Network Working Group R. Stewart Internet-Draft Netflix, Inc. Intended status: Standards Track M. Tuexen Expires: November 19, 2016 I. Ruengeler Muenster Univ. of Appl. Sciences May 18, 2016 Stream Control Transmission Protocol (SCTP) Network Address Translation Support draft-ietf-tsvwg-natsupp-09.txt Abstract The Stream Control Transmission Protocol (SCTP) provides a reliable communications channel between two end-hosts in many ways similar to the Transmission Control Protocol (TCP). With the widespread deployment of Network Address Translators (NAT), specialized code has been added to NAT 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 (NAPT). This document describes the protocol extensions required for the SCTP endpoints and the mechanisms for NATs necessary to provide similar features of NAPT in the single-point and multi-point traversal scenario. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 19, 2016. Stewart, et al. Expires November 19, 2016 [Page 1] Internet-Draft SCTP NAT Support May 2016 Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . 6 4.1.1. Single Point Traversal . . . . . . . . . . . . . . . 6 4.1.2. Multi Point Traversal . . . . . . . . . . . . . . . . 7 4.2. Limitations of Classical NAPT for SCTP . . . . . . . . . 8 4.3. The SCTP Specific Variant of NAT . . . . . . . . . . . . 8 5. Data Formats . . . . . . . . . . . . . . . . . . . . . . . . 12 5.1. Modified Chunks . . . . . . . . . . . . . . . . . . . . . 12 5.1.1. Extended ABORT Chunk . . . . . . . . . . . . . . . . 12 5.1.2. Extended ERROR Chunk . . . . . . . . . . . . . . . . 13 5.2. New Error Causes . . . . . . . . . . . . . . . . . . . . 13 5.2.1. VTag and Port Number Collision Error Cause . . . . . 13 5.2.2. Missing State Error Cause . . . . . . . . . . . . . . 14 5.2.3. Port Number Collision Error Cause . . . . . . . . . . 15 5.3. New Parameters . . . . . . . . . . . . . . . . . . . . . 15 5.3.1. Disable Restart Parameter . . . . . . . . . . . . . . 16 5.3.2. VTags Parameter . . . . . . . . . . . . . . . . . . . 16 6. Procedures for SCTP End Points and NATs . . . . . . . . . . . 17 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 17 6.2. Association Setup Considerations . . . . . . . . . . . . 18 6.3. Handling of Internal Port Number and Verification Tag Collisions . . . . . . . . . . . . . . . . . . . . . . . 18 6.4. Handling of Internal Port Number Collisions . . . . . . . 19 6.5. Handling of Missing State . . . . . . . . . . . . . . . . 20 6.6. Handling of Fragmented SCTP Packets . . . . . . . . . . . 22 6.7. Multi-Point Traversal Considerations . . . . . . . . . . 22 7. Various Examples of NAT Traversals . . . . . . . . . . . . . 22 7.1. Single-homed Client to Single-homed Server . . . . . . . 22 Stewart, et al. Expires November 19, 2016 [Page 2] Internet-Draft SCTP NAT Support May 2016 7.2. Single-homed Client to Multi-homed Server . . . . . . . . 25 7.3. Multihomed Client and Server . . . . . . . . . . . . . . 28 7.4. NAT Loses Its State . . . . . . . . . . . . . . . . . . . 32 7.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . 34 8. Socket API Considerations . . . . . . . . . . . . . . . . . . 39 8.1. Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY) . . . . . . . . . . . . . . . . . . . 40 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40 9.1. New Chunk Flags for Two Existing Chunk Types . . . . . . 40 9.2. Three New Error Causes . . . . . . . . . . . . . . . . . 41 9.3. Two New Chunk Parameter Types . . . . . . . . . . . . . . 42 10. Security Considerations . . . . . . . . . . . . . . . . . . . 42 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 42 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 43 12.1. Normative References . . . . . . . . . . . . . . . . . . 43 12.2. Informative References . . . . . . . . . . . . . . . . . 43 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44 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 using private addresses (see [RFC6890]) 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 (NAPT). Please note that this document focuses on the case where the NAT maps multiple private addresses to a single public address. 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. The only alternative for supporting legacy NATs is to use UDP encapsulation as specified in [RFC6951]. This document describes an SCTP specific variant NAT and specific packets and procedures to help NATs provide similar features of NAPT in the single-point and multi-point traversal scenario. An SCTP implementation supporting this extension will follow these procedures to assure that in both single-homed and multi-homed cases a NAT will maintain the proper state without needing to change port numbers. It is possible and desirable to make these changes for a number of reasons: Stewart, et al. Expires November 19, 2016 [Page 3] Internet-Draft SCTP NAT Support May 2016 o It is desirable for SCTP internal end-hosts on multiple platforms to be able to share a NAT's public IP address in the same way that a TCP session can use a NAT. 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. An SCTP-aware NAT 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, External Host and NAT 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. +---------------+------------+---------------+---------------+ | Internal Host | NAT | External Host | Communication | +---------------+------------+---------------+---------------+ | Support | Support | Support | Yes | | Support | Support | No Support | Limited | | Support | No Support | Support | None | | Support | No Support | No Support | None | | No Support | Support | Support | Limited | | No Support | Support | No Support | Limited | | No Support | No Support | Support | None | | No Support | No Support | No Support | None | +---------------+------------+---------------+---------------+ Table 1: Communication possibilities From the table we can see that when a NAT does not support the extension no communication can occur. This is because for the most part of the current situation i.e. SCTP packets sent externally from behind a NAT are discarded by the NAT. In some cases, where the NAT supports the feature but one of the two external 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. Stewart, et