Network Working Group R. Stewart
Internet-Draft Adara Networks
Intended status: Standards Track M. Tuexen
Expires: August 29, 2013 I. Ruengeler
Muenster Univ. of Appl. Sciences
February 25, 2013
Stream Control Transmission Protocol (SCTP) Network Address Translation
Support
draft-ietf-tsvwg-natsupp-05.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 NATs 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 August 29, 2013.
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Copyright Notice
Copyright (c) 2013 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Data Formats . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Modified Chunks . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. Extended ABORT Chunk . . . . . . . . . . . . . . . . . 6
4.1.2. Extended ERROR Chunk . . . . . . . . . . . . . . . . . 7
4.2. New Error Causes . . . . . . . . . . . . . . . . . . . . . 7
4.2.1. VTag and Port Number Collision Error Cause . . . . . . 7
4.2.2. Missing State Error Cause . . . . . . . . . . . . . . 8
4.2.3. Port Number Collision Error Cause . . . . . . . . . . 8
4.3. New Parameters . . . . . . . . . . . . . . . . . . . . . . 9
4.3.1. Disable Restart Parameter . . . . . . . . . . . . . . 9
4.3.2. VTags Parameter . . . . . . . . . . . . . . . . . . . 9
5. Problem Space and Procedures . . . . . . . . . . . . . . . . . 10
5.1. Problem Space Overview . . . . . . . . . . . . . . . . . . 11
5.2. Association Setup Considerations . . . . . . . . . . . . . 11
5.3. Handling of Internal Port Number and Verification Tag
Collisions . . . . . . . . . . . . . . . . . . . . . . . . 11
5.4. Handling of Internal Port Number Collisions . . . . . . . 12
5.5. Handling of Missing State . . . . . . . . . . . . . . . . 13
5.6. Multi-Point Traversal Considerations . . . . . . . . . . . 15
6. Socket API Considerations . . . . . . . . . . . . . . . . . . 15
6.1. Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY) . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7.1. New Chunk Flags for Two Existing Chunk Types . . . . . . . 16
7.2. Three New Error Causes . . . . . . . . . . . . . . . . . . 17
7.3. Two New Chunk Parameter Types . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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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 using private
addresses (see [RFC5735]) 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 SCTP specific packets and procedures to help
NATs 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. NATs 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 because for the most
part of 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. Data Formats
4.1. Modified Chunks
This section presents existing chunks defined in [RFC4960] that are
modified by this document.
4.1.1. Extended 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ABORT chunk is extended to add the new 'M-bit'. The M-bit
indicates to the receiver of the ABORT chunk that the chunk was not
generated by the peer SCTP endpoint, but instead by a middle box.
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4.1.2. Extended 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ERROR chunk defined in [RFC4960] is extended to add the new
'M-bit'. The M-bit indicates to the receiver of the ERROR chunk that
the chunk was not generated by the peer SCTP endpoint, but instead by
a middle box.
4.2. New Error Causes
This section defines the new error causes added by this document.
4.2.1. VTag and Port Number Collision Error Cause
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Chunk /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the VTag and Port
Number Collision Error Cause. The suggested value of this field
for IANA is 0x00B0.
Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The
value MUST be the length of the Cause-Specific Information plus 4.
Chunk: variable length
The Cause-Specific Information is filled with the chunk that
caused this error. This can be an INIT, INIT-ACK, or ASCONF
chunk. Note that if the entire chunk will not fit in the ERROR
chunk or ABORT chunk being sent then the bytes that do not fit are
truncated.
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4.2.2. Missing State Error Cause
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 /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the Missing State
Error Cause. The suggested value of this field for IANA is
0x00B1.
Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The
value MUST be the length of the Cause-Specific Information plus 4.
Incoming Packet: variable length
The Cause-Specific Information is filled with the IPv4 or IPv6
packet that caused this error. The IPv4 or IPv6 header MUST be
included. Note that if the packet will not fit in the ERROR chunk
or ABORT chunk being sent then the bytes that do not fit are
truncated.
