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rfc:rfc7263

Internet Engineering Task Force (IETF) N. Zong Request for Comments: 7263 X. Jiang Category: Standards Track R. Even ISSN: 2070-1721 Huawei Technologies

                                                              Y. Zhang
                                                CoolPad / China Mobile
                                                             June 2014

An Extension to the REsource LOcation And Discovery (RELOAD) Protocol

                 to Support Direct Response Routing

Abstract

 This document defines an optional extension to the REsource LOcation
 And Discovery (RELOAD) protocol to support the direct response
 routing mode.  RELOAD recommends symmetric recursive routing for
 routing messages.  The new optional extension provides a shorter
 route for responses, thereby reducing overhead on intermediate peers.
 This document also describes potential cases where this extension can
 be used.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7263.

Zong, et al. Standards Track [Page 1] RFC 7263 P2PSIP DRR June 2014

Copyright Notice

 Copyright (c) 2014 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
 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.

Zong, et al. Standards Track [Page 2] RFC 7263 P2PSIP DRR June 2014

Table of Contents

 1. Introduction ....................................................4
 2. Terminology .....................................................4
 3. Overview ........................................................5
    3.1. SRR and DRR ................................................5
         3.1.1. Symmetric Recursive Routing (SRR) ...................6
         3.1.2. Direct Response Routing (DRR) .......................6
    3.2. Scenarios Where DRR Can Be Used ............................7
         3.2.1. Managed or Closed P2P Systems .......................7
         3.2.2. Wireless Scenarios ..................................8
 4. Relationship between SRR and DRR ................................8
    4.1. How DRR Works ..............................................8
    4.2. How SRR and DRR Work Together ..............................8
 5. DRR Extensions to RELOAD ........................................9
    5.1. Basic Requirements .........................................9
    5.2. Modification to RELOAD Message Structure ...................9
         5.2.1. State-Keeping Flag ..................................9
         5.2.2. Extensive Routing Mode .............................10
    5.3. Creating a Request ........................................11
         5.3.1. Creating a Request for DRR .........................11
    5.4. Request and Response Processing ...........................11
         5.4.1. Destination Peer: Receiving a Request and
                Sending a Response .................................11
         5.4.2. Sending Peer: Receiving a Response .................12
 6. Overlay Configuration Extension ................................12
 7. Security Considerations ........................................12
 8. IANA Considerations ............................................13
    8.1. A New RELOAD Forwarding Option ............................13
    8.2. A New IETF XML Registry ...................................13
 9. Acknowledgments ................................................13
 10. References ....................................................13
    10.1. Normative References .....................................13
    10.2. Informative References ...................................14
 Appendix A. Optional Methods to Investigate Peer Connectivity .....15
   A.1. Getting Addresses to Be Used as Candidates for DRR .........15
   A.2. Public Reachability Test ...................................16
 Appendix B. Comparison of Cost of SRR and DRR .....................17
   B.1. Closed or Managed Networks .................................17
   B.2. Open Networks ..............................................19

Zong, et al. Standards Track [Page 3] RFC 7263 P2PSIP DRR June 2014

1. Introduction

 The REsource LOcation And Discovery (RELOAD) protocol [RFC6940]
 recommends symmetric recursive routing (SRR) for routing messages and
 describes the extensions that would be required to support additional
 routing algorithms.  In addition to SRR, two other routing options --
 direct response routing (DRR) and relay peer routing (RPR) -- are
 also discussed in Appendix A of [RFC6940].  As we show in Section 3,
 DRR is advantageous over SRR in some scenarios in that DRR can reduce
 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 provide benefits than if SRR
 is used alone.
 Note that in this document we focus on the DRR mode and its
 extensions to RELOAD to produce a standalone solution.  Please refer
 to [RFC7264] for details on the RPR mode.
 We first discuss the problem statement in Section 3.  How to combine
 DRR and SRR is presented in Section 4.  An extension to RELOAD to
 support DRR is defined in Section 5.  Some optional methods to check
 peer connectivity are introduced in Appendix A.  In Appendix B, we
 give a comparison of the cost of SRR and DRR in both managed and open
 networks.

