Internet DRAFT - draft-ietf-straw-b2bua-dtls-srtp
draft-ietf-straw-b2bua-dtls-srtp
STRAW R. Ravindranath
Internet-Draft T. Reddy
Intended status: Standards Track G. Salgueiro
Expires: October 6, 2016 Cisco
V. Pascual
Oracle
Parthasarathi. Ravindran
Nokia Networks
April 4, 2016
DTLS-SRTP Handling in Session Initiation Protocol (SIP) Back-to-Back
User Agents (B2BUAs)
draft-ietf-straw-b2bua-dtls-srtp-12
Abstract
Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUA)
exist on the signaling and media paths between the endpoints. This
document describes the behavior of B2BUAs when Secure Real-time
Transport (SRTP) security context is set up with the Datagram
Transport Layer Security (DTLS) protocol.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 6, 2016.
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Goals and Scope of this Document . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. B2BUAs Procedures to Allow End-to-End DTLS-SRTP . . . . . . . 4
4. Signaling Plane B2BUA Handling of DTLS-SRTP . . . . . . . . . 5
4.1. Proxy-B2BUAs . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Signaling-only and SDP-modifying Signaling-only B2BUAs . 5
5. Media Plane B2BUA Handling of DTLS-SRTP . . . . . . . . . . . 6
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1.1. Media Relay . . . . . . . . . . . . . . . . . . . . . 6
5.1.2. RTP/RTCP-Aware Media-Aware B2BUA . . . . . . . . . . 8
6. Forking Considerations . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
1.1. Overview
[RFC5763] describes how Session Initiation Protocol (SIP) [RFC3261]
can be used to establish a Secure Real-time Transport Protocol (SRTP)
[RFC3711] security context with the Datagram Transport Layer Security
(DTLS) [RFC6347] protocol. It describes a mechanism for transporting
a certificate fingerprint using Session Description Protocol (SDP)
[RFC4566]. The fingerprint identifies the certificate that will be
presented during the DTLS handshake. DTLS-SRTP is currently defined
for point-to-point media sessions, in which there are exactly two
participants. Each DTLS-SRTP session (described in Section 3 of
[RFC5764]) contains a single DTLS connection (if RTP and RTCP are
multiplexed) or two DTLS connections (if RTP and RTCP are not
multiplexed), and either two SRTP contexts (if media traffic is
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flowing in both directions on the same 5-tuple) or one SRTP context
(if media traffic is only flowing in one direction).
In many SIP deployments, SIP Back-to-Back User Agents (B2BUA)
entities exist on the SIP signaling path between the endpoints. As
described in [RFC7092], these B2BUAs can modify SIP and SDP
information. For example, as described in Section 3.1.3 of
[RFC7092], SDP-modifying signaling-only B2BUAs can potentially modify
the SDP. B2BUAs can also be present on the media path, in which case
they 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. [RFC7092] describes two different
categories of media plane B2BUAs, according to the level of
activities performed on the media plane.
When B2BUAs are present in a call between two SIP User Agents (UAs)
they often make end-to-end DTLS-SRTP sessions impossible. End-to-end
DTLS-SRTP session means that man-in-middle devices cannot break the
DTLS-SRTP session between the endpoints. In other words, the man-in-
middle device cannot create a separate DTLS-SRTP session between the
client and the middle device, on one side, and the middle device and
the remote peer on the other side. B2BUAs may be deployed for
address hiding or media latching [RFC7362], although TURN (and ICE)
is expected to be used more often for this purpose as it provides
better security properties. Such B2BUAs are able to perform their
functions without requiring termination of DTLS-SRTP sessions i.e.
these B2BUAs need not act as DTLS proxy and decrypt the RTP payload.
1.2. Goals and Scope of this Document
A B2BUA could be deployed for address hiding or media latching, as
described in [RFC7362]. Such B2BUAs only terminate the media plane
at the IP and transport (UDP/TCP) layers and may inspect the RTP
headers or RTP Control Protocol (RTCP) packets. The goal of this
specification is to provide guidance on how such B2BUAs function
without breaking the end-to-end DTLS-SRTP session. A B2BUA could
also terminate the media or modify the RTP headers or RTP Control
Protocol (RTCP) packets. Such B2BUAs will not allow end-to-end DTLS-
SRTP. The recommendations made in this document are not expected to
be applied by B2BUAs terminating DTLS-SRTP sessions given deployment
reality.
