Internet Engineering Task Force                  Flemming Andreasen
   MMUSIC Working Group                                   Mark Baugher
   INTERNET-DRAFT                                             Dan Wing
   EXPIRES: December 2005 March 2006                                   Cisco Systems
                                                       September, 2005

           Session Description Protocol Security Descriptions
                           for Media Streams

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Copyright Notice

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


   This document defines a Session Description Protocol (SDP)
   cryptographic attribute for unicast media streams.  The attribute
   describes a cryptographic key and other parameters, which serve to
   configure security for a unicast media stream in either a single
   message or a roundtrip exchange.  The attribute can be used with a
   variety of SDP media transports and this document defines how to use
   it for the Secure Real-time Transport Protocol (SRTP) unicast media
   streams.  The SDP crypto attribute requires the services of a data
   security protocol to secure the SDP message.

Table of Contents

1  Introduction......................................................3
2  Notational Conventions............................................4
3  Applicability.....................................................5

4  SDP "Crypto" Attribute and Parameters.............................5
 4.1  Tag............................................................6
 4.2  Crypto-suite...................................................6
 4.3  Key Parameters.................................................6
 4.4  Session Parameters.............................................7
 4.5  Example........................................................7
5  General Use of the crypto Attribute...............................8
 5.1  Use With Offer/Answer..........................................8
   5.1.1  Generating the Initial Offer - Unicast Streams............8
   5.1.2  Generating the Initial Answer - Unicast Streams...........9
   5.1.3  Processing of the Initial Answer - Unicast Streams.......10
   5.1.4  Modifying the Session....................................10
 5.2  Use Outside Offer/Answer......................................11
 5.3  General Backwards Compatibility Considerations................11
6  SRTP Security Descriptions.......................................11
 6.1  SRTP Key Parameter............................................12
 6.2  Crypto-suites.................................................15
   6.2.1  AES_CM_128_HMAC_SHA1_80..................................15
   6.2.2  AES_CM_128_HMAC_SHA1_32..................................16
   6.2.3  F8_128_HMAC_SHA1_80......................................16
   6.2.4  Adding new Crypto-suite Definitions......................16
 6.3  Session Parameters............................................16
   6.3.1  KDR=n....................................................16
   6.3.2  UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP...................17
   6.3.3  UNAUTHENTICATED_SRTP.....................................17
   6.3.4  FEC_ORDER=order..........................................17
   6.3.5  FEC_KEY=key-params.......................................17
   6.3.6  Window Size Hint (WSH)...................................18
   6.3.7  Defining New SRTP Session Parameters.....................18
 6.4  SRTP Crypto Context Initialization............................18
   6.4.1  Late Binding of one or more SSRCs to a crypto context....20
   6.4.2  Sharing cryptographic contexts among Sessions or SSRCs...21
 6.5  Removal of Crypto Contexts....................................21
7  SRTP-Specific Use of the crypto Attribute........................21
 7.1  Use with Offer/Answer.........................................21
   7.1.1  Generating the Initial Offer - Unicast Streams...........22
   7.1.2  Generating the Initial Answer - Unicast Streams..........22
   7.1.3  Processing of the Initial Answer - Unicast Streams.......23
   7.1.4  Modifying the Session....................................23
   7.1.5  Offer/Answer Example.....................................24
 7.2  SRTP-Specific Use Outside Offer/Answer........................25
 7.3  Support for SIP Forking.......................................26
 7.4  SRTP-Specific Backwards Compatibility Considerations..........26
 7.5  Operation with KEYMGT= and k= lines...........................27
8  Security Considerations..........................................27
 8.1  Authentication of packets.....................................28
 8.2  Keystream Reuse...............................................28
 8.3  Signaling Authentication and Signaling Encryption.............28 Encryption.............29
9  Grammar..........................................................29
 9.1  Generic "Crypto" Attribute Grammar............................29
 9.2  SRTP "Crypto" Attribute Grammar...............................30

10   IANA Considerations.............................................31
 10.1  Registration of the "crypto" attribute......................31
 10.2  New IANA Registries and Registration Procedures.............31
   10.2.1 Key Method Registry and Registration.....................31 Registration.....................32
   10.2.2 Media Stream Transport Registry and Registration.........32
 10.3  Initial Registrations.......................................32
   10.3.1 Key Method...............................................32
   10.3.2 SRTP Media Stream Transport..............................32
11   Acknowledgements................................................33
12   Authors' Addresses..............................................33 Addresses..............................................34
13   Normative References............................................34
14   Informative References..........................................34 References..........................................35
15   Full Copyright Statement........................................36

1  Introduction

   The Session Description Protocol (SDP) [SDP] describes multimedia
   sessions, which can be audio, video, whiteboard, fax, modem, and
   other media streams.  Security services such as data origin
   authentication, integrity and confidentiality are often needed for
   those streams.  The Secure Real-time Transport Protocol (SRTP) [srtp]
   provides security services for RTP media and is signaled by use of
   secure RTP transport (e.g., "RTP/SAVP" or "RTP/SAVPF") in an SDP
   media (m=) line.  However, there are no means within SDP itself to
   configure SRTP beyond using default values.  This document specifies
   a new SDP attribute called "crypto", which is used to signal and
   negotiate cryptographic parameters for media streams in general, and
   SRTP in particular.  The definition of the crypto attribute in this
   document is limited to two-party unicast media streams where each
   source has a unique cryptographic key; support for multicast media
   streams or multipoint unicast streams is for further study.

   The crypto attribute is defined in a generic way to enable its use
   with SRTP and any other secure transports that can establish
   cryptographic parameters with only a single message or in a single
   round-trip exchange using the offer/answer model [RFC3264].
   Extension to transports other than SRTP, however, is beyond the scope
   of this document.  Each type of secure media transport needs its own
   specification for the crypto-attribute parameter.  These definitions
   are frequently unique to the particular type of transport and must be
   specified in a Standards Track RFC and registered with IANA according
   to the procedures defined in Section 10.  This document defines the
   security parameters and keying material for SRTP only.

   It would be self-defeating not to secure cryptographic keys and other
   parameters at least as well as the data is secured.  Data security
   protocols such as SRTP rely upon a separate key management system to
   securely establish encryption and/or authentication keys.  Key
   management protocols provide authenticated key establishment (AKE)
   procedures to authenticate the identity of each endpoint and protect
   against man-in-the-middle, reflection/replay, connection hijacking
   and some denial of service attacks [skeme].  Along with the key, an
   AKE protocol such as MIKEY [mikey], GDOI [GDOI], KINK [kink], IKE
   [ike], Secure Multiparts [s/mime, pgp/mime] or TLS [TLS] securely
   disseminates information describing both the key and the data-
   security session.  AKE is needed because it is pointless to provide a
   key over a medium where an attacker can snoop the key, alter the
   definition of the key to render it useless, or change the parameters
   of the security session to gain unauthorized access to session-
   related information.

   SDP, however, was not designed to provide AKE services, and the media
   security descriptions defined in this document do not add AKE
   services to SDP.  This specification is no replacement for a key
   management protocol or for the conveyance of key management messages
   in SDP [keymgt].  The SDP security descriptions defined here are
   suitable for restricted cases only where IPsec, TLS, or some other
   encapsulating data-security protocol (e.g., SIP S/MIME) protects the
   SDP message.  This document adds security descriptions to those
   encrypted and/or authenticated SDP messages through the new SDP
   "crypto" attribute, which provides the cryptographic parameters of a
   media stream.

   The "crypto" attribute can be adapted to any media transport, but its
   precise definition is unique to a particular transport.

   Below, we first provide notational conventions in Section 2 followed
   by an applicability statement for the crypto attribute in Section 3.
   In Section 4, we introduce the general SDP crypto attribute, and in
   Section 5 we define how it is used with and without the offer/answer
   model.  In Section 6, we define the crypto attribute details needed
   for SRTP, and in Section 7 we define SRTP-specific use of the
   attribute with and without the offer/answer model.  Section 8 recites
   security considerations, and Section 9 gives an Augmented-BNF grammar
   for the general crypto attribute as well as the SRTP-specific use of
   the crypto attribute.  IANA considerations are provided in Section

2  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
   NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to
   be interpreted as described in [RFC2119].  The terminology in this
   document conforms to [RFC2828], "Internet Security Glossary".

   n^r is exponentiation where n is multiplied by itself r times; n and
   r are integers.  0..k is an integer range of all integers from 0
   through k inclusive.

   The terms 'transport' and 'media transport' are used to mean
   'transport protocol' as defined in RFC2327, page 20.

   Explanatory notes are provided in several places throughout the
   document; these notes are indented two spaces from the surrounding

3  Applicability

   RFC YYYY {Note to RFC Editor: replace "YYYY" with RFC number for
   [keymgt]} provides similar cryptographic key distribution
   capabilities, and is intended for use when the signaling is to be
   confidential and/or integrity-protected separately from the keying

   In contrast, this specification carries the keying material  within
   the SDP message, and it is intended for use when the keying material
   is protected along with the signaling.  Implementations MUST employ
   security mechanisms that provide confidentiality and integrity for
   the keying material.  When this specification is used in the context
   of SIP [RFC3261], the application SHOULD employ either the SIPS URI
   or S/MIME to provide protection for the SDP message and the keying
   material that it contains.  The use of transport layer or IP layer
   security in lieu of the SIPS URI or S/MIME protection is NOT
   RECOMMENDED since the protection of the SDP message and the keying
   material that it contains cannot be ensured through all intermediate
   entities such as SIP proxies.

