P2PSIP X. Jiang
Internet-Draft N. Zong
Intended status: Standards Track Huawei Technologies
Expires: October 8, 2010 R. Even
Gesher Erove
Y. Zhang
China Mobile
April 6, 2010
An extension to RELOAD to support Direct Response and Relay Peer routing
draft-jiang-p2psip-relay-04
Abstract
This document proposes an optional extension to RELOAD to support
direct response and relay peer routing modes. RELOAD recommends
symmetric recursive routing for routing messages. The new optional
extensions provide a shorter route for responses reducing the
overhead on intermediary peers and describe the potential cases where
these extensions can be used.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Backgrounds . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.1. Symmetric Recursive Routing (SRR) . . . . . . . . . . 7
3.1.2. Direct Response Routing (DRR) . . . . . . . . . . . . 7
3.1.3. Relay Peer Routing (RPR) . . . . . . . . . . . . . . . 8
3.2. Scenarios Where DRR can be Used . . . . . . . . . . . . . 9
3.2.1. Managed or Closed P2P System . . . . . . . . . . . . . 9
3.2.2. Wireless Scenarios . . . . . . . . . . . . . . . . . . 9
3.3. Scenarios Where RPR Benefits . . . . . . . . . . . . . . . 10
3.3.1. Managed or Closed P2P System . . . . . . . . . . . . . 10
3.3.2. Using Bootstrap Peers as Relay Peers . . . . . . . . . 10
3.3.3. Wireless Scenarios . . . . . . . . . . . . . . . . . . 10
4. Relationship Between SRR and DRR/RPR . . . . . . . . . . . . . 10
4.1. How DRR Works . . . . . . . . . . . . . . . . . . . . . . 10
4.2. How RPR Works . . . . . . . . . . . . . . . . . . . . . . 11
4.3. How These Three Routing Modes Work Together . . . . . . . 11
5. Comparison on cost of SRR and DRR/RPR . . . . . . . . . . . . 12
5.1. Closed or managed networks . . . . . . . . . . . . . . . . 12
5.2. Open networks . . . . . . . . . . . . . . . . . . . . . . 13
6. Extensions to RELOAD . . . . . . . . . . . . . . . . . . . . . 14
6.1. Basic Requirements . . . . . . . . . . . . . . . . . . . . 14
6.2. Modification To RELOAD Message Structure . . . . . . . . . 14
6.2.1. State-keeping Flag . . . . . . . . . . . . . . . . . . 14
6.2.2. Extensive Routing Mode . . . . . . . . . . . . . . . . 15
6.3. Creating a Request . . . . . . . . . . . . . . . . . . . . 15
6.3.1. Creating a Request for DRR . . . . . . . . . . . . . . 15
6.3.2. Creating a request for RPR . . . . . . . . . . . . . . 16
6.4. Request And Response Processing . . . . . . . . . . . . . 16
6.4.1. Destination Peer: Receiving a Request And Sending
a Response . . . . . . . . . . . . . . . . . . . . . . 17
6.4.2. Sending Peer: Receiving a Response . . . . . . . . . . 17
6.4.3. Relay Peer Processing . . . . . . . . . . . . . . . . 17
7. Discovery Of Relay Peer . . . . . . . . . . . . . . . . . . . 18
8. Optional Methods to Investigate Node Connectivity . . . . . . 18
8.1. Getting Addresses To Be Used As Candidates for DRR . . . . 19
8.2. Public Reachability Test . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10.1. A new RELOAD Forwarding Option . . . . . . . . . . . . . . 21
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . . 22
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
1.1. Backgrounds
RELOAD [I-D.ietf-p2psip-base] recommends symmetric recursive routing
(SRR) for routing messages and describes the extensions that would be
required to support additional routing algorithms. Other than SRR,
two other routing options: direct response routing (DRR) and relay
peer routing (RPR) are also discussed in Appendix D in [I-D.ietf-
p2psip-base]. As we show in section 3, DRR and RPR are advantageous
over RPR in some scenarios reducing load (CPU and link BW) on
intermediary peers . For example, in a closed network where every
node is in the same address realm, DRR performs better than SRR. On
the other hand, RPR works better in a network where relay peers are
provisioned in advance so that relay peers are publicly reachable in
the P2P system. In other scenarios, using a combination of these
three routing modes together is more likely to bring benefits than if
SRR is used alone. Some discussion on connectivity is in Non-
Transitive Connectivity and DHTs
[http://srhea.net/papers/ntr-worlds05.pdf].
