Network Working Group R. Stewart
Internet-Draft Adara Networks
Intended status: BCP M. Tuexen
Expires: April 12, 2013 I. Ruengeler
Muenster Univ. of Appl. Sciences
October 9, 2012
Stream Control Transmission Protocol (SCTP) Network Address Translation
draft-ietf-behave-sctpnat-07.txt
Abstract
Stream Control Transmission Protocol [RFC4960] provides a reliable
communications channel between two end-hosts in many ways similar to
TCP [RFC0793]. With the widespread deployment of Network Address
Translators (NAT), specialized code has been added to NAT for TCP
that allows multiple hosts to reside behind a NAT and yet use only a
single globally unique IPv4 address, even when two hosts (behind a
NAT) choose the same port numbers for their connection. This
additional code is sometimes classified as Network Address and Port
Translation or NAPT. To date, specialized code for SCTP has NOT yet
been added to most NATs so that only pure NAT is available. The end
result of this is that only one SCTP capable host can be behind a
NAT.
This document describes an SCTP specific variant of NAT which
provides similar features of NAPT in the single point and multi-point
traversal scenario.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 12, 2013.
Copyright Notice
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Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . . . . 4
4.1. Single Point Traversal . . . . . . . . . . . . . . . . . . 4
4.2. Multi Point Traversal . . . . . . . . . . . . . . . . . . 5
5. Limitations of Classical NAPT for SCTP . . . . . . . . . . . . 6
6. The SCTP Specific Variant of NAT . . . . . . . . . . . . . . . 6
7. NAT to SCTP . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Handling of Fragmented SCTP Packets . . . . . . . . . . . . . 11
9. Various Examples of NAT Traversals . . . . . . . . . . . . . . 11
9.1. Single-homed Client to Single-homed Server . . . . . . . . 11
9.2. Single-homed Client to Multi-homed Server . . . . . . . . 13
9.3. Multihomed Client and Server . . . . . . . . . . . . . . . 15
9.4. NAT Loses Its State . . . . . . . . . . . . . . . . . . . 19
9.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
11. Security Considerations . . . . . . . . . . . . . . . . . . . 25
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
13.1. Normative References . . . . . . . . . . . . . . . . . . . 26
13.2. Informative References . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
Stream Control Transmission Protocol [RFC4960] provides a reliable
communications channel between two end-hosts in many ways similar to
TCP [RFC0793]. With the widespread deployment of Network Address
Translators (NAT), specialized code has been added to NAT for TCP
that allows multiple hosts to reside behind a NAT and use private
addresses (see [RFC5735]) and yet use only a single globally unique
IPv4 address, even when two hosts (behind a NAT) choose the same port
numbers for their connection. This additional code is sometimes
classified as Network Address and Port Translation or NAPT. To date,
specialized code for SCTP has not yet been added to most NATs so that
only true NAT is available. The end result of this is that only one
SCTP capable host can be behind a NAT.
This document proposes an SCTP specific variant NAT that provides the
NAPT functionality without changing SCTP port numbers. The authors
feel it is possible and desirable to make these changes for a number
of reasons.
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.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Terminology
For this discussion we will use several terms, which we will define
and point out in Figure 1.
Private-Address (Priv-Addr): The private 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 Private-Address.
Internal-VTag (Int-VTag): The Verification Tag that the internal
host has chosen for its communication. The VTag is a unique 32
bit tag that must accompany any incoming SCTP packet for this
association to the Private-Address.
External-Address (Ext-Addr): The address that an internal host is
attempting to contact.
External-Port (Ext-Port): The port number of the peer process at the
External-Address.
External-VTag (Ext-VTag): The Verification Tag that the host holding
the External-Address has chosen for its communication. The VTag
is a unique 32 bit tag that must accompany any incoming SCTP
packet for this association to the External-Address.
Public-Address (Pub-Addr): The public address assigned to the NAT
box which it uses as a source address when sending packets towards
the External-Address.
Internal Network | External Network
|
Private | Public External
+---------+ Address | Address /--\/--\ Address +---------+
| SCTP | +-----+ / \ | SCTP |
|end point|==========| NAT |======= | Internet | ======== |end point|
| A | +-----+ \ / | B |
+---------+ Internal | \--/\--/ External +---------+
Internal Port | Port External
VTag | VTag
Figure 1: Architecture
4. SCTP NAT Traversal Scenarios
This section defines the notion of single and multi-point NAT
traversal.
4.1. Single Point Traversal
In this case, all packets in the SCTP association go through a single
NAT, as shown below:
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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.
4.2. 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:
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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.
5. 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.
6. 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.
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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.
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/--\/--\
+--------+ +-----+ / \ +--------+
| 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
[I-D.ietf-tsvwg-natsupp] for the format).
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 [I-D.ietf-tsvwg-natsupp] for the
format).
The processing of outgoing SCTP packets containing no INIT-chunk is
described in the following figure.
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/--\/--\
+--------+ +-----+ / \ +--------+
| 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
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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
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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.
7. NAT to SCTP
This document at various places discusses the sending of specialized
SCTP chunks (e.g. an ABORT with M-Bit set). These chunks and
procedures are not defined in this document, but instead are defined
in [I-D.ietf-tsvwg-natsupp]. The NAT implementer should refer to
[I-D.ietf-tsvwg-natsupp] for detailed descriptions of packet formats
and procedures.
8. 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.
9. Various Examples of NAT Traversals
9.1. 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
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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.
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/--\/--\
+--------+ +-----+ / \ +--------+
| 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
9.2. 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
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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
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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
9.3. Multihomed Client and Server
The client (Host A) sends an INIT to the server (Host B), but does
not include the second address.
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+-------+
/--| 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
</artwork>
</figure>
<t>NAT 1 creates entry:</t>
<figure align='center'>
<artwork>
+---------+--------+-----------+----------+--------+
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]
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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 <---------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
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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
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9.4. 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.
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/--\/--\
+--------+ +-----+ / \ +--------+
| 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
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9.5. 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:
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+---------+--------+-----------+----------+--------+
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
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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:
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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 -->; 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.
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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
10. IANA Considerations
This document requires no actions from IANA.
11. Security Considerations
State maintenance within a NAT is always a subject of possible Denial
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.
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12. Acknowledgments
The authors wish to thank Jason But Bryan Ford, David Hayes, Alfred
Hines, Henning Peters, Timo Voelker, Dan Wing, and Qiaobing Xie for
their invaluable comments.
13. References
13.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
13.2. Informative References
[RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[I-D.ietf-tsvwg-natsupp]
Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation
Support", draft-ietf-tsvwg-natsupp-02 (work in progress),
March 2012.
Authors' Addresses
Randall R. Stewart
Adara Networks
Chapin, SC 29036
US
Email: randall@lakerest.net
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Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
DE
Email: tuexen@fh-muenster.de
Irene Ruengeler
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
DE
Email: i.ruengeler@fh-muenster.de
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