draft-ietf-mmusic-latching-08.txt   rfc7362.txt 
Network Working Group E. Ivov Internet Engineering Task Force (IETF) E. Ivov
Internet-Draft Jitsi Request for Comments: 7362 Jitsi
Intended status: Informational H. Kaplan Category: Informational H. Kaplan
Expires: December 19, 2014 Oracle ISSN: 2070-1721 Oracle
D. Wing D. Wing
Cisco Cisco
June 17, 2014 September 2014
Latching: Hosted NAT Traversal (HNT) for Media in Real-Time Latching: Hosted NAT Traversal (HNT)
Communication for Media in Real-Time Communication
draft-ietf-mmusic-latching-08
Abstract Abstract
This document describes behavior of signaling intermediaries in Real- This document describes the behavior of signaling intermediaries in
Time Communication (RTC) deployments, sometimes referred to as Real-Time Communication (RTC) deployments, sometimes referred to as
Session Border Controllers (SBCs), when performing Hosted NAT Session Border Controllers (SBCs), when performing Hosted NAT
Traversal (HNT). HNT is a set of mechanisms, such as media relaying Traversal (HNT). HNT is a set of mechanisms, such as media relaying
and latching, that such intermediaries use to enable other RTC and latching, that such intermediaries use to enable other RTC
devices behind NATs to communicate with each other. devices behind NATs to communicate with each other.
This document is non-normative, and is only written to explain HNT in This document is non-normative and is only written to explain HNT in
order to provide a reference to the IETF community, as well as an order to provide a reference to the Internet community and an
informative description to manufacturers, and users. informative description to manufacturers and users.
Latching, which is one of the components of the HNT components, has a Latching, which is one of the HNT components, has a number of
number of security issues covered here. Because of those, and unless security issues covered here. Because of those, and unless all
all security considerations explained here are taken into account and security considerations explained here are taken into account and
solved, the IETF advises against use of latching mechanism over the solved, the IETF advises against use of the latching mechanism over
Internet and recommends other solutions such as the Interactive the Internet and recommends other solutions, such as the Interactive
Connectivity Establishment (ICE) protocol. Connectivity Establishment (ICE) protocol.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
Internet-Drafts are working documents of the Internet Engineering This document is a product of the Internet Engineering Task Force
Task Force (IETF). Note that other groups may also distribute (IETF). It represents the consensus of the IETF community. It has
working documents as Internet-Drafts. The list of current Internet- received public review and has been approved for publication by the
Drafts is at http://datatracker.ietf.org/drafts/current/. Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Internet-Drafts are draft documents valid for a maximum of six months Information about the current status of this document, any errata,
and may be updated, replaced, or obsoleted by other documents at any and how to provide feedback on it may be obtained at
time. It is inappropriate to use Internet-Drafts as reference http://www.rfc-editor.org/info/rfc7362.
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 19, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 26 skipping to change at page 2, line 25
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Impact on Signaling . . . . . . . . . . . . . . . . . . . . . 5 3. Impact on Signaling . . . . . . . . . . . . . . . . . . . . . 5
4. Media Behavior, Latching . . . . . . . . . . . . . . . . . . 6 4. Media Behavior and Latching . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.1. Key References . . . . . . . . . . . . . . . . . . . . . 13 7.2. Informative References . . . . . . . . . . . . . . . . . 14
8.2. Additional References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction 1. Introduction
Network Address Translators (NATs) are widely used in the Internet by Network Address Translators (NATs) are widely used in the Internet by
consumers and organizations. Although specific NAT behaviors vary, consumers and organizations. Although specific NAT behaviors vary,
this document uses the term "NAT" for devices that map any IPv4 or this document uses the term "NAT" for devices that map any IPv4 or
IPv6 address and transport port number to another IPv4 or IPv6 IPv6 address and transport port number to another IPv4 or IPv6
address and transport port number. This includes consumer NATs, address and transport port number. This includes consumer NATs,
Firewall-NATs, IPv4-IPv6 NATs, Carrier-Grade NATs (CGNs) [RFC6888], firewall/NATs, IPv4-IPv6 NATs, Carrier-Grade NATs (CGNs) [RFC6888],
etc. etc.
The Session Initiation Protocol (SIP) [RFC3261], and others that try The Session Initiation Protocol (SIP) [RFC3261], and others that try
to use a more direct path for media than with signaling, are to use a more direct path for media than with signaling, are
difficult to use across NATs. These protocols use IP addresses and difficult to use across NATs. These protocols use IP addresses and
transport port numbers encoded in bodies such as the Session transport port numbers encoded in bodies such as the Session
Description Protocol (SDP) [RFC4566] and, in the case of SIP, various Description Protocol (SDP) [RFC4566] and, in the case of SIP, various
header fields. Such addresses and ports are unusable unless all header fields. Such addresses and ports are unusable unless all
peers in a session are located behind the same NAT. peers in a session are located behind the same NAT.
Mechanisms such as Session Traversal Utilities for NAT (STUN) Mechanisms such as Session Traversal Utilities for NAT (STUN)
[RFC5389], Traversal Using Relays around NAT (TURN) [RFC5766], and [RFC5389], Traversal Using Relays around NAT (TURN) [RFC5766], and
Interactive Connectivity Establishment (ICE) [RFC5245] did not exist Interactive Connectivity Establishment (ICE) [RFC5245] did not exist
when protocols like SIP began being deployed. Some mechanisms, such when protocols like SIP began being deployed. Some mechanisms, such
as the early versions of STUN [RFC3489], had started appearing but as the early versions of STUN [RFC3489], had started appearing, but
they were unreliable and suffered a number of issues typical for they were unreliable and suffered a number of issues typical for
UNilateral Self-Address Fixing (UNSAF) and described in [RFC3424]. UNilateral Self-Address Fixing (UNSAF), as described in [RFC3424].
