P2PSIP C. Jennings
Internet-Draft Cisco
Intended status: Standards Track B. Lowekamp
Expires: August 27, 2008 SIPeerior; William & Mary
E. Rescorla
Network Resonance
J. Rosenberg
Cisco
S. Baset
H. Schulzrinne
Columbia University
February 24, 2008
REsource LOcation And Discovery (RELOAD)
draft-bryan-p2psip-reload-03
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Abstract
This document defines REsource LOcation And Discovery (RELOAD), a
peer-to-peer (P2P) binary signaling protocol for use on the Internet.
A P2P signaling protocol provides its clients with an abstract hash
table 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 data kinds 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 . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1. Architecture . . . . . . . . . . . . . . . . . . . . . . 7
1.1.1. Usage Layer . . . . . . . . . . . . . . . . . . . . . 8
1.1.2. Overlay Routing and Storage Layer . . . . . . . . . . 9
1.1.3. Forwarding Layer . . . . . . . . . . . . . . . . . . 10
1.2. Security . . . . . . . . . . . . . . . . . . . . . . . . 10
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 11
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1. Distributed Storage Layer . . . . . . . . . . . . . . . . 13
3.1.1. DHT Concepts . . . . . . . . . . . . . . . . . . . . 13
3.1.2. DHT Topology . . . . . . . . . . . . . . . . . . . . 14
3.1.3. Routing . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.4. Storing and Retrieving Structured Data . . . . . . . 15
3.1.5. Joining, Leaving, and Maintenance . . . . . . . . . . 16
3.2. Forwarding Layer . . . . . . . . . . . . . . . . . . . . 17
3.2.1. Forming Direct Connections . . . . . . . . . . . . . 17
3.2.2. Via Lists . . . . . . . . . . . . . . . . . . . . . . 18
3.2.3. Clients . . . . . . . . . . . . . . . . . . . . . . . 19
3.3. Transport Layer . . . . . . . . . . . . . . . . . . . . . 20
3.4. Enrollment . . . . . . . . . . . . . . . . . . . . . . . 20
3.4.1. Certificate Issuance . . . . . . . . . . . . . . . . 21
3.4.2. Bootstrap . . . . . . . . . . . . . . . . . . . . . . 21
3.5. Security . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5.1. Certificate-Based Security . . . . . . . . . . . . . 21
3.5.2. Shared-Key Security . . . . . . . . . . . . . . . . . 23
3.6. Migration . . . . . . . . . . . . . . . . . . . . . . . . 24
3.7. Usages Layer . . . . . . . . . . . . . . . . . . . . . . 24
3.7.1. SIP Usage . . . . . . . . . . . . . . . . . . . . . . 25
3.7.2. Certificate Store Usage . . . . . . . . . . . . . . . 27
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3.7.3. TURN Usage . . . . . . . . . . . . . . . . . . . . . 27
3.7.4. Diagnostic Usage . . . . . . . . . . . . . . . . . . 27
3.7.5. HIP Tunnel Usage . . . . . . . . . . . . . . . . . . 27
4. Base Protocol . . . . . . . . . . . . . . . . . . . . . . . . 28
4.1. Forwarding Header . . . . . . . . . . . . . . . . . . . . 28
4.1.1. Changes to Forwarding Header . . . . . . . . . . . . 33
4.1.2. Message Routing . . . . . . . . . . . . . . . . . . . 33
4.1.3. Fragmentation and Reassembly . . . . . . . . . . . . 35
4.1.4. Route Logging . . . . . . . . . . . . . . . . . . . . 36
4.2. Message Contents Format . . . . . . . . . . . . . . . . . 38
4.2.1. Common Header . . . . . . . . . . . . . . . . . . . . 38
4.2.2. Payload . . . . . . . . . . . . . . . . . . . . . . . 39
4.2.3. Signature . . . . . . . . . . . . . . . . . . . . . . 39
4.3. Response Codes and Response Errors . . . . . . . . . . . 42
5. End-to-End Timeout and Retransmission . . . . . . . . . . . . 43
6. Transports . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.2. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.2.1. Reliability for Unreliable Transports . . . . . . . . 44
6.3. HIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7. Method Definitions . . . . . . . . . . . . . . . . . . . . . 46
7.1. Connection Management . . . . . . . . . . . . . . . . . . 46
7.1.1. PING . . . . . . . . . . . . . . . . . . . . . . . . 46
7.1.2. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 48
7.1.3. TUNNEL . . . . . . . . . . . . . . . . . . . . . . . 51
7.2. Data Storage and Retrieval . . . . . . . . . . . . . . . 53
7.2.1. STORE . . . . . . . . . . . . . . . . . . . . . . . . 53
7.2.2. FETCH . . . . . . . . . . . . . . . . . . . . . . . . 60
7.2.3. REMOVE . . . . . . . . . . . . . . . . . . . . . . . 64
7.2.4. FIND . . . . . . . . . . . . . . . . . . . . . . . . 65
7.3. DHT Maintenance . . . . . . . . . . . . . . . . . . . . . 67
7.3.1. JOIN . . . . . . . . . . . . . . . . . . . . . . . . 67
7.3.2. LEAVE . . . . . . . . . . . . . . . . . . . . . . . . 68
7.3.3. UPDATE . . . . . . . . . . . . . . . . . . . . . . . 68
7.3.4. ROUTE_QUERY . . . . . . . . . . . . . . . . . . . . . 69
8. ICE and Connection Formation . . . . . . . . . . . . . . . . 70
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 71
8.2. Collecting STUN Servers . . . . . . . . . . . . . . . . . 72
8.3. Gathering Candidates . . . . . . . . . . . . . . . . . . 73
8.4. Encoding the CONNECT Message . . . . . . . . . . . . . . 73
8.5. Verifying ICE Support . . . . . . . . . . . . . . . . . . 74
8.6. Role Determination . . . . . . . . . . . . . . . . . . . 74
8.7. Connectivity Checks . . . . . . . . . . . . . . . . . . . 75
8.8. Concluding ICE . . . . . . . . . . . . . . . . . . . . . 75
8.9. Subsequent Offers and Answers . . . . . . . . . . . . . . 75
8.10. Media Keepalives . . . . . . . . . . . . . . . . . . . . 75
8.11. Sending Media . . . . . . . . . . . . . . . . . . . . . . 75
8.12. Receiving Media . . . . . . . . . . . . . . . . . . . . . 76
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9. DHT Algorithms . . . . . . . . . . . . . . . . . . . . . . . 76
9.1. Generic Algorithm Requirements . . . . . . . . . . . . . 76
9.2. Chord Algorithm . . . . . . . . . . . . . . . . . . . . . 76
9.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . 76
9.2.2. Routing . . . . . . . . . . . . . . . . . . . . . . . 77
9.2.3. Redundancy . . . . . . . . . . . . . . . . . . . . . 77
9.2.4. Joining . . . . . . . . . . . . . . . . . . . . . . . 77
9.2.5. Routing CONNECTs . . . . . . . . . . . . . . . . . . 78
9.2.6. UPDATEs . . . . . . . . . . . . . . . . . . . . . . . 79
9.2.7. Leaving . . . . . . . . . . . . . . . . . . . . . . . 82
10. Enrollment and Bootstrap . . . . . . . . . . . . . . . . . . 82
10.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 82
10.2. Overlay Configuration . . . . . . . . . . . . . . . . . . 83
10.3. Credentials . . . . . . . . . . . . . . . . . . . . . . . 84
10.3.1. Credentials for HIP . . . . . . . . . . . . . . . . . 85
10.4. Locating a Peer . . . . . . . . . . . . . . . . . . . . . 85
11. Usages . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
11.1. Generic Usage Requirements . . . . . . . . . . . . . . . 86
11.2. SIP Usage . . . . . . . . . . . . . . . . . . . . . . . . 87
11.2.1. SIP-REGISTRATION kind . . . . . . . . . . . . . . . . 87
11.2.2. GRUUs . . . . . . . . . . . . . . . . . . . . . . . . 89
11.2.3. SIP Connect . . . . . . . . . . . . . . . . . . . . . 89
11.2.4. SIP Tunnel . . . . . . . . . . . . . . . . . . . . . 89
11.3. TURN Usage . . . . . . . . . . . . . . . . . . . . . . . 90
11.4. Certificate Store Usage . . . . . . . . . . . . . . . . . 93
11.5. HIP Tunnel . . . . . . . . . . . . . . . . . . . . . . . 94
11.6. Diagnostic Usage . . . . . . . . . . . . . . . . . . . . 94
11.6.1. State Acquisition Mechanisms . . . . . . . . . . . . 95
11.6.2. Black-box diagnostics . . . . . . . . . . . . . . . . 96
11.6.3. Diagnostic Metrics for a P2PSIP Deployment . . . . . 98
12. Security Considerations . . . . . . . . . . . . . . . . . . . 99
12.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 99
12.2. Attacks on P2P Overlays . . . . . . . . . . . . . . . . . 99
12.3. Certificate-based Security . . . . . . . . . . . . . . . 100
12.4. Shared-Secret Security . . . . . . . . . . . . . . . . . 101
12.5. Storage Security . . . . . . . . . . . . . . . . . . . . 101
12.5.1. Authorization . . . . . . . . . . . . . . . . . . . . 102
12.5.2. Distributed Quota . . . . . . . . . . . . . . . . . . 102
12.5.3. Correctness . . . . . . . . . . . . . . . . . . . . . 103
12.5.4. Residual Attacks . . . . . . . . . . . . . . . . . . 103
12.6. Routing Security . . . . . . . . . . . . . . . . . . . . 104
12.6.1. Background . . . . . . . . . . . . . . . . . . . . . 104
12.6.2. Admissions Control . . . . . . . . . . . . . . . . . 104
12.6.3. Peer Identification and Authentication . . . . . . . 105
12.6.4. Protecting the Signaling . . . . . . . . . . . . . . 105
12.6.5. Residual Attacks . . . . . . . . . . . . . . . . . . 106
12.7. SIP-Specific Issues . . . . . . . . . . . . . . . . . . . 106
12.7.1. Fork Explosion . . . . . . . . . . . . . . . . . . . 106
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12.7.2. Malicious Retargeting . . . . . . . . . . . . . . . . 106
12.7.3. Privacy Issues . . . . . . . . . . . . . . . . . . . 107
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 107
13.1. Overlay Algorithm Types . . . . . . . . . . . . . . . . . 107
13.2. Data Kind-Id . . . . . . . . . . . . . . . . . . . . . . 107
13.3. Data Model . . . . . . . . . . . . . . . . . . . . . . . 108
13.4. Message Codes . . . . . . . . . . . . . . . . . . . . . . 108
14. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . 109
15. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 109
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 109
17. Appendix: Operation with SIP clients outside the DHT domain . 110
18. Appendix: Notes on DHT Algorithm Selection . . . . . . . . . 110
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 110
19.1. Normative References . . . . . . . . . . . . . . . . . . 110
19.2. Informative References . . . . . . . . . . . . . . . . . 111
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 113
Intellectual Property and Copyright Statements . . . . . . . . . 115
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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 Distributed Hash Table (DHT) service, which allows
participating nodes to read and write entries into a hash table that
is stored collectively among the participants. RELOAD is a
lightweight, binary protocol. It provides several functions that are
critical for a successful P2P protocol for the Internet. These are:
Security Framework: Security is one of the most challenging problems
in a P2P protocol. A P2P network will often be established among
a set of peers that do not trust each other. Yet, despite this
lack of trust, the network must operate reliably to allow storage
and retrieval of data. RELOAD defines an abstract enrollment
server, which all entities trust to generate unique identifiers
for each user. Using that small amount of trust as an anchor,
RELOAD defines a security framework that allows for authorization
of P2P protocol functions and authentication of data stored in the
overlay. This does not remove all attacks but greatly reduces the
possible attack space.
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].
Consequently, RELOAD has the notion of a usage, one of which is
defined to support each application (this document also defines
the SIP usage for multimedia communications). Each usage
identifies a set of data kind that need to be stored and retrieved
from the DHT. Each kind defines a data structure, authorization
policies, size quota, and information required for storage and
retrieval in the DHT. The usage concept allows RELOAD to be used
with new applications through a simple documentation process that
supplies the details for each application.
NAT Traversal: Operations for NAT traversal are part of the base
design, including establishing new RELOAD connections and
tunneling SIP or other application protocols required by P2PSIP.
RELOAD makes use of Interactive Connectivity Establishment (ICE)
[I-D.ietf-mmusic-ice] to facilitate the creation of the P2P
network and the establishment of channels for use by the
application protocol (SIP and RTP, for example). RELOAD also
defines how peers in the P2P network act as STUN and TURN servers
and how those resources can be discovered through the DHT. With
these features, RELOAD can run in modes in which nearly all the
peers are behind NATs, yet are able to fully participate without
imposing any constraints on the actual DHT algorithm.
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High Performance Routing: The very nature of DHT 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 to reduce
the amount of bandwidth and processing required of peers. It does
so by using a very lightweight binary protocol, and furthermore,
by defining a packet structure that facilitates low-complexity
forwarding, including hardware-based forwarding. In particular, a
generic transport header is used for routing the message through
the overlay without the contents needing to be parsed by (or even
visible to) intermediate peers. The header includes no
information about specific IP addresses because none are needed to
route along an overlay. The header only includes lists of peers
which the message should be routed through/too, as well as some
minor options and version flags. Clearly separating the header
components necessary for routing from the message contents
simplifies processing and increases security.
