DCCP Working Group G. Fairhurst Internet-Draft G. Renker Updates: 4340 (if approved) University of Aberdeen Intended status: Standards Track Sept 14, 2008 Expires: March 18, 2009 DCCP Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal draft-ietf-dccp-simul-open-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 18, 2009. Fairhurst & Renker Expires March 18, 2009 [Page 1]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 Abstract This document specifies an update to the Datagram Congestion Control Protocol (DCCP), a connection-oriented and datagram-based transport protocol. The update adds a packet type, DCCP-Listen, which assists DCCP applications that need to communicate through one or more middleboxes (e.g. Network Address Translators, NATs, or firewalls), where establishing necessary middlebox state requires peering endpoints to initiate communication in a near-simultaneous manner. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Scope of this Document . . . . . . . . . . . . . . . . . . 3 1.2. Scope of the Problem to be Tackled . . . . . . . . . . . . 4 1.3. Structure of this Document . . . . . . . . . . . . . . . . 4 2. Procedure for Near-Simultaneous Open . . . . . . . . . . . . . 5 2.1. Conventions and Terminology . . . . . . . . . . . . . . . 5 2.2. DCCP-Listen Packet Format . . . . . . . . . . . . . . . . 5 2.3. Protocol Method . . . . . . . . . . . . . . . . . . . . . 7 2.3.1. Protocol Method for DCCP-Server Endpoints . . . . . . 7 2.3.2. Protocol Method for DCCP-Client Endpoints . . . . . . 9 2.3.3. Processing by Routers and Middleboxes . . . . . . . . 9 2.4. Examples of Use . . . . . . . . . . . . . . . . . . . . . 9 2.5. Backwards Compatibility with RFC 4340 . . . . . . . . . . 10 3. Discussion of Design Decisions . . . . . . . . . . . . . . . . 12 3.1. Rationale for a New Packet Type . . . . . . . . . . . . . 12 3.2. Generation of Listen Packets . . . . . . . . . . . . . . . 13 3.3. Repetition of DCCP-Listen Packets . . . . . . . . . . . . 13 4. Security Considerations . . . . . . . . . . . . . . . . . . . 15 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1. Normative References . . . . . . . . . . . . . . . . . . . 18 6.2. Informative References . . . . . . . . . . . . . . . . . . 18 Appendix A. Discussion of Existing NAT Traversal Techniques . . . 20 A.1. NAT traversal Based on Simultaneous-Open . . . . . . . . . 21 A.2. Role Reversal . . . . . . . . . . . . . . . . . . . . . . 21 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26 Intellectual Property and Copyright Statements . . . . . . . . . . 27 Fairhurst & Renker Expires March 18, 2009 [Page 2]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 1. Introduction UDP Network Address Translator (NAT) traversal is well understood and widely implemented. NAT traversal for connection-oriented protocols (e.g. TCP) uses similar principles, but in some cases requires more complex and expensive solutions, such as data relay servers [TURN]. DCCP [RFC4340] is both datagram-based and connection-oriented. As such it faces the same problems as TCP NAT traversal, without the ability to simply reuse traversal solutions that work for UDP. An additional issue is that the original specification of DCCP did not allow a server to perform a simultaneous-open, an inherent characteristic of TCP that greatly simplifies NAT traversal. After discussing the problem space for DCCP, this document specifies a new DCCP extension to facilitate DCCP NAT traversal, by explicitly supporting a widely used principle known as 'hole punching'. This represents only a minor change to the standard DCCP operational procedure. The extension uses a dedicated DCCP packet type, whose usage is tied to a specific condition, can thus be turned off, and is inter-operable with hosts that do not implement this extension. The object of this extension is DCCP native support for middlebox traversal [ID-Behave-DCCP], reducing dependence on external aids such as data relay servers. 1.1. Scope of this Document This document specifically targets DCCP NAT traversal. However, due to the similarity of the principles involved, the extension may be of similar use to traversal of other types of middlebox, such as firewalls. This technique applies to scenarios where one or both DCCP peers are located behind a middlebox. It is relevant to both client/server and peer-to-peer applications, such as VoIP, file sharing, or online gaming and assists connections that utilise prior out-of-band signaling (e.g. via a well-known rendezvous server ([RFC3261], [H.323])) to notify both endpoints of the connection parameters ([RFC3235], [NAT-APP]). The document assumes a traditional (outbound) type of NAT as defined in [RFC2663] and further discussed in [RFC3022]. We understand NAT traversal as involving one or more NAT devices of this type in the path (i.e. hierarchies of nested NAT devices are possible). It is assumed that all NATs in the path between endpoints are BEHAVE- compliant [NAT-APP]. Fairhurst & Renker Expires March 18, 2009 [Page 3]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 This document does not discuss specific behavioural requirements of devices to support DCCP NAT traversal, which are addressed in [ID-Behave-DCCP]. 