4  SDP "Crypto" Attribute and Parameters

   A new media-level SDP attribute called "crypto" describes the
   cryptographic suite, key parameters, and session parameters for the
   preceding unicast media line.  The "crypto" attribute MUST only
   appear at the SDP media level (not at the session level).  The
   "crypto" attribute follows the format (see Section 9.1 for the formal
   ABNF grammar):

     a=crypto:<tag> <crypto-suite> <key-params> [<session-params>]

   The fields tag, crypto-suite, key-params, and session-params are
   described in the following sub-sections.  Below we show an example of
   the crypto attribute for the "RTP/SAVP" transport, i.e., the secure
   RTP extension to the Audio/Video Profile [srtp].  In the following,
   newlines are included for formatting reasons only:

     a=crypto:1 AES_CM_128_HMAC_SHA1_80

   The crypto-suite is AES_CM_128_HMAC_SHA1_80, key-params is defined
   by the text starting with "inline:", and session-params is omitted.

4.1  Tag

   The tag is a decimal number used as an identifier for a particular
   crypto attribute (see Section 9.1 for details).  The tag MUST be
   unique among all crypto attributes for a given media line.  It is
   used with the offer/answer model to determine which of several
   offered crypto attributes were chosen by the answerer (see Section

   In the offer/answer model, the tag is a negotiated parameter.

4.2  Crypto-suite

   The crypto-suite field is an identifier that describes the encryption
   and authentication algorithms (e.g., AES_CM_128_HMAC_SHA1_80) for
   the transport in question (see Section 9.1 for details).  The
   possible values for the crypto-suite parameter are defined within the
   context of the transport, i.e., each transport defines a separate
   namespace for the set of crypto-suites.  For example, the crypto-
   suite "AES_CM_128_HMAC_SHA1_80" defined within the context
   "RTP/SAVP" transport applies to Secure RTP only; the string may be
   reused for another transport (e.g., "RTP/SAVPF" [srtpf]), but a
   separate definition would be needed.

   In the offer/answer model, the crypto-suite is a negotiated

4.3  Key Parameters

   The key-params field provides one or more sets of keying material for
   the crypto-suite in question.  The field consists of a method
   indicator followed by a colon, and the actual keying information as
   shown below (the formal grammar is provided in Section 9.1):

     key-params = <key-method> ":" <key-info>

   Keying material might be provided by different means than key-params,
   however this is out of scope.  Only one method is defined in this
   document, namely "inline", which indicates that the actual keying
   material is provided in the key-info field itself.  There is a single
   name space for the key-method, i.e., the key-method is transport
   independent.  New key-methods (e.g., use of a URL) may be defined in
   a Standards Track RFC in the future.  Although the key-method itself
   may be generic, the accompanying key-info definition is specific not
   only to the key-method, but also to the transport in question.  Key-
   info encodes keying material for a crypto suite, which defines that
   keying material.  New key methods MUST be registered with the IANA
   according to the procedures defined in Section 10.2.1.

   Key-info is defined as a general character string (see Section 9.1
   for details); further transport and key-method specific syntax and
   semantics MUST be provided in a Standards Track RFC for each
   combination of transport and key-method that use it; definitions for
   SRTP are provided in Section 6.  Note that such definitions are
   provided within the context of both a particular transport (e.g.,
   "RTP/SAVP") and a specific key-method (e.g., "inline").  IANA will
   register the list of supported key methods for each transport.

   When multiple keys are included in the key parameters, it MUST be
   possible to determine which of the keys is being used in a given
   media packet by a simple inspection of the media packet received; a
   trial-and-error approach between the possible keys MUST NOT be

     For SRTP, this could be achieved by use of Master Key Identifiers
     (MKI) [srtp].  Use of <"From, "To"> values are not supported in
     SRTP security descriptions for reasons explained in Section 6.1,

   In the offer/answer model, the key parameter is a declarative

4.4  Session Parameters

   Session parameters are specific to a given transport and use of them
   is OPTIONAL in the security descriptions framework, where they are
   just defined as general character strings.  If session parameters are
   to be used for a given transport, then transport-specific syntax and
   semantics MUST be provided in a Standards Track RFC; definitions for
   SRTP are provided in Section 6.

   In the offer/answer model, session parameters may be either
   negotiated or declarative; the definition of specific session
   parameters MUST indicate whether they are negotiated or declarative.
   Negotiated parameters apply to data sent in both directions, whereas
   declarative parameters apply only to media sent by the entity that
   generated the SDP.  Thus, a declarative parameter in an offer applies
   to media sent by the offerer, whereas a declarative parameter in an
   answer applies to media sent by the answerer.

4.5  Example

   This example shows use of the crypto attribute for the "RTP/SAVP"
   media transport type (as defined in Section 5).  The "a=crypto" line
   is actually one long line; it is shown as two lines due to page

     o=jdoe 2890844526 2890842807 IN IP4
     s=SDP Seminar
     i=A Seminar on the session description protocol
     u= (Jane Doe)
     c=IN IP4
     t=2873397496 2873404696
     m=video 51372 RTP/SAVP 31
     a=crypto:1 AES_CM_128_HMAC_SHA1_80
     m=audio 49170 RTP/SAVP 0
     a=crypto:1 AES_CM_128_HMAC_SHA1_32
     m=application 32416 udp wb

   This SDP message describes three media streams, two of which use the
   "RTP/SAVP" transport.  Each has a crypto attribute for the
   "RTP/SAVP" transport.  These secure-RTP specific descriptions are
   defined in Section 6.

5  General Use of the crypto Attribute

   In this section, we describe the general use of the crypto attribute
   outside of any transport or key-method specific rules.

5.1  Use With Offer/Answer

   The general offer/answer rules for the crypto attribute are in
   addition to the rules specified in RFC 3264, which MUST be followed,
   unless otherwise noted.  RFC 3264 defines operation for both unicast
   and multicast streams; the sections below describe operation for two-
   party unicast streams only, since support for multicast streams (and
   multipoint unicast streams) is for further study.

5.1.1  Generating the Initial Offer - Unicast Streams

   When generating an initial offer for a unicast stream, there MUST be
   one or more crypto attributes present for each media stream for which
   security is desired.  Each crypto attribute for a given media stream
   MUST contain a unique tag.

   The ordering of multiple "a=crypto" lines is significant:  The most
   preferred crypto line is listed first.  Each crypto attribute
   describes the crypto-suite, key(s) and possibly session parameters
   offered for the media stream.  In general, a "more preferred" crypto-
   suite SHOULD be cryptographically stronger than a "less preferred"

   The crypto-suite always applies to media in the directions supported
   by the media stream (e.g., send and receive).  The key(s), however,
   apply to media in the direction from the offerer to the answerer; if
   the media stream is marked as "recvonly", a key MUST still be

    This is done for consistency.  Also, in the case of SRTP, for
    example, secure RTCP will still be flowing in both the send and
    receive direction for a unidirectional stream.

   The offer may include session parameters.  There are no general offer
   rules for the session parameters; instead, specific rules may be
   provided as part of the transport-specific definitions of any session

   When issuing an offer, the offerer MUST be prepared to support media
   security in accordance with any of the crypto attributes included in
   the offer.  There are however two problems associated with this.
   First of all, the offerer does not know which key the answerer will
   be using for media sent to the offerer.  Second, the offerer may not
   be able to deduce which of the offered crypto attributes were
   accepted.  Since media may arrive prior to the answer, delay or
   clipping can occur.  If this is unacceptable to the offerer, the
   offerer SHOULD use a mechanism outside the scope of this document to
   prevent the above problem.

     For example, in SIP [RFC3261], a "security" precondition as defined
     in [sprecon] could solve the above problem.

5.1.2  Generating the Initial Answer - Unicast Streams

   When the answerer receives the initial offer with one or more crypto
   attributes for a given unicast media stream, the answerer MUST either
   accept exactly one of the offered crypto attributes, or the offered
   stream MUST be rejected.

    If the answerer wishes to indicate support for other crypto
    attributes, those can be listed by use of the SDP Simple Capability
    Declaration [RFC3407] extensions.

   Only crypto attributes that are valid can be accepted; valid
   attributes do not violate any of the general rules defined for
   security descriptions as well as any specific rules defined for the
   transport and key-method in question.  When selecting one of the
   valid crypto attributes, the answerer SHOULD select the most
   preferred crypto attribute it can support, i.e., the first valid
   supported crypto attribute in the list, considering the answerer's
   capabilities and security policies.

   If there are one or more crypto attributes in the offer, but none of
   them are valid, or none of the valid ones are supported, the offered
   media stream MUST be rejected.