For these and other reasons, Session Border Controllers (SBCs) that For these and other reasons, Session Border Controllers (SBCs) that
were already being used by SIP domains for other SIP and media- were already being used by SIP domains for other SIP and media-
related purposes began to use proprietary mechanisms to enable SIP related purposes began to use proprietary mechanisms to enable SIP
devices behind NATs to communicate across the NATs. These mechanisms devices behind NATs to communicate across the NATs. These mechanisms
are often transparent to endpoints and rely on a dynamic address and are often transparent to endpoints and rely on a dynamic address and
port discovery technique called "latching". port discovery technique called "latching".
The term often used for this behavior is Hosted NAT Traversal (HNT), The term often used for this behavior is "Hosted NAT Traversal
although a number of manufacturers sometimes use other names such as (HNT)"; a number of manufacturers sometimes use other names such as
"Far-end NAT Traversal" or "NAT assist" instead. The systems which "Far-end NAT Traversal" or "NAT assist" instead. The systems that
perform HNT are frequently SBCs as described in [RFC5853], although perform HNT are frequently SBCs as described in [RFC5853], although
other systems such as media gateways and "media proxies" sometimes other systems such as media gateways and "media proxies" sometimes
perform the same role. For the purposes of this document, all such perform the same role. For the purposes of this document, all such
systems are referred to as SBCs, and the NAT traversal behavior is systems are referred to as SBCs and the NAT traversal behavior is
called HNT. called HNT.
As of this document's creation time, a vast majority of SIP domains At the time of this document's publication, a vast majority of SIP
use HNT to enable SIP devices to communicate across NATs, despite the domains use HNT to enable SIP devices to communicate across NATs
publication of ICE. There are many reasons for this, but those despite the publication of ICE. There are many reasons for this, but
reasons are not relevant to this document's purpose and will not be those reasons are not relevant to this document's purpose and will
discussed. It is however worth pointing out that the current not be discussed. It is, however, worth pointing out that the
deployment levels of HNT and NATs make the complete extinction of current deployment levels of HNT and NATs make the complete
this practice highly unlikely in the foreseeable future. extinction of this practice highly unlikely in the foreseeable
future.
The purpose of this document is to describe the mechanisms often used The purpose of this document is to describe the mechanisms often used
for HNT at the SDP and media layer, in order to aid understanding the for HNT at the SDP and media layer in order to aid understanding the
implications and limitations imposed by it. Although the mechanisms implications and limitations imposed by it. Although the mechanisms
used in HNT are well known in the community, publication in an IETF used in HNT are well known in the community, publication in an IETF
document is useful as a means of providing common terminology and a document is useful as a means of providing common terminology and a
reference for related documents. reference for related documents.
This document does not attempt to make a case for HNT or present it This document does not attempt to make a case for HNT or present it
as a solution that is somehow better than alternatives such as ICE. as a solution that is somehow better than alternatives such as ICE.
Due to the security issues presented in Section 5, the latching Due to the security issues presented in Section 5, the latching
mechanism is considered inappropriate for general use on the Internet mechanism is considered inappropriate for general use on the Internet
unless all security considerations are taken into account and solved. unless all security considerations are taken into account and solved.
The IETF is instead advising for the use of the Interactive The IETF is instead advising for the use of the Interactive
Connectivity Establishment [RFC5245] and Traversal Using Relays Connectivity Establishment (ICE) [RFC5245] and Traversal Using Relays
around NAT (TURN) [RFC5766] protocols. around NAT (TURN) [RFC5766] protocols.
It is also worth mentioning that there are purely signaling-layer It is also worth mentioning that there are purely signaling-layer
components of HNT as well. One such component is briefly described components of HNT as well. One such component is briefly described
for SIP in [RFC5853], but that is not the focus of this document. for SIP in [RFC5853], but that is not the focus of this document.
SIP uses numerous expressive primitives for message routing. As a SIP uses numerous expressive primitives for message routing. As a
result, the HNT component for SIP is typically more implementation- result, the HNT component for SIP is typically more implementation-
specific and deployment-specific than the SDP and media components. specific and deployment-specific than the SDP and media components.
For the purposes of this document it is hence assumed that signaling For the purposes of this document it is hence assumed that signaling
intermediaries handle traffic in a way that allows protocols such as intermediaries handle traffic in a way that allows protocols such as
SIP to function correctly across the NATs. SIP to function correctly across the NATs.
The rest of this document is going to focus primarily on use of HNT The rest of this document focuses primarily on the use of HNT for
for SIP. However, the mechanisms described here are relatively SIP. However, the mechanisms described here are relatively generic
generic and are often used with other protocols, such as XMPP and are often used with other protocols such as the Extensible
[RFC6120], Media Gateway Control Protocol (MGCP) [RFC3435], H.248/ Messaging and Presence Protocol (XMPP) [RFC6120], Media Gateway
MEGACO [RFC5125], and H.323 [H.323]. Control Protocol (MGCP) [RFC3435], Megaco/H.248 [RFC5125], and H.323
[H.323].
