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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 4567

Internet Engineering Task Force                                 J. Arkko
MMUSIC Working Group                                          E. Carrara
INTERNET-DRAFT                                               F. Lindholm
Expires: December 2005                                        M. Naslund
                                                              K. Norrman
                                                                Ericsson
                                                               June 2005






            Key Management Extensions for Session Description
          Protocol (SDP) and Real Time Streaming Protocol (RTSP)
                   <draft-ietf-mmusic-kmgmt-ext-15.txt>


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
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Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.


Abstract

   This document defines general extensions for SDP and RTSP to carry
   messages, as specified by a key management protocol, in order to
   secure the media. These extensions are presented as a framework, to
   be used by one or more key management protocols. As such, their use


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   is meaningful only when complemented by an appropriate key management
   protocol.

   General guidelines are also given on how the framework should be used
   together with SIP and RTSP. The usage with the MIKEY key management
   protocol is also defined.


TABLE OF CONTENTS

   1. Introduction.....................................................2
   1.1. Notational Conventions.........................................4
   2. Applicability....................................................4
   3. Extensions to SDP and RTSP.......................................4
   3.1. SDP Extensions.................................................5
   3.2. RTSP Extensions................................................5
   4. Usage with SDP, SIP, RTSP, and SAP...............................6
   4.1. Use of SDP.....................................................7
   4.1.1 General processing............................................7
   4.1.2 Use of SDP with offer/answer and SIP..........................9
   4.1.3 Use of SDP with SAP..........................................12
   4.1.4 Bidding-down attack prevention...............................12
   4.2. RTSP usage....................................................13
   5. Example scenarios...............................................15
   6. Adding further Key management protocols.........................19
   7. Integration of MIKEY............................................20
   7.1 MIKEY Interface................................................21
   8. Security Considerations.........................................22
   9. IANA Considerations.............................................23
   9.1. SDP Attribute Registration....................................23
   9.2. RTSP Registration.............................................24
   9.3. Protocol Identifier Registration..............................24
   10. Acknowledgments................................................25
   11. Author's Addresses.............................................25
   12. References.....................................................26
   12.1. Normative References.........................................26
   12.2. Informative References.......................................26


1. Introduction

      [RFC Editor remark] All instances of RFC xxxx should be replaced
      with the RFC number of this document, when published.

   There has recently been work to define a security profile for the
   protection of real-time applications running over RTP, [SRTP].
   However, a security protocol needs a key management solution to
   exchange keys and security parameters, manage and refresh keys, etc.

   A key management protocol is executed prior to the security
   protocol's execution. The key management protocol's main goal is to,



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   in a secure and reliable way, establish a security association for
   the security protocol. This includes one or more cryptographic keys
   and the set of necessary parameters for the security protocol, e.g.,
   cipher and authentication algorithms to be used. The key management
   protocol has similarities with, e.g., SIP [SIP] and RTSP [RTSP] in
   the sense that it negotiates necessary information in order to be
   able to setup the session.

   The focus in the following sections is to describe a new SDP
   attribute and RTSP header extension to support key management, and to
   show how these can be integrated within SIP and RTSP. The resulting
   framework is completed by one or more key management protocols, which
   use the extensions provided.

   Some of the motivations to create a framework with the possibility to
   include the key management in the session establishment are:

   * Just as the codec information is a description of how to encode and
      decode the audio (or video) stream, the key management data is a
      description of how to encrypt and decrypt the data.

   * The possibility to negotiate the security for the entire multimedia
      session at the same time.

   * The knowledge of the media at session establishment makes it easy
      to tie the key management to the multimedia sessions.

   * This approach may be more efficient than setting up the security
      later, as that approach might force extra roundtrips, possibly
      also a separate set-up for each stream, hence implying more delay
      to the actual setup of the media session.

   * The possibility to negotiate keying material end-to-end without
      applying end-to-end protection of the SDP (instead, hop-by-hop
      security mechanisms can be used which may be useful if
      intermediate proxies needs access to the SDP).

   Currently in SDP [SDPnew], there exists one field to transport keys,
   the "k=" field. However, this is not enough for a key management
   protocol as there are many more parameters that need to be
   transported, and the "k=" field is not extensible. The approach used
   is to extend the SDP description through a number of attributes that
   transport the key management offer/answer and also to associate it
   with the media sessions. SIP uses the offer/answer model [OAM]
   whereby extensions to SDP will be enough. However, RTSP [RTSP] does
   not use the offer/answer model with SDP, so a new RTSP header is
   introduced to convey key management data. [SDES] uses the approach of
   extending SDP, to carry the security parameters for the media
   streams. However, the mechanism defined in [SDES] requires end-to-end
   protection of the SDP by some security protocol such as S/MIME, in
   order to get end-to-end protection. The solution described here



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   focuses only on the end-to-end protection of key management
   parameters and as a consequence does not require external end-to-end
   protection means. It is important to note though, and we stress this
   again, that only the key management parameters are protected.

   The document also defines the use of the described framework together
   with the key management protocol Multimedia Internet KEYing (MIKEY)
   [MIKEY].

1.1. Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


2. Applicability

   [SDES] provides similar cryptographic key distribution capabilities,
   and it intended for use when keying material is protected along with
   the signaling.

   In contrast, this specification expects endpoints to have
   preconfigured keys or common security infrastructure. It provides its
   own security, and is independent of the protection of signaling (if
   any). As a result, it can be applied in environments where signaling
   protection is not turned on, or used hop-by-hop (i.e., scenarios
   where the SDP is not protected end-to-end). This specification will
   independently of the signaling protection applied, ensure end-to-end
   security establishment for the media.


3. Extensions to SDP and RTSP

   This section describes common attributes that can be included in SDP
   or RTSP when an integrated key management protocol is used. The
   attribute values follow the general SDP and RTSP guidelines (see
   [SDPnew] and [RTSP]).

   For both SDP and RTSP, the general method of adding the key
   management protocol is to introduce new attributes, one identifier to
   identify the specific key management protocol, and one data field
   where the key management protocol data is placed. The key management
   protocol data contains the necessary information to establish the
   security protocol, e.g., keys and cryptographic parameters. All
   parameters and keys are protected by the key management protocol.