In this draft, we first discuss the problem statement, then the
relationship between the three routing modes is presented. In
Section 5, we give comparison on the cost of SRR, DRR and RPR in both
managed and open networks. An extension to RELOAD to support DRR and
RPR is proposed in Section 6.
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. We also use terms defined in NAT
behavior discovery [I-D.ietf-behave-nat-behavior-discovery]. Other
terms used in this document are defined inline when used and are also
defined below for reference.
There are two types of roles in the RELOAD architecture: peer and
client. Node is used when describing both peer and client. In
discussions specific to behavior of a peer or client, the term peer
or client is used instead.
Publicly Reachable: A node is publicly reachable if it can receive
unsolicited messages from any other node in the same overlay. Note:
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"publicly" does not mean that the nodes must be on the public
Internet, because the RELOAD protocol may be used in a closed system.
Relay Peer: A type of publicly reachable peer that can receive
unsolicited messages from all other nodes in the overlay and forward
the responses from destination peers towards the request sender.
Direct Response Routing (DRR): refers to a routing mode in which
responses to P2PSIP requests are returned to the sending peer
directly from the destination peer based on the sending peer's own
local transport address(es). For simplicity, the abbreviation DRR is
used instead in the following text.
Relay Peer Routing (RPR): refers to a routing mode in which responses
to P2PSIP requests are sent by the destination peer to a relay peer
transport address who will forward the responses towards the sending
peer. For simplicity, the abbreviation RPR is used instead in the
following text.
Symmetric Recursive Routing(SRR): refers to a routing mode in which
responses follow the request path in the reverse order to get back to
the sending peer. For simplicity, the abbreviation SRR is used
instead in the following text.
3. Problem Statement
RELOAD is expected to work under a great number of application
scenarios. The situations where RELOAD is to be deployed differ
greatly. For instance, some deployments are global, such as a Skype-
like system intended to provide public service. Some run in closed
networks of small scale. SRR works in any situation, but DRR and RPR
may work better in some specific scenarios.
3.1. Overview
RELOAD is a simple request-response protocol. After sending a
request, a node waits for a response from a destination node. There
are several ways for the destination node to send a response back to
the source node. In this section, we will provide detailed
information on three routing modes: SRR, DRR and RPR.
Some assumptions are made in the following illustrations.
1) Peer A sends a request destined to a peer who is the responsible
peer for Resource-ID k;
2) Peer X is the root peer being responsible for resource k;
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3) The intermediate peers for the path from A to X are peer B, C, D.
3.1.1. Symmetric Recursive Routing (SRR)
For SRR, when the request sent by peer A is received by an
intermediate peer B, C or D, each intermediate peer will insert
information on the peer from whom they got the request in the via-
list as described in RELOAD. As a result, the destination peer X
will know the exact path which the request has traversed. Peer X
will then send back the response in the reverse path by constructing
a destination list based on the via-list in the request.
A B C D X
| Request | | | |
|----------->| | | |
| | Request | | |
| |----------->| | |
| | | Request | |
| | |----------->| |
| | | | Request |
| | | |----------->|
| | | | |
| | | | Response |
| | | |<-----------|
| | | Response | |
| | |<-----------| |
| | Response | | |
| |<-----------| | |
| Response | | | |
|<-----------| | | |
| | | | |
SRR works in any situation, especially when there are NATs or
firewalls. A downside of this solution is that the message takes
several hops to return to the client, increasing the bandwidth usage
and CPU/battery load of multiple nodes.
3.1.2. Direct Response Routing (DRR)
In DRR, peer X receives the request sent by peer A through
intermediate peer B, C and D, as in SRR. However, peer X sends the
response back directly to peer A based on peer A's local transport
address. In this case, the response won't be routed through
intermediate peers. Shorter route means less overhead on
intermediary peers, especially in the case of wireless network where
the CPU and uplink BW is limited. In the absence of NATs or other
connectivity issues, this is the optimal routing technique. Note
that secure connection requires multiple round trips. Please refer
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to Section 5 for cost comparison between SRR, DRR/RPR.