This specification assumes that a B2BUA is not providing identity
assurance and is not authorized to terminate the DTLS-SRTP session.
A B2BUA that provides identity assurance on behalf of endpoints
behind it can modify any portion of SIP and SDP before it generates
the identity signature. As the B2BUA is generating the identity
signature it is not possible to detect if a B2BUA has terminated the
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DTLS-SRTP session. B2BUAs providing identity assurance and
terminating DTLS-SRTP session are out of scope of this document.
The following sections describe the behavior B2BUAs can follow to
avoid breaking end-to-end DTLS-SRTP sessions.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Transport Address: The combination of an IP address and port number.
The following generalized terms are defined in [RFC3261], Section 6.
B2BUA: a SIP Back-to-Back User Agent, which is the logical
combination of a User Agent Server (UAS) and User Agent Client
(UAC).
UAS: a SIP User Agent Server.
UAC: a SIP User Agent Client.
All of the pertinent B2BUA terminology and taxonomy used in this
document is based on [RFC7092].
It is assumed the reader is already familiar with the fundamental
concepts of the RTP protocol [RFC3550] and its taxonomy [RFC7656], as
well as those of SRTP [RFC3711], and DTLS [RFC6347].
3. B2BUAs Procedures to Allow End-to-End DTLS-SRTP
A B2BUA MUST follow the rules mentioned below to allow end-to-end
DTLS-SRTP session.
1. B2BUAs MUST forward the certificate fingerprint and SDP setup
attribute it receives from one endpoint unmodified towards the
other endpoint and vice-versa.
2. [RFC4474] provides a means for signing portions of SIP requests
in order to provide identity assurance and certificate pinning by
providing an identity signature over the SDP that carries the
fingerprint of keying for DTLS-SRTP [RFC5763]. B2BUAs can
identify that [RFC4474] is used for identity assurance if the SIP
request contains both Identity and Identity-Info headers. In
cases where endpoints use [RFC4474], B2BUAs MUST ensure that it
does not modify any of the information used to construct the
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identity signature. This includes the entire SDP body and
portions of SIP header as described in [RFC4474]. In this case,
a B2BUA cannot act as a media relay B2BUA.
3. [I-D.ietf-stir-rfc4474bis] is introduced to overcome the
limitations of [RFC4474] (discussed in Section 1 of
[I-D.ietf-stir-rfc4474bis]). Unlike [RFC4474],
[I-D.ietf-stir-rfc4474bis] does not generate identity signature
over material that intermediaries in the field commonly alter.
In this case, a B2BUA can act as a media relay B2BUA. B2BUAs can
identify that [I-D.ietf-stir-rfc4474bis] is used for identity
assurance if the SIP request contains an Identity header but does
not include an Identity-Info header. The Identity-Info header is
deprecated in [I-D.ietf-stir-rfc4474bis]. A B2BUA MUST ensure
that it does not modify any of the headers used to construct the
identity signature.
4. Both media relays and media-aware relays MUST NOT modify the
authenticated portion of RTP and RTCP packets, and MUST NOT
modify the authentication tag in the RTP and RTCP packets.
4. Signaling Plane B2BUA Handling of DTLS-SRTP
Section 3.1 of [RFC7092] describes different categories of signaling
plane B2BUAs. This section explains how these B2BUAs are expected to
comply with the recommendations in Section 3.
4.1. Proxy-B2BUAs
Proxy-B2BUAs, as defined in Section 3.1.1 of [RFC7092], modify only
the Via and Record-Route SIP headers. These B2BUAs can continue to
perform their function and still allow end-to-end DTLS-SRTP sessions
since it does not modify any of the headers used to construct the
identity signature.
