MMUSIC Working Group                                         C. Holmberg
Internet-Draft                                                  Ericsson
Intended status: Standards Track                           H. Alvestrand
Expires: December 16, 2013 April 17, 2014                                           Google
                                                             C. Jennings
                                                                   Cisco
                                                           June
                                                        October 14, 2013

 Multiplexing Negotiation Using Session Description Protocol (SDP) Port
                                Numbers
            draft-ietf-mmusic-sdp-bundle-negotiation-04.txt
            draft-ietf-mmusic-sdp-bundle-negotiation-05.txt

Abstract

   This specification defines a new SDP Grouping Framework extension,
   "BUNDLE", that can be used with the Session Description Protocol
   (SDP) Offer/Answer mechanism to negotiate the usage of bundled media,
   which refers to the usage of a single 5-tuple for media associated
   with multiple SDP media descriptions ("m=" lines).

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
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on December 16, 2013. April 17, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4   5
   4.  Applicability Statement . . . . . . . . . . . . . . . . . . .   5
   5.  SDP Grouping Framework BUNDLE Extension Semantics . . . . . .   5
   6.  SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . .   5
     6.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.2.  Bundled SDP Information . . . . . . . . . . . . . . . . .   5
       6.2.1.  General . . . . . . . . . . . . . . . . . . . . . . .   5
       6.2.2.  Bandwidth (b=)  . . . . . . . . . . . . . . . . . . .   6
       6.2.3.  rtcp-mux Attribute  . . . . . . . . . . . . . . . . .   6
       6.2.4.  rtcp Attribute  . . . . . . . . . . . . . . . . . . .   6
       6.2.5.  DTLS-SRTP fingerprint Attribute . . . . . . . . . . .   6
       6.2.6.  SDES crypto Attribute . . . . . . . . . . . . . . . .   6
       6.2.7.  Other Attributes (a=) . . . . . . . . . . . . . . . .   6
     6.3.  RFC 5888 restrictions . . . . . . . . . . . . . . . . . .   6
     6.4.  SDP Offerer Procedures  . . . . . . . . . . . . . . . . .   6   7
       6.4.1.  SDP Offerer Bundle Address Request and Usage  General . . . . . . . . . . . . . . . . . . . . . . .   7
       6.4.2.  Request BUNDLE address selection  . . . . . . . . . .   8
       6.4.3.  Bundle Address Synchronization (BAS)  . . . . . . . .   7
       6.4.3.   8
       6.4.4.  Adding a media description to a BUNDLE group  . . . .   8
       6.4.4.
       6.4.5.  Moving A Media Description Out Of A BUNDLE Group  . .   8
       6.4.5.   9
       6.4.6.  Disabling A Media Description In A BUNDLE Group . . .   9
     6.5.  SDP Answerer Procedures . . . . . . . . . . . . . . . . .   9
       6.5.1.  Answerer  Offerer Bundle Address Selection and Usage  . . . . .   9 . . . . .  10
       6.5.2.  Anwerer Bundle Address Selection  . . . . . . . . . .  10
       6.5.3.  Moving A Media Description Out Of A BUNDLE Group  . .  10
       6.5.3.
       6.5.4.  Rejecting A Media Description In A BUNDLE Group . . .  10  11
   7.  Single vs Multiple RTP Sessions . . . . . . . . . . . . . . .  11
     7.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.2.  Single RTP Session  . . . . . . . . . . . . . . . . . . .  11
   8.  Usage With ICE  . . . . . . . . . . . . . . . . . . . . . . .  11  12
     8.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  11  12
     8.2.  Candidates  . . . . . . . . . . . . . . . . . . . . . . .  11  12
     8.3.  Candidates  . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  12  13
     10.1.  Example: Bundle Address Selection  . . . . . . . . . . .  12  13
     10.2.  Example: Bundle Mechanism Rejected . . . . . . . . . . .  14
     10.3.  Example: Offerer Adds A Media Description To A BUNDLE
            Group  . . . . . . . . . . . . . . . . . . . . . . . . .  15

     10.4.  Example: Offerer Moves A Media Description Out Of A
            BUNDLE Group . . . . . . . . . . . . . . . . . . . . . .  17
     10.5.  Example: Offerer Disables A Media Description In A
            BUNDLE Group . . . . . . . . . . . . . . . . . . . . . .  18  19
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19  20
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19  20
   13. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  20
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20  21
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     14.2.  Informative References . . . . . . . . . . . . . . . . .  21  22
   Appendix A.  Design Considerations  . . . . . . . . . . . . . . .  21  22
     A.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  21  22
     A.2.  UA Interoperability . . . . . . . . . . . . . . . . . . .  22  23
     A.3.  Usage of port number value zero . . . . . . . . . . . . .  23  24
     A.4.  B2BUA And Proxy Interoperability  . . . . . . . . . . . .  24  25
       A.4.1.  Traffic Policing  . . . . . . . . . . . . . . . . . .  24  25
       A.4.2.  Bandwidth Allocation  . . . . . . . . . . . . . . . .  25
     A.5.  Candidate Gathering . . . . . . . . . . . . . . . . . . .  25  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25  26

1.  Introduction

   In the IETF RTCWEB WG, a need to use a single 5-tuple for sending and
   receiving media associated with multiple SDP media descriptions ("m="
   lines) has been identified.  This would e.g. allow the usage of a
   single set of Interactive Connectivity Establishment (ICE) [RFC5245]
   candidates for multiple media descriptions.  Normally different media
   types (audio, video etc) will be described using different media
   descriptions.

   This specification defines a new SDP Grouping Framework [RFC5888]
   extension, "BUNDLE", that can be used with the Session Description
   Protocol (SDP) Offer/Answer mechanism [RFC3264] to negotiate the
   usage of bundled media, which refers to the usage of a single 5-tuple
   for media associated with multiple SDP media descriptions ("m="
   lines).

