draft-ietf-straw-b2bua-dtls-srtp-12.txt   rfc7879.txt 
STRAW R. Ravindranath Internet Engineering Task Force (IETF) R. Ravindranath
Internet-Draft T. Reddy Request for Comments: 7879 T. Reddy
Intended status: Standards Track G. Salgueiro Category: Standards Track G. Salgueiro
Expires: October 6, 2016 Cisco ISSN: 2070-1721 Cisco
V. Pascual V. Pascual
Oracle Oracle
Parthasarathi. Ravindran P. Ravindran
Nokia Networks Nokia Networks
April 4, 2016 May 2016
DTLS-SRTP Handling in Session Initiation Protocol (SIP) Back-to-Back DTLS-SRTP Handling in SIP Back-to-Back User Agents
User Agents (B2BUAs)
draft-ietf-straw-b2bua-dtls-srtp-12
Abstract Abstract
Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUA) Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
exist on the signaling and media paths between the endpoints. This exist on the signaling and media paths between the endpoints. This
document describes the behavior of B2BUAs when Secure Real-time document describes the behavior of B2BUAs when Secure Real-time
Transport (SRTP) security context is set up with the Datagram Transport (SRTP) security context is set up with the Datagram
Transport Layer Security (DTLS) protocol. Transport Layer Security (DTLS) protocol.
Status of This Memo Status of This Memo
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Goals and Scope of this Document . . . . . . . . . . . . 3 1.2. Goals and Scope of this Document . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. B2BUAs Procedures to Allow End-to-End DTLS-SRTP . . . . . . . 4 3. B2BUAs Procedures to Allow End-to-End DTLS-SRTP . . . . . . . 5
4. Signaling Plane B2BUA Handling of DTLS-SRTP . . . . . . . . . 5 4. Signaling-Plane B2BUA Handling of DTLS-SRTP . . . . . . . . . 5
4.1. Proxy-B2BUAs . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Proxy-B2BUAs . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Signaling-only and SDP-modifying Signaling-only B2BUAs . 5 4.2. Signaling-Only and SDP-Modifying Signaling-Only B2BUAs . 6
5. Media Plane B2BUA Handling of DTLS-SRTP . . . . . . . . . . . 6 5. Media-Plane B2BUA Handling of DTLS-SRTP . . . . . . . . . . . 6
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1.1. Media Relay . . . . . . . . . . . . . . . . . . . . . 6 5.1.1. Media Relay . . . . . . . . . . . . . . . . . . . . . 6
5.1.2. RTP/RTCP-Aware Media-Aware B2BUA . . . . . . . . . . 8 5.1.2. RTP- and RTCP-Aware Media-Aware B2BUA . . . . . . . . 8
6. Forking Considerations . . . . . . . . . . . . . . . . . . . 9 6. Forking Considerations . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . 11
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 11 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
1.1. Overview 1.1. Overview
[RFC5763] describes how Session Initiation Protocol (SIP) [RFC3261] [RFC5763] describes how the Session Initiation Protocol (SIP)
can be used to establish a Secure Real-time Transport Protocol (SRTP) [RFC3261] can be used to establish a Secure Real-time Transport
[RFC3711] security context with the Datagram Transport Layer Security Protocol (SRTP) [RFC3711] security context with the Datagram
(DTLS) [RFC6347] protocol. It describes a mechanism for transporting Transport Layer Security (DTLS) protocol [RFC6347]. It describes a
a certificate fingerprint using Session Description Protocol (SDP) mechanism for transporting a certificate fingerprint using the
[RFC4566]. The fingerprint identifies the certificate that will be Session Description Protocol (SDP) [RFC4566]. The fingerprint
presented during the DTLS handshake. DTLS-SRTP is currently defined identifies the certificate that will be presented during the DTLS
for point-to-point media sessions, in which there are exactly two handshake. DTLS-SRTP is currently defined for point-to-point media
participants. Each DTLS-SRTP session (described in Section 3 of sessions, in which there are exactly two participants. Each DTLS-
[RFC5764]) contains a single DTLS connection (if RTP and RTCP are SRTP session (described in Section 3 of [RFC5764]) contains a single
DTLS connection (if RTP and RTP Control Protocol (RTCP) are
multiplexed) or two DTLS connections (if RTP and RTCP are not multiplexed) or two DTLS connections (if RTP and RTCP are not
multiplexed), and either two SRTP contexts (if media traffic is multiplexed), and either two SRTP contexts (if media traffic is
flowing in both directions on the same 5-tuple) or one SRTP context flowing in both directions on the same 5-tuple) or one SRTP context
(if media traffic is only flowing in one direction). (if media traffic is only flowing in one direction).