4.2. Signaling-only and SDP-modifying Signaling-only B2BUAs
These categories of B2BUAs are likely to modify headers that are used
to construct the identity signature. For example, a signaling-only
B2BUA can modify the Contact URI. Such B2BUAs are likely to violate
rule 2 or rule 3 in Section 3. Depending upon the application
requirements, such a B2BUA may be able to limit modification of
header fields to those allowed to be modified by [RFC4474] or
[I-D.ietf-stir-rfc4474bis].
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5. Media Plane B2BUA Handling of DTLS-SRTP
5.1. General
This section describes how the different types of media plane B2BUAs
defined in [RFC7092] are expected to comply with the recommendations
in Section 3.
5.1.1. Media Relay
A media relay, as defined in Section 3.2.1 of [RFC7092], 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.
A media relay B2BUA follows the rule 1 mentioned in Section 3 and
forwards 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.
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+-------+ +------------------+ +-----+
| 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 |
|<----------------------------------------------------|
|(B2BUA changes transport(UDP/TCP) and IP header) |
| | |
| | |
| (9,10)ServerHello + use_srtp |
|---------------------------------------------------->|
|(B2BUA changes transport(UDP/TCP) and IP header) |
| | |
| | |
| (11) | |
| [Certificate exchange between Alice and Bob over |
| DTLS ] | |
| | |
| (12) | |
|<---------SRTP/SRTCP-----------SRTP/SRTCP----------->|
| [B2BUA changes transport(UDP/TCP) and IP headers] |
Figure 1: INVITE with SDP call-flow for Media Relay B2BUA
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.
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If RTP and RTCP traffic is multiplexed as described in [RFC5761] on a
single port then only a single DTLS connection is 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 can happen in parallel; thus, the B2BUA has to be prepared
to receive DTLS, STUN and media on the ports it advertised to Bob in
the SDP offer before it receives a SDP answer from Bob. 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. The
B2BUA cannot decrypt the RTP payload as the payload is encrypted
using the SRTP keys derived from the DTLS connection setup between
Alice and Bob.
If the endpoints use [RFC4474], a B2BUA cannot function as a media-
relay without violating rule 2 in Section 3. If [4474bis] is used, a
B2BUA can modify the IP address in the c= line and the port in the m=
line, as shown in Figure 1, as long as it does not otherwise violate
rule 3 in Section 3.
5.1.2. RTP/RTCP-Aware Media-Aware B2BUA
Unlike the media relay discussed in Section 5.1.1, a media-aware
relay as defined in Section 3.2.2 of [RFC7092], is aware of the type
of media traffic it is receiving. There are two types of media-aware
relays, those that merely inspect the RTP headers and unencrypted
portions of RTCP packets, and those that inspect and modify the RTP
headers and unencrypted portions of RTCP packets.
5.1.2.1. RTP Header and RTCP Packets Inspection
A RTP/RTCP aware media relay does not modify the RTP headers and RTCP
packets but only inspects the packets. Such B2BUAs follow rule 4 in
Section 3 and can continue to do their function while allowing end-
to-end DTLS-SRTP. Inspection by the B2BUA will not reveal the clear-
text for encrypted parts of the SRTP/SRTCP packets.
5.1.2.2. RTP Header and RTCP Packet Modification
A B2BUA cannot modify RTP headers or RTCP packets, as to do so it
would need to act as a DTLS endpoint, terminate the DTLS-SRTP session
and decrypt/re-encrypt RTP packets. If a B2BUA modifies unencrypted
or encrypted portions of the RTP or RTCP packets then the integrity
check will fail and the packet will be dropped by the endpoint. The
unencrypted and encrypted portions of the RTP or RTCP packets are
integrity protected using the HMAC algorithm negotiated during DTLS
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handshake (discussed in Section 4.1.2 of [RFC5764]). B2BUAs have to
follow the rules in Section 3 to avoid breaking integrity of SRTP/
SRTCP streams.