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 terminology and definitions from the base RELOAD specification
 [RFC6940] extensively in this document.  We also use terms defined in
 the NAT behavior discovery document [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 network.

Zong, et al. Standards Track [Page 4] RFC 7263 P2PSIP DRR June 2014

    Direct Response Routing (DRR): "DRR" refers to a routing mode in
    which responses to Peer-to-Peer SIP (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 in the rest of this
    document.
    Symmetric Recursive Routing (SRR): "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 in the rest of this document.
    Relay Peer Routing (RPR): "RPR" refers to a routing mode in which
    responses to P2PSIP requests are sent by the destination peer to
    the transport address of a relay peer that will forward the
    responses towards the sending peer.  For simplicity, the
    abbreviation "RPR" is used in the rest of this 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 small-scale closed networks.  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 illustrations that follow:
 1)  Peer A sends a request destined to a peer who is the responsible
     peer for a Resource-ID k.
 2)  Peer X is the root peer responsible for Resource-ID k.
 3)  The intermediate peers for the path from A to X are peers B, C,
     and D.

Zong, et al. Standards Track [Page 5] RFC 7263 P2PSIP DRR June 2014

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 [RFC6940].  As a result, the
 destination peer X will know the exact path that 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    |            |
       |            |            |----------->|            |
       |            |            |            | Request    |
       |            |            |            |----------->|
       |            |            |            |            |
       |            |            |            |  Response  |
       |            |            |            |<-----------|
       |            |            |  Response  |            |
       |            |            |<-----------|            |
       |            |  Response  |            |            |
       |            |<-----------|            |            |
       |  Response  |            |            |            |
       |<-----------|            |            |            |
       |            |            |            |            |
                          Figure 1: SRR 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 peers 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 are limited.
 For example, in the absence of NATs, or if the NAT implements

Zong, et al. Standards Track [Page 6] RFC 7263 P2PSIP DRR June 2014

 endpoint-independent filtering, this is the optimal routing
 technique.  Note that establishing a secure connection requires
 multiple round trips.  Please refer to Appendix B for a cost
 comparison between SRR and DRR.
         A            B            C             D           X
         |  Request   |            |            |            |
         |----------->|            |            |            |
         |            | Request    |            |            |
         |            |----------->|            |            |
         |            |            | Request    |            |
         |            |            |----------->|            |
         |            |            |            | Request    |
         |            |            |            |----------->|
         |            |            |            |            |
         |            |            |            |  Response  |
         |<-----------+------------+------------+------------|
         |            |            |            |            |
                          Figure 2: DRR 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.  During the process of establishing a direct connection with
 the sending peer, if the responding peer receives a request with SRR
 as the preferred routing mode (or it fails to establish the direct
 connection), the responding peer SHOULD NOT use DRR but instead
 switch to SRR.  The simple policy is to try DRR and, if this fails,
 switch to SRR for all connections.  In a finer-grained policy, a peer
 would keep a list of unreachable peers based on trying DRR and then
 would use only SRR for those peers.  The advantage of using DRR is
 network stability, since it puts less overhead on the intermediate
 peers that will not route the responses.  The intermediate peers will
 need to route fewer messages and will save CPU resources as well as
 link bandwidth usage.

Zong, et al. Standards Track [Page 7] RFC 7263 P2PSIP DRR June 2014

3.2.2. Wireless Scenarios

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

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 (publicly 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.  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 for how to transition
 between the routing modes in RELOAD.
 According to [RFC6940], 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.  That is,
 a node in an overlay that 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 the
 preferred routing mode from the configuration file.  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.  However, DRR SHOULD NOT be used in the open Internet or if the
 administrator does not feel he has enough information about the
 overlay network topology.  A new overlay configuration element
 specifying the usage of DRR is defined in Section 6.
 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 of the success rate of different routing modes.
 In addition to data on the success rate, the peer can also get data
 of finer granularity -- for example, the number of retransmissions
 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
 as indicated by statistics on lost messages or by responses that used
 DRR, the peer can either continue to offer DRR first or switch to

Zong, et al. Standards Track [Page 8] RFC 7263 P2PSIP DRR June 2014

 SRR.  Note that a peer should use the DRR success statistics to
 decide whether to continue using DRR or fall back to SRR.  Making
 such a decision per specific connection is not recommended; this
 should be an application decision.