1.2. Scope of the Problem to be Tackled This document refers to DCCP hosts located behind one or more NAT devices as having "private" addresses, and to DCCP hosts located in the global address realm as having "public" addresses. We consider DCCP NAT traversal for the following scenarios: 1. Private client connects to public server. 2. Public server connects to private client. 3. Private client connects to private server. A defining characteristic of traditional NAT devices [RFC3022] is that private hosts can connect to external hosts, but not vice versa. Hence the case (1) is always possible, whereas cases (2) and (3) require NAT traversal techniques. In this document we do not consider use of pre-configured, static NAT address maps, which would also allow outside hosts to connect to the private network in cases (2) and (3). A DCCP implementation conforming to [RFC4340] requires a relay server to perform NAT traversal. The extension specified by this document updates RFC 4340 to enable DCCP NAT traversal without the aid of relay servers. 1.3. Structure of this Document For background information on existing NAT traversal techniques, please consult Appendix A. The normative specification of the extension is presented in the next section. An informative discussion of underlying design decisions then follows in Section 3. Security considerations are provided in Section 4 and IANA considerations in the concluding Section 5. Fairhurst & Renker Expires March 18, 2009 [Page 4]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 2. Procedure for Near-Simultaneous Open This section is normative and specifies the simultaneous-open technique for DCCP. This updates the connection-establishment procedures of [RFC4340]. 2.1. Conventions and Terminology The document uses the terms and definitions provided in [RFC4340]. Familiarity with this specification is assumed. In particular, the following convention from ([RFC4340], 3.2) is used: "Each DCCP connection runs between two hosts, which we often name DCCP A and DCCP B. Each connection is actively initiated by one of the hosts, which we call the client; the other, initially passive host is called the server." 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]. 2.2. DCCP-Listen Packet Format This document adds a new DCCP packet type, DCCP-Listen, whose format is shown below. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Port | Dest Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data Offset | CCVal | CsCov | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Res | Type |X| Reserved | Sequence Number High Bits | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number Low Bits | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Format of a DCCP-Listen Packet Since DCCP-Listen packets are issued before an actual connection is established, they MUST NOT carry payload data, and endpoints MUST ignore any payload data encountered in DCCP-Listen packets. Fairhurst & Renker Expires March 18, 2009 [Page 5]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 At the time of writing. there areo known uses of the header option ([RFC4340], sec. 5.8) with a DCCP-Listen packet. Servers SHOULD set both Sequence Number fields to 0; clients MUST ignore the value of the Sequence Number fields; and middleboxes MUST NOT interpret sequence numbers on DCCP-Listen packets. Furthermore, the following protocol fields MUST all be set to zero: CCVal (a connection has not been established), CsCov (there is no payload). The "Res" and "Reserved" fields are specified by [RFC4340] and its successors. The interpretation of these fields is not modified by this document. The Type field has the decimal value 10, which indicates that this is a DCCP-Listen packet. Fairhurst & Renker Expires March 18, 2009 [Page 6]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 Note to the RFC Editor: This value needs to be confirmed by IANA when this document is published. Please then remove this note. ==> End of note to the RFC Editor. <== Since the use of short sequence numbers ([RFC4340], 5.1) depends on the value of the Allow Short Seqno feature ([RFC4340], 7.6.1) and since DCCP-Listen packets are sent before a connection is established, there is no way of negotiating the use of short sequence numbers. Consequently, the default value of 0 for the Allow Short Seqno feature MUST be used, X MUST be set to 1, and DCCP-Listen packets with X=0 MUST be ignored. The Service Code field contains the service code ([RFC4340], 8.1.2) that the client peer wants to use for this connection. This value MUST correspond to a service code that the server is actually offering for connections identified by the same source IP address and the same Source Port as that of the DCCP-Listen packet. Since the server may use multiple