A B C D X
| Request | | | |
|----------->| | | |
| | Request | | |
| |----------->| | |
| | | Request | |
| | |----------->| |
| | | | Request |
| | | |----------->|
| | | | |
| | | | Response |
|<-----------+------------+------------+------------|
| | | | |
3.1.3. Relay Peer Routing (RPR)
If peer A knows it is behind a NAT or NATs, and knows one or more
relay peers with whom they have a prior connections, peer A can try
RPR. Assume A is associated with relay peer R. When sending the
request, peer A includes information describing peer R transport
address in the request. When peer X receives the request, peer X
sends the response to peer R, which forwards it directly to Peer A on
the existing connection. Note that RPR also allows a shorter route
for responses compared to SRR, which means less overhead on
intermediary peers. Establishing a connection to the relay with TLS
requires multiple round trips. Please refer to Section 5 for cost
comparison between SRR, DRR/RPR.
This technique relies on the relative population of nodes such as A
that require relay peers and peers such as R that are capable of
serving as a relay peers. It also requires mechanism to enable peers
to know which nodes can be used as their relays. This mechanism may
be based on configuration, for example as part of the overlay
configuration an initial list of relay peers can be supplied.
Another option is in a response to ATTACH request the peer can signal
that it can be used as a relay peer.
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A B C D X R
| Request | | | | |
|----------->| | | | |
| | Request | | | |
| |----------->| | | |
| | | Request | | |
| | |----------->| | |
| | | | Request | |
| | | |----------->| |
| | | | | Response |
| | | | |---------->|
| | | | Response | |
|<-----------+------------+------------+------------+-----------|
| | | | | |
3.2. Scenarios Where DRR can be Used
This section lists several scenarios where using DRR would work, and
when the increased efficiency would be advantageous.
3.2.1. Managed or Closed P2P System
The properties that make P2P technology attractive, such as the lack
of need for centralized severs, self-organization, etc. are
attractive for managed systems as well as unmanaged systems. Many of
these systems are deployed on private network where nodes are in the
same address realm and/or can directly route to each other. In such
a scenario, the network administrator can indicate preference for DRR
in the peer's configuration file. Peers in such a system would
always try DRR first, but peers must also support SRR in case DRR
fails. If during the process of establishing a direct connection the
responding peer receives a retransmit on a request with SRR as the
preferred routing mode he should stop trying to establish a direct
connection and use SRR. A node can keep a list of unreachable nodes
based on trying DRR and use only SRR for these nodes. The advantage
in using DRR is on the network stability since it puts less overhead
on the intermediary peers that will not route the responses. The
intermediary peers will need to route less messages and save CPU
resources as well as the link bandwidth usage.
3.2.2. Wireless Scenarios
While some mobile deployments may use clients, in mobile networks
with full peers, there is an advantage to using DRR in order to
reduce the load on intermediary nodes. Using DRR helps with reducing
radio battery usage and bandwidth by the intermediary peers. The
service provider may recommend in the configuration using DRR based
on his knowledge of the topology.
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3.3. Scenarios Where RPR Benefits
In this section, we will list several scenarios where using RPR would
provide improved performance.
3.3.1. Managed or Closed P2P System
As described in Section 3.2.1, many P2P systems run in a closed or
managed environment so that network administrators can better manage
their system. For example, the network administrator can deploy
several relay peers which are publicly reachable in the system and
indicate their presence in the configuration file. After learning
where these relay peers are, peers behind NATs can use RPR with the
help from these relay peers. As with DRR, peers must also support
SRR in case RPR fails.
Another usage is to install relay peers on the managed network
boundary allowing external peers to send responses to peers inside
the managed network.
3.3.2. Using Bootstrap Peers as Relay Peers
Bootstrap peers must be publicly reachable in a RELOAD architecture.
As a result, one possible architecture would be to use the bootstrap
peers as relay peers for use with RPR. The requirements for being a
relay peer are publicly accessible and maintaining a direct
connection with its client. As such, bootstrap peers are well suited
to play the role of relay peers.
3.3.3. Wireless Scenarios
While some mobile deployments may use clients, in mobile networks
using peers, RPR, like DRR, may reduce radio battery usage and
bandwidth usage by the intermediary peers. The service provider may
recommend in the configuration using RPR based on his knowledge of
the topology. Such relay peers may also help connectivity to
external networks.
4. Relationship Between SRR and DRR/RPR
4.1. How DRR Works
DRR is very simple. The only requirement is for the source peers to
provide their (publically reachable) transport address to the
destination peers, so that the destination peer knows where to send
the response. Responses are sent directly to the requesting peer.
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4.2. How RPR Works
RPR is a bit more complicated than DRR. Peers using RPR must
maintain a connection with their relay peer(s). This can be done in
the same way as establishing a neighbor connection between peers by
using the Attach method.