   The key management data SHALL be base64 [RFC3548] encoded and comply
   with the base64 grammar as defined in [SDPnew]. The key management
   protocol identifier, KMPID, is defined as below in Augmented Backus-
   Naur Form grammar (ABNF) [RFC2234].



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   KMPID =  1*(ALPHA / DIGIT)

   Values for the identifier, KMPID, are registered and defined in
   accordance to Section 8. Note that the KMPID is case sensitive and it
   is RECOMMENDED that values registered are lower case letters.

3.1. SDP Extensions

   This section provides an ABNF grammar (as used in [SDPnew]) for the
   key management extensions to SDP.

   Note that the new definitions are compliant with the definition of an
   attribute field, i.e.

   attribute    = (att-field ":" att-value) | att-field

   The ABNF for the key management extensions (conforming to the att-
   field and att-value) are as follow:

     key-mgmt-attribute = key-mgmt-att-field ":" key-mgmt-att-value

     key-mgmt-att-field = "key-mgmt"
     key-mgmt-att-value = 0*1SP prtcl-id SP keymgmt-data

     prtcl-id     = KMPID
                    ; e.g. "mikey"

     keymgmt-data = base64
     SP           = 0x20

   where KMPID is as defined in Section 2 of this memo, base64 is as
   defined in SDP [SDPnew]. Prtcl-id refers to the set of values defined
   for KMPID in Section 8.

   The attribute MAY be used at session level, media level, or at both
   levels. An attribute defined at media level overrides an attribute
   defined at session level. In other words, if the media level
   attribute is present, the session level attribute MUST be ignored for
   this media. Section 3.1 describes in detail how the attributes are
   used and how the SDP is handled in different usage scenarios. The
   choice of the level depends for example on the particular key
   management protocol. Some protocols may not be able to derive enough
   key material for all the sessions; furthermore, possibly a different
   protection to each session could be required. The particular protocol
   might achieve this only by specifying it at the media level. Other
   protocols, such as MIKEY, have instead those capabilities (as it can
   express multiple security policies and derive multiple keys), so it
   may use the session level.

3.2. RTSP Extensions



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   To support the key management attributes, the following RTSP header
   is defined:

   KeyMgmt = "KeyMgmt" ":" key-mgmt-spec 0*("," key-mgmt-spec)

   key-mgmt-spec = "prot" "=" KMPID ";" ["uri" "=" <"> rtsp_URL <"> ";"]
   "data" "=" base64

   where KMPID is as defined in Section 2 of this memo, "base64" as
   defined in [SDPnew], and "rtsp_URL" as defined in [RTSP].

   The "uri" parameter identifies the context for which the key
   management data applies, and the RTSP URI SHALL match a (session or
   media) URI present in the description of the session. If the RTSP
   aggregated control URI is included it indicates that the key
   management message is on session level (and similarly the RTSP media
   control URI, that it applies to the media level). If no "uri"
   parameter is present in a key-mgmt-spec the specification applies to
   the context identified by the RTSP request URI.

   The KeyMgmt header MAY be used in the messages and directions
   described in the table below.


   Method            | Direction  |  Requirement
   ---------------------------------------------
   DESCRIBE response |   S->C     |  RECOMMENDED
   SETUP             |   C->S     |  REQUIRED
   SETUP Response    |   S->C     |  REQUIRED (error)

   Note: Section 3.2 describes in detail how the RTSP extensions are
   used.


   We define one new RTSP status code to report error due to any failure
   during the key management processing (Section 3.2):

   Status-Code  =  "463" ; Key management failure

   A 463 response MAY contain a KeyMgmt header with a key management
   protocol message that further indicates the nature of the error.


4. Usage with SDP, SIP, RTSP, and SAP

   This section gives rules and recommendations of how/when to include
   the defined key management attribute when SIP and/or RTSP are used
   together with SDP.





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   When a key management protocol is integrated with SIP/SDP and RTSP,
   the following general requirements are placed on the key management:

   * At the current time, it MUST be possible to execute the key
      management protocol in at most one request-response message
      exchange. Future relaxation of this requirement is possible but
      would introduce significant complexity for implementations
      supporting multi-roundtrip mechanisms.

   * It MUST be possible from the SIP/SDP and RTSP application, using
      the key management API, to receive key management data, and
      information of whether a message is accepted or not.

   The content of the key management messages depends on the key
   management protocol that is used. However, the content of such key
   management messages might be expected to be roughly as follow: the
   key management Initiator (e.g. the offerer) includes the key
   management data in a first message, containing the media description
   it should apply to. This data in general consists of the security
   parameters (including key material) needed to secure the
   communication, together with the necessary authentication information
   (to assure that the message is authentic).

   At the Responder's side, the key management protocol checks the
   validity of the key management message, together with the
   availability of the parameters offered, and then provides the key
   management data to be included in the answer. This answer may
   typically authenticate the Responder to the Initiator, and also state
   if the initial offer was accepted or not. Certain protocols might
   require the Responder to include a selection of the security
   parameters that he is willing to support. Again, the actual content
   of such responses is dependent on the particular key management
   protocol.

   Section 6 describes a realization of the MIKEY protocol using these
   mechanisms. Procedures to be used when mapping new key management
   protocols onto this framework are described in Section 5.

4.1. Use of SDP

   This section describes the processing rules for the different
   applications which use SDP for the key management.

4.1.1 General processing

   The processing when SDP is used is slightly different according to
   the way SDP is transported, and if it uses an offer/answer or
   announcement. The processing can be divided into four different
   steps:

   1) How to create the initial offer.



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   2) How to handle a received offer.
   3) How to create an answer.
   4) How to handle a received answer.

   It should be noted that the last two steps may not always be
   applicable, as there are cases where an answer can not or will not be
   sent back.

   The general processing for creating an initial offer SHALL follow the
   following actions:

   * The identifier of the key management protocol used MUST be placed
      in the prtcl-id field of SDP. A table of legal protocols
      identifiers is maintained by IANA (see Section 8).