al. Expires November 19, 2016 [Page 4] Internet-Draft SCTP NAT Support May 2016 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 This document uses the following terms, which are depicted in Figure 1. Familiarity with the terminology used in [RFC4960] and [RFC5061] is assumed. Private-Address (Priv-Addr): The private address that is known to the internal host. Internal-Port (Int-Port): The port number that is in use by the host holding the Private-Address. Internal-VTag (Int-VTag): The SCTP Verification Tag (VTag) that the internal host has chosen for its communication. The VTag is a unique 32-bit tag that must accompany any incoming SCTP packet for this association to the Private-Address. External-Address (Ext-Addr): The address that an internal host is attempting to contact. External-Port (Ext-Port): The port number of the peer process at the External-Address. External-VTag (Ext-VTag): The Verification Tag that the host holding the External-Address has chosen for its communication. The VTag is a unique 32-bit tag that must accompany any incoming SCTP packet for this association to the External-Address. Public-Address (Pub-Addr): The public address assigned to the NAT box which it uses as a source address when sending packets towards the External-Address. Stewart, et al. Expires November 19, 2016 [Page 5] Internet-Draft SCTP NAT Support May 2016 Internal Network | External Network | Private | Public External +---------+ Address | Address /--\/--\ Address +---------+ | SCTP | +-----+ / \ | SCTP | |end point|=========| NAT |=======| Internet |==========|end point| | A | +-----+ \ / | B | +---------+ Internal | \--/\--/ External+---------+ Internal Port | Port External VTag | VTag Figure 1: Basic network setup 4. Motivation 4.1. SCTP NAT Traversal Scenarios This section defines the notion of single and multi-point NAT traversal. 4.1.1. Single Point Traversal 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 | +---------+ | \--/\--/ +---------+ | Single NAT scenario A variation of this case is shown below, i.e., multiple NATs in a single path: Stewart, et al. Expires November 19, 2016 [Page 6] Internet-Draft SCTP NAT Support May 2016 Internal | External : Internal | External | : | +---------+ | : | /--\/--\ +---------+ | SCTP | +-----+ : +-----+ / \ | SCTP | |end point|==| NAT |=======:=======| NAT |==| Internet |==|end point| | A | +-----+ : +-----+ \ / | B | +---------+ | : | \--/\--/ +---------+ | : | Serial NATs scenario In this single point traversal scenario, we must acknowledge that while one of the main benefits of SCTP multi-homing is redundant paths, the NAT function represents a single point of failure in the path of the SCTP multi-home association. However, the rest of the path may still benefit from path diversity provided by SCTP multi- homing. The two SCTP endpoints in this case can be either single-homed or multi-homed. However, the important thing is that the NAT (or NATs) in this case sees all the packets of the SCTP association. 4.1.2. 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 |=========/ \---\/---/ +------+ | Parallel NATs scenario This case does NOT apply to a single-homed SCTP association (i.e., BOTH endpoints in the association use only one IP address). The advantage here is that the existence of multiple NAT traversal points can preserve the path diversity of a multi-homed association for the entire path. This in turn can improve the robustness of the communication. Stewart, et al. Expires November 19, 2016 [Page 7] Internet-Draft SCTP NAT Support May 2016 4.2. Limitations of Classical NAPT for SCTP Using classical NAPT may result in changing one of the SCTP port numbers during the processing which requires the recomputation of the transport layer checksum. Whereas for UDP and TCP this can be done very efficiently, for SCTP the checksum (CRC32c) over the entire packet needs to be recomputed. This would 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 NATs involved must recognize the packets they see as belonging to the same SCTP association and perform port number translation in a consistent way. One possible way of doing this is to use pre-defined table of ports and addresses configured within each NAT. Other mechanisms could make use of NAT to NAT communication. Such mechanisms are not to be deployable on a wide scale base and thus not a recommended solution. Therefore the SCTP variant of NAT has been developed. 4.3. 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 whether the restart procedure is disabled or not. An entry in that table is called a NAT state control block. The function Create() obtains the just mentioned parameters and returns a NAT-State control block. The entries in this table fulfill some uniqueness conditions. There must not be more than one entry with the same pair of Internal-Port and External-Port. This rule can be relaxed, if all entries with the same Internal-Port and External-Port have the support for the restart Stewart, et al. Expires November 19, 2016 [Page 8] Internet-Draft SCTP NAT Support May 2016 procedure enabled. In this case there must be no more than one entry with the same Internal-Port, External-Port and Ext-VTag and no more than one entry with the same Internal-Port, External-Port and Int- VTag. The processing of outgoing SCTP packets containing an INIT-chunk is described in the following figure. The scenario shown is valid for all message flows in this section. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ INIT[Initiate-Tag] Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port Ext-VTag=0 Create(Initiate-Tag, Int-Port, Priv-Addr, 0) Returns(NAT-State control block) Translate To: INIT[Initiate-Tag] Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port Ext-VTag=0 Normally a NAT control block will be created. However, it is possible that there is already a NAT control block with the same External-Address, External-Port, Internal-Port, and Internal-VTag but different Private-Address. In this case the INIT MUST be dropped by the NAT and an ABORT MUST be sent back to the SCTP host with the M-Bit set and an appropriate error cause (see Section 5.1.1 for the format). The source address of the packet containing the ABORT chunk MUST be the destination address of the packet containing the INIT chunk. It is also possible that a connection to External-Address and External-Port exists without an Internal-VTag conflict but the External-Address does not support the DISABLE_RESTART feature (noted in the NAT control block when the prior connection was established). In such a case the INIT SHOULD be dropped by the NAT and an ABORT SHOULD be sent back to the SCTP host with the M-Bit set and an appropriate error cause (see Section 5.1.1 for the format). Stewart, et al. Expires November 19, 2016 [Page 9] Internet-Draft SCTP NAT Support May 2016 The processing of outgoing SCTP packets containing no INIT-chunk is described in the following figure. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port Ext-VTag Translate To: Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port Ext-VTag The processing of incoming SCTP packets containing INIT-ACK chunks is described in the following figure. The Lookup() function getting as input the Internal-VTag, Internal-Port, External-VTag (=0), External- Port, and External-Address, returns the corresponding entry of the NAT table and updates the External-VTag by substituting it with the value of the Initiate-Tag of the INIT-ACK chunk. The wildcard character signifies that the parameter's value is not considered in the Lookup() function or changed in the Update() function, respectively. Stewart, et al. Expires November 19, 2016 [Page 10] Internet-Draft SCTP NAT Support May 2016 /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ INIT-ACK[Initiate-Tag] Pub-Addr:Int-Port <---- Ext-Addr:Ext-Port Int-VTag Lookup(Int-VTag, Int-Port, *, 0, Ext-Port) Update(*, *, *, Initiate-Tag, *) Returns(NAT-State control block containing Private-Address) INIT-ACK[Initiate-Tag] Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port Int-VTag In the case Lookup fails, the SCTP packet is dropped. The Update routine inserts the External-VTag (the Initiate-Tag of the INIT-ACK chunk) in the NAT state control block. The processing of incoming SCTP packets containing an ABORT or SHUTDOWN-COMPLETE chunk with the T-Bit set is described in the following figure. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port Ext-VTag Lookup(0, Int-Port, *, Ext-VTag, Ext-Port) Returns(NAT-State control block containing Private-Address) Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port Ext-VTag The processing of other incoming SCTP packets is described in the following figure. Stewart, et al. Expires November 19, 2016 [Page 11] Internet-Draft SCTP NAT Support May 2016 /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port Int-VTag Lookup(Int-VTag, Int-Port, *, *, Ext-Port) Returns(NAT-State control block containing Local-Address) Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port Int-VTag For an incoming packet containing an INIT-chunk a table lookup is made only based on the addresses and port numbers. If an entry with an External-VTag of zero is found, it is considered a match and the External-VTag is updated. This allows the handling of INIT-collision through NAT. 5. Data Formats This section defines the formats used to support NAT traversal. Section 5.1 and Section 5.2 describe chunks and error causes sent by NATs and received by SCTP end points. Section 5.3 describes parameters sent by SCTP end points and used by NATs and SCTP end points. 5.1. Modified Chunks This section presents existing chunks defined in [RFC4960] that are modified by this document. 5.1.1. Extended ABORT Chunk 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 6 | Reserved |M|T| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / zero or more Error Causes / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Stewart, et al. Expires November 19, 2016 [Page 12] Internet-Draft SCTP NAT Support May 2016 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: ASSIGNMENT OF M-BIT TO BE CONFIRMED BY IANA. ] 5.1.2. Extended ERROR Chunk 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 9 | Reserved |M|T| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / zero or more Error Causes / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The ERROR chunk defined in [RFC4960] is extended to add the new 'M-bit'. The M-bit indicates to the receiver of the ERROR chunk that the chunk was not generated by the peer SCTP endpoint, but instead by a middle box. [NOTE: ASSIGNMENT OF M-BIT TO BE CONFIRMED BY IANA. ] 5.2. New Error Causes This section defines the new error causes added by this document. 5.2.1. VTag and Port Number Collision Error Cause 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 0x00B0 | Cause Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ Chunk / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Stewart, et al. Expires November 19, 2016 [Page 13] Internet-Draft SCTP NAT Support May 2016 Cause Code: 2 bytes (unsigned integer) This field holds the IANA defined cause code for the VTag and Port Number Collision Error Cause. The suggested value of this field for IANA is 0x00B0. 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: ASSIGNMENT OF CAUSE-CODE TO BE CONFIRMED BY IANA. ] 5.2.2. Missing State Error Cause 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 0x00B1 | Cause Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ Incoming Packet / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Cause Code: 2 bytes (unsigned integer) This field holds the IANA defined cause code for the Missing State Error Cause. The suggested value of this field for IANA is 0x00B1. 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. Stewart, et al. Expires November 19, 2016 [Page 14] Internet-Draft SCTP NAT Support May 2016 [NOTE: ASSIGNMENT OF CAUSE-CODE TO BE CONFIRMED BY IANA. ] 5.2.3. Port Number Collision Error Cause 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 0x00B2 | Cause Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ chunk / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Cause Code: 2 bytes (unsigned integer) This field holds the IANA defined cause code for the Port Number Collision Error Cause. The suggested value of this field for IANA is 0x00B2. 