6. Forking Considerations
Due to forking [RFC3261], a SIP request carrying an SDP offer sent by
an endpoint (offerer) can reach multiple remote endpoints. As a
result, multiple DTLS-SRTP sessions can be established, one between
the endpoint that sent the SIP request and each of the remote
endpoints that received the request. B2BUAs have to follow rule 1 in
Section 3 while handling offer/answer and forward the certificate
fingerprints and SDP setup attributes it received in the SDP answer
from each endpoint (answerer) unmodified towards the offerer. Since
each DTLS connection is setup on a unique 5-tuple, B2BUA replaces 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. The B2BUA acting as a media relay
here follows rule 4 mentioned in Section 3.
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)
Figure 2: B2BUA handling multiple answers
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. The B2BUA advertises different B2BUA transport
address in each answer, as shown in Figure2, where XXX and YYY
represent different DTLS-SRTP sessions. The B2BUA replaces 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
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(192.0.2.3:8888). The B2BUA tracks the remote sources (Bob and
Charlie) and associates them to the local sources that are used to
send packets to Alice.
7. Security Considerations
This document describes the behavior B2BUAs must follow to avoid
breaking end-to-end DTLS-SRTP. Media relays that modify RTP or RTCP,
or modify SIP header fields or SDP fields that are protected by the
identity signature, are incompatible with end-to-end DTLS-SRTP. Such
relays are out of scope for this document. Security considerations
discussed in [RFC5763] are also applicable to this document. In
addition, the B2BUA behaviors outlined in this document do not impact
the security and integrity of a DTLS-SRTP session or the data
exchanged over it. A malicious B2BUA can try to break into the DTLS
connection, but such an attack can be prevented using the identity
validation mechanism discussed in [RFC4474] or
[I-D.ietf-stir-rfc4474bis]. Either the endpoints or authentication
service proxies involved in the call can use the identity validation
mechanisms discussed in [RFC4474] or [I-D.ietf-stir-rfc4474bis] to
validate the identity of peers and detect malicious B2BUA's that can
attempt to terminate the DTLS connection to decrypt the RTP payload.
8. IANA Considerations
This document makes no request of IANA.
9. Acknowledgments
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,
Christer Holmberg, Colin Perkins, Ben Campbell and Alissa Cooper for
their constructive comments, suggestions, and early reviews that were
critical to the formulation and refinement of this document. The
authors would also like to thank Dan Romascanu, Vijay K. Gurbani,
Francis Dupont, Paul Wouters and Stephen Farrell for their review and
feedback of this document.
10. Contributors
Rajeev Seth provided substantial contributions to this document.
11. References
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11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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, <http://www.rfc-editor.org/info/rfc3550>.
[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,
<http://www.rfc-editor.org/info/rfc3711>.
[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, <http://www.rfc-editor.org/info/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,
<http://www.rfc-editor.org/info/rfc5764>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
11.2. Informative References
[I-D.ietf-stir-rfc4474bis]
Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-08
(work in progress), March 2016.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
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[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474,
DOI 10.17487/RFC4474, August 2006,
<http://www.rfc-editor.org/info/rfc4474>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/RFC5761, April 2010,
<http://www.rfc-editor.org/info/rfc5761>.
[RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents",
RFC 7092, DOI 10.17487/RFC7092, December 2013,
<http://www.rfc-editor.org/info/rfc7092>.
[RFC7362] Ivov, E., Kaplan, H., and D. Wing, "Latching: Hosted NAT
Traversal (HNT) for Media in Real-Time Communication",
RFC 7362, DOI 10.17487/RFC7362, September 2014,
<http://www.rfc-editor.org/info/rfc7362>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.
Authors' Addresses
Ram Mohan Ravindranath
Cisco
Cessna Business Park
Sarjapur-Marathahalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: rmohanr@cisco.com
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Tirumaleswar Reddy
Cisco
Cessna Business Park
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
Gonzalo Salgueiro
Cisco Systems, Inc.
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
US
Email: gsalguei@cisco.com
Victor Pascual
Oracle
Barcelona, Spain
Email: victor.pascual.avila@oracle.com
Parthasarathi Ravindran
Nokia Networks
Bangalore, Karnataka
India
Email: partha@parthasarathi.co.in
Ravindranath, et al. Expires October 6, 2016 [Page 13]
