Internet DRAFT - draft-ietf-avt-profile-savpf

draft-ietf-avt-profile-savpf







INTERNET-DRAFT                                              Joerg Ott 
                                    Helsinki University of Technology 
draft-ietf-avt-profile-savpf-12.txt                Elisabetta Carrara 
Intended Status: Proposed Standard                                KTH 
                                                     19 November 2007 
                                                     Expires May 2008 
 
 
     Extended Secure RTP Profile for RTCP-based Feedback (RTP/SAVPF) 
    
    
   Status of this Memo 
    
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   applicable patent or other IPR claims of which he or she is aware 
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   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..................................................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.7 
       3.3.2 RTSP-based Negotiation of Session Descriptions.........7 
       3.3.3 Announcing Session Descriptions........................8 
       3.3.4 Describing Alternative Session Profiles................9 
      3.4  Examples.................................................9 
   4  Interworking of AVP, SAVP, AVPF, and SAVPF Entities..........13 
   5  Security Considerations......................................13 
   6  IANA Considerations..........................................14 
   7  Acknowledgements.............................................15 
   8  Authors' Addresses...........................................15 
   9  Bibliography.................................................15 
      9.1  Normative references....................................15 
      9.2  Informative References..................................16 
   10 IPR Notice...................................................17 
   11 Disclaimer of Validity.......................................17 
   12 Full Copyright Statement.....................................17 
   13 Acknowledgment...............................................18 
    
 
    
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. 

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   The RTP specification provides only rudimentary support for 
   encrypting RTP and RTCP packets.  SRTP (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, using e.g. 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. 
    
    
   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: 


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        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. 
    
   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. 
    

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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 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. 
    
    
   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 


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   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. 
    
   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 subsection 3.3.2 and support for SAP 
   announcements (with no negotiation at all) is addressed in 
   subsection 3.3.3. 
    
    


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   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 (e.g. using the same 
   transport parameters) 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 multiple 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 shall be 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. 
    
   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 

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   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) 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 
        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. 
    
    
   3.3.3 Announcing Session Descriptions 
    
   Protocols that do not allow negotiate 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 


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   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. 
    
    
   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 
   typical require integrity protection and, for some mechanisms, will 
   also need confidentiality protection.  For example, you could say 
   integrity protection is required for DTLS-SRTP's a=fingerprint, 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 DTMF for point-to-point 
   communication in which the DTMF stream uses Generic NACKs.  The key 

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   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 
      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 answered in an 
   offer/answer exchange, again using MIKEY to negotiate the keying 
   material: 
    




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      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 inquires 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 allowing to explicitly identify these as alternatives 
   in the session description in presently being defined [14].  The 
   client then issues a SETUP request 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. 
 






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      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 
      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 

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   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. 
    
    
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].  The SAVPF 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.  

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   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 3264 [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. 
    
   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 
                   mailto:jo@acm.org 

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                   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. 
    
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, Colin Perkins, and Cullen Jennings for their comments. 
    
    
8  Authors' Addresses 
    
   Joerg Ott                            jo@netlab.hut.fi
   Helsinki University of Technology    tel:+358-9-451-2460 
   Otakaari 5A 
   FI-02150 Espoo 
   Finland 
    
   Elisabetta Carrara                   carrara@kth.se
   Royal Institute of Technology 
   Stockholm 
   Sweden 
    
    
9  Bibliography 
    
   9.1  Normative references 
    




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   [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), RFC 4585, July 
        2006. 
    
   [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.  
    
   [6]  M. Handley, V. Jacobson, and C. Perkins, "SDP: Session 
        Description Protocol", RFC 4566, July 2006. 
    
   [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)," RFC 4567, July 
        2006. 
    
   [8]  F. Andreassen, M. Baugher, and D. Wing, "Session Description 
        Protocol Security Descriptions for Media Streams," RFC 4568, 
        July 2006. 
    
   [9]  J. Rosenberg and H. Schulzrinne, "An offer/answer model with 
        SDP," RFC 3264, June 2002. 
 
   [10] H. Schulzrinne, A. Rao, and R. Lanphier, "Real Time Streaming 
        Protocol (RTSP)," RFC 2326, April 1998. 
 
 
   9.2  Informative References 
 
   [11] M. Handley, C. Perkins, and E. Whelan, "Session Announcement 
        Protocol," RFC 2974, October 2000. 
    
   [12] B. Ramsdell (ed.), "Secure/Multipurpose Internet Mail 
        Extensions (S/MIME) Version 3.1 Message Specification," RFC 
        3851, July 2004. 
    
   [13] E. Rescorla, "HTTP Over TLS," RFC 2818, May 2000. 
    


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   [14] F. Andreasen, "SDP Capability Negotiation," draft-ietf-mmusic-
        sdp-capability-negotiation-07, Work in Progress, October 2007. 
    
   [15] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J. 
        Peterson, R. Sparks, M. Handley, E. Schooler, "SIP: Session 
        Initiation Protocol," RFC 3261, June 2002. 
 
10 IPR Notice 
    
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11 Disclaimer of Validity 
    
   This document and the information contained herein are provided on 
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12 Full Copyright Statement 
    
   Copyright (C) The IETF Trust (2007).  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. 


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













































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