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INTERNET-DRAFT                        Joerg Ott/Helsinki Univ of Tech
draft-ietf-avt-profile-savpf-03.txt       Elisabetta Carrara/Ericsson

                                                         October 2005
                                                   Expires April 2006


     Extended Secure RTP Profile for RTCP-based Feedback (RTP/SAVPF)


   Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   Abstract

   An RTP profile (SAVP) is defined for secure real-time
   communications, and another profile (AVPF) is specified to provide
   timely feedback from the receivers to a sender.  This memo defines
   the combination of both profiles to enable secure RTP
   communications with feedback.








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   Table of Contents

   1  Introduction.................................................2
      1.1  Definitions..............................................3
      1.2  Terminology..............................................4
   2  SAVPF Rules..................................................4
      2.1  Packet Formats...........................................5
      2.2  Extensions...............................................5
      2.3  Implications from combining AVPF and SAVP................5
   3  SDP Definitions..............................................6
      3.1  Profile Definition.......................................6
      3.2  Attribute Definitions....................................6
      3.3  Profile Negotiation......................................6
       3.3.1 Offer/Answer-based Negotiation of Session Descriptions.6
       3.3.2 RTSP-based Negotiation of Session Descriptions.........7
       3.3.3 Announcing Session Descriptions........................7
       3.3.4 Describing Alternative Session Profiles................8
      3.4  Examples.................................................9
   4  Interworking of AVP, SAVP, AVPF, and SAVPF Entities.........12
   5  Security Considerations.....................................12
   6  IANA Considerations.........................................13
   7  Acknowledgements............................................14
   8  Authors' Addresses..........................................14
   9  Bibliography................................................14
      9.1  Normative references....................................14
      9.2  Informative References..................................15
   10 IPR Notice..................................................15
   11 Disclaimer of Validity......................................16
   12 Full Copyright Statement....................................16
   13 Acknowledgment..............................................16



1  Introduction

   The Real-time Transport Protocol (RTP/RTCP) [1] and the associated
   profile for audiovisual communications with minimal control [2]
   define mechanisms for transmitting time-based media across an IP
   network.  RTP provides means to preserve timing and detect packet
   losses, among other things, and RTP payload formats provide for
   proper framing of (continuous) media in a packet-based environment.
   RTCP enables receivers to provide feedback on reception quality and
   allows all members of an RTP session to learn about each other.


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   The RTP specification provides only rudimentary support for
   encrypting RTP and RTCP packets.  SRTP [4] defines an RTP profile
   ("SAVP") for secure RTP media sessions, defining methods for proper
   RTP and RTCP packet encryption, integrity and replay protection.
   The initial negotiation of SRTP and its security parameters needs
   to be done out of band, using e.g. the Session Description Protocol
   (SDP) [6] together with extensions for conveying keying material
   [7][8].

   The RTP specification also provides limited support for timely
   feedback from receivers to senders, typically by means of reception
   statistics reporting in somewhat regular intervals depending on the
   group size, the average RTCP packet size, and the available RTCP
   bandwidth.  The extended RTP profile for RTCP-based feedback
   ("AVPF") [3] allows receivers statistically to provide immediate
   feedback while maintaining the average RTCP data rate for all
   senders.  As for SAVP, the use of AVPF and its parameters needs to
   be negotiated out-of-band by means of SDP [6] and the extensions
   defined in [3].

   Both SRTP and AVPF are RTP profiles and need to be negotiated.
   This implies that either one or the other may be used, but both
   profiles cannot be negotiated for the same RTP session (using one
   SDP session level description).  However, using secure
   communications and timely feedback together is desirable.
   Therefore, this document specifies a new RTP profile ("SAVPF") that
   combines the features of SAVP and AVPF.

   As SAVP and AVPF are largely orthogonal, the combination of both is
   mostly straightforward.  No sophisticated algorithms need to be
   specified in this document.  Instead, reference is made to both
   existing profiles and only the implications of their combination
   and possible deviations from rules of the existing profiles are
   described as is the negotiation process.