Transport Flexibility: RELOAD has native support for both DTLS and
TLS for the underlying transport protocol, with support for DTLS
over UDP as mandatory to implement. TLS over TCP is preferred
because it has better bulk data performance and connection
stability, but UDP is more likely to provide direct connections
between peers in the presence of NATs. Explicit support for
fragmentation is provided and required when using UDP. Because
there is no single universally available and suitable transport
protocol, the peer protocol must be flexible in this regard. New
transports can be supported trivially.
Pluggable DHT 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" DHT
will allow interoperability, while the extensibility allows
selection of DHTs optimized for a particular application.
These properties were designed specifically to meet the requirements
for a P2P protocol to support SIP. 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. Architecture
Architecturally this specification is divided into several layers, as
shown in the following figure.
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Application
-------------------------------------- Usage-defined API
+-------+ +-------+
Usage | SIP | | XMPP | ...
Layer | Usage | | Usage |
+-------+ +-------+
-------------------------------------- Distributed Storage API
Overlay Overlay +-------------+
Routing & Routing & +----+ | +-----+ |
Storage Replication | DB | | |Chord| ... | Topology
Layer Logic +----+ | | | | Plugins
| +-----+ |
+-------------+
--------------------------------------
+------+ +-----+
Forwarding Forwarding & | STUN | | ICE |
Layer Encoding Logic +------+ +-----+
-------------------------------------- Common Packet Encoding
Transport +-------+ +------+
Layer |TLS | |DTLS |
+-------+ +------+
The three layers defined by RELOAD include:
Usage Layer: Provides an application-specific interface that maps an
application's requirements onto the generic services of the DHT.
Overlay Routing & Storage Layer: Implements the overlay. Chooses
what links to establish to form the DHT's overlay network, manages
the storage and migration of data for this peer and on behalf of
other peers, and performs searches for requested data across the
DHT.
Forwarding Layer: Provides packet forwarding services between nodes.
Also handles setting up connections across NATs using ICE.
1.1.1. Usage Layer
The top layer, called the Usage Layer, has application usages, such
as the SIP Location Usage, that use the abstract distributed storage
API to store and retrieve data from the DHT. The goal of this layer
is to implement application-specific usages of the Overlay Routing
and Storage Layer below it. The Usage defines how a specific
application maps its data into something that can be stored in the
DHT, 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
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single application may require multiple usages. 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. A usage is not
itself encoded on the wire, only the kind-ids and data models are,
but is rather a specification of the functionality that is required
for a given application. That specification typically specifies
semantics, access control rules, and the format and size of the data
which may be stored.
One usage may depend on another. For example, the SIP usage depends
on a Certificate Store usage (not shown in the diagram) to obtain the
certificates required to authenticate messages. Because certificates
are stored in standard X.509 form, there is no reason for each usage
to specify this service independently.
1.1.2. Overlay Routing and Storage Layer
The Overlay Routing and Storage Layer stores and retrieves
information, performs maintenance of the DHT as peers join and leave
the DHT, and routes messages on the overlay. The DHT implementation
is provided by a pluggable component so that each overlay can select
an appropriate DHT that relies on the common RELOAD core code.
The Overlay Routing and Replication Logic provides a fairly generic
interface that allows the DHT implementation to control the overlay
and resource operations and messages. Since each DHT is defined and
functions differently, we generically refer to the table of other
peers that the DHT maintains and uses to route requests (neighbors)
as a Routing Table. The Logic component makes queries to the DHT's
Routing Table to determine the next hop, then encodes and sends the
message itself. Similarly, the DHT issues periodic update requests
through the logic component to maintain and update its Routing Table.
The DHT shown in the illustration is Chord, but a variety of DHT
algorithms are possible through a pluggable interface. A single node
could be functioning in multiple overlays simultaneously, each using
its own DHT algorithm. Each peer is identified by and its location
in the overlay determined by its Peer-ID that is assigned by the
enrollment server when the user or peer first enrolls in the overlay.
The Peer-ID also determines the set of resources which it will be
responsible for storing. The exact mapping between these is
determined by the DHT algorithm used by the overlay, therefore the
logic component always queries the DHT to determine where a
particular resource should be stored.
As peers enter and leave, resources may be stored on different peers,
so the information related to them is exchanged as peers enter and
leave. Redundancy is used to protect against loss of information in
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the event of a peer failure and to protect against compromised or
subversive peers. The Logic component notifies the DHT as neighbors
join and leave, and the DHT updates its Routing Table and issues
resource migration requests as appropriate.
1.1.3. Forwarding Layer
This 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 required by the
DHT's Routing Table. 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.
1.2. Security
RELOAD provides two security mechanisms, one based on public key
certificates and one based on a globally shared key.
RELOAD's preferred security framework is built upon an enrollment
server. The enrollment server issues each new peer a certificate
that assigns it a Peer-ID. By generating the Peer-IDs randomly and
controlling what peers are issued certificates, the enrollment server
protects against many of the attacks on the overlay network.
Similarly, all users are issued certificates for their identities by
the enrollment server. All resources stored on the overlay must be
signed by their creator, thus ensuring that an attacker cannot forge
data belonging to another user. The enrollment process is only
required to join the overlay (and perhaps to refresh an expired
certificate). The peers and users do not need to have real-time
access to the enrollment server.
TLS or DTLS are used for communication between peers. In combination
with the certificates, this provides both confidentiality and
authentication for communication across the overlay. Applications
such as P2PSIP can also make use of the users' certificates to
achieve secure end-to-end connections at the application layer.
RELOAD also provides for an authorization framework based on
certificates. Each usage defines the access control rules for which
users/peers are allowed to read/write each Resource-ID. This
protects data belonging to one user from being written by another, as
well as allowing for a distributed quota mechanism based on the
maximum size of each kind of data.
In addition to the enrollment server model, RELOAD offers a security
model using a pre-shared-key. Although this provides significantly
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less security than is provided through an enrollment server, it
allows ad hoc or ephemeral overlays to be set up with minimal effort
on the part of the users.
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.
The following important terms from the Concepts document are defined
below for reference.
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.
DHT Algorithm: A DHT algorithm defines the rules for determining
which peers in a DHT store a particular piece of data and for
determining a topology of interconnections amongst peers in order
to find a piece of data.
DHT Instance: A specific hash table and the collection of peers that
are collaborating to provide read and write access to it. There
can be any number of DHT instances running in an IP network at a
time, and each operates in isolation of the others.
P2P Network: Another name for a DHT instance.
P2P Network Name: A string that identifies a unique P2P network.
P2P network names are DNS names - for example, "example.org".
Lookup of such a name in DNS returns services associated with the
DHT, such as enrollment servers, bootstrap peers, or gateways (for
example, a SIP gateway between a traditional SIP and a P2P SIP
network called "example.com").
Resource-ID: A value that is not human friendly to read and
identifies some resources and which is used as a key for storing
and retrieving the resource. 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.
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Peer: A host that is participating in the DHT. By virtue of its
participation it can store data and is responsible for some
portion of the overlay.
Peer-ID: A value that uniquely identifies a peer. Peer-IDs 0 and
2^N - 1 are reserved and are invalid peer-IDs. A value of zero is
not used in the wire protocol but can be used to indicate an
invalid peer in implementations and APIs. The peer-id of 2^N-1 is
used on the wire protocol as a wildcard.
Resource: An object associated with a string identifier. In
unstructured P2P networks, the identifier is used directly as a
Resource-Id. In structured P2P networks the identifier can be
mapped into a Resource-ID by using the string as the input to the
hash function. A SIP resource, for example, is identified by its
AOR.
User: A human being.
>
We also introduce the following important new terms.
Connection Table: The set of peers to which a peer is directly
connected. This includes peers with which CONNECT handshakes have
been done but which have not sent any UPDATEs.
Routing Table: The set of peers which a peer can use to route DHT
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.
Hashed-ID: The generic term for an identifier in the hash space of
the DHT. Examples of Hashed-IDs include Resource-IDs and Peer-
IDs. This only applies to structured overlays.
Unhashed-ID: An Unhashed-ID is a string used as an input to a hash
function, the result of which is a Hashed-ID. This only applies
to structured overlays.
Usage: A usage is an application that wishes to use the DHT for some
purpose. Each application wishing to use the DHT 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.
Destination List: A list of IDs through which a message is to be
routed. This allows for request/response source routing. A
single ID is a trivial form of destination list.
3. Overview
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3.1. Distributed Storage Layer
RELOAD is designed to be extensible to both structured and
unstructured overlays. However, this version is only completely
worked out for structured overlays such as DHTs. The following text
assumes structured overlays; in particular Resource-IDs are assumed
to be fixed length for any given overlay, although the protocol
allows them to be variable length to allow extension to unstructured
overlays.
Each logical address in the DHT where data can be stored is referred
to as a Resource-ID. A given peer will be responsible for storing
data from many Resource-ID locations. Typically literature on DHTs
uses the term "key" to refer to a location in the DHT; however, in
this specification the term key is used to refer to public or private
keys used for cryptographic operations and the term Resource-ID is
used to refer to a location for storage in the DHT.
3.1.1. DHT Concepts
While very early P2P systems used flood based techniques, some newer
P2P systems locate resources using a Distributed Hash Table, or DHT
to improve efficiency. Peers are organized using a Distributed Hash
Table (DHT) structure. In such a system, every resource has a
Resource-ID, which is obtained by hashing some keyword or value (an
Unhashed-ID) that uniquely identifies the resource. Resources can be
thought of as being stored in a hash table at the entry corresponding
to their Resource-ID. The peers that make up the overlay network are
also assigned an ID, called a Peer-ID, in the same hash space as the
Resource-IDs. A peer is responsible for storing all resources that
have Resource-IDs near the peer's Peer-ID. The hash space is divided
up so that all of the hash space is always the responsibility of some
particular peer, although as peers enter and leave the system a
particular peer's area may change. Messages are exchanged between
the peers in the DHT as the peers enter and leave to preserve the
structure of the DHT and exchange stored entries. Various DHT
implementations may visualize the hash space as a grid, circle, line,
or hypercube.
Peers keep information about the location of other peers in the hash
space and typically know about many peers nearby in the hash space,
and progressively fewer more distant peers. We refer to this table
of other peers as a Routing Table. When a peer wishes to operate on
a resource it consults the list of peers it is aware of and contacts
the peer with the Peer-ID nearest the desired Resource-ID. If that
peer does not know how to find the resource, it either returns
information about a closer peer it knows about, or forwards the
request to a closer peer. In this fashion, the request eventually
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reaches the peer responsible for the resource, which then replies to
the requester.
3.1.2. DHT Topology
Each DHT will have a somewhat different structure, but many of the
concepts are common. The DHT defines a large space of Resource-IDs,
which can be thought of as addresses. In many DHTs, the Resource-IDs
are simply 128- or 160-bit integers. Each DHT also has a distance
metric such that we can say that Resource-ID A is closer to
Resource-ID B than to Resource-ID C.
Each peer in the DHT is assigned a Peer-ID and is "responsible" for
the nearby space of Resource-IDs. So, for instance, if we have a
peer P, then it could also be responsible for storing data associated
with Resource-ID P+epsilon as long as no other peer P was closer.
The DHT Resource-ID space is divided so that some peer is responsible
for each Resource-ID.
3.1.3. Routing
The way routing works in a DHT is specified by the specific DHT
algorithm but the basic concepts are common to most systems. Each
peer maintains connections to some other set of peers N. There need
not be anything special about the peers in N, except that the peer
has a direct connection to them: it can reach them without going
through any other peer. When it wishes to deliver a message to some
peer P, it selects some member of N, N_i that is closer to P than
itself (as a degenerate case, P may be in N). The peer sends the
message to N_i. At this point two things can happen:
Recursive Routing: N_i repeats the same process as P, sending the
message to one of its peers N_j. This same process repeats until
the message is delivered to N.
Iterative Routing: N_i consults its table of direct connections and
selects a new peer N_j which is closer to N. It responds to the
original sending peer with a redirect to N_j. The original peer
then sends the message to N_j, where the process repeats until the
sending peer is redirected to N.
The advantage of iterative routing is that it consumes less resources
for the intermediate peers; they only have to send redirect messages
rather than forwarding requests and responses. The advantage of
recursive routing is that it does not require the sending or
receiving peer to have a rich set of connections to other nodes in
the overlay. Thus, iterative routing is problematic in NATed
networks because there is no way to guarantee that a peer will be
able to form a connection to whatever peer it is redirected to. In
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RELOAD, iterative routing is supported using the ROUTE-QUERY request.
In most DHTs, the peers in N are selected in a particular way. One
common strategy is to have them arranged exponentially further away
from yourself so that any message can be routed in a O(log(N)) steps.
The details of the routing structure depend on the DHT algorithm,
however, since it defines the distance metric and the structure of
the connection table.
In RELOAD, messages may either be REQUESTS or RESPONSES to REQUESTS.
Requests are routed as described above. In principle, responses
could be routed independently from requests. This is called
"Asymmetric" routing because requests and responses will generally
follow different paths through the network. Asymmetric routing makes
diagnosis of errors difficult because you need to be able to acquire
debugging information at multiple locations. In the alternative
strategy, called "Symmetric" routing, as requests travel through the
network they accumulate a history of the peers they passed through
and responses are routed in the opposite direction so that they
follow the same path in reverse. RELOAD supports both flavors of
routing.