   The Offerer and Answerer [RFC3264] use the BUNDLE mechanism to
   negotiate a single BUNDLE address to be used for the bundled media
   associated with a BUNDLE group.

   The BUNDLE mechanism allows an SDP Offerer and SDP Answerer to assign
   identical addresses to multiple "m=" lines, if those "m=" lines are
   associated with a BUNDLE group.  However, until it is known whether
   both the Offerer and Answerer support the BUNDLE mechanism, unique
   addresses are assigned to each "m=" line, including those associated
   with a BUNDLE group.

   NOTE: As defined in RFC 4566 [RFC4566], the semantics of multiple
   "m=" lines using the same port number value are undefined, and there
   is no grouping defined by such means.  Instead, an explicit grouping
   mechanism needs to be used to express the intended semantics.  This
   specification provides such extension.

   SDP Offers and SDP Answer can contain multiple BUNDLE groups.  For
   each BUNDLE group, a BUNDLE address is negotiated.  Multiple BUNDLE
   groups cannot share the same bundle address.

   The default assumption is that all Real-Time Protocol (RTP) [RFC3550]
   based media flows within a BUNDLE group belongs to the same RTP
   Session [RFC3550].  Future extensions can change that assumption.

   The BUNDLE mechanism is backward compatible.  Endpoints that do not
   support the BUNDLE mechanism are expected to generate SDP Offers and
   SDP Answers without an SDP group:BUNDLE attribute, and are expected
   to assign unique addresses to each "m=" line, according to the
   procedures in [RFC4566] and [RFC3264]

2.  Terminology

   5-tuple: A collection of the following values: source address, source
   port, destination address, destination port and protocol.

   Bundled media: Two or more RTP streams using a single 5-tuple.  The
   RTCP streams associated with the RTP streams also use a single
   5-tuple, which might be the same, but can also be different, as the
   one used by the RTP streams.

   Unique address: This refers to an IP address and IP port combination,
   that can only be associated with a single "m=" line within an SDP
   Session.

   BUNDLE address: This refers to an IP address and IP port combination,
   that is associated with each "m=" line within a BUNDLE group, within
   an SDP Session.  The zero IP port value BUNDLE address MUST NOT be
   used in a BUNDLE address.

   NOTE: "m=" lines that share a BUNDLE address MUST also share other
   parameters related to the media transport plane, e.g. ICE candidate
   information.

3.  Conventions

   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 BCP 14, RFC 2119
   [RFC2119].

4.  Applicability Statement

   The mechanism in this specification only applies to the Session
   Description Protocol (SDP) [RFC4566], when used together with the SDP
   Offer/Answer mechanism [RFC3264].

5.  SDP Grouping Framework BUNDLE Extension Semantics

   This section defines a new SDP Grouping Framework extension, BUNDLE.

   The BUNDLE extension can be indicated using an SDP session-level
   'group' attribute.  Each SDP Media Description ("m=" line) that is
   grouped together, using SDP media-level mid attributes, belongs to a
   given BUNDLE group.

6.  SDP Offer/Answer Procedures

6.1.  General

   This section describes the usage of the SDP Offer/Answer mechanism
   [RFC3264] to negotiate the usage of the BUNDLE mechanism, to
   negotiate the BUNDLE address, and to add, remove and reject SDP Media
   Descriptions ("m=" lines) [RFC4566] associated with a BUNDLE group.

   The generic rules and procedures defined in [RFC3264] and [RFC5888]
   apply when the SDP Offer/Answer mechanism is used with the BUNDLE
   mechanism.  For example, if an SDP Offer is rejected, the previously
   negotiated SDP parameters and characteristics (including those
   associated with BUNDLE groups) apply.

   When an endpoint, acting as an Offerer or Answerer [RFC3264],
   generates an SDP Offer, or an SDP Answer, the endpoint MUST assign an
   SDP media-level mid value for each "m=" line in a BUNDLE group.  In
   addition, the endpoint MUST assign an SDP session-level group:BUNDLE
   attribute for each BUNDLE group, and place each mid associated with
   the SDP group:BUNDLE attribute mid list.

6.2.  Bundled SDP Information

6.2.1.  General
   This section describes restrictions associated with the usage of SDP
   parameters and extensions within a BUNDLE group.  It also describes,
   when parameter and attribute values have been assigned to each "m="
   line in the BUNDLE group, how to calculate a value for the whole
   BUNDLE group.

6.2.2.  Bandwidth (b=)

   The total proposed bandwidth is the sum of the proposed bandwidth for
   each "m=" line associated with a negotiated BUNDLE group.

6.2.3.  rtcp-mux Attribute

   For each "m=" line in a BUNDLE group, an Offerer and Answerer MUST
   assign an SDP rtcp-mux attribute [RFC5761].

6.2.4.  rtcp Attribute

   When used, for each RTP media "m=" line in a BUNDLE group, an Offerer
   and Answerer MUST assign an SDP rtcp attribute [RFC3605] with an
   identical attribute value.

6.2.5.  DTLS-SRTP fingerprint Attribute

   When DTLS-SRTP is used, for each RTP media "m=" line in a BUNDLE
   group, an Offerer and Answerer MUST assign an SDP DTLS-SRTP
   fingerprint attribute with identical attribute values.

6.2.6.  SDES crypto Attribute

   When SDES is used, for each RTP media "m=" line in a BUNDLE group, an
   Offerer and Answerer MUST assign an SDP crypto attribute, with unique
   attribute values.

6.2.7.  Other Attributes (a=)

   There are also special rules for handling many different attributes
   as defined in [I-D.nandakumar-mmusic-sdp-attributes]. [I-D.nandakumar-mmusic-sdp-mux-attributes].  It might
   not possible to use bundle with some attributes.