   1.1  Definitions

   The definitions of [1], [2], [3], and [4] apply.

   The following definitions are specifically used in this document:

   RTP session:
        An association among a set of participants communicating with
        RTP as defined in [1].

   (SDP) media description:
        This term refers to the specification given in a single m=
        line in an SDP message.  An SDP media description may define
        only one RTP session.  Grouping of m= lines in SDP may cause

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        several SDP session level descriptions to define (alternatives
        of) the same RTP session for the same media type.

   Media session:
        A media session refers to a collection of SDP media
        descriptions that are semantically grouped to represent
        alternatives of the same communications means.  Out of such a
        group, one will be negotiated or chosen for a communication
        relationship and the corresponding RTP session will be
        instantiated.  Or the media session will be rejected.
        In the simplest case, a media session is equivalent to an SDP
        media description and equivalent to an RTP session.


   1.2  Terminology

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
    NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
    in this document are to be interpreted as described in RFC 2119
    [5].


2  SAVPF Rules

   SAVP is defined as an intermediate layer between RTP (following the
   regular RTP profile AVP) and the transport layer (usually UDP).
   This yields a two layer hierarchy within the Real-time Transport
   Protocol.  In SAVPF, the upper (AVP) layer is replaced by the
   extended RTP profile for feedback (AVPF).

   AVPF modifies timing rules for transmitting RTCP packets and adds
   extra RTCP packet formats specific to feedback.  These functions
   are independent of whether or not RTCP packets are subsequently
   encrypted and/or integrity protected.  The functioning of the AVPF
   layer remains unchanged in SAVPF.

   The AVPF profile derives from [1] the (optional) use of the
   encryption prefix for RTCP. The encryption prefix MUST NOT be used
   within the SAVPF profile (it is not used in SAVP, as it is only
   applicable to the encryption method specified in [1]).

   The SAVP part uses extra fields added to the end of RTP and RTCP
   packets and executes cryptographic transforms on (some of) the
   RTP/RTCP packet contents.  This behavior remains unchanged in
   SAVPF.  The average RTCP packet size calculation done by the AVPF
   layer for timing purposes MUST take into account the fields added
   by the SAVP layer.




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   The SRTP part becomes only active whenever the RTP or RTCP was
   scheduled by the "higher" AVPF layer or received from the transport
   protocol, irrespective of its timing and contents.


   2.1  Packet Formats

   AVPF defines extra packet formats to provide feedback information.
   Those extra packet formats defined in [3] (and further ones defined
   elsewhere for use with AVPF) MAY be used with SAVPF.

   SAVP defines a modified packet format for SRTP and SRTCP packets
   that essentially consists of the RTP/RTCP packet formats plus some
   trailing protocol fields for security purposes.  For SAVPF, all
   RTCP packets MUST be encapsulated as defined in section 3.4 of [4].


   2.2  Extensions

   Extensions to AVPF RTCP feedback packets defined elsewhere MAY be
   used with the SAVPF profile provided that those extensions are in
   conformance with the extension rules of [3].

   Additional extensions (e.g., transforms) defined for SAVP following
   the rules of section 6 of [4] MAY also be used with the SAVPF
   profile.  The overhead per RTCP packet depends on the extensions
   and transforms chosen. New extensions and transforms added in the
   future MAY introduce yet unknown further per-packet overhead.

   Finally, further extensions specifically to SAVPF MAY be defined
   elsewhere.