   * The keymgmt-data field MUST be created as follows: the key
      management protocol MUST be used to create the key management
      message. This message SHALL be base64 encoded [RFC3548] by the SDP
      application and then encapsulated in the keymgmt-data attribute.
      Note though that the semantics of the encapsulated message is
      dependent on the key management protocol that is used.

   The general processing for handling a received offer SHALL follow the
   following actions:

   * The key management protocol is identified according to the prtcl-id
      field. A table of legal protocols identifiers is maintained by
      IANA (Section 8).

   * The key management data from the keymgmt-data field MUST be
      extracted, base64 decoded to reconstruct the original message, and
      then passed to the key management protocol for processing. Note
      that depending on key management protocol, some extra parameters
      might also be requested by the specific API, such as the
      source/destination network address/port(s) for the specified media
      (however, this will be implementation specific depending on the
      actual API). The extra parameters that a key management protocol
      might need (other than the ones defined here) MUST be documented,
      describing their use, as well as the interaction of that key
      management protocol with SDP and RTSP.

   * If errors occur, or the key management offer is rejected, the
       session SHALL be aborted. Possible error messages are dependent
       on the specific session establishment protocol.

   At this stage, the key management will have either accepted or
   rejected the offered parameters. This MAY cause a response message to
   be generated, depending on the key management protocol and the
   application scenario.





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   If an answer is to be generated, the following general actions SHALL
   be performed:

   * The identifier of the key management protocol used MUST be placed
      in the prtcl-id field.

   * The keymgmt-data field MUST be created as follows. The key
      management protocol MUST be used to create the key management
      message. This message SHALL be base64 encoded [RFC3548] by the SDP
      application and then encapsulated in the keymgmt-data attribute.
      The semantics of the encapsulated message is dependent on the key
      management protocol that is used.

   The general processing for handling a received answer SHALL follow
   the following actions:

   * The key management protocol is identified according to the prtcl-id
      field.

   * The key management data from the keymgmt-data field MUST be
      extracted, base64 decoded to reconstruct the original message, and
      then passed to the key management protocol for processing.

   * If the key management offer is rejected and the intent is to re-
      negotiate it, it MUST be done through another Offer/Answer
      exchange. It is RECOMMENDED to NOT abort the session in that case,
      but to re-negotiate using another Offer/Answer exchange. For
      example, in SIP [RFC3261], the "security precondition" as defined
      in [SPREC} solves the problem for a session initiation. The
      procedures in [SPREC] are outside the scope of this document. In
      an established session, an additional Offer/Answer exchange using
      a re-INVITE or UPDATE as appropriate MAY be used.

   * If errors occur, or the key management offer is rejected and there
       is no intent to re-negotiate it, the session SHALL be aborted. If
       possible an error message indicating the failure SHOULD be sent
       back.

   Otherwise, if all the steps are successful, the normal setup
   proceeds.

4.1.2 Use of SDP with offer/answer and SIP

   This section defines additional processing rules, to the general
   rules defined in Section 3.1.1, applicable only to applications using
   SDP with the offer-answer model [OAM] (and in particular SIP).

   When an initial offer is created, the following offer-answer specific
   procedure SHALL be applied:





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   * Before creating the key management data field, the list of protocol
      identifiers MUST be provided by the SDP application to (each) key
      management protocol, as defined in Section 3.1.4 (to defeat
      bidding-down attacks).

   For a received SDP offer that contains the key management attributes,
   the following offer-answer specific procedure SHALL be applied:

   * Before, or in conjunction with, passing the key management data to
      the key management protocol, the complete list of protocol
      identifier from the offer message is provided by the SDP
      application to the key management protocol (as defined in Section
      3.1.4).

   When an answer is created, the following offer-answer specific
   procedure SHALL be applied:

   * If the key management rejects the offer and the intent is to re-
      negotiate it, the Answer SHOULD include the cause of failure in an
      included message from the key management protocol. The
      renegotiation MUST be done through another Offer/Answer exchange
      (e.g, using [SPREC]). In an established session, it can also be
      done through a re-INVITE or UPDATE as appropriate.

   * If the key management rejects the offer and the session needs to be
      aborted, the answerer SHOULD return a "488 Not Acceptable Here"
      message, optionally also including one or more Warning headers (a
      306 "Attribute not understood" when one of the parameters is not
      supported, and a 399 "Miscellaneous warning" with arbitrary
      information to be presented to a human user or logged, see Section
      20.43 in [SIP]). Further details about the cause of failure MAY be
      described in an included message from the key management protocol.
      The session is then aborted (and it is up to local policy or end
      user to decide how to continue).

   Note that the key management attribute (related to the same key
   management protocol) MAY be present both at session level and at
   media level. Consequently, the process SHALL be repeated for each
   such key management attribute detected. In case the key management
   processing of any such attribute does not succeed (e.g.
   authentication failure, parameters not supported etc.), on either
   session or media level, the entire session setup SHALL be aborted,
   including those parts of the session that successfully completed
   their part of the key management.

   If more than one key management protocol is supported, multiple
   instances of the key management attribute MAY be included in the
   initial offer when using the offer-answer model, each transporting a
   different key management protocol, thus indicating supported
   alternatives.




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   If the offerer includes more than one key management protocol
   attribute at session level (analogous for the media level), these
   SHOULD be listed in order of preference (the first being the
   preferred). The answerer selects the key management protocol it
   wishes to use, and processes only it, on either session or media
   level, or on both, according to where located. If the answerer does
   not support any of the offerer's suggested key management protocols,
   the answerer indicates this to the offerer so a new Offer-Answer can
   be triggered; alternatevely, it may return a "488 Not Acceptable
   Here" error message, whereby the sender MUST abort the current setup
   procedure.

   Note that the placement of multiple key management offers in a single
   message has the disadvantage that the message expands and the
   computational workload for the offerer will increase drastically.
   Unless the guidelines of Section 3.1.4 are followed, multiple lines
   may open up bidding-down attacks. Note also that the multiple offer
   option has been added to optimize signaling overhead in case the
   Initiator knows some key (e.g. a public key) that the Responder has,
   but is unsure of what protocol the Responder supports. The mechanism
   is not intended to negotiate options within one and the same
   protocol.

   The offerer MUST include the key management data within an offer that
   contains the media description it applies to.

