draft-ietf-tsvwg-natsupp-06.txt   draft-ietf-tsvwg-natsupp-07.txt 
Network Working Group R. Stewart Network Working Group R. Stewart
Internet-Draft Adara Networks Internet-Draft Netflix, Inc.
Intended status: Standards Track M. Tuexen Intended status: Standards Track M. Tuexen
Expires: March 13, 2014 I. Ruengeler Expires: August 29, 2015 I. Ruengeler
Muenster Univ. of Appl. Sciences Muenster Univ. of Appl. Sciences
September 09, 2013 February 25, 2015
Stream Control Transmission Protocol (SCTP) Network Address Translation Stream Control Transmission Protocol (SCTP) Network Address Translation
Support Support
draft-ietf-tsvwg-natsupp-06.txt draft-ietf-tsvwg-natsupp-07.txt
Abstract Abstract
Stream Control Transmission Protocol [RFC4960] provides a reliable Stream Control Transmission Protocol [RFC4960] provides a reliable
communications channel between two end-hosts in many ways similar to communications channel between two end-hosts in many ways similar to
TCP [RFC0793]. With the widespread deployment of Network Address TCP [RFC0793]. With the widespread deployment of Network Address
Translators (NAT), specialized code has been added to NAT for TCP 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 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 single globally unique IPv4 address, even when two hosts (behind a
NAT) choose the same port numbers for their connection. This NAT) choose the same port numbers for their connection. This
skipping to change at page 1, line 48 skipping to change at page 1, line 48
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on March 13, 2014. This Internet-Draft will expire on August 29, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Data Formats . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Modified Chunks . . . . . . . . . . . . . . . . . . . . . 5 4.1. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . 6
4.1.1. Extended ABORT Chunk . . . . . . . . . . . . . . . . 5 4.1.1. Single Point Traversal . . . . . . . . . . . . . . . 6
4.1.2. Extended ERROR Chunk . . . . . . . . . . . . . . . . 5 4.1.2. Multi Point Traversal . . . . . . . . . . . . . . . . 7
4.2. New Error Causes . . . . . . . . . . . . . . . . . . . . 6 4.2. Limitations of Classical NAPT for SCTP . . . . . . . . . 7
4.2.1. VTag and Port Number Collision Error Cause . . . . . 6 4.3. The SCTP Specific Variant of NAT . . . . . . . . . . . . 8
4.2.2. Missing State Error Cause . . . . . . . . . . . . . . 7 5. Data Formats . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.3. Port Number Collision Error Cause . . . . . . . . . . 7 5.1. Modified Chunks . . . . . . . . . . . . . . . . . . . . . 12
4.3. New Parameters . . . . . . . . . . . . . . . . . . . . . 8 5.1.1. Extended ABORT Chunk . . . . . . . . . . . . . . . . 12
4.3.1. Disable Restart Parameter . . . . . . . . . . . . . . 8 5.1.2. Extended ERROR Chunk . . . . . . . . . . . . . . . . 12
4.3.2. VTags Parameter . . . . . . . . . . . . . . . . . . . 8 5.2. New Error Causes . . . . . . . . . . . . . . . . . . . . 13
5. Problem Space and Procedures . . . . . . . . . . . . . . . . 9 5.2.1. VTag and Port Number Collision Error Cause . . . . . 13
5.1. Problem Space Overview . . . . . . . . . . . . . . . . . 9 5.2.2. Missing State Error Cause . . . . . . . . . . . . . . 13
5.2. Association Setup Considerations . . . . . . . . . . . . 10 5.2.3. Port Number Collision Error Cause . . . . . . . . . . 14
5.3. Handling of Internal Port Number and Verification Tag 5.3. New Parameters . . . . . . . . . . . . . . . . . . . . . 14
Collisions . . . . . . . . . . . . . . . . . . . . . . . 10 5.3.1. Disable Restart Parameter . . . . . . . . . . . . . . 14
5.4. Handling of Internal Port Number Collisions . . . . . . . 11 5.3.2. VTags Parameter . . . . . . . . . . . . . . . . . . . 15
5.5. Handling of Missing State . . . . . . . . . . . . . . . . 12 6. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.6. Multi-Point Traversal Considerations . . . . . . . . . . 14 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 16
6. Socket API Considerations . . . . . . . . . . . . . . . . . . 14 6.2. Association Setup Considerations . . . . . . . . . . . . 17
6.1. Get or Set the NAT Friendliness 6.3. Handling of Internal Port Number and Verification Tag
(SCTP_NAT_FRIENDLY) . . . . . . . . . . . . . . . . . . . 14 Collisions . . . . . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 6.4. Handling of Internal Port Number Collisions . . . . . . . 18
7.1. New Chunk Flags for Two Existing Chunk Types . . . . . . 15 6.5. Handling of Missing State . . . . . . . . . . . . . . . . 19
7.2. Three New Error Causes . . . . . . . . . . . . . . . . . 16 6.6. Handling of Fragmented SCTP Packets . . . . . . . . . . . 21
7.3. Two New Chunk Parameter Types . . . . . . . . . . . . . . 17 6.7. Multi-Point Traversal Considerations . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17 7. Various Examples of NAT Traversals . . . . . . . . . . . . . 21
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 7.1. Single-homed Client to Single-homed Server . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.2. Single-homed Client to Multi-homed Server . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . 17 7.3. Multihomed Client and Server . . . . . . . . . . . . . . 26
10.2. Informative References . . . . . . . . . . . . . . . . . 18 7.4. NAT Loses Its State . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 7.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . 32
8. Socket API Considerations . . . . . . . . . . . . . . . . . . 37
8.1. Get or Set the NAT Friendliness
(SCTP_NAT_FRIENDLY) . . . . . . . . . . . . . . . . . . . 38
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
9.1. New Chunk Flags for Two Existing Chunk Types . . . . . . 38
9.2. Three New Error Causes . . . . . . . . . . . . . . . . . 39
9.3. Two New Chunk Parameter Types . . . . . . . . . . . . . . 40
10. Security Considerations . . . . . . . . . . . . . . . . . . . 40
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 40
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 41
12.1. Normative References . . . . . . . . . . . . . . . . . . 41
12.2. Informative References . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction 1. Introduction
Stream Control Transmission Protocol [RFC4960] provides a reliable Stream Control Transmission Protocol [RFC4960] provides a reliable
communications channel between two end-hosts in many ways similar to communications channel between two end-hosts in many ways similar to
TCP [RFC0793]. With the widespread deployment of Network Address TCP [RFC0793]. With the widespread deployment of Network Address
Translators (NAT), specialized code has been added to NAT for TCP Translators (NAT), specialized code has been added to NAT for TCP
that allows multiple hosts to reside behind a NAT using private that allows multiple hosts to reside behind a NAT using private
addresses (see [RFC6890]) and yet use only a single globally unique addresses (see [RFC6890]) and yet use only a single globally unique
IPv4 address, even when two hosts (behind a NAT) choose the same port IPv4 address, even when two hosts (behind a NAT) choose the same port
numbers for their connection. This additional code is sometimes numbers for their connection. This additional code is sometimes
classified as Network Address and Port Translation (NAPT). To date, classified as Network Address and Port Translation (NAPT). To date,
specialized code for SCTP has not yet been added to most NATs so that 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 only true NAT is available. The end result of this is that only one
SCTP capable host can be behind a NAT. SCTP capable host can be behind a NAT.