   2.3  Implications from combining AVPF and SAVP

   The AVPF profile aims at -- statistically -- allowing receivers to
   provide timely feedback to senders.  The frequency at which
   receivers are, on average, allowed to send feedback information
   depends on the RTCP bandwidth, the group size, and the average size
   of an RTCP packet.  SRTCP (see Section 3.4 of [4]) adds extra
   fields (some of which are of configurable length) at the end of
   each RTCP packet that are probably at least some 10 to 20 bytes in
   size (14 bytes as default).  Note that extensions and transforms
   defined in the future, as well as the configuration of each field
   length, MAY add greater overhead.  With this, the average size of
   an RTCP packet will increase and thus reduce the frequency at which
   (timely) feedback can be provided.  Application designers need to
   be aware of this, and take precautions so that the RTCP bandwidth
   shares are maintained.  This MUST be done by adjusting the RTCP


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   variable "avg_rtcp_size" to include the size of the fields that are
   added by SRTCP (index, E-bit, authentication tag, and when present,
   the MKI, as well any other field introduced by possible new SRTP
   extensions).


3  SDP Definitions

   3.1  Profile Definition

   The AV profiles defined in [2], [3], and [4] are referred to as
   "AVP", "AVPF", and "SAVP", respectively, in the context of e.g. the
   Session Description Protocol (SDP) [3].  The combined profile
   specified in this document is referred to as "SAVPF".


   3.2  Attribute Definitions

   SDP attributes for negotiating SAVP sessions are defined in [7] and
   [8].  Those attributes MAY also be used with SAVPF.  The rules
   defined in [7] and [8] apply.

   SDP attributes for negotiating AVPF sessions are defined in [3].
   Those attributes MAY also be used with SAVPF.  The rules defined in
   [3] apply.


   3.3  Profile Negotiation

   Session descriptions for RTP sessions may be conveyed using
   protocols dedicated for multimedia communications such as the SDP
   offer/answer model [10] used with SIP, RTSP [11], or SAP [12] but
   may also be distributed using email, NetNews, web pages, etc.

   The offer/answer model allows the resulting session parameters to
   be negotiated using the SDP attributes defined in [7] and [8].  In
   the following subsection, the negotiation process is described in
   terms of the offer/answer model.

   Afterwards, the cases that do not use the offer/answer model are
   addressed: RTSP-specific negotiation support is provided by [7] as
   discussed in subsection 3.3.2 and support for SAP announcements
   (with no negotiation at all) is addressed in subsection 3.3.3.


   3.3.1 Offer/Answer-based Negotiation of Session Descriptions

   Negotiations (e.g. of RTP profiles, codecs, transport addresses,
   etc.) are carried out on a per-media session basis (i.e. per m=

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   line in SDP).  If negotiating one media session fails, others MAY
   still succeed.

   Different RTP profiles MAY be used in different media sessions.
   For negotiation of a media description, the four profiles AVP,
   AVPF, SAVP, and SAVPF are mutually exclusive.  Note, however, that
   SAVP and SAVPF entities MAY be mixed in a single RTP session (see
   section 4).  Therefore, both MAY be offered as alternatives for the
   same media session (e.g. using the same transport parameters).

   An offerer that is capable of supporting multiple of these profiles
   for a certain media session SHOULD always offer all alternatives
   acceptable in a certain situation.  At least, SAVP and SAVPF SHOULD
   be offered as this does not impact security. However, the offers
   MUST NOT include both a secure alternative (SAVP and SAVPF) and an
   insecure alternative (e.g. AVP and AVPF) in the same offer (as this
   may open up for bidding down attacks) unless the signaling channel
   is protected against modification of SDP messages.

   If a media description in an offer uses SAVPF and the answerer does
   not support SAVPF, the media session MUST be rejected.

   If a media description in an offer does not use SAVPF but the
   answerer wants to use SAVPF, the answerer MUST reject the media
   session.  The answerer MAY provide a counter-offer with a media
   description indicating SAVPF in a subsequently initiated
   offer/answer exchange.


   3.3.2 RTSP-based Negotiation of Session Descriptions

   RTSP [11] does not support the offer/answer model.  However, RTSP
   supports negotiating media session parameters (including profile
   and address information) by means of the "Transport:" header.  SDP-
   based key management as defined in [7] adds a parameter to support
   conveying a key management protocol (including keying material).