service codes, the specific value of the Service Code field needs to be communicated out-of-band, from client to server, prior to sending the DCCP-Listen packet, e.g. described using the Session Description Protocol, SDP. 2.3. Protocol Method The term "session" is used as defined in ([RFC2663], 2.3): DCCP sessions are uniquely identified by the tuple of <source IP-address, source port, target IP-address, target port>. DCCP, in addition, introduces service codes which can be used to identify different services available via the same port [Fai08]. We call the five-tuple <source IP-address, source port, service code, target IP-address, target port> a fully specified DCCP connection, and refer to an endpoint that has been assigned all five parameters as a "fully specified endpoint". DCCP-Listen packets are only sent for the specific case of fully specified DCCP server endpoints. 2.3.1. Protocol Method for DCCP-Server Endpoints This document updates [RFC4340] for the case of fully specified DCCP server endpoints. The update modifies the way the server performs a passive-open. Prior to connection setup, it is common for DCCP server endpoints to not be fully specified: before the connection is established, a Fairhurst & Renker Expires March 18, 2009 [Page 7]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 server usually sets the target IP-address:port to wildcard values (i.e. leaves these unspecified); the endpoint only becomes fully specified after performing the handshake with an incoming connection. For such cases, this document does not update [RFC4340], i.e. the server adheres to the existing state transitions in the left half of Figure 2 (CLOSED => LISTEN => RESPOND). A fully specified DCCP server endpoint permits exactly one client, identified by target IP-address:port plus service code, to set up the connection. Such a server SHOULD perform the actions and state transitions shown in the right half of Figure 2, and specified below. unspecified remote +--------+ fully specified remote +---------------------| CLOSED |---------------------+ | +--------+ send DCCP-Listen | | | | | v v +--------+ timeout +---------+ | LISTEN |<------------------------------+-----------| INVITED | +--------+ more than 2 retransmissions | +---------+ | | 1st / 2nd ^ | | | retransm. | | | +-------------+ | | resend Listen | | | | | | receive Request +---------+ receive Request | +------------------->| RESPOND |<--------------------+ send Response +---------+ send Response Figure 2: Updated state transition diagram for DCCP-Listen A fully-specified server endpoint performs a passive-open from the CLOSED state by inviting the remote client to connect. This is performed by sending a single DCCP-Listen packet to the specified remote IP-adress:port, using the format specified in Section 2.2. The server then transitions to the INVITED state. The INVITED state is, like LISTEN, a passive state, characterised by waiting in the absence of an established connection. If the server endpoint in state INVITED receives a DCCP-Request, it transitions to RESPOND, where further processing resumes as specified in [RFC4340]. The server SHOULD repeat sending a DCCP-Listen packet while in state INVITED, at a 200 millisecond interval and up to at most 2 repetitions (Section 3 discusses this choice of timer interval). The Fairhurst & Renker Expires March 18, 2009 [Page 8]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 retransmission timer is restarted with the same 200ms interval after the second retransmission. When, upon the next timeout, the server is still in the INVITED state, it SHOULD progress to LISTEN, and resume processing as specified in [RFC4340]. Fully-specified server endpoints SHOULD treat ICMP error messages received in response to a DCCP-Listen packet as "soft errors" that do not cause a state transition. Server endpoints SHOULD in general ignore any incoming DCCP-Listen packets. As an exception to this rule, a DCCP Server in state LISTEN MAY generate a Reset (Code 7, "Connection Refused") in response to a DCCP-Listen packet. This Reset is an indication that two servers are simultaneously awaiting connections on the same port. Further details on the rationale are discussed in Section 3. 2.3.2. Protocol Method for DCCP-Client Endpoints This document updates [RFC4340], by adding the following rule for the reception of DCCP-Listen packets by clients: Clients MUST silently discard any received DCCP-Listen packets, regardless of their current state. 2.3.3. Processing by Routers and Middleboxes DCCP-Listen packets do not require special treatment and should thus be forwarded end-to-end across Internet paths, by routers and middleboxes alike. Middleboxes may utilise the connection information (address, port, service code) to establish local forwarding state. This has been the main motivation for adding the Service Code field: in combination with the source and destination addresses (found in the enclosing IP- header), the DCCP-Listen packet carries the necessary information to uniquely identify a DCCP session. 