6.3.  RFC 5888 restrictions

   Based on the rules and procedures in [RFC5888], the following
   restrictions also apply to BUNDLE groups in SDP Answers:

   o  1) A BUNDLE group must not be added to an SDP Answer, unless the
      same BUNDLE group was included in the associated SDP Offer; and

   o  2) An SDP "m=" line must not be added to a BUNDLE group in the SDP
      Answer, unless it was in the same BUNDLE group in the associated
      SDP Offer.

6.4.  SDP Offerer Procedures

6.4.1.  SDP Offerer Bundle Address Request and Usage

   An  General

   When an Offerer can assign a BUNDLE generates an Offer, it assigns an address to multiple each
   "m=" lines line, according to the procedures in [RFC3264].  To each "m="
   line within a BUNDLE group, once group the Answerer has selected Offerer assigns either an address that
   is unique to that "m=" line, or a shared address that is also
   assigned to other "m=" lines within the BUNDLE group.  Such shared
   address for
   the Offerer.  An Offerer MUST NOT assign can be, but does not have to be, a previously selected BUNDLE
   address Section 6.5.1.

   OPEN ISSUE (Q6): There is a discussion on whether assigning a shared
   address to multiple "m=" lines shall be allowed until the Answerer
   has selected the BUNDLE address.

   OPEN ISSUE: Should it be allowed to indicated support of BUNDLE.

   o  (http://www.ietf.org/mail-archive/web/mmusic/current/
      msg12245.html)

   The Offerer MUST NOT assign a new BUNDLE an address (unique or shared), that it
   has assigned to
   multiple an "m=" lines in line within a BUNDLE group, before the Answerer has
   selected the BUNDLE address?

   In order to negotiate (or, to re-negotiate) an "m=" line
   outside the BUNDLE address
   associated with group.

   The Offerer MUST, for a BUNDLE group, on the Offerer, in SDP session level
   [RFC4566], insert an SDP group:BUNDLE attribute associated with the
   BUNDLE group.  The Offerer MUST assign an SDP Offer,
   assigns a unique address 'mid' attribute
   [RFC5888] to each "m=" line within the BUNDLE group, and place the
   mid value in the group:BUNDLE attribute mid list.

   The Offerer MAY assign an SDP 'bundle-only' attribute [ref-to-be-
   added] to one or more "m=" lines within a BUNDLE group.  In
   addition, the

   OPEN ISSUE (Q8): It still needs to be decided whether a zero port
   value can be assigned to a 'bundle-only' "m=" line.

   o  (http://www.ietf.org/mail-archive/web/mmusic/current/
      msg12075.html)

   o  (http://www.ietf.org/mail-archive/web/mmusic/current/
      msg12226.html)

   o  (http://www.ietf.org/mail-archive/web/mmusic/current/
      msg12339.html)

6.4.2.  Request BUNDLE address selection

   When an Offerer indicates generates an Offer, it MUST indicate which unique address
   (unique or shared) within a BUNDLE group it wishes the
   Answerer Answer to
   select as the Offerer's BUNDLE address.  The Offerer
   places the mid, associated with the unique address requested to be
   selected as for the BUNDLE address, first in the SDP group:BUNDLE
   attribute mid list. group
   Section 6.5.1.  The Answerer will then select the BUNDLE address
   for the Offerer ([ref-to-be-added]).

   If MUST do this even if the Offerer, Answerer has, in
   a subsequent SDP Offer, wants to re-negotiate previous Answer within the dialog, already selected the Offerer's
   BUNDLE address.

   In order to request an address associated with a BUNDLE group, it MAY assign (unique or shared) to be selected as
   the
   previously negotiated Offerer's BUNDLE address as for a unique address to one of
   the "m=" lines in the BUNDLE group.

   If group, the Offerer assigns places
   the mid value, associated with the previously selected BUNDLE address to more
   than one "m=" line in a BUNDLE group, representing the
   requested address, first mid in the SDP group:BUNDLE attribute mid list MUST represent an "m=" line to which
   the BUNDLE address is assigned.  Hence, in order to re-negotiate the
   BUNDLE address, the Offerer needs to assign a unique address to each
   "m=" line in the BUNDLE group, as described above.

   An Offerer MUST NOT assign a BUNDLE address to an "m=" line that is
   not associated with a BUNDLE group.  An Offerer MUST NOT assign a
   BUNDLE address, that has been negotiated for a BUNDLE group, to an
   "m=" line that is
   associated with another the BUNDLE group.

   Section 10.1 shows an example of a Bundle Address Request.

6.4.2.

6.4.3.  Bundle Address Synchronization (BAS)

   When an Offerer has requested the Answerer to select the Offerer's receives an Answer, in which an offered BUNDLE address, and group
   is accepted, if the Offerer did not assign in the requested BUNDLE associated Offer assigned an
   address to each "m=" line in (unique or shared), that does not represent the BUNDLE group of the SDP Offer used
   address selected for the Offerer, to request an "m=" line within the BUNDLE address, when the associated SDP Answer is
   received
   group, the Offerer MUST send a subsequent SDP Offer.  In the
   subsequent SDP Offer the Offerer MUST assign Offer, in which it assigns
   the selected BUNDLE address selected for the Offerer to each "m=" line in within
   the BUNDLE group.  This procedure is referred to as Bundle Address
   Synchronization (BAS).

   When (BAS), and the Offerer performs Offer is referred to as a BAS, the BAS Offer.

   The Offerer MAY modify any SDP
   parameters parameter in the same SDP a BAS Offer.

   NOTE: It is important that the SDP Offer used for the BAS Offer gets accepted by the
   Answerer, so the Offerer needs to consider the necessity to modify
   SDP parameters that could get the Answerer to reject the SDP BAS Offer.
   Removing "m=" lines, or reducing the number of codecs, in the SDP BAS
   Offer used for the BAS is considered to have a low risk of being
   rejected.