2. Background 2. Background
The general problems with NAT traversal for protocols such as SIP The general problems with NAT traversal for protocols such as SIP
are: are:
1. The addresses and port numbers encoded in SDP bodies (or their 1. The addresses and port numbers encoded in SDP bodies (or their
equivalents) by NATed User Agents (UAs) are not usable across the equivalents) by NATed User Agents (UAs) are not usable across the
Internet, because they represent the private network addressing Internet because they represent the private network addressing
information of the UA rather than the addresses/ports that will information of the UA rather than the addresses/ports that will
be mapped to/from by the NAT. be mapped to/from by the NAT.
2. The policies inherent in NATs, and explicit in Firewalls, are 2. The policies inherent in NATs, and explicit in firewalls, are
such that packets from outside the NAT cannot reach the UA until such that packets from outside the NAT cannot reach the UA until
the UA sends packets out first. the UA sends packets out first.
3. Some NATs apply endpoint dependent filtering on incoming packets, 3. Some NATs apply endpoint-dependent filtering on incoming packets,
as described in [RFC4787] and thus a UA may only be able to as described in [RFC4787]; thus, a UA may only be able to receive
receive packets from the same remote peer IP:port as it sends packets from the same remote peer IP:port as it sends packets out
packets out to. to.
In order to overcome these issues, signaling intermediaries such as In order to overcome these issues, signaling intermediaries such as
SIP SBCs on the public side of the NATs perform HNT for both SIP SBCs on the public side of the NATs perform HNT for both
signaling and media. An example deployment model of HNT and SBCs is signaling and media. An example deployment model of HNT and SBCs is
shown in Figure 1. shown in Figure 1.
+-----+ +-----+ +-----+ +-----+
| SBC |-------| SBC | | SBC |-------| SBC |
+-----+ +-----+ +-----+ +-----+
/ \ / \
skipping to change at page 5, line 26 skipping to change at page 5, line 26
/ \ / \
+------+ +------+ +------+ +------+
| UA-A | | UA-B | | UA-A | | UA-B |
+------+ +------+ +------+ +------+
Figure 1: Signaling and Media Flows in a Common Deployment Scenario Figure 1: Signaling and Media Flows in a Common Deployment Scenario
3. Impact on Signaling 3. Impact on Signaling
Along with codec and other media-layer information, session Along with codec and other media-layer information, session
establishment signaling also conveys, potentially private and non- establishment signaling also conveys potentially private and non-
globally routable addressing information. Signaling intermediaries globally routable addressing information. Signaling intermediaries
would hence modify such information so that peer UAs are given the would hence modify such information so that peer UAs are given the
(public) addressing information of a media relay controlled by the (public) addressing information of a media relay controlled by the
intermediary. intermediary.
In typical deployments, the media relay and signaling intermediary In typical deployments, the media relay and signaling intermediary
(i.e., the SBC) are co-located, thereby sharing the same IP address. (i.e., the SBC) are co-located, thereby sharing the same IP address.
Also, the address of the media relay would typically belong to the Also, the address of the media relay would typically belong to the
same IP address family as the one used for signaling, as it is known same IP address family as the one used for signaling (as it is known
to work for that UA. In other words, signalling and media would to work for that UA). In other words, signaling and media would both
either both travel over IPv4 or IPv6. travel over either IPv4 or IPv6.
The port numbers introduced in the signaling by the intermediary are The port numbers introduced in the signaling by the intermediary are
typically allocated dynamically. Allocation strategies are entirely typically allocated dynamically. Allocation strategies are entirely
implementation dependent and they often vary from one product to the implementation dependent and they often vary from one product to the
next. next.
The offer/answer media negotiation model [RFC3264] is such that once The offer/answer media negotiation model [RFC3264] is such that once
an offer is sent, the generator of the offer needs to be prepared to an offer is sent, the generator of the offer needs to be prepared to
receive media on the advertised address/ports. In practice such receive media on the advertised address/ports. In practice, such
media may or may not be received, depending on the implementations media may or may not be received depending on the implementations
participating in a given session, local policies, and call scenario. participating in a given session, local policies, and the call
For example if a SIP SDP Offer originally came from a UA behind a scenario. For example, if a SIP SDP offer originally came from a UA
NAT, the SIP SBC cannot send media to it until an SDP Answer is given behind a NAT, the SIP SBC cannot send media to it until an SDP answer
to the UA and latching (Section 4) occurs. Another example is when a is given to the UA and latching (Section 4) occurs. Another example
SIP SBC sends an SDP Offer in a SIP INVITE to a residential is, when a SIP SBC sends an SDP offer in a SIP INVITE to a
customer's UA and receives back SDP in a 18x response, the SBC may residential customer's UA and receives back SDP in a 18x response,
decide, for policy reasons, not to send media to that customer UA the SBC may decide, for policy reasons, not to send media to that
until a SIP 200 response has been received (e.g., to prevent toll- customer UA until a SIP 200 response has been received (e.g., to
fraud). prevent toll fraud).
4. Media Behavior, Latching 4. Media Behavior and Latching
An UA that is behind a NAT would stream media from an address and a An UA that is behind a NAT would stream media from an address and a
port number (an address:port tuple) that are only valid in its local port number (an address:port tuple) that are only valid in its local
network. Once packets cross the NAT, that address:port tuple will be network. Once packets cross the NAT, that address:port tuple will be
mapped to a public one. The UA however is not typically aware of the mapped to a public one. The UA, however, is not typically aware of
public mapping and would often advertise the private address:port the public mapping and would often advertise the private address:port
tuple in signaling. This way, while a session is still being setup, tuple in signaling. This way, while a session is still being set up,
the signaling intermediary is not yet aware what addresses and ports the signaling intermediary is not yet aware what addresses and ports
the caller and the callee would end up using for media traffic: it the caller and the callee would end up using for media traffic: it
has only seen them advertise the private addresses they use behind has only seen them advertise the private addresses they use behind
their respective NATs. Therefore media relays used in HNT would their respective NATs. Therefore, media relays used in HNT would
often use a mechanism called "latching". often use a mechanism called "latching".