This document describes SCTP specific packets and procedures to help This document describes an SCTP specific variant NAT and specific
NATs provide similar features of NAPT in the single-point and multi- packets and procedures to help NATs provide similar features of NAPT
point traversal scenario. An SCTP implementation supporting this in the single-point and multi-point traversal scenario. An SCTP
extension will follow these procedures to assure that in both single- implementation supporting this extension will follow these procedures
homed and multi-homed cases a NAT will maintain the proper state to assure that in both single-homed and multi-homed cases a NAT will
without needing to change port numbers. maintain the proper state without needing to change port numbers.
A NAT will need to follow these procedures for generating appropriate The authors feel it is possible and desirable to make these changes
SCTP packet formats. NATs should refer to [I-D.ietf-behave-sctpnat] for a number of reasons:
for the BCP in using these formats.
o It is desirable for SCTP internal end-hosts on multiple platforms
to be able to share a NAT's public IP address, 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.
An SCTP-aware NAT will need to follow these procedures for generating
appropriate SCTP packet formats.
When considering this feature it is possible to have multiple levels When considering this feature it is possible to have multiple levels
of support. At each level, the Internal Host, External Host and NAT of support. At each level, the Internal Host, External Host and NAT
may or may not support the features described in this document. The may or may not support the features described in this document. The
following table illustrates the results of the various combinations following table illustrates the results of the various combinations
of support and if communications can occur between two endpoints. of support and if communications can occur between two endpoints.
+---------------+------------+---------------+---------------+ +---------------+------------+---------------+---------------+
| Internal Host | NAT | External Host | Communication | | Internal Host | NAT | External Host | Communication |
+---------------+------------+---------------+---------------+ +---------------+------------+---------------+---------------+
skipping to change at page 4, line 10 skipping to change at page 4, line 38
| No Support | Support | Support | Limited | | No Support | Support | Support | Limited |
| No Support | Support | No Support | Limited | | No Support | Support | No Support | Limited |
| No Support | No Support | Support | None | | No Support | No Support | Support | None |
| No Support | No Support | No Support | None | | No Support | No Support | No Support | None |
+---------------+------------+---------------+---------------+ +---------------+------------+---------------+---------------+
Table 1: Communication possibilities Table 1: Communication possibilities
From the table we can see that when a NAT does not support the From the table we can see that when a NAT does not support the
extension no communication can occur. This is because for the most extension no communication can occur. This is because for the most
part of the current situation i. e. SCTP packets sent externally from part of the current situation i. e. SCTP packets sent externally
behind a NAT are discarded by the NAT. In some cases, where the NAT from behind a NAT are discarded by the NAT. In some cases, where the
supports the feature but one of the two external hosts does not NAT supports the feature but one of the two external hosts does not
support the feature, communication may occur but in a limited way. 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 For example only one host may be able to have a connection when a
collision case occurs. collision case occurs.
2. Conventions 2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Terminology 3. Terminology
This document uses the following terms, which are depicted in Figure This document uses the following terms, which are depicted in
1. Figure 1.
Private-Address (Priv-Addr): The private address that is known to Private-Address (Priv-Addr): The private address that is known to
the internal host. the internal host.
Internal-Port (Int-Port): The port number that is in use by the host Internal-Port (Int-Port): The port number that is in use by the host
holding the Private-Address. holding the Private-Address.
Internal-VTag (Int-VTag): The Verification Tag that the internal Internal-VTag (Int-VTag): The Verification Tag that the internal
host has chosen for its communication. The VTag is a unique host has chosen for its communication. The VTag is a unique
32-bit tag that must accompany any incoming SCTP packet for this 32-bit tag that must accompany any incoming SCTP packet for this
skipping to change at page 5, line 9 skipping to change at page 5, line 36
External-VTag (Ext-VTag): The Verification Tag that the host holding External-VTag (Ext-VTag): The Verification Tag that the host holding
the External-Address has chosen for its communication. The VTag the External-Address has chosen for its communication. The VTag
is a unique 32-bit tag that must accompany any incoming SCTP is a unique 32-bit tag that must accompany any incoming SCTP
packet for this association to the External-Address. packet for this association to the External-Address.
Public-Address (Pub-Addr): The public address assigned to the NAT Public-Address (Pub-Addr): The public address assigned to the NAT
box which it uses as a source address when sending packets towards box which it uses as a source address when sending packets towards
the External-Address. the External-Address.
Internal Network | External Network Internal Network | External Network
| |
Private | Public External Private | Public External
+---------+ Address | Address /--\/--\ Address +---------+ +---------+ Address | Address /--\/--\ Address +---------+
| SCTP | +-----+ / \ | SCTP | | SCTP | +-----+ / \ | SCTP |
|end point|=========| NAT |=======| Internet |==========|end point| |end point|=========| NAT |=======| Internet |==========|end point|
| A | +-----+ \ / | B | | A | +-----+ \ / | B |
+---------+ Internal | \--/\--/ External+---------+ +---------+ Internal | \--/\--/ External+---------+
Internal Port | Port External Internal Port | Port External
VTag | VTag VTag | VTag
Figure 1: Basic network setup Figure 1: Basic network setup
4. Data Formats 4. Motivation
4.1. Modified Chunks 4.1. SCTP NAT Traversal Scenarios
This section defines the notion of single and multi-point NAT
traversal.
4.1.1. Single Point Traversal
In this case, all packets in the SCTP association go through a single
NAT, as shown below:
Internal Network | External Network
|
+---------+ | /--\/--\ +---------+
| SCTP | +-----+ / \ | SCTP |
|end point|=========| NAT |========= | Internet | ========|end point|
| A | +-----+ \ / | B |
+---------+ | \--/\--/ +---------+
|
Single NAT scenario
A variation of this case is shown below, i.e., multiple NATs in a
single path:
Internal | External : Internal | External
| : |
+---------+ | : | /--\/--\ +---------+
| SCTP | +-----+ : +-----+ / \ | SCTP |
|end point|==| NAT |=======:=======| NAT |==| Internet |==|end point|
| A | +-----+ : +-----+ \ / | B |
+---------+ | : | \--/\--/ +---------+
| : |
Serial NATs scenario
In this single point traversal 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.
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.
4.1.2. 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:
Internal | External
+------+ /---\/---\
+---------+ /=======|NAT A |=========\ / \ +---------+
| SCTP | / +------+ \/ \ | SCTP |
|end point|/ ... | Internet |===|end point|
| A |\ \ / | B |
+---------+ \ +------+ / \ / +---------+
\=======|NAT B |=========/ \---\/---/
+------+
|
Parallel NATs scenario
This case does NOT apply to a single-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 traversal 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.
4.2. Limitations of Classical NAPT for SCTP
Using classical NAPT may result in changing one of the SCTP port
numbers during the processing which requires the recomputation of the
transport layer checksum. Whereas for UDP and TCP this can be done
very efficiently, for SCTP the checksum (CRC32c) over the entire
packet needs to be recomputed. This would add considerable to the
NAT computational burden, however hardware support may mitigate this
in some implementations.
An SCTP endpoint may have multiple addresses but only has a single
port number. To make multipoint traversal work, all the NATs
involved must recognize the packets they see as belonging to the same
SCTP association and perform port number translation in a consistent
way. One possible way of doing this is to use pre-defined table of
ports and addresses configured within each NAT. Other mechanisms
could make use of NAT to NAT communication. Such mechanisms are
considered by the authors not to be deployable on a wide scale base
and thus not a recommended solution. Therefore the SCTP variant of
NAT has been developed.
4.3. The SCTP Specific Variant of NAT
In this section we assume that we have multiple SCTP capable hosts
behind a NAT which has one Public-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 Public-
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 Private-Address of
the host the packet has to be delivered to. The lookup of the
Private-Address is based on the External-VTag, External-Port,
External-Address, Internal-VTag and the Internal-Port.