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: ASSIGNMENT OF CAUSE-CODE TO BE CONFIRMED BY IANA. ] 5.3. New Parameters This section defines new parameters and their valid appearance defined by this document. Stewart, et al. Expires November 19, 2016 [Page 15] Internet-Draft SCTP NAT Support May 2016 5.3.1. Disable Restart Parameter This parameter is used to indicate that the RESTART procedure is requested to be disabled. Both endpoints of an association MUST include this parameter in the INIT chunk and INIT-ACK chunk when establishing an association and MUST include it in the ASCONF chunk when adding an address to successfully disable the restart procedure. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0xC007 | Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Parameter Type: 2 bytes (unsigned integer) This field holds the IANA defined parameter type for the Disable Restart Parameter. The suggested value of this field for IANA is 0xC007. Parameter Length: 2 bytes (unsigned integer) This field holds the length in bytes of the parameter. The value MUST be 4. [NOTE: ASSIGNMENT OF PARAMETER TYPE TO BE CONFIRMED BY IANA. ] This parameter MAY appear in INIT, INIT-ACK and ASCONF chunks and MUST NOT appear in any other chunk. 5.3.2. VTags Parameter This parameter is used to help a NAT 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | External Verification Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Stewart, et al. Expires November 19, 2016 [Page 16] Internet-Draft SCTP NAT Support May 2016 Parameter Type: 2 bytes (unsigned integer) This field holds the IANA defined parameter type for the VTags Parameter. The suggested value of this field for IANA is 0xC008. 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 ASCONF can use this same value in the ASCONF-ACK 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 Private-Address. External Verification Tag: 4 bytes (unsigned integer) 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. [NOTE: ASSIGNMENT OF PARAMETER TYPE TO BE CONFIRMED BY IANA. ] This parameter MAY appear in ASCONF chunks and MUST NOT appear in any other chunk. 6. Procedures for SCTP End Points and NATs 6.1. Overview When an SCTP endpoint is behind an SCTP-aware NAT a number of problems may arise as it tries to communicate with its peer: o More than one host behind a NAT may pick the same VTag and source port when talking to the same peer server. This creates a Stewart, et al. Expires November 19, 2016 [Page 17] Internet-Draft SCTP NAT Support May 2016 situation where the NAT will not be able to tell the two associations apart. This situation is discussed in Section 6.3. o When an SCTP endpoint is a server communicating with multiple peers and the peers are behind the same NAT, then the two endpoints cannot be distinguished by the server. This case is discussed in Section 6.4. o A restart of a NAT during a conversation could cause a loss of its state. This problem and its solution is discussed in Section 6.5. o An SCTP endpoint may be behind two NATs providing redundancy. The method to set up this scenario is discussed in Section 6.7. Each of these mechanisms requires additional chunks and parameters, defined in this document, and possibly modified handling procedures from those specified in [RFC4960]. 6.2. Association Setup Considerations The association setup procedure defined in [RFC4960] allows multi- homed SCTP end points to exchange its IP-addresses by using IPv4 or IPv6 address parameters in the INIT and INIT-ACK chunks. However, this can't be used when NATs are present. Every association MUST initially be set up single-homed. There MUST NOT be any IPv4 Address parameter, IPv6 Address parameter, or Supported Address Types parameter in the INIT-chunk. The INIT-ACK chunk MUST NOT contain any IPv4 Address parameter or IPv6 Address parameter. If the association should finally be multi-homed, the procedure in Section 6.7 MUST be used. The INIT and INIT-ACK chunk SHOULD contain the Disable Restart parameter defined in Section 5.3.1. 6.3. 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 between 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 Stewart, et al. Expires November 19, 2016 [Page 18] Internet-Draft SCTP NAT Support May 2016 the NAT box can control the 16-bit Natted Port and therefore avoid collisions deterministically. The same can happen when an INIT-ACK chunk or an ASCONF chunk is processed by the NAT. However, in this unlikely event the NAT box MUST send an ABORT chunk with the M-bit set if the collision is triggered by an INIT or INIT- ACK chunk or send an ERROR chunk with the M-bit set if the collision is triggered by an ASCONF chunk. The M-bit is a new bit defined by this document to express to SCTP that the source of this packet is a "middle" box, not the peer SCTP endpoint (see Section 5.1.1). If a packet containing an INIT-ACK chunk triggers the collision, the corresponding packet containing the ABORT chunk MUST contain the same source and destination address and port numbers as the packet containing the INIT-ACK chunk. In the other two cases, the source and destination address and port numbers MUST be swapped. The sender of the packet containing the INIT chunk or the receiver of the INIT-ACK chunk, upon reception of an ABORT chunk with M-bit set and the appropriate error cause code for colliding NAT table state is included, MUST 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 the ASCONF chunk, upon reception of an ERROR chunk with M-bit set, MUST stop adding the path to the association. The sender of the ERROR or ABORT chunk MUST include the error cause with cause code 'VTag and Port Number Collision' (see Section 5.2.1). 6.4. 