   NOTE: The main purpose of the BAS Offer is to make sure that
   intermediaries, that might not support the BUNDLE mechanism, have
   correct information regarding which IP address and port is going to be used for
   the bundled media.

   Section 10.1 shows an example of an SDP Offer used to perform a BAS.

6.4.3. BAS Offer.

6.4.4.  Adding a media description to a BUNDLE group

   When an Offerer generates an Offer, in which it adds an "m=" line to
   a BUNDLE group, the Offerer MUST
   assign either a unique address, or the BUNDLE assigns an address associated with
   the BUNDLE group, (unique or shared) to
   the added "m=" line.  In addition, the Offerer
   MUST assign a mid value line, assigns an SDP 'mid' attribute to the "m=" line, and place
   places the mid value in the
   SDP group:BUNDLE attribute mid list
   associated with the BUNDLE group,
   in order to group the "m=" line according to the BUNDLE group.

   NOTE: procedures in
   Section 6.4.2.  If the Offerer assigns a unique address to the added "m=" line,
   it allows wishes the Answerer to move the "m=" line out of select the BUNDLE group
   without having
   address assigned to reject the "m=" line ([ref-to-be-added]).

   To the previously added "m=" lines in as the Offerer's BUNDLE group, the Offerer
   assigns either unique addresses or address,
   the BUNDLE address mid value associated with the BUNDLE group, "m=" line is placed first in the
   list, according to the procedures in Section 6.4.1. 6.4.2.

   Section 10.3 shows an example of an SDP Offer used to add an "m=" line to
   a BUNDLE group.

6.4.4.

6.4.5.  Moving A Media Description Out Of A BUNDLE Group

   When an Offerer moves generates an Offer, in which an "m=" line is moved
   out of a BUNDLE group, the Offerer MUST assign a unique address to
   the moved "m=" line.  In addition, the Offerer MUST NOT anymore use
   include a mid value to group the "m=" line
   with the BUNDLE group.

   To value, representing the remaining "m=" lines line, in the BUNDLE group, the Offerer assigns
   either unique addresses or the BUNDLE address SDP
   group:BUNDLE attribute mid list associated with the BUNDLE group, according to the procedures in Section 6.4.1. group.

   Section 10.4 shows an example of an SDP Offer used to move an "m=" line
   out of a BUNDLE group.

6.4.5.

6.4.6.  Disabling A Media Description In A BUNDLE Group

   When an Offerer disables generates an Offer, in which an "m=" line in associated
   with a BUNDLE group, group is disabled, the Offerer MUST assign an address
   with a zero port value [RFC3264] [RFC4566] to the disabled "m=" line.  In
   addition, the Offerer MUST NOT anymore use include a mid value to group
   the "m=" line with the BUNDLE group.

   To value,
   representing the remaining "m=" lines line, in the BUNDLE group, the Offerer assigns
   either unique addresses or the BUNDLE address SDP group:BUNDLE attribute mid
   list associated with the BUNDLE group, according group.

   OPEN ISSUE (Q8): It still needs to the procedures in Section 6.4.1. be decided whether a zero port
   value can be assigned to a 'bundle-only' "m=" line.

   o  (http://www.ietf.org/mail-archive/web/mmusic/current/
      msg12075.html)

   o  (http://www.ietf.org/mail-archive/web/mmusic/current/
      msg12226.html)

   o  (http://www.ietf.org/mail-archive/web/mmusic/current/
      msg12339.html)

   Section 10.5 shows an example of an SDP Offer used to move disable an "m="
   line out of in a BUNDLE group.

6.5.  SDP Answerer Procedures
6.5.1.  Answerer Bundle Address Selection and Usage

6.5.1.1.  Offerer Bundle Address Selection

   If the Offerer, in

   When an SDP Offer, assigned a unique address to each
   "m=" line in Answerer generates an Answer that contains a BUNDLE group, it means that the Offerer has requested
   the Answerer to Answer MUST select a BUNDLE address for the Offerer. Offerer's BUNDLE address.  The first mid
   value in the SDP group:BUNDLE attribute mid list of the SDP Offer
   represents the unique address which the Offerer requests wishes the Answer to select
   as the Offerer's BUNDLE address. address Section 6.4.2.

   The Answerer SHOULD select the unique address associated with represented by the first mid to become the Offerer's BUNDLE address,
   value, unless the Answerer in the SDP associated Answer will move reject the
   "m=" line represented by associated with the mid out
   of value, or remove the "m=" line from
   the BUNDLE group, or if there is some other reason why group.  In such case the Answerer can not MUST select the unique an address
   associated with the mid.
   In first unrejected mid value that case, the Answerer MUST try remains in the next
   SDP group:BUNDLE attribute mid in list of the list. Offer.

   In the SDP Answer, the Answerer MUST place the mid value associated
   with the selected Offerer's BUNDLE address first in the SDP
   group:BUNDLE attribute mid list associated with the BUNDLE group.

   Section 10.1 shows an example of an Offerer's BUNDLE address
   selection.

6.5.1.2.

6.5.2.  Anwerer Bundle Address Selection

   The

   When an Answerer creates an Answer that contains a BUNDLE group, the
   Answerer MUST select assign a local BUNDLE shared address, and in the SDP
   Answer assign it Answerer's BUNDLE
   address, to each "m=" line associated with within the BUNDLE group.

   The Answerer is allowed to change its BUNDLE address in any SDP
   Answer.

   The Answerer MUST NOT assign a BUNDLE address shared address, that it has assigned
   to an "m=" line that is
   not associated with a BUNDLE group.  The Answerer MUST NOT assign a
   BUNDLE address, associated with within a BUNDLE group, to an "m=" line
   associated with another outside the
   BUNDLE group.

