Stream Control Transmission Protocol (SCTP) Network Address Translation Support Netflix, Inc.
Chapin SC 29036 US randall@lakerest.net
Münster University of Applied Sciences
Stegerwaldstrasse 39 48565 Steinfurt DE tuexen@fh-muenster.de
Münster University of Applied Sciences
Stegerwaldstrasse 39 48565 Steinfurt DE i.ruengeler@fh-muenster.de
Internet-Draft The Stream Control Transmission Protocol (SCTP) provides a reliable communications channel between two end-hosts in many ways similar to the Transmission Control Protocol (TCP). With the widespread deployment of Network Address Translators (NAT), specialized code has been added to NAT functions for TCP that allows multiple hosts to reside behind a NAT function and yet share a single IPv4 address, even when two hosts (behind a NAT function) choose the same port numbers for their connection. This additional code is sometimes classified as Network Address and Port Translation (NAPT). This document describes the protocol extensions required for the SCTP endpoints and the mechanisms for NAT functions necessary to provide similar features of NAPT in the single point and multi point traversal scenario. Finally, a YANG module for SCTP NAT is defined.
Introduction Stream Control Transmission Protocol (SCTP) provides a reliable communications channel between two end-hosts in many ways similar to TCP . With the widespread deployment of Network Address Translators (NAT), specialized code has been added to NAT functions for TCP that allows multiple hosts to reside behind a NAT functions using internal addresses (see ) and yet share single IPv4 address, even when two hosts (behind a NAT function) choose the same port numbers for their connection. This additional code is sometimes classified as Network Address and Port Translation (NAPT). Please note that this document focuses on the case where the NAT function maps a single or multiple internal addresses to a single external address and vice versa. To date, specialized code for SCTP has not yet been added to most NAT functions so that only a translation of IP addresses is supported. The end result of this is that only one SCTP-capable host can successfully operate behind such a NAT function and this host can only be single-homed. The only alternative for supporting legacy NAT functions is to use UDP encapsulation as specified in . The NAT function in the document refers to NAPT functions described in Section 2.2 of , NAT64 , or DS-Lite . This document specifies procedures allowing a NAT function to support SCTP by providing similar features to those provided by a NAPT for TCP and other supported protocols. The document also specifies a set of data formats for SCTP packets and a set of SCTP endpoint procedures to support NAT traversal. An SCTP implementation supporting these procedures can assure that in both single-homed and multi-homed cases a NAT function will maintain the appropriate state without the NAT function needing to change port numbers. It is possible and desirable to make these changes for a number of reasons:
  • It is desirable for SCTP internal end-hosts on multiple platforms to be able to share a NAT function's external IP address in the same way that a TCP session can use a NAT function.
  • If a NAT function does not need to change any data within an SCTP packet it will reduce the processing burden of NAT'ing SCTP by not needing to execute the CRC32c checksum required by SCTP.
  • Not having to touch the IP payload makes the processing of ICMP messages in NAT functions easier.
An SCTP-aware NAT function will need to follow these procedures for generating appropriate SCTP packet formats. When considering this feature it is possible to have multiple levels of support. At each level, the Internal Host, Remote Host and NAT function may or may not support the features described in this document. The following table illustrates the results of the various combinations of support and if communications can occur between two endpoints. Communication possibilities
Internal Host NAT Function Remote Host Communication
Support Support Support Yes
Support Support No Support Limited
Support No Support Support None
Support No Support No Support None
No Support Support Support Limited
No Support Support No Support Limited
No Support No Support Support None
No Support No Support No Support None
From the table it can be seen that when a NAT function does not support the extension no communication can occur. This assumes that the NAT function does not handle SCTP packets at all and all SCTP packets sent externally from behind a NAT function are discarded by the NAT function. In some cases, where the NAT function supports the feature but one of the two hosts does not support the feature, communication may occur but in a limited way. For example only one host may be able to have a connection when a collision case occurs.
Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.
Terminology This document uses the following terms, which are depicted in . Familiarity with the terminology used in and is assumed.
Internal-Address (Int-Addr)
The internal address that is known to the internal host.
Internal-Port (Int-Port)
The port number that is in use by the host holding the Internal-Address.
Internal-VTag (Int-VTag)
The SCTP Verification Tag (VTag) (see Section 3.1 of ) that the internal host has chosen for its communication. The VTag is a unique 32-bit tag that must accompany any incoming SCTP packet for this association to the Internal-Address.
Remote-Address (Rem-Addr)
The address that an internal host is attempting to contact.
Remote-Port (Rem-Port)
The port number of the peer process at the Remote-Address.
Remote-VTag (Rem-VTag)
The Verification Tag (VTag) (see Section 3.1 of ) that the host holding the Remote-Address has chosen for its communication. The VTag is a unique 32-bit tag that must accompany any incoming SCTP packet for this association to the Remote-Address.
External-Address (Ext-Addr)
The external address assigned to the NAT function, that it uses as a source address when sending packets towards the Remote-Address.
Basic network setup Internal Network | External Network | Internal | External Remote +--------+ Address | Address /--\/--\ Address +--------+ | SCTP | +-----+ / \ | SCTP | |endpoint|=========| NAT |=======| Internet |==========|endpoint| | A | +-----+ \ / | B | +--------+ Internal | \--/\--/ Remote +--------+ Internal Port | Port Remote VTag | VTag
Motivation
SCTP NAT Traversal Scenarios This section defines the notion of single and multi point NAT traversal.
Single Point Traversal In this case, all packets in the SCTP association go through a single NAT function, as shown below:
Single NAT scenario Internal Network | External Network | +--------+ | /--\/--\ +--------+ | SCTP | +-----+ / \ | SCTP | |endpoint|=========| NAT |========= | Internet | ========|endpoint| | A | +-----+ \ / | B | +--------+ | \--/\--/ +--------+ |
A variation of this case is shown below, i.e., multiple NAT functions in a single path:
Serial NAT Functions scenario Internal | External : Internal | External | : | +--------+ | : | /--\/--\ +--------+ | SCTP | +-----+ : +-----+ / \ | SCTP | |endpoint|==| NAT |=======:=======| NAT |==| Internet |==|endpoint| | A | +-----+ : +-----+ \ / | B | +--------+ | : | \--/\--/ +--------+ | : |
Although one of the main benefits of SCTP multi-homing is redundant paths, in the single point traversal scenario the NAT function represents a single point of failure in the path of the SCTP multi-homed association. However, the rest of the path may still benefit from path diversity provided by SCTP multi-homing. The two SCTP endpoints in this case can be either single-homed or multi-homed. However, the important thing is that the NAT function in this case sees all the packets of the SCTP association.
