Network Working Group Q. Xie
Internet-Draft Motorola
Expires: April 25, 2006 R. Stewart
Cisco Systems, Inc.
M. Holdrege
Strix Systems
M. Tuexen
Muenster Univ. of Applied Sciences
October 22, 2005
SCTP NAT Traversal Considerations
draft-xie-behave-sctp-nat-cons-01.txt
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Copyright (C) The Internet Society (2005).
Abstract
This document defines and classifies scenarios for the usage of SCTP
in networks with NATs and similar middleboxes.
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Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . . . . 3
3.1. Single Point Traversal . . . . . . . . . . . . . . . . . . 3
3.2. Multi Point Traversal . . . . . . . . . . . . . . . . . . . 4
4. Considerations for SCTP NAT Traversal . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 7
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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 and transport 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 defines and classifies scenarios 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 Traversal Scenarios
3.1. Single Point Traversal
In this case, all packets in the SCTP association go through a single
NAT, as shown below:
+---------+ +---------+
| SCTP | +-----+ | SCTP |
|end point|==========| NAT |==========|end point|
| A | +-----+ | B |
+---------+ +---------+
A variation of this case is shown below, i.e., multiple NATs in a
single path:
+---------+ +---------+
| SCTP | +-----+ +-----+ | SCTP |
|end point|====| NAT |=::==| NAT |====|end point|
| A | +-----+ +-----+ | B |
+---------+ +---------+
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The two SCTP endpoints in this case can be either single-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.
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 Traversal
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 point|/ ... \|end point|
| A |\ /| 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. It may be required
that a pre-defined table of ports and addresses would be shared
between the NAT's. Other external management schemes that help
multiple NAT's coordinate a multi-homed SCTP association could be
investigated.
4. Considerations for SCTP NAT Traversal
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.
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In a single-homed SCTP association, each endpoint uses only one IP
address and the association will always go through a single NAT
traverse point. It is important that the endpoints do not list the
IP-address again within the INIT or INIT-ACK chunks when setting up
the association. This makes sure that the NAT engine is not required
to change the INIT or INIT-ACK chunk when modifying the IP-addresses
of the packets containing the INIT and INIT-ACK chunks.
It is also important that the checksum of the whole SCTP packet has
to be recalculated if a part of the SCTP packet, for example port
numbers or IP-addresses listed in the INIT or INIT-ACK chunk, is
modyfied. It is not possible for the SCTP checksum to calculate the
difference of the checksum based only on the difference of the
packets like it is possible for the checksum used for TCP or UDP.
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
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.
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Authors' Addresses
Qiaobing Xie
Motorola, Inc.
1501 W. Shure Drive, 2-F9
Arlington Heights, IL 60004
USA
Phone: +1-847-632-3028
Email: qxie1@email.mot.com
Randall R. Stewart
Cisco Systems, Inc.
4875 Forest Drive
Suite 200
Columbia, SC 29206
USA
Email: rrs@cisco.com
Matt Holdrege
Strix Systems
Suite 110, 26610 Agoura Road
Calabasas, CA 91302
USA
Email: matt@strixsystems.com
Michael Tuexen
Muenster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
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