In many SIP deployments, SIP Back-to-Back User Agents (B2BUA) In many SIP deployments, SIP Back-to-Back User Agents (B2BUA)
entities exist on the SIP signaling path between the endpoints. As entities exist on the SIP-signaling path between the endpoints. As
described in [RFC7092], these B2BUAs can modify SIP and SDP described in [RFC7092], these B2BUAs can modify SIP and SDP
information. For example, as described in Section 3.1.3 of information. For example, as described in Section 3.1.3 of
[RFC7092], SDP-modifying signaling-only B2BUAs can potentially modify [RFC7092], SDP-modifying signaling-only B2BUAs can potentially modify
the SDP. B2BUAs can also be present on the media path, in which case 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 they modify parts of the SDP information (like IP address, port) and
subsequently modify the RTP headers as well. Such B2BUAs are subsequently modify the RTP headers as well. Such B2BUAs are
referred to as media plane B2BUAs. [RFC7092] describes two different referred to as "media-plane B2BUAs". [RFC7092] describes two
categories of media plane B2BUAs, according to the level of different categories of media-plane B2BUAs, according to the level of
activities performed on the media plane. activities performed on the media plane.
When B2BUAs are present in a call between two SIP User Agents (UAs) 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 they often make end-to-end DTLS-SRTP sessions impossible. An "end-
DTLS-SRTP session means that man-in-middle devices cannot break the to-end DTLS-SRTP session" means that man-in-the-middle devices cannot
DTLS-SRTP session between the endpoints. In other words, the man-in- break the DTLS-SRTP session between the endpoints. In other words,
middle device cannot create a separate DTLS-SRTP session between the the man-in-the-middle device cannot create a separate DTLS-SRTP
client and the middle device, on one side, and the middle device and session between the client and the middle device on one side, and the
the remote peer on the other side. B2BUAs may be deployed for middle device and the remote peer on the other side. B2BUAs may be
address hiding or media latching [RFC7362], although TURN (and ICE) deployed for address hiding or media latching [RFC7362], although
is expected to be used more often for this purpose as it provides Traversal Using Relays around NAT (TURN) and Interactive Connectivity
better security properties. Such B2BUAs are able to perform their Establishment (ICE) are expected to be used more often for this
functions without requiring termination of DTLS-SRTP sessions i.e. purpose as it provides better security properties. Such B2BUAs are
these B2BUAs need not act as DTLS proxy and decrypt the RTP payload. 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 1.2. Goals and Scope of this Document
A B2BUA could be deployed for address hiding or media latching, as A B2BUA could be deployed for address hiding or media latching as
described in [RFC7362]. Such B2BUAs only terminate the media plane described in [RFC7362]. Such B2BUAs only terminate the media plane
at the IP and transport (UDP/TCP) layers and may inspect the RTP at the IP and transport (UDP/TCP) layers and may inspect the RTP
headers or RTP Control Protocol (RTCP) packets. The goal of this headers or RTP Control Protocol (RTCP) packets. The goal of this
specification is to provide guidance on how such B2BUAs function specification is to provide guidance on how such B2BUAs function
without breaking the end-to-end DTLS-SRTP session. A B2BUA could without breaking the end-to-end DTLS-SRTP session. A B2BUA could
also terminate the media or modify the RTP headers or RTP Control also terminate the media, or modify the RTP headers or RTP Control
Protocol (RTCP) packets. Such B2BUAs will not allow end-to-end DTLS- Protocol (RTCP) packets. Such B2BUAs will not allow end-to-end DTLS-
SRTP. The recommendations made in this document are not expected to SRTP. The recommendations made in this document are not expected to
be applied by B2BUAs terminating DTLS-SRTP sessions given deployment be applied by B2BUAs terminating DTLS-SRTP sessions given deployment
reality. reality.