Multi Point Traversal This case involves multiple NAT functions and each NAT function only sees some of the packets in the SCTP association. An example is shown below:
Parallel NAT functions scenario Internal | External +------+ /---\/---\ +--------+ /=======|NAT A |=========\ / \ +--------+ | SCTP | / +------+ \/ \ | SCTP | |endpoint|/ ... | Internet |===|endpoint| | 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.
Limitations of Classical NAPT for SCTP Using classical NAPT may result in changing one of the SCTP port numbers during the processing which requires the recomputation of the transport layer checksum by the NAPT device. Whereas for UDP and TCP this can be done very efficiently, for SCTP the checksum (CRC32c) over the entire packet needs to be recomputed (see Appendix B of for details of the CRC32c computation). This would considerably add to the NAT computational burden, however hardware support may mitigate this in some implementations. An SCTP endpoint may have multiple addresses but only has a single port number. To make multipoint traversal work, all the NAT functions involved must recognize the packets they see as belonging to the same SCTP association and perform port number translation in a consistent way. One possible way of doing this is to use a pre-defined table of ports and addresses configured within each NAT function. Other mechanisms could make use of NAT to NAT communication. Such mechanisms have not been deployed on a wide scale base and thus are not a recommended solution. Therefore an SCTP variant of NAT function has been developed.
The SCTP-Specific Variant of NAT In this section it is allowed that there are multiple SCTP capable hosts behind a NAT function that has one Exernal-Address. Furthermore this section focuses on the single point traversal scenario. The modification of SCTP packets sent to the Internet is simple: the source address of the packet has to be replaced with the External-Address. It may also be necessary to establish some state in the NAT function to later handle incoming packets. For the SCTP NAT processing the NAT function has to maintain a NAT binding table of Internal-VTag, Internal-Port, Remote-VTag, Remote-Port, Internal-Address, and whether the restart procedure is disabled or not. An entry in that NAT binding table is called a NAT-State control block. The function Create() obtains the just mentioned parameters and returns a NAT-State control block. A NAT function MAY allow creating NAT-State control blocks via a management interface. For SCTP packets coming from the public Internet the destination address of the packets has to be replaced with the Internal-Address of the host to which the packet has to be delivered. The lookup of the Internal-Address is based on the Remote-VTag, Remote-Port, Internal-VTag and the Internal-Port. The entries in the NAT binding table need to fulfill some uniqueness conditions. There must not be more than one entry NAT binding table with the same pair of Internal-Port and Remote-Port. This rule can be relaxed, if all NAT binding table entries with the same Internal-Port and Remote-Port have the support for the restart procedure enabled. In this case there must be no more than one entry with the same Internal-Port, Remote-Port and Remote-VTag and no more than one NAT binding table entry with the same Internal-Port, Remote-Port and Int-VTag. The processing of outgoing SCTP packets containing an INIT chunk is described in the following figure. The scenario shown is valid for all message flows in this section. | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ INIT[Initiate-Tag] Int-Addr:Int-Port ------> Rem-Addr:Rem-Port Rem-VTag=0 Create(Initiate-Tag, Int-Port, 0, Rem-Port, Int-Addr, RestartSupported) Returns(NAT-State control block) Translate To: INIT[Initiate-Tag] Ext-Addr:Int-Port ------> Rem-Addr:Rem-Port Rem-VTag=0 ]]> Normally a NAT binding table entry will be created. However, it is possible that there is already a NAT binding table entry with the same Remote-Port, Internal-Port, and Internal-VTag but different Internal-Address. In this case the packet containing the INIT chunk MUST be dropped by the NAT and a packet containing an ABORT chunk SHOULD be sent to the SCTP host that originated the packet with the M-Bit set and an appropriate error cause (see for the format). The source address of the packet containing the ABORT chunk MUST be the destination address of the packet containing the INIT chunk. If an outgoing SCTP packet contains an INIT or ASCONF chunk and a matching NAT binding table entry is found, the packet is processed as a normal outgoing packet. It is also possible that a connection to Remote-Address and Remote-Port exists without an Internal-VTag conflict but there exists a NAT binding table entry with the same port numbers but a different Internal-Address. In such a case the packet containing the INIT chunk MUST be dropped by the NAT function and a packet containing an ABORT chunk SHOULD be sent to the SCTP host that originated the packet with the M-Bit set and an appropriate error cause (see for the format). The processing of outgoing SCTP packets containing no INIT chunks is described in the following figure. | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ Int-Addr:Int-Port ------> Rem-Addr:Rem-Port Rem-VTag Translate To: Ext-Addr:Int-Port ------> Rem-Addr:Rem-Port Rem-VTag ]]> The processing of incoming SCTP packets containing an INIT ACK chunk is described in the following figure. The Lookup() function getting as input the Internal-VTag, Internal-Port, Remote-VTag, and Remote-Port, returns the corresponding entry of the NAT binding table and updates the Remote-VTag by substituting it with the value of the Initiate-Tag of the INIT ACK chunk. The wildcard character signifies that the parameter's value is not considered in the Lookup() function or changed in the Update() function, respectively. | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ INIT ACK[Initiate-Tag] Ext-Addr:Int-Port <---- Rem-Addr:Rem-Port Int-VTag Lookup(Int-VTag, Int-Port, *, Rem-Port) Update(*, *, Initiate-Tag, *) Returns(NAT-State control block containing Int-Addr) INIT ACK[Initiate-Tag] Int-Addr:Int-Port <------ Rem-Addr:Rem-Port Int-VTag ]]> In the case Lookup fails, the SCTP packet is dropped. If it succeeds, the Update routine inserts the Remote-VTag (the Initiate-Tag of the INIT ACK chunk) in the NAT-State control block. The processing of incoming SCTP packets containing an ABORT or SHUTDOWN COMPLETE chunk with the T-Bit set is described in the following figure. | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ Ext-Addr:Int-Port <------ Rem-Addr:Rem-Port Rem-VTag Lookup(*, Int-Port, Rem-VTag, Rem-Port) Returns(NAT-State control block containing Int-Addr) Int-Addr:Int-Port <------ Rem-Addr:Rem-Port Rem-VTag ]]> For an incoming packet containing an INIT chunk a table lookup is made only based on the addresses and port numbers. If an entry with an Remote-VTag of zero is found, it is considered a match and the Remote-VTag is updated. If an entry with a non-matching Remote-VTag is found or no entry is found, the incoming packet is dropped. If an entry with a matching Remote-VTag is found, the incoming packet is forwarded. This allows the handling of INIT collision through NAT functions. The processing of other incoming SCTP packets is described in the following figure. | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ Ext-Addr:Int-Port <------ Rem-Addr:Rem-Port Int-VTag Lookup(Int-VTag, Int-Port, *, Rem-Port) Returns(NAT-State control block containing Internal-Address) Int-Addr:Int-Port <------ Rem-Addr:Rem-Port Int-VTag ]]>
Data Formats This section defines the formats used to support NAT traversal. and describe chunks and error causes sent by NAT functions and received by SCTP endpoints. describes parameters sent by SCTP endpoints and used by NAT functions and SCTP endpoints.
