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rfc:rfc7264

Internet Engineering Task Force (IETF) N. Zong Request for Comments: 7264 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 Relay Peer Routing

Abstract

 This document defines an optional extension to the REsource LOcation
 And Discovery (RELOAD) protocol to support the relay peer 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/rfc7264.

Zong, et al. Standards Track [Page 1] RFC 7264 P2PSIP RPR 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 7264 P2PSIP RPR June 2014

Table of Contents

 1. Introduction ....................................................3
 2. Terminology .....................................................4
 3. Overview ........................................................5
    3.1. RPR ........................................................5
    3.2. Scenarios Where RPR Can Be Used ............................6
         3.2.1. Managed or Closed P2P Systems .......................6
         3.2.2. Using Bootstrap Nodes as Relay Peers ................7
         3.2.3. Wireless Scenarios ..................................7
 4. Relationship between SRR and RPR ................................7
    4.1. How RPR Works ..............................................7
    4.2. How SRR and RPR Work Together ..............................7
 5. RPR Extensions to RELOAD ........................................8
    5.1. Basic Requirements .........................................8
    5.2. Modification to RELOAD Message Structure ...................8
         5.2.1. Extensive Routing Mode ..............................8
    5.3. Creating a Request .........................................9
         5.3.1. Creating a Request for RPR ..........................9
    5.4. Request and Response Processing ............................9
         5.4.1. Destination Peer: Receiving a Request and
                Sending a Response ..................................9
         5.4.2. Sending Peer: Receiving a Response .................10
         5.4.3. Relay Peer Processing ..............................10
 6. Overlay Configuration Extension ................................10
 7. Discovery of Relay Peers .......................................11
 8. Security Considerations ........................................11
 9. IANA Considerations ............................................11
    9.1. A New RELOAD Forwarding Option ............................11
 10. Acknowledgments ...............................................11
 11. References ....................................................12
    11.1. Normative References .....................................12
    11.2. Informative References ...................................12
 Appendix A. Optional Methods to Investigate Peer Connectivity .....13
 Appendix B. Comparison of Cost of SRR and RPR .....................14
   B.1. Closed or Managed Networks .................................14
   B.2. Open Networks ..............................................15

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,
 RPR is advantageous over SRR in some scenarios in that RPR can reduce
 load (CPU and link bandwidth) on intermediate peers.  RPR works
 better in a network where relay peers are provisioned in advance so

Zong, et al. Standards Track [Page 3] RFC 7264 P2PSIP RPR June 2014

 that relay peers are publicly reachable in the P2P system.  In other
 scenarios, using a combination of RPR and SRR together is more likely
 to provide benefits than if SRR is used alone.
 Note that in this document we focus on the RPR mode and its
 extensions to RELOAD to produce a standalone solution.  Please refer
 to [RFC7263] for details on the DRR mode.
 We first discuss the problem statement in Section 3.  How to combine
 RPR and SRR is presented in Section 4.  An extension to RELOAD to
 support RPR is defined in Section 5.  Discovery of relay peers is
 introduced in Section 7.  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 RPR 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.
    Relay Peer: A relay peer is a type of publicly reachable peer that
    can receive unsolicited messages from all other peers in the
    overlay and forward the responses from destination peers towards
    the sender of the request.
    Relay Peer Routing (RPR): "RPR" refers to a routing mode in which
    responses to Peer-to-Peer SIP (P2PSIP) requests are sent by the
    destination peer to a relay peer transport address that will
    forward the responses towards the sending peer.  For simplicity,
    the abbreviation "RPR" is used in the rest of this document.

Zong, et al. Standards Track [Page 4] RFC 7264 P2PSIP RPR June 2014

    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.
    Direct Response Routing (DRR): "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 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
 RPR may work better in some specific scenarios.

3.1. RPR

 RELOAD is a simple request-response protocol.  After sending a
 request, a peer waits for a response from a destination peer.  There
 are several ways for the destination peer to send a response back to
 the source peer.  In this section, we will provide detailed
 information on RPR.  Note that the same types of illustrative
 settings can be found in Appendix B.1 of [RFC7263].
 If peer A knows it is behind a NAT or NATs and knows one or more
 relay peers with whom they have had prior connections, peer A can try
 RPR.  Assume that peer A is associated with relay peer R.  When
 sending the request, peer A includes information describing peer R's
 transport address in the request.  When peer X receives the request,
 peer X sends the response to peer R, which forwards it directly to
 peer A on the existing connection.  Figure 1 illustrates RPR.  Note
 that RPR also allows a shorter route for responses compared to SRR;
 this means less overhead on intermediate peers.  Establishing a
 connection to the relay with Transport Layer Security (TLS) requires
 multiple round trips.  Please refer to Appendix B for a cost
 comparison between SRR and RPR.

