TRAM                                                            T. Reddy
Internet-Draft                                                   D. Wing
Intended status: Standards Track                            P. Martinsen
Expires: July 24, 2015                                             Cisco
                                                                V. Singh
                                                        January 20, 2015

              Discovery of path characteristics using STUN


   A host with multiple interfaces needs to choose the best interface
   for communication.  Oftentimes, this decision is based on a static
   configuration and does not consider the path characteristics, which
   may affect the user experience.

   This document describes a mechanism for an endpoint to discover the
   path characteristics using Session Traversal Utilities for NAT (STUN)
   messages.  The measurement information can then be used to influence
   the endpoint's Interactive Connectivity Establishment (ICE) candidate
   pair selection algorithm.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 24, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Path characteristics determination mechanism  . . . . . . . .   3
     3.1.  The PATH-CHARACTERISTIC attribute in request  . . . . . .   4
     3.2.  The PATH-CHARACTERISTIC attribute in response . . . . . .   5
     3.3.  Example Operation . . . . . . . . . . . . . . . . . . . .   6
   4.  Usecases  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The ICE [RFC5245] mechanism uses a prioritization formula to order
   the candidate pairs and perform connectivity checks, in which the
   most preferred address pairs are tested first and when a sufficiently
   good pair is discovered, that pair is used for communications and
   further connectivity tests are stopped.  This approach works well for
   an endpoint with a single interface, but is too simplistic for
   endpoints with multiple interfaces, wherein a candidate pair with a
   lower priority might infact have better path characteristics (e.g.,
   round-trip time, loss, etc.).  The ICE connectivity checks can assist
   in measuring the path characteristics, but as currently defined, the
   STUN responses to re-transmitted requests are indistinguishable from
   each other.

   This draft extends STUN [RFC5389] to distinguish STUN responses to
   re-transmitted requests and this assists the client in determining
   the path characteristics like round-trip time (RTT) and packet loss
   in each direction between endpoints.  These metrics can then be used
   by the controlling agent to influence the ICE candidate pair

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   The PATH-CHARACTERISTICS attribute introduced in this specification
   can be used in ICE connectivity checks (STUN Binding request and
   response).  When multiple TURN servers are discovered then this new
   attribute can also be used with Allocate request to determine the
   priority amongst the relayed candidates.

   This specification can be used with the regular nomination procedure
   defined in ICE, wherein ICE connectivity checks need to be performed
   on all or subset of the chosen candidate pairs.  Finalizing an
   approporiate candidate pair based on the path characteristics depends
   on the number of candidate pairs, time interval for pacing ICE
   connectivity checks and the corresponding RTO values.  By picking
   appropriate values the endpoints will not observe any noticeable
   impact to the media setup time.

   TODO: Add details of ICE continous nomination procedure discussed in
   mmusic WG which allows picking better candidate pairs as and when
   they appear.
   mmusic-nombis.html explains simplifying and improving the procedures
   for candidate nomination in ICE to make dynamic decisions.

2.  Notational Conventions

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

   This note uses terminology defined in ICE [RFC5245] and STUN

3.  Path characteristics determination mechanism

   When multiple paths are available for communication, the endpoint
   sends ICE connectivity checks across each path (candidate pair) and
   perhaps chooses the path with the lowest round trip time.  Choosing
   the path with the lowest round trip time is a reasonable approach,
   but re-transmits can cause an otherwise-good path to appear flawed.
   However, STUN's retransmission algorithm [RFC5389] cannot determine
   the round-trip time (RTT) if a STUN request packet is re-transmitted,
   because each request and retransmission packet is identical.
   Further, several STUN requests may be sent before the connectivity
   between candidate pairs is ascertained (see Section 16 of [RFC5245]).
   To resolve the issue of identical request and response packets in a
   STUN transaction, this document changes that retransmission behavior
   for idempotent packets.  In addition to determining RTT, it is also
   desirable to detect which path direction caused packet loss,
   described as "bi-directional path characteristics," below.  This is

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   achieved by defining a new STUN attribute and requires compliant STUN
   (TURN, ICE) endpoints to count request packets.

   This specification defines a new comprehension-optional STUN
   Type TBD-CA.  This type is in the comprehension-optional range, which
   means that STUN agents can safely ignore the attribute if they do not
   understand it.

