Internet Engineering Task Force (IETF) R. Ravindranath
Request for Comments: 7584 T. Reddy
Category: Standards Track G. Salgueiro
ISSN: 2070-1721 Cisco
July 2015 Session Traversal Utilities for NAT (STUN) Message Handling
for SIP Back-to-Back User Agents (B2BUAs)
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 document is to ensure that B2BUAs properly handle
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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2015 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 22. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 43. SDP-Modifying Signaling-only B2BUA . . . . . . . . . . . . . 54. Media Plane B2BUAs . . . . . . . . . . . . . . . . . . . . . 54.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 54.2. Mandatory ICE Termination with B2BUA . . . . . . . . . . 64.3. Optional ICE Termination with B2BUA . . . . . . . . . . . 94.4. STUN Handling in B2BUA with Forked Signaling . . . . . . 115. Security Considerations . . . . . . . . . . . . . . . . . . . 126. References . . . . . . . . . . . . . . . . . . . . . . . . . 126.1. Normative References . . . . . . . . . . . . . . . . . . 126.2. Informative References . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 141. Introduction
In many 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 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 a
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 SDP
[RFC4566] and, in the case of SIP, various header fields. Such
addresses and ports are unreachable if any peers are separated by
Mechanisms such as STUN [RFC5389], Traversal Using Relays around NAT
(TURN) [RFC5766], and ICE [RFC5245] did not exist when protocols like
SIP began to be 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) as 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 Session Border Controllers (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 User Agent (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 reoriginate
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., a dual-stack environment where
the 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
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 the 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 operate on both the signaling (SIP and SDP) and media
planes according to the level of involvement and active participation
in the media plane:
o A B2BUA that acts as a simple media relay. It is 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
When B2BUAs that operate on the media plane (media relay, media
aware, or media termination) are involved in a session between two
endpoints performing ICE, then it MUST follow the behavior described
in Section 4.
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].
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 Network Address Port Translation (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 the IP address in the "c=" line, the port in the "m="
line, and the ICE semantics of SDP, as doing so can cause an ice-
4. Media Plane B2BUAs
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 RTP/RTCP
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 describe the behaviors for these two cases.
4.2. Mandatory ICE Termination with B2BUA
A B2BUA that wishes to always be in the media path follows these
o When a B2BUA sends out the SDP, it MUST advertise support for ICE
and MAY include B2BUA's 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 an a=ice-lite attribute and MUST
include B2BUA host candidates for each component of each media
o If the B2BUA supports full ICE, then it MAY include B2BUA's
candidates of different types for each component of each media
o The B2BUA MUST generate new username and 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. The 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.
The above figure shows an example call flow with two endpoints, Alice
and Bob, using ICE processing, and a B2BUA handing STUN messages from
both the endpoints. For the sake of brevity, the entire list of 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 SDP having ICE candidates.
o The B2BUA modifies the received SDP from Alice by removing the
received candidate list, gathering its own candidates, and
generating new username and 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 (Step
3) to Bob.
o Bob responds (Step 5) to the INVITE with his own list of
o The B2BUA responds to the INVITE from Alice with SDP having a
B2BUA candidate list. The B2BUA generates new username and
password values for ice-ufrag and ice-pwd attributes in the 200 OK
response (Step 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. Steps 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 (Steps 11 and 12).
o STUN keepalives would be used between Alice and B2BUA (Step 13)
and between Bob and B2BUA (Step 14) to keep NAT and Firewall
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="
4.3. Optional ICE Termination with B2BUA
A B2BUA willing to be in the media path only for NAT traversal, but
that 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 it received in the incoming SDP to the outgoing
SDP and then sends out the SDP.
o The B2BUA's candidates MAY have lower priority than the candidates
provided by the endpoint, this way the endpoint and remote peer
candidate pairs are tested first before trying candidate pairs
with B2BUA's 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.
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
endpoints. For the sake of brevity, the entire ICE SDP attributes
are not shown. Also, the STUN messages exchanged as part of the ICE
connectivity checks are not shown. Key steps to note from the call
o Alice sends an INVITE with an SDP having its own candidate list.
o The 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 and ice-pwd attributes from
Alice in the INVITE (Step 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 (Step 5) to the INVITE with his own list of
o The B2BUA responds to the INVITE from Alice with an SDP having a
B2BUA's candidate list and the candidate list received from Bob.
The B2BUA would also propagate the received ice-ufrag and ice-pwd
attributes from Bob in (Step 5) to Alice in the 200 OK response
o ICE Connectivity checks happen between Alice and Bob in (Step 9).
ICE Connectivity checks also happen between Alice and the B2BUA
and Bob and the B2BUA as shown in Steps 10 and 11. Steps 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
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
Sections 3.2 or 3.3 for all of those received answers.
Ram Mohan Ravindranath
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathahalli Outer Ring Road
Bangalore, Karnataka 560103
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
Cisco Systems, Inc.
7200-12 Kit Creek Road
Research Triangle Park, NC 27709