Symmetric routing is easier to debug. Symmetric routing is also
required when the overlay topology is changing. For example, when a
new peer is joining the overlay, asymmetric routing cannot work
because the response would not be able to reach the new peer until it
has completed the joining process. Symmetric routing solves this
situation because the response is routed from the admitting peer
through the bootstrap peer, thus relying on a path that is already
known and established. In order to implement symmetric routing,
RELOAD provides the Via List (Section 3.2.2) feature. Asymmetric
routing, however, requires no state to be stored in the message (as a
Via List) or in on-path peers.
[[TODO: again, this is a topic that needs WG discussion. It seems
like there are situations where symmetric is very desirable (e.g.,
startup). It's less clear that asymmetric will have a performance/
state difference that will be significant.]]
[[TODO: it's not clear that symmetric/asymmetric is really that
evocative a terminology. Henning suggested key-based/trace-based but
EKR doesn't like that. Other suggestions?]]
3.1.4. Storing and Retrieving Structured Data
The Data Storage Layer provides operations to STORE, FETCH, and
REMOVE data. Each location in the DHT is referenced by a single
integer Resource-ID. However, each location may contain data
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elements corresponding to multiple kinds (e.g., certificate, SIP
registration). Similarly, there may be multiple elements of a given
kind.
+--------------------------------+
| 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. Note that a kind may be employed by multiple usages and new
usages are encouraged to use previously defined kinds where possible.
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. 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 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.
3.1.5. Joining, Leaving, and Maintenance
When a new peer wishes to join the DHT, it must have a peer-id that
it is allowed to use. It uses one of the peer-ids in the certificate
it received from the enrollment server. The main steps in joining
the DHT are:
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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.
First, the peer ("JP," for Joining Peer) uses the bootstrap
procedures to find some (any) peer in the DHT. It then typically
contacts the peer which would have formerly been responsible for the
peer's Resource-ID (since that is where in the DHT the peer will be
joining), the Admitting Peer (AP). It copies the other peer's state,
including the data values it is now responsible for and the
identities of the peers with which the other peer has direct
connections.
The details of this operation depend mostly on the DHT involved, but
a typical case would be:
1. JP sends a JOIN request to AP announcing its intention to join.
2. AP sends an 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 DHT.
5. JP makes its own connections to the appropriate peers in the DHT.
After this process is completed, JP is a full member of the DHT and
can process STORE/FETCH requests.
3.2. Forwarding Layer
The forwarding layer is responsible for looking at message and doing
one of three things:
o Deciding the message was destined for this peer and passing the
message up to the layer above this.
o Looking at the peer-id that represents the next peer to send the
message too and if there is an existing connection, sending the
message over the connection.
o Requesting the DHT Routing logic to tell the forwarding layer
which peer the message needs to be forwarded to (based on the
target peer-id or resource-id), and then sending the message.
3.2.1. Forming Direct Connections
As described in Section 3.1.3, a peer maintains a set of direct
connections to other peers in the DHT. Consider the case of a peer
JP just joining the DHT. It communicates with the admitting peer AP
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and gets the list of the peers in AP's routing table. Naively, it
could simply connect to the IP address listed for each peer, but this
works poorly if some of those peers are behind a NAT or firewall.
Instead, we use the CONNECT request to establish a connection.
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 DHT 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 DHT peers.
In general, a peer needs to maintain connections to all of the peers
near it in the DHT and to enough other peers to have efficient
routing (the details depend on the specific DHT). If a peer cannot
form a connection to some other peer, this isn't necessarily a
disaster; DHTs 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.
3.2.2. Via Lists
In a general messaging system, messages need a source and a
destination and peers need to be able to send a response to the peer
that sent the request. This can be particularly tricky in overlay
networks when a new peer is joining, or the overlay network is
stabilizing and different peers have different ideas on what the
overlay topology is. A simple and reliable way to make sure that a
response can reach the node that sent the request in these situations
is to have the response traverse the reverse path of the request.
The approach used here is to have each node the request traverses add
its peer-id to the "via list" in the request. Then the response is
routed by looking at the list and using it as list of peers that the
response will be routed thorough. To support this, each message has
a destination list of nodes it needs to be routed through as well as
a via list of what nodes it has traversed.
When a peer receives a message from the Transport Layer, it adds the
peer-id of the node it received the message from to the end of the
via list. When a peer goes to transmit a message to the Transport
Layer, it looks at the first entry on the destination list. If the
entry is this peer, it removes this entry from the list and looks at
the next entry and if the entry is not this peer, it sends the
message to the first peer on the destination list.
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When a peer goes to send a response to a request, it can simply copy
the via list in reverse to form the destination list for the response
if it wishes to route the response along the reverse path as the
request.
Peers that are willing to maintain state may do list compression for
privacy reason and to reduce the message size. They do this by
taking some number of entries off the via list and replacing them
with a unique entry that this peer can later identify. Later, if the
peer sees the unique entry in a destination list, it removes the
unique entry and replaces it with the all the entries removed from
the original via list (and reverses the order of these entries).
Note that this technique will generally require storing some per-
message state on the intermediate peer, so this is a bandwidth/
per-peer state tradeoff. The exception is if the list is not
compressed but rather the peer-ids are simply encrypted.
The via list approach provides several features. First it allows a
response to follow the same path as the request. This is
particularly important for peers that are sending requests while they
are joining and before other peers can route to them as well as
situations where message are being exchanged to stabilize the overlay
network. It also makes it easier to diagnose and manage the system
when all peers see the response to any request they forward.
3.2.3. Clients
RELOAD also allows for the possibility of client nodes. A client is
a node with a peer-id which connects to an admitting peer (or peers)
like an ordinary peer but never sends a JOIN or an UPDATE. It is
therefore in the AP's connection table but not its routing table and
never is used to store any DHT data. However, because it is
reachable through the AP, it can still send and receive messages.
The client MUST still have the usual credentials. Also, because it
never sends JOINs it is never responsible for storing data.
Because the client may only have a connection to a single AP, which,
due to topology shifts may no longer be the responsible peer, clients
SHOULD use symmetric routing and should advertise route lists that
contain both the AP to which they are connected and themselves.
E.g., if the client has peer-id X and the AP has peer-id Y, the
client should advertise the destination list (Y, X). This guarantees
reachability.
Note that clients MAY also contact APs which are not in fact
responsible for the client's peer-id.
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3.3. Transport Layer
The transport layer sends and receives messages over TLS and DTLS.
For TLS it simply pushes the messages into the stream. For DTLS it
takes care of fragmentation issues. The reason for including TLS is
the improved performance it can offer for bulk transport of data.
The reason for including DTLS is that the percentage of the time that
two devices behind NATs can form a direct connection without a relay
is much higher for DTLS than for TLS. If all NATs were
[I-D.ietf-behave-tcp] compliant, then TLS over TCP would be
preferred.
3.4. Enrollment
Before a new user can join the DHT for the first time, they must
enroll in the P2P Network for the DHT they want to join. Enrollment
will typically be done by contacting a centralized enrollment server.
Other approaches (for instance static out of band configuration) are
possible but are outside the scope of this specification. During
enrollment a new node learns about a particular overlay, sets up a
names and credentials, and discovers the bootstrap nodes. This would
typically be done when a new peer joined an overlay for the very
first time. Bootstrap is the process that happens each time a node
boots and is how the peer finds an node that can be used to join the
overlay.
Before a node can join an overlay, it needs to be provided with a
name for the overlay. Some examples are "example.com", "example",
and "example.local". This name is resolved via a DNS SRV lookup for
service name p2p_enroll and a protocol of tcp. If the TLD for the
name is .local, then this DNS SRV lookup is done using
[I-D.cheshire-dnsext-multicastdns] and the service name p2p_menroll.
The intention here is to support ad hoc/local overlays. The
resulting DNS lookup will provide the address of a enrollment server.
Once this server is found, HTTPS is used to retrieve a XML file that
contains the parameters for the overlay. These include things such
as: what algorithms the overlay uses, overlay parameters, what
usages are a peer on this overlay is required to support, the type of
credentials required, addresses of credentials servers, the root
certificate for the DHT, information about the DHT algorithm that is
being used, a P2P-Network-Id that uniquely identifies this ring, and
any other parameters it may need to connect to the DHT. The DHT also
informs the peers what Usages it is required to support to be a peer
on this P2P Network. It also provides an initial list of bootstrap
nodes that consists of multiple bootstrap entries that each have the
IP address and port for contacting a bootstrap peer. Some of the
address may be multicast addresses. In the case of multicast DNS,
every peer may also act as an enrollment server.
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If shared-key security (Section 3.5.2) is being used, then the peer
can proceed directly to bootstrap. If certificate-based security
(Section 3.5.1 is being used, the peer MUST contact the credential
server to obtain a certificate.
3.4.1. Certificate Issuance
Once the peer has the XML file that identifies if credentials are
needed, it can contact the credential server. The user establishes
his identity to the server's satisfaction and provides the server
with its public key. The centralized server then returns a
certificate binding the user's user name to his public key. The
properties of the certificate are discussed in Section 3.5. The
amount of authentication performed here can vary radically depending
on the DHT network being joined. Some networks may do no
verification at all and some may require extensive identity
verification (e.g., checking a driver's license) before issuing
credentials for a given user name. The only invariant that the
enrollment server needs to ensure is that no two users may have the
same identity.
3.4.2. Bootstrap
The above steps are only done the first time a peer joins a new
overlay or when the overlay parameters are close to their expiration
time (as listed in the configuration document) and need to be
refreshed. The next step is the bootstrap step which is done every
time the peer boots.
Bootstrapping consists of looking at the list of cached nodes and
bootstraps nodes and sending a RELOAD PING to them to see if they
respond. Once a node responds, it can be used to join the overlay.
After a node has joined, it keeps track of a small number of peers to
which it could directly connect. Theses are saved as the cached
nodes and used next time the peer boots. The point of the cached
nodes is to reduce the load on the bootstrap nodes.
3.5. Security
3.5.1. Certificate-Based Security
The certificate-based security model revolves around the enrollment
process allocating a unique name to the user and issuing a
certificate [RFC3280] for a public/private key pair for the user.
All peers in a particular DHT can verify these certificates. A given
peer acts on behalf of a user, and that user is responsible for its
operation.
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The certificate serves two purposes:
o It entitles the user to store data at specific locations in the
DHT. Each usage defines the specific rules for determining which
certificates can access each Hashed-ID/kind-id pair. For
instance, some usages might allow anyone to write at a given
location, whereas others might restrict writes to a single
certificate.
o It entitles the user to operate a peer that has a peer-id found in
the certificate. When the peer is acting as a DTLS or TLS server,
it can use this certificate so that a client connecting to it
knows it is connected to the correct server.
When a user enrolls, or enrolls a device with no keying material, the
user is given a certificate. This certificate contains information
that identifies the user and the device they are using. If a user
has more than one device, typically they would get one certificate
for each device. This allows each device to act as a separate peer.
The contents of the certificate include:
o A public key provided by the user.
o Zero or more user names that the DHT is allowing this user to use.
For example, "alice@example.org". Typically a certificate will
have one name. In the SIP usage, this name corresponds to the
AOR.
o Zero or more peer-ids. Typically there will be one peer-id. Each
device will use a different peer-id, even if two devices belong to
the same user. Peer-IDs should be chosen randomly by the
enrollment server.
o A serial number that is unique to this certificate across all the
certificates issued for this DHT.
o An expiration time for the certificate.
Note that because peer-IDs are chosen randomly, they will be randomly
distributed with respect to the user name. This has the result that
any given peer is highly unlikely to be responsible for storing data
corresponding to its own user, which promotes high availability.
3.5.1.1. Storage Permissions
When a peer uses a STORE request to place data at a particular
location X, it must sign with the private key that corresponds to a
certificate that is suitable for storing at location X. Each data
kind in a usage defines the exact rules for determining what
certificate is appropriate.
The most natural rule is that a certificate with user name X "owns"
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data located at Hash(X) (X is the Unhashed-ID and Hash(X) is the
Hashed-ID) and only he can write there. 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 DHT. 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 undetectably
modify it.
3.5.1.2. Peer Permissions
The second purpose of a certificate is to allow the device to act as
a peer with the specified peer-ID. When a peer wishes to connect to
peer X, it forms a TLS/DTLS connection to the peer and then performs
TLS mutual authentication and verifies that the presented certificate
contains peer-ID X.
Note that because the formation of a connection between two nodes
generally requires traversing other nodes in the DHT, as specified in
Section 3.2.1, those nodes can interfere with connection initiation.
However, if they attempt to impersonate the target peer they will be
unable to complete the TLS mutual authentication: therefore such
attacks can be detected.
3.5.1.3. Expiry and Renewal
At some point before the certificate expires, the user will need to
get a new certificate from the enrollment server.
3.5.2. Shared-Key Security
RELOAD also defines a shared-key security model which can be used in
closed networks where the peers are not mutually suspicious. In this
model, the peers all share a single key which is used to authenticate
the peer-to-peer DTLS connections via TLS-PSK/TLS-SRP. If shared-key
security mode is in use, a shared-key capable cipher suite such as
TLS-PSK or TLS-SRP MUST be used. This is useful for admission
control, but is completely unsafe in any setting where peers are not
mutually trusted, since it allows any peer to impersonate any other
peer.
