Network Working Group R. Stewart
Internet-Draft The Resource Group
Intended status: Standards Track M. Tuexen
Expires: January 15, 2009 I. Ruengeler
Muenster Univ. of Applied Sciences
July 14, 2008
Stream Control Transmission Protocol (SCTP) Network Address Translation
draft-stewart-behave-sctpnat-04.txt
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Abstract
Stream Control Transmission Protocol [RFC4960] provides a reliable
communications channel between two end-hosts in many ways similar to
TCP [RFC0793]. 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
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
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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 and multi-point
traversal scenario.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . . . . 4
4.1. Single Point Traversal . . . . . . . . . . . . . . . . . . 4
4.2. Multi Point Traversal . . . . . . . . . . . . . . . . . . 5
5. The SCTP specific variant of NAT . . . . . . . . . . . . . . . 6
6. Handling of local port number collisions . . . . . . . . . . . 8
7. Handling of local port number and verification tag
collisions . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Handling of missing state . . . . . . . . . . . . . . . . . . 10
9. Multi Point Traversal considerations . . . . . . . . . . . . . 11
10. Handling of fragmented SCTP packets . . . . . . . . . . . . . 11
11. Simplification for small NATs . . . . . . . . . . . . . . . . 11
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
13. Security considerations . . . . . . . . . . . . . . . . . . . 12
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
15.1. Normative References . . . . . . . . . . . . . . . . . . . 12
15.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
Stream Control Transmission Protocol [RFC4960] provides a reliable
communications channel between two end-hosts in many ways similar to
TCP [RFC0793]. 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. Conventions
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].
3. 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.
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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].
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
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.
4. SCTP NAT Traversal Scenarios
4.1. 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:
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+---------+ +---------+
| SCTP | +-----+ +-----+ | SCTP |
|end point|====| NAT |=::==| NAT |====|end point|
| A | +-----+ +-----+ | B |
+---------+ +---------+
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.
4.2. 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 existence 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.
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5. 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
focusing in this section on the single point traversal scenario.
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.
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
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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.
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.
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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.
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 containing Local-Address)
<-----[From(Global-Address:Global-Port),
To(Local-Address,Local-Port)
SCTP(Local-VTag)]
For an incoming packet containing an INIT-chunk a table lookup is
made only based on the addresses and port numbers. If an entry with
a local vtag of zero is found, it is considered a match and the local
v-tag is updated.
This allows the handling of INIT-collision through NAT.
6. Handling of local port number collisions
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 hosts 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
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the same. If they both chose the same Local-Port the server can not
distinguish both associations based on the address and port numbers.
For the server it looks like the association is being restarted. To
overcome this limitation the client sends a NAT_SUPPORTED parameter
in the INIT-chunk which is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0xC007 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
When the server receives this parameter it will also use the
verification tag to look up the association. However, this will make
it impossible to restart such associations.
7. Handling of local port number and verification tag collisions
Consider the case where two hosts 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.
However, if this unlikely happens the NAT box MUST respond to the
INIT chunk by sending an ABORT chunk with the M-bit set. The source
address of the packet containing the ABORT chunk MUST be the
destination address of the SCTP packet containing the INIT chunk.
The sender of the packet containing the INIT chunk MAY start the
association setup procedure after choosing a new initiate tag.
The ABORT chunk defined in [RFC4960] is therefore extended by using
the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The following error cause with cause code 0x00b0 (Colliding NAT table
entry) SHOULD be included in the ABORT chunk:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=0x00b0 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ INIT chunk /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8. Handling of missing state
If the NAT box receives a packet for which the lookup procedure does
not find an entry in the NAT table, a packet containing an ERROR
packet is sent back with the M-bit set. The source address of the
packet containing the ERROR chunk MUST be the destination address of
the incoming SCTP packet. The verification tag is reflected.
The ERROR chunk defined in [RFC4960] is therefore extended by using
the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following error cause with cause code 0x00b1 (Missing NAT table
entry) SHOULD be included in the ERROR chunk:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=0x00b0 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Incoming Packet /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If an end-point receives a packet with this ERROR chunk it MAY send
an SCTP packet with a ASCONF chunk containing an Add IP Address
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parameter followed by a vtag parameter:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 0xC007 | Parameter Length = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If the NAT box receives a packet for which it has no NAT table entry
and the packet contains an ASCONF chunk with a vtag parameter, the
NAT box MUST update its NAT table according to the verification tags
in the vtag parameter.
9. Multi Point Traversal considerations
If a multi-homed SCTP end-point behind a NAT connects to a peer, it
first sets up the association single-homed. Then it adds each IP
address using ASCONF chunks. The address to add is the wildcard
address and the lookup address also. The ASCONF chunks SHOULD also
contain a vtag parameter.
10. Handling of fragmented SCTP packets
A NAT box MUST support IP reassembly of received fragmented SCTP
packets. The fragments may arrive in any order.
When an SCTP packet has to be fragmented by the NAT box and the IP
header forbids fragmentation a correspond ICMP packet SHOULD be sent.
11. Simplification for small NATs
Small NAT boxes, i.e. NAT boxes which only have to support a small
number of concurrent SCTP associations, MAY not take the global
address into account when processing packets. Therefore the global-
address could also be removed from the NAT table.
This simplification may make implementing a NAT box easier, however,
the collision probability is higher than using a mapping which takes
the global address into account.
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12. IANA Considerations
TBD
13. 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.
14. Acknowledgments
The authors wish to thank Qiaobing Xie, Henning Peters, Bryan Ford,
David Hayes, and Jason But for their invaluable comments.
15. References
15.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
15.2. Informative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
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Authors' Addresses
Randall R. Stewart
The Resource Group
1700 Pennsylvania Ave NW
Suite 56
Washington, DC 20006
USA
Phone:
Email: randall.stewart@trgworld.com
Michael Tuexen
Muenster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
Irene Ruengeler
Muenster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
Email: i.ruengeler@fh-muenster.de
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