   Re-keying MUST be handled as a new offer, with the new proposed
   parameters. The answerer treats this as a new offer where the key
   management is the issue of change. The re-keying exchange MUST be
   finalized before the security protocol can change the keys. The same
   key management protocol used in the original offer SHALL also be used
   in the new offer carrying re-keying. If the new offer carrying re-
   keying fails (e.g., the authentication verification fails), the
   answerer SHOULD send a "488 Not Acceptable Here" message, including
   one or more Warning headers (at least a 306). The offerer MUST then
   abort the session.

   Note that, in multicast scenarios, unlike unicast, there is only a
   single view of the stream [OAM], hence there MUST be a uniform
   agreement of the security parameters.

   After the offer is issued, the offerer SHOULD be prepared to receive
   media, as the media may arrive prior to the answer. However, this
   brings issues, as the offer does not know yet the answererÆs choice
   in terms of e.g. algorithms, nor possibly the key is know. This can
   cause delay or clipping can occur; if this is unacceptable, the
   offerer SHOULD use mechanisms outside the scope of this document,
   e.g. the security precondition for SIP [SPREC].






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4.1.3 Use of SDP with SAP

   There are cases where SDP is used without conforming to the
   offer/answer model; instead it is a one-way SDP distribution (i.e.
   without back channel), such as when used with SAP and HTTP.

   The processing follows the two first steps of the general SDP
   processing (see Section 3.1.1). It can be noted that the processing
   in this case differs from the offer/answer case in the fact that only
   one key management protocol SHALL be offered (i.e. no negotiation
   will be possible). This implies that the bidding down attack is not
   an issue; therefore the countermeasure is not needed.  The key
   management protocol used MUST support one-way messages.

4.1.4 Bidding-down attack prevention

   The possibility to support multiple key management protocols may,
   unless properly handled, introduce bidding-down attacks.
   Specifically, a man-in-the-middle could "peel off" cryptographically
   strong offers (deleting the key management lines from the message),
   leaving only weaker ones as the Responder's choice. To avoid this,
   the list of identifiers of the proposed key management protocols MUST
   be authenticated. The authentication MUST be done separately by each
   key management protocol.

   Accordingly, it MUST be specified (in the key management protocol
   specification itself or in a companion document) how the list of key
   management protocol identifiers can be processed to be authenticated
   from the offerer to the answerer by the specific key management
   protocol. Note that even if only one key management protocol is used,
   that still MUST authenticate its own protocol identifier.

   The list of protocol identifiers MUST then be given to each of the
   selected (offered) key management protocols by the application with
   ";" separated identifiers. All the offered protocol identifiers MUST
   be included, in the same order as they appear in the corresponding
   SDP description.

   The protocol list can formally be described as

   prtcl-list   =  KMPID *(";" KMPID)

   where KMPID is as defined in Section 2.

   For example, if the offered protocols are MIKEY and two yet-to-be-
   invented protocols KEYP1, KEYP2, the SDP is:

        v=0
        o=alice 2891092738 2891092738 IN IP4 lost.example.com
        s=Secret discussion
        t=0 0



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        c=IN IP4 lost.example.com
        a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
        a=key-mgmt:keyp1 727gkdOshsuiSDF9sdhsdKnD/dhsoSJokdo7eWD...
        a=key-mgmt:keyp2 DFsnuiSDSh9sdh Kksd/dhsoddo7eOok727gWsJD...
        m=audio 39000 RTP/SAVP 98
        a=rtpmap:98 AMR/8000
        m=video 42000 RTP/SAVP 31
        a=rtpmap:31 H261/90000

   The protocol list, "mikey;keyp1;keyp2", would be generated from
   the SDP description and used as input to each specified key
   management protocol (together with the data for that protocol).
   Each of the three protocols includes this protocol identifier
   list in its authentication coverage (according to its protocol
   specification).

   If more than one protocol is supported by the offerer, it is
   RECOMMENDED that all acceptable protocols are included in the first
   offer, rather than making single, subsequent alternative offers in
   response to error messages, see "Security Considerations".

   End-to-end integrity protection of the key-mgmt attributes
   altogether, provided externally to the key management themselves,
   also gives protection against this bidding down attack. This is for
   example the case if SIP uses S/MIME [RFC3851] to end-to-end integrity
   protect the SDP description. As however this end-to-end protection is
   not an assumption of the framework, the mechanisms defined in this
   section SHALL be applied.

4.2. RTSP usage

   RTSP does not use the offer/answer model, as SIP does. This causes
   some problems, as it is not possible (without modifying RTSP) to send
   back an answer. To solve this, a new header has been introduced
   (Section 2.2). This also assumes that the key management also has
   some kind of binding to the media, so that the response to the server
   will be processed as required.

   The server SHALL be the Initiator of the key management exchange for
   sessions in PLAY mode, i.e. transporting media from server to client.
   The below text describes the behavior for PLAY mode. For any other
   mode the behavior is not defined in this specification.

   To obtain a session description, the client initially contacts the
   server via a DESCRIBE message. The initial key management message
   from the RTSP server is sent to the client in the SDP of the 200 OK
   in response to the DESCRIBE. Note that only one key management
   protocol SHALL be used per session / media level. A server MAY allow
   the SDP with key-management attribute(s) to be distributed to the
   client though other means than RTSP, although this is not specified
   here.



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   The "uri" parameter of the KeyMgmt header is used to indicate for the
   key management protocol on what context the carried message applies.
   For key management messages on the SDP session level, the answer MUST
   contain the RTSP aggregated control URL to indicate this. For Key
   management messages initially on SDP media level, the key management
   response message in the KeyMgmt header MAY use the RTSP media level
   URL. For RTSP sessions not using aggregated control, i.e. no session
   level control URI is defined, the key management protocol SHALL only
   be invoked on individual media streams. In this case also, the key
   management response SHALL be on individual media streams (i.e. one
   RTSP key management header per media).

