





Network Working Group                                             J. Ott
Request for Comments: 5124             Helsinki University of Technology
Category: Standards Track                                     E. Carrara
                                                                     KTH
                                                           February 2008


                    Extended Secure RTP Profile for
 Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   An RTP profile (SAVP) for secure real-time communications and another
   profile (AVPF) to provide timely feedback from the receivers to a
   sender are defined in RFC 3711 and RFC 4585, respectively.  This memo
   specifies the combination of both profiles to enable secure RTP
   communications with feedback.


























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Table of Contents

   1. Introduction ....................................................3
      1.1. Definitions ................................................4
      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 ..................6
   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 ........................................7
           3.3.2. RTSP-Based Negotiation of Session Descriptions ......8
           3.3.3. Announcing Session Descriptions .....................9
           3.3.4. Describing Alternative Session Profiles .............9
      3.4. Examples ..................................................10
   4. Interworking of AVP, SAVP, AVPF, and SAVPF Entities ............13
   5. Security Considerations ........................................14
   6. IANA Considerations ............................................15
   7. Acknowledgements ...............................................15
   8. References .....................................................15
      8.1. Normative References ......................................15
      8.2. Informative References ....................................16

























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1.  Introduction

   The Real-time Transport Protocol, the associated RTP Control Protocol
   (RTP/RTCP, RFC 3550) [1], and the profile for audiovisual
   communications with minimal control (RFC 3551) [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.

   The RTP specification provides only rudimentary support for
   encrypting RTP and RTCP packets.  Secure RTP (RFC 3711) [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, e.g., using the Session
   Description Protocol (SDP, RFC 4566) [6] together with extensions for
   conveying keying material (RFC 4567 [7], RFC 4568 [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")
   (RFC 4585, [3]) allows session participants 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 (RFC 4566, [6]) and the
   extensions defined in RFC 4585 [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.





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1.1.  Definitions

   The definitions of RFC 3550 [1], RFC 3551 [2], RFC 4585 [3], and RFC
   3711 [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 RFC 3550 [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.

   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.  If no common
        session parameters suitable for the involved endpoints can be
        found, 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.





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RFC 5124                                                   February 2008


   The AVPF profile derives from RFC 3550 [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.

   The SRTP part becomes active only when 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 RFC 4585 [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 RFC 3711
   [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 RFC 4585 [3].

   Additional extensions (e.g., transforms) defined for SAVP following
   the rules of Section 6 of RFC 3711 [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.








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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 RFC 3711 [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.  By using SRTP, 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 variable
   "avg_rtcp_size" to reflect the size of the SRTCP packets.

3.  SDP Definitions

3.1.  Profile Definition

   The AV profiles defined in RFC 3551 [2], RFC 4585 [3], and RFC 3711
   [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 RFC 4567
   [7] and RFC 4568 [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 RFC 4585
   [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 (RFC 3264, [9]) used with the Session Initiation Protocol (SIP)
   [15], the Real Time Streaming Protocol (RTSP) [10], or the Session
   Announcement Protocol (SAP) [11], but may also be distributed using
   email, NetNews, web pages, etc.





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   The offer/answer model allows the resulting session parameters to be
   negotiated using the SDP attributes defined in RFC 4567 [7] and RFC
   4568 [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 RFC 4567
   [7] as discussed in Section 3.3.2, and support for SAP announcements
   (with no negotiation at all) is addressed in Section 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 (e.g., per m= 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).
   Also, the offer/answer mechanism MAY be used to offer alternatives
   for the same media session and allow the answerer to choose one of
   the profiles.

   Provided that a mechanism for offering alternative security profiles
   becomes available (as is presently under development [14]), an
   offerer that is capable of supporting more than one of these profiles
   for a certain media session SHOULD always offer all alternatives
   acceptable in a certain situation.  The alternatives SHOULD be listed
   in order of preference and the offerer SHOULD prefer secure profiles
   over non-secure ones.  The offer SHOULD 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 enable bidding down and other
   attacks.  Therefore, if both secure and non-secure RTP profiles are
   offered (e.g., for best-effort SRTP [14]), the negotiation signaling
   MUST be protected appropriately to avoid such attacks.

   If an offer contains multiple alternative profiles, the answerer
   SHOULD prefer a secure profile (if supported) over non-secure ones.
   Among the secure or insecure profiles, the answerer SHOULD select the
   first acceptable alternative to respect the preference of the
   offerer.