For the SCTP NAT processing the NAT box has to maintain a table of
Internal-VTag, Internal-Port, Private-Address, External-VTag,
External-Port and whether the restart procedure is disabled or not.
An entry in that table is called a NAT state control block. The
function Create() obtains the just mentioned parameters and returns a
NAT-State control block.
The entries in this table fulfill some uniqueness conditions. There
must not be more than one entry with the same pair of Internal-Port
and External-Port. This rule can be relaxed, if all entries with the
same Internal-Port and External-Port have the support for the restart
procedure enabled. In this case there must be no more than one entry
with the same Internal-Port, External-Port and Ext-VTag and no more
than one entry with the same Internal-Port, External-Port and Int-
VTag.
The processing of outgoing SCTP packets containing an INIT-chunk is
described in the following figure. The scenario shown is valid for
all message flows in this section.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
INIT[Initiate-Tag]
Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port
Ext-VTag=0
Create(Initiate-Tag, Int-Port, Priv-Addr, 0)
Returns(NAT-State control block)
Translate To:
INIT[Initiate-Tag]
Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port
Ext-VTag=0
It should be noted that normally a NAT control block will be created.
However, it is possible that there is already a NAT control block
with the same External-Address, External-Port, Internal-Port, and
Internal-VTag but different Private-Address. In this case the INIT
SHOULD be dropped by the NAT and an ABORT SHOULD be sent back to the
SCTP host with the M-Bit set and an appropriate error cause (see
Section 5.1.1 for the format). The source address of the packet
containing the ABORT chunk MUST be the destination address of the
packet containing the INIT chunk.
It is also possible that a connection to External-Address and
External-Port exists without an Internal-VTag conflict but the
External-Address does not support the DISABLE_RESTART feature (noted
in the NAT control block when the prior connection was established).
In such a case the INIT SHOULD be dropped by the NAT and an ABORT
SHOULD be sent back to the SCTP host with the M-Bit set and an
appropriate error cause (see Section 5.1.1 for the format).
The processing of outgoing SCTP packets containing no INIT-chunk is
described in the following figure.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port
Ext-VTag
Translate To:
Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port
Ext-VTag
The processing of incoming SCTP packets containing INIT-ACK chunks is
described in the following figure. The Lookup() function getting as
input the Internal-VTag, Internal-Port, External-VTag (=0), External-
Port, and External-Address, returns the corresponding entry of the
NAT table and updates the External-VTag by substituting it with the
value of the Initiate-Tag of the INIT-ACK chunk. The wildcard
character signifies that the parameter's value is not considered in
the Lookup() function or changed in the Update() function,
respectively.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
INIT-ACK[Initiate-Tag]
Pub-Addr:Int-Port <---- Ext-Addr:Ext-Port
Int-VTag
Lookup(Int-VTag, Int-Port, *, 0, Ext-Port)
Update(*, *, *, Initiate-Tag, *)
Returns(NAT-State control block containing Private-Address)
INIT-ACK[Initiate-Tag]
Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
Int-VTag
In the case Lookup fails, the SCTP packet is dropped. The Update
routine inserts the External-VTag (the Initiate-Tag of the INIT-ACK
chunk) in the NAT state control block.
The processing of incoming SCTP packets containing an ABORT or
SHUTDOWN-COMPLETE chunk with the T-Bit set is described in the
following figure.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
Ext-VTag
Lookup(0, Int-Port, *, Ext-VTag, Ext-Port)
Returns(NAT-State control block containing Private-Address)
Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
Ext-VTag
The processing of other incoming SCTP packets is described in the
following figure.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
Int-VTag
Lookup(Int-VTag, Int-Port, *, *, Ext-Port)
Returns(NAT-State control block containing Local-Address)
Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
Int-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
an External-VTag of zero is found, it is considered a match and the
External-VTag is updated.
This allows the handling of INIT-collision through NAT.
5. Data Formats
5.1. Modified Chunks
This section presents existing chunks defined in [RFC4960] that are This section presents existing chunks defined in [RFC4960] that are
modified by this document. modified by this document.
4.1.1. Extended ABORT Chunk 5.1.1. Extended ABORT Chunk
0 1 2 3 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 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 | | Type = 6 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \ \ \
/ zero or more Error Causes / / zero or more Error Causes /
\ \ \ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ABORT chunk is extended to add the new 'M-bit'. The M-bit 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 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. generated by the peer SCTP endpoint, but instead by a middle box.
4.1.2. Extended ERROR Chunk 5.1.2. Extended ERROR Chunk
0 1 2 3 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 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 | | Type = 9 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \ \ \
/ zero or more Error Causes / / zero or more Error Causes /
\ \ \ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ERROR chunk defined in [RFC4960] is extended to add the new 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 '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 the chunk was not generated by the peer SCTP endpoint, but instead by
a middle box. a middle box.
4.2. New Error Causes 5.2. New Error Causes
This section defines the new error causes added by this document. This section defines the new error causes added by this document.
4.2.1. VTag and Port Number Collision Error Cause 5.2.1. VTag and Port Number Collision Error Cause
0 1 2 3 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 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 | | Cause Code = 0x00B0 | Cause Length = Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Chunk / \ Chunk /
/ \ / \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer) Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the VTag and Port This field holds the IANA defined cause code for the VTag and Port
Number Collision Error Cause. The suggested value of this field Number Collision Error Cause. The suggested value of this field
for IANA is 0x00B0. for IANA is 0x00B0.
Cause Length: 2 bytes (unsigned integer) Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The This field holds the length in bytes of the error cause. The
value MUST be the length of the Cause-Specific Information plus 4. value MUST be the length of the Cause-Specific Information plus 4.
Chunk: variable length Chunk: variable length
The Cause-Specific Information is filled with the chunk that The Cause-Specific Information is filled with the chunk that
caused this error. This can be an INIT, INIT-ACK, or ASCONF 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. 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 chunk or ABORT chunk being sent then the bytes that do not fit are
truncated. truncated.
4.2.2. Missing State Error Cause 5.2.2. Missing State Error Cause
0 1 2 3 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 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 | | Cause Code = 0x00B1 | Cause Length = Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Incoming Packet / \ Incoming Packet /
/ \ / \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer) Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the Missing State This field holds the IANA defined cause code for the Missing State
Error Cause. The suggested value of this field for IANA is Error Cause. The suggested value of this field for IANA is
0x00B1. 0x00B1.
Cause Length: 2 bytes (unsigned integer) Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The This field holds the length in bytes of the error cause. The
value MUST be the length of the Cause-Specific Information plus 4. value MUST be the length of the Cause-Specific Information plus 4.
Incoming Packet: variable length Incoming Packet: variable length
The Cause-Specific Information is filled with the IPv4 or IPv6 The Cause-Specific Information is filled with the IPv4 or IPv6
packet that caused this error. The IPv4 or IPv6 header MUST be 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 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 or ABORT chunk being sent then the bytes that do not fit are
truncated. truncated.
4.2.3. Port Number Collision Error Cause 5.2.3. Port Number Collision Error Cause
0 1 2 3 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 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 | | Cause Code = 0x00B2 | Cause Length = Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ chunk / \ chunk /
/ \ / \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer) Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the Port Number This field holds the IANA defined cause code for the Port Number
Collision Error Cause. The suggested value of this field for IANA Collision Error Cause. The suggested value of this field for IANA
is 0x00B2. is 0x00B2.
Cause Length: 2 bytes (unsigned integer) Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The This field holds the length in bytes of the error cause. The
value MUST be the length of the Cause-Specific Information plus 4. value MUST be the length of the Cause-Specific Information plus 4.