Modified Chunks This section presents existing chunks defined in for which additional flags are specified by this document.
Extended ABORT Chunk 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 6 | Reserved |M|T| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / zero or more Error Causes / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The ABORT chunk is extended to add the new 'M bit'. The M bit indicates to the receiver of the ABORT chunk that the chunk was not generated by the peer SCTP endpoint, but instead by a middle box. [NOTE to RFC-Editor: Assignment of M bit to be confirmed by IANA.]
Extended ERROR Chunk 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 9 | Reserved |M|T| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / zero or more Error Causes / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The ERROR chunk defined in is extended to add the new 'M bit'. The M bit indicates to the receiver of the ERROR chunk that the chunk was not generated by the peer SCTP endpoint, but instead by a middle box. [NOTE to RFC-Editor: Assignment of M bit to be confirmed by IANA.]
New Error Causes This section defines the new error causes added by this document.
VTag and Port Number Collision Error Cause 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 0x00B0 | Cause Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ Chunk / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the 'VTag and Port Number Collision' Error Cause. IANA is requested to assign the value 0x00B0 for this cause code.
Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The value MUST be the length of the Cause-Specific Information plus 4.
Chunk: variable length
The Cause-Specific Information is filled with the chunk that caused this error. This can be an INIT, INIT ACK, or ASCONF chunk. Note that if the entire chunk will not fit in the ERROR chunk or ABORT chunk being sent then the bytes that do not fit are truncated.
[NOTE to RFC-Editor: Assignment of cause code to be confirmed by IANA.]
Missing State Error Cause 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 0x00B1 | Cause Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ Incoming Packet / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the 'Missing State' Error Cause. IANA is requested to assign the value 0x00B1 for this cause code.
Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The value MUST be the length of the Cause-Specific Information plus 4.
Incoming Packet: variable length
The Cause-Specific Information is filled with the IPv4 or IPv6 packet that caused this error. The IPv4 or IPv6 header MUST be included. Note that if the packet will not fit in the ERROR chunk or ABORT chunk being sent then the bytes that do not fit are truncated.
[NOTE to RFC-Editor: Assignment of cause code to be confirmed by IANA.]
Port Number Collision Error Cause 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 0x00B2 | Cause Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ Chunk / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the 'Port Number Collision' Error Cause. IANA is requested to assign the value 0x00B2 for this cause code.
Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The value MUST be the length of the Cause-Specific Information plus 4.
Chunk: variable length
The Cause-Specific Information is filled with the chunk that caused this error. This can be an INIT, INIT ACK, or ASCONF chunk. Note that if the entire chunk will not fit in the ERROR chunk or ABORT chunk being sent then the bytes that do not fit are truncated.
[NOTE to RFC-Editor: Assignment of cause code to be confirmed by IANA.]
New Parameters This section defines new parameters and their valid appearance defined by this document.
Disable Restart Parameter This parameter is used to indicate that the restart procedure is requested to be disabled. Both endpoints of an association MUST include this parameter in the INIT chunk and INIT ACK chunk when establishing an association and MUST include it in the ASCONF chunk when adding an address to successfully disable the restart procedure. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0xC007 | Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This field holds the IANA defined parameter type for the Disable Restart Parameter. IANA is requested to assign the value 0xC007 for this parameter type.
Parameter Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the parameter. The value MUST be 4.
[NOTE to RFC-Editor: Assignment of parameter type to be confirmed by IANA.] This parameter MAY appear in INIT, INIT ACK and ASCONF chunks and MUST NOT appear in any other chunk.
VTags Parameter This parameter is used to help a NAT function to recover from state loss. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Parameter Type = 0xC008 | Parameter Length = 16 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ASCONF-Request Correlation ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Internal Verification Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote Verification Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This field holds the IANA defined parameter type for the VTags Parameter. IANA is requested to assign the value 0xC008 for this parameter type.
Parameter Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the parameter. The value MUST be 16.
ASCONF-Request Correlation ID: 4 bytes (unsigned integer)
This is an opaque integer assigned by the sender to identify each request parameter. The receiver of the ASCONF Chunk will copy this 32-bit value into the ASCONF Response Correlation ID field of the ASCONF ACK response parameter. The sender of the packet containing the ASCONF chunk can use this same value in the ASCONF ACK chunk to find which request the response is for. Note that the receiver MUST NOT change this 32-bit value.
Internal Verification Tag: 4 bytes (unsigned integer)
The Verification Tag that the internal host has chosen for its communication. The Verification Tag is a unique 32-bit tag that must accompany any incoming SCTP packet for this association to the Internal-Address.
