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Signaling Additional SRTP Context information via SDP

Document Type Active Internet-Draft (individual)
Authors Kyzer R. Davis , Esteban Valverde , Gonzalo Salgueiro
Last updated 2024-01-10
Replaces draft-davis-valverde-srtp-assurance
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mmusic                                                       K. R. Davis
Internet-Draft                                               E. Valverde
Updates: 4568 (if approved)                                 G. Salgueiro
Intended status: Standards Track                           Cisco Systems
Expires: 13 July 2024                                    10 January 2024

         Signaling Additional SRTP Context information via SDP


   This document specifies additional cryptographic attributes for
   signaling additional Secure Real-time Transport Protocol (SRTP)
   cryptographic context information via the Session Description
   Protocol (SDP) in alongside those defined by RFC4568.

   The SDP extension defined in this document address situations where
   the receiver needs to quickly and robustly synchronize with a given
   sender.  The mechanism also enhances SRTP operation in cases where
   there is a risk of losing sender-receiver synchronization.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 13 July 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Discussion Venues . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Changelog . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Problem Statement . . . . . . . . . . . . . . . . . . . .   3
     1.4.  Previous Solutions  . . . . . . . . . . . . . . . . . . .   7
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   8
   3.  Protocol Design . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Generic SRTP Context Syntax . . . . . . . . . . . . . . .   9
     3.2.  SSRC, ROC, SEQ Syntax . . . . . . . . . . . . . . . . . .  12
     3.3.  Pairing SRTP Context Attributes to SDP Security
           Attributes  . . . . . . . . . . . . . . . . . . . . . . .  13
     3.4.  Handling Unknown Cryptographic Attributes . . . . . . . .  13
     3.5.  SRTP Multiplexing . . . . . . . . . . . . . . . . . . . .  14
     3.6.  SDP Bundling  . . . . . . . . . . . . . . . . . . . . . .  15
     3.7.  SDP Considerations  . . . . . . . . . . . . . . . . . . .  15
       3.7.1.  Sender Behavior . . . . . . . . . . . . . . . . . . .  15
       3.7.2.  Receiver Behavior . . . . . . . . . . . . . . . . . .  15
       3.7.3.  Update Frequency  . . . . . . . . . . . . . . . . . .  15
     3.8.  Future Extendability  . . . . . . . . . . . . . . . . . .  16
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Appendix A.  Protocol Design Overview . . . . . . . . . . . . . .  22
     A.1.  Why not an RTP Header Extension?  . . . . . . . . . . . .  22
     A.2.  Why not an SDP Security Session Parameter?  . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

1.1.  Discussion Venues

   This section is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at

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1.2.  Changelog

   This section is to be removed before publishing as an RFC.


   *  Consolidate references to late binding, fix nits #20


   *  Better define that the tags must match #16
   *  Revise ANBF #15
   *  Handling and Signaling Unknown Values #18
   *  Cite default behavior of underlying RFCs if value is unknown/
      omitted #17


   *  Change contact name from IESG to IETF in IANA Considerations #2
   *  Discuss RFC4568 "Late Joiner" in problem statement: #3
   *  Split Serial forking scenario into its own section #4
   *  Add MIKEY considerations to Protocol Design section #6
   *  Change doc title #7
   *  Add SEQ abbreviation earlier #8
   *  Discuss why this can't be a RTP Header Extension #11
   *  Add Appendix further discussing why SDP Security Session
      Parameters extension not used #5
   *  Method to Convey Multiple SSRCs for a given stream #1
   *  Discuss why SEQ is signaled in the SDP #9

1.3.  Problem Statement

   While [RFC4568] provides most of the information required to
   instantiate an SRTP cryptographic context for RTP Packets, the state
   of a few crucial items in the SRTP cryptographic context are missing.
   One such item is the Rollover Counter (ROC) defined by Section 3.2.1
   [RFC3711] which is not signaled in any packet across the wire and
   shared between applications.

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   The ROC is one item that is used to create the SRTP Packet Index
   along with the the [RFC3550] transmitted sequence numbers (SEQ) for a
   given synchronization sources (SSRC).  The Packet index is integral
   to the encryption, decryption and authentication process of SRTP key
   streams.  Failure to synchronize the value properly at any point in
   the SRTP media exchange leads to encryption or decryption failures,
   degraded user experience and at cross-vendor interoperability issues
   with many hours of engineering time spent debugging a value that is
   never negotiated on the wire (and oftentimes not even logged in
   application logs.)

   The current method of ROC handling is to instantiate a new media
   stream's cryptographic context at 0 as per Section 3.3.1 of
   [RFC3711].  Then track the state ROC for a given cryptographic
   context as the time continues on and the stream progresses.