   When responding to the initial key management message, the client
   uses the new RTSP header (KeyMgmt) to send back an answer. How this
   is done depends on the usage context:

   * Key management protocol responses for the initial establishment of
      security parameters for an aggregated RTSP session SHALL be sent
      in the first SETUP of the session. This means that if the key
      management is declared for the whole session but is setup in non-
      aggregated fashion, i.e. one media per RTSP session, each SETUP
      MUST carry the same response for the session level context. When
      performing a setup of the second or any subsequent media in a RTSP
      session the same key management parameters as established for the
      first media also applies to these setups.

   * Key management responses for the initial establishment of security
      parameters for an individual media SHALL only be included in SETUP
      for the corresponding media stream.

   If a server receives a SETUP message in which it expects a key
   management message, but none is included, a 403 Forbidden SHOULD be
   returned to the client, whereby the current setup MUST be aborted.

   When the server creates an initial SDP message, the procedure SHALL
   be the same as described in Section 3.1.1.

   The client processing of the initial SDP message from the server
   SHALL follow the same procedures as described in Section 3.1.1,
   except that, if there is an error, the session is aborted (no error
   is sent back).

   The client SHALL create the response, using the key management header
   in RTSP, as follows:

   * The identifier of the key management protocol used (e.g. MIKEY)
      MUST be placed in the "prot" field of the header. The prot values
      are maintained by IANA (Section 8).





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   * The keymgmt-data field MUST be created as follows: the key
      management protocol MUST be used to create the key management
      message. This message SHALL be base64 encoded by the RTSP
      application and then encapsulated in the "data" field of the
      header. The semantic of the encapsulated message is dependent on
      the key management protocol that is used.

   * Include, if necessary, the URL to indicate the context in the "uri"
      parameter.

   The server SHALL process a received key management header in RTSP as
   follow:

   * The key management protocol is identified according to the "prot"
      field.

   * The key management data from the "data" field MUST be extracted,
      base64 decoded to reconstruct the original message, and then
      passed to the key management protocol for processing.

   * If the key management protocol is successful, the processing can
      proceed according to normal rules.

   * Otherwise, if the key management fails (e.g. due to authentication
      failure or parameter not supported), an error is sent back as the
      SETUP response using RTSP error code 463 (see Section 2.2) and the
      session is aborted. It is up to the key management protocol to
      specify (within the RTSP status code message or through key
      management messages) details about the type of error that
      occurred.

   Re-keying within RTSP is for further study, given that media updating
   mechanisms within RTSP are unspecified at the time this document is
   written.


5. Example scenarios

   The following examples utilize MIKEY [MIKEY] as the key management
   protocol to be integrated into SDP and RTSP (see Section 5.1.).

   Example 1 (SIP/SDP)

   A SIP call is taking place between Alice and Bob. Alice sends an
   INVITE message consisting of the following offer:

   v=0
   o=alice 2891092738 2891092738 IN IP4 w-land.example.com
   s=Cool stuff
   e=alice@w-land.example.com
   t=0 0



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   c=IN IP4 w-land.example.com
   a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAAAAAGEEoo2pee4hp2
   UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0JKpgaVkDaawi9whVBtBt
   0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUOSrzKTAv9zV
   m=audio 49000 RTP/SAVP 98
   a=rtpmap:98 AMR/8000
   m=video 52230 RTP/SAVP 31
   a=rtpmap:31 H261/90000

   i.e. Alice proposes to set up one audio stream and one video stream
   that run over SRTP (signaled by the use of the SAVP profile). She
   uses MIKEY to set up the security parameters for SRTP (Section 6).
   The MIKEY message contains the security parameters, together with the
   necessary key material. Note that MIKEY is exchanging the crypto
   suite for both streams, as it is placed at the session level. Also,
   MIKEY provides its own security, i.e. when Bob processes Alice's
   MIKEY message, he will also find the signaling of the security
   parameters used to secure the MIKEY exchange. Alice's endpoint's
   authentication information is also carried within the MIKEY message,
   to prove that the message is authentic. The above MIKEY message is an
   example of message when the pre-shared method MIKEY is used.

   Upon receiving the offer, Bob checks the validity of the received
   MIKEY message, and, in case of successful verification, he accepts
   the offer and sends an answer back to Alice (with his authentication
   information, and, if necessary, also some key material from his
   side):

   v=0
   o=bob 2891092897 2891092897 IN IP4 foo.example.com
   s=Cool stuff
   e=bob@foo.example.com
   t=0 0
   c=IN IP4 foo.example.com
   a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyONQ6gAAAAAJAAAQbWlja2
   V5QG1vdXNlLmNvbQABn8HdGE5BMDXFIuGEga+62AgY5cc=
   m=audio 49030 RTP/SAVP 98
   a=rtpmap:98 AMR/8000
   m=video 52230 RTP/SAVP 31
   a=rtpmap:31 H261/90000

   Upon receiving the answer, Alice verifies the correctness of it. In
   case of success, at this point Alice and Bob share the security
   parameters and the keys needed for a secure RTP communication.


   Example 2 (SDP)

   This example shows how Alice would have done if she wished to protect
   only the audio stream. She would have placed the MIKEY line at media
   level for the audio stream only (also specifying the use of the SRTP



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   profile there, SAVP). The semantic of the MIKEY messages is as in the
   previous case, but applies only to the audio stream.

   v=0
   o=alice 2891092738 2891092738 IN IP4 w-land.example.com
   s=Cool stuff
   e=alice@w-land.example.com
   t=0 0
   c=IN IP4 w-land.example.com
   m=audio 49000 RTP/SAVP 98
   a=rtpmap:98 AMR/8000
   a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAy...
   m=video 52230 RTP/AVP 31
   a=rtpmap:31 H261/90000

   Bob would then act as described in the previous example, including
   the MIKEY answer at the media level for the audio stream (as Alice
   did).

   Note that even if the key management attribute were specified at
   session level, the video part would not be affected by this (as a
   security profile is not used, instead the RTP/AVP profile is
   signaled).


   Example 3 (RTSP)

   A client wants to set up a streaming session and requests a media
   description from the streaming server.

   DESCRIBE rtsp://server.example.com/fizzle/foo RTSP/1.0
   CSeq: 312
   Accept: application/sdp
   From: user@example.com

   The server sends back an OK message including an SDP description,
   together with the MIKEY message. The MIKEY message contains the
   necessary security parameters that the server is willing of offering
   to the client, together with authentication information (to prove
   that the message is authentic) and the key material. The SAVP profile
   also signals the use of SRTP for securing the media sessions.