This specification assumes that a B2BUA is not providing identity This specification assumes that a B2BUA is not providing identity
assurance and is not authorized to terminate the DTLS-SRTP session. assurance and is not authorized to terminate the DTLS-SRTP session.
A B2BUA that provides identity assurance on behalf of endpoints A B2BUA that provides identity assurance on behalf of endpoints
behind it can modify any portion of SIP and SDP before it generates behind it can modify any portion of SIP and SDP before it generates
the identity signature. As the B2BUA is generating the identity the identity signature. As the B2BUA is generating the identity
signature it is not possible to detect if a B2BUA has terminated the signature, it is not possible to detect if a B2BUA has terminated the
DTLS-SRTP session. B2BUAs providing identity assurance and DTLS-SRTP session. B2BUAs providing identity assurance and
terminating DTLS-SRTP session are out of scope of this document. terminating DTLS-SRTP sessions are out of scope of this document.
The following sections describe the behavior B2BUAs can follow to The following sections describe the behavior B2BUAs can follow to
avoid breaking end-to-end DTLS-SRTP sessions. avoid breaking end-to-end DTLS-SRTP sessions.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Transport Address: The combination of an IP address and port number. Transport Address: The combination of an IP address and port number.
The following generalized terms are defined in [RFC3261], Section 6. The following generalized terms are defined in [RFC3261], Section 6.
B2BUA: a SIP Back-to-Back User Agent, which is the logical B2BUA: A SIP Back-to-Back User Agent, which is the logical
combination of a User Agent Server (UAS) and User Agent Client combination of a User Agent Server (UAS) and a User Agent Client
(UAC). (UAC).
UAS: a SIP User Agent Server. UAS: A SIP User Agent Server.
UAC: a SIP User Agent Client. UAC: A SIP User Agent Client.
All of the pertinent B2BUA terminology and taxonomy used in this All of the pertinent B2BUA terminology and taxonomy used in this
document is based on [RFC7092]. document are based on [RFC7092].
It is assumed the reader is already familiar with the fundamental It is assumed the reader is already familiar with the fundamental
concepts of the RTP protocol [RFC3550] and its taxonomy [RFC7656], as concepts of the RTP protocol [RFC3550] and its taxonomy [RFC7656], as
well as those of SRTP [RFC3711], and DTLS [RFC6347]. well as those of SRTP [RFC3711] and DTLS [RFC6347].
3. B2BUAs Procedures to Allow End-to-End DTLS-SRTP 3. B2BUAs Procedures to Allow End-to-End DTLS-SRTP
A B2BUA MUST follow the rules mentioned below to allow end-to-end A B2BUA MUST follow the rules mentioned below to allow end-to-end
DTLS-SRTP session. DTLS-SRTP sessions.
1. B2BUAs MUST forward the certificate fingerprint and SDP setup 1. B2BUAs MUST forward the certificate fingerprint and SDP setup
attribute it receives from one endpoint unmodified towards the attribute it receives from one endpoint unmodified towards the
other endpoint and vice-versa. other endpoint and vice versa.
