P2PSIP                                                      N. Zong, Ed.
Internet-Draft                                                  X. Jiang
Intended status: Standards Track                                 R. Even
Expires: January 16, February 15, 2014                           Huawei Technologies
                                                                Y. Zhang
                                                           July 15,
                                                         August 14, 2013

          An extension to RELOAD to support Relay Peer Routing


   This document proposes an optional extension to RELOAD to support
   relay peer routing mode.  RELOAD recommends symmetric recursive
   routing for routing messages.  The new optional extension provides a
   shorter route for responses reducing the overhead on intermediate
   peers and describes the potential use cases where this extension can
   be used.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  RPR . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Relay Peer Routing (RPR)  . . . . . . . . . . . . . .   4
     3.2.  Scenarios where RPR can be beneficial . . . . . . . . . .   5
       3.2.1.  Managed or closed P2P systems . . . . . . . . . . . .   5
       3.2.2.  Using bootstrap nodes as relay peers  . . . . . . . .   6
       3.2.3.  Wireless scenarios  . . . . . . . . . . . . . . . . .   6
   4.  Relationship between SRR and RPR  . . . . . . . . . . . . . .   6
     4.1.  How RPR works . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  How SRR and RPR work together . . . . . . . . . . . . . .   7
   5.  Comparison on cost of SRR and RPR . . . . . . . . . . . . . .   7
     5.1.  Closed or managed networks  . . . . . . . . . . . . . . .   7
     5.2.  Open networks . . . . . . . . . . . . . . . . . . . . . .   8
   6.  RPR extensions to RELOAD  . . . . . . . . . . . . . . . . . .   8
     6.1.  Basic requirements  . . . . . . . . . . . . . . . . . . .   8
     6.2.  Modification to RELOAD message structure  . . . . . . . .   8
       6.2.1.  State-keeping flag  . . . . . . . . . . . . . . . . .   9
       6.2.2.  Extensive routing mode  . . . . . . . . . . . . . . .   9
     6.3.  Creating a request  . . . . . . . . . . . . . . . . . . .  10
       6.3.1.  Creating a request for RPR  . . . . . . . . . . . . .  10
     6.4.  Request and response processing . . . . . . . . . . . . .  10
       6.4.1.  Destination peer: receiving a request and sending a
               response  . . . . . . . . . . . . . . . . . . . . . .  10
       6.4.2.  Sending peer: receiving a response  . . . . . . . . .  11
       6.4.3.  Relay peer processing . . . . . . . . . . . . . . . .  11
   7.  Overlay configuration extension . . . . . . . . . . . . . . .  12
   8.  Discovery of relay peers  . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  A new RELOAD Forwarding Option . . . . . . . . . . . . .  12
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     12.2.  Informative References . . . . . . . . . . . . . . . . .  13
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Appendix A.  Optional methods to investigate peer connectivity  .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   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 A of [I-D.ietf-
   p2psip-base].  As we show in section 3, RPR is advantageous over SRR
   in some scenarios reducing load (CPU and link bandwidth) on
   intermediate peers.  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 RPR
   and SRR together is more likely to bring benefits than if SRR is used

   Note that in this document, we focus on RPR routing mode and its
   extensions to RELOAD to produce a standalone solution.  Please refer
   to DRR document [I-D.ietf-p2psip-drr] for DRR routing mode.

   We first discuss the problem statement in Section 3, then how to
   combine RPR and SRR is presented in Section 4.  In Section 5, we give
   comparison on the cost of SRR and RPR in both managed and open
   networks.  An extension to RELOAD to support RPR is proposed in
   Section 6.  Discovery of relay peers is introduced in Section 7.
   Some optional methods to check peer connectivity are introduced in
   Appendix A.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   We use the terminology and definitions from the RELOAD base draft
   [I-D.ietf-p2psip-base] extensively in this document.  We also use
   terms defined in NAT behavior discovery [RFC5780].  Other terms used
   in this document are defined inline when used and are also defined
   below for reference.