   When an offered crypto attribute is accepted, the crypto attribute in
   the answer MUST contain the following:

   * The tag and crypto-suite from the accepted crypto attribute in the
    offer (the same crypto-suite MUST be used in the send and receive
   * The key(s) the answerer will be using for media sent to the
    offerer.  Note that a key MUST be provided, irrespective of any
    direction attributes in the offer or answer.

   Furthermore, any session parameters that are negotiated MUST be
   included in the answer.  Declarative session parameters provided by
   the offerer are not included in the answer, however the answerer may
   provide its own set of declarative session parameters.

   Once the answerer has accepted one of the offered crypto attributes,
   the answerer MAY begin sending media to the offerer in accordance
   with the selected crypto attribute.  Note however, that the offerer
   may not be able to process such media packets correctly until the
   answer has been received.

5.1.3  Processing of the Initial Answer - Unicast Streams

   When the offerer receives the answer, the offerer MUST verify, that
   one of the initially offered crypto suites and its accompanying tag
   was accepted and echoed in the answer.  Also, the answer MUST include
   one or more keys, which will be used for media sent from the answerer
   to the offerer.

   If the offer contained any mandatory negotiated session parameters
   (see section 6.3.7), the offerer MUST verify that said parameters are
   included in the answer and support them. If the answer contains any
   mandatory declarative session parameters, the offerer MUST be able to
   support those.

   If any of the above fails, the negotiation MUST fail.

5.1.4  Modifying the Session

   Once a media stream has been established, it MAY be modified at any
   time, as described in RFC 3264, Section 8.  Such a modification MAY
   be triggered by the security service, e.g., in order to perform a re-
   keying or change the crypto-suite.  If media stream security using
   the general security descriptions defined here is still desired, the
   crypto attribute MUST be included in these new offer/answer
   exchanges.  The procedures are similar to those defined in Section
   5.1.1, 5.1.2, 5.1.3 of this document, subject to the considerations
   provided in RFC 3264 Section 8.

5.2  Use Outside Offer/Answer

   The crypto attribute can also be used outside the context of
   offer/answer where there is no negotiation of the crypto suite,
   cryptographic key or session parameters.  In this case, the sender
   determines security parameters for the stream.  Since there is no
   negotiation mechanisms, the sender MUST include exactly one crypto
   attribute and the receiver MUST either accept it or else SHOULD NOT
   receive the associated stream.  The sender SHOULD select the security
   description that it deems most secure for its purposes.

5.3  General Backwards Compatibility Considerations

   In the offer/answer model, it is possible that the answerer supports
   a given secure transport (e.g., "RTP/SAVP") and accepts the offered
   media stream, yet the answerer does not support the crypto attribute
   defined in this document and hence ignores it.  The offerer can
   recognize this situation by seeing an accepted media stream in the
   answer that does not include a crypto line.  In that case, the
   security negotiation defined here MUST fail.

   Similar issues exist when security descriptions are used outside of
   the offer/answer model.  But the source of a non-negotiated security
   description has no indication that the receiver has ignored the
   crypto attribute.

6  SRTP Security Descriptions

   In this section, we provide definitions for security descriptions for
   SRTP media streams.  In the next section, we define how to use SRTP
   security descriptions with and without the offer/answer model.

   SRTP security descriptions MUST only be used with the SRTP transport
   (e.g., "RTP/SAVP" or "RTP/SAVPF").  The following specifies security
   descriptions for the "RTP/SAVP" profile defined in [srtp], however it
   is expected that other secure RTP profiles (e.g., "RTP/SAVPF") can
   use the same descriptions, which are in accordance with the SRTP
   protocol specification [srtp].

   There is no assurance that an endpoint is capable of configuring its
   SRTP service with a particular crypto attribute parameter, but SRTP
   guarantees minimal interoperability among SRTP endpoints through the
   default SRTP parameters [srtp].  More capable SRTP endpoints support
   a variety of parameter values beyond the SRTP defaults and these
   values can be configured by the SRTP security descriptions defined
   here.  An endpoint that does not support the crypto attribute will
   ignore it according to the SDP.  Such an endpoint will not correctly
   process the particular media stream.  By using the Offer/Answer
   model, the offerer and answerer can negotiate the crypto parameters
   to be used before commencement of the multimedia session (see Section

   There are over twenty cryptographic parameters listed in the SRTP
   specification.  Many of these parameters have fixed values for
   particular cryptographic transforms.  At the time of session
   establishment, however, there is usually no need to provide unique
   settings for many of the SRTP parameters, such as salt length and
   pseudo-random function (PRF).  Thus, it is possible to simplify the
   list of parameters by defining "cryptographic suites" that fix a set
   of SRTP parameter values for the security session.  This approach is
   followed by the SRTP security descriptions, which uses the general
   security description parameters as follows:

     * crypto-suite:     Identifies the encryption and authentication
     * key parameter:    SRTP keying material and parameters
     * session parameters:    The following parameters are defined:
          - KDR:    The SRTP Key Derivation Rate is the rate that a
                    pseudo-random function is applied  to a master key
          - UNENCRYPTED_SRTP:      SRTP messages are not encrypted
          - UNENCRYPTED_SRTCP:     SRTCP messages are not encrypted
          - UNAUTHENTICATED_SRTP:  SRTP messages are not authenticated
          - FEC_ORDER:   Order of forward error correction (FEC)
                         relative to SRTP services
          - FEC_KEY:     Master Key for FEC when the FEC stream is sent
                         to a separate address and/or port.
          - WSH:         Window Size Hint
          - Extensions:  Extension parameters can be defined

   Please refer to the SRTP specification for a complete list of
   parameters and their descriptions [Section 8.2, srtp].  Regarding the
   UNENCRYPTED_SRTCP parameter, offerers and answerers of SDP security
   descriptions MUST NOT use the SRTCP E-bit to override
   UNENCRYPTED_SRTCP or the default, which is to encrypt all SRTCP
   messages (see Section 6.3.2).  The key parameter, the crypto-suite,
   and the session parameters shown above are described in detail in the
   following subsections.

6.1  SRTP Key Parameter

   SRTP security descriptions define use of the "inline" key method as
   described in the following.  Use of any other keying method, e.g.,
   URL, for SRTP security descriptions is for further study.

   The "inline" type of key contains the keying material (master key
   and salt) and all policy related to that master key, including how
   long it can be used (lifetime) and whether or not it uses a master
   key identifier (MKI) to associate an incoming SRTP packet with a
   particular master key.  Compliant implementations obey the policies
   associated with a master key, and MUST NOT accept incoming packets
   that violate the policy (e.g., after the master key lifetime has

   The key parameter contains one or more cryptographic master keys,
   each of which MUST be a unique cryptographically random [RFC1750]
   value with respect to other master keys in the entire SDP message
   (i.e., including master keys for other streams).  Each key follows
   the format (the formal definition is provided in Section 9.2):

     "inline:" <key||salt> ["|" lifetime] ["|" MKI ":" length]

     key||salt      concatenated master key and salt, base64 encoded
                    (see [RFC3548], Section 3)
     lifetime       master key lifetime (max number of SRTP or SRTCP
                    packets using this master key)
     MKI:length     MKI and length of the MKI field in SRTP packets

   The following definition provides an example for


   The first field ("d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj") of the
   parameter is the cryptographic master key appended with the master
   salt; the two are first concatenated and then base64 encoded.  The
   length of the concatenated key and salt is determined by the crypto-
   suite for which the key applies.  If the length (after being decoded
   from base64) does not match that specified for the crypto-suite, the
   crypto attribute in question MUST be considered invalid.  Each master
   key and salt MUST be a cryptographically random number and MUST be
   unique to the entire SDP message.  When base64 decoding the key and
   salt, padding characters (i.e., one or two "=" at the end of the
   base64 encoded data) are discarded (see [RFC3548] for details).
   Base64 encoding assumes that the base64 encoding input is an integral
   number of octets.  If a given crypto-suite requires the use of a
   concatenated key and salt with a length that is not an integral
   number of octets, said crypto-suite MUST define a padding scheme that
   results in the base64 input being an integral number of octets.  For
   example, if the length defined was 250 bits, then 6 padding bits
   would be needed, which could be defined to be the last 6 bits in a
   256 bit input.

   The second field, is the OPTIONAL lifetime of the master key as
   measured in maximum number of SRTP or SRTCP packets using that master
   key (i.e., the number of SRTP packets and the number of SRTCP packets
   each have to be less than the lifetime).  The lifetime value MAY be
   written as a non-zero, positive integer or as a power of 2 (see the
   grammar in Section 9.2 for details).  The "lifetime" value MUST NOT
   exceed the maximum packet lifetime for the crypto-suite.  If the
   lifetime is too large or otherwise invalid then the entire crypto
   attribute MUST be considered invalid.  The default MAY be implicitly
   signaled by omitting the lifetime (note that the lifetime field never
   includes a colon, whereas the third field always does).  This is
   convenient when the SRTP cryptographic key lifetime is the default
   value.  As a shortcut to avoid long decimal values, the syntax of the
   lifetime allows using the literal "2^", which indicates "two to the
   power of".  The example above, shows a case where the lifetime is
   specified as 2^20.  The following example, which is for the
   AES_CM_128_HMAC_SHA1_80 crypto-suite, has a default for the lifetime
   field, which means that SRTP's and SRTCP's default values will be
   used (see [srtp]):


   The example shows a 30-character key and concatenated salt that is
   base64 encoded:  The 30-character key/salt concatenation is expanded
   to 40 characters by the three-in-four encoding of base64.