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





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   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 [10] does not support the offer/answer model.  However, RTSP
   supports exchanging media session parameters (including profile and
   address information) by means of the Transport header.  SDP-based key
   management as defined in RFC 4567 [7] adds an RTSP header (KeyMgmt)
   to support conveying a key management protocol (including keying
   material).

   The RTSP Transport header MAY be used to determine the profile for
   the media session.  Conceptually, the rules defined in Section 3.3.1
   apply accordingly.  Detailed operation is as follows:  An SDP
   description (e.g., retrieved from the RTSP server by means of
   DESCRIBE) contains the description of the media streams of the
   particular RTSP resource.

   The RTSP client MUST select exactly one of the profiles per media
   stream it wants to receive.  It MUST do so in the SETUP request.  The
   RTSP client MUST indicate the chosen RTP profile by indicating the
   profile and the full server transport address (IP address and port
   number) in the Transport header included in the SETUP request.  The
   RTSP server's response to the client's SETUP message MUST confirm
   this profile selection or refuse the SETUP request (the latter of
   which it should not do after offering the profiles in the first
   place).

        Note: To change any of the profiles in use, the client needs to
        tear down this media stream (and possibly the whole RTSP
        session) using the TEARDOWN method and re-establish it using
        SETUP.  This may change as soon as media updating (similar to a
        SIP UPDATE or re-INVITE) becomes specified.

   When using the SDP key management [7], the KeyMgmt header MUST be
   included in the appropriate RTSP messages if a secure profile is
   chosen.  If different secure profiles are offered in the SDP
   description (e.g., SAVP and SAVPF) and different keying material is
   provided for these, after choosing one profile in the SETUP message,
   only the KeyMgmt header for the chosen one MUST be provided.  The
   rules for matching KeyMgmt headers to media streams according to RFC
   4567 [7] apply.





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3.3.3.  Announcing Session Descriptions

   Protocols that do not allow negotiating session descriptions
   interactively (e.g., SAP [11], 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.

   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 RFC 4567 [7] and RFC 4568 [8] may also
   be used for the security parameter distribution of announced session
   descriptions.

   The security scheme description defined in RFC 4568 [8] requires a
   secure communications channel to prevent third parties from
   eavesdropping on the keying parameters and manipulation.  Therefore,
   SAP security (as defined in RFC 2974 [11]), S/MIME [12], HTTPS [13],
   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 incompatible and MUST NOT be used together.

   If negotiation between the involved peers is possible (as with the
   offer/answer model per Section 3.3.1 or RTSP per Section 3.3.2),
   alternative (secure and non-secure) profiles MAY be specified by one
   entity (e.g., the offerer) and a choice of one profile MUST be made
   by the other.  If no such negotiation is possible (e.g., with SAP as
   per Section 3.3.3), incompatible profiles MUST NOT be specified as
   alternatives.

   The negotiation of alternative profiles is for further study.

   RTP profiles MAY be mixed arbitrarily across different RTP sessions.








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3.4.  Examples

   This section includes examples for the use of SDP to negotiate the
   use of secure and non-secure profiles.  Depending on what keying
   mechanism is being used and how it parameterized, the SDP messages
   typically require integrity protection and, for some mechanisms, will
   also need confidentiality protection.  For example, one could say
   integrity protection is required for the a=fingerprint of Datagram
   Transport Layer Security - Secure Real-time Transport Protocol
   (DTLS-SRTP) [16], and confidentiality is required for RFC 4568 [8]
   (Security Descriptions) a=crypto.

   Example 1: The following session description indicates a secure
   session made up from audio and dual tone multi-frequency (DTMF) 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 RFC 4567 [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 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 needs to 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



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      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 3: This example is replicated from example 1 above, but shows
   the interaction between the offerer and the answerer in an
   offer/answer exchange, again using MIKEY to negotiate the keying
   material:

      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

      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

   Example 4: This example shows the exchange for video streaming
   controlled via RTSP.  A client acquires a media description from a
   server using DESCRIBE and obtains a static SDP description without
   any keying parameters, but the media description shows that both
   secure and non-secure media sessions using (S)AVPF are available.  A
   mechanism that allows explicit identification of these alternatives
   (i.e., secure and non-secure sessions) in the session description is
   presently being defined [14].  The client then issues a SETUP request



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   and indicates its choice by including the respective profile in the
   Transport parameter.  Furthermore, the client includes a KeyMgmt
   header to convey its security parameters, which is matched by a
   corresponding KeyMgmt header from the server in the response.  Only a
   single media session is chosen so that the aggregate RTSP URI is
   sufficient for identification.