Zong, et al. Standards Track [Page 5] RFC 7264 P2PSIP RPR June 2014

   A            B            C             D           X           R
   |  Request   |            |            |            |           |
   |----------->|            |            |            |           |
   |            | Request    |            |            |           |
   |            |----------->|            |            |           |
   |            |            | Request    |            |           |
   |            |            |----------->|            |           |
   |            |            |            | Request    |           |
   |            |            |            |----------->|           |
   |            |            |            |            | Response  |
   |            |            |            |            |---------->|
   |            |            |            |  Response  |           |
   |<-----------+------------+------------+------------+-----------|
   |            |            |            |            |           |
                          Figure 1: RPR Mode
 This technique relies on the relative population of peers such as
 peer A that require relay peers, and peers such as peer R that are
 capable of serving as relay peers.  It also requires a mechanism to
 enable peers to know which peers can be used as their relays.  This
 mechanism may be based on configuration -- for example, as part of
 the overlay configuration, an initial list of relay peers can be
 supplied.  Another option is a response message in which the
 responding peer can announce that it can serve as a relay peer.

3.2. Scenarios Where RPR Can Be Used

 In this section, we will list several scenarios where using RPR would
 improve performance.

3.2.1. Managed or Closed P2P Systems

 As described in Section 3.2.1 of [RFC7263], many P2P systems run in a
 closed or managed environment so that network administrators can
 better manage their system.  For example, the network administrator
 can deploy several relay peers that are publicly reachable in the
 system and indicate their presence in the configuration file.  After
 learning where these relay peers are, peers behind NATs can use RPR
 with help from these relay peers.  Peers MUST also support SRR in
 case RPR fails.
 Another usage is to install relay peers on the managed network
 boundary, allowing external peers to send responses to peers inside
 the managed network.

Zong, et al. Standards Track [Page 6] RFC 7264 P2PSIP RPR June 2014

3.2.2. Using Bootstrap Nodes as Relay Peers

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

3.2.3. Wireless Scenarios

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

4. Relationship between SRR and RPR

4.1. How RPR Works

 Peers using RPR MUST maintain a connection with their relay peer(s).
 This can be done in the same way as establishing a neighbor
 connection between peers using the Attach method [RFC6940].
 A requirement for RPR is that the source peer convey its relay peer's
 (or peers') transport address(es) in the request so the destination
 peer knows where the relay peers are and will send the response to a
 relay peer first.  The request MUST also include the requesting
 peer's Node-ID or IP address, which enables the relay peer to route
 the response back to the right peer.
 Note that being a relay peer does not require that the relay peer
 have more functionality than an ordinary peer.  Relay peers comply
 with the same procedure as an ordinary peer to forward messages.  The
 only difference is that there may be a larger traffic burden on relay
 peers.  Relay peers can decide whether to accept a new connection
 based on their current burden.

4.2. How SRR and RPR Work Together

 RPR is not intended to replace SRR.  It is better to use these two
 modes together to adapt to each peer's specific situation.  Note that
 the informative suggestions for how to transition between SRR and RPR
 are the same as those for DRR.  Please refer to Section 4.2 of
 [RFC7263] for more details.  If a relay peer is provided by the
 service provider, peers SHOULD prefer RPR over SRR.  However, RPR
 SHOULD NOT be used in the open Internet or if the administrator does

Zong, et al. Standards Track [Page 7] RFC 7264 P2PSIP RPR June 2014

 not feel he has enough information about the overlay network
 topology.  A new overlay configuration element specifying the usage
 of RPR is defined in Section 6.

5. RPR Extensions to RELOAD

 Adding support for RPR 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 RPR routing requests.

5.2. Modification to RELOAD Message Structure

 RELOAD provides an extensible framework to accommodate future
 extensions.  In this section, we define an RPR routing option for the
 extensive routing mode specified in [RFC7263].  The state-keeping
 flag [RFC7263] is needed to support the RPR mode.