2. [RFC4474] provides a means for signing portions of SIP requests 2. The enhancements described in [RFC4474] provide a means for
in order to provide identity assurance and certificate pinning by signing portions of SIP requests in order to provide identity
providing an identity signature over the SDP that carries the assurance and certificate pinning by providing an identity
fingerprint of keying for DTLS-SRTP [RFC5763]. B2BUAs can signature over the SDP that carries the fingerprint of keying for
identify that [RFC4474] is used for identity assurance if the SIP DTLS-SRTP [RFC5763]. B2BUAs can identify that the enhancements
request contains both Identity and Identity-Info headers. In in [RFC4474] are used for identity assurance if the SIP request
cases where endpoints use [RFC4474], B2BUAs MUST ensure that it contains both Identity and Identity-Info headers. In cases where
does not modify any of the information used to construct the endpoints use [RFC4474], B2BUAs MUST ensure that it does not
identity signature. This includes the entire SDP body and modify any of the information used to construct the identity
portions of SIP header as described in [RFC4474]. In this case, signature. This includes the entire SDP body and portions of the
a B2BUA cannot act as a media relay B2BUA. 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 3. [SIP-ID] is introduced to overcome the limitations of [RFC4474]
limitations of [RFC4474] (discussed in Section 1 of (discussed in Section 1 of [SIP-ID]). Unlike [RFC4474], [SIP-ID]
[I-D.ietf-stir-rfc4474bis]). Unlike [RFC4474], does not generate an identity signature over material that
[I-D.ietf-stir-rfc4474bis] does not generate identity signature intermediaries in the field commonly alter. In this case, a
over material that intermediaries in the field commonly alter. B2BUA can act as a media-relay B2BUA. B2BUAs can identify that
In this case, a B2BUA can act as a media relay B2BUA. B2BUAs can [SIP-ID] is used for identity assurance if the SIP request
identify that [I-D.ietf-stir-rfc4474bis] is used for identity contains an Identity header but does not include an Identity-Info
assurance if the SIP request contains an Identity header but does header. The Identity-Info header is deprecated in [SIP-ID]. A
not include an Identity-Info header. The Identity-Info header is B2BUA MUST ensure that it does not modify any of the headers used
deprecated in [I-D.ietf-stir-rfc4474bis]. A B2BUA MUST ensure to construct the identity signature.
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 4. Both media relays and media-aware relays MUST NOT modify the
authenticated portion of RTP and RTCP packets, and MUST NOT authenticated portion of RTP and RTCP packets, and MUST NOT
modify the authentication tag in the RTP and RTCP packets. modify the authentication tag in the RTP and RTCP packets.
4. Signaling Plane B2BUA Handling of DTLS-SRTP 4. Signaling-Plane B2BUA Handling of DTLS-SRTP
Section 3.1 of [RFC7092] describes different categories of signaling Section 3.1 of [RFC7092] describes different categories of signaling-
plane B2BUAs. This section explains how these B2BUAs are expected to plane B2BUAs. This section explains how these B2BUAs are expected to
comply with the recommendations in Section 3. comply with the recommendations in Section 3.
4.1. Proxy-B2BUAs 4.1. Proxy-B2BUAs
Proxy-B2BUAs, as defined in Section 3.1.1 of [RFC7092], modify only 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 the Via and Record-Route SIP headers. These B2BUAs can continue to
perform their function and still allow end-to-end DTLS-SRTP sessions 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 since none of the headers used to construct the identity signature
identity signature. are modified.
4.2. Signaling-only and SDP-modifying Signaling-only B2BUAs 4.2. Signaling-Only and SDP-Modifying Signaling-Only B2BUAs
These categories of B2BUAs are likely to modify headers that are used These categories of B2BUAs are likely to modify headers that are used
to construct the identity signature. For example, a signaling-only to construct the identity signature. For example, a signaling-only
B2BUA can modify the Contact URI. Such B2BUAs are likely to violate B2BUA can modify the Contact URI. Such B2BUAs are likely to violate
rule 2 or rule 3 in Section 3. Depending upon the application rule 2 or rule 3 in Section 3. Depending upon the application
requirements, such a B2BUA may be able to limit modification of requirements, such a B2BUA may be able to limit modification of
header fields to those allowed to be modified by [RFC4474] or header fields to those allowed to be modified by [RFC4474] or
[I-D.ietf-stir-rfc4474bis]. [SIP-ID].
5. Media Plane B2BUA Handling of DTLS-SRTP 5. Media-Plane B2BUA Handling of DTLS-SRTP
5.1. General 5.1. General
This section describes how the different types of media plane B2BUAs This section describes how the different types of media-plane B2BUAs
defined in [RFC7092] are expected to comply with the recommendations defined in [RFC7092] are expected to comply with the recommendations
in Section 3. in Section 3.
5.1.1. Media Relay 5.1.1. Media Relay
A media relay, as defined in Section 3.2.1 of [RFC7092], from an From an application-layer point of view, a media relay (as defined in
application layer point-of-view, forwards all packets it receives on Section 3.2.1 of [RFC7092]) forwards all packets it receives on a
a negotiated connection, without inspecting or modifying the packet negotiated connection, without inspecting or modifying the packet
contents. A media relay only modifies the transport layer (UDP/TCP) contents. A media relay only modifies the transport layer (UDP/TCP)
and IP headers. and IP headers.