2.4. Examples of Use In the examples below, DCCP A is the client and DCCP B is the server. NAT/Firewall device NA is placed before DCCP A, and NAT/Firewall device NB is placed before DCCP B. Both NA and NB use a policy that permits DCCP packets to traverse the device for outgoing links, but only permit incoming DCCP packets when a previous packet has been sent out for the same connection. DCCP A and DCCP B decide to communicate using some out-of-band Fairhurst & Renker Expires March 18, 2009 [Page 9]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 mechanism, whereupon the client and server are started. DCCP A initiates a connection by sending a DCCP-Request. DCCP B actively indicates its listening state by sending a DCCP-Listen message. This fulfils the requirement of punching a hole in NB, so that DCCP A can retransmit the DCCP-Request and connect through it. DCCP A DCCP B ------ NA NB ------ +------------------+ +-+ +-+ +-----------------+ |(1) Initiation | | | | | | | |DCCP-Request --> +--+-+---X| | | | | |<-+-+----+-+--+<-- DCCP-Listen | | | | | | | | | |DCCP-Request --> +--+-+----+-+->| | | |<-+-+----+-+--+<-- DCCP-Response| |DCCP-Ack --> +--+-+----+-+->| | | | | | | | | | |(2) Data transfer | | | | | | | |DCCP-Data --> +--+-+----+-+->| | +------------------+ +-+ +-+ +-----------------+ Figure 3: Event sequence when the client is started before the server The diagram below shows the reverse sequence of events, where the server sends the DCCP-Listen before the client sends a DCCP-Request: DCCP A DCCP B ------ NA NB ------ +------------------+ +-+ +-+ +-----------------+ |(1) Initiation | | | | | | | | | | |X---+-+--+<-- DCCP-Listen | |DCCP-Request --> +--+-+----+-+->| | | | <+-+----+-+--+<-- DCCP-Response| |DCCP-Ack --> +--+-+----+-+> | | | | | | | | | | |(2) Data transfer | | | | | | | |DCCP-Data --> +--+-+----+-+> | | +------------------+ +-+ +-+ +-----------------+ Figure 4: Event sequence when the server is started before the client 2.5. Backwards Compatibility with RFC 4340 No changes are required if a DCCP Client conforming to this document communicates with a DCCP Server conforming to [RFC4340]. Fairhurst & Renker Expires March 18, 2009 [Page 10]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 If a client implements only [RFC4340], an incoming DCCP-Listen packet would be ignored due to step 1 in [RFC4340], 8.1, which at the same time also conforms to the behaviour specified by this document. This document further does not modify communication for any server that implements a passive-open without fully binding the addresses, ports and service codes to be used. The authors therefore do not expect practical deployment problems with existing conformant DCCP implementations. Fairhurst & Renker Expires March 18, 2009 [Page 11]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 3. Discussion of Design Decisions 3.1. Rationale for a New Packet Type This is an informative setcion that reviews the rationale for the design of this technique. The DCCP-Listen packet specified in Section 2.2 has the same format as the DCCP-Request packet ([RFC4340], 5.1), the only difference is in the value of the Type field. The usage, however, differs. The DCCP-Listen packet serves as advisory message, not as part of the actual connection setup: sequence numbers have no meaning, and no payload may be present. A DCCP-Request packet could in theory also have been used for the same purpose. The following arguments were against this. The first problem was that of semantic overloading: the Request is defined in [RFC4340] serves a well-defined purpose, being the initial packet of the 3-way handshake. Additional use in the manner of a DCCP-Listen packet would require DCCP processors to have two different processing paths: one where a DCCP-Request is interpreted as part of the initial handshake, and another where the same packet is interpreted as indicator message. This would complicate packet processing in hosts and in particular stateful middleboxes (which may have restricted computational resources). The second problem is that a client receiving a DCCP-Request from a server could generate a DCCP-Reset if it has not yet entered the REQUEST state (step 7 in [RFC4340], 8.5). This document lets client endpoints ignore DCCP-Listen packets. Adding a similar rule for the DCCP-Request packet is cumbersome: clients would not be able to distinguish between a Request meant to be an indicator message and a genuinely erratic connection initiation. The third problem is similar and refers to a client receiving the indication after having itself sent a (connection-initiation) Request. Step 7 in section 8.5 of [RFC4340] requires the client to reply to an "indicator message" Request from the server with a Sync. Since sequence numbers are ignored for this type of message, additional and complex processing becomes necessary: either to ask the client not to respond to a DCCP-Request when the request is of type "indicator