DCCP Working Group G. Fairhurst Internet-Draft G. Renker Intended status: Standards Track University of Aberdeen Expires: August 21, 2008 February 18, 2008 DCCP Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal draft-ietf-dccp-simul-open-00 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 August 21, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Fairhurst & Renker Expires August 21, 2008 [Page 1]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 Abstract This document specifies an update to the Datagram Congestion Control Protocol (DCCP), a connection-oriented and datagram-based transport protocol. The update assists DCCP applications which need to communicate through one or more middleboxes (e.g. Network Address Translators 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. Discussion of Existing NAT Traversal Techniques . . . . . 4 1.3.1. Near Simultaneous-Open of Connections . . . . . . . . 5 1.3.2. Role Reversal . . . . . . . . . . . . . . . . . . . . 6 2. Procedure for Near-Simultaneous Open . . . . . . . . . . . . . 8 2.1. Conventions and Terminology . . . . . . . . . . . . . . . 8 2.2. DCCP-Listen Packet Format . . . . . . . . . . . . . . . . 8 2.3. Protocol Method . . . . . . . . . . . . . . . . . . . . . 10 2.3.1. Protocol Method for DCCP-Server Endpoints . . . . . . 10 2.3.2. Protocol Method for DCCP-Client Endpoints . . . . . . 12 2.3.3. Processing by Routers and Middleboxes . . . . . . . . 12 2.4. Examples of Use . . . . . . . . . . . . . . . . . . . . . 12 2.5. Backwards Compatibility with RFC 4340 . . . . . . . . . . 14 3. Discussion of Design Decisions . . . . . . . . . . . . . . . . 15 3.1. Rationale for a New Packet Type . . . . . . . . . . . . . 15 3.2. Generation of Listen Packets . . . . . . . . . . . . . . . 16 3.3. Repetition of Listen Packets . . . . . . . . . . . . . . . 16 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 5. Security Considerations . . . . . . . . . . . . . . . . . . . 20 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 6.1. Normative References . . . . . . . . . . . . . . . . . . . 21 6.2. Informative References . . . . . . . . . . . . . . . . . . 21 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 Intellectual Property and Copyright Statements . . . . . . . . . . 25 Fairhurst & Renker Expires August 21, 2008 [Page 2]
Internet-Draft DCCP Simultaneous-Open Technique February 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; and thus NAT traversal of DCCP faces the same problems as TCP NAT traversal, without being able to simply reuse UDP-based NAT traversal techniques. In addition, DCCP has the disadvantage of not being able to perform a simultaneous-open, a TCP-inherent characteristic which greatly simplifies NAT traversal. After discussing the problem space for DCCP, this document specifies an extension to facilitate DCCP NAT traversal, by explicitly supporting a widely implemented traversal principle known as 'hole punching'. This extension produces the same outward effect as an simultaneous-open, but without internal changes to the standard operational procedure of DCCP. The extension uses a dedicated indicator message, whose usage is tied to a specific condition, can thus be turned off, and is inter-operable with non-extended hosts. The object of this extension is in built-in support for middlebox traversal, to reduce reliance on external aids such as data relay servers. 1.1. Scope of this Document The technique described by this document applies to scenarios where one or both DCCP peers are located behind a middlebox. This document is specifically targeted at NAT traversal. However, due to the similarity of involved principles, the technique and presented extension of DCCP may also be of similar use to the traversal of other types of middlebox, such as firewalls. The proposed extension is relevant to both client/server and peer-to- peer applications, such as VoIP, file sharing, or online gaming. It 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]). For the scope of this document we assume traditional (outbound) types 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 Fairhurst & Renker Expires August 21, 2008 [Page 3]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 are BEHAVE-compliant [NAT-APP]. This memo does not discuss behavioural requirements of NAT devices to support DCCP traversal. These may be described by a separate document. We further assume that NAT devices provide only a minimum of DCCP protocol support, in that layer-4 checksums are updated to account for changes in the pseudo-header. 1.2. Scope of the Problem to be Tackled We refer 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] can perform NAT traversal with the help of an external data relay server. The extension described in this document facilitates NAT traversal without indirection via relay servers. 