Chunk: variable length Chunk: variable length
The Cause-Specific Information is filled with the chunk that The Cause-Specific Information is filled with the chunk that
caused this error. This can be an INIT, INIT-ACK, or ASCONF 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. 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 chunk or ABORT chunk being sent then the bytes that do not fit are
truncated. truncated.
4.3. New Parameters 5.3. New Parameters
This section defines new parameters and their valid appearance This section defines new parameters and their valid appearance
defined by this document. defined by this document.
4.3.1. Disable Restart Parameter 5.3.1. Disable Restart Parameter
This parameter is used to indicate that the RESTART procedure is This parameter is used to indicate that the RESTART procedure is
requested to be disabled. Both endpoints of an association MUST requested to be disabled. Both endpoints of an association MUST
include this parameter in the INIT chunk and INIT-ACK chunk when include this parameter in the INIT chunk and INIT-ACK chunk when
establishing an association and MUST include it in the ASCONF chunk establishing an association and MUST include it in the ASCONF chunk
when adding an address to successfully disable the restart procedure. when adding an address to successfully disable the restart procedure.
0 1 2 3 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 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 | | Type = 0xC007 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer) Parameter Type: 2 bytes (unsigned integer)
This field holds the IANA defined parameter type for the Disable This field holds the IANA defined parameter type for the Disable
Restart Parameter. The suggested value of this field for IANA is Restart Parameter. The suggested value of this field for IANA is
0xC007. 0xC007.
Parameter Length: 2 bytes (unsigned integer) Parameter Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the parameter. The value This field holds the length in bytes of the parameter. The value
MUST be 4. MUST be 4.
This parameter MAY appear in INIT, INIT-ACK and ASCONF chunks and This parameter MAY appear in INIT, INIT-ACK and ASCONF chunks and
MUST NOT appear in any other chunk. MUST NOT appear in any other chunk.
4.3.2. VTags Parameter 5.3.2. VTags Parameter
This parameter is used to help a NAT recover from state loss. This parameter is used to help a NAT recover from state loss.
0 1 2 3 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 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 | | Parameter Type = 0xC008 | Parameter Length = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASCONF-Request Correlation ID | | ASCONF-Request Correlation ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internal Verification Tag | | Internal Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Verification Tag | | External Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer) Parameter Type: 2 bytes (unsigned integer)
This field holds the IANA defined parameter type for the VTags This field holds the IANA defined parameter type for the VTags
Parameter. The suggested value of this field for IANA is 0xC008. Parameter. The suggested value of this field for IANA is 0xC008.
Parameter Length: 2 bytes (unsigned integer) Parameter Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the parameter. The value This field holds the length in bytes of the parameter. The value
MUST be 16. MUST be 16.
ASCONF-Request Correlation ID: 4 bytes (unsigned integer) ASCONF-Request Correlation ID: 4 bytes (unsigned integer)
skipping to change at page 9, line 43 skipping to change at page 16, line 25
External Verification Tag: 4 bytes (unsigned integer) The External Verification Tag: 4 bytes (unsigned integer) The
Verification Tag that the host holding the External-Address has Verification Tag that the host holding the External-Address has
chosen for its communication. The VTag is a unique 32-bit tag chosen for its communication. The VTag is a unique 32-bit tag
that must accompany any incoming SCTP packet for this association that must accompany any incoming SCTP packet for this association
to the External-Address. to the External-Address.
This parameter MAY appear in ASCONF chunks and MUST NOT appear in any This parameter MAY appear in ASCONF chunks and MUST NOT appear in any
other chunk. other chunk.
5. Problem Space and Procedures 6. Procedures
5.1. Problem Space Overview 6.1. Overview
When an SCTP endpoint is behind a NAT which supports When an SCTP endpoint is behind an SCTP-aware NAT a number of
[I-D.ietf-behave-sctpnat] a number of problems may arise as it tries problems may arise as it tries to communicate with its peer:
to communicate with its peer:
o More than one server behind a NAT may pick the same VTag and o More than one host behind a NAT may pick the same VTag and source
source port when talking to the same peer server. This creates a port when talking to the same peer server. This creates a
situation where the NAT will not be able to tell the two situation where the NAT will not be able to tell the two
associations apart. This situation is discussed in Section 5.3. associations apart. This situation is discussed in Section 6.3.
o When an SCTP endpoint is a server communicating with multiple o When an SCTP endpoint is a server communicating with multiple
peers and the peers are behind the same NAT, then the two peers and the peers are behind the same NAT, then the two
endpoints cannot be distinguished by the server. This case is endpoints cannot be distinguished by the server. This case is
discussed in Section 5.4. discussed in Section 6.4.
o A restart of a NAT during a conversation could cause a loss of its 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. state. This problem and its solution is discussed in Section 6.5.
o An SCTP endpoint may be behind two NATs providing redundancy. The o An SCTP endpoint may be behind two NATs providing redundancy. The
method to set up this scenario is discussed in Section 5.6. method to set up this scenario is discussed in Section 6.7.
Each of these solutions requires additional chunks and parameters, Each of these mechanisms requires additional chunks and parameters,
defined in this document, and possibly modified handling procedures defined in this document, and possibly modified handling procedures
from those specified in [RFC4960]. from those specified in [RFC4960] fdafdfafdafdafdafdafdasf.
5.2. Association Setup Considerations 6.2. Association Setup Considerations
Every association MUST initially be set up single-homed. There MUST Every association MUST initially be set up single-homed. There MUST
NOT be any IPv4 Address parameter, IPv6 Address parameter, or NOT be any IPv4 Address parameter, IPv6 Address parameter, or
Supported Address Types parameter in the INIT-chunk. The INIT-ACK Supported Address Types parameter in the INIT-chunk. The INIT-ACK
chunk MUST NOT contain any IPv4 Address parameter or IPv6 Address chunk MUST NOT contain any IPv4 Address parameter or IPv6 Address
parameter. parameter.
If the association should finally be multi-homed, the procedure in If the association should finally be multi-homed, the procedure in
Section 5.6 MUST be used. Section 6.7 MUST be used.
The INIT and INIT-ACK chunk SHOULD contain the Disable Restart The INIT and INIT-ACK chunk SHOULD contain the Disable Restart
parameter defined in Section 4.3.1. parameter defined in Section 5.3.1.
5.3. Handling of Internal Port Number and Verification Tag Collisions 6.3. Handling of Internal Port Number and Verification Tag Collisions
Consider the case where two hosts in the Private-Address space want 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 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 same host in the Internet. This means that the External-Port and the
External-Address are the same. If they both choose the same External-Address are the same. If they both choose the same
Internal-Port and Internal-VTag, the NAT box cannot distinguish Internal-Port and Internal-VTag, the NAT box cannot distinguish
between incoming packets anymore. But this is very unlikely. The between incoming packets anymore. But this is very unlikely. The
Internal-VTags are chosen at random and if the Internal-Ports are Internal-VTags are chosen at random and if the Internal-Ports are
also chosen from the ephemeral port range at random this gives a 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 46-bit random number which has to match. In the TCP like NAPT case
skipping to change at page 11, line 13 skipping to change at page 17, line 41
collisions deterministically. collisions deterministically.
The same can happen when an INIT-ACK chunk or an ASCONF chunk is The same can happen when an INIT-ACK chunk or an ASCONF chunk is
processed by the NAT. processed by the NAT.
However, in this unlikely event the NAT box MUST send an ABORT chunk 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- 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 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 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 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 "middle" box, not the peer SCTP endpoint (see Section 5.1.1). If a
packet containing an INIT-ACK chunk triggers the collision, the packet containing an INIT-ACK chunk triggers the collision, the
corresponding packet containing the ABORT chunk MUST contain the same corresponding packet containing the ABORT chunk MUST contain the same
source and destination address and port numbers as the packet source and destination address and port numbers as the packet
containing the INIT-ACK chunk. In the other two cases, the source containing the INIT-ACK chunk. In the other two cases, the source
and destination address and port numbers MUST be swapped. and destination address and port numbers MUST be swapped.