A media relay B2BUA follows the rule 1 mentioned in Section 3 and A media-relay B2BUA follows rule 1 mentioned in Section 3 and
forwards the certificate fingerprint and SDP setup attribute it forwards the certificate fingerprint and SDP setup attribute it
receives from one endpoint unmodified towards the other endpoint and receives from one endpoint unmodified towards the other endpoint and
vice-versa. The example below shows a SIP call establishment flow, vice versa. The following example shows a SIP call establishment
with both SIP endpoints (user agents) using DTLS-SRTP, and a media flow, with both SIP endpoints (user agents) using DTLS-SRTP, and a
relay B2BUA. media-relay B2BUA.
+-------+ +------------------+ +-----+ +-------+ +-------------------+ +-----+
| Alice | | MediaRelay B2BUA | | Bob | | Alice | | Media-Relay B2BUA | | Bob |
+-------+ +------------------+ +-----+ +-------+ +-------------------+ +-----+
|(1) INVITE | (3)INVITE | |(1) INVITE | (3) INVITE |
| a=setup:actpass | a=setup:actpass | | a=setup:actpass | a=setup:actpass |
| a=fingerprint1 | a=fingerprint1 | | a=fingerprint1 | a=fingerprint1 |
| (alice's IP/port) | (B2BUAs IP/port) | | (Alice's IP/port) | (B2BUAs IP/port) |
|------------------------>|-------------------------->| |------------------------>|-------------------------->|
| | | | | |
| (2) 100 trying | | | (2) 100 trying | |
|<------------------------| | |<------------------------| |
| | (4) 100 trying | | | (4) 100 trying |
| |<--------------------------| | |<--------------------------|
| | | | | |
| | (5)200 OK | | | (5) 200 OK |
| | a=setup:active | | | a=setup:active |
| | a=fingerprint2 | | | a=fingerprint2 |
| | (Bob's IP/port) | | | (Bob's IP/port) |
|<------------------------|<--------------------------| |<------------------------|<--------------------------|
| (6) 200 OK | | | (6) 200 OK | |
| a=setup:active | | | a=setup:active | |
| a=fingerprint2 | | | a=fingerprint2 | |
| B2BUAs IP/port | | | B2BUAs IP/port | |
| (7, 8)ClientHello + use_srtp | | (7, 8) ClientHello + use_srtp |
|<----------------------------------------------------| |<----------------------------------------------------|
|(B2BUA changes transport(UDP/TCP) and IP header) | |(B2BUA changes transport(UDP/TCP) and IP header) |
| | | | | |
| | | | | |
| (9,10)ServerHello + use_srtp | | (9,10) ServerHello + use_srtp |
|---------------------------------------------------->| |---------------------------------------------------->|
|(B2BUA changes transport(UDP/TCP) and IP header) | |(B2BUA changes transport(UDP/TCP) and IP header) |
| | | | | |
| | | | | |
| (11) | | | (11) | |
| [Certificate exchange between Alice and Bob over | | [Certificate exchange between Alice and Bob over |
| DTLS ] | | | DTLS ] | |
| | | | | |
| (12) | | | (12) | |
|<---------SRTP/SRTCP-----------SRTP/SRTCP----------->| |<---------SRTP/SRTCP-----------SRTP/SRTCP----------->|
| [B2BUA changes transport(UDP/TCP) and IP headers] | | [B2BUA changes transport(UDP/TCP) and IP headers] |
Figure 1: INVITE with SDP call-flow for Media Relay B2BUA Figure 1: INVITE with SDP Call Flow for Media-Relay B2BUA
NOTE: For brevity the entire value of the SDP fingerprint attribute Note: For brevity, the entire value of the SDP fingerprint attribute
is not shown. The example here shows only one DTLS connection for is not shown. The example here shows only one DTLS connection for
the sake of simplicity. In reality depending on whether the RTP and the sake of simplicity. In reality, depending on whether the RTP and
RTCP flows are multiplexed or demultiplexed there will be one or two RTCP flows are multiplexed or demultiplexed, there will be one or two
DTLS connections. DTLS connections.