1.3. Discussion of Existing NAT Traversal Techniques This section is a brief review of existing techniques to establish connectivity across NAT devices, the basic idea being 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 Fairhurst & Renker Expires August 21, 2008 [Page 4]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 allow a reverse connection back to the host in the private address realm. The adaptation of the basic hole punching principle to TCP NAT traversal was introduced in section 4 of [FSK05] and relies on the simultaneous-open feature of TCP [RFC0793]. UDP and TCP hole punching use nearly the same technique. The main difference lies in the way the clients perform connectivity checks, after obtaining the address pairs from the server. 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 connectivity 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). 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 (section 2 of [Ros07]). We note that a simultaneous-open is not the only existing solution for TCP NAT traversal [GTF04], [GF05]; other approaches are reviewed in the next subsection. 1.3.1. Near Simultaneous-Open of Connections 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 Fairhurst & Renker Expires August 21, 2008 [Page 5]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 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 simultaneous-open into DCCP does not seem a very sensible idea. 1.3.2. 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, to accept incoming connections arriving via the new path. 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 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). Lastly, the server needs additional processing to ensure that the connection coming through the listening socket matches the one for Fairhurst & Renker Expires August 21, 2008 [Page 6]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 which it previously opened an active connection. We therefore do not recommend this approach for DCCP. Fairhurst & Renker Expires August 21, 2008 [Page 7]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 2. Procedure for Near-Simultaneous Open This section presents the packet-processing details of the simultaneous-open technique for DCCP. The technique does not employ role reversal - both endpoints start out with their designated roles, as specified in [RFC4340]. Neither does it require protocol support for a genuinely simultaneous handshake. The presented extension 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 The document updates DCCP by adding a new 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ DCCP-Listen Packet Format A DCCP-Listen Packet MUST NOT include any DCCP options (since this Fairhurst & Renker Expires August 21, 2008 [Page 8]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 packet does not modify the receiver protocol state) and also MUST NOT include application data. Therefore Data Offset MUST be set to 5, the length of the DCCP-Listen packet in 32-bit words. Furthermore, the following protocol fields MUST all be set to zero: CCVal (the connection has not been established), CsCov (there is no payload). A server conforming to this revision of the specification SHOULD set both Sequence Number fields to 0; clients MUST ignore the value of the Sequence Number fields; and middleboxes SHOULD NOT interpret sequence numbers on DCCP-Listen packets. 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 value XX-IANA-assigned-XX, which indicates that this is a DCCP-Listen packet. Fairhurst & Renker Expires August 21, 2008 [Page 9]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 Note to the RFC Editor: Please replace XX-IANA-assigned-XX in the above paragraph with the value assigned in the registry and remove this note. ==> End of note to the RFC Editor. <== DCCP-Listen packets use a single format only and therefore do not support the alternative use of short sequence numbers defined in section 5.1 of [RFC4340]. Hence X MUST be set to 1, and all 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 the DCCP-Listen packet. Since the server may use multiple service codes, the value of the Service Code field needs to be communicated out-of-band, from client to server, prior to sending the DCCP-Listen packet. 2.3. Protocol Method We use the term "session" 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 that may be offered via the same port. 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 endpoints 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 conditionally applies to the way the server performs passive-open. Prior to connection setup, it is common for DCCP-server endpoints to not be fully specified: before the connection is established, a server usually sets the target IP-address:port to wildcard numbers (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 Fairhurst & Renker Expires August 21, 2008 [Page 10]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 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 passive-open from CLOSED by inviting the remote client to connect, via a single DCCP-Listen packet. The packet is sent to the specified remote IP-adress:port, using the format specified in Section 2.2. The server then transitions to INVITED. (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. The 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 per [RFC4340]. Fairhurst & Renker Expires August 21, 2008 [Page 11]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 Fully-specified server endpoints SHOULD treat ICMP error messages received in reply to a DCCP-Listen packet as "soft errors" that do not cause a state transition. Any server receiving a DCCP-Listen packet in the LISTEN state MUST reply with a Reset (using Reset Code 7, "Connection Refused"), which is the expected behaviour with regard to [RFC4340]. Listen packets received in any other state MUST be ignored (cf. next subsection). 