   If a client wishes to determine the path characteristics, it inserts
   the PATH-CHARACTERISTIC attribute in a STUN request.  In the PATH-
   CHARACTERISTIC attribute client sends the number of times the STUN
   request is retransmitted with the same Transaction ID.  The server
   would echo back the retransmission count in the response so that
   client can distinguish STUN responses from the re-transmitted
   requests.  Hence, the endpoint can use the STUN requests and
   responses to determine the round-trip time (RTT).  The server may
   also convey the number of times it received the request with the same
   Transaction ID and the number of responses it has sent for the STUN
   request to the client.  Further, this information enables the client
   to determine packet loss in each direction.

3.1.  The PATH-CHARACTERISTIC attribute in request

   The PATH-CHARACTERISTIC attribute in a STUN request takes a 1-byte
   Value, which means that the Length is 1 and 3 bytes of padding are
   required after the value (Section 15 of [RFC5389]).  When sending a
   STUN request, the PATH-CHARACTERISTIC attribute allows a client to
   indicate to the server that it wants to determine path
   characteristics.  If the client receives a STUN response with error
   code 420 (Unknown Attribute) and PATH-CHARACTERISTIC is listed in the
   UNKNOWN-ATTRIBUTE attribute of the message, the client SHOULD
   retransmit the original request without the PATH-CHARACTERISTIC
   attribute.  However this case is not expected to occur, due to the
   use of the comprehension-optional attribute type.

   This specification updates one the STUN message structuring rules
   explained in Section 6 of [RFC5389] that resends of the same request
   reuse the same transaction ID and are bit-wise identical to the
   previous request.  For idempotent packets the ReTransCnt in the PATH-
   CHARACTERISTIC attribute will be incremented by 1 for every re-
   transmission and the re-transmitted STUN request MUST be bit-wise
   identical to the previous request except for the ReTransCnt value.

   The format of the value in PATH-CHARACTERISTIC attribute in the
   request is:

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            0 1 2 3 4 5 6 7
           | ReTransCnt    |

            Figure 1: PATH-CHARACTERISTIC attribute in request

   The field is described below:

   ReTransCnt:  Number of times request is re-transmitted with the same
      transaction ID to the server.

3.2.  The PATH-CHARACTERISTIC attribute in response

   When a server receives a STUN request that includes a PATH-
   CHARACTERISTIC attribute, it processes the request as per the STUN
   specification [RFC5389] plus the specific rules mentioned here.  The
   server checks the following:

   o  If the PATH-CHARACTERISTIC attribute is not recognized, ignore the
      attribute because its type indicates that it is comprehension-
      optional.  This should be the existing behavior as explained in
      section 3.1 of [RFC5389].

   o  The server that supports PATH-CHARACTERISTIC attribute MUST echo
      back ReTransCnt in the response.

   o  If the server is stateless or does not want to remember the
      transaction ID then it would populate value 0 for the ReqTransCnt
      and RespTransCnt fields in PATH-CHARACTERISTIC attribute sent in
      the response .If the server is stateful then it populates
      ReqTransCnt with the number of times it received the STUN request
      with the same transaction ID and RespTransCnt with the number of
      responses it has sent for the STUN request.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|        ReTransCnt             |
       |   ReqTransCnt                 |        RespTransCnt           |

            Figure 2: PATH-CHARACTERISTIC attribute in response

   The fields are described below:

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   ReTransCnt:  Copied from request.

   ReqTransCnt:  Number of times request is received from the client
      with the same transaction ID.

   RespTransCnt:  Number of responses sent to the client for the same
      transaction ID.

3.3.  Example Operation

   The operation is described in Figure 3.  In the first case, all the
   requests and responses are received correctly.  In the upstream loss
   case, the first request is lost, but the second one is received
   correctly, the client on receiving the response notes that while 2
   requests were sent, only one was received by the server, also the
   server realizes that the RespTransCnt does not match the ReTransCnt,
   therefore 1 request was lost.  This may also occur at startup in the
   presence firewalls or NATs that block unsolicited incoming traffic.
   In the downstream loss case, the responses get lost, client expecting
   multiple response notes that while the server responded to 3 requests
   but only 1 response was received.  In the both loss case, requests
   and responses get lost in tandem, the server notes one request packet
   was not received, while the client expecting 3 responses received
   only one, it notes that one request and response packets were lost.

