TRAM                                                            T. Reddy
Internet-Draft                                                   D. Wing
Intended status: Standards Track                            P. Martinsen
Expires: April 7, 2016                                             Cisco
                                                                V. Singh
                                                         October 5, 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

   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 April 7, 2016.

Copyright Notice

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

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   ( in effect on the date of
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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  . . . . . . . . . . . . . . . . . . . . . . .   9

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 document 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.

   The technique described in this document can be used with the regular
   nomination procedure defined in ICE [RFC5245], wherein ICE
   connectivity checks need to be performed on all or subset of the
   chosen candidate pairs.  Finalizing an appropriate 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 in the media setup

   The technique described in this document can also be used with the
   ICE continuous nomination procedure explained in
   [I-D.uberti-mmusic-nombis] which allows the application to pick
   better candidate pairs as and when they appear.  Hence, ICE endpoints
   will be capable of switching the application data to a candidate pair
   that becomes available later and offers better path characteristics.

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 the retransmission behavior
   for idempotent packets.  In addition to determining RTT, it is also
   desirable to detect which path direction caused packet loss,

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   described as "bi-directional path characteristics," below.  This is
   achieved by defining a new STUN attribute and requires compliant STUN
   (TURN, ICE) endpoints to count request packets.

   This document defines a new comprehension-optional STUN attribute
   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 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 4-byte
   Value.  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

   This document updates one the STUN message structuring rules
   explained in Section 6 of [RFC5389] wherein 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
        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
       |        Reserved, should be 0  |  ReTransCnt   |  RespTransCnt |

            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.

   RespTransCnt:  RespTransCnt MUST be set to zero in request and
      ignored by the receiver.

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
   protocol [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 RespTransCnt
      field in PATH-CHARACTERISTIC attribute sent in the response.  If
      the server is stateful then it populates 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
       |        Reserved, should be 0  |  ReTransCnt   |  RespTransCnt |

            Figure 2: PATH-CHARACTERISTIC attribute in response

   The fields are described below:

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   ReTransCnt:  Copied from request.

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

3.3.  Example Operation

   The example 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

   Another example could be the client sends two requests but the second
   request arrives at the server before the first request because of out
   of order delivery.  In this case the stateful server populates value
   1 for the RespTransCnt field in PATH-CHARACTERISTIC attribute sent in
   response to the second request and value 2 for the RespTransCnt field
   in PATH-CHARACTERISTIC attribute sent in response to the first

4.  Usecases

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

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   o  When an endpoint has multiple interfaces (for example 3G, 4G,
      WiFi, VPN, etc.), an ICE agent can choose the interfaces for
      application data 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 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 application data 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 document 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

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   between peers using ICE, and ICE uses STUN short-term credential
   mechanism the risk of on-path attack influencing the messages is
   minimal.  If PATH-CHARACTERISTIC is used with Allocate request then
   STUN long-term credential mechanism or STUN Extension for Third-Party
   Authorization [RFC7635] or (D)TLS connection can be used between the
   TURN client and the TURN server to prevent attackers from trying to
   impersonate a TURN server and sending bogus PATH-CHARACTERISTIC
   attribute in the Allocate response.  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-CHARACTERISTIC attribute in order to
   prevent on-path attacker from influencing decision-making.

7.  Acknowledgements

   Thanks to Brandon Williams, Simon Perreault, Aijun Wang, Martin
   Thomson, Oleg Moskalenko and Ram Mohan R for valuable inputs and

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, 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.

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              Varun, V., Karkkainen, T., Ott, J., Ahsan, S., and L.
              Eggert, "Multipath RTP (MPRTP)", draft-ietf-avtcore-
              mprtp-01 (work in progress), July 2015.

              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

              Uberti, J. and J. Lennox, "Improvements to ICE Candidate
              Nomination", draft-uberti-mmusic-nombis-00 (work in
              progress), March 2015.

   [RFC7635]  Reddy, T., Patil, P., Ravindranath, R., and J. Uberti,
              "Session Traversal Utilities for NAT (STUN) Extension for
              Third-Party Authorization", RFC 7635,
              DOI 10.17487/RFC7635, August 2015,

Authors' Addresses

   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


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


   Varun Singh
   Nemu Dialogue System Oy
   Itaemerenkatu 5
   Helsinki  00150


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