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 rules for the reception of DCCP-Listen packets by clients. Clients MUST silently discard any received DCCP-Listen packets, regardless of their current state. The packet indicates only a willingness to accept a connection: if the client has already established a connection (OPEN or PARTOPEN), this has no meaning. If a client is awaiting the response to a DCCP-Request, it does not need to take specific action. While in state REQUEST, other than ignoring DCCP-Listen packets, it MUST use the protocol method defined in [RFC4340]. This ensures that retransmissions will happen in the expected manner. 2.3.3. Processing by Routers and Middleboxes Routers and middleboxes both act as forwarding agents for DCCP packets. This document does not specify rules for forwarding DCCP packets. We note, however, that DCCP-Listen packets do not require special treatment, and should therefore be forwarded end-to-end across Internet paths. 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 to the DCCP-Listen packet: in combination with the source and destination addresses found in the enclosing IP-header, the DCCP-Listen packet thereby communicates all the information necessary 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. Fairhurst & Renker Expires August 21, 2008 [Page 12]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 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 mechanism, whereupon the client and server are started. DCCP A initiates a connection by sending a DCCP-Request. DCCP B actively indicates its state by sending a 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 --> +--+-+----+-+->| | +------------------+ +-+ +-+ +-----------------+ Sequence of events when a client is started before the server The diagram below reverses this sequencing: DCCP A DCCP B ------ NA NB ------ +------------------+ +-+ +-+ +-----------------+ |(1) Initiation | | | | | | | | | | |X---+-+--+<-- DCCP-Listen | |DCCP-Request --> +--+-+----+-+->| | | | <+-+----+-+--+<-- DCCP-Response| |DCCP-Ack --> +--+-+----+-+> | | | | | | | | | | |(2) Data transfer | | | | | | | |DCCP-Data --> +--+-+----+-+> | | +------------------+ +-+ +-+ +-----------------+ Sequence of events when a server is started before the client Fairhurst & Renker Expires August 21, 2008 [Page 13]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 2.5. Backwards Compatibility with RFC 4340 This document updates the connection-establishment procedures of [RFC4340]. There are no changes if a client implementing the extensions described in this document communicates with a server conforming to [RFC4340]. This document also 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 receipt of a DCCP-Listen packet by a client that implements only [RFC4340] would lead to a DCCP-Reset (likely using code 4, "Packet Error" if the unknown packet type passes through). This would abort the connection. The authors do not however expect these compatibility issues to introduce practical deployment problems. Fairhurst & Renker Expires August 21, 2008 [Page 14]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 3. Discussion of Design Decisions 3.1. Rationale for a New Packet Type The DCCP-Listen packet specified in Section 2.2 has the same format as the DCCP-Request packet ([RFC4340], sec. 5.1), the only difference is in the value of the Type field. The usage however differs, since the DCCP-Listen serves as advisory message, not as part of the actual connection setup: sequence numbers have no meaning, and neither options nor payload are present. It is important to point out that a DCCP-Request packet could in theory also serve as indicator message, in the same way as the DCCP- Listen packet. The following arguments were against this alternative. The first problem is that of semantic overloading: the Request is defined in [RFC4340] to serve a well-defined purpose, as the first packet of the 3-way handshake. Additionally using it in the way as here specified for the DCCP-Listen packet would require DCCP processors to have two different processing paths: one where a Request is interpreted as part of the initial handshake, and one where the same packet is interpreted as indicator message. This complicates 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 Reset if it has not yet entered the REQUEST state. This document addresses that issue by asking clients to ignore DCCP-Listen packets in any state. Adding a similar rule for the Request packet is more cumbersome: clients can not 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 