A requirement for RPR is for the source peer to convey their relay
peer (or peers) transport address in the request, so the destination
peer knows where the relay peer are and send the response to a relay
peer first. The request should include also the requesting peer
information enabling the relay peer to route the response back to the
right peer.
(Editor's Note: Being a relay peer does not require that the relay
peer have more functionality than an ordinary peer. As discussed
later, relay peers comply with the same procedure as an ordinary peer
to forward messages. The only difference is that there may be a
larger traffic burden on relay peers. Relay peers can decide whether
to accept a new connection based on their current burden.)
4.3. How These Three Routing Modes Work Together
DRR and RPR are not intended to replace SRR. As seen from Section 3,
DRR or RPR have better performance in some scenarios, but have
limitations as well, see for example section 4.3 in Non-Transitive
Connectivity and DHTs [http://srhea.net/papers/ntr-worlds05.pdf]. As
a result, it is better to use these three modes together to adapt to
each peer's specific situation. In this section, we give some
suggestions on how to transition between the routing modes in RELOAD.
Editor's Note: What this draft proposes are optional extensions to
support DRR/RPR. There is no requirement for implementation to use
the strategy described to choose the appropriate mode.
A peer can collect statistical data on the success of the different
routing modes based on previous transactions and keep a list of non-
reachable addresses. Based on the data, the peer will have a clearer
view about the success rate of different routing modes. Other than
the success rate, the peer can also get data of fine granularity, for
example, the number of retransmission the peer needs to achieve a
desirable success rate.
A typical strategy for the node is as follows. A node chooses to
start with DRR or RPR. Based on the success rate as seen from the
lost message statistics or responses that used SRR, the node can
either continue to offer DRR/RPR first or switch to SRR.
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The node can decide whether to try DRR or RPR based on other
information such as configuration file information. If an overlay
runs within a private network and all nodes in the system can reach
each other directly, nodes may send most of the transactions with
DRR. If a relay peer is provided by the service provider, nodes may
prefer RPR over SRR.
5. Comparison on cost of SRR and DRR/RPR
The major advantages in using DRR/RPR are in going through less
intermediary peers on the response. By doing that it reduces the
load on those peers' resources like processing and communication
bandwidth.
5.1. Closed or managed networks
As described in Section 3, many P2P systems run in a closed or
managed environment (e.g. carrier networks) so that network
administrators would know that they could safely use DRR/RPR.
SRR brings out more routing hops than DRR and RPR. Assuming that
there are N nodes in the P2P system and Chord is applied for routing,
the number of hops for a response in SRR, DRR and RPR are listed in
the following table. Establishing a secure connection between
sending/relay peer and responding peer with (D)TLS requires multiple
messages. Note that establishing (D)TLS secure connections for P2P
overlay is not optimal in some cases, e.g. direct response routing
where (D)TLS is heavy for temporary connections. Instead, some
alternate security techniques, e.g. using public keys of the
destination to encrypt the messages, signing timestamps to prevent
reply attacks can be adopted. Therefore, in the following table, we
show the cases of: 1) no (D)TLS in DRR/RPR; 2) still using DTLS in
DRR/RPR as sub-optimal and, as the worst-cost case, 7 messages are
used during the DTLS handshaking [DTLS]. (TLS Handshake is two
round-trip negotiation protocol while DTLS handshake is three round-
trip negotiation protocol.)
Mode | Success | No. of Hops | No. of Msgs
----------------------------------------------------
SRR | Yes | logN | logN
DRR | Yes | 1 | 1
RPR | Yes | 2 | 2
DRR(DTLS) | Yes | 1 | 7+1
RPR(DTLS) | Yes | 2 | 7+2
From the above comparison, it is clear that:
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1) In most cases of N > 2^2=4, DRR/RPR has fewer hops than SRR.
Shorter route means less overhead and resource usage on intermediary
peers, which is an important consideration for adopting DRR/RPR in
the cases where the resource such as CPU and BW is limited, e.g. the
case of mobile, wireless network.
2) In the cases of N > 2^9=512, DRR/RPR also has fewer messages than
SRR.
3) In the cases where 4 < N < 512, DRR/RPR has more messages than SRR
(but still has fewer hops than SRR). So the consideration to use
DRR/RPR or SRR depends on other factors like using less resources
(bandwidth and processing) from the intermediaries peers. Section 4
provides use cases where DRR/RPR has better chance to work or where
the intermediary resources considerations are important.
