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Network Working Group R. R. Stewart
Internet-Draft Netflix, Inc.
Intended status: Standards Track M. Tüxen
Expires: 29 January 2021 I. Rüngeler
Münster Univ. of Appl. Sciences
28 July 2020
Stream Control Transmission Protocol (SCTP) Network Address Translation
Support
draft-ietf-tsvwg-natsupp-19
Abstract
The Stream Control Transmission Protocol (SCTP) provides a reliable
communications channel between two end-hosts in many ways similar to
the Transmission Control Protocol (TCP). With the widespread
deployment of Network Address Translators (NAT), specialized code has
been added to NAT functions for TCP that allows multiple hosts to
reside behind a NAT function and yet share a single IPv4 address,
even when two hosts (behind a NAT function) choose the same port
numbers for their connection. This additional code is sometimes
classified as Network Address and Port Translation (NAPT).
This document describes the protocol extensions required for the SCTP
endpoints and the mechanisms for NAT functions necessary to provide
similar features of NAPT in the single point and multi point
traversal scenario.
Finally, a YANG module for SCTP NAT is defined.
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 https://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 29 January 2021.
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Copyright Notice
Copyright (c) 2020 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 (https://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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . 6
4.1.1. Single Point Traversal . . . . . . . . . . . . . . . 7
4.1.2. Multi Point Traversal . . . . . . . . . . . . . . . . 7
4.2. Limitations of Classical NAPT for SCTP . . . . . . . . . 8
4.3. The SCTP-Specific Variant of NAT . . . . . . . . . . . . 8
5. Data Formats . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Modified Chunks . . . . . . . . . . . . . . . . . . . . . 13
5.1.1. Extended ABORT Chunk . . . . . . . . . . . . . . . . 13
5.1.2. Extended ERROR Chunk . . . . . . . . . . . . . . . . 13
5.2. New Error Causes . . . . . . . . . . . . . . . . . . . . 14
5.2.1. VTag and Port Number Collision Error Cause . . . . . 14
5.2.2. Missing State Error Cause . . . . . . . . . . . . . . 14
5.2.3. Port Number Collision Error Cause . . . . . . . . . . 15
5.3. New Parameters . . . . . . . . . . . . . . . . . . . . . 16
5.3.1. Disable Restart Parameter . . . . . . . . . . . . . . 16
5.3.2. VTags Parameter . . . . . . . . . . . . . . . . . . . 16
6. Procedures for SCTP Endpoints and NAT Functions . . . . . . . 18
6.1. Association Setup Considerations for Endpoints . . . . . 18
6.2. Handling of Internal Port Number and Verification Tag
Collisions . . . . . . . . . . . . . . . . . . . . . . . 19
6.2.1. NAT Function Considerations . . . . . . . . . . . . . 19
6.2.2. Endpoint Considerations . . . . . . . . . . . . . . . 20
6.3. Handling of Internal Port Number Collisions . . . . . . . 20
6.3.1. NAT Function Considerations . . . . . . . . . . . . . 20
6.3.2. Endpoint Considerations . . . . . . . . . . . . . . . 21
6.4. Handling of Missing State . . . . . . . . . . . . . . . . 21
6.4.1. NAT Function Considerations . . . . . . . . . . . . . 21
6.4.2. Endpoint Considerations . . . . . . . . . . . . . . . 22
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6.5. Handling of Fragmented SCTP Packets by NAT Functions . . 23
6.6. Multi Point Traversal Considerations for Endpoints . . . 24
7. Various Examples of NAT Traversals . . . . . . . . . . . . . 24
7.1. Single-homed Client to Single-homed Server . . . . . . . 24
7.2. Single-homed Client to Multi-homed Server . . . . . . . . 26
7.3. Multihomed Client and Server . . . . . . . . . . . . . . 28
7.4. NAT Function Loses Its State . . . . . . . . . . . . . . 31
7.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . 33
8. SCTP NAT YANG Module . . . . . . . . . . . . . . . . . . . . 38
8.1. Tree Structure . . . . . . . . . . . . . . . . . . . . . 38
8.2. YANG Module . . . . . . . . . . . . . . . . . . . . . . . 39
9. Socket API Considerations . . . . . . . . . . . . . . . . . . 41
9.1. Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY) . . . 42
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
10.1. New Chunk Flags for Two Existing Chunk Types . . . . . . 42
10.2. Three New Error Causes . . . . . . . . . . . . . . . . . 44
10.3. Two New Chunk Parameter Types . . . . . . . . . . . . . 45
10.4. One New URI . . . . . . . . . . . . . . . . . . . . . . 45
10.5. One New YANG Module . . . . . . . . . . . . . . . . . . 45
11. Security Considerations . . . . . . . . . . . . . . . . . . . 45
12. Normative References . . . . . . . . . . . . . . . . . . . . 46
13. Informative References . . . . . . . . . . . . . . . . . . . 48
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 49
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
1. Introduction
Stream Control Transmission Protocol (SCTP) [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
functions for TCP that allows multiple hosts to reside behind a NAT
functions using internal addresses (see [RFC6890]) and yet share
single IPv4 address, even when two hosts (behind a NAT function)
choose the same port numbers for their connection. This additional
code is sometimes classified as Network Address and Port Translation
(NAPT). Please note that this document focuses on the case where the
NAT function maps a single or multiple internal addresses to a single
external address and vice versa. To date, specialized code for SCTP
has not yet been added to most NAT functions so that only a
translation of IP addresses is supported. The end result of this is
that only one SCTP-capable host can successfully operate behind such
a NAT function and this host can only be single-homed. The only
alternative for supporting legacy NAT functions is to use UDP
encapsulation as specified in [RFC6951].
The NAT function in the document refers to NAPT functions described
in Section 2.2 of [RFC3022], NAT64 [RFC6146], or DS-Lite [RFC6333].
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This document specifies procedures allowing a NAT function to support
SCTP by providing similar features to those provided by a NAPT for
TCP and other supported protocols. The document also specifies a set
of data formats for SCTP packets and a set of SCTP endpoint
procedures to support NAT traversal. An SCTP implementation
supporting these procedures can assure that in both single-homed and
multi-homed cases a NAT function will maintain the appropriate state
without the NAT function needing to change port numbers.
It is possible and desirable to make these changes for a number of
reasons:
* It is desirable for SCTP internal end-hosts on multiple platforms
to be able to share a NAT function's external IP address in the
same way that a TCP session can use a NAT function.