The sender of the packet containing the INIT chunk or the receiver of 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, the INIT-ACK chunk, upon reception of an ABORT chunk with M-bit set,
SHOULD reinitiate the association setup procedure after choosing a SHOULD reinitiate the association setup procedure after choosing a
new initiate tag. These procedures SHOULD be followed only if the new initiate tag. These procedures SHOULD be followed only if the
appropriate error cause code for colliding NAT table state is appropriate error cause code for colliding NAT table state is
included AND the association is in the COOKIE-WAIT state (i. e. it 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 awaiting an INIT-ACK). If the endpoint is in any other state an SCTP
endpoint SHOULD NOT respond. endpoint SHOULD NOT respond.
The sender of the ASCONF chunk, upon reception of an ERROR chunk with The sender of the ASCONF chunk, upon reception of an ERROR chunk with
M-bit set, MUST stop adding the path to the association. M-bit set, MUST stop adding the path to the association.
The sender of the ERROR or ABORT chunk MUST include the error cause 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). with cause code 'VTag and Port Number Collision' (see Section 5.2.1).
5.4. Handling of Internal Port Number Collisions 6.4. Handling of Internal Port Number Collisions
When two SCTP hosts are behind a NAT and using the recommendations in When two SCTP hosts are behind an SCTP-aware NAT it is possible that
[I-D.ietf-behave-sctpnat] it is possible that two SCTP hosts in the two SCTP hosts in the Private-Address space will want to set up an
Private-Address space will want to set up an SCTP association with SCTP association with the same server running on the same host in the
the same server running on the same host in the Internet. For the Internet. For the NAT appropriate tracking may be performed by
NAT appropriate tracking may be performed by assuring that the VTags assuring that the VTags are unique between the two hosts. But for
are unique between the two hosts as defined in the external SCTP server on the internet this means that the
[I-D.ietf-behave-sctpnat]. But for the external SCTP server on the External-Port and the External-Address are the same. If they both
internet this means that the External-Port and the External-Address have chosen the same Internal-Port the server cannot distinguish
are the same. If they both have chosen the same Internal-Port the between both associations based on the address and port numbers. For
server cannot distinguish between both associations based on the the server it looks like the association is being restarted. To
address and port numbers. For the server it looks like the overcome this limitation the client sends a Disable Restart parameter
association is being restarted. To overcome this limitation the in the INIT-chunk.
client sends a Disable Restart parameter in the INIT-chunk.
When the server receives this parameter it MUST do the following: When the server receives this parameter it MUST do the following:
o Include a Disable Restart parameter in the INIT-ACK to inform the o Include a Disable Restart parameter in the INIT-ACK to inform the
client that it will support the feature. client that it will support the feature.
o Disable the restart procedures defined in [RFC4960] for this o Disable the restart procedures defined in [RFC4960] for this
association. association.
Servers that support this feature will need to be capable of Servers that support this feature will need to be capable of
skipping to change at page 12, line 34 skipping to change at page 19, line 13
internal mapping table. internal mapping table.
o If the INIT matches the external address and port of an already o If the INIT matches the external address and port of an already
existing connection, validate that the external server supports existing connection, validate that the external server supports
the Disable Restart feature, if it does allow the INIT to be the Disable Restart feature, if it does allow the INIT to be
forwarded. forwarded.
o If the external server does not support the Disable Restart o If the external server does not support the Disable Restart
extension the NAT MUST send an ABORT with the M-bit set. 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 The 'Port Number Collision' error cause (see Section 5.2.3) MUST be
included in the ABORT chunk. included in the ABORT chunk.
If the collision is triggered by an ASCONF chunk, a packet containing If the collision is triggered by an ASCONF chunk, a packet containing
an ERROR chunk with the 'Port Number Collision' error cause MUST be an ERROR chunk with the 'Port Number Collision' error cause MUST be
sent back. sent back.
5.5. Handling of Missing State 6.5. Handling of Missing State
If the NAT box receives a packet from the internal network for which 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 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. 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 The source address of the packet containing the ERROR chunk MUST be
the destination address of the incoming SCTP packet. The the destination address of the incoming SCTP packet. The
verification tag is reflected and the T-bit is set. Please note that 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 such a packet containing an ERROR chunk SHOULD NOT be sent if the
received packet contains an ABORT, SHUTDOWN-COMPLETE or INIT-ACK received packet contains an ABORT, SHUTDOWN-COMPLETE or INIT-ACK
chunk. An ERROR chunk MUST NOT be sent if the received packet chunk. An ERROR chunk MUST NOT be sent if the received packet
contains an ERROR chunk with the M-bit set. contains an ERROR chunk with the M-bit set.
When sending the ERROR chunk, the new error cause Missing state (see 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 Section 5.2.2) MUST be included and the new M-bit of the ERROR chunk
MUST be set (see Section 4.1.2). MUST be set (see Section 5.1.2).
Upon reception of this ERROR chunk by an SCTP endpoint the receiver Upon reception of this ERROR chunk by an SCTP endpoint the receiver
SHOULD take the following actions: SHOULD take the following actions:
o Validate that the verification tag is reflected by looking at the o Validate that the verification tag is reflected by looking at the
VTag that would have been included in the outgoing packet. VTag that would have been included in the outgoing packet.
o Validate that the peer of the SCTP association supports the o Validate that the peer of the SCTP association supports the
dynamic address extension, if it does not discard the incoming dynamic address extension, if it does not discard the incoming
ERROR chunk. ERROR chunk.
o Generate a new ASCONF chunk containing the VTags parameter (see o Generate a new ASCONF chunk containing the VTags parameter (see
Section 4.3.2) and the Disable Restart parameter if the Section 5.3.2) and the Disable Restart parameter if the
association is using the disabled restart feature. By processing association is using the disabled restart feature. By processing
this packet the NAT can recover the appropriate state. The this packet the NAT can recover the appropriate state. The
procedures for generating an ASCONF chunk can be found in procedures for generating an ASCONF chunk can be found in
[RFC5061]. [RFC5061].
If the NAT box receives a packet for which it has no NAT table entry 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 and the packet contains an ASCONF chunk with the VTags parameter, the
NAT box MUST update its NAT table according to the verification tags NAT box MUST update its NAT table according to the verification tags
in the VTags parameter and the optional Disable Restart parameter. in the VTags parameter and the optional Disable Restart parameter.
skipping to change at page 13, line 43 skipping to change at page 20, line 22
new to the association (following all procedures defined in new to the association (following all procedures defined in
[RFC5061]). Or, if the address is already part of the association, [RFC5061]). Or, if the address is already part of the association,
the SCTP endpoint MUST NOT respond with an error, but instead should the SCTP endpoint MUST NOT respond with an error, but instead should
respond with an ASCONF-ACK chunk acknowledging the address but take respond with an ASCONF-ACK chunk acknowledging the address but take
no action (since the address is already in the association). no action (since the address is already in the association).
Note that it is possible that upon receiving an ASCONF chunk Note that it is possible that upon receiving an ASCONF chunk
containing the VTags parameter the NAT will realize that it has an containing the VTags parameter the NAT will realize that it has an
'Internal Port Number and Verification Tag collision'. In such a 'Internal Port Number and Verification Tag collision'. In such a
case the NAT MUST send an ERROR chunk with the error cause code set 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). to 'VTag and Port Number Collision' (see Section 5.2.1).