   RTSP/1.0 200 OK
   CSeq: 312
   Date: 23 Jan 1997 15:35:06 GMT
   Content-Type: application/sdp
   Content-Length: 478

   v=0
   o=actionmovie 2891092738 2891092738 IN IP4 movie.example.com
   s=Action Movie



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   e=action@movie.example.com
   t=0 0
   c=IN IP4 movie.example.com
   a=control:rtsp://movie.example.com/action
   a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAy...
   m=audio 0 RTP/SAVP 98
   a=rtpmap:98 AMR/8000
   a=control:rtsp://movie.example.com/action/audio
   m=video 0 RTP/SAVP 31
   a=rtpmap:31 H261/90000
   a=control:rtsp://movie.example.com/action/video

   The client checks the validity of the received MIKEY message, and, in
   case of successful verification, it accept the message. The client
   then includes its key management data in the SETUP request going back
   to the server, the client authentication information (to prove that
   the message is authentic) and, if necessary, some key material.

   SETUP rtsp://movie.example.com/action/audio RTSP/1.0
   CSeq: 313
   Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057
   keymgmt: prot=mikey; uri="rtsp://movie.example.com/action";
            data="AQEFgM0XflABAAAAAAAAAAAAAAYAyONQ6g..."

   The server processes the request including checking the validity of
   the key management header.

   RTSP/1.0 200 OK
   CSeq: 313
   Session: 12345678
   Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057;
                         server_port=5000-5001

   Note than in this case the key management line was specified at the
   session level, the key management information only goes into the
   SETUP related to the first stream. The "uri" indicates to the server
   that the context is for the whole aggregated session the key
   management applies. The RTSP client then proceeds setting up the
   second media (video) in aggregation with the audio. As the two media
   are run in aggregation and the key context was established in the
   first exchange, no more key management messages are needed.


   Example 4 (RTSP)

   The use of the MIKEY message at the media level would change the
   previous example as follows.

   The 200 OK would contain the two distinct SDP attributes for MIKEY at
   the media level:




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   RTSP/1.0 200 OK
   CSeq: 312
   Date: 23 Jan 1997 15:35:06 GMT
   Content-Type: application/sdp
   Content-Length: 561

   v=0
   o=actionmovie 2891092738 2891092738 IN IP4 movie.example.com
   s=Action Movie
   e=action@movie.example.com
   t=0 0
   c=IN IP4 movie.example.com
   a=control:rtsp://movie.example.com/action
   m=audio 0 RTP/SAVP 98
   a=rtpmap:98 AMR/8000
   a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAA...
   a=control:rtsp://movie.example.com/action/audio
   m=video 0 RTP/SAVP 31
   a=rtpmap:31 H261/90000
   a=key-mgmt:mikey AQAFgM0AdlABAAAAAAAAAAAAA...
   a=control:rtsp://movie.example.com/action/video

   Each RTSP header are inserted in the SETUP related to the audio and
   video separately:

   SETUP rtsp://movie.example.com/action/audio RTSP/1.0
   CSeq: 313
   Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057
   keymgmt: prot=mikey; uri="rtsp://movie.example.com/action/audio";
            data="AQEFgM0XflABAAAAAAAAAAAAA..."


   and similarly for the video session:

   SETUP rtsp://movie.example.com/action/video RTSP/1.0
   CSeq: 315
   Transport: RTP/SAVP/UDP;unicast;client_port=3058-3059
   keymgmt: prot=mikey; uri="rtsp://movie.example.com/action/video";
            data="AQEFgM0AdlABAAAAAAAAAAAAAA..."

   Note: The "uri" parameter could be excluded from the two SETUP
   messages in this example.


6. Adding further Key management protocols

   This framework cannot be used with all key management protocols. The
   key management protocol needs to comply with the requirements
   described in Section 3. In addition to this, the following needs to
   be defined:




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   * The key management protocol identifier to be used as the protocol
      identifier should be registered at IANA according to Section 8.

   * The information that the key management needs from SDP and RTSP,
      and vice versa, as described in Section 3. The exact API is
      implementation specific, but it MUST at least support the exchange
      of the specified information.

   * The key management protocol to be added MUST be such, that the
      processing in Section 3 (describing its interactions with SDP and
      RTSP) can be applied. Note in particular, Section 3.1.4 requires
      each key management protocol to specify how the list of protocol
      identifiers is authenticated inside that key management protocol.
      The key management MUST always be given the protocol identifier(s)
      of the key management protocol(s) included in the offer in the
      correct order as they appear.

   Finally, it is obviously crucial to analyze possible security
   implications induced by the introduction of a new key management
   protocol in the described framework.

   Today, the MIKEY protocol [MIKEY] has adopted the key management
   extensions to work together with SIP and RTSP (see Section 6). Other
   protocols MAY use the described attribute and header, e.g. Kerberos
   [KERB], however this is subject to future standardization.


7. Integration of MIKEY

   [MIKEY] describes a key management protocol for real-time
   applications (both for peer-to-peer communication and group
   communication). MIKEY carries the security parameters needed for
   setting up the security protocol (e.g., SRTP) protecting the media
   stream. MIKEY can be integrated within SDP and RTSP, following the
   rules and guidelines described in this document.

   MIKEY satisfies the requirements described in Section 3. The MIKEY
   message is formed as defined in [MIKEY], then passed from MIKEY to
   the SDP application that base64 encodes it, and encapsulates it in
   the keymgmt-data attribute. The examples in Section 4 use MIKEY,
   where the semantic of the exchange is also briefly explained.

   The key management protocol identifier (KMPID) to be used as the
   protocol identifier SHALL be "mikey" and is registered at IANA, see
   in detail Section 8.

   The information that the key management needs from SDP and RTSP, and
   vice versa, follows Section 3. To avoid bidding-down attacks, the
   directives in Section 3.1.4 are followed. The list of protocol
   identifiers is authenticated within MIKEY by placing the list in a
   General Extension Payload (of type "SDP IDs", [MIKEY]), which then



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   automatically will be integrity protected/signed. The receiver SHALL
   then match the list in the General Extension Payload with the list
   included in SDP and SHOULD (according to policy) if they differ, or
   if integrity/signature verification fails, reject the offer.