      Normal     |  Upstream loss  |  Downstream loss|      Both loss  |
  Client  Server |  Client  Server |  Client  Server |  Client  Server |
  1         1,1  |  1         x    |  1         1,1  |  1         x    |
    1,1          |                 |    x            |                 |
  2         2,2  |  2         2,1  |  2         2,2  |  2         2,1  |
    2,2          |    2,1          |    x            |    x            |
  3         3,3  |  3         3,2  |  3         3,3  |  3         3,2  |
    3,3          |    3,2          |    3,3          |    3,2          |

         Figure 3: Retransmit Operation between client and Server

4.  Usecases

   The STUN attribute defined in this specification can be used by
   applications in the following scenarios:

   o  When an endpoint has multiple interfaces (for example 3G, 4G,
      WiFi, VPN, etc.), an ICE agent can choose the interfaces for media
      streams according to the path characteristics.  After STUN
      responses to STUN checks are received, the ICE agent using regular
      nomination can sort the ICE candidate pairs according to the path
      characteristics (loss and RTT) discovered using STUN.  The

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      controlling agent can then assign the highest priority to
      candidate pair which best fulfills the desired path
      characteristics.  However, it should be noted that the path
      capacity or throughput is not determined by these STUN checks.  If
      an endpoint needs to pick paths based on capacity, it would have
      to send media on those paths.

   o  When a host has multiple interfaces available an MPRTP
      [I-D.ietf-avtcore-mprtp] application can choose the interfaces for
      the corresponding subflows according to the path characteristics
      discovered using STUN.  For example, the scheduling algorithm
      described in [ACM-MPRTP] uses path capacity, latency, and loss
      rate for choosing the most suitable subset of paths.

   o  The STUN extension proposed in this specification can also be used
      to choose a TURN server that provides the best user experience
      (section 3.1 of [I-D.patil-tram-turn-serv-selection]).

5.  IANA Considerations

   [Paragraphs in braces should be removed by the RFC Editor upon

   [The PATH-CHARACTERISTIC attribute requires that IANA allocate a
   value in the "STUN attributes Registry" from the comprehension-
   optional range (0x8000-0xFFFF), to be replaced for TBD-CA throughout
   this document]

   This document defines the PATH-CHARACTERISTIC STUN attribute,
   described in Section 3.  IANA has allocated the comprehension-
   optional codepoint TBD-CA for this attribute.

6.  Security Considerations

   Security considerations discussed in [RFC5389] are to be taken into
   account.  STUN requires the 96 bits transaction ID to be uniformly
   and randomly chosen from the interval 0 .. 2**96-1, and be
   cryptographically strong.  This is good enough security against an
   off-path attacker.  An on-path attacker can either inject a fake
   response or modify the values in PATH-CHARACTERISTIC attribute to
   mislead the client and server, this attack can be mitigated using
   STUN authentication.  As PATH-CHARACTERISTIC is expected to be used
   between peers using ICE, and ICE uses STUN short-term credential
   mechanism the risk of on-path attack influencing the messages is
   minimal.  However, an attacker could corrupt, remove, or delay an ICE
   request or response, in order to discourage that path from being
   used.  Unauthenticated STUN message MUST NOT include the PATH-

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   CHARACTERISTIC attribute in order to prevent on-path attacker from
   influencing decision-making.

7.  Acknowledgements

   Thanks to Brandon Williams, Simon Perreault, Aijun Wang for valuable
   inputs and comments.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

8.2.  Informative References

              Singh, V., Ahsan, S., and J. Ott, "MPRTP: multipath
              considerations for real-time media", in Proc. of ACM
              Multimedia Systems, MMSys, 2013.

              Singh, V., Karkkainen, T., Ott, J., Ahsan, S., and L.
              Eggert, "Multipath RTP (MPRTP)", draft-ietf-avtcore-
              mprtp-00 (work in progress), December 2014.

              Patil, P., Reddy, T., and G. Salgueiro, "Traversal Using
              Relays around NAT (TURN) Server Selection", draft-patil-
              tram-turn-serv-selection-00 (work in progress), October

Authors' Addresses

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   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134


   Paal-Erik Martinsen
   Cisco Systems, Inc.
   Philip Pedersens vei 22
   Lysaker, Akershus  1325


   Varun Singh
   Nemu Dialogue System Oy
   Espoo  02235


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