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3.6. Migration
At some point in time, a given P2P Network may want to migrate from
one underlying DHT algorithm to another or update to a later
extension of the protocol. This can also be used for crypto agility
issues. The migration approach is done by having peers initializing
algorithm A. When the clients go to periodically renew their
credentials, they find out that the P2P Network now requires them to
use algorithm A but also to store all the data with algorithm B. At
this point there are effectively two DHT rings in use, rings A and B.
All data is written to both but queries only go to A. At some point
when the clients periodically renew their credentials, they learn
that the P2P Network has moved to storing to both A and B but that
FETCH requests are done with P2P Network B and that any SEND should
first be attempted on P2P Network B and if that fails, retried on P2P
Network A. In the final stage when clients renew credentials, they
find out that P2P Network A is no longer required and only P2P
Network B is in use. Some types of usages and environments may be
able to migrate very quickly and do all of these steps in under a
week, depending on how quickly software that supports both A and B is
deployed and how often credentials are renewed. On the other hand,
some very ad-hoc environments involving software from many different
providers may take years to migrate.
[[TODO: This needs more filling out]]
3.7. Usages Layer
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 needs to specify several things:
o Register kind-id code points for any kinds that the Usage defines.
o Define the data structure for each of the kinds.
o Define access control rules for each kinds.
o Provide a size limit for each kinds.
o Define how the Unhashed-ID is formed that is hashed to form the
Resource-ID where each kind is stored.
o Describe 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 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
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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.
3.7.1. SIP Usage
From the perspective of P2PSIP, the most important usage is the SIP
Usage. 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 Bob's SIP UA can use to pass SIP messages back and
forth to Alice's SIP UA.
This is done using three key operations that are provided by the SIP
Usage. They are:
o Mapping SIP URIs that are not GRUUs to other SIP URIs or to the
DHT peer responsible for the SIP UA.
o Mapping SIP GRUUs to the DHT peer responsible for the SIP UA.
o Forming a connection directly to a DHT peer that is used to send
SIP messages to the SIP UA.
All SIP URIs for a given overlay MUST be constructed so that they
terminate in the domain name of the overlay. For instance, if the
overlay name is "example.com", then all AORs must be of the form
{sip,sips}:username@example.com. Accordingly, to dereference a URI,
a P2PSIP implementation MUST check to see if the domain matches an
overlay which it is a member of. If so, it uses the following
procedures. Otherwise, it MUST follow [RFC3263] procedures. Note
that unless the P2PSIP overlay provides some kind of gateway to
ordinary SIP (e.g., a publicly accessible SIP server) this is likely
to be only partially successful, since, for instance, the callee may
not be able to call back.
3.7.1.1. SIP Location
A peer acting as a SIP UA stores their registration information in
the DHT by storing either another URI (for retargeting) or a
destination lists to reach them at a Resource-ID in the DHT formed
from the user's SIP AOR. When another peer wishes to find a peer
that is registered for a SIP URI, the lookup of the user's name is
done by taking the user's SIP Address or Record (AOR) and using it as
the Unhashed-ID that is hashed to get a Resource-ID. When the
Unhashed-ID is dereferenced, the result is a set of values. Each
value is either another SIP URI or a destination list. If the value
is a SIP URI, the calling peer looks up that URI and continues the
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process until it gets a destination list.
If the value is a destination list, then it is used to reach a peer
that represents a SIP UA registered for that AOR. Typically this
destination list will have just one entry but in the case of peers or
clients that can not be directly reached (for instance via a strict
NAT or firewall), a destination list with more than one entry may
need to be used.
The Unhashed-ID for this usage is a user's SIP AOR, such as
"sip:alice@example.com". This allows the set to store many values in
a dictionary structure. The authorization policy is that STORE
requests are only allowed if the user name in the signing
certificate, when turned into a SIP URL and hashed, matches the
Resource-ID. This policy ensures that only a user with the
certificate with the user name "alice@example.com" can write to the
Resource-ID that will be used to look up calls to
"sip:alice@example.com".
[[Open Issue: Should the Unhashed-ID be "sip:alice@example.com",
"alice@example.com", or a string that includes the code point defined
for the kind? The issue here is determining whether different usages
that store data at a Unhashed-ID that is primarily formed from
"alice@example.com" should hash to the same Resource-ID as the SIP
Usage. For example, if a buddy list had a Unhashed-ID that was
roughly the same, would we want the buddy list information to end up
on the same peers that stored the SIP location data or on different
peers?]]
3.7.1.2. SIP GRUUs
GRUUs that refer to peers in the P2P network are constructed by
simply forming a GRUU, where the value of gr URI parameter contains a
base64 encoded version of the destination list that will reach the
peer. Typically the destination list is just a single entry with the
peer-id of peer.
3.7.1.3. SIP Connect
This usage allows two clients to form a new TLS or DTLS connection
between them and then use this connection for sending SIP messages to
one another. This does not store any information in the DHT, but it
allows the CONNECT request to be used to set up a TLS or DTLS
connection between two peers and then use that connection to send SIP
messages back and forth.
The CONNECT request will ensure that the connection is formed to a
peer that has a certificate which includes the user that the
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connection is being formed to.
3.7.1.4. SIP Tunnel
This TUNNEL request allows two peers to exchange SIP messages across
the overlay using the TUNNEL method without first setting up a direct
connection using CONNECT. This allows a SIP message to be sent
immediately, without the delay associated with CONNECT and for a
simple SIP exchange, it may result in fewer messages being sent.
3.7.2. Certificate Store Usage
This usage allows each user to store their certificate in the DHT so
that it can be retrieved to be checked by various peers and
applications. Peers acting on behalf of a particular user store that
user's certificate in the DHT, and any peer that needs the
certificate can do a FETCH to retrieve the certificate. Typically it
is retrieved to check a signature on a request or the signature on a
chunk of data that the DHT has received.
3.7.3. TURN Usage
This usage defines a new kind for finding STUN-Relay servers. Any
peer that supports this usage saves a pointer to the IP address and
port of the TURN server in the DHT. When a peer wishes to discover a
TURN server, it picks a random Resource-ID and performs a FIND at
that Resource-ID for the appropriate type for the service. If
nothing is found, this can be repeated until an appropriate set of
servers are found.
3.7.4. Diagnostic Usage
This usage defines several new kinds that be queried to find
information about the peer that may be useful for monitoring and
diagnostics. This includes information such as software version,
neighbor information, and performance statistics.
3.7.5. HIP Tunnel Usage
This usage allows two peers running HIP to tunnel HIP messages across
the overlay. This allows the HIP peers to use the overlay as a
rendezvous system to set up a direct path using HIP NAT traversal
mechanisms.
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4. Base Protocol
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, just
define theses in a fixed position as adding a type and length for
them is unnecessary and would simply increase bandwidth and
introduces new potentials for interoperability issues.
Each message has three parts:
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.
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.
4.1. Forwarding Header
The layout of the forwarding header is shown below
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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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Via | Route | |
| List | List | Flags |
| Length | Length | |
28 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// 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
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.
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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: How should 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.
Version1.0 : 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 Destination List Length and the Via List Length contain the
lengths of the route and via lists respectively, in the number of
objects.
[[Open Issue: How should we handle peer-id lengths? This basically
assumes they're fixed length per DHT algorithm (but not fixed-length
for RELOAD) so that you can unambiguously parse things. Should we
have a length byte?]]
The flags word contains control flags. There is one currently
defined flag.
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ROUTE-LOG : 0x1
The ROUTE-LOG flag indicates that the route log should be included
(see Section 4.1.4
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. Destinations are defined at the end of
this section.
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.
If a message was being sent thought the sequences of peers A,B,C,D,
the message from A to B would have a empty via list and a route of
list of B,C,D. The message from B to C would have a via list of A
then route of C,D and so on. This means that when the route list is
followed exactly, all that is needed to update these lists is to
change their lengths. This avoids the need to change or move any of
the other list entries. In other cases, some entries may need to be
copied or moved.
The destination list and via lists a list of objects of type
destination_object:
STRUCTURE: destination_object
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Info |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: peer
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | |
004 +-+-+-+-+-+-+-+-+ +
| Peer |
008 + +
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| |
012 + +
| |
016 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
017 +-+-+-+-+-+-+-+-+
STRUCTURE: compressed
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | Compressed Len| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Compressed |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: resource
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | Resource Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Resource |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A destination_object can have one of three types (this is
extensible):
peer: A peer-id. All peer-ids are of fixed length for a given
overlay and therefore peer-ids have no length.
compressed: A compressed list of peer-ids and/or resources. This
value is variable length but because it was compressed by one of
the peers, it is only meaningful to that peer and cannot be
decoded by other peers.
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.
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4.1.1. Changes to Forwarding Header
The RELOAD-01 forwarding header was completely fixed, whereas this
header includes lists that change en-route. However, this type of
operation is easily accomplished in both software and hardware,
therefore we still view it as a low-overhead header. The changes
include the following.
o Rearranged fields to have a cleaner separation between payload and
header.
o Removed DHT, Hash, and Security parameters. These are now in the
overlay bootstrap system rather than per-message.
o Source and destination IDs are now destination lists to
accommodate source routing and recursion without state on
intermediate peers.
o Added route log to header to allow payload/header separation.
4.1.2. Message Routing
4.1.2.1. Request Origination
In order to send a message to a given peer-id or resource-id, a peer
must construct an appropriate destination list. The most common such
destination list is a single entry containing the peer/resource-id.
This simply uses the normal DHT routing mechanisms to forward the
message to that destination.
Messages can also be source routed. In order to construct a source
route, the originator provides a destination list containing a
sequence of resource-ids. The semantics of this destination list are
that the message is to traverse in order (potentially with
intermediate hops) each entry on the destination list. As each peer
is traversed, that entry is removed from the destination list. This
makes it possible to address a peer which is potentially behind a NAT
or a firewall in such a way that it cannot be connected to directly
under any circumstances.
[[TODO: Salman has suggested the originator doing parallel requests/
responses. This is an open issue.]
4.1.2.2. Response Origination
When a peer sends a response to a request, it SHOULD construct the
destination list by reversing the order of the entries on the via
list. This has the result that the response traverses (at least) the
same peers as the request traversed, except in reverse order
(symmetric routing). For asymmetric routing, the peer MAY simply use
the first entry on the via list.
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4.1.2.3. 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
is for an overlay in which the peer does not participate) it is an
error. The peer SHOULD generate an appropriate error but MAY simply
drop the message.
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 a private id which is
being used for destination list compression.
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 for which another peer
is responsible.
These cases are handled separately.
4.1.2.3.1. Private ID
If the first entry on the destination list is a private id, the peer
replaces that entry with the store local value that it indexes and
then re-examines the destination list to determine which case now
applies.
4.1.2.3.2. Responsible ID
If the first entry on the destination list is a Hashed-ID for which
the peer is responsible, the peer strips the entry off the route
list. If there are remaining entries on the destination list, the
peer then re-examines the destination list to determine which case
now applies. If the destination list is now empty, then the message
was destined for this peer and it MUST pass it to the next layer up.
4.1.2.3.3. Other Hashed-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
means that it MUST select one of the peers in its route table which
is closer to the first entry than to itself and send the message to
that peer. 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.
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When forwarding a message, the peer MUST:
o Decrement the TTL value
o Update the via list.
The natural way to update the via list is simply to add the peer-id
of the peer from which the message was received to the end of the
list. However, peers may use any algorithm of their choice provided
that if the peer received a destination list constructed by reversing
the via list it would be able to route the outgoing message
correctly, enabling symmetric routing.
For instance, if node D receives a message from node C with via list
(A, B), the simple approach is simply to 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 (B, A). However, node D could also list
compression and send E the via list (X). E would then use the
destination list (D, X). When D processes this destination list, it
MUST detect that X is a compressed entry, recover the via list (A, B,
C), and reverse that to produce the correct destination list (C, B,
A) before sending it to C.
Note that if a peer is using list compression and then exits the
overlay, the message cannot be forwarded and will be dropped. The
ordinary timeout and retransmission networks provide stability over
this type of failure.
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 prevent denial of
service attacks, receivers SHOULD time out incomplete fragments.
[[TODO: Describe algorithm]]
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4.1.4. 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. 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 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.
STRUCTURE: route_log
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Entries Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Entries |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The route log is simply a variable length list of route log entries.
The first two bytes are the length, followed by a sequence of route
leg entries, each of which may be individually parsed.
STRUCTURE: route_log_entry
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Version |
/ /
| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport | |
+-+-+-+-+-+-+-+-+ +
| Id |
+ +
| |
+ +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Uptime |
+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Certificate Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Certificate |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Address |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: ip4_address_type
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | Addr |
004 +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Port |
007 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: ip6_address_type
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | |
004 +-+-+-+-+-+-+-+-+ +
| Addr |
008 + +
| |
012 + +
| |
016 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Port |
019 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Each route log entry consists of the following values:
Version - A textual representation of the software version
Transport - The transport type, 1 for TLS, 2 for DTLS
Id - The peer-id of the peer.
Uptime - The uptime of the peer in seconds.
Certificate - The peer's certificate. Note that this may be omitted
by setting the length to zero.
Address - The address and port of the peer. This can be either an
IPv4 or IPv6 address.
