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

       An extension to RELOAD to support Direct Response Routing


   This document proposes an optional extension to RELOAD to support
   direct response 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 cases where this extension can be

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
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 15, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  SRR and DRR . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Symmetric Recursive Routing (SRR) . . . . . . . . . .   4
       3.1.2.  Direct Response Routing (DRR) . . . . . . . . . . . .   5
     3.2.  Scenarios where DRR can be used . . . . . . . . . . . . .   6
       3.2.1.  Managed or closed P2P systems . . . . . . . . . . . .   6
       3.2.2.  Wireless scenarios  . . . . . . . . . . . . . . . . .   6
   4.  Relationship between SRR and DRR  . . . . . . . . . . . . . .   6
     4.1.  How DRR works . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  How SRR and DRR work together . . . . . . . . . . . . . .   7
   5.  Comparison on cost of SRR and DRR . . . . . . . . . . . . . .   7
     5.1.  Closed or managed networks  . . . . . . . . . . . . . . .   7
     5.2.  Open networks . . . . . . . . . . . . . . . . . . . . . .   9
   6.  DRR extensions to RELOAD  . . . . . . . . . . . . . . . . . .   9
     6.1.  Basic requirements  . . . . . . . . . . . . . . . . . . .   9
     6.2.  Modification to RELOAD message structure  . . . . . . . .   9
       6.2.1.  State-keeping flag  . . . . . . . . . . . . . . . . .  10
       6.2.2.  Extensive routing mode  . . . . . . . . . . . . . . .  10
     6.3.  Creating a request  . . . . . . . . . . . . . . . . . . .  11
       6.3.1.  Creating a request for DRR  . . . . . . . . . . . . .  11
     6.4.  Request and response processing . . . . . . . . . . . . .  11
       6.4.1.  Destination peer: receiving a request and sending a
               response  . . . . . . . . . . . . . . . . . . . . . .  11
       6.4.2.  Sending peer: receiving a response  . . . . . . . . .  12
   7.  Overlay configuration extension . . . . . . . . . . . . . . .  12
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  13
   9.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  A new RELOAD forwarding option  . . . . . . . . . . . . .  13
     9.2.  A new IETF XML registry . . . . . . . . . . . . . . . . .  13
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  Normative references . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative references . . . . . . . . . . . . . . . . .  14
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Appendix A.  Optional methods to investigate peer connectivity  .  14
     A.1.  Getting addresses to be used as candidates for DRR  . . .  15
     A.2.  Public reachability test  . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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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, DRR is advantageous over SRR
   in some scenarios by reducing load (CPU and link bandwidth) on
   intermediate peers.  For example, in a closed network where every
   peer is in the same address realm, DRR performs better than SRR.  In
   other scenarios, using a combination of DRR and SRR together is more
   likely to bring benefits than if SRR is used alone.

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

   We first discuss the problem statement in Section 3, then how to
   combine DRR and SRR is presented in Section 4.  In Section 5, we give
   comparison on the cost of SRR and DRR in both managed and open
   networks.  An extension to RELOAD to support DRR is proposed in
   Section 6.  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.

      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 rest of the document.

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      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 DRR
   may work better in some specific scenarios.

3.1.  SRR and DRR

   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 two routing modes: SRR and DRR.

   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;

   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.  Figure 1
   illustrates SRR.

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

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   |            |            |----------->|            |
   |            |            |            | Request    |
   |            |            |            |----------->|
   |            |            |            |            |
   |            |            |            |  Response  |
   |            |            |            |<-----------|
   |            |            |  Response  |            |
   |            |            |<-----------|            |
   |            |  Response  |            |            |
   |            |<-----------|            |            |
   |  Response  |            |            |            |
   |<-----------|            |            |            |
   |            |            |            |            |

                       Figure 1. SRR routing mode

   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 peer, increasing the bandwidth usage
   and CPU/battery load of multiple peers.

