Internet DRAFT - draft-ietf-straw-b2bua-stun
draft-ietf-straw-b2bua-stun
STRAW Ram. Ravindranath
Internet-Draft T. Reddy
Intended status: Standards Track G. Salgueiro
Expires: November 19, 2015 Cisco
May 18, 2015
Session Traversal Utilities for NAT (STUN) Message Handling for Session
Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
draft-ietf-straw-b2bua-stun-08
Abstract
Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
are often designed to be on the media path, rather than just
intercepting signaling. This means that B2BUAs often act on the
media path leading to separate media legs that the B2BUA correlates
and bridges together. When acting on the media path, B2BUAs are
likely to receive Session Traversal Utilities for NAT (STUN) packets
as part of Interactive Connectivity Establishment (ICE) processing.
This document defines behavior for a B2BUA performing ICE processing.
The goal of this draft is to ensure that B2BUAs properly handles SIP
messages that carry ICE semantics in Session Description
Protocol(SDP) and STUN messages received as part of the ICE
procedures for NAT and Firewall traversal of multimedia sessions.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 19, 2015.
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Copyright Notice
Copyright (c) 2015 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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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. SDP-Modifying Signaling-only B2BUA . . . . . . . . . . . . . 5
4. Media Plane B2BUAs . . . . . . . . . . . . . . . . . . . . . 5
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Mandatory ICE Termination with B2BUA . . . . . . . . . . 6
4.3. Optional ICE Termination with B2BUA . . . . . . . . . . . 8
4.4. STUN Handling in B2BUA with Forked Signaling . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
In many Session Initiation Protocol (SIP) deployments, SIP entities
exist in the SIP signaling and media path between the originating and
final terminating endpoints, which go beyond the definition of a
traditional SIP proxy. These SIP entities, commonly known as Back-
to-Back User Agents (B2BUAs), are described in [RFC7092] and often
perform functions not defined in Standards Track RFCs.
SIP [RFC3261], and other session control protocols that try to use
direct path for media, are typically difficult to use across Network
Address Translators (NATs). These protocols use IP addresses and
transport port numbers encoded in the signaling, such as the Session
Description Protocol (SDP) [RFC4566] and, in the case of SIP, various
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header fields. Such addresses and ports are unreachable if any peers
are separated by NATs.
Mechanisms such as Session Traversal Utilities for NAT (STUN)
[RFC5389], Traversal Using Relays around NAT (TURN) [RFC5766], and
Interactive Connectivity Establishment (ICE) [RFC5245] did not exist
when protocols like SIP began being deployed. Some mechanisms, such
as the early versions of STUN, started appearing but they were
unreliable and suffered a number of issues typical for UNilateral
Self-Address Fixing (UNSAF) and described in [RFC3424]. For these
reasons, B2BUAs are being used by SIP domains for SIP and media-
related purposes. These B2BUAs use proprietary mechanisms to enable
SIP devices behind NATs to communicate across the NAT.
[RFC7362] describes how B2BUAs can perform Hosted NAT Traversal (HNT)
in certain deployments. Section 5 of [RFC7362] describes some of the
issues with SBCs implementing HNT and offers some mitigation
strategies. The most commonly used approach to solve these issues is
restricted-latching defined in section 5 of [RFC7362], whereby the
B2BUA will not latch to any packets from a source public IP address
other than the one the SIP UA uses for SIP signaling. However, this
is susceptible to attacks, where an attacker who is able to see the
source IP address of the SIP UA may generate packets using the same
IP address. There are other threats described in Section 5 of
[RFC7362] for which Secure Real-time Transport Protocol (SRTP)
[RFC3711] can be used as a solution. However, this would require the
B2BUAs to terminate and re-originate SRTP, which is not always
desirable.
This document describes proper behavior of B2BUAs performing ICE
processing. This includes defining consistent handling of SIP
messages carrying ICE semantics in SDP and STUN messages received as
part of the ICE procedures performed on the media path for NAT and
Firewall traversal of multimedia sessions.
