Network Working Group R. Stewart
Internet-Draft Researcher
Intended status: BCP M. Tuexen
Expires: August 20, 2009 I. Ruengeler
Muenster Univ. of Applied Sciences
February 16, 2009
Stream Control Transmission Protocol (SCTP) Network Address Translation
draft-ietf-behave-sctpnat-01.txt
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . . . . 5
4.1. Single Point Traversal . . . . . . . . . . . . . . . . . . 5
4.2. Multi Point Traversal . . . . . . . . . . . . . . . . . . 6
5. The SCTP specific variant of NAT . . . . . . . . . . . . . . . 7
6. Handling of internal port number collisions . . . . . . . . . 10
7. Handling of internal port number and verification tag
collisions . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Handling of missing state . . . . . . . . . . . . . . . . . . 11
9. Multi Point Traversal considerations . . . . . . . . . . . . . 12
10. Handling of fragmented SCTP packets . . . . . . . . . . . . . 13
11. Simplification for small NATs . . . . . . . . . . . . . . . . 13
12. Various examples of NAT traversals . . . . . . . . . . . . . . 13
12.1. Single-homed client to single-homed server . . . . . . . . 13
12.2. Single-homed client to multi-homed server . . . . . . . . 15
12.3. Multihomed client and server . . . . . . . . . . . . . . . 16
12.4. NAT loses its state . . . . . . . . . . . . . . . . . . . 18
12.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . . 19
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
14. Security considerations . . . . . . . . . . . . . . . . . . . 22
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
16.1. Normative References . . . . . . . . . . . . . . . . . . . 23
16.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
Stream Control Transmission Protocol [RFC4960] provides a reliable
communications channel between two end-hosts in many ways similar to
TCP [RFC0793]. With the widespread deployment of Network Address
Translators (NAT), specialized code has been added to NAT for TCP
that allows multiple hosts to reside behind a NAT and yet use only a
single globally unique IPv4 address, even when two hosts (behind a
NAT) choose the same port numbers for their connection. This
additional code is sometimes classified as Network Address and Port
Translation 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 a figure.
o Private-Address (Priv-Addr) - The private address that is known to
the internal host.
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o Internal-Port (Int-Port) - The port number that is in use by the
host holding the Private-Address. Normally this is the port that
will be translated by the NAPT to a different port number.
o 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.
o External-Address (Ext-Addr) - The address that an internal host is
attempting to contact.
o External-Port (Ext-Port) - The port number of the peer process at
the External-Address.
o 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.
o 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
4. SCTP NAT Traversal Scenarios
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 |
+---------+ | \--/\--/ +---------+
|
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 |
+---------+ | :: | \--/\--/ +---------+
| :: |
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.
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.
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:
Internal | External
+------+ /---\/---\
+---------+ /=======|NAT A |=========\ / \ +---------+
| SCTP | / +------+ \/ \ | SCTP |
|end point|/ ... | Internet |=====|end point|
| A |\ \ / | B |
+---------+ \ +------+ / \ / +---------+
\=======|NAT B |=========/ \---\/---/
+------+
|
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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.
To make this work, however, all the NATs involved must recognize the
packets they see as belonging to the same SCTP association and
perform address translation in a consistent way. This may require
that a pre-defined table of ports and addresses were shared between
the NATs. Other external management schemes that help multiple NATs
coordinate a multi-homed SCTP association could be investigated.
5. The SCTP specific variant of NAT
In this section we assume that we have multiple SCTP capable hosts
behind a NAT which has one Public-Address. Furthermore we are
focusing in this section on the single point traversal scenario.
The modification of SCTP packets sent to the public Internet is easy.
The source address of the packet has to be replaced with the Public-
Address. It may also be necessary to establish some state in the NAT
box to handle incoming packets, which is discussed later.
For SCTP packets coming from the public Internet the destination
address of the packets has to be replaced with the Private-Address of
the host the packet has to be delivered to. The lookup of the
Private-Address is based on the External-VTag, External-Port,
External-Address, Internal-VTag and the Internal-Port.