   [RFC4568], states 'there is no concept of a "late joiner" in SRTP
   security descriptions' as the main reason for not conveying the ROC,
   SSRC, or SEQ via the key management protocol but as one will see
   below; this argument is not true in practice.

   When joining ongoing streams, resuming held/transferred streams, or
   devices without embedded application logic for clustering/high
   availability where a given cryptographic context is resumed; without
   any explicit signaling about the ROC state, devices must make an
   educated guess as defined by Section 3.3.1 of [RFC3711].  The method
   specially estimates the received ROC by calculating ROC-1, ROC, ROC+1
   to see which performs a successful decrypt.  While this may work on
   paper, this process usually only done at the initial instantiation of
   a cryptographic context rather than at later points later during the
   session.  Instead many applications take the easy route and set the
   value at 0 as if this is a new stream.  While technically true from
   that receivers perspective, the sender of this stream may be
   encrypting packets with a ROC greater than 0.  Further this does not
   cover scenarios where the ROC is greater than +1.

   Where possible the ROC state (and the rest of the cryptographic
   context) is usually synced across clustered devices or high
   availability pairs via proprietary methods rather than open

   These problems detailed technically above lead to a few very common
   scenarios where the ROC may become out of sync.  These are are
   briefly detailed below with the focus on the ROC Value.

   Joining an ongoing session:

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   *  When a receiver joins an ongoing session, such as a broadcast
      conference, there is no signaling method which can quickly allow
      the new participant to know the state of the ROC assuming the
      state of the stream is shared across all participants.

   Hold/Resume, Transfer Scenarios:

   *  A session is created between sender A and receiver B.  ROC is
      instantiated at 0 normally and continues as expected.
   *  At some point the receiver is put on hold while the sender is
      connected to some other location such as music on hold or another
      party altogether.
   *  At some future point the receiver is reconnected to the sender and
      the original session is resumed.
   *  The sender may re-assume the original cryptographic context rather
      rather than create one net new.
   *  Here if the sender starts the stream from the last observed
      sequence number the receiver observed the ROC will be in sync.
   *  However there are scenarios where the sender may have been
      transmitting packets on the previous cryptographic context and if
      a ROC increment occurred; the receiver would never know.  This can
      lead to problems when the streams are reconnected as the ROC is
      now out of sync between both parties.
   *  Further, a sender may be transferred to some upstream device
      transparently to them.  If the sender does not reset their
      cryptographic context that new receiver will now be out of sync
      with possible ROC values.

   Serial Forking Case:

   *  [RFC4568] itself cites a problematic scenario in their own
      Appendix A, Scenario B, Problem 3 where a ROC out of sync scenario
      could occur.
   *  The proposed solution for problem 3 involves a method to convey
      the ROC however known the problem; the authors still did not
      include this in the base SDP Security specification.

   Application Failover (without stateful syncs):

   *  In this scenario a cryptographic context was was created with
      Device A and B of a high availability pair.
   *  An SRTP stream was created and ROC of 0 was created and media
      streamed from the source towards Device A.
   *  Time continues and the sequence wraps from 65535 to 0 and the ROC
      is incremented to 1.
   *  Both the sender and device A are tracking this locally and the
      encrypt/decrypt process proceeds normally.

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   *  Unfortunate network conditions arise and Device B must assume
      sessions of Device A transparently.
   *  Without any proprietary syncing logic between Device A and B which
      disclose the state of the ROC, Device B will likely instantiate
      the ROC at 0.
   *  Alternatively Device B may try to renegotiate the stream over the
      desired signaling protocol however this does not ensure the remote
      sender will change their cryptographic context and reset the ROC
      to 0.
   *  The transparent nature of the upstream failover means the local
      application will likely proceed using ROC 1 while upstream
      receiver has no method of knowing ROC 1 is the current value.

   Secure SIPREC Recording:

   *  If a SIPREC recorder is brought into recording an ongoing session
      through some form of transfer or on-demand recording solution the
      ROC may have incremented.
   *  Without an SDP mechanism to share this information the SIPREC will
      be unaware of the full SRTP context required to ensure proper
      decrypt of media streams being monitored.

