Network Working Group R. Stewart Internet-Draft Adara Networks Intended status: Best Current Practice M. Tuexen Expires: March 13, 2014 I. Ruengeler Muenster Univ. of Appl. Sciences September 09, 2013 Stream Control Transmission Protocol (SCTP) Network Address Translation draft-ietf-behave-sctpnat-09.txt Abstract Stream Control Transmission Protocol [RFC4960] provides a reliable communications channel between two end-hosts in many ways similar to TCP [RFC0793]. With the widespread deployment of Network Address Translators (NAT), specialized code has been added to NAT for TCP that allows multiple hosts to reside behind a NAT and yet use only a single globally unique IPv4 address, even when two hosts (behind a NAT) choose the same port numbers for their connection. This additional code is sometimes classified as Network Address and Port Translation or NAPT. To date, specialized code for SCTP has NOT yet been added to most NATs so that only pure NAT is available. The end result of this is that only one SCTP capable host can be behind a NAT. This document describes an SCTP specific variant of NAT which provides similar features of NAPT in the single point and multi-point traversal scenario. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on March 13, 2014. Stewart, et al. Expires March 13, 2014 [Page 1] Internet-Draft SCTP NAT September 2013 Copyright Notice Copyright (c) 2013 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . . . 4 4.1. Single Point Traversal . . . . . . . . . . . . . . . . . 4 4.2. Multi Point Traversal . . . . . . . . . . . . . . . . . . 5 5. Limitations of Classical NAPT for SCTP . . . . . . . . . . . 6 6. The SCTP Specific Variant of NAT . . . . . . . . . . . . . . 6 7. NAT to SCTP . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. Handling of Fragmented SCTP Packets . . . . . . . . . . . . . 10 9. Various Examples of NAT Traversals . . . . . . . . . . . . . 10 9.1. Single-homed Client to Single-homed Server . . . . . . . 10 9.2. Single-homed Client to Multi-homed Server . . . . . . . . 12 9.3. Multihomed Client and Server . . . . . . . . . . . . . . 15 9.4. NAT Loses Its State . . . . . . . . . . . . . . . . . . . 18 9.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . 20 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 11. Security Considerations . . . . . . . . . . . . . . . . . . . 24 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 13.1. Normative References . . . . . . . . . . . . . . . . . . 24 13.2. Informative References . . . . . . . . . . . . . . . . . 25 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 1. Introduction Stream Control Transmission Protocol [RFC4960] provides a reliable communications channel between two end-hosts in many ways similar to TCP [RFC0793]. With the widespread deployment of Network Address Translators (NAT), specialized code has been added to NAT for TCP that allows multiple hosts to reside behind a NAT and use private Stewart, et al. Expires March 13, 2014 [Page 2] Internet-Draft SCTP NAT September 2013 addresses (see [RFC5735]) and yet use only a single globally unique IPv4 address, even when two hosts (behind a NAT) choose the same port numbers for their connection. This additional code is sometimes classified as Network Address and Port Translation or NAPT. To date, specialized code for SCTP has not yet been added to most NATs so that only true NAT is available. The end result of this is that only one SCTP capable host can be behind a NAT. This document proposes an SCTP specific variant NAT that provides the NAPT functionality without changing SCTP port numbers. The authors feel it is possible and desirable to make these changes for a number of reasons. o It is desirable for SCTP internal end-hosts on multiple platforms to be able to share a NAT's public IP address, much as TCP does today. o If a NAT does not need to change any data within an SCTP packet it will reduce the processing burden of NAT'ing SCTP by NOT needing to execute the CRC32c checksum required by SCTP. o Not having to touch the IP payload makes the processing of ICMP messages in NATs easier. 2. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. Terminology For this discussion we will use several terms, which we will define and point out in Figure 1. Private-Address (Priv-Addr): The private address that is known to the internal host. Internal-Port (Int-Port): The port number that is in use by the host holding the Private-Address. Internal-VTag (Int-VTag): The Verification Tag that the internal host has chosen for its communication. The VTag is a unique 32 bit tag that must accompany any incoming SCTP packet for this association to the Private-Address. External-Address (Ext-Addr): The address that an internal host is attempting to contact. Stewart, et al. Expires March 13, 2014 [Page 3] Internet-Draft SCTP NAT September 2013 External-Port (Ext-Port): The port number of the peer process at the External-Address. External-VTag (Ext-VTag): The Verification Tag that the host holding the External-Address has chosen for its communication. The VTag is a unique 32 bit tag that must accompany any incoming SCTP packet for this association to the External-Address. Public-Address (Pub-Addr): The public address assigned to the NAT box which it uses as a source address when sending packets towards the External-Address. Internal Network | External Network | Private | Public External +---------+ Address | Address /--\/--\ Address +---------+ | SCTP | +-----+ / \ | SCTP | |end point|==========| NAT |======= | Internet | ======== |end point| | A | +-----+ \ / | B | +---------+ Internal | \--/\--/ External +---------+ Internal Port | Port External VTag | VTag Figure 1: Architecture 4. SCTP NAT Traversal Scenarios This section defines the notion of single and multi-point NAT traversal. 