4.2.3. Port Number Collision Error Cause
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ chunk /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the Port Number
Collision Error Cause. The suggested value of this field for IANA
is 0x00B2.
Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The
value MUST be the length of the Cause-Specific Information plus 4.
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Chunk: variable length
The Cause-Specific Information is filled with the chunk that
caused this error. This can be an INIT, INIT-ACK, or ASCONF
chunk. Note that if the entire chunk will not fit in the ERROR
chunk or ABORT chunk being sent then the bytes that do not fit are
truncated.
4.3. New Parameters
This section defines new parameters and their valid appearance
defined by this document.
4.3.1. Disable Restart Parameter
This parameter is used to indicate that the RESTART procedure is
requested to be disabled. Both endpoints of an association MUST
include this parameter in the INIT chunk and INIT-ACK chunk when
establishing an association and MUST include it in the ASCONF chunk
when adding an address to successfully disable the restart procedure.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This field holds the IANA defined parameter type for the Disable
Restart Parameter. The suggested value of this field for IANA is
0xC007.
Parameter Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the parameter. The value
MUST be 4.
This parameter MAY appear in INIT, INIT-ACK and ASCONF chunks and
MUST NOT appear in any other chunk.
4.3.2. VTags Parameter
This parameter is used to help a NAT recover from state loss.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This field holds the IANA defined parameter type for the VTags
Parameter. The suggested value of this field for IANA is 0xC008.
Parameter Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the parameter. The value
MUST be 16.
ASCONF-Request Correlation ID: 4 bytes (unsigned integer)
This is an opaque integer assigned by the sender to identify each
request parameter. The receiver of the ASCONF Chunk will copy
this 32-bit value into the ASCONF Response Correlation ID field of
the ASCONF-ACK response parameter. The sender of the ASCONF can
use this same value in the ASCONF-ACK to find which request the
response is for. Note that the receiver MUST NOT change this 32-
bit value.
Internal Verification Tag: 4 bytes (unsigned integer)
The Verification Tag that the internal host has chosen for its
communication. The Verification Tag is a unique 32-bit tag that
must accompany any incoming SCTP packet for this association to
the Private-Address.
External Verification Tag: 4 bytes (unsigned integer) 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.
This parameter MAY appear in ASCONF chunks and MUST NOT appear in any
other chunk.
5. Problem Space and Procedures
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5.1. 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 5.3.
o When an SCTP endpoint is a server communicating with multiple
peers and the peers are behind the same NAT, then the two
endpoints cannot be distinguished by the server. This case is
discussed in Section 5.4.
o A restart of a NAT during a conversation could cause a loss of its
state. This problem and its solution is discussed in Section 5.5.
o An SCTP endpoint may be behind two NATs providing redundancy. The
method to set up this scenario is discussed in Section 5.6.
Each of these solutions requires additional chunks and parameters,
defined in this document, and possibly modified handling procedures
from those specified in [RFC4960].
5.2. Association Setup Considerations
Every association MUST initially be set up single-homed. There MUST
NOT be any IPv4 Address parameter, IPv6 Address parameter, or
Supported Address Types parameter in the INIT-chunk. The INIT-ACK
chunk MUST NOT contain any IPv4 Address parameter or IPv6 Address
parameter.
If the association should finally be multi-homed, the procedure in
Section 5.6 MUST be used.
The INIT and INIT-ACK chunk SHOULD contain the Disable Restart
parameter defined in Section 4.3.1.
5.3. 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
between incoming packets anymore. But this is very unlikely. The
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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 and therefor avoid
collisions deterministically.
The same can happen when an INIT-ACK chunk or an ASCONF chunk is
processed by the NAT.
However, in this unlikely event the NAT box MUST send an ABORT chunk
with the M-bit set if the collision is triggered by an INIT or INIT-
ACK chunk or send an ERROR chunk with the M-bit set if the collision
is triggered by an ASCONF chunk. 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 (see Section 4.1.1). In a
packet containing an INIT-ACK chunk triggers the collision, the
corresponding packet containing the ABORT chunk MUST contain the same
source and destination address and port numbers as the packet
containing the INIT-ACK chunk. In the other two cases, the source
and destination address and port numbers MUST be swapped.
