STRAW                                                  Ram. Ravindranath
Internet-Draft                                                  T. Reddy
Intended status: Standards Track                            G. Salgueiro
Expires: November 18, 19, 2015                                         Cisco
                                                            May 17, 18, 2015

Session Traversal Utilities for NAT (STUN) Message Handling for Session
      Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
                     draft-ietf-straw-b2bua-stun-07
                     draft-ietf-straw-b2bua-stun-08

Abstract

   Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
   are often designed to be on the media path, rather than just
   intercepting signaling.  This means that B2BUAs often act on the
   media path leading to separate media legs that the B2BUA correlates
   and bridges together.  When acting on the media path, B2BUAs are
   likely to receive Session Traversal Utilities for NAT (STUN) packets
   as part of Interactive Connectivity Establishment (ICE) processing.

   This document defines behavior for a B2BUA performing ICE processing.
   The goal of this draft is to ensure that B2BUAs properly handles SIP
   messages that carry ICE semantics in Session Description
   Protocol(SDP) and STUN messages received as part of the ICE
   procedures for NAT and Firewall traversal of multimedia sessions.

Status of This Memo

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   This Internet-Draft will expire on November 18, 19, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  SDP-Modifying Signaling-only B2BUA  . . . . . . . . . . . . .   5
   4.  Media Plane B2BUAs  . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Mandatory ICE Termination with B2BUA  . . . . . . . . . .   6
     4.3.  Optional ICE Termination with B2BUA . . . . . . . . . . .   8
     4.4.  STUN Handling in B2BUA with Forked Signaling  . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   In many Session Initiation Protocol (SIP) deployments, SIP entities
   exist in the SIP signaling and media path between the originating and
   final terminating endpoints, which go beyond the definition of a
   traditional SIP proxy.  These SIP entities, commonly known as Back-
   to-Back User Agents (B2BUAs), are described in [RFC7092] and often
   perform functions not defined in Standards Track RFCs.

   SIP [RFC3261], and other session control protocols that try to use
   direct path for media, are typically difficult to use across Network
   Address Translators (NATs).  These protocols use IP addresses and
   transport port numbers encoded in the signaling, such as the Session
   Description Protocol (SDP) [RFC4566] and, in the case of SIP, various
   header fields.  Such addresses and ports are unreachable if any peers
   are separated by NATs.

   Mechanisms such as Session Traversal Utilities for NAT (STUN)
   [RFC5389], Traversal Using Relays around NAT (TURN) [RFC5766], and
   Interactive Connectivity Establishment (ICE) [RFC5245] did not exist
   when protocols like SIP began being deployed.  Some mechanisms, such
   as the early versions of STUN, started appearing but they were
   unreliable and suffered a number of issues typical for UNilateral
   Self-Address Fixing (UNSAF) and described in [RFC3424].  For these
   reasons, B2BUAs are being used by SIP domains for SIP and media-
   related purposes.  These B2BUAs use proprietary mechanisms to enable
   SIP devices behind NATs to communicate across the NAT.

   [RFC7362] describes how B2BUAs can perform Hosted NAT Traversal (HNT)
   in certain deployments.  Section 5 of [RFC7362] describes some of the
   issues with SBCs implementing HNT and offers some mitigation
   strategies.  The most commonly used approach to solve these issues is
   restricted-latching defined in section 5 of [RFC7362], whereby the
   B2BUA will not latch to any packets from a source public IP address
   other than the one the SIP UA uses for SIP signaling.  However, this
   is susceptible to attacks, where an attacker who is able to see the
   source IP address of the SIP UA may generate packets using the same
   IP address.  There are other threats described in Section 5 of
   [RFC7362] for which Secure Real-time Transport Protocol (SRTP)
   [RFC3711] can be used as a solution.  However, this would require the
   B2BUAs to terminate and re-originate SRTP, which is not always
   desirable.

   This document describes proper behavior of B2BUAs performing ICE
   processing.  This includes defining consistent handling of SIP
   messages carrying ICE semantics in SDP and STUN messages received as
   part of the ICE procedures performed on the media path for NAT and
   Firewall traversal of multimedia sessions.