5. DRR Extensions to RELOAD

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

5.1. Basic Requirements

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

5.2. 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.

5.2.1. 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
 define a new flag, state-keeping flag, in the ForwardingOption
 structure to indicate whether the state-keeping is required in the
 intermediate peers.
 Flag: 0x08 IGNORE-STATE-KEEPING
 If IGNORE-STATE-KEEPING is set, any peer receiving this message but
 who is not the destination of the message SHOULD forward the message
 with the full Via List and SHOULD NOT maintain any internal state.

Zong, et al. Standards Track [Page 9] RFC 7263 P2PSIP DRR June 2014

5.2.2. Extensive Routing Mode

 This document introduces a new forwarding option for an extensive
 routing mode.  This option conforms to the description in
 Section 6.3.2.3 of [RFC6940].
 We first define a new type to define the new option,
 extensive_routing_mode:
 The option value that defines the ExtensiveRoutingModeOption
 structure is illustrated below:
 enum {(0),DRR(1),(255)} RouteMode;
 struct {
         RouteMode               routemode;
         OverlayLinkType         transport;
         IpAddressPort           ipaddressport;
         Destination             destinations<1..2^8-1>;
 } ExtensiveRoutingModeOption;
 The above structure reuses the OverlayLinkType, Destination, and
 IpAddressPort structures as defined in Sections 6.5.1.1, 6.3.2.2, and
 6.3.1.1 of [RFC6940], respectively.
 RouteMode: refers to which type of routing mode is indicated to the
 destination peer.
 OverlayLinkType: refers to the transport type that is used to deliver
 responses from the destination peer to the sending peer.
 IpAddressPort: refers to the transport address that the destination
 peer will use for sending responses.  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-ID.

Zong, et al. Standards Track [Page 10] RFC 7263 P2PSIP DRR June 2014

5.3. Creating a Request

5.3.1. 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 ForwardingOption 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 ForwardingOption 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.

5.4. 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 [RFC6940] for RELOAD base
 procedures.

5.4.1. 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 cannot
 understand the extensive_routing_mode option in the request, it MUST
 attempt to use SRR to return an "Error_Unknown_Extension" response
 (defined in Sections 6.3.3.1 and 14.9 of [RFC6940]) to the sending
 peer.
 If the routing mode is DRR, the destination peer MUST construct the
 Destination List for the response with only one entry, using the
 requesting peer's Node-ID from the Via List in the request as the
 value.

Zong, et al. Standards Track [Page 11] RFC 7263 P2PSIP DRR June 2014

 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 Sections 6.3.3.1 and
 14.9 of [RFC6940]) 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 respond
 to now, the responding peer SHOULD abort the DRR response and
 use SRR.

5.4.2. Sending Peer: Receiving a Response

 Upon receiving a response, the peer follows the rules in [RFC6940].
 The peer SHOULD note if DRR worked, in order to decide whether to
 offer DRR again.  If the peer does not receive a response until the
 timeout, it SHOULD resend the request using SRR.

6. Overlay Configuration Extension

 This document extends the RELOAD overlay configuration (see
 Section 11.1 of [RFC6940]) by adding one new element, "route-mode",
 inside each "configuration" element.
 The Compact Regular Language for XML Next Generation (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.
 The mode can be DRR or RPR and, if specified in the configuration,
 should be the preferred routing mode used by the application.