   The server will need to be able to know the identity of the Client
   before creating and sending a MIKEY message. To signal the (MIKEY)
   identity of the client to the server in the DESCRIBE, it is
   RECOMMENDED to include the From header field in RTSP. Other methods
   to establish the identity could be using the IP address or retrieving
   the identity from the RTSP authentication if used.

7.1 MIKEY Interface

   This subsection describes some aspects, which implementers SHOULD
   consider. If the MIKEY implementation is separate from the
   SDP/SIP/RTSP, an application programming interface (API) between
   MIKEY and those protocols is needed with certain functionality
   (however, exactly what it looks like is implementation dependent).

   The following aspects need to be considered:

   * the possibility for MIKEY to receive information about the sessions
      negotiated. This is to some extent implementation dependent. But
      it is RECOMMENDED that, in the case of SRTP streams, the number of
      SRTP streams is included (and the direction of these). It is also
      RECOMMENDED to provide the destination addresses and ports to
      MIKEY. When referring to streams described in SDP, MIKEY SHALL
      allocate two consecutive numbers for the related Crypto Session
      indexes (as each stream can be bi-directional). An example: if the
      SDP contains two m lines (specifying whatever direction of the
      streams), and MIKEY is at the session level, then MIKEY allocates
      e.g. the Crypto Sessions Identifiers (CS IDs, see [MIKEY)] '1' and
      '2' for the first m line, and '3' and '4' for the second m line.

   * the possibility for MIKEY to receive incoming MIKEY messages and
      return a status code from/to the SIP/RTSP application.

   * the possibility for the SIP or RTSP applications to receive
      information from MIKEY. This would typically include the receiving
      of the Crypto Session Bundle Identifier (CSB ID, see [MIKEY], to
      later be able to identify the active MIKEY session), and the SSRCs
      and the rollover counter (ROC, see [SRTP]) for SRTP usage. It is
      also RECOMMENDED that extra information about errors can be
      received.

   * the possibility for the SIP or RTSP application to receive outgoing
      MIKEY messages.

   * the possibility to tear down a MIKEY CSB (e.g. if the SIP session
      is closed, the CSB SHOULD also be closed).



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8. Security Considerations

   The framework for transfer of key management data as described here
   is intended to provide the security parameters for the end-to-end
   protection of the media session. It is furthermore good practice to
   secure the session setup (e.g. SDP, SIP, RTSP, SAP).  However, it
   might be that the security of the session setup is not possible to
   achieve end-to-end, but only hop-by-hop. For example, SIP requires
   intermediate proxies to have access to part of the SIP message, and
   sometimes also to the SDP description (c.f. [E2M]), although end-to-
   end confidentiality can hide bodies from intermediaries. General
   security considerations for the session setup can be found in SDP
   [SDPnew], SIP [SIP], and RTSP [RTSP]. The framework defined in this
   memo is useful when the session setup is not protected in an end-to-
   end fashion, but the media streams needs to be end-to-end protected,
   hence the security parameters (such as keys) are not wanted revealed
   to nor manipulated by intermediaries.

   The security will also depend on the level of security the key
   management protocol offers. It follows that, under the assumption
   that the key management schemes are secure, the SDP can be passed
   along unencrypted without affecting the key management as such, and
   the media streams will still be secure even if some attackers gained
   knowledge of the SDP contents. Further security considerations can be
   found for each key management protocol (for MIKEY these can be found
   in [MIKEY]). However, if the SDP messages are not sent integrity
   protected between the parties, it is possible for an active attacker
   to change attributes without being detected. As the key management
   protocol may (indirectly) rely on some of the session information
   from SDP (e.g., address information), an attack on SDP may have
   indirect consequences on the key management. Even if the key
   management protocol does not rely on parameters of SDP and will not
   be affected by manipulation of these, different DoS attacks aimed at
   SDP may lead to undesired interruption in the setup. See also the
   attacks described at the end of this section.

   The only integrity protected attribute of the media stream is, in the
   framework proposed here, the set of key management protocols. It is
   for instance possible to (1) swap key management offers across SDP
   messages, or, (2) inject a previous key management offer into a new
   SDP message. Making the (necessary) assumption that all involved key
   management protocols are secure, the second attack will be detected
   by replay protection mechanisms of the key management protocol(s).
   Making the further assumption that, according to normal best current
   practice, the production of each key management offer is done with
   independent (pseudo)random choices (for session keys and other
   parameters), the first attack will either be detected in the
   Responder's (now incorrect) verification reply message (if such is




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   used), or, be a pure DoS attack, resulting in Initiator and Responder
   using different keys.

   It is RECOMMENDED for the identity at the SPD level to be the one
   authenticated at the key management protocol level. This might
   however need to keep into consideration privacy aspects, which are
   out of scope for this famework.

   The use of multiple key management protocols in the same offer may
   open up the possibility of a bidding-down attack, as specified in
   Section 3.1.4. To exclude such possibility, the authentication of the
   protocol identifier list is used. Note though, that the security
   level of the authenticated protocol identifier will be as high (or
   low), as the "weakest" protocol. Therefore, the offer MUST NOT
   contain any security protocols (or configurations thereof) weaker
   than permitted by local security policy.

   Note that it is impossible to assure the authenticity of a declined
   offer, since even if it comes from the true respondent, the fact that
   the answerer declines the offer usually means that he does not
   support the protocol(s) offered, and consequently cannot be expected
   to authenticate the response either. This means that if the Initiator
   is unsure of which protocol(s) the Responder supports, we RECOMMEND
   that the Initiator offers all acceptable protocols in a single offer.
   If not, this opens up the possibility for a "man-in-the-middle"
   (MITM) to affect the outcome of the eventually agreed upon protocol,
   by faking unauthenticated error messages until the Initiator
   eventually offers a protocol "to the liking" of the MITM. This is not
   really a security problem, but rather a mild form of denial of
   service that can be avoided by following the above recommendation.
   Note also that the declined offer could be result of an attacker who
   sits on the path and removes all the key management offers. The
   bidding-down attack prevention, as described above, would not work in
   this case (as the answerer receives no key management attribute).
   Also here it is impossible to assure the authenticity of a declined
   offer, though here the reason is the "peeling-off" attack. It is up
   to the local policy to decide the behavior in the case that the
   response declines any security (therefore there is impossibility of
   authenticating it). If for example the local policy requires a secure
   communication and cannot accept an unsecured one, then the session
   setup SHALL be aborted.


