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P2PSIP                                                           N. Zong
Internet-Draft                                                  X. Jiang
Intended status: Standards Track                                 R. Even
Expires: March 22, 2012                              Huawei Technologies
                                                                Y. Zhang
                                                            China Mobile
                                                      September 19, 2011


       An extension to RELOAD to support Direct Response Routing
                        draft-zong-p2psip-drr-01

Abstract

   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 intermediary
   peers and describes the potential cases where this extension 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 March 22, 2012.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
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   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
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   This document may contain material from IETF Documents or IETF
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Backgrounds  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.1.  Symmetric Recursive Routing (SRR)  . . . . . . . . . .  5
       3.1.2.  Direct Response Routing (DRR)  . . . . . . . . . . . .  6
     3.2.  Scenarios Where DRR can be Used  . . . . . . . . . . . . .  7
       3.2.1.  Managed or Closed P2P System . . . . . . . . . . . . .  7
       3.2.2.  Wireless Scenarios . . . . . . . . . . . . . . . . . .  7
   4.  Relationship Between SRR and DRR . . . . . . . . . . . . . . .  8
     4.1.  How DRR Works  . . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  How SRR and DRR Work Together  . . . . . . . . . . . . . .  8
   5.  Comparison on cost of SRR and DRR  . . . . . . . . . . . . . .  8
     5.1.  Closed or managed networks . . . . . . . . . . . . . . . .  9
     5.2.  Open networks  . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Extensions to RELOAD . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Basic Requirements . . . . . . . . . . . . . . . . . . . . 10
     6.2.  Modification To RELOAD Message Structure . . . . . . . . . 11
       6.2.1.  State-keeping Flag . . . . . . . . . . . . . . . . . . 11
       6.2.2.  Extensive Routing Mode . . . . . . . . . . . . . . . . 11
     6.3.  Creating a Request . . . . . . . . . . . . . . . . . . . . 12
       6.3.1.  Creating a Request for DRR . . . . . . . . . . . . . . 12
     6.4.  Request And Response Processing  . . . . . . . . . . . . . 12
       6.4.1.  Destination Peer: Receiving a Request And Sending
               a Response . . . . . . . . . . . . . . . . . . . . . . 13
       6.4.2.  Sending Peer: Receiving a Response . . . . . . . . . . 13
   7.  Optional Methods to Investigate Node Connectivity  . . . . . . 13
     7.1.  Getting Addresses To Be Used As Candidates for DRR . . . . 14
     7.2.  Public Reacheability Test  . . . . . . . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  A new RELOAD Forwarding Option . . . . . . . . . . . . . . 16
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     11.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17











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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 is advantageous over SRR
   in some scenarios by 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.  In other
   scenarios, using a combination of DRR and SRR 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].

   Note that in this draft, we focus on DRR routing mode and its
   extensions to RELOAD to produce a standalone solution.  Please refer
   to RPR draft [I-D.zong-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 node connectivity is
   introduced in Section 7, as informational text.


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.




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   Publicly Reachable: A node is publicly reachable if it can receive
   unsolicited messages from any other node in the same overlay.  Note:
   "publicly" does not mean that the nodes 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 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 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 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-



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   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 establishing a secure connection requires multiple round trips.
   Please refer to Section 5 for cost comparison between SRR and DRR.









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

4.2.  How SRR and DRR Work Together

   DRR is not intended to replace SRR.  As seen from Section 3, DRR has
   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 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.

   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.  Based on the success rate as seen from the lost
   message statistics or responses that used DRR, the node can either
   continue to offer DRR first or switch to SRR.

   The node can decide whether to try DRR 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.


5.  Comparison on cost of SRR and DRR

   The major advantages in using DRR 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.







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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.

   SRR brings out more routing hops than DRR.  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 and DRR are listed in the following
   table.  Establishing a secure connection between sending 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; 2) still using DTLS in DRR 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
     DRR(DTLS) |  Yes    |     1       |    7+1

   From the above comparison, it is clear that:

   1) In most cases of N > 2 (2^1), DRR has fewer hops than SRR.
   Shorter route means less overhead and resource usage on intermediary
   peers, which is an important consideration for adopting DRR 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 > 256 (2^8), DRR also has fewer messages than
   SRR.

