Internet DRAFT - draft-ram-straw-b2bua-dtls-srtp
draft-ram-straw-b2bua-dtls-srtp
STRAW R. Ravindranath
Internet-Draft T. Reddy
Intended status: Standards Track G. Salgueiro
Expires: September 9, 2015 Cisco
V. Pascual
Quobis
Parthasarathi. Ravindran
Nokia Solutions and Networks
March 8, 2015
DTLS-SRTP Handling in Session Initiation Protocol (SIP) Back-to-Back
User Agents (B2BUAs)
draft-ram-straw-b2bua-dtls-srtp-03
Abstract
Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
often function on the media plane, rather than just on the signaling
path. This document describes the behavior B2BUAs should follow when
acting on the media plane that use Secure Real-time Transport (SRTP)
security context setup with Datagram Transport Layer Security (DTLS)
protocol.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on September 9, 2015.
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Media Plane B2BUAs . . . . . . . . . . . . . . . . . . . . . 4
3.1. Media Relay . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Media Aware Relay . . . . . . . . . . . . . . . . . . . . 6
3.2.1. RTP and RTCP Header Inspection . . . . . . . . . . . 6
3.2.2. RTP and RTCP Header Modification . . . . . . . . . . 6
3.3. Media Plane B2BUA with NAT handling . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 Datagram Transport Layer Security
(DTLS) [RFC4347] protocol. It describes a mechanism of transporting
a certificate fingerprint in the Session Description Protocol (SDP)
[RFC4566], which 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 contains a single DTLS association, and either two
SRTP contexts (if media traffic is flowing in both directions on the
same host/port quartet) or one SRTP context (if media traffic is only
flowing in one direction).
In many SIP deployments, SIP entities exist in the SIP signaling path
between the originating and final terminating endpoints. These SIP
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entities, as described in [RFC7092], modify SIP and SDP bodies and
also are likely to be on the media path. Such entities, when present
in the signaling/media path, are likely to do several things. For
example, some B2BUAs modify parts of the SDP body (like IP address,
port) and subsequently modify the RTP headers as well.
1.2. Goals
[RFC7092] describes two different categories of such B2BUAs,
according to the level of activities performed on the media plane:
A B2BUA that act as a simple media relay effectively unaware of
anything that is transported and only modifies the UDP/IP header
of the packets.
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.
The following sections describe the behaviour B2BUAs should follow in
order to avoid any impact on end-to-end DTLS-SRTP streams.
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].
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
[I-D.ietf-avtext-rtp-grouping-taxonomy], as well as those of SRTP
[RFC3711], and DTLS [RFC4347].
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3. Media Plane B2BUAs
3.1. 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 UDP connection, without inspecting or modifying them.
It forwards the UDP payload as-is changing only the UDP/IP header.
A media relay B2BUA MUST forward the certificate fingerprint and
setup attribute it receives in the SDP from the originating endpoint
as-is to the remote side and vice-versa. The example below shows an
"INVITE with SDP" SIP call flow, with both SIP user agents doing
DTLS-SRTP and a media relay B2BUA that changes only the IP address/
port.
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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) | (B2BUA's 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 | |
| B2BUA's address,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 just changes UDP/IP header] |
Figure 1: INVITE with SDP callflow for Media Relay B2BUA
NOTE: For brevity the entire fingerprint attribute is not shown.
For each RTP or RTCP flow, the peers do a DTLS handshake on the same
source and destination port pair to establish a DTLS association. In
this case, Bob, after he receives an INVITE, triggers a DTLS
connection. Note the DTLS handshake and the response to the INVITE
may happen in parallel; thus, the B2BUA SHOULD be prepared to receive
media on the ports it advertised to Bob in the OFFER. Since a media
relay B2BUA does not differentiate between a DTLS, RTP or any packet
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sent it receives, it just changes the UDP/IP addresses and forwards
the packet on either leg.
[I-D.ietf-stir-rfc4474bis] 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 headers used to construct
the signature.
In the above example Alice may be authorized by the authorization
server (SIP proxy) in its domain using the procedures in section 5 of
[I-D.ietf-stir-rfc4474bis]. In such a case, if B2BUA changes 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
[I-D.ietf-stir-rfc4474bis] resulting in a 438 error response being
returned.