      Publicly Reachable: A peer is publicly reachable if it can receive
      unsolicited messages from any other peer in the same overlay.
      Note: "publicly" does not mean that the peers 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 peers in the overlay and
      forward the responses from destination peers towards the sender of
      the request.

      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 rest of the document.

      Symmetric Recursive Routing (SRR): refers to a routing mode in
      which responses follow the reverse path of the request to get to
      the sending peer.  For simplicity, the abbreviation SRR is used
      instead in the rest of the document.

3.  Overview

   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, while others run in
   closed networks of small scale.  SRR works in any situation, but RPR
   may work better in some specific scenarios.

3.1.  RPR

   RELOAD is a simple request-response protocol.  After sending a
   request, a peer waits for a response from a destination peer.  There
   are several ways for the destination peer to send a response back to
   the source peer.  In this section, we will provide detailed
   information on RPR.

   Note that the same illustrative settings can be found in DRR document

3.1.1.  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.  Figure 1 illustrates RPR.  Note that RPR
   also allows a shorter route for responses compared to SRR, which
   means less overhead on intermediate peers.  Establishing a connection
   to the relay with TLS requires multiple round trips.  Please refer to
   Section 5 for cost comparison between SRR and RPR.

   A            B            C             D           X           R
   |  Request   |            |            |            |           |
   |----------->|            |            |            |           |
   |            | Request    |            |            |           |
   |            |----------->|            |            |           |
   |            |            | Request    |            |           |
   |            |            |----------->|            |           |
   |            |            |            | Request    |           |
   |            |            |            |----------->|           |
   |            |            |            |            | Response  |
   |            |            |            |            |---------->|
   |            |            |            |  Response  |           |
   |            |            |            |            |           |

                             Figure 1. RPR routing mode

   This technique relies on the relative population of peers such as A
   that require relay peers, and peers such as R that are capable of
   serving as a relay peers.  It also requires a mechanism to enable
   peers to know which peers 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 message, the responding peer can
   announce that it can serve as a relay peer.

3.2.  Scenarios where RPR can be beneficial

   In this section, we will list several scenarios where using RPR would
   provide an improved performance.

3.2.1.  Managed or closed P2P systems

   As described in Section 3.2.1 of DRR draft [I-D.ietf-p2psip-drr],
   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.
   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.

3.2.2.  Using bootstrap nodes as relay peers

   Bootstrap nodes are typically publicly reachable in a RELOAD
   architecture.  As a result, one possible architecture would be to use
   the bootstrap nodes as relay peers for use with RPR.  A relay peer
   SHOULD be publicly accessible and maintain a direct connection with
   its client.  As such, bootstrap nodes are well suited to play the
   role of relay peers.

3.2.3.  Wireless scenarios

   In some mobile deployments, using RPR may help reducing radio battery
   usage and bandwidth by the intermediate peers.  The service provider
   may recommend using RPR based on his knowledge of the topology.

4.  Relationship between SRR and RPR

4.1.  How RPR works

   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.

   Note that being a relay peer does not require that the relay peer has
   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.

4.2.  How SRR and RPR work together

   RPR is not intended to replace SRR.  It is better to use these two
   modes together to adapt to each peer's specific situation.  Note that
   the informative suggestions on how to transition between SRR and RPR
   are the same with that of DRR.  Please refer to DRR document [I-D
   .ietf-p2psip-drr] for more details.  If a relay peer is provided by
   the service provider, peers MAY prefer RPR over SRR.  Otherwise,
   using RPR SHOULD be discouraged in the open Internet or if the
   administrator does not feel he have enough information about the
   overlay.  A new overlay configuration element specifying the usage of
   DRR is defined in Section 7.

5.  Comparison on cost of SRR and RPR

   The major advantage of the use of RPR is that it reduces the number
   of intermediate peers traversed by 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 RPR.

   The number of hops for a response in SRR and RPR are listed in the
   following table.  Note that the same illustrative settings can be
   found in DRR document [I-D.ietf-p2psip-drr].

