Network Working Group R. Stewart
Internet-Draft Adara Networks
Intended status: Standards Track M. Tuexen
Expires: April 12, 2013 I. Ruengeler
Muenster Univ. of Appl. Sciences
October 9, 2012
Stream Control Transmission Protocol (SCTP) Network Address Translation
for Endpoints
draft-ietf-tsvwg-natsupp-03.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 (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 the protocol extensions required for the SCTP
endpoints 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 in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 12, 2013.
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Problem Space Overview . . . . . . . . . . . . . . . . . . . . 5
5. Handling of Internal Port Number and Verification Tag
Collisions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Handling of Missing State . . . . . . . . . . . . . . . . . . 7
7. Multi Point Traversal Considerations . . . . . . . . . . . . . 8
8. Handling of Internal Port Number Collisions . . . . . . . . . 8
9. SCTP Socket API Considerations . . . . . . . . . . . . . . . . 10
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
11. Security Considerations . . . . . . . . . . . . . . . . . . . 10
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
13.1. Normative References . . . . . . . . . . . . . . . . . . 10
13.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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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 (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 maintain the proper
state without needing to change port numbers.
A NAT will need to follow these procedures for generating appropriate
SCTP packet formats. NAT's should refer to [I-D.ietf-behave-sctpnat]
for the BCP in using these formats.
When considering this feature it is possible to have multiple levels
of support. At each level, the Internal Host, External Host and NAT
may or may not support the features described in this document. The
following table illustrates the results of the various combinations
of support and if communications can occur between two endpoints.
+---------------+------------+---------------+---------------+
| Internal Host | NAT | External Host | Communication |
+---------------+------------+---------------+---------------+
| Support | Support | Support | Yes |
| Support | Support | No Support | Limited |
| Support | No Support | Support | None |
| Support | No Support | No Support | None |
| No Support | Support | Support | Limited |
| No Support | Support | No Support | Limited |
| No Support | No Support | Support | None |
| No Support | No Support | No Support | None |
+---------------+------------+---------------+---------------+
Table 1: Communication possibilities
From the table we can see that when a NAT does not support the
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extension no communication can occur. This is for the most part the
current situation i.e. SCTP packets sent externally from behind a
NAT are discarded by the NAT. In some cases, where the NAT supports
the feature but one of the two external hosts does not support the
feature communication may occur but in a limited way. For example
only one host may be able to have a connection when a collision case
occurs.
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
This document uses the following terms, which are depicted in
Figure 1.
Private-Address (Priv-Addr): The private address that is known to
the internal host.
Internal-Port (Int-Port): The port number that is in use by the host
holding the Private-Address.
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
association to the Private-Address.
External-Address (Ext-Addr): The address that an internal host is
attempting to contact.
External-Port (Ext-Port): The port number of the peer process at the
External-Address.
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.
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.
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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
Figure 1: Basic network setup
4. Problem Space Overview
When an SCTP endpoint is behind a NAT which supports
[I-D.ietf-behave-sctpnat] a number of problems may arise as it tries
to communicate with its peer:
o More than one server behind a NAT may pick the same VTag 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 8.
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
discussed in Section 6.
o An SCTP endpoint may be behind two NAT's giving it redundancy.
The method to set up this scenario is discussed in Section 7.
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
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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 procedures 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:
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 /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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6. 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:
o Validate the verification tag is reflected by looking at the VTag
that would have been included in the outgoing packet.
o Validate that the peer of the SCTP association 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 VTags so that the NAT may recover the appropriate state. The
procedures for generating an ASCONF chunk can be found in
[RFC5061].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 association (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 chunk acknowledging the address but take
no action (since the address is already in the association).
7. 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
add 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.
8. Handling of Internal Port Number Collisions
When two SCTP hosts are behind a NAT and using the recommendations in
[I-D.ietf-behave-sctpnat] 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
[I-D.ietf-behave-sctpnat]. 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
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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:
o Include in the INIT-ACK a DISABLE_RESTART parameter to inform the
client that it will support the feature.
o Disable the restart procedures 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.
The NAT, when processing the INIT-ACK, should note in its internal
table that the external server supports the DISABLE_RESTART
extension. This note is used when establishing future associations
(i.e. when processing an INIT from an internal host) to decide if the
connection should be allowed. The NAT MUST do the following when
processing an INIT:
o If the INIT is destined to an external address and port for which
the NAT has no outbound connection, allow the INIT creating an
internal mapping table.
o If the INIT matches the external address and port of an already
existing connection, validate that the external server supports
the DISABLE_RESTART feature. If it does allow the INIT to be
forwarded.
o If the external server does not support the DISABLE_RESTART
extension the NAT MUST send an ABORT with the M-bit set.
The following error cause with cause code 0x00B2 (Duplicate Local
Port with DISABLE_RESTART not Supported) MUST be included in the
ABORT chunk:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code = 0x00B2 | Cause Length = Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ INIT chunk /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9. SCTP Socket API Considerations
TBD
10. IANA Considerations
M-Bit for ABORT and ERROR chunk (0x02).
Error cause Colliding NAT table entry, (0x00B1).
Error cause Duplicate Local Port with DISABLE_RESTART not Supported,
(0x00B2).
Disable restart parameter (0xC007).
ASCONF Parameter (0xC008).
11. Security Considerations
TBD
12. Acknowledgments
The authors wish to thank Jason But, Bryan Ford, David Hayes, Alfred
Hines, Henning Peters, Timo Voelker, Dan Wing, and Qiaobing Xie for
their invaluable comments.
13. References
13.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, August 2007.
[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.
[RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission
Protocol (SCTP) Chunk Flags Registration", RFC 6096,
January 2011.
[I-D.ietf-tsvwg-sctp-udp-encaps]
Tuexen, M. and R. Stewart, "UDP Encapsulation of SCTP
Packets", draft-ietf-tsvwg-sctp-udp-encaps-04 (work in
progress), July 2012.
13.2. Informative References
[RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control
Transmission Protocol (SCTP)", RFC 6083, January 2011.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458, December 2011.
[I-D.ietf-behave-sctpnat]
Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation",
draft-ietf-behave-sctpnat-06 (work in progress),
March 2012.
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Authors' Addresses
Randall R. Stewart
Adara Networks
Chapin, SC 29036
USA
Email: randall@lakerest.net
Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
DE
Email: tuexen@fh-muenster.de
Irene Ruengeler
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
DE
Email: i.ruengeler@fh-muenster.de
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