   Hence, the RTSP "Transport:" header MAY be used to negotiate the
   profile for the media session.  The interoperability rules defined
   in section 3.3.1 SHALL apply.


   3.3.3 Announcing Session Descriptions

   Protocols that do not allow negotiate session descriptions
   interactively (e.g. SAP [12], descriptions posted on a web page or
   sent by mail) pose the responsibility for adequate access to the
   media sessions on the initiator of a session.



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   The initiator SHOULD provide alternative session descriptions for
   multiple RTP profiles as far as acceptable to the application and
   the purpose of the session.  If security is desired, SAVP may be
   offered as alternative to SAVPF -- but AVP or AVPF sessions SHOULD
   NOT be announced unless other security means not relying on SRTP
   are employed.

   The SDP attributes defined in [7] and [8] may also be used for the
   security parameter distribution of announced session descriptions.

   The security scheme description defined in [8] requires a secure
   communications channel to prevent third parties from eavesdropping
   on the keying parameters and manipulation.  Therefore, SAP security
   (as defined in [12]), S/MIME [13], HTTPS [14], or other suitable
   mechanisms SHOULD be used for distributing or accessing these
   session descriptions.


   3.3.4 Describing Alternative Session Profiles

   SAVP and SAVPF entities MAY be mixed in the same RTP session (see
   also section 4) and so may AVP and AVPF entities.  Other
   combinations -- i.e. between secure and insecure profiles -- in the
   same RTP session are not possible and SHALL NOT be used.

   Both insecure and secure profiles MAY be used in the same offer but
   only for different RTP sessions (i.e. sessions with different
   addresses and/or port number). A grouping mechanism as defined in
   [9] SHOULD be used to indicate semantic equivalence between the
   individual sessions and ensure that any receiver only joins one of
   them.

        Editor's note: An appropriate grouping mechanism may still
        have to be defined as FID [9] does not really match the
        semantics, nor does ANAT.  An Alternative Profile (AP) might
        be the correct approach.  This could be defined in a separate
        draft.  So far, we rely exclusively on the implicit grouping.

   For SAVP and SAVPF, the same RTP session MAY be used but it may be
   advisable to also use different ones in order to allow optimal
   support for feedback-enabled receivers.

   In case the same RTP session shall be used for both SAVP and SAVPF,
   two media descriptions need to be specified in SDP.  For the same
   RTP session both will use the same address and port numbers.  Those
   two media sessions SHOULD be grouped by using the mechanism defined
   in [9] to indicate semantic equivalence between the individual
   sessions and ensure that any receiver only joins one of them.



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   3.4  Examples

   Example 1: The following session description indicates a secure
   session made up from audio and DTMF [18] for point-to-point
   communication in which the DTMF stream uses Generic NACKs.  The key
   management protocol indicated is MIKEY.  This session description
   (the offer) could be contained in a SIP INVITE or 200 OK message to
   indicate that its sender is capable of and willing to receive
   feedback for the DTMF stream it transmits.  The corresponding
   answer may be carried in a 200 OK or an ACK.  The parameters for
   the security protocol are negotiated as described by the SDP
   extensions defined in [7].

      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Media with feedback
      t=0 0
      c=IN IP4 host.example.com
      m=audio 49170 RTP/SAVPF 0 96
      a=rtpmap:0 PCMU/8000
      a=rtpmap:96 telephone-event/8000
      a=fmtp:96 0-16
      a=rtcp-fb:96 nack
      a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...


   Example 2: This example is replicated from example 1 above but
   shows the interaction between the offerer and the answered in an
   offer/answer exchange: The offerer indicates that it supports both
   AVPF and SAVPF and uses two implicitly grouped m= lines to show its
   intention of using them as alternatives.  The answerer rejects the
   non-secured profile (AVPF) in its response by setting the
   corresponding port number to 0.