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 is not routed through
   intermediate peers.  Figure 2 illustrates DRR.  Using a shorter route
   means less overhead on intermediate peers, especially in the case of
   wireless networks where the CPU and uplink bandwidth is limited.  For
   example, in the absence of NATs, or if the NAT implements endpoint-
   independent filtering, this is the optimal routing technique.  Note
   that establishing a secure connection requires multiple round trips.
   Please refer to Section 5 for cost comparison between SRR and DRR.

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

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   |            |            |            |            |

                        Figure 2. DRR routing mode

3.2.  Scenarios where DRR can be used

   This section lists several scenarios where using DRR would work, and
   identifies when the increased efficiency would be advantageous.

3.2.1.  Managed or closed P2P systems

   The properties that make P2P technology attractive, such as the lack
   of need for centralized servers, self-organization, etc. are
   attractive for managed systems as well as unmanaged systems.  Many of
   these systems are deployed on private networks where peers 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
   with the sending peer, the responding peer receives a response with
   SRR as the preferred routing mode (or it fails to establish the
   direct connection), the responding peer SHOULD NOT use DRR but switch
   to SRR.  The simple policy is to try DRR and if fails switch to SRR
   for all connections.  A finer grained policy is when a peer keeps a
   list of unreachable peers based on trying DRR and use only SRR for
   these peers.  The advantage in using DRR is on the network stability
   since it puts less overhead on the intermediate peers that will not
   route the responses.  The intermediate peers will need to route less
   messages and save CPU resources as well as the link bandwidth usage.

3.2.2.  Wireless scenarios

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

4.  Relationship between SRR and DRR

4.1.  How DRR works

   DRR is very simple.  The only requirement is for the source peers to
   provide their potential (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

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4.2.  How SRR and DRR work together

   DRR is not intended to replace SRR.  It is better to use these two
   modes together to adapt to each peer's specific situation.  In this
   section, we give some informative suggestions on how to transition
   between the routing modes in RELOAD.

   According to base draft [I-D.ietf-p2psip-base], SRR MUST be
   supported.  An overlay MAY be configured to use alternative routing
   algorithms, and alternative routing algorithms MAY be selected on a
   per-message basis.  I.e., a node in an overlay which supports SRR and
   some other routing algorithm, for example DRR, might use SRR some of
   the time and DRR some of the time.  A node joining the overlay should
   get from the configuration file the preferred routing mode.  If an
   overlay runs within a private network and all peers in the system can
   reach each other directly, peers MAY send most of the transactions
   with DRR.  On the contrary, using DRR SHOULD be discouraged in the
   open Internet or if the administrator does not feel he have enough
   information about the overlay network topology.  A new overlay
   configuration element specifying the usage of DRR is defined in
   Section 7.

   Alternatively, 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 this 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
   finer granularity, for example, the number of retransmission the peer
   needs to achieve a desirable success rate.

   A typical strategy for the peer is as follows.  A peer chooses to
   start with DRR based on the configuration.  Based on the success rate
   seen from the lost message statistics or responses that used DRR, the
   peer can either continue to offer DRR first or switch to SRR.  Note
   that a peer should use the DRR success statistic to decide if to
   continue using DRR or fall back to SRR.  It is not recommended to
   make such decision per specific connection but this is an application

5.  Comparison on cost of SRR and DRR

   The major advantages in using DRR are in going through less
   intermediate peers on the response.  By doing that it reduces the
   load on those peers' resources like processing and communication

5.1.  Closed or managed networks

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   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.

   SRR brings out more routing hops than DRR.  Assuming that there are N
   peers in the P2P system and Chord is applied for routing, the number
   of hops for a response in SRR and DRR are listed in the following
   table.  Establishing a secure connection between the sending peer and
   the 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, and 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; 2) still using DTLS in DRR as sub-optimal.  As the
   worst-cost case, 7 messages are used during the DTLS handshaking
   [DTLS].  (TLS Handshake is a two round-trip negotiation protocol
   while DTLS handshake is a three round-trip negotiation protocol.)