   The third field, which is also OPTIONAL, is the Master Key Identifier
   (MKI) and its byte length.

   "MKI" is the master key identifier associated with the SRTP master
   key.  The MKI is here defined as a positive integer which is encoded
   as a big-endian integer in the actual SRTP packets.  If the MKI is
   given, then the length of the MKI MUST also be given and separated
   from the MKI by a colon (":").  The MKI length is the size of the MKI
   field in the SRTP packet, specified in bytes.  If the MKI length is
   not given or its value exceeds 128 (bytes), then the entire crypto
   attribute MUST be considered invalid.  The substring "1:4" in the
   first example assigns to the key a master key identifier of 1 that is
   4 bytes long, and the second example assigns a 4-byte master key
   identifier of 1066 to the key.  One or more master keys with their
   associated MKI can be initially defined, and then later updated, or
   deleted and new ones defined.

   SRTP offers a second feature for specifying the lifetime of a master
   key in terms of two values, called "From" and "To," which are defined
   on the SRTP sequence number space [srtp].  This SRTP Security
   Descriptions specification, however, does not support the
   <"From","To"> feature since the lifetime of an AES master key is 2^48
   SRTP packets, which means that there is no cryptographic reason to
   replace a master key for practical point-to-point applications.  For
   this reason, there is no need to support two means for signaling key
   update.  The MKI is chosen over <"From","To"> by this specification
   for the very few applications that need it since the MKI feature is
   simpler (though the MKI adds additional bytes to each packet whereas
   <"From", "To"> does not).

   As mentioned above, the key parameter can contain one or more master
   keys.  When the key parameter contains more than one master key, all
   of the master keys in that key parameter MUST include an MKI value.

   When using the MKI, the MKI length MUST be the same for all keys in a
   given crypto attribute.

6.2  Crypto-suites

   The SRTP crypto-suites define the encryption and authentication
   transforms to be used for the SRTP media stream.  The SRTP
   specification has defined three crypto-suites, which are described
   further in the following subsections in the context of the SRTP
   security descriptions.  The table below provides an overview of the
   crypto-suites and their parameters:

   |                     |AES_CM_128_  | AES_CM_128_  | F8_128_       |
   |                     |HMAC_SHA1_80 | HMAC_SHA1_32 |  HMAC_SHA1_80 |
   | Master key length   |   128 bits  |   128 bits   |   128 bits    |
   | Salt value          |   112 bits  |   112 bits   |   112 bits    |
   | Default lifetime    | 2^31 packets| 2^31 packets | 2^31 packets  |
   | Cipher              | AES Counter | AES Counter  |    F8         |
   |                     | Mode        | Mode         |               |
   | Encryption key      |   128 bits  |   128 bits   |   128 bits    |
   | MAC                 |  HMAC-SHA1  |  HMAC-SHA1   |  HMAC-SHA1    |
   | Authentication tag  |    80 bits  |    32 bits   |    80 bits    |
   | SRTP auth. key      |   160 bits  |   160 bits   |   160 bits    |
   | SRTCP auth. key     |   160 bits  |   160 bits   |   160 bits    |

6.2.1  AES_CM_128_HMAC_SHA1_80

   AES_CM_128_HMAC_SHA1_80 is the SRTP default AES Counter Mode cipher
   and HMAC-SHA1 message authentication having an 80-bit authentication
   tag.  The master-key length is 128 bits and has a default lifetime
   of a maximum of 2^31 SRTP packets or SRTCP packets, whichever comes
   first [Page 39, srtp].

     Technically, SRTP allows 2^48 SRTP packets or 2^31 SRTCP packets,
     whichever comes first.  SRTP security descriptions, however,
     simplify the parameters to share a single upper bound of 2^31
     packets.  It is RECOMMENDED that automated key management allow
     easy and efficient rekeying at intervals far smaller than 2^31
     packets given today's media rates or even HDTV media rates.

   The SRTP and SRTCP encryption key lengths are 128 bits.  The SRTP
   and SRTCP authentication key lengths are 160 bits (see Security
   Considerations in Section 8).  The master salt value is 112 bits in
   length and the session salt value is 112 bits in length.  The
   pseudo-random function (PRF) is the default SRTP pseudo-random
   function that uses AES Counter Mode with a 128-bit key length.

   The length of the base64 decoded key and salt value for this crypto-
   suite MUST be 30 characters, i.e., 240 bits; otherwise the crypto
   attribute is considered invalid.

6.2.2  AES_CM_128_HMAC_SHA1_32

   This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except
   that the authentication tag is 32 bits.

   The length of the base64-decoded key and salt value for this crypto-
   suite MUST be 30 characters, i.e., 240 bits; otherwise the crypto
   attribute is considered invalid.

6.2.3  F8_128_HMAC_SHA1_80

   This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except the
   cipher is F8 [srtp].

   The length of the base64 decoded key and salt value for this crypto-
   suite MUST be 30 characters, i.e. 240 bits; otherwise the crypto
   attribute is considered invalid.

6.2.4  Adding new Crypto-suite Definitions

   If new transforms are added to SRTP, new definitions for those
   transforms SHOULD be given for the SRTP security descriptions and
   published in a Standards Track RFC.  Sections 6.2.1 through 6.2.3
   illustrate how to define crypto-suite values for particular
   cryptographic transforms.  Any new crypto-suites MUST be registered
   with IANA following the procedures in section 10.

6.3  Session Parameters

   SRTP security descriptions define a set of "session" parameters,
   which OPTIONALLY may be used to override SRTP session defaults for
   the SRTP and SRTCP streams.  These parameters configure an RTP
   session for SRTP services. The session parameters provide session-
   specific information to establish the SRTP cryptographic context.

6.3.1  KDR=n

   KDR specifies the Key Derivation Rate, as described in section 4.3.1
   of [srtp].

   The value n MUST be an integer in the set {1,2,...,24}, which
   denotes a power of 2 from 2^1 to 2^24, inclusive.  The SRTP key
   derivation rate controls how frequently a new session key is derived
   from an SRTP master key(s) [srtp] given in the declaration.  When
   the key derivation rate is not specified (i.e., the KDR parameter is
   omitted), a single initial key derivation is performed [srtp].

   In the offer/answer model, KDR is a declarative parameter.


   SRTP and SRTCP packet payloads are encrypted by default.  The
   UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP session parameters modify the
   default behavior of the crypto-suites with which they are used:

   * UNENCRYPTED_SRTCP signals that the SRTCP packet payloads are not

   * UNENCRYPTED_SRTP signals that the SRTP packet payloads are not

   In the offer/answer model, these parameters are negotiated. If
   UNENCRYPTED_SRTCP is signaled for the session, then the SRTCP E bit
   MUST be clear (0) in all SRTCP messages.  If the default is used,
   all SRTCP messages are encrypted and the E bit MUST be set (1) on
   all SRTCP messages.


   SRTP and SRTCP packet payloads are authenticated by default.  The
   UNAUTHENTICATED_SRTP session parameter signals that SRTP messages
   are not authenticated.  Use of UNAUTHENTICATED_SRTP is NOT
   RECOMMENDED (see Security Considerations).

    The SRTP specification requires use of message authentication for
    SRTCP, but not for SRTP [srtp].

   In the offer/answer model, this parameter is negotiated.

6.3.4  FEC_ORDER=order

   FEC_ORDER signals the use of forward error correction for the RTP
   packets [rfc2733].  The forward error correction values for "order"
   are FEC_SRTP or SRTP_FEC.  FEC_SRTP signals that FEC is applied
   before SRTP processing by the sender of the SRTP media and after SRTP
   processing by the receiver of the SRTP media; FEC_SRTP is the
   default.  SRTP_FEC is the reverse processing.

   In the offer/answer model, FEC_ORDER is a declarative parameter.

6.3.5  FEC_KEY=key-params

   FEC_KEY signals the use of separate master key(s) for a Forward
   Error Correction (FEC) stream.  The master key(s) are specified with
   the exact same format as the SRTP Key Parameter defined in Section
   6.1, and the semantic rules are the same - in particular, the master
   key(s) MUST be different from all other master key(s) in the SDP.  A
   FEC_KEY MUST be specified when the FEC stream is sent to a different
   IP-address and/or port than the media stream to which it applies
   (i.e., the "m=" line), e.g., as described in RFC 2733 Section 11.1.
   When a FEC stream is sent to the same IP-address and port as the
   media stream to which it applies, a FEC_KEY MUST NOT be specified.
   If a FEC_KEY is specified in this latter case, the crypto attribute
   in question MUST be considered invalid.