      Offer:

      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Media with feedback
      t=0 0
      c=IN IP4 host.example.com
      a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
      m=audio 49170 RTP/SAVPF 0 96
      a=rtpmap:0 PCMU/8000
      a=rtpmap:96 telephone-event/8000
      a=fmtp:96 0-16
      a=rtcp-fb:96 nack
      m=audio 49170 RTP/AVPF 0 96
      a=rtpmap:0 PCMU/8000
      a=rtpmap:96 telephone-event/8000

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      a=fmtp:96 0-16
      a=rtcp-fb:96 nack

      Answer:

      v=0
      o=alice 3203093521 3203093521 IN IP4 host.another.example.com
      s=Media with feedback
      t=0 0
      c=IN IP4 host.another.example.com
      a=key-mgmt:mikey ushdgfdhgfuiweyfhjsgdkj2837do7eWsnDSJD...
      m=audio 53012 RTP/SAVPF 0 96
      a=rtpmap:0 PCMU/8000
      a=rtpmap:96 telephone-event/8000
      a=fmtp:96 0-16
      a=rtcp-fb:96 nack
      m=audio 0 RTP/AVPF 0 96


   Example 3: This example shows the same feedback parameters as
   example 1 but uses the secure descriptions syntax [8].  Note that
   the key part of the a=crypto attribute is not protected against
   eavesdropping and thus the session description should be exchanged
   over a secure communication channel.

      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Media with feedback
      t=0 0
      c=IN IP4 host.example.com
      m=audio 49170 RTP/SAVPF 0 96
      a=rtpmap:0 PCMU/8000
      a=rtpmap:96 telephone-event/8000
      a=fmtp:96 0-16
      a=rtcp-fb:96 nack
      a=crypto:AES_CM_128_HMAC_SHA1_32
        inline:d/16/14/NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj/2^20/1
        :32

   Example 4: The following session description indicates a multicast
   audio/video session (using PCMU for audio and either H.261 or
   H.263+) with the video source accepting Generic NACKs for both
   codecs and Reference Picture Selection for H.263.  The parameters
   for the security protocol are negotiated as described by the SDP
   extensions defined in [7], used at the session level. Such a
   description may have been conveyed using the Session Announcement
   Protocol (SAP).

      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com

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      s=Multicast video with feedback
      t=3203130148 3203137348
      a=key-mgmt:mikey uiSDF9sdhs7494ghsd/dhsoKkdOokdo7eWsnDSJD...
      m=audio 49170 RTP/SAVP 0
      c=IN IP4 224.2.1.183
      a=rtpmap:0 PCMU/8000
      m=video 51372 RTP/SAVPF 98 99
      c=IN IP4 224.2.1.184
      a=rtpmap:98 H263-1998/90000
      a=rtpmap:99 H261/90000
      a=rtcp-fb:* nack
      a=rtcp-fb:98 nack rpsi


   Example 5: The following session description defines the same media
   session as example 3 but allows for mixed mode operation of SAVP
   and SAVPF RTP entities (see also next section).  Note that both
   media descriptions use the same addresses; however, two m= lines
   are needed to convey information about both applicable RTP
   profiles.  The parameters for the security protocol are negotiated
   as described by SDP extensions defined in [7], used at the session
   level.


      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Multicast video with feedback
      t=3203130148 3203137348
      a=key-mgmt:mikey uiSDF9sdhs7854ghsd/dhsoKkdOokdo7eWsnDSJD...
      m=audio 49170 RTP/SAVP 0
      c=IN IP4 224.2.1.183
      a=rtpmap:0 PCMU/8000
      m=video 51372 RTP/SAVP 98 99
      c=IN IP4 224.2.1.184
      a=rtpmap:98 H263-1998/90000
      a=rtpmap:99 H261/90000
      m=video 51372 RTP/SAVPF 98 99
      c=IN IP4 224.2.1.184
      a=rtpmap:98 H263-1998/90000
      a=rtpmap:99 H261/90000
      a=rtcp-fb:* nack
      a=rtcp-fb:98 nack rpsi

   Note that these two m= lines SHOULD be grouped by some appropriate
   mechanism to indicate that both are alternatives actually conveying
   the same contents.  The basis for sample mechanism by which this
   can be achieved is defined in [9]; however, specific semantics for
   indicating alternative profiles need to be defined in a separate
   document.