Remote Verification Tag: 4 bytes (unsigned integer)
The Verification Tag that the host holding the Remote-Address has chosen for its communication. The VTag is a unique 32-bit tag that must accompany any incoming SCTP packet for this association to the Remote-Address.
[NOTE to RFC-Editor: Assignment of parameter type to be confirmed by IANA.] This parameter MAY appear in ASCONF chunks and MUST NOT appear in any other chunk.
Procedures for SCTP Endpoints and NAT Functions When an SCTP endpoint is behind an SCTP-aware NAT a number of problems may arise as it tries to communicate with its peer:
  • IP addresses can not be included in the SCTP packet. This is discussed in .
  • More than one host behind a NAT function could select the same VTag and source port when talking to the same peer server. This creates a situation where the NAT function will not be able to tell the two associations apart. This situation is discussed in .
  • When an SCTP endpoint is a server communicating with multiple peers and the peers are behind the same NAT function, then the two endpoints cannot be distinguished by the server. This case is discussed in .
  • A restart of a NAT function during a conversation could cause a loss of its state. This problem and its solution is discussed in .
  • NAT functions need to deal with SCTP packets being fragmented at the IP layer. This is discussed in .
  • An SCTP endpoint can be behind two NAT functions in parallel providing redundancy. The method to set up this scenario is discussed in .
Each of these mechanisms requires additional chunks and parameters, defined in this document, and modified handling procedures from those specified in as described below.
Association Setup Considerations for Endpoints The association setup procedure defined in allows multi-homed SCTP endpoints to exchange its IP-addresses by using IPv4 or IPv6 address parameters in the INIT and INIT ACK chunks. However, this does not work when NAT functions are present. Every association setup from a host behind a NAT function MUST NOT use multiple internal addresses. The INIT chunk MUST NOT contain an IPv4 Address parameter, IPv6 Address parameter, or Supported Address Types parameter. The INIT ACK chunk MUST NOT contain any IPv4 Address parameter or IPv6 Address parameter using non-global addresses. The INIT chunk and the INIT ACK chunk MUST NOT contain any Host Name parameters. If the association should finally be multi-homed, the procedure in MUST be used. The INIT and INIT ACK chunk SHOULD contain the Disable Restart parameter defined in .
Handling of Internal Port Number and Verification Tag Collisions Consider the case where two hosts in the Internal-Address space want to set up an SCTP association with the same service provided by some hosts in the Internet. This means that the Remote-Port is the same. If they both choose the same Internal-Port and Internal-VTag, the NAT function cannot distinguish between incoming packets anymore. However, this is unlikely. The Internal-VTags are chosen at random and if the Internal-Ports are also chosen from the ephemeral port range at random this gives a 46-bit random number that has to match. A NAPT device can control the Port number and therefore avoid collisions deterministically. The same can happen with the Remote-VTag when a packet containing an INIT ACK chunk or an ASCONF chunk is processed by the NAT function.
NAT Function Considerations If the NAT function detects a collision of internal port numbers and verification tags, it SHOULD send a packet containing an ABORT chunk with the M bit set if the collision is triggered by a packet containing an INIT or INIT ACK chunk. If such a collision is triggered by a packet containing an ASCONF chunk, it SHOULD send a packet containing an ERROR chunk with the M bit. The M bit is a new bit defined by this document to express to SCTP that the source of this packet is a "middle" box, not the peer SCTP endpoint (see ). If a packet containing an INIT ACK chunk triggers the collision, the corresponding packet containing the ABORT chunk MUST contain the same source and destination address and port numbers as the packet containing the INIT ACK chunk. If a packet containing an INIT chunk or an ASCONF chunk, the source and destination address and port numbers MUST be swapped. The sender of the packet containing an ERROR or ABORT chunk MUST include the error cause with cause code 'VTag and Port Number Collision' (see ).
Endpoint Considerations The sender of the packet containing the INIT chunk or the receiver of a packet containing the INIT ACK chunk, upon reception of a packet containign an ABORT chunk with M bit set and the appropriate error cause code for colliding NAT binding table state is included, SHOULD reinitiate the association setup procedure after choosing a new initiate tag, if the association is in COOKIE-WAIT state. In any other state, the SCTP endpoint MUST NOT respond. The sender of packet containing the ASCONF chunk, upon reception of a packet containing an ERROR chunk with M bit set, MUST stop adding the path to the association.
Handling of Internal Port Number Collisions When two SCTP hosts are behind an SCTP-aware NAT it is possible that two SCTP hosts in the Internal-Address space will want to set up an SCTP association with the same server running on the same host in the Internet. If the two hosts choose the same internal port, this is considered an internal port number collision. For the NAT function, appropriate tracking may be performed by assuring that the VTags are unique between the two hosts.
NAT Function Considerations The NAT function, when processing the packet containing the INIT ACK chunk, should note in its NAT binding table that the association supports the disable restart extension. This note is used when establishing future associations (i.e. when processing a packet containing an INIT chunk from an internal host) to decide if the connection should be allowed. The NAT function does the following when processing a packet containing an INIT chunk:
  • If the packet containing the INIT chunk is originating from an internal port to an remote port for which the NAT function has no matching NAT binding table entry, it MUST allow the packet containing the INIT chunk creating an NAT binding table entry.
  • If the packet containing the INIT chunk matches an existing NAT binding table entry, it MUST validate that the disable restart feature is supported and, if it does, allow the packet containing the INIT chunk to be forwarded.
  • If the disable restart feature is not supported, the NAT function SHOULD send a packet containing an ABORT chunk with the M bit set.
The 'Port Number Collision' error cause (see ) MUST be included in the ABORT chunk sent in response to the packet containing an INIT chunk. If the collision is triggered by a packet containing an ASCONF chunk, a packet containing an ERROR chunk with the 'Port Number Collision' error cause MUST be sent in response to the packet containing the ASCONF chunk.
Endpoint Considerations For the remote SCTP server on the Internet this means that the Remote-Port and the Remote-Address are the same. If they both have chosen the same Internal-Port the server cannot distinguish between both associations based on the address and port numbers. For the server it looks like the association is being restarted. To overcome this limitation the client sends a Disable Restart parameter in the INIT chunk. When the server receives this parameter it does the following:
  • It MUST include a Disable Restart parameter in the INIT ACK to inform the client that it will support the feature.