The sender of the packet containing the INIT chunk or the receiver of
the INIT-ACK chunk, upon reception of an ABORT chunk 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 an INIT-ACK). If the endpoint is in any other state an SCTP
endpoint SHOULD NOT respond.
The sender of the ASCONF chunk, upon reception of an ERROR chunk with
M-bit set, MUST stop adding the path to the association.
The sender of the ERROR or ABORT chunk MUST include the error cause
with cause code 'VTag and Port Number Collision' (see Section 4.2.1).
5.4. 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
server cannot distinguish between both associations based on the
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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.
When the server receives this parameter it MUST do the following:
o Include a Disable Restart parameter in the INIT-ACK 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 association 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 'Port Number Collision' error cause (see Section 4.2.3) MUST be
included in the ABORT chunk.
If the collision is triggered by an ASCONF chunk, a packet containing
an ERROR chunk with the 'Port Number Collision' error cause MUST be
sent back.
5.5. Handling of Missing State
If the NAT box receives a packet from the internal network for which
the lookup procedure does not find an entry in the NAT table, a
packet containing an ERROR chunk is sent back with the M-bit set.
The source address of the packet containing the ERROR chunk MUST be
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the destination address of the incoming SCTP packet. The
verification tag is reflected and the T-bit is set. Please note that
such a packet containing an ERROR chunk SHOULD NOT be sent if the
received packet contains an ABORT, SHUTDOWN-COMPLETE or INIT-ACK
chunk. An ERROR chunk MUST NOT be sent if the received packet
contains an ERROR chunk with the M-bit set.
When sending the ERROR chunk, the new error cause Missing state (see
Section 4.2.2) MUST be included and the new M-bit of the ERROR chunk
MUST be set (see Section 4.1.2).
Upon reception of this ERROR chunk by an SCTP endpoint the receiver
SHOULD take the following actions:
o Validate that 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 containing the VTags parameter (see
Section 4.3.2) and the Disable Restart parameter if the
association is using the disabled restart feature. By processing
this packet the NAT can recover the appropriate state. The
procedures for generating an ASCONF chunk can be found in
[RFC5061].
If the NAT box receives a packet for which it has no NAT table entry
and the packet contains an ASCONF chunk with the VTags parameter, the
NAT box MUST update its NAT table according to the verification tags
in the VTags parameter and the optional Disable Restart 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).
Note that it is possible that upon receiving an ASCONF chunk
containing the VTags parameter the NAT will realize that it has an
'Internal Port Number and Verification Tag collision'. In such a
case the NAT MUST send an ERROR chunk with the error cause code set
to 'VTag and Port Number Collision' (see Section 4.2.1).
If an SCTP endpoint receives an ERROR with 'Internal Port Number and
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Verification Tag collision' as the error cause and the packet in the
Error Chunk contains an ASCONF with the VTags parameter, careful
examination of the association is required. The endpoint MUST do the
following:
o Validate that 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 If the association is attempting to add an address (i. e.
following the procedures in Section 5.6) then the endpoint MUST-
NOT consider the address part of the association and SHOULD make
no further attempt to add the address (i. e. cancel any ASCONF
timers and remove any record of the path), since the NAT has a
VTag collision and the association cannot easily create a new VTag
(as it would if the error occurred when sending an INIT).
o If the endpoint has no other path, i. e. the procedure was
executed due to missing a state in the NAT, then the endpoint MUST
abort the association. This would occur only if the local NAT
restarted and accepted a new association before attempting to
repair the missing state (Note that this is no different than what
happens to all TCP connections when a NAT looses its state).
5.6. Multi-Point Traversal Considerations
If a multi-homed SCTP endpoint 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 VTags parameter and optionally the
Disable Restart parameter as illustrated above.
6. Socket API Considerations
This section describes how the socket API defined in [RFC6458] is
extended to provide a way for the application to control NAT
friendliness.
Please note that this section is informational only.
A socket API implementation based on [RFC6458] is extended by
supporting one new read/write socket option.