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 Private-Address space will want to set up an SCTP association with the same server running on the same host in the Internet. For the NAT, appropriate tracking may be performed by assuring that the VTags are unique between the two hosts. But for the external SCTP server on the Internet this means that the External-Port and the External-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 MUST do the following: Stewart, et al. Expires November 19, 2016 [Page 19] Internet-Draft SCTP NAT Support May 2016 o Include a Disable Restart parameter in the INIT-ACK to inform the client that it will support the feature. o Disable the restart procedures defined in [RFC4960] for this association. Servers that support this feature will need to be capable of maintaining multiple connections to what appears to be the same peer (behind the NAT) differentiated only by the VTags. The NAT, when processing the INIT-ACK, should note in its internal table that the association supports the Disable Restart extension. This note is used when establishing future associations (i.e. when processing an INIT from an internal host) to decide if the connection should be allowed. The NAT MUST do the following when processing an INIT: o If the INIT is destined to an external address and port for which the NAT has no outbound connection, allow the INIT creating an internal mapping table. o If the INIT matches the external address and port of an already existing connection, validate that the external server supports the Disable Restart feature, if it does allow the INIT to be forwarded. o If the external server does not support the Disable Restart extension the NAT MUST send an ABORT with the M-bit set. The 'Port Number Collision' error cause (see Section 5.2.3) MUST be included in the ABORT chunk. If the collision is triggered by an ASCONF chunk, a packet containing an ERROR chunk with the 'Port Number Collision' error cause MUST be sent back. 6.5. Handling of Missing State If the NAT box receives a packet from the internal network for which the lookup procedure does not find an entry in the NAT table, a packet containing an ERROR chunk 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 and the T-bit is set. Please note that such a packet containing an ERROR chunk SHOULD NOT be sent if the received packet contains an ABORT, SHUTDOWN-COMPLETE or INIT-ACK chunk. An ERROR chunk MUST NOT be sent if the received packet contains an ERROR chunk with the M-bit set. Stewart, et al. Expires November 19, 2016 [Page 20] Internet-Draft SCTP NAT Support May 2016 When sending the ERROR chunk, the new error cause Missing state (see Section 5.2.2) MUST be included and the new M-bit of the ERROR chunk MUST be set (see Section 5.1.2). Upon reception of this ERROR chunk by an SCTP endpoint the receiver SHOULD take the following actions: o Validate that the verification tag is reflected by looking at the VTag that would have been included in the outgoing packet. o Validate that the peer of the SCTP association supports the dynamic address extension, if it does not discard the incoming ERROR chunk. o Generate a new ASCONF chunk containing the VTags parameter (see Section 5.3.2) and the Disable Restart parameter if the association is using the disabled restart feature. By processing this packet the NAT can recover the appropriate state. The procedures for generating an ASCONF chunk can be found in [RFC5061]. If the NAT box receives a packet for which it has no NAT table entry and the packet contains an ASCONF chunk with the VTags parameter, the NAT box MUST update its NAT table according to the verification tags in the VTags parameter and the optional Disable Restart parameter. The peer SCTP endpoint receiving such an ASCONF chunk SHOULD either add the address and respond with an acknowledgment, if the address is new to the association (following all procedures defined in [RFC5061]). Or, if the address is already part of the association, the SCTP endpoint MUST NOT respond with an error, but instead should respond with an ASCONF-ACK chunk acknowledging the address but take no action (since the address is already in the association). Note that it is possible that upon receiving an ASCONF chunk containing the VTags parameter the NAT will realize that it has an 'Internal Port Number and Verification Tag collision'. In such a case the NAT MUST send an ERROR chunk with the error cause code set to 'VTag and Port Number Collision' (see Section 5.2.1). If an SCTP endpoint receives an ERROR 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 MUST do the following: o Validate that the verification tag is reflected by looking at the VTag that would have been included in the outgoing packet. Stewart, et al. Expires November 19, 2016 [Page 21] Internet-Draft SCTP NAT Support May 2016 o Validate that the peer of the SCTP association supports the dynamic address extension, if it does not discard the incoming ERROR chunk. o If the association is attempting to add an address (i.e. following the procedures in Section 6.7) 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 has a VTag collision and the association cannot easily create a new VTag (as it would if the error occurred when sending an INIT). o If the endpoint has no other path, i. e. the procedure was executed due to missing a state in the NAT, then the endpoint MUST abort the association. This would occur only if the local NAT 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 looses its state). 6.6. 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. 6.7. Multi-Point Traversal Considerations If a multi-homed SCTP endpoint behind a NAT connects to a peer, it SHOULD first set up the association single-homed with only one address causing the first NAT to populate its state. Then it SHOULD add each IP address using ASCONF chunks sent via their respective NATs. The address to add is the wildcard address and the lookup address SHOULD also contain the VTags parameter and optionally the Disable Restart parameter as illustrated above. 7. Various Examples of NAT Traversals Please note that this section is informational only. 7.1. Single-homed Client to Single-homed Server The internal client starts the association with the external server via a four-way-handshake. Host A starts by sending an INIT chunk. Stewart, et al. Expires November 19, 2016 [Page 22] Internet-Draft SCTP NAT Support May 2016 /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ +---------+--------+-----------+----------+--------+ NAT | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+--- -------+----------+--------+ INIT[Initiate-Tag = 1234] 10.0.0.1:1 ------> 100.0.0.1:2 Ext-VTtag = 0 A NAT entry is created, the source address is substituted and the packet is sent on: NAT creates entry: +---------+--------+-----------+----------+--------+ NAT | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 0 | 2 | +---------+--------+-----------+----------+--------+ INIT[Initiate-Tag = 1234] 101.0.0.1:1 --------------------------> 