A B2BUA can use ICE [RFC5245], which provides authentication tokens
(conveyed in the ice-ufrag and ice-pwd attributes) that allow the
identity of a peer to be confirmed before engaging in media exchange.
This can solve some of the security concerns with HNT solution.
Further, ICE has other benefits like selecting an address when more
than one address is available (e.g., dual-stack environment where
host can have both IPv4 and IPv6 addresses), verifying that a path
works before connecting the call etc. For these reasons endpoints
often use ICE to pick a candidate pair for media traffic between two
agents.
B2BUAs often operate on the media path and have the ability to modify
SIP headers and SDP bodies as part of their normal operation. Such
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entities, when present on the media path, are likely to take an
active role in the session signaling depending on their level of
activity on the media path. For example, some B2BUAs modify portions
of the SDP body (e.g., IP address, port) and subsequently modify the
media packet headers as well. Section 18.6 of ICE [RFC5245] explains
two different behaviors when B2BUAs are present. Some B2BUAs are
likely to remove all the SDP ICE attributes before sending the SDP
across to the other side. Consequently, the call will appear to both
endpoints as though the other side doesn't support ICE. There are
other types of B2BUAs that pass the ICE attributes without
modification, yet modify the default destination for media contained
in the m= and c= lines and rtcp attribute (defined in [RFC3605]).
This will be detected as an ICE mismatch and ICE processing will be
aborted for the session. The session may continue if the endpoints
are able to reach each other over the default candidate (sent in m=
and c= lines).
Section 3.1.3 of [RFC7092] defines a SDP-Modifying Signaling-only
B2BUA that operates in the signaling plane only and is not in the
media path, but it does modify SDP bodies and is thus aware of and
understands SDP syntax and semantics. Such B2BUA MUST follow the
behavior mentioned in Section 3.
Section 3.2 of [RFC7092] describes three different categories of
B2BUAs that operates on both signaling(SIP and SDP) and media plane
according to the level of involvement and active participation in the
media plane:
o A B2BUA that acts as a simple media relay effectively unaware of
anything that is transported and only modifies the transport
header (could be UDP/IP) of the media packets.
o 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.
o A B2BUA that performs a media-termination role and operates at the
media payload layer, such as RTP/RTCP payload (e.g., a
transcoder).
When B2BUAs that operate on media plane (media relay, media-aware or
media-termination) is involved in a session between two endpoints
performing ICE, then it MUST follow the behavior described in
Section 4.
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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].
All of the pertinent B2BUA terminology and taxonomy used in this
document is defined in [RFC7092].
Network Address Translators (NATs) are widely used in the Internet by
consumers and organizations. Although specific NAT behaviors vary,
this document uses the term "NAT", which maps to NAT and NAPT terms
from [RFC3022], for devices that map any IPv4 or IPv6 address and
transport port number to another IPv4 or IPv6 address and transport
port number. This includes consumer NATs, Firewall-NATs, IPv4-IPv6
NATs, Carrier-Grade NATs (CGNs) [RFC6888], etc.
3. SDP-Modifying Signaling-only B2BUA
An SDP-Modifying Signaling-only B2BUA is one that operates in the
signaling plane only and is not in the media path, but it modifies
SDP bodies as described in section 3.1.3 of [RFC7092]. Such B2BUAs
MUST NOT change IP address in c= line, port in m= line and ICE
semantics of SDP as doing so can cause ICE-mismatch.
4. Media Plane B2BUAs
4.1. Overview
When one or both of the endpoints are behind a NAT, and there is a
B2BUA between the endpoints, the B2BUAs MUST support ICE or at a
minimum support ICE LITE functionality as described in [RFC5245].
Such B2BUAs MUST use the mechanism described in Section 2.2 of
[RFC5245] to demultiplex STUN packets that arrive on the Real-time
Transport Protocol(RTP)/RTP Control Protocol (RTCP) port.