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6.1. Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY)
This socket option uses the option_level IPPROTO_SCTP and the
option_name SCTP_NAT_FRIENDLY. It can be used to enable/disable the
NAT friendliness for future associations and retrieve the value for
future and specific ones.
struct sctp_assoc_value {
sctp_assoc_t assoc_id;
uint32_t assoc_value;
};
assoc_id: This parameter is ignored for one-to-one style sockets.
For one-to-many style sockets the application may fill in an
association identifier or SCTP_FUTURE_ASSOC for this query. It is
an error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.
assoc_value: A non-zero value indicates a NAT-friendly mode.
7. IANA Considerations
[NOTE to RFC-Editor:
"RFCXXXX" is to be replaced by the RFC number you assign this
document.
]
[NOTE to RFC-Editor:
The suggested values for the chunk type and the chunk parameter
types are tentative and to be confirmed by IANA.
]
This document (RFCXXXX) is the reference for all registrations
described in this section. The suggested changes are described
below.
7.1. New Chunk Flags for Two Existing Chunk Types
As defined in [RFC6096] two chunk flags have to be assigned by IANA
for the ERROR chunk. The suggested value for the T bit is 0x01 and
for the M bit is 0x02.
This requires an update of the "ERROR Chunk Flags" registry for SCTP:
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ERROR Chunk Flags
+------------------+-----------------+-----------+
| Chunk Flag Value | Chunk Flag Name | Reference |
+------------------+-----------------+-----------+
| 0x01 | T bit | [RFCXXXX] |
| 0x02 | M bit | [RFCXXXX] |
| 0x04 | Unassigned | |
| 0x08 | Unassigned | |
| 0x10 | Unassigned | |
| 0x20 | Unassigned | |
| 0x40 | Unassigned | |
| 0x80 | Unassigned | |
+------------------+-----------------+-----------+
As defined in [RFC6096] one chunk flag has to be assigned by IANA for
the ABORT chunk. The suggested value of the M bit is 0x02.
This requires an update of the "ABORT Chunk Flags" registry for SCTP:
ABORT Chunk Flags
+------------------+-----------------+-----------+
| Chunk Flag Value | Chunk Flag Name | Reference |
+------------------+-----------------+-----------+
| 0x01 | T bit | [RFC4960] |
| 0x02 | M bit | [RFCXXXX] |
| 0x04 | Unassigned | |
| 0x08 | Unassigned | |
| 0x10 | Unassigned | |
| 0x20 | Unassigned | |
| 0x40 | Unassigned | |
| 0x80 | Unassigned | |
+------------------+-----------------+-----------+
7.2. Three New Error Causes
Three error causes have to be assigned by IANA. It is suggested to
use the values given below.
This requires three additional lines in the "Error Cause Codes"
registry for SCTP:
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Error Cause Codes
+-------+--------------------------------+-----------+
| Value | Cause Code | Reference |
+-------+--------------------------------+-----------+
| 176 | VTag and Port Number Collision | [RFCXXXX] |
| 177 | Missing State | [RFCXXXX] |
| 178 | Port Number Collision | [RFCXXXX] |
+-------+--------------------------------+-----------+
7.3. Two New Chunk Parameter Types
Two chunk parameter types have to be assigned by IANA. It is
suggested to use the values given below. IANA should assign these
values from the pool of parameters with the upper two bits set to
'11'.
This requires two additional lines in the "Chunk Parameter Types"
registry for SCTP:
Chunk Parameter Types
+----------+--------------------------+-----------+
| ID Value | Chunk Parameter Type | Reference |
+----------+--------------------------+-----------+
| 49159 | Disable Restart (0xC007) | [RFCXXXX] |
| 49160 | VTags (0xC008) | [RFCXXXX] |
+----------+--------------------------+-----------+
8. Security Considerations
The document does not add any additional security considerations to
the ones given in [RFC4960], [RFC4895], and [RFC5061].
9. 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.
10. References
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10.1. 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.
[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.
10.2. Informative References
[RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[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-07 (work in progress),
October 2012.
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Authors' Addresses
Randall R. Stewart
Adara Networks
Chapin, SC 29036
US
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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