  • It MUST disable the restart procedures defined in for this association.
Servers that support this feature will need to be capable of maintaining multiple connections to what appears to be the same peer (behind the NAT function) differentiated only by the VTags.
Handling of Missing State
NAT Function Considerations If the NAT function receives a packet from the internal network for which the lookup procedure does not find an entry in the NAT binding table, a packet containing an ERROR chunk SHOULD be sent back with the M bit set. The source address of the packet containing the ERROR chunk MUST be the destination address of the incoming SCTP packet. The verification tag is reflected and the T bit is set. Such a packet containing an ERROR chunk SHOULD NOT be sent if the received packet contains an ABORT, SHUTDOWN COMPLETE or INIT ACK chunk. A packet containing an ERROR chunk MUST NOT be sent if the received packet contains an ERROR chunk with the M bit set. In any case, the packet SHOULD NOT be forwarded to the remote address. When sending a packet containing an ERROR chunk, the error cause 'Missing State' (see ) MUST be included and the M bit of the ERROR chunk MUST be set (see ). If the NAT device receives a packet for which it has no NAT binding table entry and the packet contains an ASCONF chunk with the VTags parameter, the NAT function MUST update its NAT binding table according to the verification tags in the VTags parameter and the optional Disable Restart parameter.
Endpoint Considerations Upon reception of this packet containing the ERROR chunk by an SCTP endpoint the receiver takes the following actions:
  • It SHOULD validate that the verification tag is reflected by looking at the VTag that would have been included in the outgoing packet. If the validation fails, discard the incoming packet containing the ERROR chunk.
  • It SHOULD validate that the peer of the SCTP association supports the dynamic address extension. If the validation fails, discard the incoming packet containing the ERROR chunk.
  • It SHOULD generate a packet containing a new ASCONF chunk containing the VTags parameter (see ) and the Disable Restart parameter (see ) if the association is using the disable restart feature. By processing this packet the NAT function can recover the appropriate state. The procedures for generating an ASCONF chunk can be found in .
The peer SCTP endpoint receiving such a packet containing an ASCONF chunk SHOULD either add the address and respond with an acknowledgment, if the address is new to the association (following all procedures defined in ). Or, if the address is already part of the association, the SCTP endpoint MUST NOT respond with an error, but instead SHOULD respond with packet containing an ASCONF ACK chunk acknowledging the address and take no action (since the address is already in the association). Note that it is possible that upon receiving a packet containing an ASCONF chunk containing the VTags parameter the NAT function will realize that it has an 'Internal Port Number and Verification Tag collision'. In such a case the NAT function SHOULD send a packet containing an ERROR chunk with the error cause code set to 'VTag and Port Number Collision' (see ). If an SCTP endpoint receives a packet containing an ERROR chunk with 'Internal Port Number and Verification Tag collision' as the error cause and the packet in the Error Chunk contains an ASCONF with the VTags parameter, careful examination of the association is required. The endpoint does the following:
  • It MUST validate that the verification tag is reflected by looking at the VTag that would have been included in the outgoing packet. If the validation fails, it MUST discard the packet.
  • It MUST validate that the peer of the SCTP association supports the dynamic address extension. If the peer does not support it, the NAT function MUST discard the incoming packet containing the ERROR chunk.
  • If the association is attempting to add an address (i.e. following the procedures in ) then the endpoint MUST NOT consider the address part of the association and SHOULD make no further attempt to add the address (i.e. cancel any ASCONF timers and remove any record of the path), since the NAT function has a VTag collision and the association cannot easily create a new VTag (as it would if the error occurred when sending a packet containing an INIT chunk).
  • If the endpoint has no other path, i.e. the procedure was executed due to missing a state in the NAT function, then the endpoint MUST abort the association. This would occur only if the local NAT function restarted and accepted a new association before attempting to repair the missing state (Note that this is no different than what happens to all TCP connections when a NAT function looses its state).
Handling of Fragmented SCTP Packets by NAT Functions SCTP minimizes the use of IP-level fragmentation. However, it can happen that using IP-level fragmentation is needed to continue an SCTP association. For example, if the path MTU is reduced and there are still some DATA chunk in flight, which require packets larger than the new path MTU. If IP-level fragmentation can not be used, the SCTP association will be terminated in a non-graceful way. Therefore, a NAT function MUST be able to handle IP-level fragmented SCTP packets. The fragments may arrive in any order. When an SCTP packet has to be fragmented by the NAT function and the IP header forbids fragmentation, the NAT MUST send back a corresponding ICMP message to the internal host. This allows for a faster recovery from this packet drop.
Multi Point Traversal Considerations for Endpoints If a multi-homed SCTP endpoint behind a NAT function connects to a peer, it MUST first set up the association single-homed with only one address causing the first NAT function to populate its state. Then it SHOULD add each IP address using packets containing ASCONF chunks sent via their respective NAT functions. The address to add is the wildcard address and the lookup address SHOULD also contain the VTags parameter and optionally the Disable Restart parameter.
Various Examples of NAT Traversals Please note that this section is informational only. The addresses being used in the following examples are IPv4 addresses for private-use networks and for documentation as specified in . However, the method described here is not limited to this NAT44 case. The NAT binding table entries shown in the following examples do not include the flag indicating whether the restart procedure is supported or not. This flag is not relevant for these examples.
