One document matched: draft-ietf-straw-b2bua-dtls-srtp-06.xml
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<!--rfc category="info" ipr="full3978"-->
<rfc category="std" docName="draft-ietf-straw-b2bua-dtls-srtp-06"
ipr="trust200902">
<front>
<title abbrev="DTLS-SRTP Handling in SIP B2BUA">DTLS-SRTP Handling in
Session Initiation Protocol (SIP) Back-to-Back User Agents
(B2BUAs)</title>
<author fullname="Ram Mohan Ravindranath" initials="R."
surname="Ravindranath">
<organization>Cisco</organization>
<address>
<postal>
<street>Cessna Business Park</street>
<street>Sarjapur-Marathahalli Outer Ring Road</street>
<city>Bangalore</city>
<region>Karnataka</region>
<code>560103</code>
<country>India</country>
</postal>
<email>rmohanr@cisco.com</email>
</address>
</author>
<author fullname="Tirumaleswar Reddy" initials="T." surname="Reddy">
<organization>Cisco</organization>
<address>
<postal>
<street>Cessna Business Park, Varthur Hobli</street>
<street>Sarjapur Marathalli Outer Ring Road</street>
<city>Bangalore</city>
<region>Karnataka</region>
<code>560103</code>
<country>India</country>
</postal>
<email>tireddy@cisco.com</email>
</address>
</author>
<author fullname="Gonzalo Salgueiro" initials="G." surname="Salgueiro">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>7200-12 Kit Creek Road</street>
<city>Research Triangle Park</city>
<region>NC</region>
<code>27709</code>
<country>US</country>
</postal>
<email>gsalguei@cisco.com</email>
</address>
</author>
<author fullname="Victor Pascual" initials="V." surname="Pascual">
<organization abbrev="Quobis">Quobis</organization>
<address>
<postal>
<!-- Street is mandatory under postal XML tag. So, change the taga from country to street -->
<street>Spain</street>
</postal>
<email>victor.pascual.avila@gmail.com</email>
</address>
</author>
<author fullname="Parthasarathi Ravindran" initials="Parthasarathi"
surname="Ravindran">
<organization>Nokia Networks</organization>
<address>
<postal>
<street></street>
<city>Bangalore</city>
<region>Karnataka</region>
<country>India</country>
</postal>
<email>partha@parthasarathi.co.in</email>
</address>
</author>
<date year="2015" />
<area>Real-time Applications and Infrastructure (RAI)</area>
<workgroup>STRAW</workgroup>
<abstract>
<t>Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
often act on the media plane, rather than just on the signaling
path. This document describes the behavior such B2BUAs should adhere to when
acting on the media plane that uses an Secure Real-time Transport (SRTP) security
context set up with the Datagram Transport Layer Security (DTLS) protocol.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<section title="Overview">
<t><xref target="RFC5763"></xref> describes how Session Initiation
Protocol (SIP) <xref target="RFC3261"></xref> can be used to establish
a Secure Real-time Transport Protocol (SRTP) <xref
target="RFC3711"></xref> security context with the Datagram Transport
Layer Security (DTLS) <xref target="RFC6347"></xref> protocol. It
describes a mechanism for transporting a certificate fingerprint using
Session Description Protocol (SDP) <xref target="RFC4566"></xref>. The fingerprint,
identifies the certificate that will be presented during the
DTLS handshake. DTLS-SRTP is defined for point-to-point media
sessions, in which there are exactly two participants. Each DTLS-SRTP
session (described in Section 3 of <xref target="RFC5764"></xref>)
contains a single DTLS connection, and either two SRTP contexts (if
media traffic is flowing in both directions on the same 5-tuple) or
one SRTP context (if media traffic is only flowing in one
direction).</t>
<t>In many SIP deployments, SIP Back-to-Back User Agents (B2BUA) entities
exist on the SIP signaling path between the endpoints. As described in
<xref target="RFC7092"></xref>, these B2BUAs may modify
SIP and SDP information. They may also be present on the media path, in which case
they may modify parts of the SDP information (like IP address, port) and subsequently