If an SCTP endpoint receives an ERROR with 'Internal Port Number and If an SCTP endpoint receives an ERROR with 'Internal Port Number and
Verification Tag collision' as the error cause and the packet in the Verification Tag collision' as the error cause and the packet in the
Error Chunk contains an ASCONF with the VTags parameter, careful Error Chunk contains an ASCONF with the VTags parameter, careful
examination of the association is required. The endpoint MUST do the examination of the association is required. The endpoint MUST do the
following: following:
o Validate that the verification tag is reflected by looking at the o Validate that the verification tag is reflected by looking at the
VTag that would have been included in the outgoing packet. VTag that would have been included in the outgoing packet.
o Validate that the peer of the SCTP association supports the o Validate that the peer of the SCTP association supports the
dynamic address extension, if it does not discard the incoming dynamic address extension, if it does not discard the incoming
ERROR chunk. ERROR chunk.
o If the association is attempting to add an address (i. e. o If the association is attempting to add an address (i. e.
following the procedures in Section 5.6) then the endpoint MUST- following the procedures in Section 6.7) then the endpoint MUST-
NOT consider the address part of the association and SHOULD make NOT consider the address part of the association and SHOULD make
no further attempt to add the address (i. e. cancel any ASCONF 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 timers and remove any record of the path), since the NAT has a
VTag collision and the association cannot easily create a new VTag VTag collision and the association cannot easily create a new VTag
(as it would if the error occurred when sending an INIT). (as it would if the error occurred when sending an INIT).
o If the endpoint has no other path, i. e. the procedure was 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 executed due to missing a state in the NAT, then the endpoint MUST
abort the association. This would occur only if the local NAT abort the association. This would occur only if the local NAT
restarted and accepted a new association before attempting to restarted and accepted a new association before attempting to
repair the missing state (Note that this is no different than what repair the missing state (Note that this is no different than what
happens to all TCP connections when a NAT looses its state). happens to all TCP connections when a NAT looses its state).
5.6. Multi-Point Traversal Considerations 6.6. 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 corresponding ICMP packet SHOULD be
sent.
6.7. Multi-Point Traversal Considerations
If a multi-homed SCTP endpoint behind a NAT connects to a peer, it 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 SHOULD first set up the association single-homed with only one
address causing the first NAT to populate its state. Then it SHOULD address causing the first NAT to populate its state. Then it SHOULD
add each IP address using ASCONF chunks sent via their respective add each IP address using ASCONF chunks sent via their respective
NATs. The address to add is the wildcard address and the lookup NATs. The address to add is the wildcard address and the lookup
address SHOULD also contain the VTags parameter and optionally the address SHOULD also contain the VTags parameter and optionally the
Disable Restart parameter as illustrated above. Disable Restart parameter as illustrated above.
6. Socket API Considerations 7. Various Examples of NAT Traversals
7.1. Single-homed Client to Single-homed Server
The internal client starts the association with the external server
via a four-way-handshake. Host A starts by sending an INIT chunk.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
+---------+--------+-----------+----------+--------+
NAT | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+--- -------+----------+--------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 ------> 100.0.0.1:2
Ext-VTtag = 0
A NAT entry is created, the source address is substituted and the
packet is sent on:
NAT creates entry:
+---------+--------+-----------+----------+--------+
NAT | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 0 | 2 |
+---------+--------+-----------+----------+--------+
INIT[Initiate-Tag = 1234]
101.0.0.1:1 --------------------------> 100.0.0.1:2
Ext-VTtag = 0
Host B receives the INIT and sends an INIT-ACK with the NAT's
external address as destination address.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
INIT-ACK[Initiate-Tag = 5678]
101.0.0.1:1 <------------------------- 100.0.0.1:2
Int-VTag = 1234
NAT updates entry:
+---------+--------+-----------+----------+--------+
NAT | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 |
+---------+--------+-----------+----------+--------+
INIT-ACK[Initiate-Tag = 5678]
10.0.0.1:1 <------ 100.0.0.1:2
Int-VTag = 1234
The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE-
ACK.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
COOKIE-ECHO
10.0.0.1:1 ------> 100.0.0.1:2
Ext-VTag = 5678
COOKIE-ECHO
101.0.0.1:1 -------------------------> 100.0.0.1:2
Ext-VTag = 5678
COOKIE-ACK
101.0.0.1:1 <------------------------- 100.0.0.1:2
Int-VTag = 1234
COOKIE-ACK
10.0.0.1:1 <------ 100.0.0.1:2
Int-VTag = 1234
7.2. Single-homed Client to Multi-homed Server
The internal client is single-homed whereas the external server is
multi-homed. The client (Host A) sends an INIT like in the single-
homed case.
+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <-----> | NAT | <-> | Internet | == =| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
+---------+--------+-----------+----------+--------+
NAT | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 ---> 100.0.0.1:2
Ext-VTag = 0
NAT creates entry:
+---------+--------+-----------+----------+--------+
NAT | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 0 | 2 |
+---------+--------+-----------+----------+--------+
INIT[Initiate-Tag = 1234]
101.0.0.1:1 ----------------------------> 100.0.0.1:2
Ext-VTag = 0
The server (Host B) includes its two addresses in the INIT-ACK chunk,
which results in two NAT entries.
+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <-----> | NAT | <-> | Internet | == =| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
INIT-ACK[Initiate-tag = 5678, IP-Addr = 100.1.0.1]
101.0.0.1:1 <---------------------------- 100.0.0.1:2
Int-VTag = 1234
NAT does need to change the table for second address:
+---------+--------+-----------+----------+--------+
NAT | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 |
+---------+--------+-----------+----------+--------+
INIT-ACK[Initiate-Tag = 5678]
10.0.0.1:1 <--- 100.0.0.1:2
Int-VTag = 1234
The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE-
ACK.
+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <-----> | NAT | <-> | Internet | == =| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
COOKIE-ECHO
10.0.0.1:1 ---> 100.0.0.1:2
ExtVTag = 5678
COOKIE-ECHO
101.0.0.1:1 ----------------------------> 100.0.0.1:2
Ext-VTag = 5678
COOKIE-ACK
101.0.0.1:1 <---------------------------- 100.0.0.1:2
Int-VTag = 1234
COOKIE-ACK
10.0.0.1:1 <--- 100.0.0.1:2
Int-VTag = 1234
7.3. Multihomed Client and Server
The client (Host A) sends an INIT to the server (Host B), but does
not include the second address.
+-------+
/--| NAT 1 |--\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |====| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--| NAT 2 |--/ \--/\--/
+-------+
+---------+--------+-----------+----------+--------+
NAT 1 | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+--- -------+----------+--------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 --------> 100.0.0.1:2
Ext-VTag = 0
NAT 1 creates entry:
+---------+--------+-----------+----------+--------+
NAT 1 | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 0 | 2 |
+---------+--------+-----------+----------+--------+
INIT[Initiate-Tag = 1234]
101.0.0.1:1 -----------------------> 100.0.0.1:2
ExtVTag = 0
Host B includes its second address in the INIT-ACK, which results in
two NAT entries in NAT 1.
+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |===| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
INIT-ACK[Initiate-Tag = 5678, IP-Addr = 100.1.0.1]
101.0.0.1:1 <------------------------- 100.0.0.1:2
Int-VTag = 1234
NAT 1 does not need to update the table for second address:
+---------+--------+-----------+----------+--------+
NAT 1 | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 |
+---------+--------+-----------+----------+--------+
INIT-ACK[Initiate-Tag = 5678]
10.0.0.1:1 &lt;---------100.0.0.1:2
Int-VTag = 1234
The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE-
ACK.