   In the offer/answer model, FEC_KEY is a declarative parameter.

6.3.6  Window Size Hint (WSH)

   SRTP defines the SRTP-WINDOW-SIZE [srtp, section 3.3.2] parameter to
   protect against replay attacks.  The minimum value is 64 [srtp],
   however this value may be considered too low for some applications
   (e.g., video).

   The Window Size Hint (WSH) session parameter provides a hint for how
   big this window should be to work satisfactorily (e.g., based on
   sender knowledge of number of packets per second).  However, there
   might be enough information given in SDP attributes like
   "a=maxprate" [maxprate] and the bandwidth modifiers to allow a
   receiver to derive the parameter satisfactorily.  Consequently, this
   value is only considered a hint to the receiver of the SDP which MAY
   choose to ignore the value provided.

   In the offer/answer model, WSH is a declarative parameter.

6.3.7  Defining New SRTP Session Parameters

   New SRTP session parameters for the SRTP security descriptions can
   be defined in a Standards Track RFC and registered with IANA
   according to the registration procedures defined in Section 10.

   New SRTP session parameters are by default mandatory.  A newly-
   defined SRTP session parameter that is prefixed with the dash
   character ("-") however is considered optional and MAY be ignored.
   If an SDP crypto attribute is received with an unknown session
   parameter that is not prefixed with a "-" character, that crypto
   attribute MUST be considered invalid.

6.4  SRTP Crypto Context Initialization

   In addition to the various SRTP parameters defined above, there are
   three pieces of information that are critical to the operation of the
   default SRTP ciphers:

   * SSRC:     Synchronization source
   * ROC:      Roll-over counter for a given SSRC
   * SEQ:      Sequence number for a given SSRC
   In a unicast session, as defined here, there are three constraints on
   these values.

   The first constraint is on the SSRC, which makes an SRTP keystream be
   unique from other participants.  As explained in SRTP, the keystream
   MUST NOT be reused on two or more different pieces of plaintext.
   Keystream reuse makes the ciphertext vulnerable to cryptanalysis.
   One vulnerability is that known-plaintext fields in one stream can
   expose portions of the reused keystream and this could further expose
   more plaintext in other streams.  Since all current SRTP encryption
   transforms use keystreams, key sharing is a general problem [srtp].
   SRTP mitigates this problem by including the SSRC of the sender in
   the keystream.  But SRTP does not solve this problem in its entirety
   because Real-time Transport Protocol has SSRC collisions, which are
   very rare [rtp] but quite possible.  During a collision, two or more
   SSRCs that share a master key will have identical keystreams for
   overlapping portions of the RTP sequence-number space.  SRTP Security
   Descriptions avoids keystream reuse by making unique master keys
   REQUIRED for the sender and receiver of the security description.
   Thus, the first constraint is satisfied.

     Also note, that there is a second problem with SSRC collisions: The
     SSRC is used to identify the crypto context and thereby the cipher,
     key, ROC, etc. to process incoming packets.  In case of SSRC
     collisions, crypto context identification becomes ambiguous and
     correct packet processing may not occur.  Furthermore, if an RTCP
     BYE packet is to be sent for a colliding SSRC, that packet may also
     have to be secured.  In a (unicast) point-to-multipoint scenario,
     this can be problematic for the same reasons, i.e., it is not known
     which of the possible crypto contexts to use.  Note that these
     problems are not unique to the SDP security descriptions; any use
     of SRTP needs to consider them.

   The second constraint is that the ROC MUST be zero at the time that
   each SSRC commences sending packets.  Thus, there is no concept of a
   "late joiner" in SRTP security descriptions, which are constrained to
   be unicast and pairwise.  The ROC and SEQ form a "packet index" in
   the default SRTP transforms and the ROC is consistently set to zero
   at session commencement, according to this document.

   The third constraint is that the initial value of SEQ SHOULD be
   chosen to be within the range of 0..2^15-1; this avoids an ambiguity
   when packets are lost at the start of the session.  If at the start
   of a session, an SSRC source might randomly select a high sequence-
   number value and put the receiver in an ambiguous situation:  If
   initial packets are lost in transit up to the point that the sequence
   number wraps (i.e., exceeds 2^16-1), then the receiver might not
   recognize that its ROC needs to be incremented.  By restricting the
   initial SEQ to the range of 0..2^15-1, SRTP packet-index
   determination will find the correct ROC value, unless all of the
   first 2^15 packets are lost (which seems, if not impossible, then
   rather unlikely).  See Section 3.3.1 of the SRTP specification
   regarding packet-index determination [srtp].

6.4.1  Late Binding of one or more SSRCs to a crypto context

   The packet index, therefore, depends on the SSRC, the SEQ of an
   incoming packet and the ROC, which is an SRTP crypto context
   variable.  Thus, SRTP has a big security dependency on SSRC

   Given the above constraints, unicast SRTP crypto contexts can be
   established without the need to negotiate SSRC values in the SRTP
   security descriptions.  Instead, an approach called "late binding" is
   RECOMMENDED by this specification.  When a packet arrives, the SSRC
   that is contained in it can be bound to the crypto context at the
   time of session commencement (i.e., SRTP packet arrival) rather than
   at the time of session signaling (i.e., receipt of an SDP).  With the
   arrival of the packet containing the SSRC, all the data items needed
   for the SRTP crypto context are held by the receiver (note that the
   ROC value by definition is zero; if non-zero values were to be
   supported, additional signaling would be required).  In other words,
   the crypto context for a secure RTP session using late binding is
   initially identified by the SDP as:

          <*, address, port>

   where '*' is a wildcard SSRC, "address" is the local receive address
   from the "c=" line, and "port" is the local receive port from the
   "m=" line.  When the first packet arrives with ssrcX in its SSRC
   field, the crypto context

          <ssrcX, address, port>

   is instantiated subject to the following constraints:

   * Media packets are authenticated:  Authentication MUST succeed;
     otherwise, the crypto context is not instantiated.

   * Media packets are not authenticated:  Crypto context is
     automatically instantiated.

   It should be noted, that use of late binding when there is no
   authentication of the SRTP media packets is subject to numerous
   security attacks and consequently it is NOT RECOMMENDED (of course,
   this can be said for unauthenticated SRTP in general).

    Note that use of late binding without authentication will result in
    local state being created as a result of receiving a packet from
    any unknown SSRC.  UNAUTHENTICATED_SRTP, therefore is NOT
    RECOMMENDED because it invites easy denial-of-service attack.  In
    contrast, late binding with authentication does not suffer from
    this weakness.

6.4.2  Sharing cryptographic contexts among Sessions or SSRCs

   With the constraints and procedures described above, it is not
   necessary to explicitly signal the SSRC, ROC and SEQ for a unicast
   RTP session.  So there are no a=crypto parameters for signaling SSRC,
   ROC or SEQ.  Thus, multiple SSRCs from the same entity will share
   a=crypto parameters when late binding is used. Multiple SSRCs from
   the same entity arises due to either multiple sources (microphones,
   cameras, etc.), or RTP payloads requiring SSRC multiplexing within
   that same session.  SDP also allows multiple RTP sessions to be
   defined in the same media description ("m="), these RTP sessions will
   also share the a=crypto parameters.  An application that uses
   a=crypto in this way serially shares a master key among RTP sessions
   or SSRCs and MUST replace the master key when the aggregate number of
   packets among all SSRCs approaches 2^31 packets.  SSRCs that share a
   master key MUST be unique from one another.

6.5  Removal of Crypto Contexts

   The mechanism defined above addresses the issue of creating crypto
   contexts, however in practice, session participants may want to
   remove crypto contexts prior to session termination.  Since a crypto
   context contains information that can not automatically be recovered
   (e.g., ROC), it is important that the sender and receiver agree on
   when a crypto context can be removed, and perhaps more importantly
   when it cannot.

    Even when late binding is used for a unicast stream, the ROC is
    lost and cannot be recovered automatically (unless it is zero) once
    the crypto context is removed.

   We resolve this problem as follows.  When SRTP security descriptions
   are being used, crypto-context removal MUST follow the same rules as
   SSRC removal from the member table [RFC 3550]; note that this can
   happen as the result of an SRTCP BYE packet or a simple time-out due
   to inactivity.  Inactive session participants that wish to ensure
   their crypto contexts are not timed out MUST thus send SRTCP packets
   at regular intervals.

7  SRTP-Specific Use of the crypto Attribute

   Section 5 describes general use of the crypto attribute, and this
   section completes it by describing SRTP-specific use.

7.1  Use with Offer/Answer

   In this section, we describe how the SRTP security descriptions are
   used with the offer/answer model to negotiate cryptographic
   capabilities and communicate SRTP master keys.  The rules defined
   below complement the general offer/answer rules defined in Section
   5.1, which MUST be followed, unless otherwise specified.  Note that
   the rules below define unicast operation only; support for multicast
   and multipoint unicast streams is for further study.

7.1.1  Generating the Initial Offer - Unicast Streams

   When the initial offer is generated, the offerer MUST follow the
   steps in Section 5.1.1 as well as the following steps.