   A B2BUA can use ICE [RFC5245], which provides authentication tokens
   (conveyed in the ice-ufrag and ice-pwd attributes) that allow the
   identity of a peer to be confirmed before engaging in media exchange.
   This can solve some of the security concerns with HNT solution.
   Further, ICE has other benefits like selecting an address when more
   than one address is available (e.g., dual-stack environment where
   host can have both IPv4 and IPv6 addresses), verifying that a path
   works before connecting the call etc.  For these reasons endpoints
   often use ICE to pick a candidate pair for media traffic between two
   agents.

   B2BUAs often operate on the media path and have the ability to modify
   SIP headers and SDP bodies as part of their normal operation.  Such
   entities, when present on the media path, are likely to take an
   active role in the session signaling depending on their level of
   activity on the media path.  For example, some B2BUAs modify portions
   of the SDP body (e.g., IP address, port) and subsequently modify the
   media packet headers as well.  Section 18.6 of ICE [RFC5245] explains
   two different behaviors when B2BUAs are present.  Some B2BUAs are
   likely to remove all the SDP ICE attributes before sending the SDP
   across to the other side.  Consequently, the call will appear to both
   endpoints as though the other side doesn't support ICE.  There are
   other types of B2BUAs that pass the ICE attributes without
   modification, yet modify the default destination for media contained
   in the m= and c= lines and rtcp attribute (defined in [RFC3605]).
   This will be detected as an ICE mismatch and ICE processing will be
   aborted for the session.  The session may continue if the endpoints
   are able to reach each other over the default candidate (sent in m=
   and c= lines).

   Section 3.1.3 of [RFC7092] defines a SDP-Modifying Signaling-only
   B2BUA that operates in the signaling plane only and is not in the
   media path, but it does modify SDP bodies and is thus aware of and
   understands SDP syntax and semantics.  Such B2BUA MUST follow the
   behavior mentioned in Section 3.

   Section 3.2 of [RFC7092] describes three different categories of
   B2BUAs that operates on both signaling(SIP and SDP) and media plane
   according to the level of involvement and active participation in the
   media plane:

   o  A B2BUA that acts as a simple media relay effectively unaware of
      anything that is transported and only modifies the transport
      header (could be UDP/IP) of the media packets.

   o  A B2BUA that performs a media-aware role.  It inspects and
      potentially modifies RTP or RTP Control Protocol (RTCP) headers;
      but it does not modify the payload of RTP/RTCP.

   o  A B2BUA that performs a media-termination role and operates at the
      media payload layer, such as RTP/RTCP payload (e.g., a
      transcoder).

   When B2BUAs that operate on media plane (media relay, media-aware or
   media-termination) is involved in a session between two endpoints
   performing ICE, then it MUST follow the behavior described in
   Section 4.

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 [RFC2119].

   All of the pertinent B2BUA terminology and taxonomy used in this
   document is defined in [RFC7092].

   Network Address Translators (NATs) are widely used in the Internet by
   consumers and organizations.  Although specific NAT behaviors vary,
   this document uses the term "NAT", which maps to NAT and NAPT terms
   from [RFC3022], for devices that map any IPv4 or IPv6 address and
   transport port number to another IPv4 or IPv6 address and transport
   port number.  This includes consumer NATs, Firewall-NATs, IPv4-IPv6
   NATs, Carrier-Grade NATs (CGNs) [RFC6888], etc.

3.  SDP-Modifying Signaling-only B2BUA

   An SDP-Modifying Signaling-only B2BUA is one that operates in the
   signaling plane only and is not in the media path, but it modifies
   SDP bodies as described in section 3.1.3 of [RFC7092].  Such B2BUAs
   MUST NOT change IP address in c= line, port in m= line and ICE
   semantics of SDP as doing so can cause ICE-mismatch.

4.  Media Plane B2BUAs

4.1.  Overview

   When one or both of the endpoints are behind a NAT, and there is a
   B2BUA between the endpoints, the B2BUAs MUST support ICE or at a
   minimum support ICE LITE functionality as described in [RFC5245].
   Such B2BUAs MUST use the mechanism described in Section 2.2 of
   [RFC5245] to demultiplex STUN packets that arrive on the Real-time
   Transport Protocol(RTP)/RTP Control Protocol (RTCP) port.