If RTP and RTCP traffic is multiplexed as described in [RFC5761] on a If RTP and RTCP traffic is multiplexed on a single port as described
single port then only a single DTLS connection is required between in [RFC5761], then only a single DTLS connection is required between
the peers. If RTP and RTCP are not multiplexed, then the peers would 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 have to establish two DTLS connections. In this case, after
receives an INVITE request, triggers the establishment of a DTLS receiving an INVITE request, Bob triggers the establishment of a DTLS
connection. Note that the DTLS handshake and the sending of INVITE connection. Note that the DTLS handshake and the sending of the
response can happen in parallel; thus, the B2BUA has to be prepared INVITE response can happen in parallel; thus, the B2BUA has to be
to receive DTLS, STUN and media on the ports it advertised to Bob in prepared to receive DTLS, Session Traversal Utilities for NAT (STUN),
the SDP offer before it receives a SDP answer from Bob. Since a media and media on the ports it advertised to Bob in the SDP offer before
relay B2BUA does not differentiate between a DTLS message, RTP or any it receives an SDP answer from Bob. Since a media-relay B2BUA does
packet it receives, it only changes the transport layer (UDP/TCP) and not differentiate between a DTLS message, RTP, or any packet it
IP headers and forwards the packet towards the other endpoint. The receives, it only changes the transport layer (UDP/TCP) and IP
B2BUA cannot decrypt the RTP payload as the payload is encrypted 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 using the SRTP keys derived from the DTLS connection setup between
Alice and Bob. Alice and Bob.
If the endpoints use [RFC4474], a B2BUA cannot function as a media- 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 relay without violating rule 2 in Section 3. If [SIP-ID] is used, a
B2BUA can modify the IP address in the c= line and the port in the m= 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 line in the SDP as long as it does not otherwise violate rule 3 in
rule 3 in Section 3. Section 3.
5.1.2. RTP/RTCP-Aware Media-Aware B2BUA 5.1.2. RTP- and RTCP-Aware Media-Aware B2BUA
Unlike the media relay discussed in Section 5.1.1, a media-aware 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 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 of media traffic it is receiving. There are two types of media-aware
relays, those that merely inspect the RTP headers and unencrypted relays, those that merely inspect the RTP headers and unencrypted
portions of RTCP packets, and those that inspect and modify the RTP portions of RTCP packets, and those that inspect and modify the RTP
headers and unencrypted portions of RTCP packets. headers and unencrypted portions of RTCP packets.
5.1.2.1. RTP Header and RTCP Packets Inspection 5.1.2.1. RTP Header and RTCP Packets Inspection
A RTP/RTCP aware media relay does not modify the RTP headers and RTCP An RTP-/RTCP-aware media relay does not modify the RTP headers and
packets but only inspects the packets. Such B2BUAs follow rule 4 in RTCP packets but only inspects the packets. Such B2BUAs follow rule
Section 3 and can continue to do their function while allowing end- 4 in Section 3 and can continue to do their function while allowing
to-end DTLS-SRTP. Inspection by the B2BUA will not reveal the clear- end-to-end DTLS-SRTP. Inspection by the B2BUA will not reveal the
text for encrypted parts of the SRTP/SRTCP packets. clear-text for encrypted parts of the SRTP/SRTCP packets.
5.1.2.2. RTP Header and RTCP Packet Modification 5.1.2.2. RTP Header and RTCP Packet Modification
A B2BUA cannot modify RTP headers or RTCP packets, as to do so it 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 would need to act as a DTLS endpoint, terminate the DTLS-SRTP
and decrypt/re-encrypt RTP packets. If a B2BUA modifies unencrypted session, and decrypt/re-encrypt RTP packets. If a B2BUA modifies
or encrypted portions of the RTP or RTCP packets then the integrity unencrypted or encrypted portions of the RTP or RTCP packets, then
check will fail and the packet will be dropped by the endpoint. The the integrity check will fail and the packet will be dropped by the
unencrypted and encrypted portions of the RTP or RTCP packets are endpoint. The unencrypted and encrypted portions of the RTP or RTCP
integrity protected using the HMAC algorithm negotiated during DTLS packets are integrity protected using the HMAC algorithm negotiated
handshake (discussed in Section 4.1.2 of [RFC5764]). B2BUAs have to during the DTLS handshake (discussed in Section 4.1.2 of [RFC5764]).
follow the rules in Section 3 to avoid breaking integrity of SRTP/ B2BUAs have to follow the rules in Section 3 to avoid breaking the
SRTCP streams. integrity of SRTP/SRTCP streams.
