Internet DRAFT - draft-davis-valverde-srtp-assurance
draft-davis-valverde-srtp-assurance
Network Working Group K. R. Davis
Internet-Draft E. Valverde
Updates: 4568 (if approved) G. Salgueiro
Intended status: Standards Track Cisco Systems
Expires: 29 December 2023 27 June 2023
SDP Security Assurance for Secure Real-time Transport Protocol (SRTP)
draft-davis-valverde-srtp-assurance-00
Abstract
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
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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 29 December 2023.
Copyright Notice
Copyright (c) 2023 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 (https://trustee.ietf.org/
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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 2
1.2. Previous Solutions . . . . . . . . . . . . . . . . . . . 5
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 6
3. Protocol Design . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. SDP Considerations . . . . . . . . . . . . . . . . . . . 7
3.2. Sender Behavior . . . . . . . . . . . . . . . . . . . . . 10
3.3. Receiver Behavior . . . . . . . . . . . . . . . . . . . . 11
3.4. Update Frequency . . . . . . . . . . . . . . . . . . . . 11
3.5. Extendability . . . . . . . . . . . . . . . . . . . . . . 11
4. Security Considerations . . . . . . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
1.1. 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.
The ROC is one item that is used to create the SRTP Packet Index
along with the the [RFC3550] transmitted sequence numbers 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.)
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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.
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
standards.
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:
* 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.
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* 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.
* A similar scenario was brought up in Appendix A of [RFC4568]
"Scenario B" and "Problem 3" of the summary within this section.
* 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.
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.
* 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:
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* 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
operations.
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].
1.2. 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
decrypt.
Lastly, there is no defined method for applications defined for
applications to advertise the usage of this protocol via any
signaling methods.
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[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]
implementations.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
Streams:
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].
DTLS-SRTP, EKT-SRTP, Privacy Enhanced Conferencing items (PERC):
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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
Sources:
SRTP Context is compatible with [RFC9335] "a=cryptex" media and
session level attribute.
3.1. SDP Considerations
This specification introduces a new SRTP Context attribute defined as
"a=srtpctx".
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 such as those obtained from late binding or
some other mechanism.
The SRTP Context value syntax utilizes standard attribute field=value
pairs separated by semi-colons as seen in Figure 1. The
implementation's goal is extendable allowing for additional vendor
specific field=value pairs alongside the ones defined in this
document or room for future specifications to add additional
field=value pairs.
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a=srtpctx:<a-crypto-tag> \
<att_field_1>=<value_1>;<att_field_1>=<att_value_2>
Figure 1: Base SRTP Context Syntax
This specification specifically defines SRTP Context Attribute Fields
of SSRC, ROC, and SEQ shown in Figure 2.
a=srtpctx:<a-crypto-tag> \
ssrc=<ssrc_value_hex>;roc=<roc_value_hex>;seq=<last_known_tx_seq_hex>
Figure 2: Example SRTP Context Syntax
Note that long lines in this document have been broken into multiple
lines using the "The Single Backslash Strategy ('')" defined by
[RFC8792].
The formal definition of the SRTP Context Attribute, including custom
extension field=value pairs is provided by the following ABNF
[RFC5234]:
srtp-assurance = srtp-attr
srtp-tag
[srtp-ssrc";"]
[srtp-roc";"]
[srtp-seq";"]
[srtp-ext";"]
srtp-attr = "a=srtpctx:"
srtp-tag = 1*9DIGIT 1WSP
srtp-ssrc = "ssrc=" ("0x"1*8HEXDIG / "unknown")
srtp-roc = "roc=" ("0x"1*4HEXDIG / "unknown")
srtp-seq = "seq=" ("0x"1*4HEXDIG / "unknown")
srtp-ext = 1*VCHAR "=" (1*VCHAR / "unknown")
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
VCHAR = %x21-7E
Leading 0s may be omitted and the alphanumeric hex may be upper or
lowercase but at least one 0 must be present. Additionally the "0x"
provided additional context that these values are hex and not
integers. Thus as per Figure 3 these two lines are functionally
identical:
a=srtpctx:1 ssrc=0x00845FED;roc=0x00000000;seq=0x005D
a=srtpctx:1 ssrc=0x845fed;roc=0x0;seq=0x05d
Figure 3: Comparison with and without Leading 0s
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When SSRC, ROC, or Sequence information needs to be conveyed about a
given stream, the a=srtpctx attribute is coupled with the relevant
a=crypto attribute in the SDP.
In Figure 4 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 two attributes
provide better insights as to the state of the SRTP cryptographic
context from the senders perspective.
a=crypto:1 AEAD_AES_256_GCM \
inline:3/sxOxrbg3CVDrxeaNs91Vle+wW1RvT/zJWTCUNP1i6L45S9qcstjBv+eo0=\
|2^20|1:32
a=srtpctx:1 ssrc=0x00845FED;roc=0x0000;seq=0x0150
Figure 4: Example SRTP Context attribute
The value of "unknown" MAY be used in place of any of the fields to
indicate default behavior SHOULD be utilized by the receiving
application (usually falling back to late binding or locally derived/
stored cryptographic contact information for the packet index.) The
example shown in Figure 5 indicates that only the SSRC of the stream
is unknown to the sender at the time of the SDP exchange but values
for ROC and Last Known Sequence are present. Alternatively, the
attribute key and value MAY be omitted entirely.
This MAY be updated via signaling at any later time but applications
SHOULD ensure any offer/answer has the appropriate SRTP Context
attribute.
Applications SHOULD NOT include SRTP Context attribute if all three
values are unknown or would be omitted. For example, starting a new
sending session instantiation or for advertising potential
cryptographic attributes that are part of a new offer.