* If a NAT function 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.
* Not having to touch the IP payload makes the processing of ICMP
messages in NAT functions easier.
An SCTP-aware NAT function will need to follow these procedures for
generating appropriate SCTP packet formats.
When considering this feature it is possible to have multiple levels
of support. At each level, the Internal Host, Remote Host and NAT
function 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.
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+===============+==============+=============+===============+
| Internal Host | NAT Function | Remote 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 it can be seen that when a NAT function does not
support the extension no communication can occur. This assumes that
the NAT function does not handle SCTP packets at all and all SCTP
packets sent externally from behind a NAT function are discarded by
the NAT function. In some cases, where the NAT function supports the
feature but one of the two 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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Terminology
This document uses the following terms, which are depicted in
Figure 1. Familiarity with the terminology used in [RFC4960] and
[RFC5061] is assumed.
Internal-Address (Int-Addr)
The internal address that is known to the internal host.
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Internal-Port (Int-Port)
The port number that is in use by the host holding the Internal-
Address.
Internal-VTag (Int-VTag)
The SCTP Verification Tag (VTag) (see Section 3.1 of [RFC4960])
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 Internal-Address.
Remote-Address (Rem-Addr)
The address that an internal host is attempting to contact.
Remote-Port (Rem-Port)
The port number of the peer process at the Remote-Address.
Remote-VTag (Rem-VTag)
The Verification Tag (VTag) (see Section 3.1 of [RFC4960]) that
the host holding the Remote-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
Remote-Address.
External-Address (Ext-Addr)
The external address assigned to the NAT function, that it uses as
a source address when sending packets towards the Remote-Address.
Internal Network | External Network
|
Internal | External Remote
+--------+ Address | Address /--\/--\ Address +--------+
| SCTP | +-----+ / \ | SCTP |
|endpoint|=========| NAT |=======| Internet |==========|endpoint|
| A | +-----+ \ / | B |
+--------+ Internal | \--/\--/ Remote +--------+
Internal Port | Port Remote
VTag | VTag
Figure 1: Basic network setup
4. Motivation
4.1. SCTP NAT Traversal Scenarios
This section defines the notion of single and multi point NAT
traversal.
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4.1.1. Single Point Traversal
In this case, all packets in the SCTP association go through a single
NAT function, as shown below:
Internal Network | External Network
|
+--------+ | /--\/--\ +--------+
| SCTP | +-----+ / \ | SCTP |
|endpoint|=========| NAT |========= | Internet | ========|endpoint|
| A | +-----+ \ / | B |
+--------+ | \--/\--/ +--------+
|
Figure 2: Single NAT scenario
A variation of this case is shown below, i.e., multiple NAT functions
in a single path:
Internal | External : Internal | External
| : |
+--------+ | : | /--\/--\ +--------+
| SCTP | +-----+ : +-----+ / \ | SCTP |
|endpoint|==| NAT |=======:=======| NAT |==| Internet |==|endpoint|
| A | +-----+ : +-----+ \ / | B |
+--------+ | : | \--/\--/ +--------+
| : |
Figure 3: Serial NAT Functions scenario
Although one of the main benefits of SCTP multi-homing is redundant
paths, in the single point traversal scenario the NAT function
represents a single point of failure in the path of the SCTP multi-
homed 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 function
in this case sees all the packets of the SCTP association.
4.1.2. Multi Point Traversal
This case involves multiple NAT functions and each NAT function only
sees some of the packets in the SCTP association. An example is
shown below:
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Internal | External
+------+ /---\/---\
+--------+ /=======|NAT A |=========\ / \ +--------+
| SCTP | / +------+ \/ \ | SCTP |
|endpoint|/ ... | Internet |===|endpoint|
| A |\ \ / | B |
+--------+ \ +------+ / \ / +--------+
\=======|NAT B |=========/ \---\/---/
+------+
|
Figure 4: Parallel NAT functions 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 by the NAPT device. Whereas for UDP and TCP
this can be done very efficiently, for SCTP the checksum (CRC32c)
over the entire packet needs to be recomputed (see Appendix B of
[RFC4960] for details of the CRC32c computation). This would
considerably add 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 NAT
functions 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 a pre-
defined table of ports and addresses configured within each NAT
function. Other mechanisms could make use of NAT to NAT
communication. Such mechanisms have not been deployed on a wide
scale base and thus are not a recommended solution. Therefore an
SCTP variant of NAT function has been developed.
4.3. The SCTP-Specific Variant of NAT
In this section it is allowed that there are multiple SCTP capable
hosts behind a NAT function that has one Exernal-Address.
Furthermore this section focuses on the single point traversal
scenario.
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The modification of SCTP packets sent to the Internet is simple: the
source address of the packet has to be replaced with the External-
Address. It may also be necessary to establish some state in the NAT
function to later handle incoming packets.
For the SCTP NAT processing the NAT function has to maintain a NAT
binding table of Internal-VTag, Internal-Port, Remote-VTag, Remote-
Port, Internal-Address, and whether the restart procedure is disabled
or not. An entry in that NAT binding table is called a NAT-State
control block. The function Create() obtains the just mentioned
parameters and returns a NAT-State control block. A NAT function MAY
allow creating NAT-State control blocks via a management interface.
For SCTP packets coming from the public Internet the destination
address of the packets has to be replaced with the Internal-Address
of the host to which the packet has to be delivered. The lookup of
the Internal-Address is based on the Remote-VTag, Remote-Port,
Internal-VTag and the Internal-Port.
The entries in the NAT binding table need to fulfill some uniqueness
conditions. There must not be more than one entry NAT binding table
with the same pair of Internal-Port and Remote-Port. This rule can
be relaxed, if all NAT binding table entries with the same Internal-
Port and Remote-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, Remote-Port and Remote-VTag and no more than one
NAT binding table entry with the same Internal-Port, Remote-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.
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
INIT[Initiate-Tag]
Int-Addr:Int-Port ------> Rem-Addr:Rem-Port
Rem-VTag=0
Create(Initiate-Tag, Int-Port, 0, Rem-Port, Int-Addr,
RestartSupported)
Returns(NAT-State control block)
Translate To:
INIT[Initiate-Tag]
Ext-Addr:Int-Port ------> Rem-Addr:Rem-Port
Rem-VTag=0
Normally a NAT binding table entry will be created.