4.2. Message Contents Format
Although from the perspective of the forwarding layer the content is
opaque, all RELOAD messages share a common content structure
consisting of two parts:
Common Header: A common header containing the request method/
response code, and a transaction ID.
Payload: The actual body of the request/response. These are
dependent on whether this is a request or response and the type of
request being carried.
4.2.1. Common Header
The layout of the common header is shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
| Message Code | Reserved |
| | |
4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Code is a 16 bit field that indicates which message this is.
This field 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_Q (the PING request)
has value 1 and PING_A (the PING response) has value 2
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0x8000 .. 0xfffe Reserved
0xffff Error
4.2.2. Payload
Payload is a simple string of uninterpreted bytes preceded by a
length field indicating the length of the data, not including the
length field. The bytes themselves are dependent on the code value.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Length |
| |
4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Length bytes of data //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.2.3. Signature
The Signature element is used to attach signatures to messages and or
stored data elements. All signatures are formatted using this
element. However, the input structure to the signature computation
varies depending on the data element being signed.
STRUCTURE: signature
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm | Signature Value Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Signature Value |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Identity |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The signature construct is just a container for the signature. It
contains the following values:
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Algorithm - The signature algorithm in use. This may have the
values RSA-SHA1 (0x01) or RSA-SHA-256 (0x02).
Value - The signature value itself. This is just the string of
bytes emitted by the signature algorithm.
Identity - The identity or certificate used to form the signature
[[TODO: Should we convert all of this to CMS?]]
A number of possible identity formats are permitted, as shown below.
The peer may indicate any of:
o Peer-id
o User name
o The certificate itself.
The first byte of the identity field is a type indicating the type of
identity in use.
STRUCTURE: signer_object
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Signer |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: signer_identity_peer
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | |
004 +-+-+-+-+-+-+-+-+ +
| Signer Identity Peer |
008 + +
| |
012 + +
| |
016 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
017 +-+-+-+-+-+-+-+-+
STRUCTURE: signer_identity_name
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 2
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | Signer Identity Name Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Signer Identity Name |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: signer_identity_certificate
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x03 |Signer Identity Certificate Len| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Signer Identity Certificate |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For signatures over messages the input to the signature function is:
STRUCTURE: message_signature_input
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Overlay |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
008 + +
| Xid |
012 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Signer Identity |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Message Contents |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The contents of this structure are as follows:
Overlay - The overlay identifier from the message.
Xid - The transaction id from the message.
Signer Identity - The identify of the signer (from the signature
structure.)
Message Contents - The contents section of the message.
[[TODO: Check the inputs to this carefully.]]
The input to signatures over data values is different, and is
described in Section 7.2.1.3.
4.3. Response Codes and Response Errors
A peer processing a request returns its status in the Message Code
field of the common header. 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.
For any code other than 200, the payload should be as defined below:
STRUCTURE: error_response
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Reason Phrase Len |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Reason Phrase |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Info Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Error Info |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of this payload are:
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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 (e.g., "Moved Temporarily).
Error Info - Payload specific error information. This MUST be empty
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, DHT algorithm, or hash algorithm.
[[Open Issue: What is the best error number and reason phrase to
use?]]
499 (UnWilling To Proxy) A peer receiving the request is unwilling
to support the Routing mechanism specified in the Routing field of
the message header. [[Open Issue: What is the best error number
and reason phrase to use?]]
5. End-to-End Timeout and Retransmission
Timeout and retransmission are handled on an end-to-end basis as well
as the transports providing a hop by hop reliability mechanism. For
end-to-end reliability, the requesting node retransmits a requests
every 3 seconds until it receives a response or after it has send the
request 5 times. Retransmissions MUST use the same transaction ID.
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6. Transports
Currently multiple transport protocols are specified and more may be
defined in the future. Implementation MUST implement TLS and DTLS.
A given overlay can choose which protocols it uses.
6.1. TLS
TLS runs on top of TCP which offers the best performance from a data
transfer point of view and does not require as frequent keep alive
messages.
6.2. DTLS
DTLS runs on top of UDP which offers the highest probability of
direct connectivity in the face of the current generation of consumer
NATs.
6.2.1. Reliability for Unreliable Transports
When RELOAD is carried over DTLS or another unreliable transport, it
needs to be used with a reliability and flow 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.
6.2.1.1. Message Format
Each message being sent is prepended with a header that indicates the
24 bit sequence number.
STRUCTURE: header
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | Seq |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Seq The sequence number of the message.
Each DTLS session has it own sequence number. Initially the value is
zero and it increments by exactly one for each message sent over that
DTLS session.
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6.2.1.2. Acknowledgement Format
STRUCTURE: ACK
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | Ack Seq |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Received |
008 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 DTLS flow. The contents of this
packet are:
Ack Seq - The sequence number of the message being acknowledged.
Received - A bitmask indicating whether or not each of the previous
32 packets has been received. 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.
6.2.1.3. 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 use 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.
6.3. HIP
RELOAD MAY also be used with a HIP transport using the architecture
for HIP BONE described in [I-D.camarillo-hip-bone]. From the
perspective of the P2P layer, HIP looks very much like normal IP.
Either TLS (over TCP) or DTLS (over UDP) is run over top of the HIP.
Thus the reliability and congestion control schemes are the same for
DTLS section. If an overlay is configured such that HIP is the only
transport that it will use, then it may make sense to configure the
p2p layer to only offer the ORCHID when gather candidate addresses
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for ICE. This will effectively disable ICE at the p2p layer.
For overlays that use HIP, the enrollment server MUST provide each
peer with a unique ORCHID and use that ORCHID to generate the peer-id
for the peer (see Section 10.3. Later when the HIP layer wishes to
tunnel a message (such as an I1 message) through the overlay, the HIP
layer can use the ORCHID to generate the peer-id, and then use the
TUNNEL message with the HIP to route the message to that the peer
that owns that ORCHID.
7. Method Definitions
In this section, we define the initial set of methods supported by
RELOAD. New methods are defined by adding new method codes. Each
method defines the contents of the payload element (see
Section 4.2.2).
PDUs are named using the following convention. For method type FOO,
the request PDU is named FOO_Q and the response PDU is named FOO_A
(as are the method codes). When discussing the PDU itself, we use
these terms. Throughout the rest of the document we refer to the FOO
method or the FOO request/response for easier readability.
7.1. Connection Management
7.1.1. PING
PING is used to test connectivity along a path. A ping can be
addressed to a specific peer-id or to the broadcast peer-id (all 1s).
In either case, the target peer-ids respond with a simple response
containing some status information.
7.1.1.1. Request Definition
The PING_Q message contains a list (potentially empty) of the pieces
of status information that the requester would like the responder to
provide.
STRUCTURE: ping_q
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ping Info Len | |
+-+-+-+-+-+-+-+-+ +
| Ping Info |
/ /
| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The two currently defined types are:
RESPONSIBLE-SET : 0x01
NUM-RESOURCES : 0x02
RESPONSIBLE-SET indicates that the peer should Respond with the
fraction of the overlay for which the responding peer is responsible
(in parts per billion).
NUM-RESOURCES indicates that the peer should Respond with the number
of resources currently being stored by the peer.
7.1.1.2. Response Definition
A successful PING_A response contains the information elements
requested by the peer.
STRUCTURE: ping_a
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
004 + +
| Response Id |
008 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Infos Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Infos |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A PING_A message contains the following elements:
Response ID - A randomly generated 64-bit response ID. This is used
to distinguish PING responses in cases where the PING request is
multicast.
Infos - A sequence of ping info data structures, as shown below.
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STRUCTURE: ping_info_data
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Ping Info Data |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: info_responsible_type
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | Info Responsible Type |
004 +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
005 +-+-+-+-+-+-+-+-+
STRUCTURE: info_num_resources_type
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | Info Num Resources Type |
004 +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
005 +-+-+-+-+-+-+-+-+
The ping info data elements are simple typed elements, with a type
identifier as the leading 16 bits and then arbitrary (type-specific)
text following. In the case of the two defined types, the responses
are 32-bit integers.
The responding peer SHOULD include any values that the requesting
peer requested and that it recognizes. They SHOULD be returned in
the requested order.
7.1.2. 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 8.
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Note: A CONNECT 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, 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.
7.1.2.1. Request Definition
A CONNECT_Q message contains the requesting peer's ICE connection
parameters formatted into a binary structure.
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STRUCTURE: connect_data
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ufrag Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Ufrag |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Password Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Password |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application | Fingerprint Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Fingerprint |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Role Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Role |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Candidate List Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Candidate List |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: candidate
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Candidate String Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Candidate String |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The values contained in connect-request 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.
Fingerprint - One fingerprint attribute (from RFC 4572 [RFC4572]).
Role - An active/passive/actpass attribute from RFC 4145 [RFC4145].
Candidate - 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.
These values should be generated using the procedures of Section 8.
7.1.2.2. Response Definition
If a peer receives a CONNECT request, it SHOULD follow the procedures
of Section 8 to process the request and generate its own response (a
CONNECT_A) containing a connect_data object. It should then begin
ICE checks. When a peer receives a CONNECT response, it SHOULD parse
the response and begin its own ICE checks.
7.1.3. 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.
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When a usage requires the TUNNEL method, it must specify the specific
application protocol(s) that will be TUNNELed and for each protocol,
specify:
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.
7.1.3.1. Request Definition
The TUNNEL_Q message contains the application PDU that the requesting
peer wishes to transmit, along with some control information
identifying the handling of the PDU.
STRUCTURE: tunnel_q
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application | Dialog Id Len |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Dialog Id |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application Pdu Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Application Pdu |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The values contained in the TUNNEL_Q are:
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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.
Application PDU - An application PDU in the format specified by the
application.
7.1.3.2. Response Definition
A TUNNEL_A 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
7.2. Data Storage and Retrieval
The STORE, FETCH, and REMOVE methods are used to manipulate
information in the DHT. They form an instantiation of the abstract
GET and PUT operations described in [I-D.ietf-p2psip-concepts].
7.2.1. STORE
The STORE method is used to store data in the overlay. As described
in Section 3.1.4, each location may contain data of multiple kinds.
Each kind-id is a code point assigned to a specific application usage
by IANA. As part of the Usage 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
item; some sets may be allowed to contain multiple identical items
while others may only have unique items. The protocol currently
defines the following data models:
o single value
o array
o dictionary
Each kind MUST specify the appropriate data model for that kind. The
format of the STORE request depends on the data model.
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7.2.1.1. Request Definition
A STORE_Q message is a sequence of kind-data pairs, 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.
STRUCTURE: store_q
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resource Len | |
+-+-+-+-+-+-+-+-+ +
| Resource |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Store Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Store Data |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A single STORE request stores data of a number of kinds to a single
resource location. The contents of the request are:
Resource - The resource to store at.
Store Kind Data - A series of elements, one for each kind of data to
be stored.
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STRUCTURE: store_kind_data
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Model | |
008 +-+-+-+-+-+-+-+-+ +
| Generation |
012 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Values Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Values |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each store kind data 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.
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.
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STRUCTURE: stored_data
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
008 + +
| Storage Time |
012 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
016 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Data-value |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Signature |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each stored_data element represents a single stored data value.
These elements are individually signed. The contents of the element
are as follows:
Length - The length of the stored data element.
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 known to
the receiving peer 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.
Lifetime - The validity period for the data, in seconds, starting
from the time of store.
Signature - A signature over the data value. Section 7.2.1.3
describes the signature computation. The element is formatted as
described in Section 4.2.3
Data Value - The data value itself, as described below.
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STRUCTURE: single_value_entry
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Value |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: array_entry
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Value |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: dictionary_entry
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Key |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Value |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The responsible 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.
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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 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.
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. 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 store_kind_data, 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.
We now discuss each data model:
7.2.1.1.1. Single Value
There may be only one single-value element for each resource-id,
kind-id pair. A store of a new single-value element MUST overwrite
the current value.
7.2.1.1.2. Array
A store of an array entry replaces (or inserts) the given value at
the location specified by the index. Arrays are zero-based. Note
that arrays can be sparse. Thus, 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 empty strings.
If the index value is -1, then the value is placed at the end of the
array.
7.2.1.1.3. Dictionary
A stored dictionary entry has a dictionary-key used as a lookup key
and a dictionary-value containing the data. There may be only one
value for any given dictionary-key and therefore a write to a
dictionary-key overwrites whatever is there.
7.2.1.2. Response Definition
In response to a successful STORE request the peer MUST return a
STORE_A message containing a series of store_kind_response elements
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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.
STRUCTURE: store_kind_response
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
008 + +
| Generation |
012 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Replicas Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Replicas |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of each element 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 DHTs 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.
The response itself is just the store_kind_response values packed
end-to-end.
If the request was rejected because of an invalid generation counter,
then the store-response MUST also be returned, but with a response
code of 412. Otherwise, the response MAY contain a response-error-
reason production or MAY be empty. [[TODO: The generation counter
may need more thinking for uniqueness.]]
7.2.1.3. Data Signature Computation
Each stored-data 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 stored value.
The data signed is defined as:
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STRUCTURE: stored_data_to_be_signed
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resource Len | |
+-+-+-+-+-+-+-+-+ +
| Resource |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Stored Data |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of this value are as follows:
Resource - The resource ID where this data is stored.
Type - The kind-id for this data.
Stored Data - The contents of the stored data value, as described in
the stored_data PDU of Section 7.2.1.1
[[TODO: Should we include the identity?.]]