6. Forking Considerations 6. Forking Considerations
Due to forking [RFC3261], a SIP request carrying an SDP offer sent by Due to forking [RFC3261], a SIP request carrying an SDP offer sent by
an endpoint (offerer) can reach multiple remote endpoints. As a an endpoint (offerer) can reach multiple remote endpoints. As a
result, multiple DTLS-SRTP sessions can be established, one between result, multiple DTLS-SRTP sessions can be established, one between
the endpoint that sent the SIP request and each of the remote the endpoint that sent the SIP request and each of the remote
endpoints that received the request. B2BUAs have to follow rule 1 in endpoints that received the request. B2BUAs have to follow rule 1 in
Section 3 while handling offer/answer and forward the certificate Section 3 while handling offer/answer and forward the certificate
fingerprints and SDP setup attributes it received in the SDP answer fingerprints and SDP setup attributes it received in the SDP answer
from each endpoint (answerer) unmodified towards the offerer. Since from each endpoint (answerer) unmodified towards the offerer. Since
each DTLS connection is setup on a unique 5-tuple, B2BUA replaces the each DTLS connection is set up on a unique 5-tuple, B2BUA replaces
answerer's transport addresses in each answer with its unique the answerer's transport addresses in each answer with its unique
transport addresses so that the offerer can establish a DTLS transport addresses so that the offerer can establish a DTLS
connection with each answerer. The B2BUA acting as a media relay connection with each answerer. The B2BUA, acting as a media relay
here follows rule 4 mentioned in Section 3. here, follows rule 4 mentioned in Section 3.
Bob (192.0.2.1:6666) Bob (192.0.2.1:6666)
/ /
/ /
/ DTLS-SRTP=XXX / DTLS-SRTP=XXX
/ /
/ /
DTLS-SRTP=XXX v DTLS-SRTP=XXX v
<-----------> (192.0.2.3:7777) <-----------> (192.0.2.3:7777)
Alice (192.0.2.0:5555) B2BUA Alice (192.0.2.0:5555) B2BUA
<-----------> (192.0.2.3:8888) <-----------> (192.0.2.3:8888)
DTLS-SRTP=YYY ^ DTLS-SRTP=YYY ^
\ \
\ DTLS-SRTP=YYY \ DTLS-SRTP=YYY
\ \
\ \
\ \
Charlie (192.0.2.2:6666) Charlie (192.0.2.2:6666)
Figure 2: B2BUA handling multiple answers Figure 2: B2BUA Handling Multiple Answers
For instance, as shown in Figure 2 Alice sends a request with an For instance, as shown in Figure 2, Alice sends a request with an
offer, and the request is forked. Alice receives answers from both offer and the request is forked. Alice receives answers from both
Bob and Charlie. The B2BUA advertises different B2BUA transport Bob and Charlie. The B2BUA advertises different B2BUA transport
address in each answer, as shown in Figure2, where XXX and YYY addresses in each answer, as shown in Figure 2, where XXX and YYY
represent different DTLS-SRTP sessions. The B2BUA replaces Bob's represent different DTLS-SRTP sessions. The B2BUA replaces Bob's
transport address (192.0.2.1:6666) in the answer with its transport transport address (192.0.2.1:6666) in the answer with its transport
address (192.0.2.3:7777) and Charlie's transport address address (192.0.2.3:7777) and Charlie's transport address
(192.0.2.2:6666) in the answer with its transport address (192.0.2.2:6666) in the answer with its transport address
(192.0.2.3:8888). The B2BUA tracks the remote sources (Bob and (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 Charlie) and associates them to the local sources that are used to
send packets to Alice. send packets to Alice.