Historically, "latching" only referred to the process by which SBCs Historically, "latching" only referred to the process by which SBCs
"latch" onto UDP packets from a given UA for security purposes, and "latch" onto UDP packets from a given UA for security purposes, and
"symmetric-latching" is when the latched address:port tuples are used "symmetric-latching" is when the latched address:port tuples are used
to send media back to the UA. Today most people talk about them both to send media back to the UA. Today, most people talk about them
as "latching", and thus this document does as well. both as "latching"; thus, this document does as well.
The latching mechanism works as follows: The latching mechanism works as follows:
1. After receiving an offer from Alice (UAC located behind a NAT), a 1. After receiving an offer from Alice (User Agent Client (UAC)
signaling intermediary located on the public Internet would located behind a NAT), a signaling intermediary located on the
allocate a set of IP address:port tuples on a media relay. The public Internet would allocate a set of IP address:port tuples on
set would then be advertised to Bob (UAS) so that he would use a media relay. The set would then be advertised to Bob (User
those media relay address:port tuples for all media it wished to Agent Server (UAS)) so that he would use those media relay
send toward Alice (UAC). address:port tuples for all media he wished to send toward Alice
(UAC).
2. Next, after receiving from Bob (UAS) an answer to its offer, the 2. Next, after receiving from Bob (UAS) an answer to its offer, the
signaling server would allocate a second address:port set on the signaling server would allocate a second address:port set on the
media relay. In it's the answer to Alice (UAC) the SBC will media relay. In its answer to Alice (UAC), the SBC will replace
replace Bob's address:port with this second set. This way Alice Bob's address:port with this second set. This way, Alice will
will send media to this media relay address:port. send media to this media relay address:port.
3. The media relay receives the media packets on the allocated 3. The media relay receives the media packets on the allocated ports
ports, and uses their respective source address:ports as a and uses their respective source address:ports as a destination
destination for all media bound in the opposite direction. In for all media bound in the opposite direction. In other words,
other words, it "latches" or locks on these source address:port it "latches" or locks on these source address:port tuples.
tuple.
4. This way, when Alice (UAC) streams media toward the media relay, 4. This way, when Alice (UAC) streams media toward the media relay,
it would be received on the second address:port tuple. The it would be received on the second address:port tuple. The
source address:port of her traffic would belong to the public source address:port of her traffic would belong to the public
interface of Alice's NAT and anything that the relay sends back interface of Alice's NAT, and anything that the relay sends back
to that address:port, would find its way to Alice. to that address:port would find its way to Alice.
5. Similarly the source of the media packets that Bob (UAS) is 5. Similarly, the source of the media packets that Bob (UAS) is
sending would be latched upon and used for media going in that sending would be latched upon and used for media going in that
direction. direction.
6. Latching is usually done only once per peer and not allowed to 6. Latching is usually done only once per peer and not allowed to
change or cause a re-latching until a new offer and answer get change or cause a re-latching until a new offer and answer get
exchanged (e.g., in a subsequent call or after a SIP peer has exchanged (e.g., in a subsequent call or after a SIP peer has
gone on and off hold). The reasons for such restrictions are gone on and off hold). The reasons for such restrictions are
mostly related to security: once a session has started a user mostly related to security: once a session has started, a user
agent is not expected to suddenly start streaming from a agent is not expected to suddenly start streaming from a
different port without sending a new offer first. A change may different port without sending a new offer first. A change may
indicate an attempt to hijack the session. In some cases indicate an attempt to hijack the session. In some cases,
however, a port change may be caused by a re-mapping in a NAT however, a port change may be caused by a re-mapping in a NAT
device standing between the SBC and the UA. More advanced SBCs device standing between the SBC and the UA. More advanced SBCs
may therefore allow some level of flexibility on the re-latching may therefore allow some level of flexibility on the re-latching
restrictions while carefully considering the potential security restrictions while carefully considering the potential security
implications of doing so. implications of doing so.
Figure 2 describes how latching occurs for SIP where HNT is provided Figure 2 describes how latching occurs for SIP where HNT is provided
by an SBC connected to two networks: 203.0.113/24 facing towards the by an SBC connected to two networks: 203.0.113/24 facing towards the
User Agent Client (UAC) network and 198.51.100/24 facing towards the UAC network and 198.51.100/24 facing towards the UAS network.
User Agent Server (UAS) network.
