kind is is multivalued (e.g., an array data
model), the storing node can return only some subset of the values,
thus biasing its responses. This can be countered by using single
values rather than sets, but that makes coordination between multiple
storing agents much more difficult. This is a tradeoff that must be
made when designing any usage.
15.6. Routing Security
Because the storage security system guarantees (within limits) the
integrity of the stored data, routing security focuses on stopping
the attacker from performing a DOS attack on the system by misrouting
requests in the overlay. There are a few obvious observations to
make about this. First, it is easy to ensure that an attacker is at
least a valid peer in the Overlay Instance. Second, this is a DOS
attack only. Third, if a large percentage of the peers on the
Overlay Instance are controlled by the attacker, it is probably
impossible to perfectly secure against this.
15.6.1. Background
In general, attacks on DHT routing are mounted by the attacker
arranging to route traffic through or two nodes it controls. In the
Eclipse attack [Eclipse] the attacker tampers with messages to and
from nodes for which it is on-path with respect to a given victim
node. This allows it to pretend to be all the nodes that are
reachable through it. In the Sybil attack [Sybil], the attacker
registers a large number of nodes and is therefore able to capture a
large amount of the traffic through the DHT.
Both the Eclipse and Sybil attacks require the attacker to be able to
exercise control over her peer IDs. The Sybil attack requires the
creation of a large number of peers. The Eclipse attack requires
that the attacker be able to impersonate specific peers. In both
cases, these attacks are limited by the use of centralized,
certificate-based admission control.
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15.6.2. Admissions Control
Admission to an RELOAD Overlay Instance is controlled by requiring
that each peer have a certificate containing its peer ID. The
requirement to have a certificate is enforced by using certificate-
based mutual authentication on each connection. Thus, whenever a
peer connects to another peer, each side automatically checks that
the other has a suitable certificate. These peer IDs are randomly
assigned by the central enrollment server. This has two benefits:
o It allows the enrollment server to limit the number of peer IDs
issued to any individual user.
o It prevents the attacker from choosing specific peer IDs.
The first property allows protection against Sybil attacks (provided
the enrollment server uses strict rate limiting policies). The
second property deters but does not completely prevent Eclipse
attacks. Because an Eclipse attacker must impersonate peers on the
other side of the attacker, he must have a certificate for suitable
peer IDs, which requires him to repeatedly query the enrollment
server for new certificates which only will match by chance. From
the attacker's perspective, the difficulty is that if he only has a
small number of certificates the region of the Overlay Instance he is
impersonating appears to be very sparsely populated by comparison to
the victim's local region.
15.6.3. Peer Identification and Authentication
In general, whenever a peer engages in overlay activity that might
affect the routing table it must establish its identity. This
happens in two ways. First, whenever a peer establishes a direct
connection to another peer it authenticates via certificate-based
mutual authentication. All messages between peers are sent over this
protected channel and therefore the peers can verify the data origin
of the last hop peer for requests and responses without further
cryptography.
In some situations, however, it is desirable to be able to establish
the identity of a peer with whom one is not directly connected. The
most natural case is when a peer Updates its state. At this point,
other peers may need to update their view of the overlay structure,
but they need to verify that the Update message came from the actual
peer rather than from an attacker. To prevent this, all overlay
routing messages are signed by the peer that generated them.
[OPEN ISSUE: this allows for replay attacks on requests. There are
two basic defenses here. The first is global clocks and loose anti-
replay. The second is to refuse to take any action unless you verify
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the data with the relevant node. This issue is undecided.]
[TODO: I think we are probably going to end up with generic
signatures or at least optional signatures on all overlay messages.]
15.6.4. Protecting the Signaling
The goal here is to stop an attacker from knowing who is signaling
what to whom. An attacker being able to observe the activities of a
specific individual is unlikely given the randomization of IDs and
routing based on the present peers discussed above. Furthermore,
because messages can be routed using only the header information, the
actual body of the RELOAD message can be encrypted during
transmission.
There are two lines of defense here. The first is the use of TLS or
DTLS for each communications link between peers. This provides
protection against attackers who are not members of the overlay. The
second line of defense, if certificate-based security is used, is to
digitally sign each message. This prevents adversarial peers from
modifying messages in flight, even if they are on the routing path.
15.6.5. Residual Attacks
The routing security mechanisms in RELOAD are designed to contain
rather than eliminate attacks on routing. It is still possible for
an attacker to mount a variety of attacks. In particular, if an
attacker is able to take up a position on the overlay routing between
A and B it can make it appear as if B does not exist or is
disconnected. It can also advertise false network metrics in attempt
to reroute traffic. However, these are primarily DoS attacks.