9. IANA Considerations

9.1. SDP Attribute Registration

   The IANA is hereby requested to create a new subregistry for the
   purpose of key management protocol integration with SDP.

      SDP Attribute Field ("att-field"):



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        Name:               key-mgmt-att-field
        Long form:          key management protocol attribute field
        Type of name:       att-field
        Type of attribute:  Media and session level
        Purpose:            See RFC xxxx, Section 2.
        Reference:          RFC xxxx, Section 2.1
        Values:             See RFC xxxx, Section 2.1 and 8.3.


9.2. RTSP Registration

   The IANA is hereby requested to create a new subregistry for the
   purpose of key management protocol integration with RTSP.

   Following the guidelines of [RTSP], the registration is defined as
   follows:

   Header name:      keymgmt
   Header syntax:    see RFC xxxx, Section 2.2
   Intended usage:   see RFC xxxx, Section 2.2
   Proxy treatment:  Proxies SHALL NOT add, change, or delete the
                      header. The proxy does not need to read this
                      header.
   Purpose:          see RFC xxxx, Section 2

   The RTSP Status-Code "463" [RFC xxxx], with the default string "Key
   management failure", needs to be registered.


9.3. Protocol Identifier Registration

   This document defines one new name space, the "SDP/RTSP key
   management protocol identifier", associated with the protocol
   identifier, KMPID, defined in Section 2 to be used with the above
   registered attributes in SDP and RTSP.

   The IANA is hereby requested to create a new subregistry for the
   KMPID parameter, with the following registration created initially:
   "mikey".

        Value name:     mikey
        Long name:      Multimedia Internet KEYing
        Purpose:        Usage of MIKEY with the key-mgmt-att-field
                         attribute and the keymgmt RTSP header
        Reference:      Section 7 in RFC 3830

   Note that this registration implies that the protocol identifier,
   KMPID, name space will be shared between SDP and RTSP.





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   Further values may be registered according to the "Specification
   Required" policy as defined in [RFC2434]. Each new registration needs
   to indicate the parameter name, and register it within IANA. Note
   that the parameter name is case sensitive and it is RECOMMENDED that
   the name to be in lower case letters. For each new registration, it
   is mandatory that a permanent, stable, and publicly accessible
   document exists that specifies the semantics of the registered
   parameter and the requested details of interaction between the key
   management protocol and SDP, as specified in RFC xxxx.

   New values MUST be register with IANA. Registrations SHALL include
   the following information:

   * Contact: the contact name and email address
   * Value name: the name of the value being registered (which MUST
      comply with the KMPID as defined in Section 2)
   * Long Name: long-form name in English
   * Purpose: short explanation of the purpose of the registered name.
   * Reference: a reference to the specification (e.g. RFC number)
      providing the usage guidelines in accordance to Section 5 (and
      also complying to the specified requirements).


10. Acknowledgments

   The authors would like to thank Francois Audet, Rolf Blom, Johan
   Bilien, Magnus Brolin, Erik Eliasson, Martin Euchner, Steffen Fries,
   Joerg Ott, Jon Peterson, and Jon-Olov Vatn. A special thanks to Colin
   Perkins and Magnus Westerlund, who contributed in many sections.


11. Author's Addresses

     Jari Arkko
     Ericsson
     02420 Jorvas             Phone:  +358 40 5079256
     Finland                  Email:  jari.arkko@ericsson.com

     Elisabetta Carrara
     Ericsson
     SE-16480 Stockholm       Phone:  +46 8 50877040
     Sweden                   EMail:  elisabetta.carrara@ericsson.com

     Fredrik Lindholm
     Ericsson
     SE-16480 Stockholm       Phone:  +46 8 58531705
     Sweden                   EMail:  fredrik.lindholm@ericsson.com

     Mats Naslund
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 58533739



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     Sweden                   EMail:  mats.naslund@ericsson.com

     Karl Norrman
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 4044502
     Sweden                   EMail:  karl.norrman@ericsson.com


12. References

12.1. Normative References

   [MIKEY] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and
   Norrman, K., "MIKEY: Multimedia Internet KEYing", IETF, RFC 3830.

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

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

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

   [SDPnew] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
   Description Protocol", Internet Draft, IETF, draft-ietf-mmusic-sdp-
   new-15.txt.

   [SIP] Rosenberg et al., "SIP: Session Initiation Protocol", IETF, RFC
   3261.

   [RFC2234] Crocker, D. and Overell, P., "Augmented BNF for Syntax
   Specifications: ABNF", RFC 2234, November 1997.

   [RFC2434] Narten, T. and Alvestrand, H., "Guidelines for Writing an
   IANA Considerations Section in RFCs", IETF, RFC 2434.

   [RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
   Encodings", IETF, RFC 3548.


12.2. Informative References

   [E2M] Ono, K. and Tachimoto, S., "End-to-middle security in the
   Session Initiation Protocol (SIP)", Internet Draft, IETF, draft-ono-
   sipping-end2middle-security-03.

   [KERB] Kohl, J., Neuman, C., "The Kerberos Network Authentication
   Service (V5)", IETF, RFC 1510.





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   [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
   (S/MIME) Version 3.1 Message Specification", IETF, RFC 3851.

   [SDES] Andreasen, F., Baugher, M., Wing, D., "Session Description
   Protocol Security Descriptions for Media Streams", work in progress,
   February 2005.

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

   [SPREC] Andreasen, F., Baugher, M., and Wing, D., "Security
   Preconditions for Session Description Protocol Media Streams", work
   in progress, February 2004.


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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,



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   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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   This Internet-Draft expires in December 2005.

















































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