192.0.2.1 192.0.2.9/203.0.113.4 198.51.100.33 192.0.2.1 192.0.2.9/203.0.113.4 198.51.100.33
Alice NAT 203.0.113.9-SBC-198.51.100.2 Bob Alice NAT 203.0.113.9-SBC-198.51.100.2 Bob
------- --- --- ------- ------- --- --- -------
| | | | | | | |
1. |--SIP INVITE+offer c=192.0.2.1--->| | 1. |--SIP INVITE+offer c=192.0.2.1--->| |
| | | | | | | |
2. | | (SBC allocates 198.51.100.2:22007 | 2. | | (SBC allocates 198.51.100.2:22007 |
| | for inbound RTP from Bob) | | | for inbound RTP from Bob) |
| | | | | | | |
skipping to change at page 8, line 43 skipping to change at page 8, line 43
| | | | | | | |
12. | | (SBC latches to | 12. | | (SBC latches to |
| | source IP address and | | | source IP address and |
| | port seen at (11)) | | | port seen at (11)) |
| | | | | | | |
13. | | |<======= RTP ==========| 13. | | |<======= RTP ==========|
| | |dest:198.51.100.2:22007| | | |dest:198.51.100.2:22007|
14. |<=====RTP, to latched address=====| | 14. |<=====RTP, to latched address=====| |
| | | | | | | |
Figure 2: Latching by a SIP SBC across two interfaces Figure 2: Latching by a SIP SBC across Two Interfaces
While XMPP implementations often rely on ICE to handle NAT traversal, While XMPP implementations often rely on ICE to handle NAT traversal,
there are some that also support a non-ICE transport called XMPP there are some that also support a non-ICE transport called XMPP
Jingle Raw UDP Transport Method [XEP-0177]. Figure 3 describes how Jingle Raw UDP Transport Method [XEP-0177]. Figure 3 describes how
latching occurs for one such XMPP implementation where HNT is latching occurs for one such XMPP implementation where HNT is
provided by an XMPP server on the public internet. provided by an XMPP server on the public Internet.
192.0.2.1 192.0.2.9/203.0.113.4 203.0.113.9 198.51.100.8 192.0.2.1 192.0.2.9/203.0.113.4 203.0.113.9 198.51.100.8
Romeo NAT XMPP Server Juliet Romeo NAT XMPP Server Juliet
----- --- --- ----- ----- --- --- -----
| | | | | | | |
1. |----session-initiate cand=192.0.2.1--->| | 1. |----session-initiate cand=192.0.2.1--->| |
| | | | | | | |
2. |<------------ack-----------------------| | 2. |<------------ack-----------------------| |
| | | | | | | |
3. | | (Server allocates 203.0.113.9:2200 | 3. | | (Server allocates 203.0.113.9:2200 |
skipping to change at page 9, line 46 skipping to change at page 9, line 46
| | | | | | | |
12. | | (XMPP server latches to | 12. | | (XMPP server latches to |
| | src IP 203.0.113.4 and | | | src IP 203.0.113.4 and |
| | src port seen at (11)) | | | src port seen at (11)) |
| | | | | | | |
13. | | |<======= RTP ========| 13. | | |<======= RTP ========|
| | |dest=203.0.113.9:2200| | | |dest=203.0.113.9:2200|
14. |<======RTP, to latched address=========| | 14. |<======RTP, to latched address=========| |
| | | | | | | |
Figure 3: Latching by an XMPP server across two interfaces Figure 3: Latching by an XMPP Server across Two Interfaces
The above is a general description, and some details vary between The above is a general description, and some details vary between
implementations or configuration settings. For example, some implementations or configuration settings. For example, some
intermediaries perform additional logic before latching on received intermediaries perform additional logic before latching on received
packet source information to prevent malicious attacks or latching packet source information to prevent malicious attacks or latching
erroneously to previous media senders - often called "rogue-rtp" in erroneously to previous media senders -- often called "rogue-rtp" in
the industry. the industry.
It is worth pointing out that latching is not an exclusively "server It is worth pointing out that latching is not exclusively a "server
affair" and some clients may also use it in cases where they are affair", and some clients may also use it in cases where they are
configured with a public IP address and they are contacted by a NATed configured with a public IP address and are contacted by a NATed
client with no other NAT traversal means. client with no other NAT traversal means.
In order for latching to function correctly, the UA behind the NAT In order for latching to function correctly, the UA behind the NAT
needs to support symmetric RTP. That is, it needs to use the same needs to support symmetric RTP. That is, it needs to use the same
ports for sending data as the ones it listens on for inbound packets. ports for sending data as the ones it listens on for inbound packets.
Today this is the case for with, for example, almost all SIP and XMPP Today, this is the case with almost all SIP and XMPP clients. Also,
clients. Also UAs need to make sure they can begin sending media UAs need to make sure they can begin sending media packets
packets independently and without waiting for packets to arrive independently without waiting for packets to arrive first. In
first. In theory, it is possible that some UAs would not send theory, it is possible that some UAs would not send packets out
packets out first; for example if a SIP session begins in 'inactive' first, for example, if a SIP session begins in 'inactive' or
or 'recvonly' SDP mode from the UA behind the NAT. In practice, 'recvonly' SDP mode from the UA behind the NAT. In practice,
however, SIP sessions from regular UAs (the kind that one could find however, SIP sessions from regular UAs (the kind that one could find
behind a NAT) virtually never begin an inactive or 'recvonly' mode, behind a NAT) virtually never begin in 'inactive' or 'recvonly' mode,
for obvious reasons. The media direction would also be problematic for obvious reasons. The media direction would also be problematic
if the SBC side indicated 'inactive' or 'sendonly' modes when it sent if the SBC side indicated 'inactive' or 'sendonly' modes when it sent
SDP to the UA. However SBCs providing HNT would always be configured SDP to the UA. However, SBCs providing HNT would always be
to avoid this. configured to avoid this.
Given that, in order for latching to work properly, media relays need Given that, in order for latching to work properly, media relays need
to begin receiving media before they start sending, it is possible to begin receiving media before they start sending, it is possible
for deadlocks to occur. This can happen when the UAC and the UAS in for deadlocks to occur. This can happen when the UAC and the UAS in
a session are connected to different signaling intermediaries that a session are connected to different signaling intermediaries that
both provide HNT. In this case the media relays controlled by the both provide HNT. In this case, the media relays controlled by the
signaling servers could end up each waiting upon the other to signaling servers could end up each waiting upon the other to
initiate the streaming. To prevent this relays would often attempt initiate the streaming. To prevent this, relays would often attempt
to start streaming toward the address:port tuples provided in the to start streaming toward the address:port tuples provided in the
offer/answer even before receiving any inbound traffic. If the offer/answer even before receiving any inbound traffic. If the
entity they are streaming to is another HNT performing server it entity they are streaming to is another HNT performing server, it
would have provided its relay's public address and ports and the would have provided its relay's public address and ports, and the
early stream would find its target. early stream would find its target.