4.1. Single Point Traversal In this case, all packets in the SCTP association go through a single NAT, as shown below: Internal Network | External Network | +---------+ | /--\/--\ +---------+ | SCTP | +-----+ / \ | SCTP | |end point|=========| NAT |========= | Internet | ========|end point| | A | +-----+ \ / | B | +---------+ | \--/\--/ +---------+ | Single NAT scenario Stewart, et al. Expires March 13, 2014 [Page 4] Internet-Draft SCTP NAT September 2013 A variation of this case is shown below, i.e., multiple NATs in a single path: Internal | External : Internal | External | : | +---------+ | : | /--\/--\ +---------+ | SCTP | +-----+ : +-----+ / \ | SCTP | |end point|==| NAT |=======:=======| NAT |==| Internet |==|end point| | A | +-----+ : +-----+ \ / | B | +---------+ | : | \--/\--/ +---------+ | : | Serial NATs scenario In this single point traversal scenario, we must acknowledge that while one of the main benefits of SCTP multi-homing is redundant paths, the NAT function represents a single point of failure in the path of the SCTP multi-home association. However, the rest of the path may still benefit from path diversity provided by SCTP multi- homing. The two SCTP endpoints in this case can be either single-homed or multi-homed. However, the important thing is that the NAT (or NATs) in this case sees all the packets of the SCTP association. 4.2. Multi Point Traversal This case involves multiple NATs and each NAT only sees some of the packets in the SCTP association. An example is shown below: 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 Stewart, et al. Expires March 13, 2014 [Page 5] Internet-Draft SCTP NAT September 2013 entire path. This in turn can improve the robustness of the communication. 5. Limitations of Classical NAPT for SCTP Using classical NAPT may result in changing one of the SCTP port numbers during the processing which requires the recomputation of the transport layer checksum. Whereas for UDP and TCP this can be done very efficiently, for SCTP the checksum (CRC32c) over the entire packet needs to be recomputed. This would add considerable to the NAT computational burden, however hardware support may mitigate this in some implementations. An SCTP endpoint may have multiple addresses but only has a single port number. To make multipoint traversal work, all the NATs involved must recognize the packets they see as belonging to the same SCTP association and perform port number translation in a consistent way. One possible way of doing this is to use pre-defined table of ports and addresses configured within each NAT. Other mechanisms could make use of NAT to NAT communication. Such mechanisms are considered by the authors not to be deployable on a wide scale base and thus not a recommended solution. Therefore the SCTP variant of NAT has been developed. 6. The SCTP Specific Variant of NAT In this section we assume that we have multiple SCTP capable hosts behind a NAT which has one Public-Address. Furthermore we are focusing in this section on the single point traversal scenario. 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. Stewart, et al. Expires March 13, 2014 [Page 6] Internet-Draft SCTP NAT September 2013 The entries in this table fulfill some uniqueness conditions. There must not be more than one entry with the same pair of Internal-Port and External-Port. This rule can be relaxed, if all entries with the same Internal-Port and External-Port have the support for the restart procedure enabled. In this case there must be no more than one entry with the same Internal-Port, External-Port and Ext-VTag and no more than one entry with the same Internal-Port, External-Port and Int- VTag. The processing of outgoing SCTP packets containing an INIT-chunk is described in the following figure. The scenario shown is valid for all message flows in this section. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ INIT[Initiate-Tag] Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port Ext-VTag=0 Create(Initiate-Tag, Int-Port, Priv-Addr, 0) Returns(NAT-State control block) Translate To: INIT[Initiate-Tag] Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port Ext-VTag=0 It should be noted that normally a NAT control block will be created. However, it is possible that there is already a NAT control block with the same External-Address, External-Port, Internal-Port, and Internal-VTag but different Private-Address. In this case the INIT SHOULD be dropped by the NAT and an ABORT SHOULD be sent back to the SCTP host with the M-Bit set and an appropriate error cause (see [I-D.ietf-tsvwg-natsupp] for the format). 