100.0.0.1:2 Ext-VTtag = 0 Host B receives the INIT and sends an INIT-ACK with the NAT's external address as destination address. Stewart, et al. Expires November 19, 2016 [Page 23] Internet-Draft SCTP NAT Support May 2016 /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ INIT-ACK[Initiate-Tag = 5678] 101.0.0.1:1 <------------------------- 100.0.0.1:2 Int-VTag = 1234 NAT updates entry: +---------+--------+-----------+----------+--------+ NAT | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 5678 | 2 | +---------+--------+-----------+----------+--------+ INIT-ACK[Initiate-Tag = 5678] 10.0.0.1:1 <------ 100.0.0.1:2 Int-VTag = 1234 The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE- ACK. Stewart, et al. Expires November 19, 2016 [Page 24] Internet-Draft SCTP NAT Support May 2016 /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ COOKIE-ECHO 10.0.0.1:1 ------> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ECHO 101.0.0.1:1 -------------------------> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ACK 101.0.0.1:1 <------------------------- 100.0.0.1:2 Int-VTag = 1234 COOKIE-ACK 10.0.0.1:1 <------ 100.0.0.1:2 Int-VTag = 1234 7.2. Single-homed Client to Multi-homed Server The internal client is single-homed whereas the external server is multi-homed. The client (Host A) sends an INIT like in the single- homed case. Stewart, et al. Expires November 19, 2016 [Page 25] Internet-Draft SCTP NAT Support May 2016 +--------+ /--\/--\ /-|Router 1| \ +------+ +-----+ / \ / +--------+ \ +------+ | Host | <-----> | NAT | <-> | Internet | == =| Host | | A | +-----+ \ / \ +--------+ / | B | +------+ \--/\--/ \-|Router 2|-/ +------+ +--------+ +---------+--------+-----------+----------+--------+ NAT | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ INIT[Initiate-Tag = 1234] 10.0.0.1:1 ---> 100.0.0.1:2 Ext-VTag = 0 NAT creates entry: +---------+--------+-----------+----------+--------+ NAT | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 0 | 2 | +---------+--------+-----------+----------+--------+ INIT[Initiate-Tag = 1234] 101.0.0.1:1 ----------------------------> 100.0.0.1:2 Ext-VTag = 0 The server (Host B) includes its two addresses in the INIT-ACK chunk, which results in two NAT entries. Stewart, et al. Expires November 19, 2016 [Page 26] Internet-Draft SCTP NAT Support May 2016 +--------+ /--\/--\ /-|Router 1| \ +------+ +-----+ / \ / +--------+ \ +------+ | Host | <-----> | NAT | <-> | Internet | == =| Host | | A | +-----+ \ / \ +--------+ / | B | +------+ \--/\--/ \-|Router 2|-/ +------+ +--------+ INIT-ACK[Initiate-tag = 5678, IP-Addr = 100.1.0.1] 101.0.0.1:1 <---------------------------- 100.0.0.1:2 Int-VTag = 1234 NAT does need to change the table for second address: +---------+--------+-----------+----------+--------+ NAT | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 5678 | 2 | +---------+--------+-----------+----------+--------+ INIT-ACK[Initiate-Tag = 5678] 10.0.0.1:1 <--- 100.0.0.1:2 Int-VTag = 1234 The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE- ACK. Stewart, et al. Expires November 19, 2016 [Page 27] Internet-Draft SCTP NAT Support May 2016 +--------+ /--\/--\ /-|Router 1| \ +------+ +-----+ / \ / +--------+ \ +------+ | Host | <-----> | NAT | <-> | Internet | == =| Host | | A | +-----+ \ / \ +--------+ / | B | +------+ \--/\--/ \-|Router 2|-/ +------+ +--------+ COOKIE-ECHO 10.0.0.1:1 ---> 100.0.0.1:2 ExtVTag = 5678 COOKIE-ECHO 101.0.0.1:1 ----------------------------> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ACK 101.0.0.1:1 <---------------------------- 100.0.0.1:2 Int-VTag = 1234 COOKIE-ACK 10.0.0.1:1 <--- 100.0.0.1:2 Int-VTag = 1234 7.3. Multihomed Client and Server The client (Host A) sends an INIT to the server (Host B), but does not include the second address. Stewart, et al. Expires November 19, 2016 [Page 28] Internet-Draft SCTP NAT Support May 2016 +-------+ /--| NAT 1 |--\ /--\/--\ +------+ / +-------+ \ / \ +--------+ | Host |=== ====| Internet |====| Host B | | A | \ +-------+ / \ / +--------+ +------+ \--| NAT 2 |--/ \--/\--/ +-------+ +---------+--------+-----------+----------+--------+ NAT 1 | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+--- -------+----------+--------+ INIT[Initiate-Tag = 1234] 10.0.0.1:1 --------> 100.0.0.1:2 Ext-VTag = 0 NAT 1 creates entry: +---------+--------+-----------+----------+--------+ NAT 1 | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 0 | 2 | +---------+--------+-----------+----------+--------+ INIT[Initiate-Tag = 1234] 101.0.0.1:1 -----------------------> 100.0.0.1:2 ExtVTag = 0 Host B includes its second address in the INIT-ACK, which results in two NAT entries in NAT 1. Stewart, et al. Expires November 19, 2016 [Page 29] Internet-Draft SCTP NAT Support May 2016 +-------+ /--------| NAT 1 |--------\ /--\/--\ +------+ / +-------+ \ / \ +--------+ | Host |=== ====| Internet |===| Host B | | A | \ +-------+ / \ / +--------+ +------+ \--------| NAT 2 |--------/ \--/\--/ +-------+ INIT-ACK[Initiate-Tag = 5678, IP-Addr = 100.1.0.1] 101.0.0.1:1 <------------------------- 100.0.0.1:2 Int-VTag = 1234 NAT 1 does not need to update the table for second address: +---------+--------+-----------+----------+--------+ NAT 1 | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 5678 | 2 | +---------+--------+-----------+----------+--------+ INIT-ACK[Initiate-Tag = 5678] 10.0.0.1:1 <---------100.0.0.1:2 Int-VTag = 1234 The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE- ACK. Stewart, et al. Expires November 19, 2016 [Page 30] Internet-Draft SCTP NAT Support May 2016 +-------+ /--------| NAT 1 |--------\ /--\/--\ +------+ / +-------+ \ / \ +--------+ | Host |=== ====| Internet |===| Host B | | A | \ +-------+ / \ / +--------+ +------+ \--------| NAT 2 |--------/ \--/\--/ +-------+ COOKIE-ECHO 10.0.0.1:1 --------> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ECHO 101.0.0.1:1 --------------------> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ACK 101.0.0.1:1 <-------------------- 100.0.0.1:2 Int-VTag = 1234 COOKIE-ACK 10.0.0.1:1 <------- 100.0.0.1:2 Int-VTag = 1234 Host A announces its second address in an ASCONF chunk. The address parameter contains an undefined address (0) to indicate that the source address should be added. The lookup address parameter