For the SCTP NAT processing the NAT box has to maintain a table of
Internal-VTag, Internal-Port, Private-Address, External-VTag,
External-Port and External-Address. An entry in that table is called
a NAT state control block.
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,Internal-Port,Private-Address,
0,External-Port,External-Address)
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, External-VTag,
Internal-VTag but different Private-Address. In this case the INIT
SHOULD be dropped and an ABORT MAY be sent back.
The processing of outgoing SCTP packets containing no INIT-chunk is
described in the following figure.
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.
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INIT-ACK[Initiate-Tag]
Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
Int-VTag
Lookup(Internal-VTag,Internal-Port,*,
0,External-Port,External-Address)
Update(*, *, *, Initiate-Tag, *, *)
Returns(NAT-State control block containing Private-Address)
INIT-ACK[Initiate-Tag]
Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
Int-VTag
In the case Lookup fails, the SCTP packet is dropped. The Update
routine inserts the External-VTag (the Initiate-Tag of the INIT-ACK
chunk) in the NAT state control block.
The processing of incoming SCTP packets containing an ABORT or
SHUTDOWN-COMPLETE chunk with the T-Bit set is described in the
following figure.
Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
Ext-VTag
Lookup(0, Internal-Port, *,External-VTag,
External-Port, External-Address)
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.
Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
Int-VTag
Lookup(Internal-VTag, Internal-Port, *,
*, External-Port, External-Address)
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
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made only based on the addresses and port numbers. If an entry with
an Internal-VTag of zero is found, it is considered a match and the
Internal-VTag is updated.
This allows the handling of INIT-collision through NAT.
6. Handling of internal port number collisions
There is one drawback of the SCTP specific variant of NAT compared to
a NAPT solution like the ones available for TCP. Consider the case
where two hosts in the Private-Address space want to set up an SCTP
association with the same server running on the same host in the
Internet. This means that the External-Port and the External-Address
are the same. If they both choose the same Internal-Port the server
cannot distinguish both associations based on the address and port
numbers. For the server it looks like the association is being
restarted. To overcome this limitation the client sends a
NAT_SUPPORTED parameter in the INIT-chunk which is defined as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0xC007 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
When the server receives this parameter it will also use the
verification tag to look up the association. However, this will make
it impossible to restart such associations.
7. Handling of internal port number and verification tag collisions
Consider the case where two hosts in the Private-Address space want
to set up an SCTP association with the same server running on the
same host in the Internet. This means that the External-Port and the
External-Address are the same. If they both choose the same
Internal-Port and Internal-VTag, the NAT box cannot distinguish
incoming packets anymore. But this is very unlikely. The Internal-
VTags are chosen at random and if the Internal-Ports are also chosen
from the ephemeral port range at random this gives a 46 bit random
number which has to match. In the TCP like NAPT case the NAT box can
control the 16 bit Natted Port.
However, in this unlikely event the NAT box MUST respond to the INIT
chunk by sending an ABORT chunk with the M-bit set. The source
address of the packet containing the ABORT chunk MUST be the
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destination address of the SCTP packet containing the INIT chunk.
The sender of the packet containing the INIT chunk MAY start the
association setup procedure after choosing a new initiate tag.
The ABORT chunk defined in [RFC4960] is therefore extended by using
the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following error cause with cause code 0x00B0 (Colliding NAT table
entry) SHOULD be included in the ABORT chunk:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=0x00B0 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ INIT chunk /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8. Handling of missing state
If the NAT box receives a packet for which the lookup procedure does
not find an entry in the NAT table, a packet containing an ERROR
packet is sent back with the M-bit set. The source address of the
packet containing the ERROR chunk MUST be the destination address of
the incoming SCTP packet. The verification tag is reflected.
The ERROR chunk defined in [RFC4960] is therefore extended by using
the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following error cause with cause code 0x00B1 (Missing NAT table
entry) SHOULD be included in the ERROR chunk:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=0x00B1 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Incoming Packet /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If an end-point receives a packet with this ERROR chunk it MAY send
an SCTP packet with an ASCONF chunk containing an Add IP Address
parameter followed by a vtag parameter:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 0xC008 | Parameter Length = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASCONF-Request Correlation ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internal Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If the NAT box receives a packet for which it has no NAT table entry
and the packet contains an ASCONF chunk with a vtag parameter, the
NAT box MUST update its NAT table according to the verification tags
in the vtag parameter.