   Improper SRTP context resets:

   *  As defined by Section 3.3.1 of [RFC3711] an SRTP re-key MUST NOT
      reset the ROC within SRTP Cryptographic context.
   *  However, some applications may incorrectly use the re-key event as
      a trigger to reset the ROC leading to out-of-sync encrypt/decrypt

   Out of Sync Sliding Windows / Sequence Numbers:

   *  There is corner case situation where two devices communicating via
      a Back to Back User Agent (B2BUA) which is performing RTP-SRTP
   *  In this scenario the B2BUA is also a session border controller
      (SBC) tasked with topology abstraction.  That is, the signaling
      itself is abstracted from both parties.
   *  In this scenario a hold/resume where a sequence rolls can not only
      cause problems with the ROC; but can also cause sliding window
   *  To be more specific, assume that both parties did have access to
      the cryptographic context and resumed the old ROC value after the
      hold thus ROC is not out of sync.
   *  What should the sliding window and sequence be set to in this

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   *  The post-hold call could in theory have a problem where the
      sequence number of received packets is lower than what was
      originally observed before the hold.
   *  Thus the sliding window would drop packets until the sequence
      number gets back to the last known sequence and the sliding window
   *  Advertising the Sequence in some capacity to reinitialize the
      sliding window (along with advertising the ROC) can ensure a
      remote application can properly re-instantiate the cryptographic
      context in this scenario.

   This is a problem that other SRTP Key Management protocols (MIKEY,
   DTLS-SRTP, EKT-SRTP) have solved but SDP Security has lagged behind
   in solution parity.  For a quick comparison of all SRTP Key
   Management negotiations refer to [RFC7201] and [RFC5479].

1.4.  Previous Solutions

   As per RFC3711, "Receivers joining an on-going session MUST be given
   the current ROC value using out-of-band signaling such as key-
   management signaling."  [RFC4771] aimed to solve the problem however
   this solution has a few technical shortcomings detailed below.

   First, this specifies the use of Multimedia Internet KEYing (MIKEY)
   defined by [RFC3830] as the out-of-band signaling method.  A proper
   MIKEY implementation requires more overhead than is needed to convey
   and solve this problem.  By selecting MIKEY as the out-of-band
   signaling method the authors may have inadvertently inhibited
   significant adoption by the industry.

   Second, [RFC4771] also transforms the SRTP Packet to include the four
   byte value after the encrypted payload and before an optional
   authentication tag.  This data about the SRTP context is unencrypted
   on the wire and not covered by newer SRTP encryption protocols such
   as [RFC6904] and [RFC9335].  Furthermore this makes the approach
   incompatible with AEAD SRTP Cipher Suites which state that trimming/
   truncating the authentication tag weakens the security of the
   protocol in Section 13.2 of [RFC7714].

   Third, this is not in line with the standard method of RTP Packet
   modifications.  The proposal would have benefited greatly from being
   an RTP Header Extension rather than a value appended after payload.
   But even an RTP header extension proves problematic in where modern
   SRTP encryption such as Cryptex defined by [RFC9335] are applied.
   That is, the ROC is a required input to decrypt the RTP packet
   contents.  It does not make sense to convey this data as an RTP
   Header Extension obfuscated by the very encryption it is required to

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   Lastly, there is no defined method for applications defined for
   applications to advertise the usage of this protocol via any
   signaling methods.

   [RFC5159] also defined some SDP attributes namely the
   "a=SRTPROCTxRate" attribute however this does not cover other
   important values in the SRTP Cryptographic context and has not seen
   widespread implementation.

   [RFC8870] solves the problem for DTLS-SRTP [RFC5763]/[RFC5764]

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Protocol Design

   A few points of note are below about this specifications relationship
   to other SRTP Key Management protocols or SRTP protocols as to leave
   no ambiguity.

   Session Description Protocol (SDP) Security Descriptions for Media
      The authors have chosen to avoid modifying RFC4568 a=crypto offers
      as to avoid backwards compatibility issues with a non-versioned
      protocol.  Instead this specification adds to what is defined in
      SDP Security Framework [RFC4568] by allowing applications to
      explicitly negotiate additional items from the cryptographic
      context such as the packet index ingredients: ROC, SSRC and
      Sequence Number via a new SDP Attribute.  By coupling this
      information with the applicable "a=crypto" offers; a receiving
      application can properly instantiate an SRTP cryptographic context
      at the start of a session, later in a session, after session
      modification or when joining an ongoing session.

   Key Management Extensions for Session Description Protocol (SDP)
   and Real Time Streaming Protocol (RTSP):
      This specifications makes no attempt to be compatible with the Key
      Management Extension for SDP "a=key-mgmt" defined by [RFC4567]

   ZRTP: Media Path Key Agreement for Unicast Secure RTP:
      This specifications makes no attempt to be compatible with the Key
      Management via SDP for ZRTP "a=zrtp-hash" defined by [RFC6189].

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      This specifications makes no attempt to be compatible with the
      SRTP Key Management via MIKEY [RFC3830].

   DTLS-SRTP, EKT-SRTP, Privacy Enhanced Conferencing items (PERC):
      All DTLS-SRTP items including Privacy Enhanced Conferencing items
      (PERC) ([RFC8723] and [RFC8871]) are out of scope for the purposes
      of this specification.

   Secure Real Time Control Protocol (SRTCP):
      This specification is not required by SRTCP since the packet index
      is carried within the SRTCP packet and does not need an out-of-
      band equivalent.