5.2. Open networks
In open network where DRR/RPR is not guaranteed, DRR/RPR can fall
back to SRR If it fails after trial, as described in Section 4.
Based on the same settings in Section 5.1, the number of hops, number
of messages for a response in SRR, DRR and RPR are listed in the
following table.
Mode | Success | No. of Hops | No. of Msgs
-----------------------------------------------------------
SRR | Yes | logN | logN
DRR | Yes | 1 | 1
| Fail&Fall back to SRR | 1+logN | 1+logN
RPR | Yes | 2 | 2
| Fail&Fall back to SRR | 2+logN | 2+logN
DRR(DTLS) | Yes | 1 | 7+1
| Fail&Fall back to SRR | 1+logN | 8+logN
RPR(DTLS) | Yes | 2 | 7+2
| Fail&Fall back to SRR | 2+logN | 9+logN
From the above comparison, it can be observed that:
1) Trying DRR/RPR would still have a good chance of fewer hops than
SRR. Suppose that P peers are publicly reachable, the number of hops
in DRR and SRR is P*1+(N-P)*(1+logN), N*logN, respectively. The
condition for fewer hops in DRR is P*1+(N-P)*(1+logN) < N*logN, which
is P/N > 1/logN. This means that when the number of peers N grows,
the required ratio of publicly reachable peers P/N for fewer hops in
DRR decreases. Similar analysis can be easily applied to RPR.
Therefore, the chance of trying DRR/RPR with fewer hops than SRR
becomes better as the scale of the network increases.
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2) In the cases of large network and the success rate of DRR/RPR is
good, it is still possible that DRR/RPR has fewer messages than SRR.
Otherwise, the consideration to use DRR/RPR or SRR depends on other
factors like using less resources from the intermediaries peers.
6. Extensions to RELOAD
Adding support for DRR and RPR requires extensions to the current
RELOAD protocol. In this section, we define the changes required to
the protocol, including changes to message structure and to message
processing.
6.1. Basic Requirements
All peers implementing DRR or RPR MUST support SRR.
All peers MUST be able to process requests for routing in SRR, and
MAY support DRR or RPR routing requests.
Peers that do not support or do not wish to provide DRR or RPR MAY
reject these messages.
6.2. Modification To RELOAD Message Structure
RELOAD provides an extensible framework to accommodate future
extensions. In this section, we define a ForwardingOption structure
to support DRR and RPR modes. Additionally we present a state-
keeping flag to inform intermediate peers if they are allowed to not
maintain state for a transaction.
6.2.1. State-keeping Flag
RELOAD allows intermediate peers to maintain state in order to
implement SRR, for example for implementing hop-by-hop
retransmission. If DRR or RPR is used, the response will not follow
the reverse path, and the state in the intermediate peers won't be
cleared until such state expires. In order to address this issue, we
propose a new flag, state-keeping flag, in the message header to
indicate whether the state should be maintained in the intermediate
peers.
flag : 0x3 IGNORE-STATE-KEEPING
If IGNORE-STATE-KEEPING is set, any peer receiving this message and
which is not the destination of the message MUST forward the message
with the full VIA list and MUST not maintain any internal state.
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6.2.2. Extensive Routing Mode
This draft introduces a new forwarding option for an extensive
routing mode. This option conforms to the description in section
5.3.2.3 in [I-D.ietf-p2psip-base].
We first define a new type to define the new option,
EXTENSIVE_ROUTING_MODE_TYPE:
The option value will be illustrated in the following figure,
defining the ExtensiveRoutingModeOption structure:
enum { 0x0, 0x01 (DRR), 0x02(RPR), 255} RouteMode;
struct {
RouteMode routemode;
OverlayLink transport;
IpAddressPort ipaddressport;
Destination destination<1..2>;
} ExtensiveRoutingModeOption;
The above structure reuses: Transport, Destination and IpAddressPort
structure defined in section 5.3.1.1 and 5.3.2.2 in [I-D.ietf-p2psip-
base].
Route mode: refers to which type of routing mode is indicated to the
destination peer. Currently, only DRR and RPR are defined.
Transport: refers to the transport type which is used to deliver
responses from the destination peer to the sending peer or the relay
peer.
IpAddressPort: refers to the transport address that the destination
peer should use to send the response to. This will be a sending node
address for DRR and a relay peer address for RPR.
Destination: refers to the relay peer or the sending node itself. if
the routing mode is DRR, then the destination only contains the
sending node's node-id; If the routing mode is RPR, then the
destination contains two destinations, which are the relay peer's
node-id and the sending node's node-id.
