Network Working Group R. Stewart
Internet-Draft Cisco Systems, Inc.
Expires: December 1, 2006 M. Tuexen
Muenster Univ. of Applied Sciences
May 30, 2006
Stream Control Transmission Protocol (SCTP) Network Address Translation
draft-stewart-behave-sctpnat-02.txt
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Abstract
Stream Control Transmission Protocol RFC2960 [6] provides a reliable
communications channel between two end-hosts in many ways similar to
TCP RFC793 [2]. With the widespread deployment of Network Address
Translators (NAT), specialized code has been added to NAT for TCP
that allows multiple hosts to reside behind a NAT and yet use only a
single globally unique IPv4 address, even when two hosts (behind the
NAT) choose the same port numbers for their connection. This
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additional code is sometimes classified as Network Address and Port
Translation or NAPT. To date, specialized code for SCTP has NOT yet
been added to most NAT's so that only pure NAT is available. The end
result of this is that only one SCTP capable host can be behind a
NAT.
This document describes an SCTP specific variant of NAT which
provides similar features of NAPT in the single point traversal
scenario described in NATCONS [1]. Furthermore both algorithms are
compared.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The SCTP specific variant of NAT . . . . . . . . . . . . . . . 4
4. Discussion of the SCTP specific variant of NAT . . . . . . . . 7
5. Security considerations . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
Stream Control Transmission Protocol RFC2960 [6] provides a reliable
communications channel between two end-hosts in many ways similar to
TCP RFC793 [2]. With the widespread deployment of Network Address
Translators (NAT), specialized code has been added to NAT for TCP
that allows multiple hosts to reside behind a NAT and yet use only a
single globally unique IPv4 address, even when both hosts (behind the
NAT) choose the same port numbers for their connection. This
additional code is sometimes classified as Network Address and Port
Translation or NAPT. To date, specialized code for SCTP has NOT yet
been added to most NAT's so that only true NAT is available. The end
result of this is that only one SCTP capable host can be behind a
NAT.
This document proposes an SCTP specific variant NAT that provides the
NAPT functionality without changing SCTP port numbers. The authors
feel it is possible and desirable to make these changes for a number
of reasons.
o It is desirable for SCTP end-hosts on multiple platforms to be
able to share a global IP address behind a NAT, much as TCP does
today.
o If a NAT does not need to change any data within an SCTP packet it
will reduce the processing burden of NAT'ing SCTP by NOT needing
to execute the CRC32c checksum required by SCTP.
o Not having to touch the IP payload makes the processing of ICMP
messages in NATs easier.
2. Terminology
For this discussion we will use several terms. For clarity we will
first define these terms.
o Global-Address - That address that a host behind a NAT is
attempting to contact.
o Global-Port - The port number of the peer process at the Global-
Address.
o Local-Address - The local address that is known to the host behind
the NAT, aka a private address RFC1918 [3].
o Local-Port - The port number that is in use by the host holding
the Local-Address. Normally this is the port that will be
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translated by the NAPT to a different port number.
o Nat-Global-Address - The global address assigned to the NAT box
which it uses as a source address when sending packets towards the
Global-Address.
o Natted-Port - The port number that the NAT is using to represent
the Local-Port when send data packets toward the Global-Address
and Global-Port.
o Local-Vtag - The Verification Tag that the host inside the natted
address space has chosen for its communication. The V-Tag is a
unique 32 bit tag that must accompany any incoming SCTP packet for
this association to the Local-Address.
o Remote-Vtag - The Verification Tag that the host holding the
Global-Address has chosen for its communication. The V-Tag is a
unique 32 bit tag that must accompany any incoming SCTP packet for
this association to the Global-Address.
3. The SCTP specific variant of NAT
In this section we assume that we have multiple SCTP capable hosts
behind a NAT which has one Nat-Global address. Furthermore we are
considering only the single point traversal scenario described in
NATCONS [1].
The modification of SCTP packets sent to the public Internet is easy.
The source address of the packet has to be replaced with the Nat-
Global-Address. It may also be necessary to establish some state in
the NAT box to handle incoming packets, which is discussed later.
For SCTP packets coming from the public Internet the destination
address of the packets has to be replaced with the Local-Address of
the host the packet has to be delivered to. The lookup of the Local-
Address is based on the Global-VTag, Global-Port, Global-Address,
Local-Vtag and the Local-Port.
For the SCTP NAT processing the NAT box has to maintain a table of
Global-VTag, Global-Port, Global-Address, Local-VTag, Local-Port and
Local-Address. An entry in that table is called a NAT state control
block.
The processing of outgoing SCTP packets containing an INIT-chunk is
described in the following figure.
