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P2PSIP                                                          X. Jiang
Internet-Draft                                                   N. Zong
Intended status: Standards Track                     Huawei Technologies
Expires: September 11, 2011                                      R. Even
                                                            Gesher Erove
                                                                Y. Zhang
                                                            China Mobile
                                                          March 10, 2011


An extension to RELOAD to support Direct Response and Relay Peer routing
                      draft-jiang-p2psip-relay-05

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 11, 2011.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 21
     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.

   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
   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



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   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
   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



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   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.


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-12 (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",



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   draft-ietf-p2psip-concepts-03 (work in progress), October 2010.

   [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.


Authors' Addresses

   Xingfeng Jiang
   Huawei Technologies

   Email: jiang.x.f@huawei.com


   Ning Zong
   Huawei Technologies

   Email: zongning@huawei.com




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   Roni Even
   Gesher Erove

   Email: ron.even.tlv@gmail.com


   Yunfei Zhang
   China Mobile

   Email: zhangyunfei@chinamobile.com









































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