   Source-Specific Media Attributes in the Session Description
   Protocol (SDP):
      The authors of this specification vetted [RFC5576] SSRC Attribute
      "a=ssrc" but felt that it would require too much modification and
      additions to the SSRC Attribute specification to allow unknown
      SSRC values and the other information which needs to be conveyed.
      Further, requiring implementation of the core SSRC Attribute RFC
      could pose as a barrier entry and separating the two into
      different SDP Attributes is the better option.  An implementation
      SHOULD NOT send RFC5576 SSRC Attributes alongside SRTP Context
      SSRC Attributes.  If both are present in SDP, a receiver SHOULD
      utilize prioritize the SRTP Context Attributes over SSRC
      Attributes since these attributes will provide better SRTP
      cryptographic context initialization.

   Completely Encrypting RTP Header Extensions and Contributing
      SRTP Context is compatible with [RFC9335] "a=cryptex" media and
      session level attribute.

3.1.  Generic SRTP Context Syntax

   This specification introduces a new SRTP Context Attribute defined as

   The SRTP Context syntax utilizes standard attribute key-value pairs
   to convey data.  The implementation's goal is extendable allowing for
   additional vendor specific key-value pairs alongside the ones defined
   in this document or room for future specifications to add additional
   key-value pairs.

   The SRTP context can convey one or more key-value pair lists as per
   the following rules:

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   *  Multiple key-value pairs are separated by semicolons to create a
      single list.
   *  Individual key names MUST be unique within a given list.
   *  Two or more lists of separate key-value pair groupings can be
      conveyed by wrapping a list in parenthesis and separating them
      with a comma.
   *  This method of parenthesis grouping MUST NOT be used when there is
      a single list of key-value pairs (with unique key names.)
   *  Multiple of the same key name MAY exist within different key-value
      list groupings.
   *  Further key-value list groupings may contain more or less keys-
      value pairs than other groupings.
   *  A given Key's value does not need to be unique within a given list
      or across list groupings.
   *  The final list member of a given single key-value list or key-
      value list grouping MUST NOT feature a trailing semicolon or

   The first line of Figure 1 details a single list, without
   parenthesis, which conveys two unique key-value pairs.  The second
   line of Figure 1 details a list of multiple key-value pair groupings
   where the key named "key1" exists in two lists, keys name "key2" and
   "key3" are unique to their list grouping and finally a grouping only
   contains a single key-value pair named "key4".

   Note that long lines in this document have been broken into multiple
   lines using the "The Single Backslash Strategy" defined by [RFC8792].

   a=srtpctx:1 key1=value1;key2=value2
   a=srtpctx:1 (key1=value1;key2=value2),\

                     Figure 1: Base SRTP Context Syntax

   The formal definition of the SRTP Context Attribute, including
   generic key-value pairs is provided by the following ABNF [RFC5234]
   found in Figure 2.

   srtp-context    = srtp-attr

   srtp-fmt-param  = srtp-param / srtp-param-list

   srtp-param      = 1srtp-ext [*(";" srtp-ext)]
                     ; One or more key=value pairs
                     ; key=value pairs separated by semicolon

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                     ; e.g: key=value;key2=value2

   srtp-param-list = 1"("srtp-param")" 1*("," "("srtp-param")")
                     ; Two or more lists of key=value pairs
                     ; Lists wrapped in parenthesis and
                     ; separated by commas
                     ; (key=value;key2=value2), (key=value;key3=value3)

   srtp-attr       = "a=srtpctx:"

   srtp-tag        = 1*9DIGIT SP
                     ; Matches tag length ABNF from RFC 4568

   srtp-ext        = param-key "=" param-value
                     ; key=value

   param-key       = 1*(ALPHA / DIGIT / "_" / "-")
                     ; Alphanumeric key name
                     ; May include underscore or hyphen

   param-value     = 1*BYTESTRING
                     ; Byte String key value

   ALPHA           = %x41-5A / %x61-7A
                     ; A-Z / a-z

   DIGIT           = %x30-39
                     ; 0-9

   BYTESTRING      = %x01-09 / %x0B-0C / %x0E-27 /
                     %x2A-2B / %x2D-3A / %x3C-FF
                     ; Excluding
                     ; %x00 (NULL)
                     ; %x0A (LF)
                     ; %x0D (CR)
                     ; %x28-29 (Left and Right Parenthesis)
                     ; %x2C (Comma)
                     ; %x3B (Semicolon)

          Figure 2: ABNF of Generic of the SRTP Context Attribute

   Note that Figure 2 does not allow raw left or right parenthesis,
   comma or semicolons within a parameter value as to avoid parsing
   errors with those specific delimiters.  If these specific values need
   to be conveyed, the value MAY be "percent encoded" as described by
   the logic in [RFC3986], Section 2.1.