Once the signature has been computed, the signature is represented
using a signature element, as described in Section 4.2.3.
7.2.2. FETCH
The FETCH request retrieves one or more data elements stored at a
given resource-id.
7.2.2.1. Request Definition
A FETCH_Q message consists of a single fetch_request element followed
by a series of fetch_kind_data elements.
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STRUCTURE: fetch_q
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resource Len | |
+-+-+-+-+-+-+-+-+ +
| Resource |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fetch Data Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Fetch Data |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of the request are as follows:
Resource - The resource ID to fetch from.
Fetch Data - A sequence of data specifiers, one for each desired
kind-id.
Each fetch_kind_data element is specified as follows.
STRUCTURE: fetch_kind_data
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Model | |
008 +-+-+-+-+-+-+-+-+ +
| Generation |
012 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ +
| Reference |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Kind - The kind-id of the data being fetched. Implementations
SHOULD reject requests corresponding to unknown kinds unless
specifically configured otherwise.
Data Model - The data model of the data.
Generation - The last generation counter that the requesting peer
saw. This is used to avoid unnecessary fetches.
Reference - A reference to the data value being requested within the
data model specified for the kind, as specified below.
STRUCTURE: fetch_array_reference
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last |
008 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: fetch_dictionary_reference
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dictionary Keys Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Dictionary Keys |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: dictionary_key
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Value Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Key Value |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As with STORE, the FETCH_Q contains a list of kind-ids and associated
references. The reference encoding depends on the kind of value
being stored.
o If the data is of data model single value, the reference is empty.
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o If the data is of data model array, the reference contains 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 -1 is used to indicate the final element. The
beginning of the range MUST be earlier in the array then the end.
o If the data is of data model dictionary then the reference
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.]]
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 cache hit
indicator rather than the stored data.
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.
7.2.2.2. Response Definition
The response to a successful FETCH request is a FETCH_A message
containing the data requested by the requester.
STRUCTURE: fetch_a
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
008 + +
| Generation |
012 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stored Data Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Stored Data |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There MUST be one fetch_kind_data element for each kind-id in the
request. If the generation-counter in the request matches the
generation-counter in the stored data, then the count of stored data
elements MUST be zero. Otherwise, all relevant data values MUST be
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returned. A nonexistent value is represented as a value with an
empty data value portion and no signature. In particular, if a
dictionary key that does not exist is requested, then there must be a
dictionary entry with that key but an empty value.
7.2.3. REMOVE
The REMOVE request is used to remove a stored element or elements
from the storing peer. Although each kind defines its own access
control requirements, in general only the original signer of the data
should be allowed to remove it. Any successful remove of an existing
element for a given kind MUST increment the generation counter by at
least one.
A remove-request 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 remove-request. Each fetch_kind_data 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.
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.
7.2.3.1. Single Value
A REMOVE of a single value element simple causes it not to exist. If
no such element exists, then this simply is a silent success.
7.2.3.2. Array
A REMOVE of an array element (or element range) replaces those
elements with empty 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"].
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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"].
7.2.3.3. Dictionary
A REMOVE of a dictionary element (or elements) replaces those
elements with empty elements. If no such elements exist, then this
is a silent success.
7.2.3.4. 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.
7.2.4. FIND
The FIND request is used to explore the DHT. 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 DHT by interactively fetching
R_n+1=nearest(1 + R_n).
7.2.4.1. Request Definition
The FIND_Q message contains a series of resource-IDs and kind-ids
identifying the resource the peer is interested in.
STRUCTURE: find_q
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resource Len | |
+-+-+-+-+-+-+-+-+ +
| Resource |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ids Len | |
+-+-+-+-+-+-+-+-+ +
| Ids |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The request contains a list of kind-ids which the FIND is for, as
indicated below.
Resource - The desired resource-id
Ids - The desired kind-ids. Each value MUST only appear once.
7.2.4.2. Response Definition
A response to a successful FIND request is a FIND_A message
containing the closest resource-ID for each kind specified in the
request.
STRUCTURE: find_kind_data
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind |
004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Closest Len | |
+-+-+-+-+-+-+-+-+ +
| Closest |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If the processing peer is not responsible for the specified
resource-id, it SHOULD return a 404 error.
When each kind is defined, it can indicate if the kind is not allowed
to be used in a FIND request. This would be done to help achieve
some types of security properties for the data stored in that kind.
For each kind-id in the request the response MUST contain a
find_response_value indicating the closest resource-id for that
kind-id unless the kind is not allowed to be used with FIND in which
case a find_kind_data 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 find_kind_data 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. DHT Maintenance
This section describes methods that are expected to be useful for all
DHTs. These methods have generic semantics (join, leave, update) and
some common fields, but where appropriate allow room for DHT-specific
data.
7.3.1. JOIN
A new peer (but which already has credentials) uses the JOIN_Q
message to join the DHT. The JOIN_Q is sent to the peer which
previously was responsible for the resource-id corresponding to the
peer-id which the new peer has. This notifies the responsible peer
that the new peer is taking over some of the overlay and it needs to
synchronize its state.
STRUCTURE: join_q
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
004 + +
| Desired Peer Id |
008 + +
| |
012 + +
| |
016 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Dht Specific Data |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The default JOIN_Q contains only the peer-id which the sending peer
wishes to assume. DHTs MAY specific other data to appear in this
request.
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The responding peer responds with success or failure. However, if it
is success it 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, DHTs MAY define data to
appear in the response payload.
7.3.2. LEAVE
The LEAVE_Q message is used to indicate that a peer is exiting the
overlay. The peer SHOULD send this message to each peer with which
it is directly connected prior to exiting the overlay.
STRUCTURE: leave_q
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
004 + +
| Leaving Peer Id |
008 + +
| |
012 + +
| |
016 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Dht Specific Data |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The default LEAVE_Q contains only the peer-id of the leaving peer.
DHTs MAY specific other data to appear in this request.
Upon receiving a LEAVE request, a peer MUST update its own routing
and routing table, and send the appropriate STORE/UPDATE sequences to
re-stabilize the overlay.
7.3.3. UPDATE
Update is the primary DHT-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 and it is up to the recipient to take
whatever actions are appropriate to deal with the state change.
The contents of the UPDATE_Q message are completely DHT-specific.
The UPDATE_A response is expected to be either success or an error.
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7.3.4. 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 ROUTE-QUERY for destination X requests the peer-id where the
receiving peer would next route to get to X. A ROUTE-QUERY can also
request that the receiving peer initiate an UPDATE request to
transfer his routing table.
One important use of the ROUTE-QUERY request is to support iterative
routing. The way that his works is that the sender selects one of
the peers in its neighbor table and sends it a ROUTE-QUERY message
with the destination_object set to the peer-id/resource-id it wishes
to route to. The neighbor responds with the next peer-id to send to.
The sending peer then CONNECTs to that peer and repeats the ROUTE-
QUERY. Eventually, the sender gets a response from a peer containing
a peer-id that is the same as that peer. At that point, the sender
can send whatever request is needed directly to that peer.
Note that this procedure only works well if all the peers are
mutually directly reachable--either by all having public IP addresses
or at least by all being behind the same NAT. Accordingly, peers
MUST only use this method if permitted by the overlay configuration
(see Section 10.2).
7.3.4.1. Request Definition
A ROUTE_QUERY_Q 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.
STRUCTURE: route_query_q
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send Update | |
+-+-+-+-+-+-+-+-+ +
| Destination Object |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of the ROUTE_QUERY_Q message are as follows:
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send_update: A single byte. This may be set to 1 to indicate that
the requester wishes the responder to initiate an UPDATE request
immediately. Otherwise, this value MUST be set to zero.
destination_object: The destination which the requester is
interested in. This may be any valid destination object,
including a peer-id, compressed ids, or resource-id
7.3.4.2. Response Definition
A response to a successful ROUTE_QUERY request is a ROUTE_QUERY_A
message containing the address of the peer to which the responding
peer would have routed the request message in recursive routing.
STRUCTURE: route_query_a
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
004 + +
| Next Peer |
008 + +
| |
012 + +
| |
016 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Next Addr |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of the ROUTE_QUERY_A 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.
next_addr: The address of the next peer.
If the requester set the send_update flag, the responder SHOULD
initiate an UPDATE immediately after.
8. ICE and Connection Formation
At numerous times during the operation of RELOAD, a node will need to
establish a connection to another node. This may be for the purposes
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of building finger tables when the node joins the P2P network, or
when the node learns of a new neighbor through an UPDATE and needs to
establish a connection to that neighbor.
In addition, a node may need to connect to another node for the
purposes of an application connection. In the case of SIP, when a
node has looked up the target AOR in the DHT, it will obtain a
Node-ID that identifies that peer. The next step will be to
establish a "direct" connection for the purposes of performing SIP
signaling.
In both of these cases, the node starts with a destination Node-ID,
and its objective is to create a connection (ideally using TCP, but
falling back to UDP when it is not available) to the node with that
given Node-ID. The establishment of this connection is done using
the CONNECT request in conjunction with ICE. It is assumed that the
reader has familiarity with ICE.
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.
8.1. Overview
To utilize ICE, the CONNECT method provides a basic offer/answer
operation that exchanges a set of candidates for a single "stream".
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.
The CONNECT request contains the candidates for this stream, and the
CONNECT response contains the corresponding answer with candidates
for that stream. Though CONNECT provides an offer/answer exchange,
it does not actually carry or utilize Session Description Protocol
(SDP) messages. Rather, it carries the raw ICE parameters required
for ICE operation, and the ICE spec is utilized as if these
parameters had actually been used in an SDP offer or answer. In
essence, ICE is utilized by mapping the CONNECT parameters into an
SDP for the purposes of following the details of ICE itself. That
avoids the need for RELOAD to respecify ICE, yet allows it to operate
without the baggage that SDP would bring.
In addition, 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.
RELOAD and SIP always run over TLS for TCP connections and DTLS
[RFC4347] for UDP "connections". Consequently, once ICE processing
has completed, both agents will begin TLS and DTLS procedures to
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establish a secure link. Its important to note that, had a TURN
server been utilized for the TCP or UDP stream, the TURN server will
transparently relay the TLS messaging and the encrypted TLS content,
and thus will not have access to the contents of the connection once
it is established. Any attack by the TURN server to insert itself as
a man-in-the-middle are thwarted by the usage of the fingerprint
mechanism of RFC 4572 [RFC4572], which will reveal that the TLS and
DTLS certificates do not match the ones used to sign the RELOAD
messages.
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.
8.2. 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 you know about will be willing to be a STUN
server for you -- 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.
OPEN ISSUE: should the peer try to keep at least one peer in each
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group, even if it has no other reason for the connection? Need to
specify when to stop adding new groups if the peer is behind a really
bad NAT.
OPEN ISSUE: RELOAD-01 had a Peer-Info structure that allowed peers
to exchange information such as a "default" IP-port pair in UPDATEs.
This structure could be expanded to include the candidate list for a
peer, thus allowing ICE negotiation to begin or even direct
communication before a CONNECT request has been received. (The
candidate pairs for the P2P port are fixed because the same source
port is used for all connections.) However, because this would
require significant changes to the ICE algorithm, we have not
introduced such an extension at this point.
8.3. 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 ring, as described in
Section 8.2 and Section 11.3.
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.
8.4. Encoding the CONNECT Message
Section 4.3 of ICE describes procedures for encoding the SDP.
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 7.1.2 describes the detailed attribute
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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 Next-Protocol 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 and RELOAD itself.
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
considered a valid answer for the purposes of following the ICE
specification.
Since all messages with RELOAD are secured between nodes, the node
MUST implement the fingerprint attribute of RFC 4572 [RFC4572], and
encode it into the CONNECT request and response as described in
Section 7.1.2. This fingerprint will be matched with the
certificates utilized to authenticate the RELOAD CONNECT request and
its response.
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.
8.5. 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.
8.6. 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.
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8.7. 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.
8.8. 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
8.3 of ICE does not apply.
8.9. 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.
8.10. Media Keepalives
STUN MUST be utilized for the keepalives described in Section 10 of
ICE.
8.11. 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 fingerprint from the CONNECT
request and its response are used as described in RFC 4572 [RFC4572],
to ensure that another node in the P2P network, acting as a TURN
server, has not inserted itself as a man-in-the-middle. 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.
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8.12. 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.
9. DHT Algorithms
9.1. Generic Algorithm Requirements
When specifying a new DHT, 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.
o The hash algorithm used to go from a Unhashed-ID, such as a user
name, to a Resource-ID. This also includes the length of the
Resource-IDs and Peer-IDs
o The procedures that peers use to route messages.
o The replication strategy used to ensure data redundancy.
9.2. Chord Algorithm
This algorithm is assigned the name chord-128-2-16+ to indicate it is
based on Chord, uses a 128 bit hash function, stores 2 redundant
copies of all data, and has finger tables with at least 16 entries.
9.2.1. 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 peer-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
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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 rapidly be found - it
needs to be there so that peers can be found in O(log(N)) time
instead of the typical O(N) time that a linked list would provide.
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.
9.2.2. Routing
If a peer is not responsible for a Resource-ID k, then it routes a
request to that location by routing it to the peer in either the
neighborhood or finger table that has the largest peer-id that is in
the interval between the peer and k.
9.2.3. 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.
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?]]
9.2.4. Joining
The join process for a joining party (JP) with peer-id n is as
follows.