The certificate-based security scheme secures the namespace, but if
an individual peer is compromised or if an attacker obtains a
certificate from the CA, then a number of subversive peers can still
appear in the overlay. While these peers cannot falsify responses to
resource queries, they can respond with error messages, effecting a
DoS attack on the resource registration. They can also subvert
routing to other compromised peers. To defend against such attacks,
a resource search must still consist of parallel searches for
replicated registrations.
15.7. SIP-Specific Issues
15.7.1. Fork Explosion
Because SIP includes a forking capability (the ability to retarget to
multiple recipients), fork bombs are a potential DoS concern.
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However, in the SIP usage of RELOAD, fork bombs are a much lower
concern because the calling party is involved in each retargeting
event and can therefore directly measure the number of forks and
throttle at some reasonable number.
15.7.2. Malicious Retargeting
Another potential DoS attack is for the owner of an attractive number
to retarget all calls to some victim. This attack is difficult to
ameliorate without requiring the target of a SIP registration to
authorize all stores. The overhead of that requirement would be
excessive and in addition there are good use cases for retargeting to
a peer without there explicit cooperation.
15.7.3. Privacy Issues
All RELOAD SIP registration data is public. Methods of providing
location and identity privacy are still being studied.
16. IANA Considerations
This section contains the new code points registered by this
document. The IANA policies are TBD.
16.1. Overlay Algorithm Types
IANA SHALL create/(has created) a "RELOAD Overlay Algorithm Type"
Registry. Entries in this registry are strings denoting the names of
overlay algorithms. The registration policy for this registry is
TBD.
The initial contents of this registry are:
chord-128-2-16+
The algorithm defined in Section 12 of this document.
16.2. Data Kind-Id
IANA SHALL create/(has created) a "RELOAD Data Kind-Id" Registry.
Entries in this registry are 32-bit integers denoting data kinds, as
described in Section 4.1.2. The registration policy for this
registry is TBD.
The initial contents of this registry are:
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+--------------------+---------+
| Kind | Kind-Id |
+--------------------+---------+
| SIP-REGISTRATION | 1 |
| TURN_SERVICE | 2 |
| CERTIFICATE | 3 |
| ROUTING_TABLE_SIZE | 4 |
| SOFTWARE_VERSION | 5 |
| MACHINE_UPTIME | 6 |
| APP_UPTIME | 7 |
| MEMORY_FOOTPRINT | 8 |
| DATASIZE_StoreD | 9 |
| INSTANCES_StoreD | 10 |
| MESSAGES_SENT_RCVD | 11 |
| EWMA_BYTES_SENT | 12 |
| EWMA_BYTES_RCVD | 13 |
| LAST_CONTACT | 14 |
| RTT | 15 |
+--------------------+---------+
16.3. Data Model
IANA SHALL create/(has created) a "RELOAD Data Model" Registry.
Entries in this registry are 8-bit integers denoting data models, as
described in Section 7.2. The registration policy for this registry
is TBD.
+--------------+------------+
| Data Model | Identifier |
+--------------+------------+
| SINGLE_VALUE | 1 |
| ARRAY | 2 |
| DICTIONARY | 3 |
+--------------+------------+
16.4. Message Codes
IANA SHALL create/(has created) a "RELOAD Message Code" Registry.
Entries in this registry are 16-bit integers denoting method codes as
described in Section 6.2.3. The registration policy for this
registry is TBD.
The initial contents of this registry are:
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+-------------------+----------------+
| Message Code Name | Code Value |
+-------------------+----------------+
| reserved | 0 |
| ping_req | 1 |
| ping_ans | 2 |
| connect_req | 3 |
| connect_ans | 4 |
| tunnel_req | 5 |
| tunnel_ans | 6 |
| store_req | 7 |
| store_ans | 8 |
| fetch_req | 9 |
| fetch_ans | 10 |
| remove_req | 11 |
| remove_ans | 12 |
| find_req | 13 |
| find_ans | 14 |
| join_req | 15 |
| join_ans | 16 |
| leave_req | 17 |
| leave_ans | 18 |
| update_req | 19 |
| update_ans | 20 |
| route_query_req | 21 |
| route_query_ans | 22 |
| reserved | 0x8000..0xfffe |
| error | 0xffff |
+-------------------+----------------+
[[TODO - add IANA registration for p2p_enroll SRV and p2p_menroll]]
16.5. Error Codes
IANA SHALL create/(has created) a "RELOAD Error Code" Registry.