However, it is possible that there is already a NAT binding table
entry with the same Remote-Port, Internal-Port, and Internal-VTag but
different Internal-Address. In this case the packet containing the
INIT chunk MUST be dropped by the NAT and a packet containing an
ABORT chunk SHOULD be sent to the SCTP host that originated the
packet 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.
If an outgoing SCTP packet contains an INIT or ASCONF chunk and a
matching NAT binding table entry is found, the packet is processed as
a normal outgoing packet.
It is also possible that a connection to Remote-Address and Remote-
Port exists without an Internal-VTag conflict but there exists a NAT
binding table entry with the same port numbers but a different
Internal-Address. In such a case the packet containing the INIT
chunk MUST be dropped by the NAT function and a packet containing an
ABORT chunk SHOULD be sent to the SCTP host that originated the
packet 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 chunks is
described in the following figure.
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
Int-Addr:Int-Port ------> Rem-Addr:Rem-Port
Rem-VTag
Translate To:
Ext-Addr:Int-Port ------> Rem-Addr:Rem-Port
Rem-VTag
The processing of incoming SCTP packets containing an INIT ACK chunk
is described in the following figure. The Lookup() function getting
as input the Internal-VTag, Internal-Port, Remote-VTag, and Remote-
Port, returns the corresponding entry of the NAT binding table and
updates the Remote-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]
Ext-Addr:Int-Port <---- Rem-Addr:Rem-Port
Int-VTag
Lookup(Int-VTag, Int-Port, *, Rem-Port)
Update(*, *, Initiate-Tag, *)
Returns(NAT-State control block containing Int-Addr)
INIT ACK[Initiate-Tag]
Int-Addr:Int-Port <------ Rem-Addr:Rem-Port
Int-VTag
In the case Lookup fails, the SCTP packet is dropped. If it
succeeds, the Update routine inserts the Remote-VTag (the Initiate-
Tag of the INIT ACK chunk) in the NAT-State control block.
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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 |
+--------+ +-----+ \ / +--------+
\--/\---/
Ext-Addr:Int-Port <------ Rem-Addr:Rem-Port
Rem-VTag
Lookup(*, Int-Port, Rem-VTag, Rem-Port)
Returns(NAT-State control block containing Int-Addr)
Int-Addr:Int-Port <------ Rem-Addr:Rem-Port
Rem-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 Remote-VTag of zero is found, it is considered a match and the
Remote-VTag is updated. If an entry with a non-matching Remote-VTag
is found or no entry is found, the incoming packet is dropped. If an
entry with a matching Remote-VTag is found, the incoming packet is
forwarded. This allows the handling of INIT collision through NAT
functions.
The processing of other incoming SCTP packets is described in the
following figure.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
Ext-Addr:Int-Port <------ Rem-Addr:Rem-Port
Int-VTag
Lookup(Int-VTag, Int-Port, *, Rem-Port)
Returns(NAT-State control block containing Internal-Address)
Int-Addr:Int-Port <------ Rem-Addr:Rem-Port
Int-VTag
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5. Data Formats
This section defines the formats used to support NAT traversal.
Section 5.1 and Section 5.2 describe chunks and error causes sent by
NAT functions and received by SCTP endpoints. Section 5.3 describes
parameters sent by SCTP endpoints and used by NAT functions and SCTP
endpoints.
5.1. Modified Chunks
This section presents existing chunks defined in [RFC4960] for which
additional flags are specified by this document.
5.1.1. 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.
[NOTE to RFC-Editor: Assignment of M bit to be confirmed by IANA.]
5.1.2. 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.
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[NOTE to RFC-Editor: Assignment of M bit to be confirmed by IANA.]
5.2. New Error Causes
This section defines the new error causes added by this document.
5.2.1. 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. IANA is requested to assign
the value 0x00B0 for this cause code.
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.
[NOTE to RFC-Editor: Assignment of cause code to be confirmed by
IANA.]
5.2.2. 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)
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This field holds the IANA defined cause code for the 'Missing
State' Error Cause. IANA is requested to assign the value 0x00B1
for this cause code.
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.
[NOTE to RFC-Editor: Assignment of cause code to be confirmed by
IANA.]
5.2.3. 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. IANA is requested to assign the value
0x00B2 for this cause code.
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.
[NOTE to RFC-Editor: Assignment of cause code to be confirmed by
IANA.]
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5.3. New Parameters
This section defines new parameters and their valid appearance
defined by this document.
5.3.1. 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. IANA is requested to assign the value 0xC007
for this parameter type.
Parameter Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the parameter. The value
MUST be 4.
[NOTE to RFC-Editor: Assignment of parameter type to be confirmed by
IANA.]
This parameter MAY appear in INIT, INIT ACK and ASCONF chunks and
MUST NOT appear in any other chunk.
5.3.2. VTags Parameter
This parameter is used to help a NAT function to 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This field holds the IANA defined parameter type for the VTags
Parameter. IANA is requested to assign the value 0xC008 for this
parameter type.
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 packet
containing the ASCONF chunk can use this same value in the ASCONF
ACK chunk 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 Internal-Address.
Remote Verification Tag: 4 bytes (unsigned integer)
The Verification Tag that the host holding the Remote-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 Remote-Address.
[NOTE to RFC-Editor: Assignment of parameter type to be confirmed by
IANA.]
This parameter MAY appear in ASCONF chunks and MUST NOT appear in any
other chunk.
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6. Procedures for SCTP Endpoints and NAT Functions
When an SCTP endpoint is behind an SCTP-aware NAT a number of
problems may arise as it tries to communicate with its peer:
* IP addresses can not be included in the SCTP packet. This is
discussed in Section 6.1.
* More than one host behind a NAT function could select the same
VTag and source port when talking to the same peer server. This
creates a situation where the NAT function will not be able to
tell the two associations apart. This situation is discussed in
Section 6.2.
* When an SCTP endpoint is a server communicating with multiple
peers and the peers are behind the same NAT function, then the two
endpoints cannot be distinguished by the server. This case is
discussed in Section 6.3.
* A restart of a NAT function during a conversation could cause a
loss of its state. This problem and its solution is discussed in
Section 6.4.
* NAT functions need to deal with SCTP packets being fragmented at
the IP layer. This is discussed in Section 6.5.
* An SCTP endpoint can be behind two NAT functions in parallel
providing redundancy. The method to set up this scenario is
discussed in Section 6.6.