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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
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 peer-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^(numBitsInPeerId-i). This will be
routed to a peer in approximately the right location around the ring.
9.2.5. 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 peer-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
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9.2.6. UPDATEs
An UPDATE is defined as
STRUCTURE: chord_update
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Predecessors Le| |
+-+-+-+-+-+-+-+-+ +
| Predecessors |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Successors Len| |
+-+-+-+-+-+-+-+-+ +
| Successors |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of this message are:
Predecessors - The predecessor set of the UPDATEing peer.
Successors - The successor set of the UPDATEing peer.
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.
9.2.6.1. 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 peer-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
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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.
9.2.6.2. Receiving UPDATEs
When a peer, N, receives an UPDATE request, it examines the peer-ids
in the UPDATE_Q and at its neighborhood table and decides if this
UPDATE_Q 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:
o The UPDATE_Q contains peers that would not change the neighbor set
because they match the neighborhood table.
o The UPDATE_Q contains peers closer to N than those in its
neighborhood table.
o The UPDATE_Q 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.
A 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
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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.
9.2.6.3. Stabilization
There are four components to stabilization:
1. exchange UPDATES will all peers in its routing table to exchange
state
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 peer (n+2^(numBitsInPeerId-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
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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
peer P MUST periodically repeat the discovery process used in the
initial join for the overay 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 peer-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.)
9.2.7. Leaving
Peers SHOULD send a LEAVE request prior to exiting the DHT. 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.
10. Enrollment and Bootstrap
10.1. 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 the DNS SRV lookup is done
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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 can 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.
10.2. 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 name="chord.example.com" expiration="86400">
<dht name="chord-128-2-8"/>
<root-cert>[DER certificate here]</root-cert>
<required-usage name="SIP"/>
<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>
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:
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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:
dht - This element has an attribute called name that describes which
DHT algorithm is being used.
root-cert - This element contains a DER 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-usage - This element has an attribute called "name" that
describes a usage that peers in this overlay are required to
support. More than one required-usage element may be present.
credential-server - 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.
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.
iterative-permitted - This element indicates that iterative routing
(see Section 7.3.4) MAY be used. If iterative routing is
permitted, then this value MUST be set to "TRUE". Otherwise, it
SHOULD be absent, but MAY be set to "FALSE".
[[TODO: Do a RelaxNG grammar.]]
10.3. Credentials
If the configuration document contains a credential-server element,
credentials are required to use the DHT. 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
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user name(s).
The credential server MUST authenticate the request using HTTP digest
[RFC2617]. If the authentication succeeds and the requested user
name(s) is acceptable, the server and returns a certificate. The
SubjectAltName field in the certificate contains the following
values:
o One or more Peer-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.
o The names this user is allowed to use in the overlay
The certificate is returned in a "Simple Enrollment Response".
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
Peer-IDs it can use.
10.3.1. Credentials for HIP
When RELOAD is used with HIP, the certificates MUST be generated so
that:
o Each node is assigned a unique ORCHID.
o The peer-id can be uniquely determined from the ORCHID.
Because in general, ORCHIDs are shorter than peer-ids, this means
that the ORCHIDS MUST be generated first and MUST be
cryptographically random in order to make the peer-ids
cryptographically random. The mapping function used to produce the
peer-id from the ORCHID MUST be the same as that used by the DHT to
produce resource-ids from Unhashed-IDs.
In addition to the usual attributes, when HIP is in use certificates
MUST contain a subjectAltName with an iPAddress value containing the
HIP ORCHID. This allows these certificates to be used by the HIP
peers during the HIP base exchange.
10.4. Locating a 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 peer-id set to that peer's
peer-id.
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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 peer-id as the destination
peer-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 peer-id in the response and use this
peer-id to start sending requests to join the DHT as described in
Section 3.1.5 and Section 7.3.
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.
11. Usages
11.1. Generic Usage Requirements
A new usage MUST specify the following information:
o The kind-ids which the usage defines and what each kind means.
o The data model for the data being stored (single value, array,
dictionary, etc.) for each kind
o Access control rules for each kind, indicating what credentials
are allowed to read and write that kind-id at a given location.
o The minimum amounts of data of each kind that a conformant
implementation MUST store.
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, kind SHOULD use the build-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.
Note: New usages MAY reuse existing kind-ids. New kind-ids only
need to be defined where different data is stored or different
behavior is required.
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11.2. SIP Usage
The SIP usage allows a RELOAD overlay to be used as a distributed SIP
registrar/proxy network. 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 Bob's SIP UA can use to pass SIP
messages back and forth to Alice's SIP UA. Provides the following
three functions:
o Mapping SIP URIs that are not GRUUs to the overlay peer
responsible for the SIP UA.
o Mapping SIP GRUUs to the DHT peer responsible for the SIP UA.
o Forming a connection directly to a DHT peer that is used to send
SIP messages to the SIP UA.
Section 3.7.1 provides an overview of how these fit together.
11.2.1. SIP-REGISTRATION kind
The first mapping is provided using the SIP-REGISTRATION kind-id:
Kind IDs The Unhashed-ID for the SIP-REGISTRATION kind-id is a URI,
typically the AOR for the user. The data stored is a sip-
registration-data, which can contain either another URI or a
destination list to the peer which is acting for the user.
[[TODO: we want to somehow put caller-prefs in here along with
the route list, but I'm not sure how to do it yet.]]
Data Model The data model for the SIP-REGISTRATION kind-id is
dictionary. The dictionary key is the peer-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 Unhashed-ID for the resource-id and (2) contains a
peer-id matching the storing dictionary key.
Data Sizes Peers MUST be prepared to store SIP-REGISTRATION values
of up to 10 kilobytes and must be prepared to store up to 10
values for each user name.
The contents of the SIP-REGISTRATION kind are
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STRUCTURE: sip_registration
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
| Registration Data |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: sip_registration_uri_type
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | Sip Registration Uri Type Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Sip Registration Uri Type |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: sip_registration_route_type
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | Contact Prefs Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Contact Prefs |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination List Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Destination List |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A registration may contain either a URI (type code 0x01) or a contact
preferences structure and a destination list (type code 0x01). The
leading byte indicates the type.
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11.2.2. GRUUs
GRUUs do not require storing data in the DHT. 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.
Anonymous GRUUs are done in roughly the same way but require either
that the enrollment server issue a different peer-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
peer-id from being revealed.
11.2.3. SIP Connect
Once the destination list for a user has been identified, the calling
peer uses the CONNECT request to form a connection to the peer
identified by the destination list. The CONNECT request MUST contain
the connect-application value of 5160 (SIP). If certificate-based
authentication is in use, the responding peer MUST present a
certificate with a peer-id matching the terminal entry in the route
list.
[[TODO: Note that this constrains destination lists from hiding the
last peer-id when used here. I think that's OK, but we should take a
look]]
Once the association has been formed, the calling peer sends generic
SIP messages down the new association and ordinary SIP procedures are
followed.
11.2.4. SIP Tunnel
This usage allows two peers to exchange SIP messages across the
overlay using the TUNNEL method. TUNNEL is provided as an
alternative to using CONNECT because it allows a SIP message to be
sent immediately, without the delay associated with CONNECT. For a
simple SIP exchange, it may result in fewer messages being sent.
An implementation SHOULD use CONNECT for a dialog that is expected to
endure for sufficient time and exchange significant numbers of
messages. An implementation MAY establish an initial dialog using
TUNNELing and then migrate it to a direct dialog opened with CONNECT
once that negotiation is complete.
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As an application of TUNNEL, this usage defines the following items:
o For SIP, the application attribute is 5060.
o The application MAY establish any dialog using TUNNEL if it
expects to replace it once a CONNECT request completes. The
application SHOULD NOT exchange messages with another SIP UA
repeatedly using a TUNNEL unless it is unable to complete a
CONNECT.
o The Replaces header should be used to migrate dialogs established
via TUNNEL to a direct connection.
o The dialogid is the GRUU of the destination of the request.
o By using the GRUU of the destination as the dialogid, the
receiving peer is able to deliver the message to the appropriate
process without parsing the SIP message.
In constructing the message, the SIP UA forms the message as if it
were being routed directly to the GRUU of the destination. The SIP
stack hands the message to RELOAD for delivery. Although the message
is passed through a sequence of untrusted peers, it is not subject to
modification by those peers because of the message's signature.
OPEN ISSUE: should specify how to request encryption of the message
end-to-end.
Note: The easiest implementation of TUNNEL is likely to default to
sending all messages across a TUNNEL when the first message is
sent to a new destination GRUU and simultaneously issuing a
CONNECT. Messages then continue through the TUNNEL until the
CONNECT completes, at which point they are delivered via the new
connection.
OPEN ISSUE: If the tunneling vs direct decision can be made
equivalently to a link-layer decision, it may not be necessary to
modify the dialog or inform the SIP UA in any way that it has now
obtained a direct route.
11.3. TURN Usage
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
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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 Unhashed-ID by
concatenating its peer-ID and the value d. This Unhashed-ID is
hashed to form a Resource-ID. The address of the peer is stored at
that Resource-ID using type TURN-SERVICE and the turn-server
production:
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 the STUN-Relay server type 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].
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STRUCTURE: turn_server
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Iteration | |
+-+-+-+-+-+-+-+-+ +
| Address |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: ip4_address_type
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | Addr |
004 +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Port |
007 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
STRUCTURE: ip6_address_type
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | |
004 +-+-+-+-+-+-+-+-+ +
| Addr |
008 + +
| |
012 + +
| |
016 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Port |
019 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[[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.
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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.
Data Sizes TURN-SERVICE values are of fixed size. Peers MUST be
prepared to store values with iteration counter of up to 100.
The data is stored in a data structure with the IP address of the
server and an indication whether the address is an IPv4 or IPv6
address. The Unhashed-ID used to form the storage Resource-ID is
simply the peer-id. The access control rule is that the certificate
used to sign the request must contain a peer-id that when hashed
would match the Resource-ID where the data is being stored.
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.
Open issues: Should there be low and high bandwidth version of STUN-
Relay one can find? Low would be usable for signaling type things
and high would be usable for audio, video, and others.
11.4. 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 Unhashed-IDs:
o The user names in the certificate.
o The peer-ids in the certificate.
Note that in the second case the certificate is not stored at the
peer's peer-id but rather at a hash of the peer's peer-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 DHT
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when joining and redo the check about every 24 hours after that.
Certificate data should be stored with an expiry time of 60 days.
When a client is checking the existence of data, if the expiry is
less than 30 days, it should be refreshed to have an expiry of 60
days. The certificate information is frequently used for many
operations, and peers should cache it for 8 hours.
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 peer-id or user name
which, when hashed, maps the resource-id at which the value is
being stored.
Data Sizes Peers MUST be prepared to store at least 10 certificates
of sizes up to 1K each.
11.5. HIP Tunnel
This usage allows two peers to exchange HIP messages across the
overlay using the TUNNEL method. This is meant to be used as part of
the HIP BONE architecture described in [I-D.camarillo-hip-bone].
As an application of TUNNEL, this usage defines the following items:
o For HIP, the application attribute is TBD (IANA port number).
o The dialogid is empty and set to zero length.
In order to route HIP messages correctly, there needs to be an
unambiguous mapping between the ORCHID assigned to each HIP node and
the peer-id assigned to that node. The ORCHID MUST be used as the
Unhashed-ID to generate the peer-id. [TODO: We need a general
scheme for mapping Unhashed-IDs to IDs so they don't collide. This
isn't the place to define it.] Messages to a given ORCHID are then
routed to the mapped peer-id. Section 10.3 describes more
considerations for the generation of ORCHIDs for use with RELOAD.
TODO - should discuss interaction of HIP and P2P retransmission
timers
11.6. Diagnostic Usage
[[TODO: reduce text of motivation description in the next version]]
The development and deployment of a peer-to-peer system is a
continuous process. The developers write code which is tested on a
scale that may be smaller than the actual deployment size. After
this local testing, the code is deployed in a real environment. Bugs
arise during development and deployment phases. The designers of the
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peer-to-peer system need mechanisms which can help identify problems
and bugs in a peer-to-peer system during development and deployment
phases. Peer-to-peer systems are an example of a distributed system
and it is not a trivial task to provide protocol mechanisms, tools
and techniques to identify problems that may arise in such systems.
The diagnostic mechanisms can broadly be classified into online and
offline mechanisms. The online mechanisms attempt to identify faults
in a running system where as offline mechanisms try to infer faults
by gathering the log files of machines participating in a distributed
system.
In a peer-to-peer system, a peer maintains routing state to forward
messages according to the overlay protocol being used. In addition,
a peer stores information published by other peers. The routing and
storage of resources consume network, space (memory), and CPU
resources. A peer also needs to keep track of how long the P2PSIP
application has been running and the last time peers in the routing
table were last contacted. During development and deployment phase,
an overlay designer needs mechanisms to query some or all of the
above mentioned information.
The overlay designer may also treat overlay as a black box and
determine if the routing mechanisms are working correctly under
various levels of churn.
Thus, there are at least two types of online diagnostic mechanisms:
1) state acquisition 2) black-box diagnostics
11.6.1. State Acquisition Mechanisms
The protocol provides a DIAGNOSTIC method [TODO] which queries the
peer for its routing state, average bandwidth, CPU utilization, and
storage state. The DIAGNOSTIC request should typically be sent over
a reliable transport protocol as the response will likely exceed UDP
MTU size. The state acquisition mechanism can be used to construct a
local view of the connectivity state of the system. It can also be
used to construct a geographical map of the system.