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3.2.  SSRC, ROC, SEQ Syntax

   This specification specifically defines SRTP Context Attribute key-
   value pairs of "ssrc", "roc", and "seq".  The formal definition of
   the "ssrc", "roc", and "seq" key-value pairs which align to "srtp-
   ext" of Figure 2 are detailed in this specification are defined by
   the ABNF of Figure 3.

   srtp-ext  = srtp-ssrc / srtp-roc / srtp-seq
   srtp-ssrc = "ssrc=" "0x"1*8HEXDIG
               ; 32 bit SSRC
   srtp-roc  = "roc=" "0x"1*8HEXDIG
               ; 32 bit ROC
   srtp-seq  = "seq=" "0x"1*4HEXDIG
               ; 16 bit Sequence
   HEXDIG    = %x30-39 / %x41-46
               ; 0-9 / A-F

                     Figure 3: ABNF of Specific Syntax

   For "ssrc", "roc", and "seq", leading 0s may be omitted and the
   alphanumeric hex may be upper or lowercase.  Thus as per Figure 4
   these three lines are functionally identical.

   a=srtpctx:1 ssrc=0x00845FED;roc=0x00000000;seq=0x005D
   a=srtpctx:1 ssrc=0x845fed;roc=0x0;seq=0x05d
   a=srtpctx:1 ssrc=0x845feD;roc=0x0;seq=0x5D

              Figure 4: Comparison with and without Leading 0s

   In Figure 5 the sender has shares the usual cryptographic information
   as per a=crypto but has included other information such as the 32 bit
   SSRC, 32 bit ROC, and 16 bit Last Known Sequence number as hex values
   within the a=srtpctx attribute.  Together these attributes provide
   better insights as to the state of the SRTP cryptographic context
   from the senders perspective.

  a=crypto:1 AEAD_AES_256_GCM \
  a=srtpctx:1 ssrc=0x00845FED;roc=0x0000;seq=0x0150

                 Figure 5: Example SRTP Context Attribute

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3.3.  Pairing SRTP Context Attributes to SDP Security Attributes

   When SRTP context information needs to be conveyed about a given
   stream, the SRTP Context attribute (a=srtpctx) is coupled with the
   relevant SDP Security attribute (a=crypto) in the SDP.  This coupling
   is done via the "tag" found in both SDP attributes.  The tag used by
   SRTP Context Attributes is functionally the same as detailed in
   [RFC4568], Section 4.1.  The tag advertised in the SRTP Context
   Attribute is used to identify the SDP Security parameter a given SRTP
   Context Attribute is meant to pair with.  As such, within given media
   stream (m=), the tag of the SRTP Context Attribute MUST exactly match
   the SDP Security parameters tag as to create a pair of cryptographic

   The example in shown in Figure 6, within the audio stream, the sender
   is advertising an explicit packet index mapping for a=crypto tag 2
   (a=srtpctx:2) which matches the SDP security parameter with the same
   tag (a=crypto:2) Within the audio and video media stream tag 1
   (a=crypto:1) does not feature any paired SRTP Context Attributes.

   c=IN IP4
   m=audio 49170 RTP/SAVP 0
   a=crypto:1 AES_CM_128_HMAC_SHA1_80 \
   a=crypto:2 AEAD_AES_256_GCM \
   a=srtpctx:2 ssrc=0xBFBDD;roc=0x0001;seq=0x3039
   m=video 49172 RTP/SAVP 126
   a=crypto:1 AEAD_AES_128_GCM \
   a=srtpctx:1 ssrc=0xDD147C14;roc=0x0001;seq=0x3039

           Figure 6: Example crypto and SRTP Context tag mapping

   It is unlikely a sender will send SRTP Context Attributes for every
   crypto attribute since many will be fully unknown (such as the start
   of a session.)  However it is theoretically possible for every
   a=crypto tag to have a similar a=srtpctx attribute for additional

3.4.  Handling Unknown Cryptographic Attributes

   Applications MUST NOT include SRTP Context Attributes if all the
   values are unknown; such as the start of a session or later in a
   session where full SRTP context is lost by an application.  These
   unknown SRTP Context Attributes MAY be signaled at any later time but
   applications SHOULD ensure any offer/answer has the appropriate SRTP
   Context Attributes.

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   Further, if an SRTP Context Attribute key-value pair is advertised at
   one point during a session and then later removed during a session
   modification; the peer receiving application SHOULD fallback to
   default application logic, or locally derived/stored cryptographic
   context information, rather than failing/rejecting the session.

   For "ssrc", "roc", and "seq" the following are quick pointers to the
   default application logic that can be used when locally derived/
   stored cryptographic context information is not available and an SRTP
   Context Attribute was omitted or removed during session modification.