Single-homed Client to Single-homed Server The internal client starts the association with the remote server via a four-way-handshake. Host A starts by sending a packet containing an INIT chunk. | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ +---------+--------+----------+--------+-----------+ NAT | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ INIT[Initiate-Tag = 1234] 10.0.0.1:1 ------> 203.0.113.1:2 Rem-VTtag = 0 ]]> A NAT binding tabled entry is created, the source address is substituted and the packet is sent on: 203.0.113.1:2 Rem-VTtag = 0 ]]> Host B receives the packet containing an INIT chunk and sends a packet containing an INIT ACK chunk with the NAT's Remote-address as destination address. | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ INIT ACK[Initiate-Tag = 5678] 192.0.2.1:1 <----------------------- 203.0.113.1:2 Int-VTag = 1234 NAT function updates entry: +---------+--------+----------+--------+-----------+ NAT | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ | 1234 | 1 | 5678 | 2 | 10.0.0.1 | +---------+--------+----------+--------+-----------+ INIT ACK[Initiate-Tag = 5678] 10.0.0.1:1 <------ 203.0.113.1:2 Int-VTag = 1234 ]]> The handshake finishes with a COOKIE ECHO acknowledged by a COOKIE ACK. | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ COOKIE ECHO 10.0.0.1:1 ------> 203.0.113.1:2 Rem-VTag = 5678 COOKIE ECHO 192.0.2.1:1 -----------------------> 203.0.113.1:2 Rem-VTag = 5678 COOKIE ACK 192.0.2.1:1 <----------------------- 203.0.113.1:2 Int-VTag = 1234 COOKIE ACK 10.0.0.1:1 <------ 203.0.113.1:2 Int-VTag = 1234 ]]>
Single-homed Client to Multi-homed Server The internal client is single-homed whereas the remote server is multi-homed. The client (Host A) sends a packet containing an INIT chunk like in the single-homed case. | NAT | <-> | Internet | == =| Host | | A | +-----+ \ / \ +--------+ / | B | +------+ \--/\--/ \-|Router 2|-/ +------+ +--------+ +---------+--------+----------+--------+-----------+ NAT | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ INIT[Initiate-Tag = 1234] 10.0.0.1:1 ---> 203.0.113.1:2 Rem-VTag = 0 ]]> NAT function creates entry: 203.0.113.1:2 Rem-VTag = 0 ]]> The server (Host B) includes its two addresses in the INIT ACK chunk. | NAT | <-> | Internet | == =| Host | | A | +-----+ \ / \ +--------+ / | B | +------+ \--/\--/ \-|Router 2|-/ +------+ +--------+ INIT ACK[Initiate-tag = 5678, IP-Addr = 203.0.113.129] 192.0.2.1:1 <-------------------------- 203.0.113.1:2 Int-VTag = 1234 ]]> The NAT function does not need to change the NAT binding table for the second address: The handshake finishes with a COOKIE ECHO acknowledged by a COOKIE ACK. | NAT | <-> | Internet | == =| Host | | A | +-----+ \ / \ +--------+ / | B | +------+ \--/\--/ \-|Router 2|-/ +------+ +--------+ COOKIE ECHO 10.0.0.1:1 ---> 203.0.113.1:2 Rem-VTag = 5678 COOKIE ECHO 192.0.2.1:1 --------------------------> 203.0.113.1:2 Rem-VTag = 5678 COOKIE ACK 192.0.2.1:1 <-------------------------- 203.0.113.1:2 Int-VTag = 1234 COOKIE ACK 10.0.0.1:1 <--- 203.0.113.1:2 Int-VTag = 1234 ]]>
Multihomed Client and Server The client (Host A) sends a packet containing an INIT chunk to the server (Host B), but does not include the second address. 203.0.113.1:2 Rem-VTag = 0 ]]> NAT function 1 creates entry: 203.0.113.1:2 Rem-VTag = 0 ]]> Host B includes its second address in the INIT ACK. NAT function 1 does not need to update the NAT binding table for the second address: The handshake finishes with a COOKIE ECHO acknowledged by a COOKIE ACK. 203.0.113.1:2 Rem-VTag = 5678 COOKIE ECHO 192.0.2.1:1 ------------------> 203.0.113.1:2 Rem-VTag = 5678 COOKIE ACK 192.0.2.1:1 <------------------ 203.0.113.1:2 Int-VTag = 1234 COOKIE ACK 10.0.0.1:1 <------- 203.0.113.1:2 Int-VTag = 1234 ]]> Host A announces its second address in an ASCONF chunk. The address parameter contains an undefined address (0) to indicate that the source address should be added. The lookup address parameter within the ASCONF chunk will also contain the pair of VTags (remote and internal) so that the NAT function may populate its NAT binding table entry completely with this single packet. 203.0.113.129:2 Rem-VTag = 5678 ]]> NAT function 2 creates a complete entry: 203.0.113.129:2 Rem-VTag = 5678 ASCONF ACK 192.0.2.129:1 <------------------- 203.0.113.129:2 Int-VTag = 1234 ASCONF ACK 10.1.0.1:1 <----- 203.0.113.129:2 Int-VTag = 1234 ]]>
NAT Function Loses Its State Association is already established between Host A and Host B, when the NAT function loses its state and obtains a new external address. Host A sends a DATA chunk to Host B. | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ +---------+--------+----------+--------+-----------+ NAT | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ DATA 10.0.0.1:1 ----------> 203.0.113.1:2 Rem-VTag = 5678 ]]> The NAT function cannot find an entry in the NAT binding table for the association. It sends a packet containing an ERROR chunk with the M-Bit set and the cause "NAT state missing". | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ ERROR [M-Bit, NAT state missing] 10.0.0.1:1 <---------- 203.0.113.1:2 Rem-VTag = 5678 ]]> On reception of the packet containing the ERROR chunk, Host A sends a packet containing an ASCONF chunk indicating that the former information has to be deleted and the source address of the actual packet added. | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ ASCONF [ADD-IP, DELETE-IP, Int-VTag=1234, Rem-VTag = 5678] 10.0.0.1:1 ----------> 203.0.113.129:2 Rem-VTag = 5678 +---------+--------+----------+--------+-----------+ NAT | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ | 1234 | 1 | 5678 | 2 | 10.0.0.1 | +---------+--------+----------+--------+-----------+ ASCONF [ADD-IP, DELETE-IP, Int-VTag=1234, Rem-VTag = 5678] 192.0.2.2:1 -----------------> 203.0.113.129:2 Rem-VTag = 5678 ]]> Host B adds the new source address to this association and deletes all other addresses from this association. | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ ASCONF ACK 192.0.2.2:1 <----------------- 203.0.113.129:2 Int-VTag = 1234 ASCONF ACK 10.1.0.1:1 <---------- 203.0.113.129:2 Int-VTag = 1234 DATA 10.0.0.1:1 ----------> 203.0.113.1:2 Rem-VTag = 5678 DATA 192.0.2.2:1 -----------------> 203.0.113.129:2 Rem-VTag = 5678 ]]>