+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |===| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
COOKIE-ECHO
10.0.0.1:1 --------> 100.0.0.1:2
Ext-VTag = 5678
COOKIE-ECHO
101.0.0.1:1 --------------------> 100.0.0.1:2
Ext-VTag = 5678
COOKIE-ACK
101.0.0.1:1 <-------------------- 100.0.0.1:2
Int-VTag = 1234
COOKIE-ACK
10.0.0.1:1 <------- 100.0.0.1:2
Int-VTag = 1234
Host A announces its second address in an ASCONF chunk. The address
parameter contains an undefined address (0) to indicate that the
source address should be added. The lookup address parameter within
the ASCONF chunk will also contain the pair of VTags (external and
internal) so that the NAT may populate its table completely with this
single packet.
+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |===| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
ASCONF [ADD-IP=0.0.0.0, INT-VTag=1234, Ext-VTag = 5678]
10.1.0.1:1 --------> 100.1.0.1:2
Ext-VTag = 5678
NAT 2 creates complete entry:
+---------+--------+-----------+----------+--------+
NAT 2 | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.1.0.1 | 5678 | 2 |
+---------+--------+-----------+----------+--------+
ASCONF [ADD-IP,Int-VTag=1234, Ext-VTag = 5678]
101.1.0.1:1 -----------------------> 100.1.0.1:2
Ext-VTag = 5678
ASCONF-ACK
101.1.0.1:1 <----------------------- 100.1.0.1:2
Int-VTag = 1234
ASCONF-ACK
10.1.0.1:1 <----- 100.1.0.1:2
Int-VTag = 1234
7.4. NAT Loses Its State
Association is already established between Host A and Host B, when
the NAT loses its state and obtains a new public address. Host A
sends a DATA chunk to Host B.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
+---------+--------+-----------+----------+--------+
NAT | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 |
+---------+--------+-----------+----------+--------+
DATA
10.0.0.1:1 ----------> 100.0.0.1:2
Ext-VTag = 5678
The NAT box cannot find entry for the association. It sends ERROR
message with the M-Bit set and the cause "NAT state missing".
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
ERROR [M-Bit, NAT state missing]
10.0.0.1:1 <---------- 100.0.0.1:2
Ext-VTag = 5678
On reception of the ERROR message, Host A sends an ASCONF chunk
indicating that the former information has to be deleted and the
source address of the actual packet added.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678]
10.0.0.1:1 ----------> 100.1.0.1:2
Ext-VTag = 5678
+---------+--------+-----------+----------+--------+
NAT | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 |
+---------+--------+-----------+----------+--------+
ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678]
102.1.0.1:1 ---------------------> 100.1.0.1:2
Ext-VTag = 5678
Host B adds the new source address and deletes all former entries.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
ASCONF-ACK
102.1.0.1:1 <--------------------- 100.1.0.1:2
Int-VTag = 1234
ASCONF-ACK
10.1.0.1:1 <---------- 100.1.0.1:2
Int-VTag = 1234
DATA
10.0.0.1:1 ----------> 100.0.0.1:2
Ext-VTag = 5678
DATA
102.1.0.1:1 ---------------------> 100.1.0.1:2
Ext-VTag = 5678
7.5. Peer-to-Peer Communication
If two hosts are behind NATs, they have to get knowledge of the
peer's public address. This can be achieved with a so-called
rendezvous server. Afterwards the destination addresses are public,
and the association is set up with the help of the INIT collision.
The NAT boxes create their entries according to their internal peer's
point of view. Therefore, NAT A's Internal-VTag and Internal-Port
are NAT B's External-VTag and External-Port, respectively. The
naming of the verification tag in the packet flow is done from the
sending peer's point of view.
Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
NAT-Tables
+---------+--------+-----------+----------+--------+
NAT A | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+--- -------+----------+--------+
+---------+--------+-----------+----------+--------+
NAT B | Int | Int | Priv | Ext | Ext |
| v-tag | port | addr | v-tag | port |
+---------+--------+--- -------+----------+--------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 --> 100.0.0.1:2
Ext-VTag = 0
NAT A creates entry:
+---------+--------+-----------+----------+--------+
NAT A | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 0 | 2 |
+---------+--------+-----------+----------+--------+
INIT[Initiate-Tag = 1234]
101.0.0.1:1 ----------------> 100.0.0.1:2
Ext-VTag = 0
NAT B processes INIT, but cannot find an entry. The SCTP packet is
silently discarded and leaves the NAT table of NAT B unchanged.
+---------+--------+-----------+----------+--------+
NAT B | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
Now Host B sends INIT, which is processed by NAT B. Its parameters
are used to create an entry.
Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
INIT[Initiate-Tag = 5678]
101.0.0.1:1 <-- 10.1.0.1:2
Ext-VTag = 0
+---------+--------+-----------+----------+--------+
NAT B | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 5678 | 2 | 10.1.0.1 | 0 | 1 |
+---------+--------+-----------+----------+--------+
INIT[Initiate-Tag = 5678]
101.0.0.1:1 <--------------- 100.0.0.1:2
Ext-VTag = 0
NAT A processes INIT. As the outgoing INIT of Host A has already
created an entry, the entry is found and updated:
Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
VTag != Int-VTag, but Ext-VTag == 0, find entry.
+---------+--------+-----------+----------+--------+
NAT A | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 |
+---------+--------+-----------+----------+--------+
INIT[Initiate-tag = 5678]
10.0.0.1:1 <-- 100.0.0.1:2
Ext-VTag = 0
Host A send INIT-ACK, which can pass through NAT B:
Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
INIT-ACK[Initiate-Tag = 1234]
10.0.0.1:1 -->; 100.0.0.1:2
Ext-VTag = 5678
INIT-ACK[Initiate-Tag = 1234]
101.0.0.1:1 ----------------> 100.0.0.1:2
Ext-VTag = 5678
NAT B updates entry:
+---------+--------+-----------+----------+--------+
NAT B | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 5678 | 2 | 10.1.0.1 | 1234 | 1 |
+---------+--------+-----------+----------+--------+
INIT-ACK[Initiate-Tag = 1234]
101.0.0.1:1 --> 10.1.0.1:2
Ext-VTag = 5678
The lookup for COOKIE-ECHO and COOKIE-ACK is successful.
Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
COOKIE-ECHO
101.0.0.1:1 <-- 10.1.0.1:2
Ext-VTag = 1234
COOKIE-ECHO
101.0.0.1:1 <------------- 100.0.0.1:2
Ext-VTag = 1234
COOKIE-ECHO
10.0.0.1:1 <-- 100.0.0.1:2
Ext-VTag = 1234
COOKIE-ACK
10.0.0.1:1 --> 100.0.0.1:2
Ext-VTag = 5678
COOKIE-ACK
101.0.0.1:1 ----------------> 100.0.0.1:2
Ext-VTag = 5678
COOKIE-ACK
101.0.0.1:1 --> 10.1.0.1:2
Ext-VTag = 5678
8. Socket API Considerations
This section describes how the socket API defined in [RFC6458] is This section describes how the socket API defined in [RFC6458] is
extended to provide a way for the application to control NAT extended to provide a way for the application to control NAT
friendliness. friendliness.
Please note that this section is informational only. Please note that this section is informational only.
A socket API implementation based on [RFC6458] is extended by A socket API implementation based on [RFC6458] is extended by
supporting one new read/write socket option. supporting one new read/write socket option.