   Rollover Counter (ROC):
      If at the start of a session set the ROC to zero.  If later in a
      session, solve for "v" as per [RFC3711], Section 3.3.1

   Synchronization Source (SSRC)
      Via "Late Binding" defined by [RFC4568], Section 6.4.1

   Sequence (SEQ):
      Via "Late Binding" defined by [RFC4568], Section 6.4.1

3.5.  SRTP Multiplexing

   For scenarios where SRTP Multiplexing are concerned, EKT-SRTP
   ([RFC8870]) SHOULD be used in lieu of SDP Security as per [RFC8872]
   Section 4.3.2.  If SRTP Context Attributes are to be used, multiple
   SRTP Context Attribute key-value pairs can be grouped in a different
   lists using parenthesis as a delimiter with a comma to separate
   multiple key-value list groupings.  The default syntax for key-value
   list groupings detailed further in Section 3.1.

   The key-value list groupings for "ssrc, "roc" and "seq" can be
   observed in Figure 7 where three SSRC and the respective ROC/SEQ are
   provided as a key-value list groupings within the a=srtpctx

   a=crypto:1 AES_CM_128_HMAC_SHA1_80 \
   a=srtpctx:1 (ssrc=0x01;roc=0x0;seq=0x1234),\

             Figure 7: Example SRTP Context with Multiple SSRC

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3.6.  SDP Bundling

   For scenarios where SDP Bundling are concerned, SRTP Context
   Attributes follow the same bundling guidelines defined by [RFC8859],
   section 5.7 for SDP Securities a=crypto attribute.

3.7.  SDP Considerations

   The presence of the "a=srtpctx" attribute in the SDP (in either an
   offer or an answer) indicates that the endpoint is signaling explicit
   cryptographic context information and this data SHOULD be used in
   place of derived values (see Section 3.4).

3.7.1.  Sender Behavior

   Senders utilizing SDP Security via "a=crypto" MUST make an attempt to
   signal any known packet index values to the peer receiver.  The
   exception being when all values are unknown, such as at the very
   start of media stream negotiation.

   For best results all sending parties of a given session stream SHOULD
   advertise known packet index values for all media streams.  This
   should continue throughout the life of the session to ensure any
   errors or out of sync errors can be quickly corrected via new
   signaling methods.  See Section 3.7.3 for update frequency

3.7.2.  Receiver Behavior

   Receivers SHOULD utilize the signaled information in application
   logic to instantiate the SRTP cryptographic context.

   In the even there is no SRTP Context Attributes present in SDP
   receivers SHOULD fall back to application defaults as outlined in
   Section 3.4.

   See Section 3.4 for handling scenarios where a value was advertised
   and has been removed during session modification.

3.7.3.  Update Frequency

   Senders SHOULD provide SRTP Context SDP when SDP Crypto attributes
   are negotiated.  There is no explicit time or total number of packets
   in which a new update is required from sender to receiver.  This
   specification will not cause overcrowding on the session
   establishment protocol's signaling channel if natural session
   updates, session changes, and session liveliness checks are followed.

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3.8.  Future Extendability

   As stated in Section 3.1, the SRTP Context SDP implementation's goal
   is extendability allowing for additional vendor specific key-value
   pairs alongside the ones defined in this document.  This ensures that
   a=crypto SDP security may remain compatible with future algorithms
   that need to signal cryptographic context information outside of what
   is currently specified in [RFC4568].

   A complying specification needs only to follow the general rules
   defined by Section 3.1 and the generic ABNF outlined in Figure 2.

   To illustrate, imagine a new example SRTP algorithm and crypto suite
   is created named "FOO_CHACHA20_POLY1305_SHA256" and the application
   needs to signal "Foo, "Bar", and "Nonce" values to properly
   instantiate the SRTP context.  Rather than modify a=crypto SDP
   security or create a new unique SDP attribute, one can simply utilize
   SRTP Context SDP's key-value pairs to convey the information.
   Implementations MUST define how to handle default scenarios where the
   value is not present, unknown, or is removed later in a session.

a=crypto:1 FOO_CHACHA20_POLY1305_SHA256 \
a=srtpctx:1 foo=1;bar=abc123;nonce=8675309

   With this extendable method, all that is now required in the
   fictional RFC defining "FOO_CHACHA20_POLY1305_SHA256" is to include a
   section which details the expected SRTP Context Attribute key-value
   pair syntax, offer/answer usage (including unknown values and later
   session modifications).  Don't forget to detail other aspects of
   importance such as usage with SDP bundling, SRTP multiplexing and
   comparability with SRTP extensions defined in Section 3.

   This approach is similar to how Media Format Parameter Capability
   ("a=fmtp") is utilized in modern SDP.  An example is [RFC6184],
   Section 8.2.1 for H.264 video Media Format Parameters.