   Section 10.1 shows an example of an Answerer's local BUNDLE address
   selection.

6.5.2.

6.5.3.  Moving A Media Description Out Of A BUNDLE Group

   When an Answerer moves generates an Answer, in which an "m=" line is moved
   out of a BUNDLE group, the Answerer MUST assign a unique assigns an address to the moved
   "m=" line.  In
   addition, line based on the Answerer MUST NOT anymore use a mid value type of address that the Offerer assigned to group
   the associated "m=" line with in the BUNDLE group.

   To associated Offer, as described below.

   If the Offerer assigned a shared address to the remaining "m=" lines line, the
   answerer MUST reject the moved "m=" line, according to the procedures
   in Section 6.5.4.

   If the BUNDLE group, Offerer assigned an SDP 'bundle-only' attribute to the Answerer assigns "m="
   line, the Answerer's BUNDLE address.

   An Answerer MUST NOT move an reject the moved "m=" line out of a BUNDLE group, unless:

   o  1) The line, according to the
   procedures in Section 6.5.4.

   If the Offerer assigned a unique address to the "m=" line line, the Answer
   MUST assign a unique address to the moved "m=" line.

   In addition, in either case above, the
      associated SDP Offer; or

   o  2) The Answerer also rejects MUST NOT anymore
   include a mid value, representing the "m=" line Section 6.5.3.

6.5.3. line, in the SDP
   group:BUNDLE attribute list associated with the BUNDLE group.

6.5.4.  Rejecting A Media Description In A BUNDLE Group

   When an Answerer rejects generates an Answer, in which an "m=" line in
   associated with a BUNDLE group, group is rejected, the Answerer MUST assign
   an address with a zero port value to the rejected "m=" line. line,
   according to the procedures in [RFC4566].  In addition, the Answerer
   MUST NOT anymore use include a mid value to group the
   "m=" line with the BUNDLE group.

   To value, representing the remaining "m=" lines line, in
   the BUNDLE group, the Answerer assigns SDP group:BUNDLE attribute midlist associated with the Answerer's BUNDLE address.
   group.

7.  Single vs Multiple RTP Sessions

7.1.  General

   By default, all RTP based media flows within a given BUNDLE group
   belong to a single RTP session [RFC3550].  Multiple BUNDLE groups
   will form multiple RTP Sessions.

   The usage of multiple RTP Sessions within a given BUNDLE group, or
   the usage of a single RTP Session that spans over multiple BUNDLE
   groups, is outside the scope of this specification.  Other
   specification needs to extend the BUNDLE mechanism in order to allow
   such usages.

7.2.  Single RTP Session

   When a single RTP Session is used, media associated with all "m="
   lines part of a bundle group share a single SSRC [RFC3550] numbering
   space.

   In addition, the following rules and restrictions apply for a single
   RTP Session:

   o  -  The dynamic payload type values used in the "m=" lines MUST NOT
      overlap.

   o  -  The "proto" value in each "m=" line MUST be identical (e.g. RTP/
      AVPF).

   o  -  A given SSRC SHOULD NOT transmit RTP packets using payload types
      that originates from different "m=" lines.

   NOTE: The last bullet above is to avoid sending multiple media types
   from the same SSRC.  If transmission of multiple media types are done
   with time overlap RTP and RTCP fails to function.  Even if done in
   proper sequence this causes RTP Timestamp rate switching issues [ref
   to draft-ietf-avtext-multiple-clock-rates].

8.  Usage With ICE

8.1.  General

   This section describes how to use the BUNDLE grouping extension
   together with the Interactive Connectivity Establishment (ICE)
   mechanism [RFC5245].

8.2.  Candidates

   When an ICE-enabled endpoint generates an SDP Offer, which contains a
   BUNDLE group, the SDP Offerer MUST include ICE candidates for each
   "m=" line associated with a "BUNDLE" group, except for any "m=" line
   with a zero port number value.  If the "m=" lines associated with the
   BUNDLE group contain different port number values, the SDP Offerer
   MUST also insert different candidate values in each "m=" line
   associated with the BUNDLE group.  If the "m=" lines associated with
   the BUNDLE group contain an identical port number value, the
   candidate values MUST also be identical.

   When an ICE-enabled endpoint generates and SDP Answer, which contains
   a BUNDLE group, the Answerer MUST include ICE candidates for each
   "m=" line associated with the "BUNDLE" group, except for any "m="
   line where the port number value is set to zero.  The Answerer MUST
   insert identical candidate values in each "m=" line associated with
   the BUNDLE group.

8.3.  Candidates

   Once it is known that both endpoints support, and accept to use, the
   BUNDLE grouping extension, ICE connectivity checks and keep-alives
   only needs to be performed for the whole BUNDLE group, instead of for
   each individual "m=" line associated with the group.

9.  Security Considerations
   This specification does not significantly change the security
   considerations of SDP which can be found in Section X of TBD.

   TODO: Think carefully about security analysis of reuse of same SDES
   key on multiple "m=" lines when the far end does not use BUNDLE and
   warn developers of any risks.

10.  Examples

10.1.  Example: Bundle Address Selection

   The example below shows:

   o  1.  An SDP Offer, in which the Offerer assigns unique addresses to
      each "m=" line in the BUNDLE group, and requests the Answerer to
      select the Offerer's BUNDLE address.

   o  2.  An SDP Answer, in which the Answerer selects the BUNDLE
      address for the Offerer, and assigns its own local BUNDLE address
      to each "m=" line in the BUNDLE group.

   o  3.  A subsequent SDP Offer, which is used to perform a Bundle
      Address Synchronization (BAS).