   The subsequent sections describe the behavior B2BUAs MUST follow for
   handling ICE messages.  A B2BUA can terminate ICE and thus have two
   ICE contexts with either endpoint.  The reason for ICE termination
   could be due to the need for B2BUA to be in the media path ( e.g.,
   address hiding for privacy, interworking between ICE to no-ICE,
   etc.).  A B2BUA can also be in optional ICE termination mode and
   passes across the candidate list and STUN short-term credentials
   (ice-ufrag and ice-pwd attributes) from one endpoint to the other
   side after adding its own candidates.  A B2BUA MAY can be in optional ICE
   termination mode when it does not have a need to be on the media
   path.  The below sections describes the behaviors for these two
   cases.

4.2.  Mandatory ICE Termination with B2BUA

   A B2BUA that wishes to always be in the media path follows the below
   steps:

   o  When a B2BUA sends out SDP, it MUST advertise support for ICE and
      MAY include B2BUA candidates of different types for each component
      of each media stream.

   o  If the B2BUA is in ICE lite mode as described in Section 2.7 of
      [RFC5245] then it MUST send a=ice-lite attribute and MUST include
      B2BUAs host candidates for each component of each media stream.

   o  If the B2BUA supports full ICE then it MAY include B2BUAs
      candidates of different types for each component of each media
      stream.

   o  The B2BUA MUST generate new username, password values for ice-
      ufrag and ice-pwd attributes when it sends out the SDP and MUST
      NOT propagate the ufrag, password values it received in the
      incoming SDP.  This ensures that the short-term credentials used
      for both the legs are different.  The short-term credentials
      include authentication tokens (conveyed in the ice-ufrag and ice-
      pwd attributes), which the B2BUA can use to verify the identity of
      the peer.  B2BUA terminates the ICE messages on each leg and does
      not propagate them.

   o  The B2BUA MUST NOT propagate the candidate list received in the
      incoming SDP to the outbound SDP and instead only advertise its
      candidate list.  The B2BUA MUST also add its default candidate in
      the c= line (IP address) and m= line (port).  In this way the
      B2BUA will be always in the media path.

   o  Depending on whether the B2BUA supports ICE lite or full ICE it
      implements the appropriate procedures mentioned in [RFC5245] for
      ICE connectivity checks.

       +-------+            +------------------+              +-----+
       | Alice |            | Mediaplane B2BUA |              | Bob |
       +-------+            +------------------+              +-----+
           |(1) INVITE               |  (3)INVITE                |
           |   a=ice-ufrag1          |    a=ice-ufrag2           |
           |   a=ice-pwd1            |     a=ice-pwd2            |
           |   (Alice's IP, port)    |   (B2BUAs IP, port)       |
           |(Alice's candidate list )|   (B2BUAs candidate list) |
           |------------------------>|-------------------------->|
           |                         |                           |
           |    (2)  100 trying      |                           |
           |<------------------------|                           |
           |                         | (4) 100 trying            |
           |                         |<--------------------------|
           |                         |  (5)200 OK                |
           |                         |   a=ice-ufrag3            |
           |                         |    a=ice-pwd3             |
           |                         |  (Bob's IP, port)         |
           |                         | (Bob's candidate list)    |
           |                         |<--------------------------|
           |    (6) 200 OK           |                           |
           |    a=ice-ufrag4         |-----------ACK------------>|
           |    a=ice-pwd4           |           (7)             |
           |    B2BUAs IP,port       |                           |
           | (B2BUAs cand list1)     |                           |
           |<------------------------|                           |
           |--------ACK------------->|                           |
           |              (8)        |                           |
           |                         |                           |
           |<----ICE Connectivity 1->|                           |
           |      checks+conclusion  |<-----ICE Connectivity 2-->|
           |         (9)             |        checks +conclusion |
           |                         |         (10)              |
           |<-------Media packets -->|<----Media packets-------->|
           |      (13)               |         (14)              |
           |                         |                           |
           |<---ICE keepalives 1---->|                           |
           |        (15)             |<----ICE keep alives 2---->|
                                            (16)

     Figure 1: INVITE with SDP having ICE and with a Media Plane B2BUA
                              terminating ICE

   The above figure shows an example call flow with two endpoints Alice
   and Bob doing ICE and a B2BUA handing STUN messages from both the
   endpoints.  For the sake of brevity the entire ICE SDP attributes are
   not shown.  Also the STUN messages exchanged as part of ICE
   connectivity checks are not shown.  Key steps to note from the call
   flow are:

   o  Alice sends an INVITE with SDP having ICE candidates.