4.  Security Considerations

   When SDP carries SRTP Context Attributes additional insights are
   present about the SRTP cryptographic context.  Due to this an
   intermediary MAY be able to analyze how long a session has been
   active by the ROC value.

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   Since the SRTP Context Attribute is carried in plain-text (alongside
   existing values like the SRTP Master Key for a given session) care
   MUST be taken as per the [RFC8866] that keying material must not be
   sent over unsecure channels unless the SDP can be both private
   (encrypted) and authenticated.

5.  IANA Considerations

   This document updates the "attribute-name (formerly "att-field")"
   sub-registry of the "Session Description Protocol (SDP) Parameters"
   registry (see Section 8.2.4 of [RFC8866]).  Specifically, it adds the
   SDP "a=srtpctx" attribute for use at the media level.

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   | Form          | Value                                            |
   | Contact name  | IETF                                             |
   | Contact email |                                |
   | address       |                                                  |
   | Attribute     | srtpctx                                          |
   | name          |                                                  |
   | Attribute     | srtpctx                                          |
   | value         |                                                  |
   | Attribute     | Provided by ABNF found in Section 3.1            |
   | syntax        |                                                  |
   | Attribute     | Provided by sub-sections of Section 3            |
   | semantics     |                                                  |
   | Usage level   | media                                            |
   | Charset       | No                                               |
   | dependent     |                                                  |
   | Purpose       | Provide additional insights about SRTP context   |
   |               | information not conveyed required by a receiver  |
   |               | to properly decrypt SRTP.                        |
   | O/A           | SDP O/A procedures are described in Section 3.1, |
   | procedures    | specifically sections Section 3.7.1 and          |
   |               | Section 3.7.2 of this document.                  |
   | Mux Category  | TRANSPORT                                        |

                   Table 1: IANA SDP Registration Form

6.  Acknowledgements

   Thanks to Paul Jones for reviewing early draft material and providing
   valuable feedback.

7.  References

7.1.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8859]  Nandakumar, S., "A Framework for Session Description
              Protocol (SDP) Attributes When Multiplexing", RFC 8859,
              DOI 10.17487/RFC8859, January 2021,

   [RFC8866]  Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
              Session Description Protocol", RFC 8866,
              DOI 10.17487/RFC8866, January 2021,

7.2.  Informative References

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <>.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              DOI 10.17487/RFC3830, August 2004,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

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   [RFC4567]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
              Carrara, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", RFC 4567, DOI 10.17487/RFC4567, July
              2006, <>.

   [RFC4771]  Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
              Transform Carrying Roll-Over Counter for the Secure Real-
              time Transport Protocol (SRTP)", RFC 4771,
              DOI 10.17487/RFC4771, January 2007,

   [RFC5159]  Dondeti, L., Ed. and A. Jerichow, "Session Description
              Protocol (SDP) Attributes for Open Mobile Alliance (OMA)
              Broadcast (BCAST) Service and Content Protection",
              RFC 5159, DOI 10.17487/RFC5159, March 2008,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC5479]  Wing, D., Ed., Fries, S., Tschofenig, H., and F. Audet,
              "Requirements and Analysis of Media Security Management
              Protocols", RFC 5479, DOI 10.17487/RFC5479, April 2009,

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,

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   [RFC6184]  Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184,
              DOI 10.17487/RFC6184, May 2011,

   [RFC6189]  Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
              Media Path Key Agreement for Unicast Secure RTP",
              RFC 6189, DOI 10.17487/RFC6189, April 2011,

   [RFC6904]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", RFC 6904,
              DOI 10.17487/RFC6904, April 2013,

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,

   [RFC7714]  McGrew, D. and K. Igoe, "AES-GCM Authenticated Encryption
              in the Secure Real-time Transport Protocol (SRTP)",
              RFC 7714, DOI 10.17487/RFC7714, December 2015,

   [RFC8723]  Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
              "Double Encryption Procedures for the Secure Real-Time
              Transport Protocol (SRTP)", RFC 8723,
              DOI 10.17487/RFC8723, April 2020,

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,

   [RFC8870]  Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
              Andreasen, "Encrypted Key Transport for DTLS and Secure
              RTP", RFC 8870, DOI 10.17487/RFC8870, January 2021,

   [RFC8871]  Jones, P., Benham, D., and C. Groves, "A Solution
              Framework for Private Media in Privacy-Enhanced RTP
              Conferencing (PERC)", RFC 8871, DOI 10.17487/RFC8871,
              January 2021, <>.