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4  Interworking of AVP, SAVP, AVPF, and SAVPF Entities

   The SAVPF profile defined in this document is a combination of the
   SAVP profile [4] and the AVPF profile [3](which in turn is an
   extension of the RTP profile as defined in [2]).

   SAVP and SAVPF use SRTP [4] to achieve security.  AVP and AVPF use
   plain RTP [1] and hence do not provide security (unless external
   security mechanisms are applied as discussed in section 9.1 of
   [1]).  SRTP and RTP are not meant to interoperate, the respective
   protocol entities are not supposed to be part of the same RTP
   session.  Hence, AVP and AVPF on one side and SAVP and SAVPF on the
   other MUST NOT be mixed.

   RTP entities using the SAVP and the SAVPF profiles MAY be mixed in
   a single RTP session.  The interworking considerations defined in
   section 5 of [3] apply.


5  Security Considerations

   The SAVPF profile inherits its security properties from the SAVP
   profile; therefore it is subject to the security considerations
   discussed in [4].  The SAVP profile does not add, nor take away,
   any security services compared to SAVP.

   There is a desire to support security for media streams and, at the
   same time, for backward compatibility with non-SAVP(F) nodes.
   Application designers should be aware that security SHOULD NOT be
   traded for interoperability.  If information is to be distributed
   to closed groups (i.e. confidentially protected), it is RECOMMENDED
   not to offer alternatives for a media session other than SAVP and
   SAVPF as described in sections 3.3 and 3.4, unless other security
   mechanisms will be used, e.g. the ones described in Section 9.1 of
   [1]. Similarly, if integrity protection is considered important, it
   is RECOMMENDED not to offer the alternatives other than SAVP and
   SAVPF, unless other mechanisms are known to be in place that can
   guarantee it, e.g. lower-layer mechanisms as described in Section 9
   of [1].

   Offering secure and insecure profiles simultaneously may open to
   bidding down attacks. Therefore, such a mix of profile offer SHOULD
   NOT be made.

   Note that the rules for sharing master keys apply as described in
   [4] (e.g., Section 9.1). In particular, the same rules for avoiding
   the two-time pad (keystream reuse) apply: a master key MUST NOT be
   shared among different RTP sessions, and the SSRC MUST be unique

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   between all the RTP streams within the same RTP session that share
   the same master key.

   The key management MUST be called to provide new master key(s)
   (previously stored and used keys MUST NOT be used again), or the
   session MUST be terminated, when 2^48 SRTP packets or 2^31 SRTCP
   packets have been secured with the same key (whichever occurs
   before).

   Different media sessions may use a mix of different profiles,
   particularly including a secure profile and an insecure profile.
   However, mixing secure and insecure media sessions may reveal
   information to third parties and thus the decision to do so MUST be
   in line with a local security policy.  For example, the local
   policy MUST specify whether it is acceptable to have e.g. the audio
   stream not secured and the related video secured.

   The security considerations in [3] are valid too. Note in
   particular, applying the SAVPF profile implies mandatory integrity
   protection on RTCP.  While this solves the problem of false packets
   from members not belonging to the group, it does not solve the
   issues related to a malicious member acting improperly.


6  IANA Considerations

   The following contact information shall be used for all
   registrations included here:

     Contact:      Joerg Ott
                   mailto:jo@acm.org
                   tel:+358-9-451-2460

   The secure RTP feedback profile as a combination of Secure RTP and
   the feedback profile needs to be registered for the Session
   Description Protocol (specifically the type "proto"): "RTP/SAVPF".