   o  B2BUA modifies the received SDP from Alice by removing the
      received candidate list, gathers its own candidates, generates new
      username, password values for ice-ufrag and ice-pwd attributes.
      The B2BUA also changes the c= line and m= line to have its default
      candidate and forwards the INVITE (3) to Bob.

   o  Bob responds (5) to the INVITE with his own list of candidates.

   o  B2BUA responds to the INVITE from Alice with SDP having B2BUAs
      candidate list.  B2BUA generates new username, password values for
      ice-ufrag and ice-pwd attributes in the 200 OK response (6).

   o  ICE Connectivity checks happen between Alice and the B2BUA in step
      9.  Depending on whether the B2BUA supports ICE or ICE lite it
      will follow the appropriate procedures mentioned in [RFC5245].
      ICE Connectivity checks also happen between Bob and the B2BUA in
      step 10.  Step 9 and 10 happen in parallel.  The B2BUA always
      terminates the ICE messages on each leg and has two independent
      ICE contexts running.

   o  Media flows between Alice and Bob via B2BUA (Step 13, 14).

   o  STUN keepalives would be used between Alice and B2BUA (step 15)
      and between Bob and B2BUA (step 16) to keep NAT and Firewall
      bindings alive.

   Since there are two independent ICE contexts on either side of the
   B2BUA it is possible that ICE checks will conclude on one side before
   concluding on the other side.  This could result in an ongoing media
   session for one end, while the other is still being set up.  Any such
   media received by the B2BUA would continue to be sent to the other
   side on the default candidate address (that was sent in c= line).

4.3.  Optional ICE Termination with B2BUA

   A B2BUA willing to be in the media path only for NAT traversal, but
   does not otherwise require to be in the media path can do the
   following steps mentioned in this section.

   o  When a B2BUA receives an incoming SDP with ICE semantics it copies
      the received candidate list and appends its own candidate list in
      the outgoing SDP.  The B2BUA also copies the ufrag/password values
      it received in the incoming SDP to the outgoing SDP and then sends
      out the SDP.

   o  The B2BUAs candidates MAY have lower-priority than the candidates
      provided by the endpoint, this way endpoint and remote peer
      candidate pairs are tested first before trying candidate pairs
      with B2BUA candidates.

   o  After offer/answer is complete, the endpoints will have both the
      B2BUAs and remote peer candidates.  It will then use ICE
      procedures described in Section 8 of [RFC5245] to nominate a
      candidate pair for sending and receiving media streams.

   o  With this approach the B2BUA will be in the media path only if the
      ICE checks between all the candidate pairs formed from both the
      endpoints fail.

       +-------+            +------------------+              +-----+
       | Alice |            | Mediaplane B2BUA |              | Bob |
       +-------+            +------------------+              +-----+
           |(1) INVITE               |  (3)INVITE                |
           |   a=ice-ufrag1          |    a=ice-ufrag1           |
           |   a=ice-pwd1            |     a=ice-pwd1            |
           |  (Alice's IP, port)     | (Alices's IP, port)       |
           |(Alice's candidate list )| (Alice's Candidate list + |
           |                         |   B2BUAs candidate list1) |
           |------------------------>|-------------------------->|
           |                         |                           |
           |    (2)  100 trying      |                           |
           |<------------------------|                           |
           |                         | (4) 100 trying            |
           |                         |<--------------------------|
           |                         |  (5)200 OK                |
           |                         |   a=ice-ufrag2            |
           |                         |    a=ice-pwd2             |
           |                         |  (Bob's IP, port)         |
           |                         | (Bob's candidate list)    |
           |                         |<--------------------------|
           |    (6) 200 OK           |                           |
           |    a=ice-ufrag2         |-----------ACK------------>|
           |    a=ice-pwd2           |           (7)             |
           | (Bobs's IP,port)        |                           |
           | (B2BUAs cand list2 +    |                           |
           |   Bob's Candidate list) |                           |
           |<------------------------|                           |
           |----------ACK----------->|                           |
           |          (8)            |                           |
           |                         |                           |
           |<----ICE Connectivity 1 (9)------------------------->|
           |                         |                           |
           |<----ICE Connectivity 2->|                           |
           |      checks+conclusion  |<-----ICE Connectivity 2-->|
           |         (10)            |      checks +conclusion   |
           |                         |         (11)              |
           |<-------------------Media packets------------------->|
           |                       (12)                          |
           |                         |                           |
           |<------------------ICE keepalives------------------->|
                                   (13)