     Mode      | Success | No. of Hops | No. of Msgs
     SRR       |  Yes    |     log(N)  |    log(N)
     DRR       |  Yes    |     1       |    1
     DRR(DTLS) |  Yes    |     1       |    7+1

    Table 1. Comparison of SRR and DRR in closed networks

   From the above comparison, it is clear that:

   1) In most cases when N > 2 (2^1), DRR 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
   DRR 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 > 256 (2^8), DRR also uses fewer messages than

   3) In the cases when N < 256, DRR uses more messages than SRR (but
   still uses fewer hops than SRR).  So the consideration on whether
   using DRR or SRR depends on other factors like using less resources
   (bandwidth and processing) from the intermediate peers.  Section 4
   provides use cases where DRR has better chance to work or where the
   intermediary resources considerations are important.

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5.2.  Open networks

   In open networks where DRR is not guaranteed to work, DRR 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 and DRR are listed in the following

     Mode      |       Success         | No. of Hops | No. of Msgs
     SRR       |         Yes           |     log(N)  |    log(N)
     DRR       |         Yes           |     1       |    1
               | Fail&Fall back to SRR |     1+log(N)|    1+log(N)
     DRR(DTLS) |         Yes           |     1       |    7+1
               | Fail&Fall back to SRR |     1+log(N)|    8+log(N)

        Table 2. Comparison of SRR and DRR in open networks

   From the above comparison, it can be observed that trying to first
   use DRR could still provide an overall number of hops lower than
   directly using 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.  Therefore, the
   chance of trying DRR with fewer hops than SRR becomes better as the
   scale of the network increases.

6.  DRR extensions to RELOAD

   Adding support for DRR 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 DRR routing requests.

6.2.  Modification to RELOAD message structure

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   RELOAD provides an extensible framework to accommodate future
   extensions.  In this section, we define a ForwardingOption structure
   to support DRR mode.  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 is used, the response will not follow the
   reverse path, and the state in the intermediate peers will not 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 keeping is required in the intermediate


   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

   This draft introduces a new forwarding option for an extensive
   routing mode.  This option conforms to the description in section of [I-D.ietf-p2psip-base].

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

   The above structure reuses OverlayLinkType, Destination and
   IpAddressPort structure defined in section, and of [I-D.ietf-p2psip-base].

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   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 sending peer.

   IpAddressPort: refers to the transport address that the destination
   peer use to send the response to.  This will be a sending peer
   address for DRR.

   Destination: refers to the sending peer itself.  If the routing mode
   is DRR, then the destination only contains the sending peer's Node-

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
   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 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 value, 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 DRR is
   introduced.  Here, we only describe the additional procedures for
   supporting DRR.  Please refer to [I-D.ietf-p2psip-base] for RELOAD
   base procedures.

6.4.1.  Destination peer: receiving a request and sending a response

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   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 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 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.

   In the event that the routing mode is set to DRR and there is not
   exactly one destination, the destination peer MUST try to return 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 respond
   to now, the responding peer SHOULD abort the DRR response and use

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.

7.  Overlay configuration extension

   This document extends the RELOAD overlay configuration by adding one
   new element, "route-mode", inside each "configuration" element.

   The Compact Relax NG Grammar for this element is:

      namespace route-mode = "urn:ietf:params:xml:ns:p2p:route-mode"

      parameter &= element route-mode:mode { xsd:string }?

   This namespace is added into the <mandatory-extension> element in the
   overlay configuration file.  The defined routing modes include DRR
   and RPR.

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   Mode can be DRR or RPR and if specified in the configuration should
   be the preferred routing mode used by the application.

8.  Security considerations

   As a routing alternative, the security part of DRR conforms to
   section 13.6 of the base draft [I-D.ietf-p2psip-base] which describes
   routing security.  The DRR routing option provide the information
   about the route back to the source.  According to section 13 of the
   base drat the forwarding header MUST be digitally signed protecting
   the DRR routing information.