      RTSP DESCRIBE request-response pair (optional):

      DESCRIBE rtsp://movies.example.org/example RTSP/2.0
      CSeq: 314
      Accept: application/sdp

      200 OK
      CSeq: 314
      Date: 25 Nov 2005 22:09:35 GMT
      Content-Type: application/sdp
      Content-Length: 316

      v=0
      o=alice 3203093520 3203093520 IN IP4 movies.example.com
      s=Media with feedback
      t=0 0
      c=IN IP4 0.0.0.0
     +-Alternative one-----------------+
     |m=video 49170 RTP/SAVPF 96       |
     |a=rtpmap:96 H263-2000/90000      |
     |a=rtcp-fb:96 nack                |
     +---------------------------------+
     +-Alternative two-----------------+
     |m=video 49172 RTP/AVPF 96        |
     |a=rtpmap:96 H263-2000/90000      |
     |a=rtcp-fb:96 nack                |
     +---------------------------------+

      RTSP SETUP request-response pair

      SETUP rtsp://movies.example.org/example RTSP/2.0
      CSeq: 315
      Transport: RTP/SAVPF;unicast;dest_addr=":53012"/":53013"
      KeyMgmt: prot=mikey;url="rtsp://movies.example.org/example";
               data="uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnD..."

      200 OK
      CSeq: 315
      Date: 25 Nov 2005 22:09:36 GMT
      Session: 4711




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      Transport: RTP/SAVPF;unicast;dest_addr=":53012"/":53013";
                 src_addr="192.0.2.15:60000"/"192.0.2.15:60001"
      KeyMgmt: prot=mikey;url="rtsp://movies.example.org/example";
               data="ushdgfdhgfuiweyfhjsgdkj2837do7eWsnDSJD..."
      Accept-Ranges: NPT, SMPTE

   Example 5: 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 RFC 4567 [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
      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

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 RFC 3551 [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 RFC
   3550 [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 RFC 4585 [3] apply.




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5.  Security Considerations

   The SAVPF profile inherits its security properties from the SAVP
   profile; therefore, it is subject to the security considerations
   discussed in RFC 3711 [4].  When compared to SAVP, the SAVPF profile
   does not add or take away any security services.

   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
   RFC 3550 [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 RFC 3550 [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 RFC
   3711 [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 unless the SSRCs used are
   unique across all the RTP streams of the RTP sessions that share the
   same master key.

   When 2^48 SRTP packets or 2^31 SRTCP packets have been secured with
   the same key (whichever occurs before), 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.

   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.






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   The security considerations in RFC 4585 [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
                   mail: jo@acm.org
                   tel:  +358-9-451-2460

   The secure RTP feedback profile, as a combination of Secure RTP and
   the feedback profile, has been 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 5124
     Reference:          RFC 5124

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

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, Colin Perkins, and Cullen Jennings for their comments.

8.  References

8.1.  Normative References

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

   [2]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
        Conferences with Minimal Control", STD 65, RFC 3551, July 2003.




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   [3]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
        "Extended RTP Profile for Real-time Transport Control Protocol
        (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 2006.

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

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

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

   [7]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
        Carrara, "Key Management Extensions for Session Description
        Protocol (SDP) and Real Time Streaming Protocol (RTSP)", RFC
        4567, July 2006.

   [8]  Andreasen, F., Baugher, M., and D. Wing, "Session Description
        Protocol (SDP) Security Descriptions for Media Streams", RFC
        4568, July 2006.

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

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

8.2.  Informative References

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

   [12] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail Extensions
        (S/MIME) Version 3.1 Message Specification", RFC 3851, July
        2004.

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

   [14] Andreasen, F., "SDP Capability Negotiation", Work in Progress,
        December 2007.

   [15] 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.





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   [16] McGrew, D. and Rescorla, E., "Datagram Transport Layer Security
        (DTLS) Extension to Establish Keys for Secure Real-time
        Transport Protocol (SRTP)", Work in Progress, November 2007.

Authors' Addresses

   Joerg Ott
   Helsinki University of Technology
   Otakaari 5A
   FI-02150 Espoo
   Finland

   EMail: jo@comnet.tkk.fi
   Phone: +358-9-451-2460


   Elisabetta Carrara
   Royal Institute of Technology
   Stockholm
   Sweden

   EMail: carrara@kth.se





























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RFC 5124                                                   February 2008


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