Network Working Group Q. Xie
Internet-Draft Motorola
Expires: January 7, 2005 R. Stewart
Cisco Systems, Inc.
M. Holdrege
Strix Systems
July 9, 2004
SCTP NAT Transverse Considerations
draft-xie-tsvwg-sctp-nat-00.txt
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Abstract
This document provides guidelines and solutions for dealing with SCTP
association transversing NAT and similar middleboxes.
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Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. SCTP NAT Transverse Scenarios . . . . . . . . . . . . . . . . 3
3.1 Single Point Transvers . . . . . . . . . . . . . . . . . . 3
3.2 Multi-point Traverse . . . . . . . . . . . . . . . . . . . 4
4. Considerations for SCTP NAT transverse . . . . . . . . . . . . 4
4.1 Simple NAT enhancement to support singly-homed SCTP
associations . . . . . . . . . . . . . . . . . . . . . . . 4
4.2 NAT enhancement to support multi-homed SCTP
associations through a single traverse point . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1 Normative References . . . . . . . . . . . . . . . . . . . . 6
6.2 Informative References . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . 8
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1. Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
[2].
2. Introduction
It is the job of Network Address Translators (NAT) [3] and
middleboxes [6] that utilize a NAT function to manipulate address and
port information in the IP header. This poses a challenge for hosts
that attempt to use certain end-to-end protocols [5]. This issue has
drawn increasingly wide attention from the IP development and service
community and much work has been done to ameliorate the situation for
UDP, TCP and other protocols.
The same issue not only exists for SCTP [4], but also may become a
more difficult issue when SCTP associations are multi-homed. This
document provides guidelines and solutions for dealing with SCTP and
NAT traversal. In the following discussion, we will simply refer to
NAT as a function, but note that many types of middleboxes employ NAT
functions.
3. SCTP NAT Transverse Scenarios
3.1 Single Point Transvers
In this case, all packets in the SCTP association go through a single
NAT, as shown below:
+------+ +-----+ +------+
|SCTP |==========| NAT |==========|SCTP |
|end A | +-----+ |end B |
+------+ +------+
A variation of this case is shown below, i.e., multiple NATs in a
single transverse path:
+------+ +-----+ +-----+ +------+
|SCTP |======| NAT |=::==| NAT |=======|SCTP |
|end A | +-----+ +-----+ |end B |
+------+ +------+
The two SCTP endpoints in this case can be either singly-homed or
multi-homed. However, the important thing is that the NAT (or NATs)
in this case sees ALL the packets of the SCTP association.
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In this single traverse point scenario, we must acknowledge that
while one of the main benefits of SCTP multi-homing is redundant
paths, the NAT function represents a single point of failure in the
path of the SCTP multi-home association. However, the rest of the
path may still benefit from path diversity provided by SCTP
multi-homing.
3.2 Multi-point Traverse
This case involves multiple NATs and each NAT only sees some of the
packets in the SCTP association. An example is shown below:
+------+
/====|NAT A |====\
+------+ / +------+ \ +------+
|SCTP |/ ... \|SCTP |
|end A |\ /|end B |
+------+ \ +------+ / +------+
\====|NAT B |====/
+------+
This case does NOT apply to a singly-homed SCTP association (i.e.,
BOTH endpoints in the association use only one IP address). The
advantage here is that the existance of multiple NAT traverse points
can preserve the path diversity of a multi-homed association for the
entire path. This in turn can improve the robustness of the
communication.
To make this work, however, all the NATs involved must recognize the
packets they see as belonging to the same SCTP association and
perform address translation in a consistent way. This can only be
possible if there exists some inter-NAT protocol which can provide
coordination amongst the NATs when the association is set up.
Further study is needed for this scenario. In the rest of this
document, we will only discuss the single traverse point scenario.
4. Considerations for SCTP NAT transverse
In any type of traverse, the NAT must understand the SCTP protocol.
Since SCTP is relatively new (compared to UDP or TCP), some older
existing NATs that are capable of handling UDP or TCP traverse will
need to be enhanced for SCTP. In this section we discuss what
considerations should be made for that NAT enhancement.
4.1 Simple NAT enhancement to support singly-homed SCTP associations
In a singly-homed SCTP association, each endpoint uses only one IP
address and the association will always go through a single NAT
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traverse point. In such cases, the enhancement needed for a NAT to
support SCTP is relatively simple: 1) recognize the protocol number
for SCTP, and 2) locate and change the transport ports (if needed).
The location of the transport port numbers are the same as both TCP
and UDP so these should be minor changes for NAT software.
It is important that the endpoints not to list the address again
within the INIT or INIT-ACK when setting up the association.
Note, the use of a single address by the endpoint may be the result
of either the endpoint only has a single address assigned to the host
or due to user of the SCTP stack only specifies to bind one address
to the endpoint.
4.2 NAT enhancement to support multi-homed SCTP associations through a
single traverse point
At the association setup, the NAT needs to recognize and intercept
the INIT, or INIT-ACK chunk. The NAT will look into the chunk and
find out all the listed IP addresses (including the source IP address
in the packet's IP header). The NAT will map this address list to a
_list_ of external addresses. Then, the NAT will reconstruct the
INIT or INIT-ACK chunk, fill it with external addresses, re-calculate
the checksum, and finally forward the packet on to the peer. This
approach is transparent to the endpoints and can support
multi-homing.
The NAT function is capable of variety in translating one to many or
many to one addresses in the chunk. Depending on network
architectural requirements, NAT administrators may choose to map many
internal addresses to a single external address. It should be noted
that this will reduce path diversity in a portion of the network.
Alternatively, by mapping 1 internal address to many external
addresses can increase the path diversity in a portion of the
network.
Editor's Note: Add discussion about ASCONF handling by the NAT.
Editor's Note: Add some scenario examples.
5. Security Considerations
See RFC XXXX on SCTP security considerations. See RFC XXXX on NAT
security considerations. See RFC XXXX for IAB comments on NAT.
6. References
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6.1 Normative References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
6.2 Informative References
[3] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[4] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
"Stream Control Transmission Protocol", RFC 2960, October 2000.
[5] Holdrege, M. and P. Srisuresh, "Protocol Complications with the
IP Network Address Translator", RFC 3027, January 2001.
[6] Swale, R., Mart, P., Sijben, P., Brim, S. and M. Shore,
"Middlebox Communications (midcom) Protocol Requirements", RFC
3304, August 2002.
Authors' Addresses
Qiaobing Xie
Motorola, Inc.
1501 W. Shure Drive, 2-F9
Arlington Heights, IL 60004
US
Phone: +1-847-632-3028
EMail: qxie1@email.mot.com
Randall R. Stewart
Cisco Systems, Inc.
8725 West Higgins Road
Suite 300
Chicago, IL 60631
USA
Phone: +1-815-477-2127
EMail: rrs@cisco.com
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Matt Holdrege
Strix Systems
Suite 110, 26610 Agoura Road
Calabasas, CA 91302
USA
EMail: matt@strixsystems.com
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