9.  IANA considerations

9.1.  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

9.2.  A new IETF XML registry

   This section requests IANA to register the following URN in the "XML
   Namespaces" class of the "IETF XML Registry" in accordance with

   URI: urn:ietf:params:xml:ns:p2p:route-mode

   Registrant Contact: The IESG

   XML: This specification

10.  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.

11.  References

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11.1.  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.

   [RFC3688] Mealling, M., "The IETF XML Registry ", BCP 81, RFC3688,
   January 2004.

11.2.  Informative references

   [DTLS] Modadugu, N., Rescorla, E., "The Design and Implementation of
   Datagram TLS", 11th Network and Distributed System Security Symposium
   (NDSS), 2004.

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

   [I-D.ietf-p2psip-rpr] Zong, N., Jiang, X., Even, R. and Zhang, Y.,
   "An extension to RELOAD to support Relay Peer Routing", draft-ietf-
   p2psip-rpr-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.

   [IGD2] UPnP Forum, "WANIPConnection:2 Service (http://upnp.org/specs/
   gw/UPnP-gw-WANIPConnection-v2-Service.pdf)", September 2010.

   [RFC6886] Cheshire, S., Krochmal M., and K. Sekar, "NAT Port Mapping
   Protocol (NAT-PMP)", RFC6886, April 2013.

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

12.  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 DRR can be used.  It summarizes some methods which can
   be used for a peer to determine its own network location compared

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   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.  This document addresses the UNSAF [RFC3424] concerns by
   specifying a fallback plan to SRR [p2psip-base-draft].  SRR is not an
   UNSAF mechanism.  The document does not define any new UNSAF

   For DRR to function correctly, a peer may attempt to determine
   whether it is publicly reachable.  If it is not, 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 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 use DRR for

   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.

   In the following sections, we first introduce several ways for a peer
   to get the addresses needed for further tests.  Then a test for
   learning whether a peer may be publicly reachable is proposed.

A.1.  Getting addresses to be used as candidates for DRR

   In order to test whether a peer may be publicly reachable, the peer
   should first get one or more addresses which will be used by other
   peers to send him messages directly.  This address is either a local
   address of a peer or a translated address which is assigned by a NAT
   to the peer.

   STUN is used to get a reflexive address on the public side of a NAT
   with the help of STUN servers.  Discovery of NAT behavior using STUN
   is specified in [RFC5780].  Under RELOAD architecture, a few
   infrastructure servers can be leveraged for discovering NAT behavior,
   such as enrollment servers, diagnostic servers, bootstrap servers,

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   The peer 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 peer can
   determine that there likely is no NAT between it 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
   peer can be considered publicly reachable.  On the other hand, with
   the techniques in [RFC5780], a peer can also decide whether it is
   behind a NAT with endpoint-independent mapping behavior.  If the peer
   is behind a NAT with endpoint- independent mapping behavior, the
   reflexive address should also be a candidate for further tests.

   UPnP-IGD [IGD2] is a mechanism that a peer can use to get the
   assigned address from its residential gateway and after obtaining
   this address to communicate it with other peers, the peer 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 peer behind NAT can use to learn its assigned
   address by NAT is NAT-PMP [RFC6886].  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.

A.2.  Public reachability test

   Using the transport addresses obtained by the above techniques, a
   peer can start a test to learn whether the candidate transport
   address is publicly reachable.  The basic idea for the test is for a
   peer to send a request and expect another peer in the overlay to send
   back a response.  If the response is received by the sending peer
   successfully and also the peer giving the response has no direct
   connection with the sending peer, the sending peer can determine that
   the address is probably publicly reachable and hence the peer may be
   publicly reachable at the tested transport address.

   In a 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 peer.  Especially in
   RELOAD architecture, every peer maintains a connection table.  So it
   is easier for a peer to check whether it has direct connection with
   another peer.

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   If a peer 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 peer, instead it is to get a peer who can deliver
   the sending peer 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 peer 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 transactions.

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

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