DCCP-Listen 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. However, sequence numbers are ignored for this type of message, so additional and complicating processing becomes necessary: either to ask the client not to respond to a Request when the Request of type "indicator message"; or ask middleboxes and servers to ignore Sync packets generated in response to Request packets serving as indicator message. Furthermore, since no initial sequence numbers have been negotiated yet, sending a SyncAck would not be meaningful. Using a separate packet type allows simpler and clearer processing. Fairhurst & Renker Expires August 21, 2008 [Page 15]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 The rationale for ignoring the Sequence Number fields on DCCP-Listen packets is that endpoints have not yet entered connection setup: the Listen packet is sent out 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 Sequence Number fields thus do not serve a purpose, both 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 has been 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 proposal does not perform a role-change from passive to active. Like TCP, we require that DCCP-Listen packets are only sent by a DCCP-server when the endpoint is fully specified (Section 2.3). 3.3. Repetition of Listen Packets Repetition is a necessary requirement to increase robustness and the chance of successful connection establishment, in case a Listen packet is lost due to congestion, link loss or some other reason. Recommending a maximum number of 3 timeouts (2 repetitions) is due to the following considerations. The repeated copies need to be spaced sufficiently far apart in time to avoid suffering from correlated loss. The interval of 200ms has been chosen to accommodate a wide range of wired and wireless network paths. Another constraint is given by the retransmission interval for the DCCP-Request. To establish state, intermediate systems need to receive a (retransmitted) DCCP-Listen packet before the DCCP-Request times out (1 second, cf. section 8.1.1 of [RFC4340]). With three timeouts, each spaced 200 milliseconds apart, the overall time is still less than this value. On the other hand, the sum of 600 Fairhurst & Renker Expires August 21, 2008 [Page 16]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 milliseconds is sufficiently large to provide for large 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 in such cases still reach the server, thus allowing connection establishment to succeed. Fairhurst & Renker Expires August 21, 2008 [Page 17]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 4. IANA Considerations This document requires IANA action by allocation of a new Packet Type from the IANA DCCP Packet Types Registry. The name of the Packet Type is "DCCP-Listen" packet, and its type field is set to XX-IANA- assigned-XX. The Registry entry is to reference this document. Fairhurst & Renker Expires August 21, 2008 [Page 18]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 Note to the RFC Editor: Please replace XX-IANA-assigned-XX in the above paragraph with the value assigned in the registry and remove this note. Fairhurst & Renker Expires August 21, 2008 [Page 19]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 5. Security Considerations The method specified in this document exposes the state of a DCCP server that has been explicitly configured to accept a connection from a known client. Establishing this state requires prior out-of- band signaling between the client and server (e.g. via SIP). The technique generates a packet addressed to the expected client. This increases the vulnerability of the 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). 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 August 21, 2008 [Page 20]
Internet-Draft DCCP Simultaneous-Open Technique February 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. [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. [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, 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. Fairhurst & Renker Expires August 21, 2008 [Page 21]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 [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. [Ros07] Rosenberg, J., "TCP Candidates with Interactive Connectivity Establishment (ICE)", Work In Progress, draft-ietf-mmusic-ice-tcp-05, November 2007. [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-06, January 2008. Fairhurst & Renker Expires August 21, 2008 [Page 22]
Internet-Draft DCCP Simultaneous-Open Technique February 2008 Appendix A. Change Log Revision 00 retrieved from previous individual submission draft-fairhurst-dccp-behave-update-01 by the same authors. Fairhurst & Renker Expires August 21, 2008 [Page 23]
Internet-Draft DCCP Simultaneous-Open Technique February 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 August 21, 2008 [Page 24]
Internet-Draft DCCP Simultaneous-Open Technique February 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. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Fairhurst & Renker Expires August 21, 2008 [Page 25]