modify the RTP headers as well. Such B2BUAs are referred to as media plane B2BUAs</t>
</section>
<section title="Goals">
<t><xref target="RFC7092"></xref> describes two different categories
of media plane B2BUAs, according to the level of activities performed on the
media plane:</t>
<t><list style="hanging">
<t>A B2BUA that acts as a simple media relay effectively unaware of
anything that is transported and only terminates the media plane at
the IP and transport (UDP/TCP) layers.</t>
<t>A B2BUA that performs a media-aware role. It inspects and
potentially modifies RTP or RTP Control Protocol (RTCP) headers;
but it does not modify the payload of RTP/RTCP.</t>
</list></t>
<t>The following sections describe the behavior B2BUAs should follow
in order to avoid any impact to end-to-end DTLS-SRTP streams.</t>
</section>
</section>
<section anchor="sec-term" title="Terminology">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119"></xref>.</t>
<t>The following generalized terms are defined in <xref
target="RFC3261"></xref>, Section 6.</t>
<t><list style="hanging">
<t>B2BUA: a SIP Back-to-Back User Agent, which is the logical
combination of a User Agent Server (UAS) and User Agent Client
(UAC).</t>
<t>UAS: a SIP User Agent Server.</t>
<t>UAC: a SIP User Agent Client.</t>
</list></t>
<t>All of the pertinent B2BUA terminology and taxonomy used in this
document is based on <xref target="RFC7092"></xref>.</t>
<t>It is assumed the reader is already familiar with the fundamental
concepts of the RTP protocol <xref target="RFC3550"></xref> and its
taxonomy <xref target="I-D.ietf-avtext-rtp-grouping-taxonomy"></xref>,
as well as those of SRTP <xref target="RFC3711"></xref>, and DTLS <xref
target="RFC6347"></xref>.</t>
</section>
<section title="Media Plane B2BUA Handling of DTLS-SRTP">
<section title="General">
<t>
This section describes the DTLS-SRTP handling by the different types of
media plane B2BUAs defined in <xref target="RFC7092"/>.
</t>
<section title="Media Relay">
<t>A media relay, as defined in section 3.2.1 of <xref
target="RFC7092"></xref>, from an application layer point-of-view,
forwards all packets it receives on a negotiated connection,
without inspecting or modifying the packet contents. A media relay
only modifies the transport layer (UDP/TCP) and IP headers.</t>
<t>A media relay B2BUA MUST forward the certificate fingerprint and
SDP setup attribute it receives from one endpoint
unmodified towards the other endpoint and vice-versa. The example below shows a
SIP call establishment flow, with both SIP endpoints (user agents) using
DTLS-SRTP, and a media relay B2BUA.</t>
<t><figure anchor="Figure1"
title="INVITE with SDP call-flow for Media Relay B2BUA">
<artwork align="center"><![CDATA[
+-------+ +------------------+ +-----+
| Alice | | MediaRelay B2BUA | | Bob |
+-------+ +------------------+ +-----+
|(1) INVITE | (3)INVITE |
| a=setup:actpass | a=setup:actpass |
| a=fingerprint1 | a= fingerprint1 |
| (alice's IP/port) | (B2BUAs IP/port) |
|------------------------>|-------------------------->|
| | |
| (2) 100 trying | |
|<------------------------| |
| | (4) 100 trying |
| |<--------------------------|
| | |
| | (5)200 OK |
| | a=setup:active |
| | a=fingerprint2 |
| | (Bob's IP/port) |
|<------------------------|<--------------------------|
| (6) 200 OK | |
| a=setup:active | |
| a=fingerprint2 | |
| B2BUAs IP/port | |
| (7, 8)ClientHello + use_srtp |
|<------------------------|<--------------------------|
| | |
| | |
| (9,10)ServerHello + use_srtp |
|------------------------>|-------------------------->|
| (11) | |
| [Certificate exchange between Alice and Bob over |
| DTLS ] | |
| | |
| (12) | |
|<---------SRTP/SRTCP---->|<----SRTP/SRTCP----------->|
| [B2BUA changes transport(UDP/TCP) and IP headers] |
]]></artwork>
</figure></t>
<t>NOTE: For brevity the entire value of the SDP fingerprint attribute is not
shown. The example here shows only one DTLS connection for the sake of simplicity.