Although many SBCs only support UDP-based media latching, and in Although many SBCs only support UDP-based media latching (in
particular RTP/RTCP, many SBCs support TCP-based media latching as particular, RTP/RTCP), many SBCs support TCP-based media latching as
well. TCP-based latching is more complicated, and involves forcing well. TCP-based latching is more complicated; it involves forcing
the UA behind the NAT to be the TCP client and sending the initial the UA behind the NAT to be the TCP client and sending the initial
SYN-flagged TCP packet to the SBC (i.e., be the 'active' mode side of SYN-flagged TCP packet to the SBC (i.e., be the 'active' mode side of
a TCP-based media session). If both UAs of a TCP-based media session a TCP-based media session). If both UAs of a TCP-based media session
are behind NATs, then SBCs typically force both UAs to be the TCP are behind NATs, then SBCs typically force both UAs to be the TCP
clients, and the SBC splices the TCP connections together. TCP clients, and the SBC splices the TCP connections together. TCP
splicing is a well-known technique, as described in [tcp-splicing]. splicing is a well-known technique, as described in [TCP-SPLICING].
HNT and latching in particular are generally found to be working HNT and latching, in particular, are generally found to work
reliably but they do have obvious caveats. The first one usually reliably, but they do have obvious caveats. The first one usually
raised by IETF participants is that UAs are not aware of it raised by IETF participants is that UAs are not aware of it
occurring. This makes it impossible for the mechanism to be used occurring. This makes it impossible for the mechanism to be used
with protocols such as ICE that try various traversal techniques in with protocols such as ICE that try various traversal techniques in
an effort to choose the one that best suits a particular situation. an effort to choose the one that best suits a particular situation.
Overwriting address information in offers and answers may actually Overwriting address information in offers and answers may actually
completely prevent UAs from using ICE because of the ice-mismatch completely prevent UAs from using ICE because of the ice-mismatch
rules described in [RFC5245] rules described in [RFC5245].
The second issue raised by IETF participants is that it causes media The second issue raised by IETF participants is that it causes media
to go through a relay instead of directly over the IP-routed path to go through a relay instead of directly over the IP-routed path
between the two participating UAs. While this adds obvious drawbacks between the two participating UAs. While this adds obvious drawbacks
such as reduced scalability and often increased latency, it is also such as reduced scalability and increased latency, it is also
considered a benefit by SBC administrators: if a customer pays for considered a benefit by SBC administrators: if a customer pays for
"phone" service, for example, the media is what is truly being paid "phone" service, for example, the media is what is truly being paid
for, and the administrators usually like to be able to detect that for, and the administrators usually like to be able to detect that
media is flowing correctly, evaluate its quality, know if and why it the media is flowing correctly, evaluate its quality, know if and why
failed, etc. Also in some cases routing media through operator it failed, etc. Also, in some cases, routing media through operator
controlled relays may route media over paths explicitly optimized for controlled relays may route media over paths explicitly optimized for
media and hence offer better performance than regular Internet media and hence offer better performance than regular Internet
routing. routing.
5. Security Considerations 5. Security Considerations
A common concern is that an SBC (or an XMPP server, all security A common concern is that an SBC (or an XMPP server -- all security
considerations apply to both) that implements HNT may latch to considerations apply to both) that implements HNT may latch to
incorrect and possibly malicious sources. The ICE [RFC5245] protocol incorrect and possibly malicious sources. The ICE [RFC5245]
for example, provides authentication tokens (conveyed in the ice- protocol, for example, provides authentication tokens (conveyed in
ufrag and ice-pwd attributes) that allow the identity of a peer to be the ice-ufrag and ice-pwd attributes) that allow the identity of a
confirmed before engaging in media exchange with her. Without such peer to be confirmed before engaging in media exchange with her.
authentication, a malicious source could, for example, attempt a Without such authentication, a malicious source could attempt a
resource exhaustion attack by flooding all possible media-latching resource exhaustion attack by flooding all possible media-latching
UDP ports on the SBC in order to prevent calls from succeeding. SBCs UDP ports on the SBC in order to prevent calls from succeeding. SBCs
have various mechanisms to prevent this from happening, or alert an have various mechanisms to prevent this from happening or to alert an
administrator when it does. Still, a sufficiently sophisticated administrator when it does. Still, a sufficiently sophisticated
attacker may be able to bypass them for some time. The most common attacker may be able to bypass them for some time. The most common
example is typically referred to as "restricted-latching", whereby example is typically referred to as "restricted-latching", whereby
the SBC will not latch to any packets from a source public IP address the SBC will not latch to any packets from a source public IP address
other than the one the SIP UA uses for SIP signaling. This way the other than the one the SIP UA uses for SIP signaling. This way, the
SBC simply ignores and does not latch onto packets coming from the SBC simply ignores and does not latch onto packets coming from the
attacker. In some cases the limitation may be loosened to allow attacker. In some cases, the limitation may be loosened to allow
media from a range of IP addresses belonging to the same network in media from a range of IP addresses belonging to the same network in
order to allow for use cases such as decomposed UAs and various forms order to allow for use cases such as decomposed UAs and various forms
of third party call control. However, since relaxing the of third-party call control. However, since relaxing the
restrictions in such a way may provide attackers with a larger attack restrictions in such a way may provide attackers with a larger attack
surface, such configurations are generally performed only on a case- surface, such configurations are generally performed only on a case-
by-case basis so that the specifics of individual deployments would by-case basis so that the specifics of individual deployments can be
be taken into account. taken into account.