   For each unicast media line (m=) using the secure RTP transport
   where the offerer wants to specify cryptographic parameters, the
   offerer MUST provide at least one valid SRTP security description
   ("a=crypto" line), as defined in Section 6.  If the media stream
   includes Forward Error Correction with a different IP-address and/or
   port than the media stream itself, a FEC_KEY parameter MUST be
   included, as described in Section 6.3.5.

   The offerer MAY include one or more other SRTP session parameters as
   defined in Section 6.3.  Note however, that if any SRTP session
   parameters are included that are not known to the answerer, but are
   nonetheless mandatory (see Section 6.3.6), the negotiation will fail
   if the answerer does not support them.

7.1.2  Generating the Initial Answer - Unicast Streams

   When the initial answer is generated, the answerer MUST follow the
   steps in Section 5.1.2 as well as the following steps.

   For each unicast media line which uses the secure RTP transport and
   contains one or more "a=crypto" lines in the offer, the answerer
   MUST either accept one (and only one) of the crypto lines for that
   media stream, or it MUST reject the media stream.  Only "a=crypto"
   lines that are considered valid SRTP security descriptions as
   defined in Section 6 can be accepted.  Furthermore, all parameters
   (crypto-suite, key parameter, and mandatory session parameters) MUST
   be acceptable to the answerer in order for the offered media stream
   to be accepted.  Note that if the media stream includes Forward
   Error Correction with a different IP-address and/or port than the
   media stream itself, a FEC_KEY parameter MUST be included, as
   described in Section 6.3.5.

   When the answerer accepts an SRTP unicast media stream with a crypto
   line, the answerer MUST include one or more master keys appropriate
   for the selected crypto algorithm; the master key(s) included in the
   answer MUST be different from those in the offer.

    When the master key(s) are not shared between the offerer and
    answerer, SSRC collisions between the offerer and answerer will not
    lead to keystream reuse, and hence SSRC collisions do not
    necessarily have to be prevented.

   If Forward Error Correction to a separate IP-address and/or port is
   included, the answer MUST include a FEC_KEY parameter, as described
   in Section 6.3.5.

   Declarative session parameters may be added to the answer as usual,
   however the answerer SHOULD NOT add any mandatory session parameter
   (see Section 6.3.6) that might be unknown to the offerer.

   If the answerer cannot find any valid crypto line that it supports,
   or if its configured policy prohibits any cryptographic key
   parameter (e.g., key length) or cryptographic session parameter
   (e.g., KDR, FEC_ORDER), it MUST reject the media stream, unless it
   is able to successfully negotiate use of SRTP by other means outside
   the scope of this document (e.g., by use of MIKEY [mikey]).

7.1.3  Processing of the Initial Answer - Unicast Streams

   When the offerer receives the answer, it MUST perform the steps in
   Section 5.1.3 as well as the following steps for each SRTP media
   stream it offered with one or more crypto lines in it.

   If the media stream was accepted and it contains a crypto line, it
   MUST be checked that the crypto line is valid according to the
   constraints specified in Section 6 (including any FEC constraints).

   If the offerer either does not support or is not willing to honor
   one or more of the SRTP parameters in the answer, the offerer MUST
   consider the crypto line invalid.

   If the crypto line is not valid, or the offerer's configured policy
   prohibits any cryptographic key parameter (e.g. key length) or
   cryptographic session parameter, the SRTP security negotiation MUST
   be deemed to have failed.

7.1.4  Modifying the Session

   When a media stream using the SRTP security descriptions has been
   established, and a new offer/answer exchange is performed, the
   offerer and answerer MUST follow the steps in Section 5.1.4 as well
   as the following steps.

   When modifying the session, all negotiated aspects of the SRTP media
   stream can be modified. For example, a new crypto suite can be used
   or a new master key can be established.  As described in RFC 3264,
   when doing a new offer/answer exchange there will be a window of
   time, where the offerer and the answerer must be prepared to receive
   media according to both the old and the new offer/answer exchange.

   This requirement applies here as well, however the following should
   be noted:

   * When authentication is not being used, it may not be possible for
    either the offerer or the answerer to determine if a given packet
    is encrypted according to the old or new offer/answer exchange.
    RFC 3264 defines a couple of techniques to address this problem,
    e.g., changing the payload types used and/or the transport
    addresses.  Note however that a change in transport addresses may
    have an impact on Quality of Service as well as firewall and NAT
    traversal.  The SRTP security descriptions use the MKI to deal with
    this (which adds a few bytes to each SRTP packet) as described in
    Section 6.1.  For further details on the MKI, please refer to

     *    If the answerer changes its master key, the offerer will not
     be able to process packets secured via this master key until the
     answer is received.  This could be addressed by using a security
     "precondition" [sprecon].

   If the offerer includes an IP address and/or port that differs from
   that used previously for a media stream (or FEC stream), the offerer
   MUST include a new master key with the offer (and in so doing, it
   will be creating a new crypto context with the ROC set to zero).
   Similarly, if the answerer includes an IP address and/or port that
   differs from that used previously for a media stream (or FEC
   stream), the answerer MUST include a new master key with the offer
   (and hence create a new crypto context with the ROC set to zero).
   The reason for this is, that when the answerer receives an offer, or
   the offerer receives an answer, with an updated IP address and/or
   port, it is not possible to determine if the other side has access
   to the old crypto context parameters (and in particular the ROC) or
   not.  For example, if one side is a decomposed gateway, or a back-
   to-back user agent is involved, it is possible that the media
   endpoint changed and no longer has access to the old crypto context.
   By always requiring a new master key in this case, the
   answerer/offerer will know that the ROC is zero for this
   offer/answer, and any key lifetime constraints will trivially be
   satisfied too.  Another consideration here applies to media relays;
   if the relay changes the media endpoint on one side transparently to
   the other side, the relay cannot operate as a simple packet
   reflector but will have to actively engage in SRTP packet processing
   and transformation (i.e., decryption and re-encryption, etc.).

   Finally note, that if the new offer is rejected, the old crypto
   parameters remain in place.

7.1.5  Offer/Answer Example

   In this example, the offerer supports two crypto suites (f8 and AES).
   The a=crypto line is actually one long line, although it is shown as
   two lines in this document due to page formatting.  The f8 example
   shows two inline parameters; as explained in Section 6.1, there may
   be one or more key (i.e. inline) parameters in a crypto attribute.
   In this way, multiple keys are offered to support key rotation using
   a master key index (MKI).

   Offerer sends:
     o=sam 2890844526 2890842807 IN IP4
     s=SRTP Discussion
     i=A discussion of Secure RTP
     u= (Marge Simpson)
     c=IN IP4
     t=2873397496 2873404696
     m=audio 49170 RTP/SAVP 0
     a=crypto:1 AES_CM_128_HMAC_SHA1_80
     a=crypto:2 F8_128_HMAC_SHA1_80

   Answerer replies:
     o=jill 25690844 8070842634 IN IP4
     s=SRTP Discussion
     i=A discussion of Secure RTP
     u= (Homer Simpson)
     c=IN IP4
     t=2873397526 2873405696
     m=audio 32640 RTP/SAVP 0
     a=crypto:1 AES_CM_128_HMAC_SHA1_80

   In this case, the session would use the AES_CM_128_HMAC_SHA1_80
   crypto suite for the RTP and RTCP traffic.  If F8_128_HMAC_SHA1_80
   were selected by the answerer, there would be two inline keys
   associated with the SRTP cryptographic context.  One key has an MKI
   value of 1 and the second has an MKI of 2.

7.2  SRTP-Specific Use Outside Offer/Answer

   Use of SRTP security descriptions outside the offer/answer model is
   not defined.

    Use of SRTP security descriptions outside offer/answer could have
    been defined for sendonly media streams, however, there would not
    be a way to indicate the key to use for SRTCP by the receiver of
    said media stream.

7.3  Support for SIP Forking

   As mentioned earlier, the security descriptions defined here do not
   support multicast media streams or multipoint unicast streams.
   However, in the SIP protocol, it is possible to receive several
   answers to a single offer due to the use of forking (see [SIP]).
   Receiving multiple answers leads to a couple of problems for the
   SRTP security descriptions:

   * Different answerers may choose different ciphers, keys, etc.,
     however there is no way for the offerer to associate a particular
     incoming media packet with a particular answer.

   * Two or more answerers may pick the same SSRC and hence the SSRC
     collision problems mentioned earlier may arise.

   As stated earlier, the above point-to-multipoint cases are outside
   the scope of the SDP security descriptions. However, there is a way
   of supporting SIP forking: Change the multipoint scenario resulting
   from SIP forking into multiple two-party unicast cases.  This is
   done as follows:

   For each answer received beyond the initial answer, issue a new
   offer to that particular answerer using a new receive transport
   address (IP address and port); note that this requires support for
   the SIP UPDATE method [RFC 3313].  Also, to ensure that two media
   sessions are not inadvertently established prior to the UPDATE being
   processed by one of them, use security preconditions [sprecon].