within the ASCONF chunk will also contain the pair of VTags (external and internal) so that the NAT may populate its table completely with this single packet. +-------+ /--------| NAT 1 |--------\ /--\/--\ +------+ / +-------+ \ / \ +--------+ | Host |=== ====| Internet |===| Host B | | A | \ +-------+ / \ / +--------+ +------+ \--------| NAT 2 |--------/ \--/\--/ +-------+ ASCONF [ADD-IP=0.0.0.0, INT-VTag=1234, Ext-VTag = 5678] 10.1.0.1:1 --------> 100.1.0.1:2 Ext-VTag = 5678 NAT 2 creates complete entry: Stewart, et al. Expires November 19, 2016 [Page 31] Internet-Draft SCTP NAT Support May 2016 +---------+--------+-----------+----------+--------+ NAT 2 | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.1.0.1 | 5678 | 2 | +---------+--------+-----------+----------+--------+ ASCONF [ADD-IP,Int-VTag=1234, Ext-VTag = 5678] 101.1.0.1:1 -----------------------> 100.1.0.1:2 Ext-VTag = 5678 ASCONF-ACK 101.1.0.1:1 <----------------------- 100.1.0.1:2 Int-VTag = 1234 ASCONF-ACK 10.1.0.1:1 <----- 100.1.0.1:2 Int-VTag = 1234 7.4. NAT Loses Its State Association is already established between Host A and Host B, when the NAT loses its state and obtains a new public address. Host A sends a DATA chunk to Host B. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ +---------+--------+-----------+----------+--------+ NAT | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 5678 | 2 | +---------+--------+-----------+----------+--------+ DATA 10.0.0.1:1 ----------> 100.0.0.1:2 Ext-VTag = 5678 The NAT box cannot find entry for the association. It sends ERROR message with the M-Bit set and the cause "NAT state missing". Stewart, et al. Expires November 19, 2016 [Page 32] Internet-Draft SCTP NAT Support May 2016 /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ ERROR [M-Bit, NAT state missing] 10.0.0.1:1 <---------- 100.0.0.1:2 Ext-VTag = 5678 On reception of the ERROR message, Host A sends an ASCONF chunk indicating that the former information has to be deleted and the source address of the actual packet added. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678] 10.0.0.1:1 ----------> 100.1.0.1:2 Ext-VTag = 5678 +---------+--------+-----------+----------+--------+ NAT | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 5678 | 2 | +---------+--------+-----------+----------+--------+ ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678] 102.1.0.1:1 ---------------------> 100.1.0.1:2 Ext-VTag = 5678 Host B adds the new source address and deletes all former entries. Stewart, et al. Expires November 19, 2016 [Page 33] Internet-Draft SCTP NAT Support May 2016 /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ ASCONF-ACK 102.1.0.1:1 <--------------------- 100.1.0.1:2 Int-VTag = 1234 ASCONF-ACK 10.1.0.1:1 <---------- 100.1.0.1:2 Int-VTag = 1234 DATA 10.0.0.1:1 ----------> 100.0.0.1:2 Ext-VTag = 5678 DATA 102.1.0.1:1 ---------------------> 100.1.0.1:2 Ext-VTag = 5678 7.5. Peer-to-Peer Communication If two hosts are behind NATs, they have to get knowledge of the peer's public address. This can be achieved with a so-called rendezvous server. Afterwards the destination addresses are public, and the association is set up with the help of the INIT collision. The NAT boxes create their entries according to their internal peer's point of view. Therefore, NAT A's Internal-VTag and Internal-Port are NAT B's External-VTag and External-Port, respectively. The naming of the verification tag in the packet flow is done from the sending peer's point of view. Stewart, et al. Expires November 19, 2016 [Page 34] Internet-Draft SCTP NAT Support May 2016 Internal | External External | Internal | | | /--\/---\ | +--------+ +-------+ / \ +-------+ +--------+ | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | NAT-Tables +---------+--------+-----------+----------+--------+ NAT A | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+--- -------+----------+--------+ +---------+--------+-----------+----------+--------+ NAT B | Int | Int | Priv | Ext | Ext | | v-tag | port | addr | v-tag | port | +---------+--------+--- -------+----------+--------+ INIT[Initiate-Tag = 1234] 10.0.0.1:1 --> 100.0.0.1:2 Ext-VTag = 0 NAT A creates entry: +---------+--------+-----------+----------+--------+ NAT A | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 0 | 2 | +---------+--------+-----------+----------+--------+ INIT[Initiate-Tag = 1234] 101.0.0.1:1 ----------------> 100.0.0.1:2 Ext-VTag = 0 NAT B processes INIT, but cannot find an entry. The SCTP packet is silently discarded and leaves the NAT table of NAT B unchanged. +---------+--------+-----------+----------+--------+ NAT B | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ Stewart, et al. Expires November 19, 2016 [Page 35] Internet-Draft SCTP NAT Support May 2016 Now Host B sends INIT, which is processed by NAT B. Its parameters are used to create an entry. Internal | External External | Internal | | | /--\/---\ | +--------+ +-------+ / \ +-------+ +--------+ | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | INIT[Initiate-Tag = 5678] 101.0.0.1:1 <-- 10.1.0.1:2 Ext-VTag = 0 +---------+--------+-----------+----------+--------+ NAT B | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 5678 | 2 | 10.1.0.1 | 0 | 1 | +---------+--------+-----------+----------+--------+ INIT[Initiate-Tag = 5678] 101.0.0.1:1 <--------------- 100.0.0.1:2 Ext-VTag = 0 NAT A processes INIT. As the outgoing INIT of Host A has already created an entry, the entry is found and updated: Stewart, et al. Expires November 19, 2016 [Page 36] Internet-Draft SCTP NAT Support May 2016 Internal | External External | Internal | | | /--\/---\ | +--------+ +-------+ / \ +-------+ +--------+ | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | VTag != Int-VTag, but Ext-VTag == 0, find entry. +---------+--------+-----------+----------+--------+ NAT A | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 5678 | 2 | +---------+--------+-----------+----------+--------+ INIT[Initiate-tag = 5678] 10.0.0.1:1 <-- 100.0.0.1:2 Ext-VTag = 0 Host A send INIT-ACK, which can pass through NAT B: Stewart, et al. Expires November 19, 2016 [Page 37] Internet-Draft SCTP NAT Support May 2016 Internal | External External | Internal | | | /--\/---\ | +--------+ +-------+ / \ +-------+ +--------+ | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | INIT-ACK[Initiate-Tag = 1234] 10.0.0.1:1 -->; 100.0.0.1:2 Ext-VTag = 5678 INIT-ACK[Initiate-Tag = 1234] 101.0.0.1:1 ----------------> 100.0.0.1:2 Ext-VTag = 5678 NAT B updates entry: +---------+--------+-----------+----------+--------+ NAT B | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 5678 | 2 | 10.1.0.1 | 1234 | 1 | +---------+--------+-----------+----------+--------+ INIT-ACK[Initiate-Tag = 1234] 101.0.0.1:1 --> 10.1.0.1:2 Ext-VTag = 5678 The lookup for COOKIE-ECHO and COOKIE-ACK is successful. Stewart, et al. Expires November 19, 2016 [Page 38] Internet-Draft SCTP NAT Support May 2016 Internal | External External | Internal | | | /--\/---\ | +--------+ +-------+ / \ +-------+ +--------+ | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | COOKIE-ECHO 101.0.0.1:1 <-- 10.1.0.1:2 Ext-VTag = 1234 COOKIE-ECHO 101.0.0.1:1 <------------- 100.0.0.1:2 Ext-VTag = 1234 COOKIE-ECHO 10.0.0.1:1 <-- 100.0.0.1:2 Ext-VTag = 1234 COOKIE-ACK 10.0.0.1:1 --> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ACK 101.0.0.1:1 ----------------> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ACK 101.0.0.1:1 --> 10.1.0.1:2 Ext-VTag = 5678 8. Socket API Considerations This section describes how the socket API defined in [RFC6458] is extended to provide a way for the application to control NAT friendliness. Please note that this section is informational only. A socket API implementation based on [RFC6458] is extended by supporting one new read/write socket option. Stewart, et al. Expires November 19, 2016 [Page 39] Internet-Draft SCTP NAT Support May 2016 8.1. Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY) This socket option uses the option_level IPPROTO_SCTP and the option_name SCTP_NAT_FRIENDLY. It can be used to enable/disable the NAT friendliness for future associations and retrieve the value for future and specific ones. struct sctp_assoc_value { sctp_assoc_t assoc_id; uint32_t assoc_value; }; assoc_id: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets the application may fill in an association identifier or SCTP_FUTURE_ASSOC for this query. It is an error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id. assoc_value: A non-zero value indicates a NAT-friendly mode. 9. IANA Considerations [NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number you assign this document. ] [NOTE to RFC-Editor: The suggested 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 suggested changes are described below. 9.1. New Chunk Flags for Two Existing Chunk Types As defined in [RFC6096] two chunk flags have to be assigned by IANA for the ERROR chunk. The suggested 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: Stewart, et al. Expires November 19, 2016 [Page 40] Internet-Draft SCTP NAT Support May 2016 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 [RFC6096] one chunk flag has to be assigned by IANA for the ABORT chunk. The suggested 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 | | +------------------+-----------------+-----------+ 9.2. Three New Error Causes Three error causes have to be assigned by IANA. It is suggested to use the values given below. This requires three additional lines in the "Error Cause Codes" registry for SCTP: Stewart, et al. Expires November 19, 2016 [Page 41] Internet-Draft SCTP NAT Support May 2016 Error Cause Codes +-------+--------------------------------+-----------+ | Value | Cause Code | Reference | +-------+--------------------------------+-----------+ | 176 | VTag and Port Number Collision | [RFCXXXX] | | 177 | Missing State | [RFCXXXX] | | 178 | Port Number Collision | [RFCXXXX] | +-------+--------------------------------+-----------+ 9.3. Two New Chunk Parameter Types Two chunk parameter types have to be assigned by IANA. It is suggested 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] | +----------+--------------------------+-----------+ 10. Security Considerations State maintenance within a NAT is always a subject of possible Denial Of Service attacks. This document recommends that at a minimum a NAT runs a timer on any SCTP state so that old association state can be cleaned up. For SCTP end-points, this document does not add any additional security considerations to the ones given in [RFC4960], [RFC4895], and [RFC5061]. In particular, SCTP is protected by the verification tags and the usage of [RFC4895] against off-path attackers. 11. Acknowledgments The authors wish to thank Jason But, Gorry Fairhurst, Bryan Ford, David Hayes, Alfred Hines, Henning Peters, Timo Voelker, Dan Wing, and Qiaobing Xie for their invaluable comments. Stewart, et al. Expires November 19, 2016 [Page 42] Internet-Draft SCTP NAT Support May 2016 12. References 12.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla, "Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August 2007, . [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, . [RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, "Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration", RFC 5061, DOI 10.17487/RFC5061, September 2007, . [RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission Protocol (SCTP) Chunk Flags Registration", RFC 6096, DOI 10.17487/RFC6096, January 2011, . 12.2. Informative References [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, . [RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V. Yasevich, "Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)", RFC 6458, DOI 10.17487/RFC6458, December 2011, . [RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman, "Special-Purpose IP Address Registries", BCP 153, RFC 6890, DOI 10.17487/RFC6890, April 2013, . Stewart, et al. Expires November 19, 2016 [Page 43] Internet-Draft SCTP NAT Support May 2016 [RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host Communication", RFC 6951, DOI 10.17487/RFC6951, May 2013, . Authors' Addresses Randall R. Stewart Netflix, Inc. Chapin, SC 29036 US Email: randall@lakerest.net Michael Tuexen Muenster University of Applied Sciences Stegerwaldstrasse 39 48565 Steinfurt DE Email: tuexen@fh-muenster.de Irene Ruengeler Muenster University of Applied Sciences Stegerwaldstrasse 39 48565 Steinfurt DE Email: i.ruengeler@fh-muenster.de Stewart, et al. Expires November 19, 2016 [Page 44]