In reality depending on whether the RTP and RTCP flows are multiplexed or demultiplexed
there will be one or two DTLS connections.</t>
<t>If RTP and RTCP traffic is multiplexed as described in <xref target="RFC5761"/> on a
single port then only a single DTLS connection would be required between the peers.
If RTP and RTCP are not multiplexed, then the peers would have to establish two DTLS connections.
In this case, Bob, after he receives an INVITE request, triggers the establishment of a
DTLS connection. Note that the DTLS handshake and the sending of INVITE response may
happen in parallel; thus, the B2BUA SHOULD be prepared to receive
DTLS, STUN and media on the ports it advertised to Bob in the INVITE request. Since a media
relay B2BUA does not differentiate between a DTLS message, RTP or any packet
it receives, it only changes the transport layer (UDP/TCP) and IP headers
and forwards the packet towards the other endpoint.</t>
<t><xref target="I-D.ietf-stir-rfc4474bis"></xref> provides a means
for signing portions of SIP requests in order to provide identity
assurance and certificate pinning by providing a signature over the
fingerprint of keying material in SDP for DTLS-SRTP [RFC5763]. A media
relay B2BUA MUST ensure that it does not modify any of the information
used to construct the signature.</t>
<t>In the above example, Alice may be authorized by the authorization
server (SIP proxy) in its domain using the procedures in Section 5 of
<xref target="I-D.ietf-stir-rfc4474bis"></xref>. In such a case, if the
B2BUA modifies some of the SIP headers or SDP content that was used by
Alice's authorization server to generate the identity, it would break
the identity verification procedure explained in Section 4.2 of <xref
target="I-D.ietf-stir-rfc4474bis"></xref> resulting in a 438 error
response being returned.</t>
</section>
<section title="Media Aware B2BUA">
<t>Unlike the media relay discussed in Section 3.1.1, a media-aware relay as defined
in Section 3.2.2 of <xref target="RFC7092"/>, is aware of the type of media traffic it is
receiving. A media-aware relay inspects received SRTP and SRTCP packets and may modify
the headers before forwarding the packets.</t>
<section title="RTP and RTCP Header Inspection">
<t>A media-aware relay does not modify the RTP and RTCP headers but
only inspects the header values. It MUST NOT terminate the
DTLS-SRTP connection on which the packets are received.</t>
</section>
<section title="RTP and RTCP Header Modification">
<t>
In addition to inspecting the RTP and RTCP headers, a media-aware relay may also
modify them. In order to modify headers a B2BUA needs to act as a DTLS endpoint and
terminate the DTLS connection and decrypt/re-encrypt RTP packets. This would
break end-to-end security and hence a B2BUA MUST NOT terminate DTLS-SRTP
sessions. This security and privacy problem can be mitigated by having separate
keys for encrypting the RTP header and media payload as discussed in
<xref target="I-D.jones-perc-private-media-reqts"></xref>.
With such an approach, the B2BUA is not aware of the keys used to decrypt
the media payload.
</t>
</section>
</section>
</section>
<section title="Media Plane B2BUA with NAT Handling">
<t>DTLS-SRTP handshakes and SDP offer/answer exchanges <xref target="RFC3264"/> may
happen in parallel. If an endpoint is behind a NAT, and the endpoint is acting as a DTLS
server, the ClientHello message from a B2BUA (acting as DTLS client) is likely to be lost, as described
in Section 7.3 of <xref target="RFC5763"></xref>. In order to overcome
this problem, the endpoint and B2BUA can support the Interactive Connectivity
Establishment (ICE) mechanism <xref target="RFC5245"/>, as discussed in Section
7.3 of <xref target="RFC5763"></xref>. If the ICE check is successful then the endpoint
will receive the ClientHello message from the B2BUA.</t>
</section>
</section>
<section title="Forking Considerations">
<t>Due to forking <xref target="RFC3261"/>, a SIP request carrying an SDP offer
sent by an endpoint (offerer) may reach multiple remote endpoints. As a result,
multiple DTLS-SRTP connections may be established, one between the endpoint that sent
the SIP request and each of the remote endpoints that received the request.