message"; or ask middleboxes and servers to ignore Sync packets generated in response to "indicator message" DCCP- Requests. Furthermore, since no initial sequence numbers have been negotiated at this stage, sending a SyncAck would not be meaningful. Using a separate packet type therefore allows simpler and clearer processing. Fairhurst & Renker Expires March 18, 2009 [Page 12]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 The rationale for ignoring the Sequence Number fields on DCCP-Listen packets is that endpoints have not yet entered connection setup: the DCCP-Listen packet is sent while the server is still in the passive- open (INVITED) state, i.e. it has not yet allocated state, other than binding to the client's IP-address:port and service code. Although the DCCP-Listen Sequence Number fields are ignored, they have been retained to reuse the generic header format from section 5.1 of [RFC4340]. 3.2. Generation of Listen Packets Since DCCP-Listen packets solve a particular problem (NAT and/or firewall traversal), the generation of DCCP-Listen packets on passive sockets is tied to a condition (binding to an a priori known remote address and service code), so as to not interfere with the general case of "normal" DCCP connections (where client addresses are generally not known in advance). In the TCP world, the analogue is a transition from LISTEN to SYN_SENT by virtue of sending data: "A fully specified passive call can be made active by the subsequent execution of a SEND" ([RFC0793], 3.8). Unlike TCP, this extension does not perform a role-change from passive to active. Like TCP, DCCP-Listen packets are only sent by a DCCP-server when the endpoint is fully specified (Section 2.3). 3.3. Repetition of DCCP-Listen Packets Repetition is a necessary requirement, to increase robustness and the chance of successful connection establishment: in case a DCCP-Listen packet is lost due to congestion, link loss, or some other reason. The decision to recommend a maximum number of 3 timeouts (2 repetitions) results from the following considerations. The repeated copies need to be spaced sufficiently far apart in time to avoid suffering from correlated loss. The interval of 200 ms has been chosen to accommodate a wide range of wireless and wired network paths. Another constraint is given by the retransmission interval for the DCCP-Request ([RFC4340], 8.1.1). To establish state, intermediate systems need to receive a (retransmitted) DCCP-Listen packet before the DCCP-Request times out (1 second). With three timeouts, each spaced 200 milliseconds apart, the overall time is still below one Fairhurst & Renker Expires March 18, 2009 [Page 13]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 second. On the other hand, the sum of 600 milliseconds is sufficiently large to provide for longer one-way delays, such as e.g. found on some wireless links. The rationale behind transitioning to the LISTEN state after two retransmissions is that other problems, independent of establishing middlebox state, may occur (such as delay or loss of the initial DCCP-Request). Any late or retransmitted DCCP-Request packets will then still reach the server, so that connection establishment successfully completes. Fairhurst & Renker Expires March 18, 2009 [Page 14]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 4. Security Considerations The technique specified in this document exposes the state of a DCCP server that has been explicitly pre-configured to accept a connection from a known client. Establishing this state requires prior out-of- band signalling between the client and server (e.g. via the Session Initiation Protocol [RFC3261]). The technique generates a packet addressed to the expected client. This increases the vulnerability of a DCCP server, by revealing which ports are in a passive listening state (the information is not encrypted and therefore could be seen on the path to the client through the network). Servers that do not wish to disclose this information MAY refrain from generating DCCP-Listen packets, without impacting subsequent DCCP state transitions. This document requires endpoint nodes to ignore reception of DCCP- Listen packets (in any state other than LISTEN). We do not believe these changes significantly increase the complexity or vulnerability of a DCCP implementation that conforms to [RFC4340]. Fairhurst & Renker Expires March 18, 2009 [Page 15]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 5. IANA Considerations This document requires IANA action by allocation of a new Packet Type from the IANA DCCP Packet Types Registry. The decimal value 10 has been assigned to a "DCCP-Listen" packet. The Registry entry references this document. Fairhurst & Renker Expires March 18, 2009 [Page 16]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 Note to the RFC Editor: This value must be confirmed by IANA in the registry when this document is published, please then remove this note. Fairhurst & Renker Expires March 18, 2009 [Page 17]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 6. References 6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006. 