Each of these mechanisms requires additional chunks and parameters,
defined in this document, and modified handling procedures from those
specified in [RFC4960] as described below.
6.1. Association Setup Considerations for Endpoints
The association setup procedure defined in [RFC4960] allows multi-
homed SCTP endpoints to exchange its IP-addresses by using IPv4 or
IPv6 address parameters in the INIT and INIT ACK chunks. However,
this does not work when NAT functions are present.
Every association setup from a host behind a NAT function MUST NOT
use multiple internal addresses. The INIT chunk MUST NOT contain an
IPv4 Address parameter, IPv6 Address parameter, or Supported Address
Types parameter. The INIT ACK chunk MUST NOT contain any IPv4
Address parameter or IPv6 Address parameter using non-global
addresses. The INIT chunk and the INIT ACK chunk MUST NOT contain
any Host Name parameters.
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If the association should finally be multi-homed, the procedure in
Section 6.6 MUST be used.
The INIT and INIT ACK chunk SHOULD contain the Disable Restart
parameter defined in Section 5.3.1.
6.2. Handling of Internal Port Number and Verification Tag Collisions
Consider the case where two hosts in the Internal-Address space want
to set up an SCTP association with the same service provided by some
hosts in the Internet. This means that the Remote-Port is the same.
If they both choose the same Internal-Port and Internal-VTag, the NAT
function cannot distinguish between incoming packets anymore.
However, this is unlikely. The Internal-VTags are chosen at random
and if the Internal-Ports are also chosen from the ephemeral port
range at random this gives a 46-bit random number that has to match.
A NAPT device can control the Port number and therefore avoid
collisions deterministically.
The same can happen with the Remote-VTag when a packet containing an
INIT ACK chunk or an ASCONF chunk is processed by the NAT function.
6.2.1. NAT Function Considerations
If the NAT function detects a collision of internal port numbers and
verification tags, it SHOULD send a packet containing an ABORT chunk
with the M bit set if the collision is triggered by a packet
containing an INIT or INIT ACK chunk. If such a collision is
triggered by a packet containing an ASCONF chunk, it SHOULD send a
packet containing an ERROR chunk with the M bit. 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 5.1.1). If 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. If a packet containing
an INIT chunk or an ASCONF chunk, the source and destination address
and port numbers MUST be swapped.
The sender of the packet containing an ERROR or ABORT chunk MUST
include the error cause with cause code 'VTag and Port Number
Collision' (see Section 5.2.1).
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6.2.2. Endpoint Considerations
The sender of the packet containing the INIT chunk or the receiver of
a packet containing the INIT ACK chunk, upon reception of a packet
containign an ABORT chunk with M bit set and the appropriate error
cause code for colliding NAT binding table state is included, SHOULD
reinitiate the association setup procedure after choosing a new
initiate tag, if the association is in COOKIE-WAIT state. In any
other state, the SCTP endpoint MUST NOT respond.
The sender of packet containing the ASCONF chunk, upon reception of a
packet containing an ERROR chunk with M bit set, MUST stop adding the
path to the association.
6.3. Handling of Internal Port Number Collisions
When two SCTP hosts are behind an SCTP-aware NAT it is possible that
two SCTP hosts in the Internal-Address space will want to set up an
SCTP association with the same server running on the same host in the
Internet. If the two hosts choose the same internal port, this is
considered an internal port number collision.
For the NAT function, appropriate tracking may be performed by
assuring that the VTags are unique between the two hosts.
6.3.1. NAT Function Considerations
The NAT function, when processing the packet containing the INIT ACK
chunk, should note in its NAT binding table that the association
supports the disable restart extension. This note is used when
establishing future associations (i.e. when processing a packet
containing an INIT chunk from an internal host) to decide if the
connection should be allowed. The NAT function does the following
when processing a packet containing an INIT chunk:
* If the packet containing the INIT chunk is originating from an
internal port to an remote port for which the NAT function has no
matching NAT binding table entry, it MUST allow the packet
containing the INIT chunk creating an NAT binding table entry.
* If the packet containing the INIT chunk matches an existing NAT
binding table entry, it MUST validate that the disable restart
feature is supported and, if it does, allow the packet containing
the INIT chunk to be forwarded.
* If the disable restart feature is not supported, the NAT function
SHOULD send a packet containing an ABORT chunk with the M bit set.
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The 'Port Number Collision' error cause (see Section 5.2.3) MUST be
included in the ABORT chunk sent in response to the packet containing
an INIT chunk.
If the collision is triggered by a packet containing an ASCONF chunk,
a packet containing an ERROR chunk with the 'Port Number Collision'
error cause MUST be sent in response to the packet containing the
ASCONF chunk.
6.3.2. Endpoint Considerations
For the remote SCTP server on the Internet this means that the
Remote-Port and the Remote-Address are the same. If they both have
chosen the same Internal-Port the server cannot distinguish between
both associations based on the address and port numbers. For the
server it looks like the association is being restarted. To overcome
this limitation the client sends a Disable Restart parameter in the
INIT chunk.
When the server receives this parameter it does the following:
* It MUST include a Disable Restart parameter in the INIT ACK to
inform the client that it will support the feature.
* It MUST 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 function) differentiated only by the VTags.
6.4. Handling of Missing State
6.4.1. NAT Function Considerations
If the NAT function receives a packet from the internal network for
which the lookup procedure does not find an entry in the NAT binding
table, a packet containing an ERROR chunk SHOULD be sent back with
the M bit set. The source address of the packet containing the ERROR
chunk MUST be the destination address of the incoming SCTP packet.
The verification tag is reflected and the T bit is set. Such a
packet containing an ERROR chunk SHOULD NOT be sent if the received
packet contains an ABORT, SHUTDOWN COMPLETE or INIT ACK chunk. A
packet containing an ERROR chunk MUST NOT be sent if the received
packet contains an ERROR chunk with the M bit set. In any case, the
packet SHOULD NOT be forwarded to the remote address.
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When sending a packet containing an ERROR chunk, the error cause
'Missing State' (see Section 5.2.2) MUST be included and the M bit of
the ERROR chunk MUST be set (see Section 5.1.2).
If the NAT device receives a packet for which it has no NAT binding
table entry and the packet contains an ASCONF chunk with the VTags
parameter, the NAT function MUST update its NAT binding table
according to the verification tags in the VTags parameter and the
optional Disable Restart parameter.