   Finally, note that all SIP User Agents that received the offer will
   know the key(s) being proposed by the initial offer.  If the offerer
   wants to ensure security with respect to all other User Agents that
   may have received the offer, a new offer/answer exchange with a new
   key needs to be performed with the answerer as well.  It should be
   noted, that the offerer cannot determine whether a single or
   multiple SIP User Agents received the offer, since intermediate
   forking proxies may only forward a single answer to the offerer.

7.4  SRTP-Specific Backwards Compatibility Considerations

   It is possible that the answerer supports the SRTP transport and
   accepts the offered media stream, yet it does not support the crypto
   attribute defined here.  The offerer can recognize this situation by
   seeing an accepted SRTP media stream in the answer that does not
   include a crypto line.  In that case, the security negotiation
   defined here MUST be deemed to have failed.

   Also, if a media stream with a given SRTP transport (e.g.,
   "RTP/SAVP") is sent to a device that does not support SRTP, that
   media stream will be rejected.

7.5  Operation with KEYMGT= and k= lines

   An offer MAY include both "a=crypto" and "a=keymgt" lines [keymgt].
   Per SDP rules, the answerer will ignore attribute lines that it does
   not understand.  If the answerer supports both "a=crypto" and
   "a=keymgt", the answer MUST include either "a=crypto" or "a=keymgt"
   but not both, as including both is undefined.

   An offer MAY include both "a=crypto" and "k=" lines [SDP].  Per SDP
   rules, the answerer will ignore attribute lines it does not
   understand.  If the answerer supports both "a=crypto" and "k=", the
   answer MUST include either "a=crypto" or "k=" but not both, as
   including both is undefined.

8  Security Considerations

   Like all SDP messages, SDP messages containing security
   descriptions, are conveyed in an encapsulating application protocol
   (e.g., SIP, MGCP, etc.).  It is the responsibility of the
   encapsulating protocol to ensure the protection of the SDP security
   descriptions.  Therefore, IT IS REQUIRED that the application invoke
   its own security mechanisms (e.g., secure multiparts such as S/MIME
   [smime]) or alternatively utilize a lower-layer security service
   (e.g., TLS, or IPsec).  IT IS REQUIRED that this security service
   provide strong message authentication and packet-payload encryption
   as well as effective replay protection.

   "Replay protection" is needed against an attacker that has enough
   access to the communications channel to intercept messages and
   deliver copies to the destination.  A successful replay attack will
   cause the recipient to perform duplicate processing on a message;
   the attack is worse when the duped recipient sends a duplicate reply
   to the initiator.  Replay protections are not found in S/MIME or in
   the other secure-multiparts standard, PGP/MIME.  S/MIME and
   PGP/MIME, therefore need to be augmented with some replay-protection
   mechanism that is appropriate to the encapsulating application
   protocol (e.g. SIP, MGCP, etc.).  Three common ways to provide
   replay protection are to place a sequence number in the message, use
   a timestamp, or for the receiver to keep a hash of the message to
   compare with incoming messages.  There typically needs to be a
   replay "window" and some policy for keeping state information from
   previous messages in a "replay table" or list.

   The discussion which follows uses "message authentication" and
   "message confidentiality" in a consistent manner with SRTP
   [RFC3711].  "Message confidentiality" means that only the holder of
   the secret decryption key can access the plain-text content of the
   message.  The decryption key is the same key as the encryption key
   using SRTP counter mode and f8 encryption transforms, which are
   vulnerable to message tampering and need SRTP message authentication
   to detect such tampering. "Message authentication" and "message
   integrity validation" generally mean the same thing in IETF security
   standards: An SRTP message is authenticated following a successful
   HMAC integrity check [RFC3711], which proves that the message
   originated from the holder of an SRTP master key and was not altered
   en route.  Such an "authentic" message, however, can be captured by
   an attacker and "replayed" when the attacker re-inserts the packet
   into the channel.  A replayed packet can have a variety of bad
   effects on the session, and SRTP uses the extended sequence number
   to detect replayed SRTP packets [RFC3711].

   The SRTP specification identifies which services and features are
   default values that are normative-to-implement (such as
   AES_CM_128_80) versus normative-to-use (such as AES_CM_128_32).

8.1  Authentication of packets

   Security descriptions as defined herein signal security services for
   RTP packets.  RTP messages are vulnerable to a variety of attacks
   such as replay and forging.  To limit these attacks, SRTP message
   integrity mechanisms SHOULD be used (SRTP replay protection is
   always enabled).

8.2  Keystream Reuse

   SRTP security descriptions signal configuration parameters for SRTP
   sessions.  Misconfigured SRTP sessions  are vulnerable to attacks on
   their encryption services when running the crypto suites defined in
   Sections 6.2.1, 6.2.2, and 6.2.3.  An SRTP encryption service is
   "misconfigured" when two or more media streams are encrypted using
   the same keystream of AES blocks.  When senders and receivers share
   derived session keys, SRTP requires that the SSRCs of session
   participants serve to make their corresponding keystreams unique,
   which is violated in the case of SSRC collision: SRTP SSRC collision
   drastically weakens SRTP or SRTCP payload encryption during the time
   that identical keystreams were used [srtp].  An attacker, for
   example, might collect SRTP and SRTCP messages and await a
   collision.  This attack on the AES-CM and AES-f8 encryption is
   avoided entirely when each media stream has its own unique master
   key in both the send and receive direction.  This specification
   restricts use of SDP security description to unicast point-to-point
   streams so that keys are not shared between SRTP hosts, and the
   master keys used in the send and receive direction for a given media
   stream are unique.

8.3  Signaling Authentication and Signaling Encryption

   There is no reason to incur the complexity and computational expense
   of SRTP, however, when its key establishment is exposed to
   unauthorized parties.  In most cases, the SRTP crypto attribute and
   its parameters are vulnerable to denial of service attacks when they
   are carried in an unauthenticated SDP message.  In some cases, the
   integrity or confidentiality of the RTP stream can be compromised.
   For example, if an attacker sets UNENCRYPTED for the SRTP stream in
   an offer, this could result in the answerer not decrypting the
   encrypted SRTP messages.  In the worst case, the answerer might
   itself send unencrypted SRTP and leave its data exposed to snooping.

   Thus, IT IS REQUIRED that MIME secure multiparts, IPsec, TLS, or
   some other data security service be used to provide message
   authentication for the encapsulating protocol that carries the SDP
   messages having a crypto attribute (a=crypto).  Furthermore, IT IS
   REQUIRED that encryption of the encapsulating payload be used
   whenever a master key parameter (inline) appears in the message.
   Failure to encrypt the SDP message containing an inline SRTP master
   key renders the SRTP authentication or encryption service useless in
   practically all circumstances.  Failure to authenticate an SDP
   message that carries SRTP parameters renders the SRTP authentication
   or encryption service useless in most practical applications.

   When the communication path of the SDP message is routed through
   intermediate systems that inspect parts of the SDP message, security
   protocols such as IPsec or TLS SHOULD NOT be used for encrypting
   and/or authenticating the security description.  In the case of
   intermediate-system processing of a message containing SDP security
   descriptions, the "a=crypto" attributes SHOULD be protected end-to-
   end so that the intermediate system can neither modify the security
   description nor access the keying material.  Network or transport
   security protocols that terminate at each intermediate system,
   therefore, SHOULD NOT be used for protecting SDP security
   descriptions.  A security protocol SHOULD allow the security
   descriptions to be encrypted and authenticated end-to-end
   independently of the portions of the SDP message that any
   intermediate system modifies or inspects:  MIME secure multiparts
   are RECOMMENDED for the protection of SDP messages that are
   processed by intermediate systems.

9  Grammar

   In this section we first provide the ABNF grammar for the generic
   crypto attribute, and then we provide the ABNF grammar for the SRTP
   specific use of the crypto attribute.

9.1  Generic "Crypto" Attribute Grammar

   The ABNF grammar for the crypto attribute is defined below:

   "a=crypto:" tag 1*WSP crypto-suite 1*WSP key-params
                                           *(1*WSP session-param)

   tag              = 1*9DIGIT
   crypto-suite     = 1*(ALPHA / DIGIT / "_")

   key-params       = key-param *(";" key-param)
   key-param        = key-method ":" key-info
   key-method       = "inline" / key-method-ext
   key-method-ext   = 1*(ALPHA / DIGIT / "_")
   key-info         = %x21-3A / %x3C-7E ; visible (printing) characters
                                        ; except semi-colon
   session-param    = 1*(VCHAR)         ; visible (printing) characters

   where WSP, ALPHA, DIGIT, and VCHAR are defined in [RFC2234].