The subsequent sections describe the behavior B2BUAs MUST follow for
handling ICE messages. A B2BUA can terminate ICE and thus have two
ICE contexts with either endpoint. The reason for ICE termination
could be due to the need for B2BUA to be in the media path ( e.g.,
address hiding for privacy, interworking between ICE to no-ICE,
etc.). A B2BUA can also be in optional ICE termination mode and
passes across the candidate list and STUN short-term credentials
(ice-ufrag and ice-pwd attributes) from one endpoint to the other
side after adding its own candidates. A B2BUA can be in optional ICE
termination mode when it does not have a need to be on the media
path. The below sections describes the behaviors for these two
cases.
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4.2. Mandatory ICE Termination with B2BUA
A B2BUA that wishes to always be in the media path follows the below
steps:
o When a B2BUA sends out SDP, it MUST advertise support for ICE and
MAY include B2BUA candidates of different types for each component
of each media stream.
o If the B2BUA is in ICE lite mode as described in Section 2.7 of
[RFC5245] then it MUST send a=ice-lite attribute and MUST include
B2BUAs host candidates for each component of each media stream.
o If the B2BUA supports full ICE then it MAY include B2BUAs
candidates of different types for each component of each media
stream.
o The B2BUA MUST generate new username, password values for ice-
ufrag and ice-pwd attributes when it sends out the SDP and MUST
NOT propagate the ufrag, password values it received in the
incoming SDP. This ensures that the short-term credentials used
for both the legs are different. The short-term credentials
include authentication tokens (conveyed in the ice-ufrag and ice-
pwd attributes), which the B2BUA can use to verify the identity of
the peer. B2BUA terminates the ICE messages on each leg and does
not propagate them.
o The B2BUA MUST NOT propagate the candidate list received in the
incoming SDP to the outbound SDP and instead only advertise its
candidate list. The B2BUA MUST also add its default candidate in
the c= line (IP address) and m= line (port). In this way the
B2BUA will be always in the media path.
o Depending on whether the B2BUA supports ICE lite or full ICE it
implements the appropriate procedures mentioned in [RFC5245] for
ICE connectivity checks.
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+-------+ +------------------+ +-----+
| Alice | | Mediaplane B2BUA | | Bob |
+-------+ +------------------+ +-----+
|(1) INVITE | (3)INVITE |
| a=ice-ufrag1 | a=ice-ufrag2 |
| a=ice-pwd1 | a=ice-pwd2 |
| (Alice's IP, port) | (B2BUAs IP, port) |
|(Alice's candidate list )| (B2BUAs candidate list) |
|------------------------>|-------------------------->|
| | |
| (2) 100 trying | |
|<------------------------| |
| | (4) 100 trying |
| |<--------------------------|
| | (5)200 OK |
| | a=ice-ufrag3 |
| | a=ice-pwd3 |
| | (Bob's IP, port) |
| | (Bob's candidate list) |
| |<--------------------------|
| (6) 200 OK | |
| a=ice-ufrag4 |-----------ACK------------>|
| a=ice-pwd4 | (7) |
| B2BUAs IP,port | |
| (B2BUAs cand list1) | |
|<------------------------| |
|--------ACK------------->| |
| (8) | |
| | |
|<----ICE Connectivity 1->| |
| checks+conclusion |<-----ICE Connectivity 2-->|
| (9) | checks +conclusion |
| | (10) |
|<-------Media packets -->|<----Media packets-------->|
| (13) | (14) |
| | |
|<---ICE keepalives 1---->| |
| (15) |<----ICE keep alives 2---->|
(16)
Figure 1: INVITE with SDP having ICE and with a Media Plane B2BUA
terminating ICE
The above figure shows an example call flow with two endpoints Alice
and Bob doing ICE and a B2BUA handing STUN messages from both the
endpoints. For the sake of brevity the entire ICE SDP attributes are
not shown. Also the STUN messages exchanged as part of ICE
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connectivity checks are not shown. Key steps to note from the call
flow are:
o Alice sends an INVITE with SDP having ICE candidates.
o B2BUA modifies the received SDP from Alice by removing the
received candidate list, gathers its own candidates, generates new
username, password values for ice-ufrag and ice-pwd attributes.