Both media relays and media-aware relays MUST forward the certificate
fingerprints and SDP setup attributes it received in the SDP answer from each endpoint (answerer)
unmodified towards the offerer. Since DTLS operates on the 5-tuple, B2BUA MUST replace
the answerer's transport addresses in each answer with its unique
transport addresses so that the offerer can establish a DTLS connection
with each answerer.</t>
<figure anchor="Figure2"
title="B2BUA handling multiple answers">
<artwork><![CDATA[
Bob (192.0.2.1:6666)
/
/
/ DTLS-SRTP=XXX
/
/
DTLS-SRTP=XXX v
<-----------> (192.0.2.3:7777)
Alice (192.0.2.0:5555) B2BUA
<-----------> (192.0.2.3:8888)
DTLS-SRTP=YYY ^
\
\ DTLS-SRTP=YYY
\
\
\
Charlie (192.0.2.2:6666)
]]></artwork>
</figure>
<t></t>
<t>For instance, as shown in Figure 2 Alice sends a request with an offer, and the request is forked.
Alice receives answers from both Bob and Charlie. B2BUA MUST advertise different
B2BUA transport address in each answer, as shown in Figure2,
where XXX and YYY represent different DTLS-SRTP connections. B2BUA
replaces the Bob's transport address (192.0.2.1:6666) in the answer with
its transport address (192.0.2.3:7777) and Charlie's transport address
(192.0.2.2:6666) in the answer with its transport address
(192.0.2.3:8888). B2BUA tracks the remote sources (Bob and Charlie) and
associates them to the local sources that are used to send packets to
Alice.</t>
</section>
<section title="Security Considerations">
<t>This document describes the behavior media plane B2BUAs (media-aware
and media-unaware) should follow when acting on the media plane that
uses SRTP security context setup with the DTLS protocol. Attempting to cover
media-aware relay and media termination scenarios involving secure sessions
(like DTLS-SRTP) will inevitably lead to the B2BUA acting as a man-in-the-middle,
and as such its behavior is unspecified and discouraged. This document does not
introduce any specific security considerations beyond those detailed in
<xref target="RFC5763"></xref>. In addition, the B2BUA behaviors outlined in this document
do not impact the security and integrity of a DTLS-SRTP connection or
the data exchanged over it. A malicious B2BUA may try to break into the
DTLS connection, but such an attack can be prevented using the identity
validation mechanism discussed in <xref
target="I-D.ietf-stir-rfc4474bis"></xref>.
</t>
</section>
<section anchor="sec.iana-considerations" title="IANA Considerations">
<t>This document makes no request of IANA.</t>
</section>
<section title="Acknowledgments">
<t>Special thanks to Lorenzo Miniero, Ranjit Avarsala, Hadriel Kaplan,
Muthu Arul Mozhi, Paul Kyzivat, Peter Dawes, Brett Tate, Dan Wing,
Charles Eckel, Simon Perreault, Albrecht Schwarz, Jens Guballa and Christer Holmberg
for their constructive comments,suggestions, and early reviews that were critical to
the formulation and refinement of this document.</t>
</section>
<section title="Contributors">
<t>Rajeev Seth provided substantial contributions to this document.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.3550"?>
<?rfc include="reference.RFC.3711"?>
<?rfc include="reference.RFC.5763"?>
<?rfc include="reference.RFC.5764"?>
<?rfc include="reference.RFC.6347"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.3261"?>
<?rfc include="reference.RFC.3264"?>
<?rfc include="reference.RFC.4566"?>
<?rfc include="reference.RFC.7092"?>
<?rfc include="reference.RFC.5245"?>
<?rfc include="reference.I-D.ietf-avtext-rtp-grouping-taxonomy"?>
<?rfc include="reference.I-D.jones-perc-private-media-reqts" ?>
<?rfc include="reference.I-D.ietf-stir-rfc4474bis" ?>
<?rfc include="reference.RFC.5761"?>
</references>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-24 04:26:25 |