5.2.1. Extensive Routing Mode

 The new RouteMode value for RPR is defined below for the
 ExtensiveRoutingModeOption structure:
 enum {(0),DRR(1),RPR(2),(255)} RouteMode;
 struct {
         RouteMode               routemode;
         OverlayLinkType         transport;
         IpAddressPort           ipaddressport;
         Destination             destinations<1..2^8-1>;
 } ExtensiveRoutingModeOption;
 Note that the DRR value in RouteMode is defined in [RFC7263].
 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 relay peer.
 IpAddressPort: refers to the transport address that the destination
 peer should use for sending responses.  This will be a relay peer
 address for RPR.

Zong, et al. Standards Track [Page 8] RFC 7264 P2PSIP RPR June 2014

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

5.3. Creating a Request

5.3.1. Creating a Request for RPR

 When using RPR for a transaction, the sending peer MUST set the
 IGNORE-STATE-KEEPING flag in the ForwardingHeader.  Additionally, the
 peer MUST construct and include a 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 0x02 (RPR).
     2.2)  transport set as appropriate for the relay peer.
     2.3)  ipaddressport set to the transport address of the relay
           peer through which the sender wishes the message relayed.
     2.4)  The destination structure MUST contain two values.  The
           first MUST be defined as type "node" and set with the
           values for the relay peer.  The second MUST be defined as
           type "node" and set with the sending peer's own values.

5.4. Request and Response Processing

 This section gives normative text for message processing after RPR is
 introduced.  Here, we only describe the additional procedures for
 supporting RPR.  Please refer to [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.

Zong, et al. Standards Track [Page 9] RFC 7264 P2PSIP RPR June 2014

 If the routing mode is RPR, the destination peer MUST construct a
 destination_list for the response with two entries as defined in
 [RFC6940].  The first entry MUST be set to the relay peer's Node-ID
 from the option in the request, and the second entry MUST be the
 sending peer's Node-ID from the option in the request.
 In the event that the routing mode is set to RPR and there are not
 exactly two destinations, the destination peer MUST try to send an
 "Error_Unknown_Extension" response (defined in 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 RPR response and
 use SRR.

5.4.2. Sending Peer: Receiving a Response

 Upon receiving a response, the peer follows the rules in [RFC6940].
 If the sender used RPR and did not get a response until the timeout,
 it MAY resend the message using either RPR (but with a different
 relay peer, if available) or SRR.

5.4.3. Relay Peer Processing

 Relay peers are designed to forward responses to peers who are not
 publicly reachable.  For the routing of the response, this document
 still uses the destination_list.  The only difference from SRR is
 that the destination_list is not the reverse of the via_list.
 Instead, it is constructed from the forwarding option as described
 below.
 When a relay peer receives a response, it MUST follow the rules in
 [RFC6940].  It receives the response, validates the message,
 readjusts the destination_list, and forwards the response to the next
 hop in the destination_list based on the connection table.  There is
 no added requirement for the relay peer.

6. Overlay Configuration Extension

 This document uses the new RELOAD overlay configuration element,
 "route-mode", inside each "configuration" element, as defined in
 Section 6 of [RFC7263].  The route mode MUST be "RPR".

Zong, et al. Standards Track [Page 10] RFC 7264 P2PSIP RPR June 2014

7. Discovery of Relay Peers

 There are several ways to distribute information about relay peers
 throughout the overlay.  P2P network providers can deploy some relay
 peers and advertise them in the configuration file.  With the
 configuration file at hand, peers can get relay peers to try RPR.
 Another way is to consider the relay peer as a service; some service
 advertisement and discovery mechanism can then also be used for
 discovering relay peers -- for example, using the same mechanism as
 that used in Traversal Using Relays around NAT (TURN) server
 discovery as discussed in [RFC6940].  Another option is to let a peer
 advertise its capability to be a relay in the response to an Attach
 or Join [RFC6940].

8. Security Considerations

 The normative security recommendations of Section 13 of [RFC6940] are
 applicable to this document.  As a routing alternative, the security
 part of RPR conforms to Section 13.6 of [RFC6940], which describes
 routing security.  RPR behaves like a DRR requesting node towards the
 destination node.  The RPR relay peer is not necessarily an arbitrary
 node -- for example, a managed network, a bootstrap node, or a
 configured relay peer; it should be a trusted node, because a trusted
 node will be less of a risk, as outlined in Section 13 of [RFC6940].
 In order to address possible DoS attacks, the relay peer SHOULD also
 limit the number of maximum connections; this is required in order to
 also reduce load on the relay peer, as explained in Section 4.1.