7. Security Considerations

 The normative security recommendations of Section 13 of [RFC6940] are
 applicable to this document.  As a routing alternative, the security
 part of DRR conforms to Section 13.6 of [RFC6940], which describes
 routing security.  For example, the DRR routing option provides
 information about the route back to the source.  According to

Zong, et al. Standards Track [Page 12] RFC 7263 P2PSIP DRR June 2014

 Section 13.6 of [RFC6940], the entire DRR routing message MUST be
 digitally signed and sent over via a protected channel to protect the
 DRR routing information.

8. IANA Considerations

8.1. A New RELOAD Forwarding Option

 A new RELOAD Forwarding Option type has been added to the "RELOAD
 Forwarding Option" registry defined in [RFC6940].
 Code: 2
 Forwarding Option: extensive_routing_mode

8.2. A New IETF XML Registry

 IANA has registered the following URN in the "XML Namespaces" class
 of the "IETF XML Registry" in accordance with [RFC3688].
 URI: urn:ietf:params:xml:ns:p2p:route-mode
 Registrant Contact: The IESG
 XML: This specification

9. Acknowledgments

 David Bryan 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.

10. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
            January 2004.
 [RFC6940]  Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
            H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
            Base Protocol", RFC 6940, January 2014.

Zong, et al. Standards Track [Page 13] RFC 7263 P2PSIP DRR June 2014

10.2. Informative References

 [Chord]    Stoica, I., Morris, R., Liben-Nowell, D., Karger, D.,
            Kaashoek, M., Dabek, F., and H. Balakrishnan, "Chord: A
            Scalable Peer-to-Peer Lookup Protocol for Internet
            Applications", IEEE/ACM Transactions on Networking
            Volume 11, Issue 1, 17-32, February 2003.
 [DTLS]     Modadugu, N. and E. Rescorla, "The Design and
            Implementation of Datagram TLS", Proc. 11th Network and
            Distributed System Security Symposium (NDSS),
            February 2004.
 [IGD2]     UPnP Forum, "WANIPConnection:2 Service", September 2010,
            <http://upnp.org/specs/gw/
            UPnP-gw-WANIPConnection-v2-Service.pdf>.
 [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
            Self-Address Fixing (UNSAF) Across Network Address
            Translation", RFC 3424, November 2002.
 [RFC5780]  MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
            Using Session Traversal Utilities for NAT (STUN)",
            RFC 5780, May 2010.
 [RFC6886]  Cheshire, S. and M. Krochmal, "NAT Port Mapping Protocol
            (NAT-PMP)", RFC 6886, April 2013.
 [RFC7264]  Zong, N., Jiang, X., Even, R., and Y. Zhang, "An Extension
            to the REsource LOcation And Discovery (RELOAD) Protocol
            to Support Relay Peer Routing", RFC 7264, June 2014.
 [wikiChord]
            Wikipedia, "Chord (peer-to-peer)", 2013,
            <http://en.wikipedia.org/w/
            index.php?title=Chord_%28peer-to-peer%29&oldid=549516287>.

Zong, et al. Standards Track [Page 14] RFC 7263 P2PSIP DRR June 2014

Appendix A. Optional Methods to Investigate Peer Connectivity

 This section is for informational purposes only and provides some
 mechanisms that can be used when the configuration information does
 not specify if DRR can be used.  It summarizes some methods that can
 be used by 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
 whether a peer is publicly reachable, other than via out-of-band
 mechanisms.  This document addresses UNilateral Self-Address Fixing
 (UNSAF) [RFC3424] considerations by specifying a fallback plan to SRR
 [RFC6940].  SRR is not an UNSAF mechanism.  This document does not
 define any new UNSAF mechanisms.
 For DRR to function correctly, a peer may attempt to determine
 whether it is publicly reachable.  If it is not, the peer should fall
 back to SRR.  If the peer believes it is publicly reachable, DRR may
 be attempted.  NATs and firewalls are two major contributors to
 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 peer to which the address belongs can use DRR for
 responses.
 Some conditions that are unique in P2PSIP architecture could be
 leveraged to facilitate the tests.  In a P2P overlay network, each
 peer has only a partial 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 that 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 that is assigned by a NAT
 to the peer.