6.4.2. Endpoint Considerations
Upon reception of this packet containing the ERROR chunk by an SCTP
endpoint the receiver takes the following actions:
* It SHOULD validate that the verification tag is reflected by
looking at the VTag that would have been included in the outgoing
packet. If the validation fails, discard the incoming packet
containing the ERROR chunk.
* It SHOULD validate that the peer of the SCTP association supports
the dynamic address extension. If the validation fails, discard
the incoming packet containing the ERROR chunk.
* It SHOULD generate a packet containing a new ASCONF chunk
containing the VTags parameter (see Section 5.3.2) and the Disable
Restart parameter (see Section 5.3.1) if the association is using
the disable restart feature. By processing this packet the NAT
function can recover the appropriate state. The procedures for
generating an ASCONF chunk can be found in [RFC5061].
The peer SCTP endpoint receiving such a packet containing 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 packet containing an ASCONF
ACK chunk acknowledging the address and take no action (since the
address is already in the association).
Note that it is possible that upon receiving a packet containing an
ASCONF chunk containing the VTags parameter the NAT function will
realize that it has an 'Internal Port Number and Verification Tag
collision'. In such a case the NAT function SHOULD send a packet
containing an ERROR chunk with the error cause code set to 'VTag and
Port Number Collision' (see Section 5.2.1).
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If an SCTP endpoint receives a packet containing an ERROR chunk with
'Internal Port Number and 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 does the following:
* It MUST validate that the verification tag is reflected by looking
at the VTag that would have been included in the outgoing packet.
If the validation fails, it MUST discard the packet.
* It MUST validate that the peer of the SCTP association supports
the dynamic address extension. If the peer does not support it,
the NAT function MUST discard the incoming packet containing the
ERROR chunk.
* If the association is attempting to add an address (i.e. following
the procedures in Section 6.6) 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 function has a VTag
collision and the association cannot easily create a new VTag (as
it would if the error occurred when sending a packet containing an
INIT chunk).
* If the endpoint has no other path, i.e. the procedure was executed
due to missing a state in the NAT function, then the endpoint MUST
abort the association. This would occur only if the local NAT
function 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
function looses its state).
6.5. Handling of Fragmented SCTP Packets by NAT Functions
SCTP minimizes the use of IP-level fragmentation. However, it can
happen that using IP-level fragmentation is needed to continue an
SCTP association. For example, if the path MTU is reduced and there
are still some DATA chunk in flight, which require packets larger
than the new path MTU. If IP-level fragmentation can not be used,
the SCTP association will be terminated in a non-graceful way.
Therefore, a NAT function MUST be able to handle IP-level fragmented
SCTP packets. The fragments may arrive in any order.
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When an SCTP packet can not be forwarded by the NAT function due to
MTU issues and the IP header forbids fragmentation, the NAT MUST send
back a "Fragmentation needed and DF set" ICMPv4 or PTB ICMPv6 message
to the internal host. This allows for a faster recovery from this
packet drop.
6.6. Multi Point Traversal Considerations for Endpoints
If a multi-homed SCTP endpoint behind a NAT function connects to a
peer, it MUST first set up the association single-homed with only one
address causing the first NAT function to populate its state. Then
it SHOULD add each IP address using packets containing ASCONF chunks
sent via their respective NAT functions. The address to add is the
wildcard address and the lookup address SHOULD also contain the VTags
parameter and optionally the Disable Restart parameter.
7. Various Examples of NAT Traversals
Please note that this section is informational only.
The addresses being used in the following examples are IPv4 addresses
for private-use networks and for documentation as specified in
[RFC6890]. However, the method described here is not limited to this
NAT44 case.
The NAT binding table entries shown in the following examples do not
include the flag indicating whether the restart procedure is
supported or not. This flag is not relevant for these examples.
7.1. Single-homed Client to Single-homed Server
The internal client starts the association with the remote server via
a four-way-handshake. Host A starts by sending a packet containing
an INIT chunk.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 ------> 203.0.113.1:2
Rem-VTtag = 0
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A NAT binding tabled entry is created, the source address is
substituted and the packet is sent on:
NAT function creates entry:
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 0 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
192.0.2.1:1 ------------------------> 203.0.113.1:2
Rem-VTtag = 0
Host B receives the packet containing an INIT chunk and sends a
packet containing an INIT ACK chunk with the NAT's Remote-address as
destination address.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
INIT ACK[Initiate-Tag = 5678]
192.0.2.1:1 <----------------------- 203.0.113.1:2
Int-VTag = 1234
NAT function updates entry:
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT ACK[Initiate-Tag = 5678]
10.0.0.1:1 <------ 203.0.113.1:2
Int-VTag = 1234
The handshake finishes with a COOKIE ECHO acknowledged by a COOKIE
ACK.
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
COOKIE ECHO
10.0.0.1:1 ------> 203.0.113.1:2
Rem-VTag = 5678
COOKIE ECHO
192.0.2.1:1 -----------------------> 203.0.113.1:2
Rem-VTag = 5678
COOKIE ACK
192.0.2.1:1 <----------------------- 203.0.113.1:2
Int-VTag = 1234
COOKIE ACK
10.0.0.1:1 <------ 203.0.113.1:2
Int-VTag = 1234
7.2. Single-homed Client to Multi-homed Server
The internal client is single-homed whereas the remote server is
multi-homed. The client (Host A) sends a packet containing an INIT
chunk like in the single-homed case.
+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <-----> | NAT | <-> | Internet | == =| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 ---> 203.0.113.1:2
Rem-VTag = 0
NAT function creates entry:
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+---------+--------+----------+--------+-----------+
NAT | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 0 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
192.0.2.1:1 --------------------------> 203.0.113.1:2
Rem-VTag = 0
The server (Host B) includes its two addresses in the INIT ACK chunk.
+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <-----> | NAT | <-> | Internet | == =| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
INIT ACK[Initiate-tag = 5678, IP-Addr = 203.0.113.129]
192.0.2.1:1 <-------------------------- 203.0.113.1:2
Int-VTag = 1234
The NAT function does not need to change the NAT binding table for
the second address:
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT ACK[Initiate-Tag = 5678]
10.0.0.1:1 <--- 203.0.113.1:2
Int-VTag = 1234
The handshake finishes with a COOKIE ECHO acknowledged by a COOKIE
ACK.