Peer-to-Peer Communication If two hosts, each of them behind a NAT function, want to communicate with each other, they have to get knowledge of the peer's external address. This can be achieved with a so-called rendezvous server. Afterwards the destination addresses are external, and the association is set up with the help of the INIT collision. The NAT functions create their entries according to their internal peer's point of view. Therefore, NAT function A's Internal-VTag and Internal-Port are NAT function B's Remote-VTag and Remote-Port, respectively. The naming (internal/remote) of the verification tag in the packet flow is done from the sending host's point of view. | NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | NAT Binding Tables +---------+--------+----------+--------+-----------+ NAT A | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ +---------+--------+----------+--------+-----------+ NAT B | Int | Int | Rem | Rem | Int | | v-tag | port | v-tag | port | Addr | +---------+--------+----------+--------+-----------+ INIT[Initiate-Tag = 1234] 10.0.0.1:1 --> 203.0.113.1:2 Rem-VTag = 0 ]]> NAT function A creates entry: 203.0.113.1:2 Rem-VTag = 0 ]]> NAT function B processes the packet containing the INIT chunk, but cannot find an entry. The SCTP packet is silently discarded and leaves the NAT binding table of NAT function B unchanged. +---------+--------+----------+--------+-----------+ NAT B | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ Now Host B sends a packet containing an INIT chunk, which is processed by NAT function B. Its parameters are used to create an entry. | NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | INIT[Initiate-Tag = 5678] 192.0.2.1:1 <-- 10.1.0.1:2 Rem-VTag = 0 +---------+--------+----------+--------+-----------+ NAT B | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ | 5678 | 2 | 0 | 1 | 10.1.0.1 | +---------+--------+----------+--------+-----------+ INIT[Initiate-Tag = 5678] 192.0.2.1:1 <--------------- 203.0.113.1:2 Rem-VTag = 0 ]]> NAT function A processes the packet containing the INIT chunk. As the outgoing packet containing an INIT chunk of Host A has already created an entry, the entry is found and updated: | NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | VTag != Int-VTag, but Rem-VTag == 0, find entry. +---------+--------+----------+--------+-----------+ NAT A | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ | 1234 | 1 | 5678 | 2 | 10.0.0.1 | +---------+--------+----------+--------+-----------+ INIT[Initiate-tag = 5678] 10.0.0.1:1 <-- 203.0.113.1:2 Rem-VTag = 0 ]]> Host A sends a packet containing an INIT ACK chunk, which can pass through NAT function B: | NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | INIT ACK[Initiate-Tag = 1234] 10.0.0.1:1 --> 203.0.113.1:2 Rem-VTag = 5678 INIT ACK[Initiate-Tag = 1234] 192.0.2.1:1 ----------------> 203.0.113.1:2 Rem-VTag = 5678 NAT function B updates entry: +---------+--------+----------+--------+-----------+ NAT B | Int | Int | Rem | Rem | Int | | VTag | Port | VTag | Port | Addr | +---------+--------+----------+--------+-----------+ | 5678 | 2 | 1234 | 1 | 10.1.0.1 | +---------+--------+----------+--------+-----------+ INIT ACK[Initiate-Tag = 1234] 192.0.2.1:1 --> 10.1.0.1:2 Rem-VTag = 5678 ]]> The lookup for COOKIE ECHO and COOKIE ACK is successful. | NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | COOKIE ECHO 192.0.2.1:1 <-- 10.1.0.1:2 Rem-VTag = 1234 COOKIE ECHO 192.0.2.1:1 <------------- 203.0.113.1:2 Rem-VTag = 1234 COOKIE ECHO 10.0.0.1:1 <-- 203.0.113.1:2 Rem-VTag = 1234 COOKIE ACK 10.0.0.1:1 --> 203.0.113.1:2 Rem-VTag = 5678 COOKIE ACK 192.0.2.1:1 ----------------> 203.0.113.1:2 Rem-VTag = 5678 COOKIE ACK 192.0.2.1:1 --> 10.1.0.1:2 Rem-VTag = 5678 ]]>
SCTP NAT YANG Module This section defines a YANG module for SCTP NAT. The terminology for describing YANG data models is defined in . The meaning of the symbols in tree diagrams is defined in .
Tree Structure This module augments NAT YANG module with SCTP specifics. The module supports both classical SCTP NAT (that is, rewrite port numbers) and SCTP-specific variant where the ports numbers are not altered. The YANG "feature" is used to indicate whether SCTP-specific variant is supported. The tree structure of the SCTP NAT YANG module is provided below: Concretely, the SCTP NAT YANG module augments the NAT YANG module (policy, in particular) with the following:
  • The sctp-timeout is used to control the SCTP inactivity timeout. That is, the time an SCTP mapping will stay active without SCTP packets traversing the NAT. This timeout can be set only for SCTP. Hence, "/nat:nat/nat:instances/nat:instance/nat:policy/nat:transport-protocols/nat:protocol-id" MUST be set to '132' (SCTP).