Entries in this registry are 16-bit integers denoting error codes.
[[TODO: Complete this once we decide on error code strategy.
16.6. Route Log Extension Types
IANA SHALL create/(has created) a "RELOAD Route Log Extension Type
Registry. This entry is currently empty.
16.7. reload: URI Scheme
This section describes the scheme for a reload: URI, which can be
used to refer to either:
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o A peer.
o A resource inside a peer.
The reload: URI is defined using a subset of the URI schema
specified in Appendix A. of RFC 3986 [REF] and the associated URI
Guidelines [REF: RFC4395] per the following ABNF syntax:
RELOAD-URI = "reload://" destination "@" overlay "/"
[specifier]
destination = 1 * HEXDIG
overlay = reg-name
specifier = 1*HEXDIG
The definitions of these productions are as follows:
destination: a hex-encoded Destination List object.
overlay: the name of the overlay.
specifier : a hex-encoded StoredDataSpecifier indicating the data
element.
If no specifier is present than this URI addresses the peer which can
be reached via the indicated destination list at the indicated
overlay name. If a specifier is present, then the URI addresses the
data value.
16.7.1. URI Registration
The following summarizes the information necessary to register the
reload: URI. [NOTE TO IANA/RFC-EDITOR: Please replace XXXX with
the RFC number for this specification in the following list.]
URI Scheme Name: reload
Status: permanent
URI Scheme Syntax: see Section 16.7.
URI Scheme Semantics: The reload: URI is intended to be used as a
reference to a RELOAD peer or resource.
Encoding Considerations: The reload: URI is not intended to be
human-readable text, therefore they are encoded entirely in US-
ASCII.
Applications/protocols that use this URI scheme: The RELOAD
protocol described in RFC XXXX.
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TBD for the rest of this template.
17. Acknowledgments
This draft is a merge of the "REsource LOcation And Discovery
(RELOAD)" draft by David A. Bryan, Marcia Zangrilli and Bruce B.
Lowekamp, the "Address Settlement by Peer to Peer" draft by Cullen
Jennings, Jonathan Rosenberg, and Eric Rescorla, the "Security
Extensions for RELOAD" draft by Bruce B. Lowekamp and James Deverick,
the "A Chord-based DHT for Resource Lookup in P2PSIP" by Marcia
Zangrilli and David A. Bryan, and the Peer-to-Peer Protocol (P2PP)
draft by Salman A. Baset, Henning Schulzrinne, and Marcin
Matuszewski.
Thanks to the many people who contributed including: Michael Chen,
TODO - fill in.
18. References
18.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-16 (work in progress), June 2007.
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., "Session Traversal Utilities for (NAT)
(STUN)", draft-ietf-behave-rfc3489bis-06 (work in
progress), March 2007.
[I-D.ietf-behave-turn]
Rosenberg, J., "Obtaining Relay Addresses from Simple
Traversal Underneath NAT (STUN)",
draft-ietf-behave-turn-03 (work in progress), March 2007.
[I-D.ietf-pkix-cmc-trans]
Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC) Transport Protocols", draft-ietf-pkix-cmc-trans-05
(work in progress), May 2006.
[I-D.ietf-pkix-2797-bis]
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Myers, M. and J. Schaad, "Certificate Management Messages
over CMS", draft-ietf-pkix-2797-bis-04 (work in progress),
March 2006.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279,
December 2005.
[I-D.ietf-tls-srp]
Taylor, D., "Using SRP for TLS Authentication",
draft-ietf-tls-srp-14 (work in progress), June 2007.
[I-D.ietf-mmusic-ice-tcp]
Rosenberg, J., "TCP Candidates with Interactive
Connectivity Establishment (ICE",
draft-ietf-mmusic-ice-tcp-03 (work in progress),
March 2007.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., and J. Peterson, "SIP: Session Initiation Protocol",
RFC 3261, June 2002.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4828] Floyd, S. and E. Kohler, "TCP Friendly Rate Control
(TFRC): The Small-Packet (SP) Variant", RFC 4828,
April 2007.
18.2. Informative References
[I-D.ietf-behave-tcp]
Guha, S., "NAT Behavioral Requirements for TCP",
draft-ietf-behave-tcp-07 (work in progress), April 2007.
[I-D.ietf-p2psip-concepts]
Bryan, D., "Concepts and Terminology for Peer to Peer
SIP", draft-ietf-p2psip-concepts-00 (work in progress),
July 2007.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
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[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572, July 2006.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
and P. Leach, "HTTP Authentication: Basic and Digest
Access Authentication", RFC 2617, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[Sybil] Douceur, J., "The Sybil Attack", IPTPS 02, March 2002.