All of the above problems would still arise if the attacker knows the All of the above problems would still arise if the attacker knows the
public source IP of the UA that is actually making the call. This public source IP of the UA that is actually making the call. This
would allow them to still flood all of the SBC's public IP addresses would allow attackers to still flood all of the SBC's public IP
and ports with packets spoofing that SIP UA's public source IP addresses and ports with packets spoofing that SIP UA's public source
address. However, this would only impact media from that IP (or IP address. However, this would only impact media from that IP (or
range of IP addresses) rather than all calls that the SBC is range of IP addresses) rather than all calls that the SBC is
servicing. servicing.
A malicious source could send media packets to an SBC media-latching A malicious source could send media packets to an SBC media-latching
UDP port in the hopes of being latched-to for the purpose of UDP port in the hopes of being latched to for the purpose of
receiving media for a given SIP session. SBCs have various receiving media for a given SIP session. SBCs have various
mechanisms to prevent this as well. Restricted latching for example mechanisms to prevent this as well. Restricted latching, for
would also help in this case since the attacker can't make the SBC example, would also help in this case because the attacker can't make
send media packets back to themselves since the SBC will not latch the SBC send media packets back to themselves since the SBC will not
onto the attacker's media packets, not having seen the corresponding latch onto the attacker's media packets, not having seen the
signaling packets first. There could still be an issue if the corresponding signaling packets first. There could still be an issue
attacker happens to be either (1) in the IP routing path and thus can if the attacker happens to be either (1) in the IP routing path where
spoof the same IP as the real UA and get the media coming back, in it can thus spoof the same IP as the real UA and get the media coming
which case the attacker hardly needs to attack at all to begin with, back, in which case the attacker hardly needs to attack at all to
or (2) the attacker is behind the same NAT as the legitimate SIP UA, begin with, or (2) behind the same NAT as the legitimate SIP UA, in
in which case the attacker's packets will be latched-to by the SBC which case the attacker's packets will be latched to by the SBC and
and the SBC will send media back to the attacker. In this latter the SBC will send media back to the attacker. In the latter case,
case, which may be of particular concern with Carrier-Grade NATs, the which may be of particular concern with Carrier-Grade NATs, the
legitimate SIP UA will likely end the call anyway when a human user legitimate SIP UA will likely end the call anyway when a human user
who does not hear anything hangs up. In the case of a non-human call who does not hear anything hangs up. In the case of a non-human call
participant, such as an answering machine, this may not happen participant, such as an answering machine, this may not happen
(although many such automated UAs would also hang up when they do not (although many such automated UAs would also hang up when they do not
receive any media). The attacker could also redirect all media to receive any media). The attacker could also redirect all media to
the real SIP UA after receiving it, in which case the attack would the real SIP UA after receiving it, in which case the attack would
likely remain undetected and succeed. Again, this would be of likely remain undetected and succeed. Again, this would be of
particular concern with larger scale NATs serving many different particular concern with larger-scale NATs serving many different
endpoints such as Carrier-Grade NATs. The larger the number of endpoints, such as Carrier-Grade NATs. The larger the number of
devices fronted by a NAT is, the more use cases would vary and the devices fronted by a NAT is, the more use cases would vary, and the
more the number of possible attack vectors would grow. more the number of possible attack vectors would grow.
Naturally, SRTP [RFC3711] would help mitigate such threats and, if Naturally, Secure RTP (SRTP) [RFC3711] would help mitigate such
used with the appropriate key negotiation mechanisms, would protect threats and, if used with the appropriate key negotiation mechanisms,
the media from monitoring while in transit. It should therefore be would protect the media from monitoring while in transit. It should
used independently of HNT. [RFC3261] Section 26 provides an overview therefore be used independently of HNT. Section 26 of [RFC3261]
of additional threats and solutions on monitoring and session provides an overview of additional threats and solutions on
interception. monitoring and session interception.
With SRTP, if the SBC that performs the latching is actually With SRTP, if the SBC that performs the latching is actually
participating in the SRTP key exchange, then it would simply refuse participating in the SRTP key exchange, then it would simply refuse
to latch onto a source unless it can authenticate it. Failing to to latch onto a source unless it can authenticate it. Failing to
implement and use SRTP would represent a serious threat to users implement and use SRTP would represent a serious threat to users
connecting from behind Carrier-Grade NATs [RFC6888] and is considered connecting from behind Carrier-Grade NATs [RFC6888] and is considered
a harmful practice. a harmful practice.
For SIP clients, HNT is usually transparent in the sense that the SIP For SIP clients, HNT is usually transparent in the sense that the SIP
UA does not know it occurs. In certain cases it may be detectable, UA does not know it occurs. In certain cases, it may be detectable,
such as when ICE is supported by the SIP UA and the SBC modifies the such as when ICE is supported by the SIP UA and the SBC modifies the
default connection address and media port numbers in SDP, thereby default connection address and media port numbers in SDP, thereby
disabling ICE due to the mismatch condition. Even in that case, disabling ICE due to the mismatch condition. Even in that case,
however, the SIP UA only knows a middle box is relaying media, but however, the SIP UA only knows that a middlebox is relaying media but
not necessarily that it is performing latching/HNT. not necessarily that it is performing latching/HNT.