9.2  SRTP "Crypto" Attribute Grammar

   This section provides an Augmented BNF [RFC2234] grammar for the
   SRTP-specific use of the SDP crypto attribute:

     crypto-suite        = srtp-crypto-suite
     key-method          = srtp-key-method
     key-info            = srtp-key-info
     session-param       = srtp-session-param

     srtp-crypto-suite   = "AES_CM_128_HMAC_SHA1_32" /
                           "F8_128_HMAC_SHA1_32" /
                           "AES_CM_128_HMAC_SHA1_80" /

     srtp-key-method     = "inline"
     srtp-key-info       = key-salt ["|" lifetime] ["|" mki]

     key-salt            = 1*(base64)   ; binary key and salt values
                                   ; concatenated together, and then
                                   ; base64 encoded [section 6.8 of
                                   ; RFC2046]

     lifetime           = ["2^"] 1*(DIGIT)   ; see section 6.1 for "2^"
     mki                 = mki-value ":" mki-length
     mki-value           = 1*DIGIT
     mki-length          = 1*3DIGIT   ; range 1..128.

     srtp-session-param  = kdr /
                           "UNENCRYPTED_SRTP" /
                           "UNENCRYPTED_SRTCP" /
                           "UNAUTHENTICATED_SRTP" /
                           fec-order /
                           fec-key /
                           wsh /

     kdr                 = "KDR=" 1*2(DIGIT)  ; range 0..24,
                                              ; power of two

     fec-order           = "FEC_ORDER=" fec-type
     fec-type            = "FEC_SRTP" / "SRTP_FEC"
     fec-key             = "FEC_KEY=" key-params

     wsh                 = "WSH=" 2*DIGIT    ; minimum value is 64
     base64              =  ALPHA / DIGIT / "+" / "/" / "="

     srtp-crypto-suite-ext  = 1*(ALPHA / DIGIT / "_")
     srtp-session-extension = ["-"] 1*(VCHAR)  ;visible chars [RFC2234]
                              ; first character must not be dash ("-")

10 IANA Considerations

10.1 Registration of the "crypto" attribute

   The IANA is hereby requested to register a new SDP attribute as

   Attribute name:      crypto
   Long form name:      Security description cryptographic attribute
                        for media streams
   Type of attribute:   Media-level
   Subject to charset:  No
   Purpose:             Security descriptions
   Appropriate values:  See Section 4

10.2 New IANA Registries and Registration Procedures

   The following sub-sections define a new IANA registry with associated
   sub-registries to be used for the SDP security descriptions.  The
   IANA is hereby requested to create an SDP Security Description
   registry as shown below and further described in the following

   SDP Security Descriptions
     +- Key Methods (described in 10.2.1)
     +- Media Stream Transports (described in 10.2.2)
          +- Transport1 (e.g. SRTP)
          |    |
          |    +- Supported Key Methods (e.g. inline)
          |    |
          |    +- crypto suites
          |    |
          |    +- session parameters
          +- Transport2
          :    :

10.2.1 Key Method Registry and Registration

   The IANA is hereby requested to create a new subregistry for SDP
   security description key methods.  An IANA key method registration
   MUST be documented in an RFC in accordance with the RFC 2434
   Standards Action and it MUST provide the name of the key method in
   accordance with the grammar for key-method-ext defined in Section

10.2.2 Media Stream Transport Registry and Registration

   The IANA is hereby requested to create a new subregistry for SDP
   security description Media Stream Transports.  An IANA media stream
   transport registration MUST be documented in an RFC in accordance
   with the RFC 2434 Standards Action and the procedures defined in
   Section 4 and 5 of this document.  The registration MUST provide the
   name of the transport and a list of supported key methods.

   In addition, each new media stream transport registry must contain a
   crypto-suite registry and a session parameter registry as well as
   IANA instructions for how to populate these registries.

10.3 Initial Registrations

10.3.1 Key Method

   The following security descriptions key methods are hereby


10.3.2 SRTP Media Stream Transport

   The IANA is hereby requested to create an SDP Security Description
   Media Stream Transport subregistry for "SRTP".  The key methods
   supported is "inline".  The reference for the SDP security
   description for SRTP is this document.  SRTP Crypto Suite Registry and Registration

   The IANA is hereby requested to create a new subregistry for SRTP
   crypto suites under the SRTP transport of the SDP Security
   Descriptions. An IANA SRTP crypto suite registration MUST indicate
   the crypto suite name in accordance with the grammar for srtp-
   crypto-suite-ext defined in Section 9.2.

   The semantics of the SRTP crypto suite MUST be described in an RFC
   in accordance with the RFC 2434 Standards Action, including the
   semantics of the "inline" key-method and any special semantics of

   The following SRTP crypto suites are hereby registered:


   The reference for these crypto-suites is provided in this document.  SRTP Session Parameter Registration

   The IANA is hereby requested to create a new subregistry for SRTP
   session parameters under the SRTP transport of the SDP Security
   Descriptions.  An IANA SRTP session parameter registration MUST
   indicate the session parameter name (srtp-session-extension as
   defined in Section 9.2); the name MUST NOT begin with the dash
   character ("-").

   The semantics of the parameter MUST be described in an RFC in
   accordance with the RFC 2434 Standards Action.  If values can be
   assigned to the parameter, then the format and possible values that
   can be assigned MUST be described in the RFC in accordance with the
   RFC 2434 Standards Action as well.  Also, it MUST be specified
   whether the parameter is declarative or negotiated in the
   offer/answer model.

   The following SRTP session parameters are hereby registered:


   The reference for these parameters is this document.

11 Acknowledgements

   This document is a product of the IETF MMUSIC working group and has
   benefited from comments from its participants.  This document also
   benefited from discussions with Elisabetta Cararra, Earl Carter,
   Bill Foster, Matt Hammer, Cullen Jennings, Paul Kyzivat, David
   McGrew, Mats Naslund, Dave Oran, Jonathan Rosenberg, Dave Singer,
   Mike Thomas, Brian Weis, and Magnus Westerlund.

12 Authors' Addresses

   Flemming Andreasen
   Cisco Systems, Inc.
   499 Thornall Street, 8th Floor
   Edison, New Jersey  08837 USA

   Mark Baugher
   5510 SW Orchid Street
   Portland, Oregon  97219 USA

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134  USA

13 Normative References

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

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

   [SDP] M. Handley, V. Jacobson, "SDP: Session Description Protocol",
   RFC 2327, April 1998.

   [RFC2828] R. Shirey, "Internet Security Glossary", RFC 2828, May

   [RFC3264] J. Rosenberg, H. Schulzrinne, "An Offer/Answer Model with
   the Session Description Protocol (SDP)", RFC 3264, June 2202,

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

   [RFC1750] D. Eastlake 3rd, S. Crocker, J. Schiller, "Randomness
   Recommendations for Security", RFC 1750, December 1994,

   [RFC3548] S. Josefsson, "The Base16, Base32, and Base64 Data
   Encodings", RFC 3548, July 2003.

   [RFC2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
   Considerations Section in RFCs", RFC 2434, October 1998.

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

14 Informative References

   [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
   Capability Declaration", RFC 3407, October 2002,

   [Bellovin] Steven M. Bellovin, "Problem Areas for the IP Security
   Protocols," in Proceedings of the Sixth Usenix Unix Security
   Symposium, pp. 1-16, San Jose, CA, July 1996.

   [GDOI] M. Baugher, B. Weis, T. Hardjono, H. Harney, "The Group
   Domain of Interpretation", RFC 3547, July 2003,

   [kink] M. Thomas, J. Vilhuber, "Kerberized Internet Negotiation of
   Keys (KINK)", Work in Progress.

   [ike] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)", RFC
   2409, November 1998,

   [ipsec] Kent, S. and R. Atkinson, "Security Architecture for the
   Internet Protocol", RFC 2401, November 1998,

   [maxprate] Westerlund, M., "A Transport-independent Bandwidth
   Modifier for the Session Description Protocol (SDP)," April 2004,

   [RFC2733] J. Rosenberg, H. Schulzrinne, "An RTP Payload Format for
   Generic Forward Error Correction", RFC 2733, December 1999,

   [s/mime] Ramsdell B., "S/MIME Version 3 Message Specification", RFC
   2633, June 1999,

   [pgp/mime] M. Elkins, "MIME Security with Pretty Good Privacy
   (PGP)", RFC 2015, October 1996,

   [tls] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
   2246, January 1999,

   [keymgt] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
   "Key Management Extensions for SDP and RTSP", Work in Progress.

   [mikey] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
   "MIKEY: Multimedia Internet KEYing", Work in Progress.

   [RFC2045] N. Freed, N. Borenstein, "Multipurpose Internet Mail
   Extensions (MIME) Part One: Format of Internet Message Bodies", RFC
   2045, November 1996,

   [RFC2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
   for Message Authentication", RFC 2014, November 1997,

   [skeme] H. Krawczyk, "SKEME: A Versatile Secure Key Exchange
   Mechanism for the Internet", ISOC Secure Networks and Distributed
   Systems Symposium, San Diego, 1996.

   [RFC3312] G. Camarillo, W. Marshall, J. Rosenberg, "Integration of
   Resource Management and Session Initiation Protocol (SIP)", RFC
   3312, October 2002,

   [RFC2974] M. Handley, C. Perkins, E. Whelan, "Session Announcement
   Protocol", RFC 2974, October 2000,

   [srtpf] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
   RTCP-based Feedback (RTP/SAVPF)", work in progress, October 2003.

   [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
   A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
   Session Initiation Protocol", RFC 3261, June 2002.

15 Full Copyright Statement

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