6.3. Creating a Request
6.3.1. Creating a Request for DRR
When using DRR for a transaction, the sending peer MUST set the
IGNORE-STATE-KEEPING flag in the ForwardingHeader. Additionally, the
peer MUST construct and include a ForwardingOptions structure in the
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ForwardingHeader. When constructing the ForwardingOption structure,
the fields MUST be set as follows:
1) The type MUST be set to EXTENSIVE_ROUTING_MODE_TYPE.
2) The ExtensiveRoutingModeOption structure MUST be used for the
option field within the ForwardingOptions structure. The fields MUST
be defined as follows:
2.1) RouteMode set to 0x01 (DRR).
2.2) Transport set as appropriate for the sender.
2.3) IPAddressPort set to the peer's associated transport address.
2.4) The destination structure MUST contain one vaule, defined as
type peer and set with the sending peer's own values.
6.3.2. Creating a request for RPR
When using RPR for a transaction, the sending peer MUST set the
IGNORE- STATE-KEEPING flag in the ForwardingHeader. Additionally,
the peer MUST construct and include a ForwardingOptions structure in
the ForwardingHeader. When constructing the ForwardingOption
structure, the fields MUST be set as follows:
1) The type MUST be set to EXTENSIVE_ROUTING_MODE_TYPE.
2) The ExtensiveRoutingModeOption structure MUST be used for the
option field within the ForwardingOptions structure. The fields MUST
be defined as follows:
2.1) RouteMode set to 0x02 (RPR).
2.2) Transport set as appropriate for the relay peer.
2.3) IPAddressPort set to the transport address of the relay peer
that the sender wishes the message to be relayed through.
2.4) Destination structure MUST contain two values. The first MUST
be defined as type peer and set with the values for the relay peer.
The second MUST be defined as type peer and set with the sending
peer's own values.
6.4. Request And Response Processing
This section gives normative text for message processing after DRR
and RPR are introduced. Here, we only describe the additional
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procedures for supporting DRR and RPR. Please refer to [I-D.ietf-
p2psip-base] for RELOAD base procedures.
6.4.1. Destination Peer: Receiving a Request And Sending a Response
When the destination peer receives a request, it will check the
options in the forwarding header. If the destination peer can not
understand extensive_routing_mode option in the request, it MUST
attempt to use SRR to return a error response to the sending peer.
If the routing mode is DRR, the peer MUST construct the Destination
list for the response with only one entry, using the sending peer's
node-id from the option in the request as the value.
If the routing mode is RPR, the destination peer MUST construct a
Destination list for the response with two entries. The first MUST
be set to the relay peer node-id from the option in the request and
the second MUST be the sending node node-id from the option of the
request.
In the event that the routing mode is set to DRR and there is not
exactly one destination, or the routing mode is set to RPR and there
are not exactly two destinations the destination peer MUST try to
send a error response to the sending peer using SRR.
After the peer constructs the destination list for the response, it
sends the response to the transport address which is indicated in the
IpAddressPort field in the option using the specific transport mode
in the Forwardingoption. If the destination peer receives a
retransmit with SRR preference on the message he is trying to
response to now, the responding peer should abort the DRR/RPR
response and use SRR.
6.4.2. Sending Peer: Receiving a Response
Upon receiving a response, the peer follows the rules in [I-D.ietf-
p2psip-base]. The peer should note if DRR worked in order to decide
if to offer DRR again. If the peer does not receive a response until
the timeout it SHOULD resend the request using SRR.
If the sender used RPR and does not get a response until the timeout,
it MAY either resend the message using RPR but with a different relay
peer (if available), or resend the message using SRR.
6.4.3. Relay Peer Processing
Relay peers are designed to forward responses to nodes who are not
publicly reachable. For the routing of the response, this draft
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still uses the destination list. The only difference from SRR is
that the destination list is not the reverse of the via-list, instead
it is constructed from the forwarding option as described below.
When a relay peer receives a response, it MUST follow the rules in
[I-D.ietf-p2psip-base]. It receives the response, validates the
message, re-adjust the destination-list and forward the response to
the next hop in the destination list based on the connection table.
There is no added requirement for relay peer.
7. Discovery Of Relay Peer
There are several ways to distribute the information about relay
peers throughout the overlay. P2P network providers can deploy some
relay peers and advertise them in the configuration file. With the
configuration file at hand, peers can get relay peers to try RPR.