6.1. Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY) 8.1. Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY)
This socket option uses the option_level IPPROTO_SCTP and the This socket option uses the option_level IPPROTO_SCTP and the
option_name SCTP_NAT_FRIENDLY. It can be used to enable/disable the option_name SCTP_NAT_FRIENDLY. It can be used to enable/disable the
NAT friendliness for future associations and retrieve the value for NAT friendliness for future associations and retrieve the value for
future and specific ones. future and specific ones.
struct sctp_assoc_value { struct sctp_assoc_value {
sctp_assoc_t assoc_id; sctp_assoc_t assoc_id;
uint32_t assoc_value; uint32_t assoc_value;
}; };
assoc_id: This parameter is ignored for one-to-one style sockets. assoc_id: This parameter is ignored for one-to-one style sockets.
For one-to-many style sockets the application may fill in an For one-to-many style sockets the application may fill in an
association identifier or SCTP_FUTURE_ASSOC for this query. It is association identifier or SCTP_FUTURE_ASSOC for this query. It is
an error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id. an error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.
assoc_value: A non-zero value indicates a NAT-friendly mode. assoc_value: A non-zero value indicates a NAT-friendly mode.
7. IANA Considerations 9. IANA Considerations
[NOTE to RFC-Editor: [NOTE to RFC-Editor:
"RFCXXXX" is to be replaced by the RFC number you assign this "RFCXXXX" is to be replaced by the RFC number you assign this
document. document.
] ]
[NOTE to RFC-Editor: [NOTE to RFC-Editor:
The suggested values for the chunk type and the chunk parameter The suggested values for the chunk type and the chunk parameter
types are tentative and to be confirmed by IANA. types are tentative and to be confirmed by IANA.
] ]
This document (RFCXXXX) is the reference for all registrations This document (RFCXXXX) is the reference for all registrations
described in this section. The suggested changes are described described in this section. The suggested changes are described
below. below.
7.1. New Chunk Flags for Two Existing Chunk Types 9.1. New Chunk Flags for Two Existing Chunk Types
As defined in [RFC6096] two chunk flags have to be assigned by IANA 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 ERROR chunk. The suggested value for the T bit is 0x01 and
for the M bit is 0x02. for the M bit is 0x02.
This requires an update of the "ERROR Chunk Flags" registry for SCTP: This requires an update of the "ERROR Chunk Flags" registry for SCTP:
ERROR Chunk Flags ERROR Chunk Flags
+------------------+-----------------+-----------+ +------------------+-----------------+-----------+
skipping to change at page 16, line 38 skipping to change at page 39, line 40
| 0x01 | T bit | [RFC4960] | | 0x01 | T bit | [RFC4960] |
| 0x02 | M bit | [RFCXXXX] | | 0x02 | M bit | [RFCXXXX] |
| 0x04 | Unassigned | | | 0x04 | Unassigned | |
| 0x08 | Unassigned | | | 0x08 | Unassigned | |
| 0x10 | Unassigned | | | 0x10 | Unassigned | |
| 0x20 | Unassigned | | | 0x20 | Unassigned | |
| 0x40 | Unassigned | | | 0x40 | Unassigned | |
| 0x80 | Unassigned | | | 0x80 | Unassigned | |
+------------------+-----------------+-----------+ +------------------+-----------------+-----------+
7.2. Three New Error Causes 9.2. Three New Error Causes
Three error causes have to be assigned by IANA. It is suggested to Three error causes have to be assigned by IANA. It is suggested to
use the values given below. use the values given below.
This requires three additional lines in the "Error Cause Codes" This requires three additional lines in the "Error Cause Codes"
registry for SCTP: registry for SCTP:
Error Cause Codes Error Cause Codes
+-------+--------------------------------+-----------+ +-------+--------------------------------+-----------+
| Value | Cause Code | Reference | | Value | Cause Code | Reference |
+-------+--------------------------------+-----------+ +-------+--------------------------------+-----------+
| 176 | VTag and Port Number Collision | [RFCXXXX] | | 176 | VTag and Port Number Collision | [RFCXXXX] |
| 177 | Missing State | [RFCXXXX] | | 177 | Missing State | [RFCXXXX] |
| 178 | Port Number Collision | [RFCXXXX] | | 178 | Port Number Collision | [RFCXXXX] |
+-------+--------------------------------+-----------+ +-------+--------------------------------+-----------+
7.3. Two New Chunk Parameter Types 9.3. Two New Chunk Parameter Types
Two chunk parameter types have to be assigned by IANA. It is Two chunk parameter types have to be assigned by IANA. It is
suggested to use the values given below. IANA should assign these suggested to use the values given below. IANA should assign these
values from the pool of parameters with the upper two bits set to values from the pool of parameters with the upper two bits set to
'11'. '11'.
This requires two additional lines in the "Chunk Parameter Types" This requires two additional lines in the "Chunk Parameter Types"
registry for SCTP: registry for SCTP:
Chunk Parameter Types Chunk Parameter Types
+----------+--------------------------+-----------+ +----------+--------------------------+-----------+
| ID Value | Chunk Parameter Type | Reference | | ID Value | Chunk Parameter Type | Reference |
+----------+--------------------------+-----------+ +----------+--------------------------+-----------+
| 49159 | Disable Restart (0xC007) | [RFCXXXX] | | 49159 | Disable Restart (0xC007) | [RFCXXXX] |
| 49160 | VTags (0xC008) | [RFCXXXX] | | 49160 | VTags (0xC008) | [RFCXXXX] |
+----------+--------------------------+-----------+ +----------+--------------------------+-----------+
8. Security Considerations 10. Security Considerations
The document does not add any additional security considerations to State maintenance within a NAT is always a subject of possible Denial
the ones given in [RFC4960], [RFC4895], and [RFC5061]. Of Service attacks. This document recommends that at a minimum a NAT
runs a timer on any SCTP state so that old association state can be
cleaned up.
9. Acknowledgments For SCTP end-points, this document does not add any additional
security considerations to the ones given in [RFC4960], [RFC4895],
and [RFC5061].
11. Acknowledgments
The authors wish to thank Jason But, Bryan Ford, David Hayes, Alfred The authors wish to thank Jason But, Bryan Ford, David Hayes, Alfred
Hines, Henning Peters, Timo Voelker, Dan Wing, and Qiaobing Xie for Hines, Henning Peters, Timo Voelker, Dan Wing, and Qiaobing Xie for
their invaluable comments. their invaluable comments.
10. References 12. References
10.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 12.1. Normative References
793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla, [RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission "Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, August 2007. Protocol (SCTP)", RFC 4895, August 2007.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007. 4960, September 2007.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. [RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP) Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061, September Dynamic Address Reconfiguration", RFC 5061, September
2007. 2007.
[RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission [RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission
Protocol (SCTP) Chunk Flags Registration", RFC 6096, Protocol (SCTP) Chunk Flags Registration", RFC 6096,
January 2011. January 2011.
10.2. Informative References 12.2. Informative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V. [RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458, December 2011. Transmission Protocol (SCTP)", RFC 6458, December 2011.
[RFC6890] Cotton, M., Vegoda, L., Bonica, R., and B. Haberman, [RFC6890] Cotton, M., Vegoda, L., Bonica, R., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153, RFC "Special-Purpose IP Address Registries", BCP 153, RFC
6890, April 2013. 6890, April 2013.
[I-D.ietf-behave-sctpnat]
Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation",
draft-ietf-behave-sctpnat-08 (work in progress), February
2013.
Authors' Addresses Authors' Addresses
Randall R. Stewart Randall R. Stewart
Adara Networks Netflix, Inc.
Chapin, SC 29036 Chapin, SC 29036
US US
Email: randall@lakerest.net Email: randall@lakerest.net
Michael Tuexen Michael Tuexen
Muenster University of Applied Sciences Muenster University of Applied Sciences
Stegerwaldstrasse 39 Stegerwaldstrasse 39
48565 Steinfurt 48565 Steinfurt
DE DE
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