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 Stewart, et al. Expires March 13, 2014 [Page 7] Internet-Draft SCTP NAT September 2013 External-Address does not support the DISABLE_RESTART feature (noted in the NAT control block when the prior connection was established). In such a case the INIT SHOULD be dropped by the NAT and an ABORT SHOULD be sent back to the SCTP host with the M-Bit set and an appropriate error cause (see [I-D.ietf-tsvwg-natsupp] for the format). The processing of outgoing SCTP packets containing no INIT-chunk is described in the following figure. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port Ext-VTag Translate To: Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port Ext-VTag The processing of incoming SCTP packets containing INIT-ACK chunks is described in the following figure. The Lookup() function getting as input the Internal-VTag, Internal-Port, External-VTag (=0), External- Port, and External-Address, returns the corresponding entry of the NAT table and updates the External-VTag by substituting it with the value of the Initiate-Tag of the INIT-ACK chunk. The wildcard character signifies that the parameter's value is not considered in the Lookup() function or changed in the Update() function, respectively. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ INIT-ACK[Initiate-Tag] Pub-Addr:Int-Port <---- Ext-Addr:Ext-Port Int-VTag Stewart, et al. Expires March 13, 2014 [Page 8] Internet-Draft SCTP NAT September 2013 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. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ Stewart, et al. Expires March 13, 2014 [Page 9] Internet-Draft SCTP NAT September 2013 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. 7. NAT to SCTP This document at various places discusses the sending of specialized SCTP chunks (e.g. an ABORT with M-Bit set). These chunks and procedures are not defined in this document, but instead are defined in [I-D.ietf-tsvwg-natsupp]. The NAT implementer should refer to [I-D.ietf-tsvwg-natsupp] for detailed descriptions of packet formats and procedures. 8. Handling of Fragmented SCTP Packets A NAT box MUST support IP reassembly of received fragmented SCTP packets. The fragments may arrive in any order. When an SCTP packet has to be fragmented by the NAT box and the IP header forbids fragmentation a corresponding ICMP packet SHOULD be sent. 9. Various Examples of NAT Traversals 9.1. Single-homed Client to Single-homed Server The internal client starts the association with the external server via a four-way-handshake. Host A starts by sending an INIT chunk. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | Stewart, et al. Expires March 13, 2014 [Page 10] Internet-Draft SCTP NAT September 2013 +--------+ +-----+ \ / +--------+ \--/\---/ +---------+--------+-----------+----------+--------+ 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. /--\/--\ +--------+ +-----+ / \ +--------+ | 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 | Stewart, et al. Expires March 13, 2014 [Page 11] Internet-Draft SCTP NAT September 2013 | 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. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <------> | NAT | <------> | Internet | <------> | Host B | +--------+ +-----+ \ / +--------+ \--/\---/ COOKIE-ECHO 10.0.0.1:1 ------> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ECHO 101.0.0.1:1 -------------------------> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ACK 101.0.0.1:1 <------------------------- 100.0.0.1:2 Int-VTag = 1234 COOKIE-ACK 10.0.0.1:1 <------ 100.0.0.1:2 Int-VTag = 1234 9.2. Single-homed Client to Multi-homed Server The internal client is single-homed whereas the external server is multi-homed. The client (Host A) sends an INIT like in the single- homed case. Stewart, et al. Expires March 13, 2014 [Page 12] Internet-Draft SCTP NAT September 2013 +--------+ /--\/--\ /-|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. +--------+ /--\/--\ /-|Router 1| \ +------+ +-----+ / \ / +--------+ \ +------+ | Host | <-----> | NAT | <-> | Internet | == =| Host | | A | +-----+ \ / \ +--------+ / | B | +------+ \--/\--/ \-|Router 2|-/ +------+ +--------+ Stewart, et al. Expires March 13, 2014 [Page 13] Internet-Draft SCTP NAT September 2013 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. +--------+ /--\/--\ /-|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 Stewart, et al. Expires March 13, 2014 [Page 14] Internet-Draft SCTP NAT September 2013 COOKIE-ACK 10.0.0.1:1 <--- 100.0.0.1:2 Int-VTag = 1234 9.3. Multihomed Client and Server The client (Host A) sends an INIT to the server (Host B), but does not include the second address. +-------+ /--| 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 Stewart, et al. Expires March 13, 2014 [Page 15] Internet-Draft SCTP NAT September 2013 Host B includes its second address in the INIT-ACK, which results in two NAT entries in NAT 1. +-------+ /--------| NAT 1 |--------\ /--\/--\ +------+ / +-------+ \ / \ +--------+ | Host |=== ====| Internet |===| Host B | | A | \ +-------+ / \ / +--------+ +------+ \--------| NAT 2 |--------/ \--/\--/ +-------+ INIT-ACK[Initiate-Tag = 5678, IP-Addr = 100.1.0.1] 101.0.0.1:1 <------------------------- 100.0.0.1:2 Int-VTag = 1234 NAT 1 does not need to update the table for second address: +---------+--------+-----------+----------+--------+ NAT 1 | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 5678 | 2 | +---------+--------+-----------+----------+--------+ INIT-ACK[Initiate-Tag = 5678] 10.0.0.1:1 <---------100.0.0.1:2 Int-VTag = 1234 