[Eclipse] Singh, A., Ngan, T., Druschel, T., and D. Wallach,
"Eclipse Attacks on Overlay Networks: Threats and
Defenses", INFOCOM 2006, April 2006.
[I-D.cheshire-dnsext-multicastdns]
Cheshire, S. and M. Krochmal, "Multicast DNS",
draft-cheshire-dnsext-multicastdns-06 (work in progress),
August 2006.
[I-D.cheshire-dnsext-dns-sd]
Krochmal, M. and S. Cheshire, "DNS-Based Service
Discovery", draft-cheshire-dnsext-dns-sd-04 (work in
progress), August 2006.
[I-D.matthews-p2psip-bootstrap-mechanisms]
Cooper, E., "Bootstrap Mechanisms for P2PSIP",
draft-matthews-p2psip-bootstrap-mechanisms-00 (work in
progress), February 2007.
[I-D.garcia-p2psip-dns-sd-bootstrapping]
Garcia, G., "P2PSIP bootstrapping using DNS-SD",
draft-garcia-p2psip-dns-sd-bootstrapping-00 (work in
progress), October 2007.
[I-D.camarillo-hip-bone]
Camarillo, G., Nikander, P., and J. Hautakorpi, "HIP BONE:
Host Identity Protocol (HIP) Based Overlay Networking
Environment", draft-camarillo-hip-bone-00 (work in
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progress), December 2007.
[I-D.pascual-p2psip-clients]
Pascual, V., Matuszewski, M., Shim, E., Zhang, H., and S.
Yongchao, "P2PSIP Clients",
draft-pascual-p2psip-clients-01 (work in progress),
February 2008.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[I-D.jiang-p2psip-sep]
Jiang, X. and H. Zhang, "Service Extensible P2P Peer
Protocol", draft-jiang-p2psip-sep-01 (work in progress),
February 2008.
[stoica-non-transitive-worlds05]
Freedman, M., Lakshminarayanan, K., Rhea, S., and I.
Stoica, "Non-Transitive Connectivity and DHTs",
WORLDS'05.
[stoica-geometry-sigcomm03]
Gummadi, K., Gummadi, R., Gribble, S., Ratnasamy, S.,
Shenker, S., and I. Stoica, "The Impact of DHT Routing
Geometry on Resilience and Proximity", SIGCOMM'03.
[ng-analytical-churn-ieeep2p06]
Wu, D., Tian, Y., and K. Ng, "Analytical Study on
Improving DHT Lookup Performance under Churn", IEEE
P2P'06.
[bryan-design-hotp2p08]
Bryan, D., Lowekamp, B., and M. Zangrilli, "The Design of
a Versatile, Secure P2PSIP Communications Architecture for
the Public Internet", Hot-P2P'08.
[opendht-sigcomm05]
Rhea, S., Godfrey, B., Karp, B., Kubiatowicz, J.,
Ratnasamy, S., Shenker, S., Stoica, I., and H. Yu,
"OpenDHT: A Public DHT and its Uses", SIGCOMM'05.
[Chord] Stoica, I., Morris, R., Liben-Nowell, D., Karger, D.,
Kaashoek, M., Dabek, F., and H. Balakrishnan, "Chord: A
Scalable Peer-to-peer Lookup Service for Internet
Applications", IEEE/ACM Transactions on Networking Volume
11, Issue 1, 17-32, Feb 2003.
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[vulnerabilities-acsac04]
Srivatsa, M. and L. Liu, "Vulnerabilities and Security
Threats in Structured Peer-to-Peer Systems: A Quantitative
Analysis", ACSAC 2004.
Authors' Addresses
Cullen Jennings
Cisco
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 421-9990
Email: fluffy@cisco.com
Bruce B. Lowekamp
SIPeerior Technologies
3000 Easter Circle
Williamsburg, VA 23188
USA
Phone: +1 757 565 0101
Email: lowekamp@sipeerior.com
Eric Rescorla
Network Resonance
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Phone: +1 650 320-8549
Email: ekr@networkresonance.com
Salman A. Baset
Columbia University
1214 Amsterdam Avenue
New York, NY
USA
Email: salman@cs.columbia.edu
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Henning Schulzrinne
Columbia University
1214 Amsterdam Avenue
New York, NY
USA
Email: hgs@cs.columbia.edu
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Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Jennings, et al. Expires December 12, 2008 [Page 127]