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+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <-----> | NAT | <-> | Internet | == =| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
COOKIE ECHO
10.0.0.1:1 ---> 203.0.113.1:2
Rem-VTag = 5678
COOKIE ECHO
192.0.2.1:1 --------------------------> 203.0.113.1:2
Rem-VTag = 5678
COOKIE ACK
192.0.2.1:1 <-------------------------- 203.0.113.1:2
Int-VTag = 1234
COOKIE ACK
10.0.0.1:1 <--- 203.0.113.1:2
Int-VTag = 1234
7.3. Multihomed Client and Server
The client (Host A) sends a packet containing an INIT chunk 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 | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 --------> 203.0.113.1:2
Rem-VTag = 0
NAT function 1 creates entry:
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+---------+--------+----------+--------+-----------+
NAT 1 | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 0 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
192.0.2.1:1 ---------------------> 203.0.113.1:2
Rem-VTag = 0
Host B includes its second address in the INIT ACK.
+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |===| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
INIT ACK[Initiate-Tag = 5678, IP-Addr = 203.0.113.129]
192.0.2.1:1 <----------------------- 203.0.113.1:2
Int-VTag = 1234
NAT function 1 does not need to update the NAT binding table for the
second address:
+---------+--------+----------+--------+-----------+
NAT 1 | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT ACK[Initiate-Tag = 5678]
10.0.0.1:1 <-------- 203.0.113.1:2
Int-VTag = 1234
The handshake finishes with a COOKIE ECHO acknowledged by a COOKIE
ACK.
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+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |===| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
COOKIE ECHO
10.0.0.1:1 --------> 203.0.113.1:2
Rem-VTag = 5678
COOKIE ECHO
192.0.2.1:1 ------------------> 203.0.113.1:2
Rem-VTag = 5678
COOKIE ACK
192.0.2.1:1 <------------------ 203.0.113.1:2
Int-VTag = 1234
COOKIE ACK
10.0.0.1:1 <------- 203.0.113.1:2
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 (remote and
internal) so that the NAT function may populate its NAT binding table
entry completely with this single packet.
+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |===| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
ASCONF [ADD-IP=0.0.0.0, INT-VTag=1234, Rem-VTag = 5678]
10.1.0.1:1 --------> 203.0.113.129:2
Rem-VTag = 5678
NAT function 2 creates a complete entry:
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+---------+--------+----------+--------+-----------+
NAT 2 | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.1.0.1 |
+---------+--------+----------+--------+-----------+
ASCONF [ADD-IP, Int-VTag=1234, Rem-VTag = 5678]
192.0.2.129:1 -------------------> 203.0.113.129:2
Rem-VTag = 5678
ASCONF ACK
192.0.2.129:1 <------------------- 203.0.113.129:2
Int-VTag = 1234
ASCONF ACK
10.1.0.1:1 <----- 203.0.113.129:2
Int-VTag = 1234
7.4. NAT Function Loses Its State
Association is already established between Host A and Host B, when
the NAT function loses its state and obtains a new external address.
Host A sends a DATA chunk to Host B.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
DATA
10.0.0.1:1 ----------> 203.0.113.1:2
Rem-VTag = 5678
The NAT function cannot find an entry in the NAT binding table for
the association. It sends a packet containing an ERROR chunk with
the M-Bit set and the cause "NAT state missing".
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
ERROR [M-Bit, NAT state missing]
10.0.0.1:1 <---------- 203.0.113.1:2
Rem-VTag = 5678
On reception of the packet containing the ERROR chunk, Host A sends a
packet containing 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, Rem-VTag = 5678]
10.0.0.1:1 ----------> 203.0.113.129:2
Rem-VTag = 5678
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
ASCONF [ADD-IP, DELETE-IP, Int-VTag=1234, Rem-VTag = 5678]
192.0.2.2:1 -----------------> 203.0.113.129:2
Rem-VTag = 5678
Host B adds the new source address to this association and deletes
all other addresses from this association.
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
ASCONF ACK
192.0.2.2:1 <----------------- 203.0.113.129:2
Int-VTag = 1234
ASCONF ACK
10.1.0.1:1 <---------- 203.0.113.129:2
Int-VTag = 1234
DATA
10.0.0.1:1 ----------> 203.0.113.1:2
Rem-VTag = 5678
DATA
192.0.2.2:1 -----------------> 203.0.113.129:2
Rem-VTag = 5678
7.5. Peer-to-Peer Communication
If two hosts, each of them behind a NAT function, want to communicate
with each other, they have to get knowledge of the peer's external
address. This can be achieved with a so-called rendezvous server.
Afterwards the destination addresses are external, and the
association is set up with the help of the INIT collision. The NAT
functions create their entries according to their internal peer's
point of view. Therefore, NAT function A's Internal-VTag and
Internal-Port are NAT function B's Remote-VTag and Remote-Port,
respectively. The naming (internal/remote) of the verification tag
in the packet flow is done from the sending host's point of view.
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Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
NAT Binding Tables
+---------+--------+----------+--------+-----------+
NAT A | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
+---------+--------+----------+--------+-----------+
NAT B | Int | Int | Rem | Rem | Int |
| v-tag | port | v-tag | port | Addr |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 --> 203.0.113.1:2
Rem-VTag = 0
NAT function A creates entry:
+---------+--------+----------+--------+-----------+
NAT A | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 0 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
192.0.2.1:1 ----------------> 203.0.113.1:2
Rem-VTag = 0
NAT function B processes the packet containing the INIT chunk, but
cannot find an entry. The SCTP packet is silently discarded and
leaves the NAT binding table of NAT function B unchanged.
+---------+--------+----------+--------+-----------+
NAT B | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
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Now Host B sends a packet containing an INIT chunk, which is
processed by NAT function 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]
192.0.2.1:1 <-- 10.1.0.1:2
Rem-VTag = 0
+---------+--------+----------+--------+-----------+
NAT B | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 5678 | 2 | 0 | 1 | 10.1.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 5678]
192.0.2.1:1 <--------------- 203.0.113.1:2
Rem-VTag = 0
NAT function A processes the packet containing the INIT chunk. As
the outgoing packet containing an INIT chunk of Host A has already
created an entry, the entry is found and updated:
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Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
VTag != Int-VTag, but Rem-VTag == 0, find entry.