The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE- ACK. +-------+ /--------| NAT 1 |--------\ /--\/--\ +------+ / +-------+ \ / \ +--------+ | Host |=== ====| Internet |===| Host B | | A | \ +-------+ / \ / +--------+ +------+ \--------| NAT 2 |--------/ \--/\--/ +-------+ COOKIE-ECHO Stewart, et al. Expires March 13, 2014 [Page 16] Internet-Draft SCTP NAT September 2013 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: +---------+--------+-----------+----------+--------+ NAT 2 | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.1.0.1 | 5678 | 2 | Stewart, et al. Expires March 13, 2014 [Page 17] Internet-Draft SCTP NAT September 2013 +---------+--------+-----------+----------+--------+ 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 9.4. NAT Loses Its State Association is already established between Host A and Host B, when the NAT loses its state and obtains a new public address. Host A sends a DATA chunk to Host B. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ +---------+--------+-----------+----------+--------+ NAT | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+-----------+----------+--------+ | 1234 | 1 | 10.0.0.1 | 5678 | 2 | +---------+--------+-----------+----------+--------+ DATA 10.0.0.1:1 ----------> 100.0.0.1:2 Ext-VTag = 5678 The NAT box cannot find entry for the association. It sends ERROR message with the M-Bit set and the cause "NAT state missing". /--\/--\ Stewart, et al. Expires March 13, 2014 [Page 18] Internet-Draft SCTP NAT September 2013 +--------+ +-----+ / \ +--------+ | 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. /--\/--\ +--------+ +-----+ / \ +--------+ | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | +--------+ +-----+ \ / +--------+ \--/\--/ Stewart, et al. Expires March 13, 2014 [Page 19] Internet-Draft SCTP NAT September 2013 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 9.5. Peer-to-Peer Communication If two hosts are behind NATs, they have to get knowledge of the peer's public address. This can be achieved with a so-called rendezvous server. Afterwards the destination addresses are public, and the association is set up with the help of the INIT collision. The NAT boxes create their entries according to their internal peer's point of view. Therefore, NAT A's Internal-VTag and Internal-Port are NAT B's External-VTag and External-Port, respectively. The naming of the verification tag in the packet flow is done from the sending peer's point of view. Internal | External External | Internal | | | /--\/---\ | +--------+ +-------+ / \ +-------+ +--------+ | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ | \--/\---/ | NAT-Tables +---------+--------+-----------+----------+--------+ NAT A | Int | Int | Priv | Ext | Ext | | VTag | Port | Addr | VTag | Port | +---------+--------+--- -------+----------+--------+ +---------+--------+-----------+----------+--------+ NAT B | Int | Int | Priv | Ext | Ext | | v-tag | port | addr | v-tag | port | Stewart, et al. Expires March 13, 2014 [Page 20] Internet-Draft SCTP NAT September 2013 +---------+--------+--- -------+----------+--------+ 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 | +---------+--------+-----------+----------+--------+ 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 Stewart, et al. Expires March 13, 2014 [Page 21] Internet-Draft SCTP NAT September 2013 +---------+--------+-----------+----------+--------+ 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: 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: Internal | External External | Internal | | | /--\/---\ | +--------+ +-------+ / \ +-------+ +--------+ | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | +--------+ +-------+ \ / +-------+ +--------+ Stewart, et al. Expires March 13, 2014 [Page 22] Internet-Draft SCTP NAT September 2013 | \--/\---/ | 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. 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 Stewart, et al. Expires March 13, 2014 [Page 23] Internet-Draft SCTP NAT September 2013 COOKIE-ACK 10.0.0.1:1 --> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ACK 101.0.0.1:1 ----------------> 100.0.0.1:2 Ext-VTag = 5678 COOKIE-ACK 101.0.0.1:1 --> 10.1.0.1:2 Ext-VTag = 5678 10. IANA Considerations This document requires no actions from IANA. 11. Security Considerations State maintenance within a NAT is always a subject of possible Denial Of Service attacks. This document recommends that at a minimum a NAT runs a timer on any SCTP state so that old association state can be cleaned up. 12. Acknowledgments The authors wish to thank Jason But Bryan Ford, David Hayes, Alfred Hines, Henning Peters, Timo Voelker, Dan Wing, and Qiaobing Xie for their invaluable comments. 13. References 13.1. Normative References [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007. [I-D.ietf-tsvwg-natsupp] Stewart, et al. Expires March 13, 2014 [Page 24] Internet-Draft SCTP NAT September 2013 Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control Transmission Protocol (SCTP) Network Address Translation Support", draft-ietf-tsvwg-natsupp-05 (work in progress), February 2013. 13.2. Informative References [RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses", RFC 5735, January 2010. Authors' Addresses Randall R. Stewart Adara Networks 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 March 13, 2014 [Page 25]