The B2BUA also changes the c= line and m= line to have its default
candidate and forwards the INVITE (3) to Bob.
o Bob responds (5) to the INVITE with his own list of candidates.
o B2BUA responds to the INVITE from Alice with SDP having B2BUAs
candidate list. B2BUA generates new username, password values for
ice-ufrag and ice-pwd attributes in the 200 OK response (6).
o ICE Connectivity checks happen between Alice and the B2BUA in step
9. Depending on whether the B2BUA supports ICE or ICE lite it
will follow the appropriate procedures mentioned in [RFC5245].
ICE Connectivity checks also happen between Bob and the B2BUA in
step 10. Step 9 and 10 happen in parallel. The B2BUA always
terminates the ICE messages on each leg and has two independent
ICE contexts running.
o Media flows between Alice and Bob via B2BUA (Step 13, 14).
o STUN keepalives would be used between Alice and B2BUA (step 15)
and between Bob and B2BUA (step 16) to keep NAT and Firewall
bindings alive.
Since there are two independent ICE contexts on either side of the
B2BUA it is possible that ICE checks will conclude on one side before
concluding on the other side. This could result in an ongoing media
session for one end, while the other is still being set up. Any such
media received by the B2BUA would continue to be sent to the other
side on the default candidate address (that was sent in c= line).
4.3. Optional ICE Termination with B2BUA
A B2BUA willing to be in the media path only for NAT traversal, but
does not otherwise require to be in the media path can do the
following steps mentioned in this section.
o When a B2BUA receives an incoming SDP with ICE semantics it copies
the received candidate list and appends its own candidate list in
the outgoing SDP. The B2BUA also copies the ufrag/password values
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it received in the incoming SDP to the outgoing SDP and then sends
out the SDP.
o The B2BUAs candidates MAY have lower-priority than the candidates
provided by the endpoint, this way endpoint and remote peer
candidate pairs are tested first before trying candidate pairs
with B2BUA candidates.
o After offer/answer is complete, the endpoints will have both the
B2BUAs and remote peer candidates. It will then use ICE
procedures described in Section 8 of [RFC5245] to nominate a
candidate pair for sending and receiving media streams.
o With this approach the B2BUA will be in the media path only if the
ICE checks between all the candidate pairs formed from both the
endpoints fail.
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+-------+ +------------------+ +-----+
| Alice | | Mediaplane B2BUA | | Bob |
+-------+ +------------------+ +-----+
|(1) INVITE | (3)INVITE |
| a=ice-ufrag1 | a=ice-ufrag1 |
| a=ice-pwd1 | a=ice-pwd1 |
| (Alice's IP, port) | (Alices's IP, port) |
|(Alice's candidate list )| (Alice's Candidate list + |
| | B2BUAs candidate list1) |
|------------------------>|-------------------------->|
| | |
| (2) 100 trying | |
|<------------------------| |
| | (4) 100 trying |
| |<--------------------------|
| | (5)200 OK |
| | a=ice-ufrag2 |
| | a=ice-pwd2 |
| | (Bob's IP, port) |
| | (Bob's candidate list) |
| |<--------------------------|
| (6) 200 OK | |
| a=ice-ufrag2 |-----------ACK------------>|
| a=ice-pwd2 | (7) |
| (Bobs's IP,port) | |
| (B2BUAs cand list2 + | |
| Bob's Candidate list) | |
|<------------------------| |
|----------ACK----------->| |
| (8) | |
| | |
|<----ICE Connectivity 1 (9)------------------------->|
| | |
|<----ICE Connectivity 2->| |
| checks+conclusion |<-----ICE Connectivity 2-->|
| (10) | checks +conclusion |
| | (11) |
|<-------------------Media packets------------------->|
| (12) |
| | |
|<------------------ICE keepalives------------------->|
(13)
Figure 2: INVITE with SDP having ICE and with a Media Plane B2BUA in
optional ICE termination mode
The above figure shows a sample call flow with two endpoints Alice
and Bob doing ICE and a B2BUA handing STUN messages from both the
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endpoints. For the sake of brevity the entire ICE SDP attributes are
not shown. Also the STUN messages exchanged as part of ICE
connectivity checks are not shown. Key steps to note from the call
flow are:
o Alice sends an INVITE with an SDP having its own candidate list.