7. Security Considerations 7. Security Considerations
This document describes the behavior B2BUAs must follow to avoid This document describes the behavior B2BUAs must follow to avoid
breaking end-to-end DTLS-SRTP. Media relays that modify RTP or RTCP, 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 or modify SIP header fields or SDP fields that are protected by the
identity signature, are incompatible with end-to-end DTLS-SRTP. Such identity signature, are incompatible with end-to-end DTLS-SRTP. Such
relays are out of scope for this document. Security considerations relays are out of scope for this document. Security considerations
discussed in [RFC5763] are also applicable to this document. In discussed in [RFC5763] are also applicable to this document. In
addition, the B2BUA behaviors outlined in this document do not impact addition, the B2BUA behaviors outlined in this document do not impact
the security and integrity of a DTLS-SRTP session or the data 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 exchanged over it. A malicious B2BUA can try to break into the DTLS
connection, but such an attack can be prevented using the identity connection, but such an attack can be prevented using the identity
validation mechanism discussed in [RFC4474] or validation mechanism discussed in [RFC4474] or [SIP-ID]. Either the
[I-D.ietf-stir-rfc4474bis]. Either the endpoints or authentication endpoints or the authentication service proxies involved in the call
service proxies involved in the call can use the identity validation can use the identity validation mechanisms discussed in [RFC4474] or
mechanisms discussed in [RFC4474] or [I-D.ietf-stir-rfc4474bis] to [SIP-ID] to validate the identity of peers and detect malicious
validate the identity of peers and detect malicious B2BUA's that can B2BUAs that can attempt to terminate the DTLS connection to decrypt
attempt to terminate the DTLS connection to decrypt the RTP payload. 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. 8. References
11. References 8.1. Normative References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>. July 2003, <http://www.rfc-editor.org/info/rfc3550>.
skipping to change at page 11, line 37 skipping to change at page 11, line 40
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010, DOI 10.17487/RFC5764, May 2010,
<http://www.rfc-editor.org/info/rfc5764>. <http://www.rfc-editor.org/info/rfc5764>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>. January 2012, <http://www.rfc-editor.org/info/rfc6347>.
11.2. Informative References 8.2. 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, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002, DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>. <http://www.rfc-editor.org/info/rfc3261>.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for [RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, Initiation Protocol (SIP)", RFC 4474,
skipping to change at page 12, line 36 skipping to change at page 12, line 30
Traversal (HNT) for Media in Real-Time Communication", Traversal (HNT) for Media in Real-Time Communication",
RFC 7362, DOI 10.17487/RFC7362, September 2014, RFC 7362, DOI 10.17487/RFC7362, September 2014,
<http://www.rfc-editor.org/info/rfc7362>. <http://www.rfc-editor.org/info/rfc7362>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656, for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015, DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>. <http://www.rfc-editor.org/info/rfc7656>.
[SIP-ID] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", Work in Progress,
draft-ietf-stir-rfc4474bis-09, May 2016
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.
Contributors
Rajeev Seth provided substantial contributions to this document.
Authors' Addresses Authors' Addresses
Ram Mohan Ravindranath Ram Mohan Ravindranath
Cisco Cisco
Cessna Business Park Cessna Business Park
Sarjapur-Marathahalli Outer Ring Road Sarjapur-Marathahalli Outer Ring Road
Bangalore, Karnataka 560103 Bangalore, Karnataka 560103
India India
Email: rmohanr@cisco.com Email: rmohanr@cisco.com
skipping to change at page 13, line 17 skipping to change at page 13, line 29
Sarjapur Marathalli Outer Ring Road Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103 Bangalore, Karnataka 560103
India India
Email: tireddy@cisco.com Email: tireddy@cisco.com
Gonzalo Salgueiro Gonzalo Salgueiro
Cisco Systems, Inc. Cisco Systems, Inc.
7200-12 Kit Creek Road 7200-12 Kit Creek Road
Research Triangle Park, NC 27709 Research Triangle Park, NC 27709
US United States
Email: gsalguei@cisco.com Email: gsalguei@cisco.com
Victor Pascual Victor Pascual
Oracle Oracle
Barcelona, Spain Barcelona, Spain
Email: victor.pascual.avila@oracle.com Email: victor.pascual.avila@oracle.com
Parthasarathi Ravindran Parthasarathi Ravindran
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