     Mode      | Success | No. of Hops | No. of Msgs
     SRR       |  Yes    |     log(N)  |    log(N)
     RPR       |  Yes    |     2       |    2
     RPR(DTLS) |  Yes    |     2       |    7+2

    Table 1. Comparison of SRR and RPR in closed networks

   From the above comparison, it is clear that:

   1) In most cases when N > 4 (2^2), RPR uses fewer hops than SRR.
   Using a shorter route means less overhead and resource usage on
   intermediate peers, which is an important consideration for adopting
   RPR in the cases where the resources such as CPU and bandwidth are
   limited, e.g. the case of mobile, wireless networks.

   2) In the cases when N > 512 (2^9), RPR also uses fewer messages than

   3) In the cases when N < 512, RPR uses more messages than SRR (but
   still uses fewer hops than SRR).  So the consideration on whether
   using RPR or SRR depends on other factors like using less resources
   (bandwidth and processing) from the intermediate peers.  Section 4
   provides use cases where RPR has better chance to work or where the
   intermediary resources considerations are important.

5.2.  Open networks

   In open networks where RPR is not guaranteed to work, RPR can fall
   back to SRR if it fails after trial, as described in Section 4.
   Based on the same settings of Section 5.1, the number of hops, number
   of messages for a response in SRR and RPR are listed in the following

     Mode      |       Success         | No. of Hops | No. of Msgs
     SRR       |         Yes           |     log(N)  |    log(N)
     RPR       |         Yes           |     2       |    2
               | Fail&Fall back to SRR |     2+log(N)|    2+log(N)
     RPR(DTLS) |         Yes           |     2       |    7+2
               | Fail&Fall back to SRR |     2+log(N)|    9+log(N)

       Table 2. Comparison of SRR and RPR in open networks

   From the above comparison, it can be observed that trying to first
   use RPR could still provide an overall number of hops lower than
   directly using SRR.  The detailed analysis is same as DRR case and
   can be found in DRR document [I-D.ietf-p2psip-drr].

6.  RPR extensions to RELOAD

   Adding support for RPR requires extensions to the current RELOAD
   protocol.  In this section, we define the extensions required to the
   protocol, including extensions to message structure and to message

6.1.  Basic requirements

   All peers MUST be able to process requests for routing in SRR, and
   MAY support RPR routing requests.

6.2.  Modification to RELOAD message structure
   RELOAD provides an extensible framework to accommodate future
   extensions.  In this section, we define a ForwardingOption structure
   and present a state-keeping flag to support RPR mode.

6.2.1.  State-keeping flag


   If IGNORE-STATE-KEEPING is set, any peer receiving this message and
   which is not the destination of the message SHOULD forward the
   message with the full via_list and SHOULD NOT maintain any internal

6.2.2.  Extensive routing mode

   We first define a new type to define the new option,

   The option value is illustrated in the following figure, defining the
   ExtensiveRoutingModeOption structure:

   enum {(0),DRR(1),RPR(2),(255)} RouteMode;
   struct {
           RouteMode               routemode;
           OverlayLinkType         transport;
           IpAddressPort           ipaddressport;
           Destination             destinations<1..2^8-1>;
   } ExtensiveRoutingModeOption;

   Note that DRR value in RouteMode is defined in DRR document [I-D

   RouteMode: refers to which type of routing mode is indicated to the
   destination peer.

   OverlayLinkType: refers to the transport type which is used to
   deliver responses from the destination peer to the relay peer.

   IpAddressPort: refers to the transport address that the destination
   peer should use to send the response to.  This will be a relay peer
   address for RPR.

   Destination: refers to the relay peer itself.  If the routing mode is
   RPR, then the destination contains two destinations, which are the
   relay peer's Node-ID and the sending peer's Node-ID.

6.3.  Creating a request

6.3.1.  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.

   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 node and set with the values for the relay peer.
   The second MUST be defined as type node and set with the sending
   peer's own values.

6.4.  Request and response processing

   This section gives normative text for message processing after RPR is
   introduced.  Here, we only describe the additional procedures for
   supporting 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 using SRR to return an "Error_Unknown_Extension" response
   (defined in Section and Section 14.9 of [I-D.ietf-p2psip-
   base]) to the sending peer.