9. Multi Point Traversal considerations
If a multi-homed SCTP end-point behind a NAT connects to a peer, it
first sets up the association single-homed. Then it adds each IP
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address using ASCONF chunks. The address to add is the wildcard
address and the lookup address also. The ASCONF chunks SHOULD also
contain a vtag parameter.
10. 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.
11. Simplification for small NATs
Small NAT boxes, i.e. NAT boxes which only have to support a small
number of concurrent SCTP associations, MAY not take the external
address into account when processing packets. Therefore the
External-Address could also be removed from the NAT table.
This simplification may make implementing a NAT box easier, however,
the collision probability is higher than using a mapping which takes
the external address into account.
12. Various examples of NAT traversals
12.1. Single-homed client to single-homed server
The internal client starts the association with the external server
via a four-way-handshake.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <-------> | NAT | <------> | Internet | <-------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
+---------+--------+-----------+----------+--------+-----------+
NAT | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+--- -------+----------+--------+-----------+
Host A sends INIT:
INIT[Initiate-Tag = 1234]
10.0.0.1:1 ------> 100.0.0.1:2
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Ext-VTtag = 0
NAT creates entry:
+---------+--------+-----------+----------+--------+-----------+
NAT | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 0 | 2 | 100.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
101.0.0.1:1 ---------------------------> 100.0.0.1:2
Ext-VTtag = 0
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 | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 | 100.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
INIT-ACK[Initiate-Tag = 5678]
10.0.0.1:1 <------ 100.0.0.1:2
Int-VTag = 1234
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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12.2. Single-homed client to multi-homed server
The internal client is single-homed whereas the external server is
multi-homed. The server includes its addresses in the INIT-ACK
chunk, which results in two NAT entries.
+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <------> | NAT | <-> | Internet | == ==| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
+---------+--------+-----------+----------+--------+-----------+
NAT | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+--- -------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 ---> 100.0.0.1:2
Ext-VTag = 0
NAT creates entry:
+---------+--------+-----------+----------+--------+-----------+
NAT | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 0 | 2 | 100.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
101.0.0.1:1 ------------------------------> 100.0.0.1:2
Ext-VTag = 0
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 updates first entry and creates entry for second address:
+---------+--------+-----------+----------+--------+-----------+
NAT | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 | 100.0.0.1 |
| 1234 | 1 | 10.0.0.1 | 5678 | 2 | 100.1.0.1 |
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+---------+--------+-----------+----------+--------+-----------+
INIT-ACK[Initiate-Tag = 5678]
10.0.0.1:1 <--- 100.0.0.1:2
Int-VTag = 1234
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
12.3. Multihomed client and server
+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |==== ====| Internet |====| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
+---------+--------+-----------+----------+--------+-----------+
NAT 1 | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+--- -------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 --------> 100.0.0.1:2
Ext-VTag = 0
NAT 1 creates entry:
+---------+--------+-----------+----------+--------+-----------+
NAT 1 | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
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+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 0 | 2 | 100.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
101.0.0.1:1 -------------------------> 100.0.0.1:2
ExtVTag = 0
INIT-ACK[Initiate-Tag = 5678, IP-Addr = 100.1.0.1]
101.0.0.1:1 <------------------------- 100.0.0.1:2
Int-VTag = 1234
NAT 1 updates first entry and creates complete entry for second address:
+---------+--------+-----------+----------+--------+-----------+
NAT 1 | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 | 100.0.0.1 |
| 1234 | 1 | 10.0.0.1 | 5678 | 2 | 100.1.0.1 |
+---------+--------+-----------+----------+--------+-----------+
INIT-ACK[Initiate-Tag = 5678]
10.0.0.1:1 <---------100.0.0.1:2
Int-VTag = 1234
COOKIE-ECHO
10.0.0.1:1 --------> 100.0.0.1:2
Ext-VTag = 5678
COOKIE-ECHO
101.0.0.1:1 ----------------------> 100.0.0.1:2
Ext-VTag = 5678
COOKIE-ACK
101.0.0.1:1 <---------------------- 100.0.0.1:2
Int-VTag = 1234
COOKIE-ACK
10.0.0.1:1 <------- 100.0.0.1:2
Int-VTag = 1234
Host A announces the second address
ASCONF [ADD-IP,INT-VTag=1234, Ext-VTag = 5678]
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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 | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.1.0.1 | 5678 | 2 | 100.1.0.1 |
+---------+--------+-----------+----------+--------+-----------+
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
12.4. NAT loses its state
Assocation is already established between Host A and Host B.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
+---------+--------+-----------+----------+--------+-----------+
NAT A | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 | 100.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
The NAT loses its state and obtaines a new public address.