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   [RFC8872]  Westerlund, M., Burman, B., Perkins, C., Alvestrand, H.,
              and R. Even, "Guidelines for Using the Multiplexing
              Features of RTP to Support Multiple Media Streams",
              RFC 8872, DOI 10.17487/RFC8872, January 2021,

   [RFC9335]  Uberti, J., Jennings, C., and S. Murillo, "Completely
              Encrypting RTP Header Extensions and Contributing
              Sources", RFC 9335, DOI 10.17487/RFC9335, January 2023,

Appendix A.  Protocol Design Overview

   This appendix section is included to details some important itmes
   integral to the decision process of creating this specification.
   This section may be removed by the editors or left for future
   generations to understand why specific things were done as they are.

   In general, the overall design for this protocol tends to follow the
   phrase found in RFC6709, Section 1.  "Experience with many protocols
   has shown that protocols with few options tend towards ubiquity,
   whereas protocols with many options tend towards obscurity.

   Each and every extension, regardless of its benefits, must be
   carefully scrutinized with respect to its implementation, deployment,
   and interoperability costs."

A.1.  Why not an RTP Header Extension?

   In order to be compatible with "a=cryptex", a protocol which extends
   the SRTP encryption over the RTP Extension Headers, the designed
   specification must ensure that information about the SRTP context is
   not within these RTP extension headers.  Thus one has to provide this
   information in an out of band mechanism.

A.2.  Why not an SDP Security Session Parameter?

   While analyzing SDP Security's Session Parameter feature number of
   interesting details were found.  That is sections 6.3.7, 7.1.1, 9.2,
   and of [RFC4568] specifically.

   A few illustrative examples below detail what this could look like
   are provided below, though these MUST NOT be used.

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 a=crypto:1 [..omitted..] SSRC=0x00845FED ROC=0x00000000 SEQ=0x005D

 a=crypto:1 ..omitted.. -SSRC=0x00845FED -ROC=0x00000000 -SEQ=0x005D

 a=crypto:1 AEAD_AES_256_GCM \
  |2^20|1:32 SSRC=0x00845FED ROC=0x0000 SEQ=0x0150

 a=crypto:1 AES_CM_128_HMAC_SHA1_80 \
   ;inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|21|3:4 \
   SSRC=0xDD148F16 ROC=0x0 SEQ=0x5A53
 a=crypto:2 AES_CM_128_HMAC_SHA1_32 \
   inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20 \
   FEC_KEY=inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20|2:4 \
   WSH=60 SSRC=0xD903 ROC=0x0002 SEQ=0xB043
 a=crypto:3 AEAD_AES_256_GCM \
   inline:HGAPy4Cedy/qumbZvpuCZSVT7rNDk8vG4TdUXp5hkyWqJCqiLRGab0KJy1g= \
 a=crypto:4 AEAD_AES_128_GCM \
   inline:bQJXGzEPXJPClrd78xwALdaZDs/dLttBLfLE5Q== \

   To analyze the faults of this method: First, a unknown and/or
   unsupported SDP Security Session Parameter is destructive.  If one
   side where to advertise the ROC value as an SDP Security Session
   Parameter and the remote party does not understand that specific SDP
   Security Session Parameter, that entire crypto line is to be
   considered invalid.  If this is the only a=crypto entry then the
   entire session may fail.  The solution in this document allows for a
   graceful fallback to known methods to determine these value.
   Implementations could get around this by duplicating the a=crypto SDP
   attribute into two values: one with the postfix and one without to
   create to potential offers; but at this point we have a second SDP
   attribute.  Instead this specification decided to cut to the chase
   and format the second attribute in a standardized way and avoid
   endless duplication (and potentially other harmful issues, see the
   final item in this document.)

   Second, there is a method to advertise "optional" SDP Security
   Session Parameters.  However, upon further scrutiny, the document
   contradicts itself in many sections.  To be specific, Section 6.3.7
   states that an SDP Security Session Parameter prefixed with a dash
   character "-" MAY be ignored.  Subsequent sections (9.2 and
   state that a dash character is illegal and MUST NOT be used.  It is
   not very well defined as such pursuit of this method has been

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   Further, we know how applications will handle unknown SDP attributes;
   we do not know how applications will handle new mandatory (or
   optional) SDP Security Session Parameter values as none have ever
   been created.  See IANA registry which only details those from the
   original RFC. (
   descriptions-4) Including these could cause larger application issues
   and are the reason modern protocols use logic like Generate Random
   Extensions And Sustain Extensibility (GREASE) to catch bad
   implementation behavior and correct it before it leads to problems
   like those described in this section.

   In closing, this method has too many challenges but a lot has been
   learned.  These items have influenced the protocol design and
   sections like Section 3.8 which aim to avoid making the same

Authors' Addresses

   Kyzer R. Davis
   Cisco Systems

   Esteban Valverde
   Cisco Systems

   Gonzalo Salgueiro
   Cisco Systems

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