   SDP Offer (1)

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       a=group:BUNDLE foo bar
       m=audio 10000 RTP/AVP 0 8 97
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       a=rtpmap:8 PCMA/8000
       a=rtpmap:97 iLBC/8000
       m=video 10002 RTP/AVP 31 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:31 H261/90000
       a=rtpmap:32 MPV/90000

   SDP Answer (2)
       v=0
       o=bob 2808844564 2808844564 IN IP4 host.biloxi.com biloxi.example.com
       s=
       c=IN IP4 host.biloxi.com biloxi.example.com
       t=0 0
       a=group:BUNDLE foo bar
       m=audio 20000 RTP/AVP 0
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       m=video 20000 RTP/AVP 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:32 MPV/90000

   SDP Offer (3)

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       a=group:BUNDLE foo bar
       m=audio 10000 RTP/AVP 0 8 97
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       a=rtpmap:8 PCMA/8000
       a=rtpmap:97 iLBC/8000
       m=video 10000 RTP/AVP 31 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:31 H261/90000
       a=rtpmap:32 MPV/90000

10.2.  Example: Bundle Mechanism Rejected

   The example below shows:

   o  1.  An SDP Offer, in which the Offerer assigns unique addresses to
      each "m=" line in the BUNDLE group, and requests the Answerer to
      select the Offerer's BUNDLE address.

   o  2.  An SDP Answer, in which the Answerer rejects the BUNDLE group,
      and assigns unique addresses to each "m=" line.

   SDP Offer (1)

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       a=group:BUNDLE foo bar
       m=audio 10000 RTP/AVP 0 8 97
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       a=rtpmap:8 PCMA/8000
       a=rtpmap:97 iLBC/8000
       m=video 10002 RTP/AVP 31 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:31 H261/90000
       a=rtpmap:32 MPV/90000

   SDP Answer (2)

       v=0
       o=bob 2808844564 2808844564 IN IP4 host.biloxi.com biloxi.example.com
       s=
       c=IN IP4 host.biloxi.com biloxi.example.com
       t=0 0
       m=audio 20000 RTP/AVP 0
       b=AS:200
       a=rtpmap:0 PCMU/8000
       m=video 30000 RTP/AVP 32
       b=AS:1000
       a=rtpmap:32 MPV/90000

10.3.  Example: Offerer Adds A Media Description To A BUNDLE Group

   The example below shows:

   o  1.  An SDP Offer, in which the Offerer adds an "m=" line,
      represented by the "zen" mid value, to a previously negotiated
      BUNDLE group, assigns a unique address to the added "m=" line, and
      assigns the previously negotiated BUNDLE address to the previously
      added "m=" lines in the BUNDLE group.

   o  2.  An SDP Answer, in which the Answerer assigns its own local
      BUNDLE address to each "m=" line (including the added "m=" line)
      in the BUNDLE group.

   o  3.  A subsequent SDP Offer, which is used to perform a Bundle
      Address Synchronization (BAS).

   SDP Offer (1)

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       a=group:BUNDLE foo bar zen
       m=audio 10000 RTP/AVP 0 8 97
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       a=rtpmap:8 PCMA/8000
       a=rtpmap:97 iLBC/8000
       m=video 10000 RTP/AVP 31 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:31 H261/90000
       a=rtpmap:32 MPV/90000
       m=video 20000 RTP/AVP 66
       a=mid:zen
       b=AS:1000
       a=rtpmap:66 H261/90000

   SDP Answer (2)

       v=0
       o=bob 2808844564 2808844564 IN IP4 host.biloxi.com biloxi.example.com
       s=
       c=IN IP4 host.biloxi.com biloxi.example.com
       t=0 0
       a=group:BUNDLE foo bar zen
       m=audio 20000 RTP/AVP 0
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       m=video 20000 RTP/AVP 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:32 MPV/90000
       m=video 20000 RTP/AVP 66
       a=mid:zen
       b=AS:1000
       a=rtpmap:66 H261/90000

   SDP Offer (3)

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       a=group:BUNDLE foo bar zen
       m=audio 10000 RTP/AVP 0 8 97
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       a=rtpmap:8 PCMA/8000
       a=rtpmap:97 iLBC/8000
       m=video 10000 RTP/AVP 31 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:31 H261/90000
       a=rtpmap:32 MPV/90000
       m=video 10000 RTP/AVP 66
       a=mid:zen
       b=AS:1000
       a=rtpmap:66 H261/90000

10.4.  Example: Offerer Moves A Media Description Out Of A BUNDLE Group

   The example below shows:

   o  1.  An SDP Offer, in which the Offerer moves an "m=" line out of a
      previously negotiated BUNDLE group, assigns a unique address to
      the moved "m=" line, and assigns the previously negotiated BUNDLE
      address to the remaining "m=" lines in the BUNDLE group.

   o  2.  An SDP Answer, in which the Answerer moves the corresponding
      "m=" line out of the BUNDLE group, and assigns unique address to
      the moved "m=" line, and assigns the previously negotiated BUNDLE
      address to the remaining "m=" lines in the BUNDLE group.

   SDP Offer (1)

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       a=group:BUNDLE foo bar
       m=audio 10000 RTP/AVP 0 8 97
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       a=rtpmap:8 PCMA/8000
       a=rtpmap:97 iLBC/8000
       m=video 10000 RTP/AVP 31 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:31 H261/90000
       a=rtpmap:32 MPV/90000
       m=video 50000 RTP/AVP 66
       b=AS:1000
       a=rtpmap:66 H261/90000

   SDP Answer (2)

       v=0
       o=bob 2808844564 2808844564 IN IP4 host.biloxi.com biloxi.example.com
       s=
       c=IN IP4 host.biloxi.com biloxi.example.com
       t=0 0
       a=group:BUNDLE foo bar
       m=audio 20000 RTP/AVP 0
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       m=video 20000 RTP/AVP 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:32 MPV/90000
       m=video 60000 RTP/AVP 66
       b=AS:1000
       a=rtpmap:66 H261/90000

10.5.  Example: Offerer Disables A Media Description In A BUNDLE Group

   The example below shows:

   o  1.  An SDP Offer, in which the Offerer moves an "m=" line out of a
      previously negotiated BUNDLE group, assigns a zero port number the
      moved "m=" line in order to disable it, and assigns the previously
      negotiated BUNDLE address to the remaining "m=" lines in the
      BUNDLE group.

   o  2.  An SDP Answer, in which the Answerer moves the corresponding
      "m=" line out of the BUNDLE group, and assigns a zero port value
      to the moved "m=" line in order to disable it, and assigns the
      previously negotiated BUNDLE address to the remaining "m=" lines
      in the BUNDLE group.