+---------+--------+----------+--------+-----------+
NAT A | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-tag = 5678]
10.0.0.1:1 <-- 203.0.113.1:2
Rem-VTag = 0
Host A sends a packet containing an INIT ACK chunk, which can pass
through NAT function B:
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Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
INIT ACK[Initiate-Tag = 1234]
10.0.0.1:1 --> 203.0.113.1:2
Rem-VTag = 5678
INIT ACK[Initiate-Tag = 1234]
192.0.2.1:1 ----------------> 203.0.113.1:2
Rem-VTag = 5678
NAT function B updates entry:
+---------+--------+----------+--------+-----------+
NAT B | Int | Int | Rem | Rem | Int |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 5678 | 2 | 1234 | 1 | 10.1.0.1 |
+---------+--------+----------+--------+-----------+
INIT ACK[Initiate-Tag = 1234]
192.0.2.1:1 --> 10.1.0.1:2
Rem-VTag = 5678
The lookup for COOKIE ECHO and COOKIE ACK is successful.
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Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
COOKIE ECHO
192.0.2.1:1 <-- 10.1.0.1:2
Rem-VTag = 1234
COOKIE ECHO
192.0.2.1:1 <------------- 203.0.113.1:2
Rem-VTag = 1234
COOKIE ECHO
10.0.0.1:1 <-- 203.0.113.1:2
Rem-VTag = 1234
COOKIE ACK
10.0.0.1:1 --> 203.0.113.1:2
Rem-VTag = 5678
COOKIE ACK
192.0.2.1:1 ----------------> 203.0.113.1:2
Rem-VTag = 5678
COOKIE ACK
192.0.2.1:1 --> 10.1.0.1:2
Rem-VTag = 5678
8. SCTP NAT YANG Module
This section defines a YANG module for SCTP NAT.
The terminology for describing YANG data models is defined in
[RFC7950]. The meaning of the symbols in tree diagrams is defined in
[RFC8340].
8.1. Tree Structure
This module augments NAT YANG module [RFC8512] with SCTP specifics.
The module supports both classical SCTP NAT (that is, rewrite port
numbers) and SCTP-specific variant where the ports numbers are not
altered. The YANG "feature" is used to indicate whether SCTP-
specific variant is supported.
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The tree structure of the SCTP NAT YANG module is provided below:
module: ietf-nat-sctp
augment /nat:nat/nat:instances/nat:instance
/nat:policy/nat:timers:
+--rw sctp-timeout? uint32
augment /nat:nat/nat:instances/nat:instance
/nat:mapping-table/nat:mapping-entry:
+--rw int-VTag? uint32 {sctp-nat}?
+--rw rem-VTag? uint32 {sctp-nat}?
Concretely, the SCTP NAT YANG module augments the NAT YANG module
(policy, in particular) with the following:
* The sctp-timeout is used to control the SCTP inactivity timeout.
That is, the time an SCTP mapping will stay active without SCTP
packets traversing the NAT. This timeout can be set only for
SCTP. Hence, "/nat:nat/nat:instances/nat:instance/nat:policy/
nat:transport-protocols/nat:protocol-id" MUST be set to '132'
(SCTP).
In addition, the SCTP NAT YANG module augments the mapping entry with
the following parameters defined in Section 3. These parameters
apply only for SCTP NAT mapping entries (i.e.,
"/nat/instances/instance/mapping-table/mapping-entry/transport-
protocol" MUST be set to '132');
* The Internal Verification Tag (Int-VTag)
* The Remote Verification Tag (Rem-VTag)
8.2. YANG Module
<CODE BEGINS> file "ietf-nat-sctp@2020-07-13.yang"
module ietf-nat-sctp {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-nat-sctp";
prefix nat-sctp;
import ietf-nat {
prefix nat;
reference
"RFC 8512: A YANG Module for Network Address Translation
(NAT) and Network Prefix Translation (NPT)";
}
organization
"IETF TSVWG Working Group";
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contact
"WG Web: <https://datatracker.ietf.org/wg/tsvwg/>
WG List: <mailto:tsvwg@ietf.org>
Author: Mohamed Boucadair
<mailto:mohamed.boucadair@orange.com>";
description
"This module augments NAT YANG module with Stream Control
Transmission Protocol (SCTP) specifics. The extension supports
both a classical SCTP NAT (that is, rewrite port numbers)
and a, SCTP-specific variant where the ports numbers are
not altered.
Copyright (c) 2020 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC XXXX; see
the RFC itself for full legal notices.";
revision 2019-11-18 {
description
"Initial revision.";
reference
"RFC XXXX: Stream Control Transmission Protocol (SCTP)
Network Address Translation Support";
}
feature sctp-nat {
description
"This feature means that SCTP-specific variant of NAT
is supported. That is, avoid rewriting port numbers.";
reference
"Section 4.3 of RFC XXXX.";
}
augment "/nat:nat/nat:instances/nat:instance"
+ "/nat:policy/nat:timers" {
when "/nat:nat/nat:instances/nat:instance"
+ "/nat:policy/nat:transport-protocols"
+ "/nat:protocol-id = 132";
description
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"Extends NAT policy with a timeout for SCTP mapping
entries.";
leaf sctp-timeout {
type uint32;
units "seconds";
description
"SCTP inactivity timeout. That is, the time an SCTP
mapping entry will stay active without packets
traversing the NAT.";
}
}
augment "/nat:nat/nat:instances/nat:instance"
+ "/nat:mapping-table/nat:mapping-entry" {
when "nat:transport-protocol = 132";
if-feature "sctp-nat";
description
"Extends the mapping entry with SCTP specifics.";
leaf int-VTag {
type uint32;
description
"The Internal Verification Tag that the internal
host has chosen for this communication.";
}
leaf rem-VTag {
type uint32;
description
"The Remote Verification Tag that the remote
peer has chosen for this communication.";
}
}
}
<CODE ENDS>
9. 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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9.1. 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.
10. IANA Considerations
[NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number
you assign this document.]
[NOTE to RFC-Editor: The requested 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 requested changes are described
below.
10.1. 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 requested 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:
ERROR Chunk Flags
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+==================+=================+===========+
| 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 | |
+------------------+-----------------+-----------+
Table 2
As defined in [RFC6096] one chunk flag has to be assigned by IANA for
the ABORT chunk. The requested value of the M bit is 0x02.
This requires an update of the "ABORT Chunk Flags" registry for SCTP:
ABORT Chunk Flags
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+==================+=================+===========+
| 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 | |
+------------------+-----------------+-----------+
Table 3
10.2. Three New Error Causes
Three error causes have to be assigned by IANA. It is requested to
use the values given below.
