Network Working Group R. Stewart
Internet-Draft Huawei
Intended status: Standards Track M. Tuexen
Expires: June 24, 2010 I. Ruengeler
Muenster Univ. of Applied Sciences
December 21, 2009
Stream Control Transmission Protocol (SCTP) Network Address Translation
Support
draft-stewart-natsupp-tsvwg-00.txt
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 a
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 NATs 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 chunks and procedures to
help NAT's provide similar features of NAPT in the single point and
multi-point traversal scenario.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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The list of Internet-Draft Shadow Directories can be accessed at
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This Internet-Draft will expire on June 24, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Problem space overview . . . . . . . . . . . . . . . . . . . . 4
5. Handling of internal port number and verification tag
collisions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Handling of internal port number collisions . . . . . . . . . . 6
7. Handling of missing state . . . . . . . . . . . . . . . . . . . 7
8. Multi Point Traversal considerations . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9
10. Security considerations . . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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 two hosts (behind a
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 NATs 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 describes an SCTP specific chunks and procedures to
help NAT's provide similar features of NAPT in the single point and
multi-point traversal scenario. An SCTP implementation supporting
this extension will follow these procedures to assure that in both
single homed and multi-homed cases a NAT will maintian the proper
state without needing to change port numbers.
A NAT will need to follow these proceedures for generating
appropriate SCTP packet formats. NAT's should refer to
xxxxbehavedraftxxx for the BCP in using these formats.
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, which we will define
and point out in a figure.
o Private-Address (Priv-Addr) - The private address that is known to
the internal host.
o Internal-Port (Int-Port) - The port number that is in use by the
host holding the Private-Address.
o Internal-VTag (Int-VTag) - The Verification Tag that the internal
host has chosen for its communication. The VTag is a unique 32
bit tag that must accompany any incoming SCTP packet for this
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association to the Private-Address.
o External-Address (Ext-Addr) - The address that an internal host is
attempting to contact.
o External-Port (Ext-Port) - The port number of the peer process at
the External-Address.
o External-VTag (Ext-VTag) - The Verification Tag that the host
holding the External-Address has chosen for its communication.
The VTag is a unique 32 bit tag that must accompany any incoming
SCTP packet for this association to the External-Address.
o Public-Address (Pub-Addr) - The public address assigned to the NAT
box which it uses as a source address when sending packets towards
the External-Address.
Internal Network | External Network
|
Private | Public External
+---------+ Address | Address /--\/--\ Address +---------+
| SCTP | +-----+ / \ | SCTP |
|end point|==========| NAT |======= | Internet | ========== |end point|
| A | +-----+ \ / | B |
+---------+ Internal | \--/\--/ External +---------+
Internal Port | Port External
VTag | VTag
4. Problem space overview
When an SCTP endpoint is behind a NAT which supports xxxnatdraftxxx a
number of problems may arise as it trys to communicate with its peer.
o More than one server behind a NAT may pick the same V-Tag and
source port when talking to the same peer server. This creates a
situation where the NAT will not be able to tell the two
associations apart. This situation is discussed in Section 5
o When an SCTP endpoint is a server and talking with multiple peers
and the peers are behind the same NAT, to the server the two
endpoints cannot be distinguished. This case is discussed in
Section 6.
o A NAT could at one point during a conversation restart causing all
of its state to be lost. This problem and its solution is
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discussed in Section 7.
o An SCTP endpoint may be behind two NAT's giving it redundancy.
The method to set up this scenario is discussed in Section 8.
Each of these solutions requires additional chunks and parameters,
defined in this document, and possibly modified handling procedures
from those specified in [RFC4960].
5. Handling of internal port number and verification tag collisions
Consider the case where two hosts in the Private-Address space want
to set up an SCTP association with the same server running on the
same host in the Internet. This means that the External-Port and the
External-Address are the same. If they both choose the same
Internal-Port and Internal-VTag, the NAT box cannot distinguish
incoming packets anymore. But this is very unlikely. The Internal-
VTags are chosen at random and if the Internal-Ports are also chosen
from the ephemeral port range at 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, in this unlikely event the NAT box MUST respond to the INIT
chunk by sending an ABORT chunk with the M-bit set. The M-bit is a
new bit defined by this document to express to SCTP that the source
of this packet is a "middle" box, not the peer SCTP endpoint. 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, upon reception of
an ABORT with M-bit set SHOULD reinitiate the association setup
procedure after choosing a new initiate tag. These proceedures
SHOULD be followed only if the appropriate error cause code for
colliding NAT table state is included AND the association is in the
COOKIE-WAIT state (i.e. it is awaiting a INIT-ACK). If the endpoint
is in any other state an SCTP endpoint SHOULD NOT respond.
The ABORT chunk defined in [RFC4960] is therefore extended by using
the following format:
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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 /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following error cause with cause code 0x00B0 (Colliding NAT table
entry) MUST 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 /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6. Handling of internal port number collisions
When two SCTP hosts are behind a NAT and using the recommendations in
xxxxbehavexxx it is possible that two SCTP hosts in the Private-
Address space will want to set up an SCTP association with the same
server running on the same host in the Internet. For the NAT
appropriate tracking may be performed by assuring that the vtags are
unique between the two hosts as defined in xxxxbehavexxx. But for
the external SCTP server on the internet this means that the
External-Port and the External-Address are the same. If they both
have chosen the same Internal-Port the server cannot 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 DISABLE_RESTART 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 MUST do the following:
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o Include in the INIT-ACK a DISABLE_RESTART parameter to inform the
client that it will support the feature.
o Disable the restart procdures defined in [RFC4960] for this
association.
Servers that support this feature will need to be capable of
maintaining multiple connections to what appears to be the same peer
(behind the NAT) differentiated only by the vtags.
7. 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=0x00B1 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Incoming Packet /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Upon reception by an SCTP end-point with this ERROR chunk the
receiver SHOULD take the following actions:
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o Validate the verifcation tag is reflected by looking at the V-tag
that would have been included in the outgoing packet.
o Validate that the peer of the SCTP assocation supports the dynamic
address extension, if it does not discard the incoming ERROR
chunk.
o Generate a new ASCONF chunk as defined below including both sets
of V-tags so that the NAT may recover the appropriate state. The
procedures for generating an ASCONF can be found in [RFC5061]
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 = 0xC008 | Parameter Length = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASCONF-Request Correlation ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internal Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External 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.
The peer SCTP endpoint receiving such an ASCONF chunk SHOULD either
add the address and respond with an acknowledgment, if the address is
new to the assocation (following all procedures defined in
[RFC5061]). Or, if the address is already part of the association,
the SCTP endpoint MUST NOT respond with an error, but instead should
respond with an ASCONF-ACK acknowledging the address but take no
action (since the address is already in the association).
8. Multi Point Traversal considerations
If a multi-homed SCTP end-point behind a NAT connects to a peer, it
SHOULD first set up the association single-homed with only one
address causing the first NAT to populate its state. Then it SHOULD
adds each IP address using ASCONF chunks sent via their respective
NATs. The address to add is the wildcard address and the lookup
address SHOULD also contain the vtag parameter pair illustrated
above.
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9. IANA Considerations
TBD
10. Security considerations
TBD
11. Acknowledgments
The authors wish to thank Qiaobing Xie, Henning Peters, Bryan Ford,
David Hayes, Alfred Hines, Dan Wing, and Jason But for their
invaluable comments.
12. References
12.1. 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.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
September 2007.
12.2. 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
Huawei
Chapin, SC 29036
USA
Phone:
Email: rstewart@huawei.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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