   3) In the cases where N < 256, DRR has more messages than SRR (but
   still has fewer hops than SRR).  So the consideration to use DRR or
   SRR depends on other factors like using less resources (bandwidth and
   processing) from the intermediaries 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 network where DRR is not guaranteed, 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 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
     DRR(DTLS) |         Yes           |     1       |    7+1
               | Fail&Fall back to SRR |     1+logN  |    8+logN

   From the above comparison, it can be observed that:

   1) Trying DRR 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.  Therefore, the chance of trying DRR with fewer hops
   than SRR becomes better as the scale of the network increases.

   2) In the cases of large network and the success rate of DRR is good,
   it is still possible that DRR has fewer messages than SRR.
   Otherwise, the consideration to use DRR or SRR depends on other
   factors like using less resources from the intermediaries peers.


6.  Extensions to RELOAD

   Adding support for DRR 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 MUST support SRR.

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






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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 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 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 keeping is not required in the
   intermediate peers.

   flag : 0x8 IGNORE-STATE-KEEPING

   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
   state.

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..2^8-1>;
   } 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-



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   base].

   Route mode: refers to which type of routing mode is indicated to the
   destination peer.  Currently, only DRR and RPR (specified in RPR
   draft [I-D.zong-p2psip-rpr]) are defined.

   Transport: 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 should use to send the response to.  This will be a sending node
   address for DRR.

   Destination: refers to the sending node itself.  If the routing mode
   is DRR, then the destination only contains 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
   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 value, 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 is
   introduced.  Here, we only describe the additional procedures for
   supporting DRR.  Please refer to [I-D.ietf-p2psip-base] for RELOAD
   base procedures.



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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 an "Error_Unknown_Extension" response
   (defined in Section 5.3.3.1 and Section 13.9 in [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 5.3.3.1 and
   Section 13.9 in [I-D.ietf-p2psip-base]) 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 it is trying to
   response to now, the responding peer should abort the DRR 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.


7.  Optional Methods to Investigate Node 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 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.




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   For DRR to function correctly, a node 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 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.

   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.

7.1.  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



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   node can be considered publicly reachable.  On the other hand, with
   the techniques in [I-D.ietf-behave-nat-behavior-discovery], a node
   can also decide whether it is behind NAT with endpoint-independent
   mapping behavior.  If the node is behind a NAT with endpoint-
   independent mapping behavior, the reflexive address should also be a
   candidate for further tests.

   UPnP-IGD is a mechanism that a node can use to get the assigned
   address from its residential gateway and after obtaining this address
   to communicate it with other nodes, the node can receive unsolicited
   messages from outside, even though it is behind a NAT.  So the
   address obtained through the UPnP mechanism should also be used for
   further tests.

   Another way that a node behind NAT can use to learn its assigned
   address by NAT is NAT-PMP.  Like in UPnP-IGD, the address obtained
   using this mechanism should also be tested further.

   The above techniques are not exhaustive.  These techniques can be
   used to get candidate transport addresses for further tests.

7.2.  Public Reacheability 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.
   The following text is informative.




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   If a node wants to test whether its transport address is publicly
   reachable, it can send a request to the overlay.  The routing for the
   test message would be different from other kinds of requests because
   it is not for storing/fetching something to/from the overlay or
   locating a specific node, instead it is to get a peer who can deliver
   the sending node an unsolicited response and which has no direct
   connection with him.  Each intermediate peer receiving the request
   first checks whether it has a direct connections with the sending
   peer.  If there is a direct connection, the request is routed to the
   next peer.  If there is no direct connection, the intermediate peer
   terminates the request and sends the response back directly to the
   sending node with the transport address under test.

   After performing the test, if the peer determines that it may be
   publicly reachable, it can try DRR in subsequent transaction.


8.  Security Considerations

   TBD


9.  IANA Considerations

9.1.  A new RELOAD Forwarding Option

   A new RELOAD Forwarding Option type is add to the Registry.

   Type: 0x2 - extensive_routing_mode


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 and Bruce Lowekamp for their constructive comments.


11.  References

11.1.  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-18 (work in
   progress), August 2011.




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   [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-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.

11.2.  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.

   [I-D.zong-p2psip-rpr] Zong, N., Jiang, X., Even, R. and Zhang, Y.,
   "An extension to RELOAD to support Relay Peer Routing",
   draft-zong-p2psip-rpr-01, September 2011.


Authors' Addresses

   Ning Zong
   Huawei Technologies

   Email: zongning@huawei.com




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   Xingfeng Jiang
   Huawei Technologies

   Email: jiang.x.f@huawei.com


   Roni Even
   Huawei Technologies

   Email: even.roni@huawei.com


   Yunfei Zhang
   China Mobile

   Email: zhangyunfei@chinamobile.com



































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