Zong, et al. Standards Track [Page 15] RFC 7263 P2PSIP DRR June 2014

 Session Traversal Utilities for NAT (STUN) is used to get a reflexive
 address on the public side of a NAT with the help of STUN servers.
 NAT behavior discovery using STUN is specified in [RFC5780].  Under
 the RELOAD architecture, a few infrastructure servers can be
 leveraged for discovering NAT behavior, such as enrollment servers,
 diagnostic servers, bootstrap servers, etc.
 The peer can use a STUN Binding request to one of the 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 is likely 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 servers are publicly reachable in the overlay, so the
 peer can be considered publicly reachable.  On the other hand, using
 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.
 The Universal Plug and Play Internet Gateway Device (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 learn its assigned address by
 NAT is via the NAT Port Mapping Protocol (NAT-PMP) [RFC6886].  As
 with 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 of the test is that a
 peer sends a request and expects another peer in the overlay to send
 back a response.  If the response is successfully received by the
 sending peer and the peer giving the response has no direct

Zong, et al. Standards Track [Page 16] RFC 7263 P2PSIP DRR June 2014

 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.  Every peer
 maintains a connection table, particularly in the RELOAD
 architecture, so it is easier for a peer to see whether it has direct
 connection with another peer.
 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 or fetching something to or from the overlay,
 or for locating a specific peer; instead, it is to get a peer who can
 deliver to the sending peer an unsolicited response and who has no
 direct connection with him.  Each intermediate peer receiving the
 request first checks to see whether it has a direct connection 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.

Appendix B. Comparison of Cost of SRR and DRR

 The major advantage of using DRR is that it reduces the number of
 intermediate peers traversed by the response.  This reduces the load,
 such as processing and communication bandwidth, on those peers'
 resources.

B.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 network
 administrators would know that they could safely use DRR.
 SRR uses more routing hops than DRR.  Assuming that there are N peers
 in the P2P system and Chord [Chord] [wikiChord] is applied for
 routing, the number of hops for a response in SRR and in DRR are
 listed in the following table.  Establishing a secure connection
 between the sending peer and the responding peer with Transport Layer
 Security (TLS) or Datagram TLS (DTLS) requires multiple messages.
 Note that establishing (D)TLS secure connections for a P2P overlay is

Zong, et al. Standards Track [Page 17] RFC 7263 P2PSIP DRR June 2014

 not optimal in some cases, e.g., DRR where (D)TLS is heavy for
 temporary connections.  Therefore, in the following table we show the
 cases of 1) no (D)TLS in DRR and 2) still using DTLS in DRR as
 sub-optimal.  As the worst-cost case, seven (7) messages are used
 during DTLS handshaking [DTLS].  (The TLS handshake is a negotiation
 protocol that requires two (2) round trips, while the DTLS handshake
 is a negotiation protocol that requires three (3) round trips.)
          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 the number of peers (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 such resources
     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 SRR.
 3)  In the cases when N < 256, DRR uses more messages than SRR (but
     still uses fewer hops than SRR), so the consideration of whether
     to use DRR or SRR depends on other factors such as using less
     resources (bandwidth and processing) from the intermediate peers.
     Section 4 provides use cases where DRR has a better chance of
     working or where the considerations of intermediary resources are
     important.

Zong, et al. Standards Track [Page 18] RFC 7263 P2PSIP DRR June 2014

B.2. Open Networks

 In open networks (e.g., the Internet) 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 as those listed in
 Appendix B.1, the number of hops, as well as the 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             |   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) and 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 improves as the scale
 of the network increases.

Zong, et al. Standards Track [Page 19] RFC 7263 P2PSIP DRR June 2014

Authors' Addresses

 Ning Zong
 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
 CoolPad / China Mobile
 EMail: hishigh@gmail.com

Zong, et al. Standards Track [Page 20]

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