9. IANA Considerations

9.1. 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

10. 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.

Zong, et al. Standards Track [Page 11] RFC 7264 P2PSIP RPR June 2014

11. References

11.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC6940]  Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
            H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
            Base Protocol", RFC 6940, January 2014.
 [RFC7263]  Zong, N., Jiang, X., Even, R., and Y. Zhang, "An Extension
            to the REsource LOcation And Discovery (RELOAD) Protocol
            to Support Direct Response Routing", RFC 7263, June 2014.

11.2. Informative References

 [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.

Zong, et al. Standards Track [Page 12] RFC 7264 P2PSIP RPR 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 RPR 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 RPR to function correctly, a peer may attempt to determine
 whether it is publicly reachable.  If it is not, RPR may be chosen to
 route the response with help from relay peers, or the peers should
 fall back to SRR.  NATs and firewalls are two major contributors to
 preventing RPR from functioning properly.  There are a number of
 techniques by which a peer can get its reflexive address on the
 public side of the NAT.  After obtaining the reflexive address, a
 peer can perform further tests to learn whether the reflexive address
 is publicly reachable.  If the address appears to be publicly
 reachable, the peer to which the address belongs can be a candidate
 to serve as a relay peer.  Peers that are not publicly reachable may
 still use RPR to shorten the response path, with help from relay
 peers.
 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.
 The approaches for a peer to get the addresses needed for further
 tests, as well as the test for learning whether a peer may be
 publicly reachable, are the same as those for DRR.  Please refer to
 Appendix A of [RFC7263] for more details.

Zong, et al. Standards Track [Page 13] RFC 7264 P2PSIP RPR June 2014

Appendix B. Comparison of Cost of SRR and RPR

 The major advantage of using RPR 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 RPR.
 The number of hops for a response in SRR and in RPR are listed in the
 following table.  Note that the same types of illustrative settings
 can be found in Appendix B.1 of [RFC7263].
         Mode       | Success | No. of Hops | No. of Msgs
         ------------------------------------------------
         SRR        |  Yes    |     log(N)  |    log(N)
         RPR        |  Yes    |     2       |    2
         RPR (DTLS) |  Yes    |     2       |    7+2
      Table 1: Comparison of SRR and RPR in Closed Networks
 From the above comparison, it is clear that:
 1)  In most cases when the number of peers (N) > 4 (2^2), RPR uses
     fewer hops than SRR.  Using a shorter route means less overhead
     and resource usage on intermediate peers, which is an important
     consideration for adopting RPR in the cases where such resources
     as CPU and bandwidth are limited, e.g., the case of mobile,
     wireless networks.
 2)  In the cases when N > 512 (2^9), RPR also uses fewer messages
     than SRR.
 3)  In the cases when N < 512, RPR uses more messages than SRR (but
     still uses fewer hops than SRR), so the consideration of whether
     to use RPR or SRR depends on other factors such as using less
     resources (bandwidth and processing) from the intermediate peers.
     Section 4 provides use cases where RPR has a better chance of
     working or where the considerations of intermediary resources are
     important.

Zong, et al. Standards Track [Page 14] RFC 7264 P2PSIP RPR June 2014

B.2. Open Networks

 In open networks (e.g., the Internet) where RPR is not guaranteed to
 work, RPR can fall back to SRR if it fails after trial, as described
 in Section 4.2.  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 RPR, are listed in the following table:
  Mode       |          Success        | No. of Hops | No. of Msgs
  ----------------------------------------------------------------
  SRR        |         Yes             |   log(N)    |   log(N)
  RPR        |         Yes             |   2         |   2
             | Fail & fall back to SRR |   2+log(N)  |   2+log(N)
  RPR (DTLS) |         Yes             |   2         |   7+2
             | Fail & fall back to SRR |   2+log(N)  |   9+log(N)
        Table 2: Comparison of SRR and RPR in Open Networks
 From the above comparison, it can be observed that trying to first
 use RPR could still provide an overall number of hops lower than
 directly using SRR.  The detailed analysis is the same as that for
 DRR and can be found in [RFC7263].

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 15]

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