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Local-Network Global-Internet
[From(Local-Address,Local-Port),
To(Global-Address:Global-Port)
INIT(Initiate-Tag)]------->
Create(Global-Port,Global-Address,Initiate-Tag,Local-Port,Local-Address)
<---Returns(NAT-State control block)
Translate To:
[From(Nat-Global-Address:Local-Port),
To(Global-Address:Global-Port)
INIT(Initiate-Tag)]------->
It should be noted that normally no NAT control block will be
created. However it is possible that that there is already a NAT
control block with the same Global-Port, Global-Address, Initiate-
Tag, Local-VTag but different Local-Address. In this case the INIT
SHOULD be dropped and an ABORT MAY be sent back.
The processing of outgoing SCTP packets containing no INIT-chunk is
described in the following figure.
Local-Network Global-Internet
[From(Local-Address,Local-Port),
To(Global-Address:Global-Port)]------->
Translate To:
[From(Nat-Global-Address:Local-Port),
To(Global-Address:Global-Port)]------->
The processing of incoming SCTP packets containing INIT-ACK chunks is
described in the following figure.
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Local-Network Global-Internet
<-----[From(Global-Address,Global-Port),
To(Nat-Global-Address,Local-Port),
SCTP(Local-VTag),
INIT-ACK(Initiate-Tag)]
Lookup(0,Global-Port,Global-Address,Local-VTag,Local-Port)
Update(Initiate-Tag,Global-Port,Global-Address,Local-VTag,Local-Port)
<---Returns(NAT-State control block containing Local-Address)
<-----[From(Global-Address:Global-Port),
To(Local-Address,Local-Port)
SCTP(Local-VTag)]
In the case Lookup fails, the SCTP packet is dropped. The Update
routine inserts the Global-VTag in the NAT state control block.
The processing of incoming SCTP packets containing and ABORT or
SHUTDOWN-COMLETE chunk with the T-Bit set is described in the
following figure.
Local-Network Global-Internet
<-----[From(Global-Address,Global-Port),
To(Nat-Global-Address,Local-Port),
SCTP(Global-VTag)]
Lookup(Global-VTag,Global-Port,Global-Address,0,Local-Port)
<---Returns(NAT-State control block containing Local-Address)
<-----[From(Global-Address:Global-Port),
To(Local-Address,Local-Port)
SCTP(Global-VTag)]
The processing of other incoming SCTP packets is described in the
following figure.
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Local-Network Global-Internet
<-----[From(Global-Address,Global-Port),
To(Nat-Global-Address,Local-Port),
SCTP(Local-VTag)]
Lookup(0,Global-Port,Global-Address,Local-VTag,Local-Port)
<---Returns(NAT-State control block contaning Local-Address)
<-----[From(Global-Address:Global-Port),
To(Local-Address,Local-Port)
SCTP(Local-VTag)]
4. Discussion of the SCTP specific variant of NAT
There is one drawback of the SCTP specific variant of NAT compared to
a NAPT solution like the ones available for TCP. Consider the case
where two host in the Local-Address space want to setup an SCTP
association with the same server running on the same host in the
Internet. This means that the Global-Port and the Global-Address are
the same. If they both chose the same Local-Port and Local-VTag, the
NAT box can not distinguish incoming packets anymore. But this is
very unlikely. The Local-Vtags are chosen by random and if the
Local-Ports are also chosen ephemeral an random this gives a 46 bit
random number which has to match. In the TCP like NAPT case the NAT
box can control the 16 bit Natted Port.
The advantages of using the SCTP specific variant of NAT is that the
NAT engines do not have to modify the SCTP packet at all. This is
important because modifying the packets requires a recalculation of
the checksum over the complete packet. There is no way of just
computing the different like it is possible for the IP, UDP and TCP
checksum. It should also be noted that the processing of ICMP
packets is easier.
5. Security considerations
State maintenance within a NAT is always a subject of possible Denial
Of Service attack. This document recommends that at a minimum a NAT
run a timer on any SCTP state so that old association state can be
cleaned up.
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6. References
[1] Xie, Q., "SCTP NAT Traversal Considerations",
draft-xie-behave-sctp-nat-cons-01 (work in progress),
October 2005.
[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[3] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
Lear, "Address Allocation for Private Internets", BCP 5,
RFC 1918, February 1996.
[4] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] 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.
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Authors' Addresses
Randall R. Stewart
Cisco Systems, Inc.
4875 Forest Drive
Suite 200
Columbia, SC 29206
USA
Phone:
Email: rrs@cisco.com
Michael Tuexen
Muenster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
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