This requires three additional lines in the "Error Cause Codes"
registry for SCTP:
Error Cause Codes
+=======+================================+===========+
| Value | Cause Code | Reference |
+=======+================================+===========+
| 176 | VTag and Port Number Collision | [RFCXXXX] |
+-------+--------------------------------+-----------+
| 177 | Missing State | [RFCXXXX] |
+-------+--------------------------------+-----------+
| 178 | Port Number Collision | [RFCXXXX] |
+-------+--------------------------------+-----------+
Table 4
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10.3. Two New Chunk Parameter Types
Two chunk parameter types have to be assigned by IANA. It is
requested 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] |
+----------+--------------------------+-----------+
Table 5
10.4. One New URI
An URI in the "ns" subregistry within the "IETF XML" registry has to
be assigned by IANA ([RFC3688]):
URI: urn:ietf:params:xml:ns:yang:ietf-nat-sctp
Registrant Contact: The IESG.
XML: N/A; the requested URI is an XML namespace.
10.5. One New YANG Module
An YANG module in the "YANG Module Names" subregistry within the
"YANG Parameters" registry has to be assigned by IANA ([RFC6020]):
Name: ietf-nat-sctp
Namespace: urn:ietf:params:xml:ns:yang:ietf-nat-sctp
Maintained by IANA: N
Prefix: nat-sctp
Reference: RFCXXXX
11. Security Considerations
State maintenance within a NAT function is always a subject of
possible Denial Of Service attacks. This document recommends that at
a minimum a NAT function runs a timer on any SCTP state so that old
association state can be cleaned up.
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Generic issues related to address sharing are discussed in [RFC6269]
and apply to SCTP as well.
For SCTP endpoints not disabling the restart procedure, this document
does not add any additional security considerations to the ones given
in [RFC4960], [RFC4895], and [RFC5061].
SCTP endpoints disabling the restart procedure, should monitor the
status of all associations to mitigate resource exhaustion attacks by
establishing a lot of associations sharing the same IP addresses and
port numbers.
In any case, SCTP is protected by the verification tags and the usage
of [RFC4895] against off-path attackers.
For IP-level fragmentation and reassembly related issues see
[RFC4963].
The YANG module specified in this document defines a schema for data
that is designed to be accessed via network management protocols such
as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer
is the secure transport layer, and the mandatory-to-implement secure
transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer
is HTTPS, and the mandatory-to-implement secure transport is TLS
[RFC8446].
The Network Configuration Access Control Model (NACM) [RFC8341]
provides the means to restrict access for particular NETCONF or
RESTCONF users to a preconfigured subset of all available NETCONF or
RESTCONF protocol operations and content.
All data nodes defined in the YANG module that can be created,
modified, and deleted (i.e., config true, which is the default) are
considered sensitive. Write operations (e.g., edit-config) applied
to these data nodes without proper protection can negatively affect
network operations. An attacker who is able to access the SCTP NAT
function can undertake various attacks, such as:
* Setting a low timeout for SCTP mapping entries to cause failures
to deliver incoming SCTP packets.
* Instantiating mapping entries to cause NAT collision.
12. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
2007, <https://www.rfc-editor.org/info/rfc4895>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
DOI 10.17487/RFC5061, September 2007,
<https://www.rfc-editor.org/info/rfc5061>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.
[RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission
Protocol (SCTP) Chunk Flags Registration", RFC 6096,
DOI 10.17487/RFC6096, January 2011,
<https://www.rfc-editor.org/info/rfc6096>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
<https://www.rfc-editor.org/info/rfc6242>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
Stewart, et al. Expires 29 January 2021 [Page 47]
Internet-Draft SCTP NAT Support July 2020
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration
Access Control Model", STD 91, RFC 8341,
DOI 10.17487/RFC8341, March 2018,
<https://www.rfc-editor.org/info/rfc8341>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8512] Boucadair, M., Ed., Sivakumar, S., Jacquenet, C.,
Vinapamula, S., and Q. Wu, "A YANG Module for Network
Address Translation (NAT) and Network Prefix Translation
(NPT)", RFC 8512, DOI 10.17487/RFC8512, January 2019,
<https://www.rfc-editor.org/info/rfc8512>.
13. Informative References
[DOI_10.1145_1496091.1496095]
Hayes, D., But, J., and G. Armitage, "Issues with network
address translation for SCTP", ACM SIGCOMM Computer
Communication Review Vol. 39, pp. 23-33,
DOI 10.1145/1496091.1496095, December 2008,
<https://doi.org/10.1145/1496091.1496095>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
DOI 10.17487/RFC3022, January 2001,
<https://www.rfc-editor.org/info/rfc3022>.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<https://www.rfc-editor.org/info/rfc4963>.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <https://www.rfc-editor.org/info/rfc6146>.
Stewart, et al. Expires 29 January 2021 [Page 48]
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[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
DOI 10.17487/RFC6269, June 2011,
<https://www.rfc-editor.org/info/rfc6269>.
[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
<https://www.rfc-editor.org/info/rfc6333>.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458,
DOI 10.17487/RFC6458, December 2011,
<https://www.rfc-editor.org/info/rfc6458>.
[RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153,
RFC 6890, DOI 10.17487/RFC6890, April 2013,
<https://www.rfc-editor.org/info/rfc6890>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/info/rfc6951>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
[RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
<https://www.rfc-editor.org/info/rfc8340>.
Acknowledgments
The authors wish to thank Mohamed Boucadair, Gorry Fairhurst, Bryan
Ford, David Hayes, Alfred Hines, Karen E. E. Nielsen, Henning
Peters, Maksim Proshin, Timo Voelker, Dan Wing, and Qiaobing Xie for
their invaluable comments.
In addition, the authors wish to thank David Hayes, Jason But, and
Grenville Armitage, the authors of [DOI_10.1145_1496091.1496095], for
their suggestions.
The authors also wish to thank Mohamed Boucadair for contributing the
text related to the YANG module.
Stewart, et al. Expires 29 January 2021 [Page 49]
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Authors' Addresses
Randall R. Stewart
Netflix, Inc.
Chapin, SC 29036
United States of America
Email: randall@lakerest.net
Michael Tüxen
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
Irene Rüngeler
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: i.ruengeler@fh-muenster.de
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