   Figure 2: INVITE with SDP having ICE and with a Media Plane B2BUA in
                       optional ICE termination mode

   The above figure shows a sample call flow with two endpoints Alice
   and Bob doing ICE and a B2BUA handing STUN messages from both the
   endpoints.  For the sake of brevity the entire ICE SDP attributes are
   not shown.  Also the STUN messages exchanged as part of ICE
   connectivity checks are not shown.  Key steps to note from the call
   flow are:

   o  Alice sends an INVITE with an SDP having its own candidate list.

   o  B2BUA propagates the received candidate list in incoming SDP from
      Alice after adding its own candidate list.  The B2BUA also
      propagates the received ice-ufrag, ice-password attributes from
      Alice in the INVITE (3) to Bob. In this example, the B2BUA does
      not modify the default candidate sent in the c= line and m= line
      and retains the values sent originally from Alice.  If B2BUA wants
      to be in the media path when ICE connectivity checks between
      endpoints fails or one of the endpoints does not support ICE, then
      it overwrites its candidate address and port as a default
      candidate in the m= and c= lines.

   o  Bob responds (5) to the INVITE with his own list of candidates.

   o  B2BUA responds to the INVITE from Alice with an SDP having B2BUAs
      candidate list and the candidate list received from Bob.  The
      B2BUA would also propagate the received ice-ufrag, ice-password
      attributes from Bob in step (5) to Alice in the 200 OK response
      (6).

   o  ICE Connectivity checks happen between Alice and Bob in step 9.
      ICE Connectivity checks also happens between Alice and B2BUA and
      Bob and B2BUA as shown in step 10, 11.  Step 9, 10 and 11 happen
      in parallel.  In this example Alice and Bob conclude ICE with a
      candidate pair that enables them to send media directly.

   o  Media flows between Alice and Bob in Step 12.

4.4.  STUN Handling in B2BUA with Forked Signaling

   Because of forking, a B2BUA MAY might receive multiple answers for a
   single outbound INVITE.  When this occurs the B2BUA SHOULD follow
   section 3.2 or 3.3 for all of those received answers.

5.  Security Considerations

   ICE as described in Section 2.5 of [RFC5245] uses STUN short-term
   credential mechanism for authentication and message integrity.  STUN
   connectivity checks include MESSAGE-INTEGRITY attribute that contains
   HMAC-SHA1 of the STUN message and the HMAC is computed using the key
   exchanged in the signaling channel.  The signaling channel between
   the endpoints and B2BUA MUST be encrypted so that the key is not
   visible to eavesdropper otherwise the security benefits of short-term
   authentication would be lost.

6.  IANA Considerations

   This document makes no request of IANA.

7.  Acknowledgments

   Special thanks to Dan Wing, Pal Martinsen, Charles Eckel, Marc Petit-
   Huguenin, Simon Perreault, Lorenzo Miniero, Ari Keranen and
   Parthasarathi R for their constructive comments, suggestions, and
   early reviews that were critical to the formulation and refinement of
   this document.

   Thanks to Ben Campbell, Barry Leiba, Nevil Brownlee, Spencer Dawkins,
   Sam Hartman, Stephen Farrell, Kathleen Moriarty and Francis Dupont
   for their thoughtful review 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.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

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

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

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

   [RFC7092]  Kaplan, H. and V. Pascual, "A Taxonomy of Session
              Initiation Protocol (SIP) Back-to-Back User Agents", RFC
              7092, December 2013.

8.2.  Informative References

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022, January
              2001.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605, October
              2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC6888]  Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common Requirements for Carrier-Grade NATs
              (CGNs)", BCP 127, RFC 6888, April 2013.

   [RFC7362]  Ivov, E., Kaplan, H., and D. Wing, "Latching: Hosted NAT
              Traversal (HNT) for Media in Real-Time Communication", RFC
              7362, September 2014.

Authors' Addresses

   Ram Mohan Ravindranath
   Cisco
   Cessna Business Park
   Sarjapur-Marathahalli Outer Ring Road
   Bangalore, Karnataka  560103
   IN

   Email: rmohanr@cisco.com
   Tirumaleswar Reddy
   Cisco
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103
   IN

   Email: tireddy@cisco.com

   Gonzalo Salgueiro
   Cisco Systems, Inc.
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   US

   Email: gsalguei@cisco.com