Figure 5 shows that tag 1 does not have any SRTP Context parameters
rather than rather an SRTP Context attribute with all three values
set to "unknown". This same example shows an unknown value carried
with tag 2 and seq has been committed leaving only the ROC as a value
shared with the second a=crypto tag.
a=crypto:1 AES_CM_128_HMAC_SHA1_32 \
inline:k4x3YXkTD1TWlNL3BZpESzOFuxkBZmTo0vGa1omW
a=crypto:2 AES_CM_128_HMAC_SHA1_80 \
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR
a=srtpctx:2 ssrc=unknown;roc=0x0001
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Figure 5: Example SRTP Context with unknown mappings
The tag for an SRTP Context attribute MUST follow the peer SDP
Security a=crypto tag for a given media stream (m=). The example in
shown in Figure 6 the sender is advertising an explicit packet index
mapping for a=crypto tag 2 for the audio stream and tag 1 for the
video media stream. Note that some SDP values have been truncated
for the sake of simplicity.
c=IN IP4 192.0.0.1
m=audio 49170 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_80 \
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=crypto:2 AEAD_AES_256_GCM \
inline:HGAPy4Cedy/qumbZvpuCZSVT7rNDk8vG4TdUXp5hkyWqJCqiLRGab0KJy1g=
a=srtpctx:2 ssrc=0xBFBDD;roc=0x0001;seq=0x3039
m=video 49172 RTP/SAVP 126
a=crypto:1 AEAD_AES_128_GCM \
inline:bQJXGzEPXJPClrd78xwALdaZDs/dLttBLfLE5Q==
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
details.
For scenarios where RTP Multiplexing are concerned, EKT-SRTP
([RFC8870]) MUST be used in lieu of SDP Security as per [RFC8872]
Section 4.3.2.
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.
3.2. 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 medias stream negotiation.
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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 3.4 for update frequency
recommendations.
3.3. 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 MUST
fallback to [RFC3711] for guesting the ROC and [RFC4568] logic for
late binding to gleam the SSRC and sequence numbers.
3.4. 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. By
following natural session updates, session changes and session
liveliness checks this specification will not cause overcrowding on
the session establishment protocol's signaling channel.
3.5. Extendability
As stated in Section 3.1, the SRTP Context SDP implementation's goal
is extendability allowing for additional vendor specific field=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].
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.
a=crypto:1 FOO_CHACHA20_POLY1305_SHA256 \
inline:1ef9a49f1f68f75f95feca6898921db8c73bfa53e71e33726c4c983069dd7d44
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
an "SDP parameters" section which details the expected "a=srtpctx"
values and their usages. This approach is similar to how Media
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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.
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 | IESG |
+---------------+-----------------------------------------------+
| Contact email | kydavis@cisco.com |
| 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 |
| procedures | Section 3.1, specifically sections |
| | Section 3.2 and Section 3.3 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
valueable 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,
<https://www.rfc-editor.org/rfc/rfc2119>.
[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,
<https://www.rfc-editor.org/rfc/rfc3711>.
[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,
<https://www.rfc-editor.org/rfc/rfc4568>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8859] Nandakumar, S., "A Framework for Session Description
Protocol (SDP) Attributes When Multiplexing", RFC 8859,
DOI 10.17487/RFC8859, January 2021,
<https://www.rfc-editor.org/rfc/rfc8859>.
[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/rfc/rfc8866>.
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, <https://www.rfc-editor.org/rfc/rfc3550>.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
DOI 10.17487/RFC3830, August 2004,
<https://www.rfc-editor.org/rfc/rfc3830>.
[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, <https://www.rfc-editor.org/rfc/rfc4567>.
Davis, et al. Expires 29 December 2023 [Page 14]
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[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,
<https://www.rfc-editor.org/rfc/rfc4771>.
[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,
<https://www.rfc-editor.org/rfc/rfc5159>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[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,
<https://www.rfc-editor.org/rfc/rfc5479>.
[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,
<https://www.rfc-editor.org/rfc/rfc5576>.
[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, <https://www.rfc-editor.org/rfc/rfc5763>.
[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,
<https://www.rfc-editor.org/rfc/rfc5764>.
[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,
<https://www.rfc-editor.org/rfc/rfc6184>.
[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,
<https://www.rfc-editor.org/rfc/rfc6189>.
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[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
<https://www.rfc-editor.org/rfc/rfc6904>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<https://www.rfc-editor.org/rfc/rfc7201>.
[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,
<https://www.rfc-editor.org/rfc/rfc7714>.
[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,
<https://www.rfc-editor.org/rfc/rfc8723>.
[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,
<https://www.rfc-editor.org/rfc/rfc8792>.
[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,
<https://www.rfc-editor.org/rfc/rfc8870>.
[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, <https://www.rfc-editor.org/rfc/rfc8871>.
[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,
<https://www.rfc-editor.org/rfc/rfc8872>.
[RFC9335] Uberti, J., Jennings, C., and S. Murillo, "Completely
Encrypting RTP Header Extensions and Contributing
Sources", RFC 9335, DOI 10.17487/RFC9335, January 2023,
<https://www.rfc-editor.org/rfc/rfc9335>.
Authors' Addresses
Davis, et al. Expires 29 December 2023 [Page 16]
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Kyzer R. Davis
Cisco Systems
Email: kydavis@cisco.com
Esteban Valverde
Cisco Systems
Email: jovalver@cisco.com
Gonzalo Salgueiro
Cisco Systems
Email: gsalguei@cisco.com
Davis, et al. Expires 29 December 2023 [Page 17]