   SDP Offer (1)

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       a=group:BUNDLE foo bar
       m=audio 10000 RTP/AVP 0 8 97
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       a=rtpmap:8 PCMA/8000
       a=rtpmap:97 iLBC/8000
       m=video 10000 RTP/AVP 31 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:31 H261/90000
       a=rtpmap:32 MPV/90000
       m=video 0 RTP/AVP 66
       a=rtpmap:66 H261/90000

   SDP Answer (2)

       v=0
       o=bob 2808844564 2808844564 IN IP4 host.biloxi.com biloxi.example.com
       s=
       c=IN IP4 host.biloxi.com biloxi.example.com
       t=0 0
       a=group:BUNDLE foo bar
       m=audio 20000 RTP/AVP 0
       a=mid:foo
       b=AS:200
       a=rtpmap:0 PCMU/8000
       m=video 20000 RTP/AVP 32
       a=mid:bar
       b=AS:1000
       a=rtpmap:32 MPV/90000
       m=video 0 RTP/AVP 66
       a=rtpmap:66 H261/90000

11.  IANA Considerations

   This document requests IANA to register the new SDP Grouping semantic
   extension called BUNDLE.

12.  Acknowledgements

   The usage of the SDP grouping extension for negotiating bundled media
   is based on a similar alternatives proposed by Harald Alvestrand and
   Cullen Jennings.  The BUNDLE mechanism described in this document is
   based on the different alternative proposals, and text (e.g. SDP
   examples) have been borrowed (and, in some cases, modified) from
   those alternative proposals.

   The SDP examples are also modified versions from the ones in the
   Alvestrand proposal.

   Thanks to Paul Kyzivat and Martin Thompson for taking the the time to
   read the text along the way, and providing useful feedback.

13.  Change Log

   [RFC EDITOR NOTE: Please remove this section when publishing]

   Changes from draft-ietf-mmusic-sdp-bundle-negotiation-04

   o  Updated Offerer procedures (http://www.ietf.org/mail-archive/web/
      mmusic/current/msg12293.html).

   o  Updated Answerer procedures (http://www.ietf.org/mail-archive/web/
      mmusic/current/msg12333.html).

   o  Usage of SDP 'bundle-only' attribute added.

   o  Reference to Trickle ICE document added.

   Changes from draft-ietf-mmusic-sdp-bundle-negotiation-02

   o  Mechanism modified, to be based on usage of SDP Offers with both
      different and identical port number values, depending on whether
      it is known if the remote endpoint supports the extension.

   o  Cullen Jennings added as co-author.

   Changes from draft-ietf-mmusic-sdp-bundle-negotiation-01

   o  No changes.  New version due to expiration.

   Changes from draft-ietf-mmusic-sdp-bundle-negotiation-00

   o  No changes.  New version due to expiration.

   Changes from draft-holmberg-mmusic-sdp-multiplex-negotiation-00

   o  Draft name changed.

   o  Harald Alvestrand added as co-author.

   o  "Multiplex" terminology changed to "bundle".

   o  Added text about single versus multiple RTP Sessions.

   o  Added reference to RFC 3550.

14.  References

14.1.  Normative References

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

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264, June
              2002.

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

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, April 2010.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888, June 2010.

   [I-D.nandakumar-mmusic-sdp-attributes]

   [I-D.nandakumar-mmusic-sdp-mux-attributes]
              Nandakumar, S. and C. Jennings, "A Framework for SDP
              Attributes when Multiplexing", Multiplexing ", draft-nandakumar-mmusic-
              sdp-attributes-00
              sdp-mux-attributes-04 (work in progress), February September 2013.

14.2.  Informative References

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

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

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

   [I-D.ietf-mmusic-trickle-ice]
              Ivov, E., Rescorla, E., and J. Uberti, "Trickle ICE:
              Incremental Provisioning of Candidates for the Interactive
              Connectivity Establishment (ICE) Protocol ", draft-ietf-
              mmusic-trickle-ice-00 (work in progress), October 2013.

Appendix A.  Design Considerations

A.1.  General

   One of the main issues regarding the BUNDLE grouping extensions has
   been whether, in SDP Offers and SDP Answers, the same port number
   value should be inserted in "m=" lines associated with a BUNDLE
   group, as the purpose of the extension is to negotiate the usage of a
   single 5-tuple for media associated with the "m=" lines.  Issues with
   both approaches, discussed in the Appendix have been raised.  The
   outcome was to specify a mechanism which uses SDP Offers with both
   different and identical port number values.

   Below are the primary issues that have been considered when defining
   the "BUNDLE" grouping extension:

   o  1) Interoperability with existing UAs.

   o  2) Interoperability with intermediary B2BUA- and proxy entities.

   o  3) Time to gather, and the number of, ICE candidates.

   o  4) Different error scenarios, and when they occur.

   o  5) SDP Offer/Answer impacts, including usage of port number value
      zero.

   NOTE: Before this document is published as an RFC, this
   Appendix might be removed.

