| TOC |
|
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Stream Control Transmission Protocol [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) provides a reliable communications channel between two end-hosts in many ways similar to TCP [RFC0793] (Postel, J., “Transmission Control Protocol,” September 1981.). 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.
1.
Introduction
2.
Conventions
3.
Terminology
4.
SCTP NAT Traversal Scenarios
4.1.
Single Point Traversal
4.2.
Multi Point Traversal
5.
The SCTP specific variant of NAT
6.
Handling of internal port number collisions
7.
Handling of internal port number and verification tag collisions
8.
Handling of missing state
9.
Multi Point Traversal considerations
10.
Handling of fragmented SCTP packets
11.
Simplification for small NATs
12.
Various examples of NAT traversals
12.1.
Single-homed client to single-homed server
12.2.
Single-homed client to multi-homed server
12.3.
Multihomed client and server
12.4.
NAT loses its state
12.5.
Peer-to-Peer Communication
13.
IANA Considerations
14.
Security considerations
15.
Acknowledgments
16.
References
16.1.
Normative References
16.2.
Informative References
§
Authors' Addresses
| TOC |
Stream Control Transmission Protocol [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) provides a reliable communications channel between two end-hosts in many ways similar to TCP [RFC0793] (Postel, J., “Transmission Control Protocol,” September 1981.). 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.
| TOC |
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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
| TOC |
For this discussion we will use several terms, which we will define and point out in a figure.
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
| TOC |
| TOC |
In this case, all packets in the SCTP association go through a single NAT, as shown below:
Internal Network | External Network
|
+---------+ | /--\/--\ +---------+
| SCTP | +-----+ / \ | SCTP |
|end point|==========| NAT |========= | Internet | =========|end point|
| A | +-----+ \ / | B |
+---------+ | \--/\--/ +---------+
|
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.
| TOC |
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 |=========/ \---\/---/
+------+
|
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.
| TOC |
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.
/--\/--\
+--------+ +-----+ / \ +--------+
| 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.
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 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.
| TOC |
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.
| TOC |
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 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] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) 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 / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOC |
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] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) 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 = 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.
| TOC |
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 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.
| TOC |
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.
| TOC |
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.
| TOC |
| TOC |
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
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
| TOC |
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 |
+---------+--------+-----------+----------+--------+-----------+
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
| TOC |
+-------+
/--------| 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 |
+---------+--------+-----------+----------+--------+-----------+
| 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]
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
| TOC |
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
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
| TOC |
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 | 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 |
+---------+--------+-----------+----------+--------+-----------+
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
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
| TOC |
TBD
| TOC |
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.
| TOC |
The authors wish to thank Qiaobing Xie, Henning Peters, Bryan Ford, David Hayes, Alfred Hines, Dan Wing, and Jason But for their invaluable comments.
| TOC |
| TOC |
| [RFC0793] | Postel, J., “Transmission Control Protocol,” STD 7, RFC 793, September 1981 (TXT). |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [RFC4960] | Stewart, R., “Stream Control Transmission Protocol,” RFC 4960, September 2007 (TXT). |
| TOC |
| [RFC1918] | Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” BCP 5, RFC 1918, February 1996 (TXT). |
| TOC |
| Randall R. Stewart | |
| Researcher | |
| Chapin, SC 29036 | |
| USA | |
| Phone: | |
| Email: | randall@lakerest.net |
| 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 |