In addition, the SCTP NAT YANG module augments the mapping entry with the following parameters defined in . These parameters apply only for SCTP NAT mapping entries (i.e., "/nat/instances/instance/mapping-table/mapping-entry/transport-protocol" MUST be set to '132');
  • The Internal Verification Tag (Int-VTag)
  • The Remote Verification Tag (Rem-VTag)
YANG Module file "ietf-nat-sctp@2020-07-13.yang" module ietf-nat-sctp { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-nat-sctp"; prefix nat-sctp; import ietf-nat { prefix nat; reference "RFC 8512: A YANG Module for Network Address Translation (NAT) and Network Prefix Translation (NPT)"; } organization "IETF TSVWG Working Group"; contact "WG Web: WG List: Author: Mohamed Boucadair "; description "This module augments NAT YANG module with Stream Control Transmission Protocol (SCTP) specifics. The extension supports both a classical SCTP NAT (that is, rewrite port numbers) and a, SCTP-specific variant where the ports numbers are not altered. Copyright (c) 2020 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info). This version of this YANG module is part of RFC XXXX; see the RFC itself for full legal notices."; revision 2019-11-18 { description "Initial revision."; reference "RFC XXXX: Stream Control Transmission Protocol (SCTP) Network Address Translation Support"; } feature sctp-nat { description "This feature means that SCTP-specific variant of NAT is supported. That is, avoid rewriting port numbers."; reference "Section 4.3 of RFC XXXX."; } augment "/nat:nat/nat:instances/nat:instance" + "/nat:policy/nat:timers" { when "/nat:nat/nat:instances/nat:instance" + "/nat:policy/nat:transport-protocols" + "/nat:protocol-id = 132"; description "Extends NAT policy with a timeout for SCTP mapping entries."; leaf sctp-timeout { type uint32; units "seconds"; description "SCTP inactivity timeout. That is, the time an SCTP mapping entry will stay active without packets traversing the NAT."; } } augment "/nat:nat/nat:instances/nat:instance" + "/nat:mapping-table/nat:mapping-entry" { when "nat:transport-protocol = 132"; if-feature "sctp-nat"; description "Extends the mapping entry with SCTP specifics."; leaf int-VTag { type uint32; description "The Internal Verification Tag that the internal host has chosen for this communication."; } leaf rem-VTag { type uint32; description "The Remote Verification Tag that the remote peer has chosen for this communication."; } } } ]]>
Socket API Considerations This section describes how the socket API defined in is extended to provide a way for the application to control NAT friendliness. Please note that this section is informational only. A socket API implementation based on is extended by supporting one new read/write socket option.
Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY) This socket option uses the option_level IPPROTO_SCTP and the option_name SCTP_NAT_FRIENDLY. It can be used to enable/disable the NAT friendliness for future associations and retrieve the value for future and specific ones. struct sctp_assoc_value { sctp_assoc_t assoc_id; uint32_t assoc_value; };
assoc_id
This parameter is ignored for one-to-one style sockets. For one-to-many style sockets the application may fill in an association identifier or SCTP_FUTURE_ASSOC for this query. It is an error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.
assoc_value
A non-zero value indicates a NAT-friendly mode.
IANA Considerations [NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number you assign this document.] [NOTE to RFC-Editor: The requested values for the chunk type and the chunk parameter types are tentative and to be confirmed by IANA.] This document (RFCXXXX) is the reference for all registrations described in this section. The requested changes are described below.
New Chunk Flags for Two Existing Chunk Types As defined in two chunk flags have to be assigned by IANA for the ERROR chunk. The requested value for the T bit is 0x01 and for the M bit is 0x02. This requires an update of the "ERROR Chunk Flags" registry for SCTP: ERROR Chunk Flags
Chunk Flag Value Chunk Flag Name Reference
0x01 T bit [RFCXXXX]
0x02 M bit [RFCXXXX]
0x04 Unassigned
0x08 Unassigned
0x10 Unassigned
0x20 Unassigned
0x40 Unassigned
0x80 Unassigned
As defined in one chunk flag has to be assigned by IANA for the ABORT chunk. The requested value of the M bit is 0x02. This requires an update of the "ABORT Chunk Flags" registry for SCTP: ABORT Chunk Flags
Chunk Flag Value Chunk Flag Name Reference
0x01 T bit [RFC4960]
0x02 M bit [RFCXXXX]
0x04 Unassigned
0x08 Unassigned
0x10 Unassigned
0x20 Unassigned
0x40 Unassigned
0x80 Unassigned
Three New Error Causes Three error causes have to be assigned by IANA. It is requested to use the values given below. This requires three additional lines in the "Error Cause Codes" registry for SCTP: Error Cause Codes
Value Cause Code Reference
176 VTag and Port Number Collision [RFCXXXX]
177 Missing State [RFCXXXX]
178 Port Number Collision [RFCXXXX]
Two New Chunk Parameter Types Two chunk parameter types have to be assigned by IANA. It is requested to use the values given below. IANA should assign these values from the pool of parameters with the upper two bits set to '11'. This requires two additional lines in the "Chunk Parameter Types" registry for SCTP: Chunk Parameter Types
ID Value Chunk Parameter Type Reference
49159 Disable Restart (0xC007) [RFCXXXX]
49160 VTags (0xC008) [RFCXXXX]
One New URI An URI in the "ns" subregistry within the "IETF XML" registry has to be assigned by IANA ():
One New YANG Module An YANG module in the "YANG Module Names" subregistry within the "YANG Parameters" registry has to be assigned by IANA ():
Security Considerations State maintenance within a NAT function is always a subject of possible Denial Of Service attacks. This document recommends that at a minimum a NAT function runs a timer on any SCTP state so that old association state can be cleaned up. Generic issues related to address sharing are discussed in and apply to SCTP as well. For SCTP endpoints not disabling the restart procedure, this document does not add any additional security considerations to the ones given in , , and . SCTP endpoints disabling the restart procedure, should monitor the status of all associations to mitigate resource exhaustion attacks by establishing a lot of associations sharing the same IP addresses and port numbers. In any case, SCTP is protected by the verification tags and the usage of against off-path attackers. For IP-level fragmentation and reassembly related issues see . The YANG module specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF or RESTCONF . The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) . The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS . The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. All data nodes defined in the YANG module that can be created, modified, and deleted (i.e., config true, which is the default) are considered sensitive. Write operations (e.g., edit-config) applied to these data nodes without proper protection can negatively affect network operations. An attacker who is able to access the SCTP NAT function can undertake various attacks, such as:
  • Setting a low timeout for SCTP mapping entries to cause failures to deliver incoming SCTP packets.
  • Instantiating mapping entries to cause NAT collision.
 Normative References Informative References
Acknowledgments The authors wish to thank Mohamed Boucadair, Gorry Fairhurst, Bryan Ford, David Hayes, Alfred Hines, Karen E. E. Nielsen, Henning Peters, Maksim Proshin, Timo Voelker, Dan Wing, and Qiaobing Xie for their invaluable comments. In addition, the authors wish to thank David Hayes, Jason But, and Grenville Armitage, the authors of , for their suggestions. The authors also wish to thank Mohamed Boucadair for contributing the text related to the YANG module.