   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 peer Node-ID from the option of the

   In the event that the routing mode is set to RPR and there are not
   exactly two destinations, the destination peer MUST try to send an
   "Error_Unknown_Extension" response (defined in Section and
   Section 14.9 of [I-D.ietf-p2psip-base]) to the sending peer using

   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 it is trying to
   response to now, the responding peer SHOULD abort the 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].  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 peers who are not
   publicly reachable.  For the routing of the response, this document
   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

   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.  Overlay configuration extension

   This document uses the new RELOAD overlay configuration element,
   "route-mode", inside each "configuration" element, as defined in DRR
   document [I-D.ietf-p2psip-drr].

8.  Discovery of relay peers

   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.

9.  Security Considerations

   As a routing alternative, the security part of RPR conforms to
   section 13.6 of the base draft [I-D.ietf-p2psip-base] which describes
   routing security.  RPR behave like a DRR requesting node towards the
   destination node.  The RPR relay node is not an arbitrary node but
   SHOULD be a trusted one (managed network, bootstrap nodes or
   configured relay) which will make it less of a risk as outlined in
   section13 of the based draft.

10.  IANA Considerations

10.1.  A new RELOAD Forwarding Option

   A new RELOAD Forwarding Option type is added to the Forwarding Option
   Registry defined in [I-D.ietf-p2psip-base].

   Type: 0x02 - 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, Bruce Lowekamp, Stephane Bryant, Marc Petit-Huguenin and
   Carlos Jesus Bernardos Cano for their constructive comments.

12.  References
12.1.  Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC2119, March 1997.

   [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-26 (work in
   progress), February 2013.

12.2.  Informative References

   [RFC5780] MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
   Using STUN", RFC5780, May 2010.

   [I-D.ietf-p2psip-drr] Zong, N., Jiang, X., Even, R. and Zhang, Y.,
   "An extension to RELOAD to support Direct Response Routing", draft-
   ietf-p2psip-drr-07, June
   ietf-p2psip-drr-09, August 2013.

   [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-04 (work in progress), October 2011.

   [RFC3424] Daigle, L., "IAB Considerations for UNilateral Self-Address
   Fixing (UNSAF) Across Network Address Translation", RFC3424, November

13.  References

Appendix A.  Optional methods to investigate peer connectivity

   This section is for informational purposes only for providing some
   mechanisms that can be used when the configuration information does
   not specify if RPR can be used.  It summarizes some methods which can
   be used for a peer to determine its own network location compared
   with NAT.  These methods may help a peer to decide which routing mode
   it may wish to try.  Note that there is no foolproof way to determine
   if a peer 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  This document addresses the UNSAF [RFC3424] concerns by
   specifying a fallback plan to SRR routing if the
   other routing mechanisms fail. [p2psip-base-draft].  SRR is not an
   UNSAF mechanism.  The document does not define any new UNSAF

   For RPR to function correctly, a peer 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.  NATs and firewalls are two major
   contributors preventing RPR from functioning properly.  There are a
   number of techniques by which a peer 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 peers to which the address belongs can be a candidate
   to serve as a relay peer.  Peers which are not publicly reachable may
   still use RPR to shorten the response path with the help from relay

   Some conditions are unique in P2PSIP architecture which could be
   leveraged to facilitate the tests.  In P2P overlay network, each peer
   only has partial a view of the whole network, and knows of a few
   peers in the overlay.  P2P routing algorithms can easily deliver a
   request from a sending peer to a peer with whom the sending peer has
   no direct connection.  This makes it easy for a peer to ask other
   peers to send unsolicited messages back to the requester.

   The approaches for a peer to get the addresses needed for the further
   tests, as well as the test for learning whether a peer may be
   publicly reachable is same as the DRR case.  Please refer to DRR
   document [I-D.ietf-p2psip-drr] for more details.

Authors' Addresses

   Ning Zong (editor)
   Huawei Technologies

   Email: zongning@huawei.com

   Xingfeng Jiang
   Huawei Technologies

   Email: jiang.x.f@huawei.com

   Roni Even
   Huawei Technologies

   Email: roni.even@mail01.huawei.com

   Yunfei Zhang

   Email: hishigh@gmail.com