Another way is to consider relay peer as a service and then some
service advertisement and discovery mechanism can also be used for
discovering relay peers, for example, using the same mechanism as
used in TURN server discovery in base RELOAD [I-D.ietf-p2psip-base].
Another option is to let a peer advertise his capability to be a
relay in the response to ATTACH or JOIN.
Editor note: This section will be extended if we adopt RPR, but like
other configuration information, there may be many ways to obtain
this.
8. Optional Methods to Investigate Node Connectivity
This section is for informational purposes only for providing some
mecahnsism that can be used when the configuration information does
not specify if DRR or RPR can be used. It summarizes some methods
which can be used for a node to determine its own network location
compared with NAT. These methods may help a node to decide which
routing mode it may wish to try. Note that there is no foolproof way
to determine if a node is publically reachable, other than via out-
of-band mechanisms. As such, peers using these mechanisms may be
able to optimize traffic, but must be able to fall back to SRR
routing if the other routing mechanisms fail.
For DRR and RPR to function correctly, a node may attempt to
determine whether it is publicly reachable. If it is not, RPR may be
chosen to route the response with the help from relay peers, or the
peers should fall back to SRR. If the peer believes it is publically
reachable, DRR may be attempted. NATs and firewalls are two major
contributors preventing DRR and RPR from functioning properly. There
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are a number of techniques by which a node can get its reflexive
address on the public side of the NAT. After obtaining the reflexive
address, a peer can perform further tests to learn whether the
reflexive address is publicly reachable. If the address appears to
be publicly reachable, the nodes to which the address belongs can use
DRR for responses and can also be a candidate to serve as a relay
peer. Nodes which are not publicly reachable may still use RPR to
shorten the response path with the help from relay peers.
There are a number of techniques which a node can use to obtain its
reflexive address which is on the public side of the NAT. After
obtaining the reflexive address, a peer can perform further tests to
learn whether the reflexive address is publicly reachable. If the
address proves publicly reachable, the nodes to which the address
belongs can use DRR for responses and also can be a candidate for
relay peer. Nodes that are not publicly reachable may still use RPR
to shorten response path with the help of relay peers.
Some conditions are unique in P2PSIP architecture which could be
leveraged to facilitate the tests. In P2P overlay network, each node
only has partial a view of the whole network, and knows of a few
nodes in the overlay. P2P routing algorithms can easily deliver a
request from a sending node to a peer with whom the sending node has
no direct connection. This makes it easy for a node to ask other
nodes to send unsolicited messages back to the requester.
In the following sections, we first introduce several ways for a node
to get the addresses needed for the further tests. Then a test for
learning whether a peer may be publicly reachable is proposed.
8.1. Getting Addresses To Be Used As Candidates for DRR
In order to test whether a peer may be publicly reachable, the node
should first get one or more addresses which will be used by other
nodes to send him messages directly. This address is either a local
address of a node or a translated address which is assigned by a NAT
to the node.
STUN is used to get a reflexive address on the public side of a NAT
with the help of STUN servers. There is also a STUN usage [I-D.ietf-
behave-nat-behavior-discovery] to discover NAT behavior. Under
RELOAD architecture, a few infrastructure servers can be leveraged
for this usage, such as enrollment servers, diagnostic servers,
bootstrap servers, etc.
The node can use a STUN Binding request to one of STUN servers to
trigger a STUN Binding response which returns the reflexive address
from the server's perspective. If the reflexive transport address is
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the same as the source address of the Binding request, the node can
determine that there likely is no NAT between him and the chosen
infrastructure server. (Certainly, in some rare cases, the allocated
address happens to be the same as the source address. Further tests
will detect this case and rule it out in the end.). Usually, these
infrastructure severs are publicly reachable in the overlay, so the
node can be considered publicly reachable. On the other hand, with
the techniques in [I-D.ietf-behave-nat-behavior-discovery], a node
can also decide whether it is behind NAT with endpoint-independent
mapping behavior. If the node is behind a NAT with endpoint-
independent mapping behavior, the reflexive address should also be a
candidate for further tests.
UPnP-IGD is a mechanism that a node can use to get the assigned
address from its residential gateway and after obtaining this address
to communicate it with other nodes, the node can receive unsolicited
messages from outside, even though it is behind a NAT. So the
address obtained through the UPnP mechanism should also be used for
further tests.
Another way that a node behind NAT can use to learn its assigned
address by NAT is NAT-PMP. Like in UPnP-IGD, the address obtained
using this mechanism should also be tested further.