In order to perform HNT, the SBC has to modify SDP to and from the In order to perform HNT, the SBC has to modify SDP to and from the
SIP UA behind a NAT, and thus the SIP UA cannot use S/MIME [RFC5751], SIP UA behind a NAT; thus, the SIP UA cannot use S/MIME [RFC5751],
and it cannot sign a sending request or verify a received request and it cannot sign a sending request, or verify a received request
using [RFC4474] unless the SBC re-signs the request. However, using the SIP Identity mechanism [RFC4474] unless the SBC re-signs
neither S/MIME or [RFC4474] are widely deployed, thus not being able the request. However, neither S/MIME nor SIP Identity are widely
to sign/verify requests appear not to be a concern at this time. deployed; thus, not being able to sign/verify requests appears not to
be a concern at this time.
From a privacy perspective, media relaying is sometimes seen as a way From a privacy perspective, media relaying is sometimes seen as a way
of protecting one's IP address and not revealing it to the remote of protecting one's IP address and not revealing it to the remote
party. That kind of IP address masking is often perceived as party. That kind of IP address masking is often perceived as
important. However, this is no longer an exclusive advantage of HNT important. However, this is no longer an exclusive advantage of HNT
since it can also be accomplished by client-controlled relaying since it can also be accomplished by client-controlled relaying
mechanisms such as TURN [RFC5766], if the client explicitly wishes to mechanisms such as TURN [RFC5766] if the client explicitly wishes to
do so. do so.
6. IANA Considerations 6. Acknowledgements
This document has no actions for IANA.
Note to the RFC-Editor: please remove this section prior to
publication as an RFC.
7. Acknowledgements
The authors would like to thank Flemming Andreasen, Miguel A. The authors would like to thank Flemming Andreasen, Miguel A.
Garcia, Alissa Cooper, Vijay K. Gurbani, Ari Keranen and Paul Garcia, Alissa Cooper, Vijay K. Gurbani, Ari Keranen, and Paul
Kyzivat for their reviews and suggestions on improving this document. Kyzivat for their reviews and suggestions on improving this document.
8. References 7. References
8.1. Key References 7.1. Normative References
[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,
June 2002. June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June with Session Description Protocol (SDP)", RFC 3264, June
2002. 2002.
skipping to change at page 14, line 30 skipping to change at page 14, line 39
[RFC5853] Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen, [RFC5853] Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen,
A., and M. Bhatia, "Requirements from Session Initiation A., and M. Bhatia, "Requirements from Session Initiation
Protocol (SIP) Session Border Control (SBC) Deployments", Protocol (SIP) Session Border Control (SBC) Deployments",
RFC 5853, April 2010. RFC 5853, April 2010.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011. Protocol (XMPP): Core", RFC 6120, March 2011.
[XEP-0177] [XEP-0177]
Beda, J., Saint-Andre, P., Hildebrand, J., and S. Egan, Beda, J., Saint-Andre, P., Hildebrand, J., and S. Egan,
"XEP-0177: Jingle Raw UDP Transport Method", XEP XEP-0177, "XEP-0177: Jingle Raw UDP Transport Method", XSF XEP 0177,
December 2009. December 2009.
8.2. Additional References 7.2. Informative References
[H.323] International Telecommunication Union, "Packet Based [H.323] International Telecommunication Union, "Packet-based
Multimedia Communication Systems", Recommendation H.323, multimedia communication systems", ITU-T Recommendation
July 2003. H.323, December 2009.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002. Translation", RFC 3424, November 2002.
[RFC3435] Andreasen, F. and B. Foster, "Media Gateway Control [RFC3435] Andreasen, F. and B. Foster, "Media Gateway Control
Protocol (MGCP) Version 1.0", RFC 3435, January 2003. Protocol (MGCP) Version 1.0", RFC 3435, January 2003.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP) "STUN - Simple Traversal of User Datagram Protocol (UDP)
skipping to change at page 15, line 33 skipping to change at page 15, line 41
Specification", RFC 5751, January 2010. Specification", RFC 5751, January 2010.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010. Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC6888] Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A., [RFC6888] Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common Requirements for Carrier-Grade NATs and H. Ashida, "Common Requirements for Carrier-Grade NATs
(CGNs)", BCP 127, RFC 6888, April 2013. (CGNs)", BCP 127, RFC 6888, April 2013.
[tcp-splicing] [TCP-SPLICING]
Maltz, D. and P. Bhagwat, "TCP Splice for application Maltz, D. and P. Bhagwat, "TCP Splice for application
layer proxy performance", Journal of High Speed Networks layer proxy performance", Journal of High Speed Networks
vol. 8, no. 3, 1999, pp. 235-240, March 1999. Vol. 8, No. 3, 1999, pp. 225-240, March 1999.
Authors' Addresses Authors' Addresses
Emil Ivov Emil Ivov
Jitsi Jitsi
Strasbourg 67000 Strasbourg 67000
France France
Email: emcho@jitsi.org EMail: emcho@jitsi.org
Hadriel Kaplan Hadriel Kaplan
Oracle Oracle
100 Crosby Drive 100 Crosby Drive
Bedford, MA 01730 Bedford, MA 01730
USA USA
Email: hadriel.kaplan@oracle.com EMail: hadrielk@yahoo.com
Dan Wing Dan Wing
Cisco Systems, Inc. Cisco Systems, Inc.
170 West Tasman Drive 170 West Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
USA USA
Email: dwing@cisco.com EMail: dwing@cisco.com
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