o B2BUA propagates the received candidate list in incoming SDP from
Alice after adding its own candidate list. The B2BUA also
propagates the received ice-ufrag, ice-password attributes from
Alice in the INVITE (3) to Bob. In this example, the B2BUA does
not modify the default candidate sent in the c= line and m= line
and retains the values sent originally from Alice. If B2BUA wants
to be in the media path when ICE connectivity checks between
endpoints fails or one of the endpoints does not support ICE, then
it overwrites its candidate address and port as a default
candidate in the m= and c= lines.
o Bob responds (5) to the INVITE with his own list of candidates.
o B2BUA responds to the INVITE from Alice with an SDP having B2BUAs
candidate list and the candidate list received from Bob. The
B2BUA would also propagate the received ice-ufrag, ice-password
attributes from Bob in step (5) to Alice in the 200 OK response
(6).
o ICE Connectivity checks happen between Alice and Bob in step 9.
ICE Connectivity checks also happens between Alice and B2BUA and
Bob and B2BUA as shown in step 10, 11. Step 9, 10 and 11 happen
in parallel. In this example Alice and Bob conclude ICE with a
candidate pair that enables them to send media directly.
o Media flows between Alice and Bob in Step 12.
4.4. STUN Handling in B2BUA with Forked Signaling
Because of forking, a B2BUA might receive multiple answers for a
single outbound INVITE. When this occurs the B2BUA SHOULD follow
section 3.2 or 3.3 for all of those received answers.
5. Security Considerations
ICE as described in Section 2.5 of [RFC5245] uses STUN short-term
credential mechanism for authentication and message integrity. STUN
connectivity checks include MESSAGE-INTEGRITY attribute that contains
HMAC-SHA1 of the STUN message and the HMAC is computed using the key
exchanged in the signaling channel. The signaling channel between
the endpoints and B2BUA MUST be encrypted so that the key is not
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visible to eavesdropper otherwise the security benefits of short-term
authentication would be lost.
6. IANA Considerations
This document makes no request of IANA.
7. Acknowledgments
Special thanks to Dan Wing, Pal Martinsen, Charles Eckel, Marc Petit-
Huguenin, Simon Perreault, Lorenzo Miniero, Ari Keranen and
Parthasarathi R for their constructive comments, suggestions, and
early reviews that were critical to the formulation and refinement of
this document.
Thanks to Ben Campbell, Barry Leiba, Nevil Brownlee, Spencer Dawkins,
Sam Hartman, Stephen Farrell, Kathleen Moriarty and Francis Dupont
for their thoughtful review comments.
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.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents", RFC
7092, December 2013.
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8.2. Informative References
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[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.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605, October
2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC6888] Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common Requirements for Carrier-Grade NATs
(CGNs)", BCP 127, RFC 6888, April 2013.
[RFC7362] Ivov, E., Kaplan, H., and D. Wing, "Latching: Hosted NAT
Traversal (HNT) for Media in Real-Time Communication", RFC
7362, September 2014.
Authors' Addresses
Ram Mohan Ravindranath
Cisco
Cessna Business Park
Sarjapur-Marathahalli Outer Ring Road
Bangalore, Karnataka 560103
IN
Email: rmohanr@cisco.com
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Tirumaleswar Reddy
Cisco
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
IN
Email: tireddy@cisco.com
Gonzalo Salgueiro
Cisco Systems, Inc.
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
US
Email: gsalguei@cisco.com
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