Below, we identify potential issues with the state acquisition
mechanisms.
Security: If any peer can query the routing or storage state of any
other peer, then clearly privacy and security concerns arise. To
address this, the state acquisition mechanisms need an access list
like mechanism so that only the overlay implementer can query the
state of all the nodes. Alternatively, the state acquisition
mechanisms are only enabled during the development phase or are only
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enabled for 'admin' users.
Scalability: It is possible to query the state of few hundred or a
few thousand nodes (as it is currently done in our live system on
Planet lab); however, a serial state acquisition of a million node is
a non starter. In large scale networks, one option is to query the
state of few hundred nodes and to construct an high level
connectivity map. CAIDA [ref] collects data at a few vantage points
to construct BGP maps.
Instantaneous vs. long term state: Another issue with these state
acquisitions mechanisms is whether they acquire the instantaneous
state snapshot or an exponential moving average or a list of
snapshots over a period of time. For diagnostic metrics such as CPU
utilization, an exponential moving average metric is also helpful in
addition to the instantaneous snapshot.
Pull vs. push: The state acquisition mechanisms can either be pull-
based or push-based or a combination of both. In pull-based
mechanisms, peer explicitly request state of another peer. This may
not be sufficient because pull-based mechanisms require a to
periodically poll a peer for any change state. In a push-based
mechanism, peers advertise any change in certain metrics to their
routing or neighbor peers. As an example of push-based mechanism, a
peer which starts to relay a call may indicate a change in its
bandwidth to its routing or neighbor peers in a PING message.
Development vs. deployment: A hard problem is to decide which
diagnostics are absolutely necessary during deployment and which are
needed during development.
Clearly, complete state acquisition has security concerns in a
deployed system. The other option an overlay implementer can use is
to run a few peers and have complete control over the functionality
of these peers. These peers are same as other peers with the
difference that an overlay implementer can explicitly query the state
of these peers. It can then use this information to 'crawl' the
overlay network and construct a local map of the network.
11.6.2. Black-box diagnostics
[[TODO: a better name for this section]]
Black-box diagnostics: DHTs are examples of structured peer-to-peer
networks and they allow nodes to store key/value pairs in the
overlay. A simple diagnostic mechanism is to treat the overlay as a
black-box: publish several key/value pairs at one peer and then look
them up from another peer. For this kind of diagnostic mechanism,
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clients are more suitable as they do not provide any routing or
storage services to the overlay and can connect to an arbitrary peer.
The Diagnostic Usage allow a peer 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, and performance statistics of a peer. The usage defines
several new kinds which can be retrieved to get the statistics. The
peer-id is directly used when retrieving data so no Unhashed-ID is
defined. The access control model for all of these is local policy
defined by the peer. The peer MAY have a list of users (such as
"admin") that it is willing to return the information for and
restrict access to users with that name. The access control can 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.
The following kinds are defined:
SOFTWARE_VERSION A single value element containing a US-ASCII string
that identifies the manufacture, model, and version of the
software.
UPTIME A single value element containing an unsigned 64-bit integer
specifying the time the nodes has been up in seconds.
AS_NUMBER A single value element containing the Autonomous System
[TODO REF] number as an unsigned 32-bit integer. Zero is returned
if the AS number is unknown.
(OPEN ISSUES: How to determine a AS number? This metric is
primarily used for advertising and locating STUN/TURN servers. A
TURN server is inserted and looked up under H(AS). What if there
are no TURN servers in the same AS? )
CPU_UTILIZATION A single value element containing an unsigned 8-bit
integer representing the percentage CPU load from 1 to 100.
(OPEN ISSUE: It is not a very precise metric.)
DATA_STORED A single value element containing an unsigned 64-bit
integer representing the number of bytes of data being stored by
this node.
MESSAGES_SENT 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.
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.
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ROUTING_TABLE_SIZE A single value element containing an unsigned 32-
bit integer representing the number of peers in the node's routing
table.
NEIGHBOR_TABLE_SIZE A single value element containing an unsigned
32-bit integer representing the number of peers in the node's
neighbor table.
EWMA_BYTES_SENT A single value element containing an unsigned 32-bit
integer representing an exponential weighted average of bytes sent
by this peer.
EWMA_BYTES_RCVD A single value element containing an unsigned 32-bit
integer representing an exponential weighted average of bytes
received by this peer.
LAST_CONTACT A single value element containing an unsigned 32-bit
integer specifying the time in number of seconds the node was last
contacted.
MEMORY_FOOTPRINT A single value element containing an unsigned 32-
bit integer representing the memory footprint of the peer program
in kilo bytes.
RTT A single value element containing an unsigned 32-bit integer
specifying the recent RTT estimate in ms between two peers.
[[TODO: We would like some sort of bandwidth measurement, but we're
kind of unclear on the units and representation.]]
11.6.3. Diagnostic Metrics for a P2PSIP Deployment
Clearly, all diagnostic metrics are useful during development and
testing. The hard question is which metrics are absolutely necessary
for a deployed P2PSIP system. We attempt to identify these metrics
and classify them under 'resource' and 'peer' metrics.
For 'resource' metric, we identify CPU_UTILIZATION, EWMA_BYTES_SENT,
EWMA_BYTES_RCVD, and MEMORY_FOOTPRINT as the key metrics and for
'peer' metric we identify UPTIME, LAST_CONTACT, and RTT as the
metrics that are crucial for a deployed P2PSIP system.
(OPEN QUESTION: any other metrics?)
(OPEN: 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. )
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12. Security Considerations
12.1. Overview
RELOAD provides a generic storage service, albeit one designed to be
useful for P2P SIP. In this section we discuss security issues that
are likely to be relevant to any usage of RELOAD. In Section 12.7 we
describe issues that are specific to SIP.
In any DHT, 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 DHT. 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 DHT 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.
12.2. Attacks on P2P Overlays
The two basic functions provided by DHT 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 DHT 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 DTLS
or TLS, 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
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the sender's private key, providing end-to-end security for
communications.
12.3. Certificate-based Security
This specification stores users' registrations and possibly other
data in a Distributed Hash table (DHT). This requires a solution to
securing this data as well as securing, as well as possible, the
routing in the DHT. 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 DHT, 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 peer-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 DHT
corresponding to the user name.
o As a DHT peer with the peer ID(s) listed in the certificate.
Note that since only users of this DHT need to validate a
certificate, this usage does not require a global PKI. It does,
however, require a central enrollment authority which acts as the
certificate authority for the DHT. 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.
In order to protect data storage, in the certificate-based security
scheme, all stored data is signed by the owner of the data. This
allows the storing peer to 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.
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12.4. Shared-Secret Security
For small environments where deployment of the PKI necessary to use a
certificate-based model is impractical, RELOAD supports a shared
secret security 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.
The shared-secret scheme prohibits unauthorized peers from joining
the overlay, but it provides no protection from a compromised peer
inserting arbitrary resource registrations, performing a Sybil
attack[Sybil], or performing other attacks on the resources or
routing. Thus, it is only safe to use in limited settings in which
peers are not adversarial. In addition, because the messages and
data are not authenticated, each intermediate peer MUST take care to
use TLS and check the other peer's knowledge of the shared secret, or
message insertion is possible.
If the shared secret key for the shared-key security scheme is
discovered by an attacker, then most of the security of the scheme is
lost: an attacker can impersonate any peer to any other peer. Thus,
the shared-secret scheme is only appropriate for small deployments,
such as a small office or ad hoc overlay set up among participants in
a meeting.
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.
12.5. 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 DHTs - relatively simple.
When shared-secret security is used, then all peers trust all other
peers, provided that they have demonstrated that they have the
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credentials to join the overlay at all. The following text therefore
applies only to certificate-based security.
12.5.1. 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 Unhashed-ID 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 Unhashed-ID construction algorithm for the
specified kind based on the user's certificate information. It then
computes the Resource-ID from the Unhashed-ID 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
Peer-Id: 013456789abcdef
Serial: 1234
If Alice wishes to STORE a value of the "SIP Location" kind, the
Unhashed-ID will be the SIP AOR "sip:alice@dht.example.com". The
Resource-ID will be determined by hashing the Unhashed-ID. When a
peer receives a request to store a record at Resource-ID X, it takes
the signing certificate and recomputes the Unhashed-ID, 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 Unhashed-ID
construction algorithm may be different for other kinds.
12.5.2. Distributed Quota
Being a peer in a DHT carries with it the responsibility to store
data for a given region of the DHT. 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
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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.
12.5.3. 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.
12.5.4. 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.
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
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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.
12.6. 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 DHT. 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 DHT. Second, this is a DOS attack only. Third,
if a large percentage of the peers on the DHT are controlled by the
attacker, it is probably impossible to perfectly secure against this.
12.6.1. 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.
12.6.2. Admissions Control
Admission to an RELOAD DHT is controlled by requiring that each peer
have a certificate containing its peer ID. The requirement to have a
certificate is enforced by using TLS 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
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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 DHT he is
impersonating appears to be very sparsely populated by comparison to
the victim's local region.
12.6.3. Peer Identification and Authentication
In general, whenever a peer engages in DHT 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 TLS 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 DHT structure, but
they need to verify that the UPDATE message came from the actual peer
rather than from an attacker. To prevent this, all DHT routing
messages are signed by the peer that generated them.
[TODO: 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
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 DHT messages.]
12.6.4. 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
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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.
12.6.5. 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 DHT 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.
12.7. SIP-Specific Issues
12.7.1. Fork Explosion
Because SIP includes a forking capability (the ability to retarget to
multiple recipients), fork bombs are a potential DoS concern.
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.
12.7.2. 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.
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12.7.3. Privacy Issues
All RELOAD SIP registration data is public. Methods of providing
location and identity privacy are still being studied.
13. IANA Considerations
This section contains the new code points registered by this
document. The IANA policies are TBD.
13.1. Overlay Algorithm Types
IANA SHALL create/(has created) a "RELOAD Overlay Algorithm Type"
Registry. Entries in this registry are strings denoting the names of
DHT algorithms. The registration policy for this registry is TBD.
The initial contents of this registry are:
chord-128-2-8 - The algorithm defined in Section 9.2 of this
document.
13.2. 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 11.1. The registration policy for this registry
is TBD.
The initial contents of this registry are:
+---------------------+---------+
| Kind | Kind-Id |
+---------------------+---------+
| SIP-REGISTRATION | TBD |
| TURN_SERVICE | TBD |
| CERTIFICATE | TBD |
| SOFTWARE_VERSION | TBD |
| UPTIME | TBD |
| AS_NUMBER | TBD |
| CPU_UTILIZATION | TBD |
| DATA_STORED | TBD |
| MESSAGES_SENT | TBD |
| INSTANCES_STORED | TBD |
| ROUTING_TABLE_SIZE | TBD |
| NEIGHBOR_TABLE_SIZE | TBD |
+---------------------+---------+
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13.3. 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 3.1.4. The registration policy for this
registry is TBD.
+--------------+------------+
| Data Model | Identifier |
+--------------+------------+
| SINGLE_VALUE | TBD |
| ARRAY | TBD |
| DICTIONARY | TBD |
+--------------+------------+
13.4. 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 4.2.1 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_Q | TBD |
| PING_A | TBD |
| CONNECT_Q | TBD |
| CONNECT_A | TBD |
| TUNNEL_Q | TBD |
| TUNNEL_A | TBD |
| STORE_Q | TBD |
| STORE_A | TBD |
| FETCH_Q | TBD |
| FETCH_A | TBD |
| REMOVE_Q | TBD |
| REMOVE_A | TBD |
| FIND_Q | TBD |
| FIND_A | TBD |
| JOIN_Q | TBD |
| JOIN_A | TBD |
| LEAVE_Q | TBD |
| LEAVE_A | TBD |
| UPDATE_Q | TBD |
| UPDATE_A | TBD |
| ROUTE_QUERY_Q | TBD |
| ROUTE_QUERY_A | TBD |
| RESERVED | 0x800..0xffe |
| ERROR | 0xffff |
+-------------------+--------------+
[[TODO - add IANA registration for p2p_enroll SRV and p2p_menroll]]
14. 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.
15. Examples
See draft [TODO add ref] for message flow examples.
16. Acknowledgments
This draft is a merge of the "REsource LOcation And Discovery
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(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.
17. Appendix: Operation with SIP clients outside the DHT domain
18. Appendix: Notes on DHT Algorithm Selection
An important point: if you assume NATs are doing ICE to set up
connections, you want a lot fewer connections than you might have on
a very open network - this might push towards something like Chord
with fewer connections than, say, bamboo.
TODO - ref draft-irtf-p2prg-survey-search
19. References
19.1. 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.
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[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]
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.
19.2. 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),
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July 2007.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
[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
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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
progress), December 2007.
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; William & Mary
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
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Jonathan Rosenberg
Cisco
Edison, NJ
USA
Email: jdrosen@cisco.com
Salman A. Baset
Columbia University
1214 Amsterdam Avenue
New York, NY
USA
Email: salman@cs.columbia.edu
Henning Schulzrinne
Columbia University
1214 Amsterdam Avenue
New York, NY
USA
Email: hgs@cs.columbia.edu
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