6.2. Informative References [Epp05] Eppinger, J-L., "TCP Connections for P2P Apps: A Software Approach to Solving the NAT Problem", Carnegie Mellon University/ISRI Technical Report CMU-ISRI-05-104, January 2005. [FSK05] Ford, B., Srisuresh, P., and D. Kegel, "Peer-to-Peer Communication Across Network Address Translators", Proceedings of USENIX-05, pages 179-192, 2005. [Fai08] Fairhurst, G., "The DCCP Service Code", Work In Progress, draft-ietf-dccp-serv-codes-06, June 2008. [GBF+07] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. Srisuresh, "NAT Behavioral Requirements for TCP", Work In Progress, draft-ietf-behave-tcp-07, April 2007. [GF05] Guha, S. and P. Francis, "Characterization and Measurement of TCP Traversal through NATs and Firewalls", Proceedings of Internet Measurement Conference (IMC-05), pages 199- 211, 2005. [GTF04] Guha, S., Takeda, Y., and P. Francis, "NUTSS: A SIP based approach to UDP and TCP connectivity", Proceedings of SIGCOMM-04 Workshops, Portland, OR, pages 43-48, 2004. [H.323] ITU-T, "Packet-based Multimedia Communications Systems", Recommendation H.323, July 2003. [ID-Behave-DCCP] "Network Address Translation (NAT) Behavioral Requirements for DCCP", Work in Progress draft-ietf-behave-dccp-02.txt, 2008. [NAT-APP] Ford, B., Srisuresh, P., and D. Kegel, "Application Design Guidelines for Traversal through Network Address Translators", Work In Progress, draft-ford-behave-app-05, Fairhurst & Renker Expires March 18, 2009 [Page 18]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 March 2007. [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [RFC3235] Senie, D., "Network Address Translator (NAT)-Friendly Application Design Guidelines", RFC 3235, January 2002. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [Ros08] Rosenberg, J., "TCP Candidates with Interactive Connectivity Establishment (ICE)", Work In Progress, draft-ietf-mmusic-ice-tcp-07, February 2008. [TURN] Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)", Work In Progress, draft-ietf-behave-turn-09, February 2008. Fairhurst & Renker Expires March 18, 2009 [Page 19]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 Appendix A. Discussion of Existing NAT Traversal Techniques This Appendix provides a brief review of existing techniques to establish connectivity across NAT devices, with the aim of providing background information. We first consider TCP NAT traversal based on simultaneous-open, and then discuss a second technique based on role reversal. Further information can be found in [GTF04] and [GF05]. A central idea shared by these techniques is to make peer-to-peer sessions look like "outbound" sessions on each NAT device. Often a rendezvous server, located in the public address realm, is used to enable clients to discover their NAT topology and the addresses of peers. The term 'hole punching' was coined in [FSK05] and refers to creating soft state in a traditional NAT device, by initiating an outbound connection. A well-behaved NAT can subsequently exploit this to allow a reverse connection back to the host in the private address realm. UDP and TCP hole punching use nearly the same technique. The adaptation of the basic UDP hole punching principle to TCP NAT traversal was introduced in section 4 of [FSK05] and relies on the simultaneous-open feature of TCP [RFC0793]. A further difference between UDP and TCP lies in the way the clients perform connectivity checks, after obtaining suitable address pairs for connection establishment. Whereas in UDP a single socket is sufficient, TCP clients require several sockets for the same address / port tuple: o a passive socket to listen for connectivity tests from peers and o multiple active connections from the same address to test reachability of other peers. The SYN sent out by client A to its peer B creates soft state in A's NAT. At the same time, B tries to connect to A: o if the SYN from B has left B's NAT before the arrival of A's SYN, both endpoints perform simultaneous-open (4-way handshake of SYN/ SYNACK); o otherwise A's SYN may not enter B's NAT, which leads to B performing a normal open (SYN_SENT => ESTABLISHED) and A performing a simultaneous-open (SYN_SENT => SYN_RCVD => ESTABLISHED). Fairhurst & Renker Expires March 18, 2009 [Page 20]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 In the latter case, it is necessary that the NAT does not interfere with a RST segment (REQ-4 in [GBF+07]). The simultaneous-open solution is convenient due to its simplicity, and is thus a preferred mode of operation in the TCP extension for ICE ([Ros08], sec. 2). A.1. NAT traversal Based on Simultaneous-Open Among the various TCP NAT traversal approaches, simultaneous-open suggests itself due to its simplicity [GF05], [NAT-APP]. A characteristic of simultaneous-open is that the clear distinction between client and server is erased: both sides enter through active (SYN_SENT) as well as passive (SYN_RCVD) states. This characteristic is in conflict with several ideas underlying DCCP, as a clear separation between client and server has been one of the initial design decisions ([RFC4340], 4.6). Furthermore, several mechanisms implicitly rely on clearly-defined client/server roles: o Feature Negotiation: with few exceptions, almost all of DCCP's negotiable features use the "server-priority" reconciliation rule ([RFC4340], 6.3.1), whereby peers exchange their preference lists of feature values, and the server decides the outcome. o Closing States: only servers may generate CloseReq packets (asking the peer to hold timewait state), while clients are only permitted to send Close or Reset packets to terminate a connection ([RFC4340], 8.3). o Service Codes: servers may be associated with multiple service codes, while clients must be associated with exactly one ([RFC4340], 8.1.2). o Init Cookies: may only be used by the server and on DCCP-Response packets ([RFC4340], 8.1.4). The latter two points are not obstacles per se, but hinder the transition from a passive to an active socket. The assumption that "all DCCP hosts are clients", on the other hand, must be dismissed since it limits application programming. As a consequence, retro- fitting this into DCCP does not seem a very sensible idea. A.2. Role Reversal After the simultaneous-open, one of the simplest TCP NAT traversal schemes involves role traversal ([Epp05] and [GTF04]), where a peer first opens an active connection for the single purpose of punching a hole in the firewall; and then reverts to a listening socket, accepting connections arriving via the new path. Fairhurst & Renker Expires March 18, 2009 [Page 21]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 This solution has several disadvantages for DCCP. First, a DCCP server would be required to change its role temporarily to 'client'. This requires modification of settings, in particular service codes and perhaps Init Cookies. Further, the server must not yet have started feature negotiation, since its choice of initial options may rely on its role (i.e. if an endpoint knows it is the server, it can make a priori assumptions about the preference lists of features it is negotiating with the client, thereby enforcing a particular policy). Finally, the server needs additional processing to ensure that the connection coming through the listening socket matches the one for which it previously opened an active connection. We therefore do not recommend this approach for DCCP. Fairhurst & Renker Expires March 18, 2009 [Page 22]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 Appendix B. Change Log Revision 00 was based on a previous individual submission draft-fairhurst-dccp-behave-update-01 by the same authors. Revision 01: o introduced many format changes to improve readability o migrated background information into the Appendix o added Section 1.3 to summarize the document structure o updated introductory paragraph of Section 2 to account for new structure o added captions to all figures o updated the specification in Section 2 to (i) permit options on DCCP-Listen packets; (ii) explain why the presence of payload data is not useful; (iii) clarify that middleboxes must not interpret sequence numbers on DCCP-Listen packets o clarified that the default value of the Allow Short Seqno feature is to be used o added references to the service code draft [Fai08] o clarified the processing of DCCP-Listen packets by server endpoints o corrected the reaction of a client implementing [RFC4340] only - DCCP-Listen packets are treated as unknown and hence do not generate a Reset o swapped order of IANA / Security-Considerations sections o added a note in the Security Considerations section that servers may refrain from generating DCCP-Listen packets Revision 02: o minor edits following WG feedback at IETF meeting o updated to reflect ID.Behave-DCCP o update to reflect comments from Colin Perkins Fairhurst & Renker Expires March 18, 2009 [Page 23]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 o added a tentative IANA code point (as suggested at IETF-73) Fairhurst & Renker Expires March 18, 2009 [Page 24]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 Note to the RFC Editor: Please remove this Change Log when done with the document. Fairhurst & Renker Expires March 18, 2009 [Page 25]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 Authors' Addresses Godred Fairhurst University of Aberdeen School of Engineering Fraser Noble Building Aberdeen AB24 3UE Scotland Email: gorry@erg.abdn.ac.uk URI: http://www.erg.abdn.ac.uk Gerrit Renker University of Aberdeen School of Engineering Fraser Noble Building Aberdeen AB24 3UE Scotland Email: gerrit@erg.abdn.ac.uk URI: http://www.erg.abdn.ac.uk Fairhurst & Renker Expires March 18, 2009 [Page 26]
Internet-Draft DCCP Simultaneous-Open Technique Sept 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Fairhurst & Renker Expires March 18, 2009 [Page 27]