A.2.  UA Interoperability

   Consider the following SDP Offer/Answer exchange, where Alice sends
   an SDP Offer to Bob:

   SDP Offer

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       m=audio 10000 RTP/AVP 97
       a=rtpmap:97 iLBC/8000
       m=video 10002 RTP/AVP 97
       a=rtpmap:97 H261/90000

   SDP Answer

       v=0
       o=bob 2808844564 2808844564 IN IP4 host.biloxi.com biloxi.example.com
       s=
       c=IN IP4 host.biloxi.com biloxi.example.com
       t=0 0
       m=audio 20000 RTP/AVP 97
       a=rtpmap:97 iLBC/8000
       m=video 20002 RTP/AVP 97
       a=rtpmap:97 H261/90000

   RFC 4961 specifies a way of doing symmetric RTP but that is an a
   later invention to RTP and Bob can not assume that Alice supports RFC
   4961.  This means that Alice may be sending RTP from a different port
   than 10000 or 10002 - some implementation simply send the RTP from an
   ephemeral port.  When Bob's endpoint receives an RTP packet, the only
   way that Bob know if it should be passed to the video or audio codec
   is by looking at the port it was received on.  This lead some SDP
   implementations to use the fact that each "m=" line had a different
   port number to use that port number as an index to find the correct m
   line in the SDP.  As a result, some implementations that do support
   symmetric RTP and ICE still use a SDP data structure where SDP with
   "m=" lines with the same port such as:

   SDP Offer

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       m=audio 10000 RTP/AVP 97
       a=rtpmap:97 iLBC/8000
       m=video 10000 RTP/AVP 98
       a=rtpmap:98 H261/90000

   will result in the second "m=" line being considered an SDP error
   because it has the same port as the first line.

A.3.  Usage of port number value zero

   In an SDP Offer or SDP Answer, the media associated with an "m=" line
   can be disabled/rejected by setting the port number value to zero.
   This is different from e.g. using the SDP direction attributes, where
   RTCP traffic will continue even if the SDP "inactive" attribute is
   indicated for the associated "m=" line.

   If each "m=" line associated with a BUNDLE group would contain
   different port number values, and one of those port would be used for
   the 5-tuple, problems would occur if an endpoint wants to disable/
   reject the "m=" line associated with that port, by setting the port
   number value to zero.  After that, no "m=" line would contain the
   port number value which is used for the 5-tuple.  In addition, it is
   unclear what would happen to the ICE candidates associated with the
   "m=" line, as they are also used for the 5-tuple.

A.4.  B2BUA And Proxy Interoperability

   Some back to back user agents may be configured in a mode where if
   the incoming call leg contains an SDP attribute the B2BUA does not
   understand, the B2BUS still generates that SDP attribute in the Offer
   for the outgoing call leg.  Consider an B2BUA that did not understand
   the SDP "rtcp" attribute, defined in RFC 3605, yet acted this way.
   Further assume that the B2BUA was configured to tear down any call
   where it did not see any RTCP for 5 minutes.  In this cases, if the
   B2BUA received an Offer like:

   SDP Offer

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com atlanta.example.com
       s=
       c=IN IP4 host.atlanta.com atlanta.example.com
       t=0 0
       m=audio 49170 RTP/AVP 0
       a=rtcp:53020

   It would be looking for RTCP on port 49172 but would not see any
   because the RTCP would be on port 53020 and after five minutes, it
   would tear down the call.  Similarly, an SBC that did not understand
   BUNDLE yet put BUNDLE in it's offer may be looking for media on the
   wrong port and tear down the call.  It is worth noting that a B2BUA
   that generated an Offer with capabilities it does not understand is
   not compliant with the specifications.

A.4.1.  Traffic Policing

   Sometimes intermediaries do not act as B2BUA, in the sense that they
   don't modify SDP bodies, nor do they terminate SIP dialogs.  Still,
   however, they may use SDP information (e.g. IP address and port) in
   order to control traffic gating functions, and to set traffic
   policing rules.  There might be rules which will trigger a session to
   be terminated in case media is not sent or received on the ports
   retrieved from the SDP.  This typically occurs once the session is
   already established and ongoing.

A.4.2.  Bandwidth Allocation

   Sometimes intermediaries do not act as B2BUA, in the sense that they
   don't modify SDP bodies, nor do they terminate SIP dialogs.  Still,
   however, they may use SDP information (e.g. codecs and media types)
   in order to control bandwidth allocation functions.  The bandwidth
   allocation is done per "m=" line, which means that it might not be
   enough if media associated with all "m=" lines try to use that
   bandwidth.  That may either simply lead to bad user experience, or to
   termination of the call.

A.5.  Candidate Gathering

   When using ICE, an candidate needs to be gathered for each port.
   This takes approximately 20 ms extra for each extra "m=" line due to
   the NAT pacing requirements.  All of this gather can be overlapped
   with other things while the page is loading to minimize the impact.
   If the client only wants to generate TURN or STUN ICE candidates for
   one of the "m=" lines and then use trickle ICE [TODO REF]
   [I-D.ietf-mmusic-trickle-ice] to get the non host ICE candidates for
   the rest of the "m=" lines, it MAY do that and will not need any
   additional gathering time.

   Some people have suggested a TURN extension to get a bunch of TURN
   allocation at once.  This would only provide a single STUN result so
   in cases where the other end did not support BUNDLE, may cause more
   use of the TURN server but would be quick in the cases where both
   sides supported BUNDLE and would fall back to a successful call in
   the other cases.

Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com

   Harald Tveit Alvestrand
   Google
   Kungsbron 2
   Stockholm  11122
   Sweden

   Email: harald@alvestrand.no
   Cullen Jennings
   Cisco
   400 3rd Avenue SW, Suite 350
   Calgary, AB  T2P 4H2
   Canada

   Email: fluffy@iii.ca