The above techniques are not exhaustive. These techniques can be
used to get candidate transport addresses for further tests.
8.2. Public Reachability Test
Using the transport addresses obtained by the above techniques, a
node can start a test to learn whether the candidate transport
address is publicly reachable. The basic idea for the test is for a
node to send a request and expect another node in the overlay to send
back a response. If the response is received by the sending node
successfully and also the node giving the response has no direct
connection with the sending node, the sending node can determine that
the address is probably publicly reachable and hence the node may be
publicly reachable at the tested transport address.
In P2P overlay, a request is routed through the overlay and finally a
destination peer will terminate the request and give the response.
In a large system, there is a high probability that the destination
peer has no direct connection with the sending node. Especially in
RELOAD architecture, every node maintains a connection table. So it
is easier for a node to check whether it has direct connection with
another node.
Note: Currently, no existing message in base RELOAD can achieve the
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test. In our opinion, this kind of test is within diagnostic scope,
so authors hope WG can define a new diagnostic message to do that.
We don't plan to define the message in this document, for the
objective of this draft is to propose an extension to support DRR and
RPR. The following text is informative.
If a node wants to test whether its transport address is publicly
reachable, it can send a request to the overlay. The routing for the
test message would be different from other kinds of requests because
it is not for storing/fetching something to/from the overlay or
locating a specific node, instead it is to get a peer who can deliver
the sending node an unsolicited response and which has no direct
connection with him. Each intermediate peer receiving the request
first checks whether it has a direct connections with the sending
peer. If there is a direct connection, the request is routed to the
next peer. If there is no direct connection, the intermediate peer
terminates the request and sends the response back directly to the
sending node with the transport address under test.
After performing the test, if the peer determines that it may be
publicly reachable, it can try DRR in subsequent transaction, and may
advertise that it is a candidate to serve as a relay peer.
9. Security Considerations
TBD
10. IANA Considerations
10.1. A new RELOAD Forwarding Option
A new RELOAD Forwarding Option type is add to the Registry.
Type: 0x1 - extensive_routing_mode
11. Acknowledgements
David Bryan has helped extensively with this document, and helped
provide some of the text, analysis, and ideas contained here. The
authors would like to thank Ted Hardie, Narayanan Vidya, Dondeti
Lakshminath and Bruce Lowekamp for their constructive comments.
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12. References
12.1. Normative References
[I-D.ietf-p2psip-base] Jennings, C., Lowekamp, B., Rescorla, E.,
Baset, S., and H. Schulzrinne, "REsource LOcation And Discovery
(RELOAD) Base Protocol", draft-ietf-p2psip-base-08 (work in
progress), March 2010.
[I-D.ietf-p2psip-concepts] Bryan, D., Matthews, P., Shim, E., Willis,
D., and S. Dawkins, "Concepts and Terminology for Peer to Peer SIP",
draft-ietf-p2psip-concepts-02 (work in progress), July 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[ChurnDHT] Rhea, S., "Handling Churn in a DHT", Proceedings of the
USENIX Annual Technical Conference. Handling Churn in a DHT, June
2004.
[DTLS] Modadugu, N., Rescorla, E., "The Design and Implementation of
Datagram TLS", 11th Network and Distributed System Security Symposium
(NDSS), 2004.
[I-D.ietf-behave-nat-behavior-discovery] MacDonald, D. and B.
Lowekamp, "NAT Behavior Discovery Using STUN",
draft-ietf-behave-nat-behavior-discovery-04 (work in progress), July
2008.
[I-D.ietf-behave-tcp] Guha, S., Biswas, K., Ford, B., Sivakumar, S.,
and P. Srisuresh, "NAT Behavioral Requirements for TCP",
draft-ietf-behave-tcp-08 (work in progress), September 2008.
[I-D.lowekamp-mmusic-ice-tcp-framework] Lowekamp, B. and A. Roach, "A
Proposal to Define Interactive Connectivity Establishment for the
Transport Control Protocol (ICE-TCP) as an Extensible Framework",
draft-lowekamp-mmusic-ice-tcp-framework-00 (work in progress),
October 2008.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787,
January 2007.
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Authors' Addresses
Xingfeng Jiang
Huawei Technologies
Email: jiang.x.f@huawei.com
Ning Zong
Huawei Technologies
Email: zongning@huawei.com
Roni Even
Gesher Erove
Email: ron.even.tlv@gmail.com
Yunfei Zhang
China Mobile
Email: zhangyunfei@chinamobile.com
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