DATA
10.0.0.1:1 ----------> 100.0.0.1:2
Ext-VTag = 5678
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NAT cannot find entry: sends ERROR message
ERROR [M-Bit, NAT state missing]
10.0.0.1:1 <---------- 100.0.0.1:2
Ext-VTag = 5678
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 A | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 | 100.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
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 new source address and deletes all former entries.
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 1
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
12.5. 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
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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 | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+--- -------+----------+--------+-----------+
+---------+--------+-----------+----------+--------+-----------+
NAT B | Int | Int | Priv | Ext | Ext | Ext |
| v-tag | port | addr | v-tag | port | addr |
+---------+--------+--- -------+----------+--------+-----------+
Host A sends INIT:
INIT[Initiate-Tag = 1234]
10.0.0.1:1 --> 100.0.0.1:2
Ext-VTag = 0
NAT A creates entry:
+---------+--------+-----------+----------+--------+-----------+
NAT A | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 0 | 2 | 100.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
101.0.0.1:1 ----------------> 100.0.0.1:2
Ext-VTag = 0
NAT B processes INIT
SCTP packet silently discarded
+---------+--------+-----------+----------+--------+-----------+
NAT B | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
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Host B sends INIT:
INIT[Initiate-Tag = 5678]
101.0.0.1:1 <-- 10.1.0.1:2
Ext-VTag = 0
NAT B processes INIT:
+---------+--------+-----------+----------+--------+-----------+
NAT B | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 5678 | 2 | 10.1.0.1 | 0 | 1 | 101.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
NAT B forwards INIT:
INIT[Initiate-Tag = 5678]
101.0.0.1:1 <--------------- 100.0.0.1:2
Ext-VTag = 0
NAT A processes INIT and updates entry:
VTag != Int-VTag, but Ext-VTag == 0, find entry.
+---------+--------+-----------+----------+--------+-----------+
NAT A | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 1234 | 1 | 10.0.0.1 | 5678 | 2 | 100.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
NAT A forwards INIT:
INIT[Initiate-tag = 5678]
10.0.0.1:1 <-- 100.0.0.1:2
Ext-VTag = 0
Host A send INIT-ACK:
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
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NAT B updates entry:
+---------+--------+-----------+----------+--------+-----------+
NAT B | Int | Int | Priv | Ext | Ext | Ext |
| VTag | Port | Addr | VTag | Port | Addr |
+---------+--------+-----------+----------+--------+-----------+
| 5678 | 2 | 10.1.0.1 | 1234 | 1 | 101.0.0.1 |
+---------+--------+-----------+----------+--------+-----------+
INIT-ACK[Initiate-Tag = 1234]
101.0.0.1:1 --> 10.1.0.1:2
Ext-VTag = 5678
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
13. IANA Considerations
TBD
14. 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
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cleaned up.
15. Acknowledgments
The authors wish to thank Qiaobing Xie, Henning Peters, Bryan Ford,
David Hayes, Alfred Hines, Dan Wing, and Jason But for their
invaluable comments.
16. References
16.1. 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.
16.2. Informative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
Authors' Addresses
Randall R. Stewart
Researcher
Chapin, SC 29036
USA
Phone:
Email: randall@lakerest.net
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Michael Tuexen
Muenster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
Irene Ruengeler
Muenster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
Email: i.ruengeler@fh-muenster.de
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