3.2. Media Aware Relay
A media-aware relay, unlike the media relay discussed in the previous
section, is actually aware of the media traffic it is handling. A
media-aware relay inspects SRTP and SRTCP packets flowing through it,
and may or may not modify the headers of the packets before
forwarding them.
3.2.1. RTP and RTCP Header Inspection
B2BUAs explained in Section 3.2.2 of [RFC7092] do not modify the RTP
and RTCP headers but only inspect the headers. Such B2BUA MUST not
terminate the DTLS-SRTP session.
3.2.2. RTP and RTCP Header Modification
In addition to inspecting the RTP and RTCP headers, the B2BUAs
explained in section 3.2.2 [RFC7092], can also potentially modify
them. To modify media headers a B2BUA needs to act as a DTLS
intermediary and terminate the DTLS connection so it can decrypt/re-
encrypt RTP packets. This breaks end-to-end security. This security
and privacy problem can be addressed by having separate keys for
encrypting the RTP header and media payload as discussed in
[I-D.jones-avtcore-private-media-reqts], in which case the B2BUA is
not aware of the keys used to decrypt the media payload.
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3.3. Media Plane B2BUA with NAT handling
DTLS-SRTP handshakes and offer/answer can happen in parallel. If a
UA is behind a NAT and acting as a DTLS server, the ClientHello
message from a B2BUA(DTLS client) is likely to be lost, as described
in section 7.3 of [RFC5763]. In order to overcome this problem, a UA
and B2BUA must support ICE as discussed in section 7.3 of [RFC5763].
If ICE check is successful then UA will receive ClientHello packet
from B2BUA.
4. Security Considerations
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. It does not
introduce any specific security considerations beyond those detailed
in [RFC5763]. The B2BUA behaviors outlined here also do not impact
the security and integrity of the DTLS-SRTP session nor the data
exchanged over it. A malicious B2BUA can try to break into the DTLS
session, but such an attack can be prevented using the identity
validation mechanism discussed in [I-D.ietf-stir-rfc4474bis].
5. IANA Considerations
This document makes no request of IANA.
6. Acknowledgments
Special thanks to Lorenzo Miniero, Ranjit Avarsala, Hadriel Kaplan,
Muthu Arul Mozhi, Paul Kyzivat, Peter Dawes, Brett Tate, Dan Wing and
Charles Eckel for their constructive comments, suggestions, and early
reviews that were critical to the formulation and refinement of this
document.
7. Contributors
Rajeev Seth provided substantial contributions to this document.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
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[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[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, 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, May 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
8.2. Informative References
[I-D.ietf-avtext-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
"A Taxonomy of Grouping Semantics and Mechanisms for Real-
Time Transport Protocol (RTP) Sources", draft-ietf-avtext-
rtp-grouping-taxonomy-06 (work in progress), March 2015.
[I-D.ietf-stir-rfc4474bis]
Peterson, J., Jennings, C., and E. Rescorla,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-02
(work in progress), October 2014.
[I-D.ietf-straw-b2bua-rtcp]
Miniero, L., Murillo, S., and V. Pascual, "Guidelines to
support RTCP end-to-end in Back-to-Back User Agents
(B2BUAs)", draft-ietf-straw-b2bua-rtcp-03 (work in
progress), February 2015.
[I-D.jones-avtcore-private-media-reqts]
Jones, P., Ismail, N., Benham, D., Buckles, N., Mattsson,
J., Cheng, Y., and R. Barnes, "Requirements for Private
Media in a Switched Conferencing Environment", draft-
jones-avtcore-private-media-reqts-01 (work in progress),
March 2015.
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[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents", RFC
7092, December 2013.
Authors' Addresses
Ram Mohan Ravindranath
Cisco
Cessna Business Park
Sarjapur-Marathahalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: rmohanr@cisco.com
Tirumaleswar Reddy
Cisco
Cessna Business Park, Varthur Hobli
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
Quobis
Spain
Email: victor.pascual@quobis.com
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Parthasarathi Ravindran
Nokia Solutions and Networks
Bangalore, Karnataka
India
Email: partha@parthasarathi.co.in
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