   SDP Protocol ("proto"):

     Name:               RTP/SAVPF
     Long form:          Secure RTP Profile with RTCP-based Feedback
     Type of name:       proto
     Type of attribute:  Media level only
     Purpose:            RFC XXXX
     Reference:          RFC XXXX

   All the SDP attribute defined for RTP/SAVP and RTP/AVPF are valid
   for RTP/SAVPF, too.



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NOTE TO THE RFC EDITOR: Please replace all occurrences of RFC XXXX by
the RFC number assigned to this document.


7  Acknowledgements

   This document is a product of the Audio-Visual Transport (AVT)
   Working Group of the IETF.  The authors would like to thank Magnus
   Westerlund and Colin Perkins for their comments.


8  Authors' Addresses

   Joerg Ott                            {sip,mailto}:jo@netlab.hut.fi
   Helsinki University of Technology    tel:+358-9-451-2460
   SE 324
   Otakaari 5A
   FI-02150 Espoo
   Finland

   Elisabetta Carrara      mailto:elisabetta.carrara@ericsson.com
   Ericsson Research       tel:+46-8-50877040
   SE-16480 Stockholm
   Sweden


9  Bibliography

   9.1  Normative references

   [1]  H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP
        - A Transport Protocol for Real-time Applications," RFC 3550
        (STD0064), July 2003.

   [2]  H. Schulzrinne and S. Casner, "RTP Profile for Audio and Video
        Conferences with Minimal Control," RFC 3551 (STD0065), March
        2003.

   [3]  J. Ott, S. Wenger, N. Sato, C. Burmeister, J. Rey, "Extended
        RTP Profile for RTCP-based Feedback (RTP/AVPF), "Internet
        Draft draft-ietf-avt-rtcp-feedback-09.txt, Work in Progress,
        July 2004.

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

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


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   [6]  M. Handley, V. Jacobson, and Colin Perkins, "SDP: Session
        Description Protocol", Internet Draft draft-ietf-mmusic-sdp-
        new-25.txt, July 2005.

   [7]  J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K. Norrman,
        "Key Management Extensions for Session Description Protocol
        (SDP) and Real Time Streaming Protocol (RTSP)," Internet Draft
        draft-ietf-mmusic-kmgmt-ext-15.txt, Work in Progress, June
        2005.

   [8]  F. Andreassen, M. Baugher, and D. Wing, "Session Description
        Protocol Security Descriptions for Media Streams," Internet
        Draft draft-ietf-mmusic-sdescriptions-11.txt, Work in
        Progress, June 2005.

   [9]  G. Camarillo, J. Holler, G. Eriksson, H. Schulzrinne,
        "Grouping of media lines in SDP," RFC 3388, December 2002.

   [10] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
        SDP," RFC 3264, June 2002.

   [11] H. Schulzrinne, A. Rao, and R. Lanphier, "Real Time Streaming
        Protocol (RTSP)," RFC 2326, April 1998.


   9.2  Informative References

   [12] M. Handley, C. Perkins, and E. Whelan, "Session Announcement
        Protocol," RFC 2974, October 2000.

   [13] B. Ramsdell (ed.), " S/MIME Version 3 Message Specification,"
        RFC 2633, June 1999.

   [14] E.Rescorla, "HTTP Over TLS," RFC 2818, May 2000.



10 IPR Notice

   The IETF takes no position regarding the validity or scope of any
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   to pertain to the implementation or use of the technology described
   in this document or the extent to which any license under such
   rights might or might not be available; nor does it represent that
   it has made any independent effort to identify any such rights.
   Information on the procedures with respect to rights in RFC
   documents can be found in BCP 78 and BCP 79.



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   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.


11 Disclaimer of Validity

   This document and the information contained herein are provided on
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
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12 Full Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is
   subject to the rights, licenses and restrictions contained in BCP
   78, and except as set forth therein, the authors retain all their
   rights.


13 Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.













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