Datagram Congestion Control T. Phelan Protocol Sonus Internet-Draft August 26, 2010 Intended status: Experimental Expires: February 27, 2011 Datagram Congestion Control Protocol (DCCP) Encapsulation in UDP for NAT Traversal (DCCP-UDP) draft-ietf-dccp-udpencap-02 Abstract This document specifies an alternative encapsulation of the Datagram Congestion Control Protocol (DCCP), referred to as DCCP-UDP. This encapsulation will allow DCCP to be carried through the current generation of Network Address Translation (NAT) middleboxes without modification of those middleboxes. 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 February 27, 2011. Copyright Notice Copyright (c) 2010 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 Phelan Expires February 27, 2011 [Page 1] Internet-Draft DCCP-UDP Encapsulation August 2010 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. DCCP-UDP . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. UDP Header . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. DCCP Generic Header . . . . . . . . . . . . . . . . . . . . 5 3.3. DCCP-UDP Checksum Procedures . . . . . . . . . . . . . . . 5 3.3.1. Partial Chescksums and the Minimum Checksum Coverage Feature . . . . . . . . . . . . . . . . . . . 6 3.4. Explicit Congestion Notification . . . . . . . . . . . . . 6 3.5. Path Maximum Transmission Unit Discovery . . . . . . . . . 6 3.6. Service Codes and the DCCP Port Registry . . . . . . . . . 6 4. DCCP-UDP and Higher-Layer Protocols . . . . . . . . . . . . . . 7 5. Signaling the Use of DCCP-UDP . . . . . . . . . . . . . . . . . 7 5.1. SDP for RTP over DCCP . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 Phelan Expires February 27, 2011 [Page 2] Internet-Draft DCCP-UDP Encapsulation August 2010 1. Introduction The Datagram Congestion Control Protocol (DCCP), specified in [RFC4340], is a transport-layer protocol that provides upper layers with the capability of using unreliable but congestion controlled flows. According to [RFC4340], DCCP packets are directly encapsulated in IPv4 or IPv6 packets. [RFC5597] specifies how DCCP should be handled by NAT devices, but there is a significant installed base of NAT devices that do not support [RFC5597]. In the short term, it would be useful to have an encapsulation for DCCP that is compatible with the installed base of NAT devices that supports [RFC4787] but do not support [RFC5597]. This document specifies that encapsulation, which is referred to as DCCP-UDP. For convenience, the [RFC4340] encapsulation (updated by [RFC5596] as required) is referred to as DCCP-STD. The DCCP-UDP encapsulation specified here supports all of the features contained in DCCP-STD except for partial checksums. 2. Terminology 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. DCCP-UDP The basic approach here is to insert a UDP ([RFC0768]) header between the IP header and the DCCP packet. Note that this is not strictly a tunneling approach. The IP addresses of the communicating end systems are carried in the IP header (which could be modified by NAT devices) and there are no other IP addresses embedded. Devices offering or using DCCP services via DCCP-UDP encapsulation listen on a UDP port (default port awaiting IANA action) for incoming packets and pass received packets along to the DCCP protocol. DCCP implementations MAY allow services to be simultaneously offered over any or all combinations of DCCP-STD and DCCP-UDP encapsulations with IPv4 and IPv6. The basic format of a DCCP-UDP packet is: Phelan Expires February 27, 2011 [Page 3] Internet-Draft DCCP-UDP Encapsulation August 2010 +-----------------------------------+ | IP Header (IPv4 or IPv6) | Variable length +-----------------------------------+ | UDP Header | 8 bytes +-----------------------------------+ | DCCP Generic Header | 12 or 16 bytes +-----------------------------------+ | Additional (type-specific) Fields | Variable length (could be 0) +-----------------------------------+ | DCCP Options | Variable length (could be 0) +-----------------------------------+ | Application Data Area | Variable length (could be 0) +-----------------------------------+ 3.1. UDP Header The format of the UDP header is taken from [RFC0768]: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Length | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ For DCCP-UDP, the fields are interpreted as follows: Source and Dest(ination) Ports: 16 bits each These fields identify the UDP ports on which the source and destination (respectively) of the packet are listening for incoming DCCP-UDP packets (normally both are the default port to be assigned by IANA). Note that they do not identify the DCCP source and destination ports. Length: 16 bits This field is the length of the UDP datagram, including the UDP header and the payload (which for DCCP-UDP is the DCCP-UDP datagram). Checksum: 16 bits This field is the Internet checksum of a network-layer pseudoheader and the UDP packet. Phelan Expires February 27, 2011 [Page 4] Internet-Draft DCCP-UDP Encapsulation August 2010 3.2. DCCP Generic Header As defined in [RFC4340], the DCCP Generic Header takes two forms, one with long sequence numbers (48 bits) and the other with short sequence numbers (24 bits). 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | |X| | . | Res | Type |=| Reserved | Sequence Number (high bits) . | | |1| | . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number (low bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ and 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | |X| | | Res | Type |=| Sequence Number (low bits) | | | |0| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ All generic header fields, except for Checksum field, have the same meaning as specified in [RFC4340] and the update specified in [RFC5596]. 3.3. DCCP-UDP Checksum Procedures For DCCP-UDP, the function of the DCCP Checksum field is performed by the UDP checksum field. On transmit, the DCCP Checksum field SHOULD be set to zero. On receive, the DCCP Checksum field MUST be ignored. If the UDP Length field is less than 20 (UDP Header length and minimum DCCP-NAT header length), the packet MUST be dropped. If the UDP Checksum field, computed using standard UDP methods, is Phelan Expires February 27, 2011 [Page 5] Internet-Draft DCCP-UDP Encapsulation August 2010 invalid, the packet MUST be dropped. If the UDP Checksum field is zero, the packet MUST be dropped. 3.3.1. Partial Chescksums and the Minimum Checksum Coverage Feature DCCP-UDP supports the syntax of partial checksums (it supports negotiation of the Minimum Checksum Coverage feature and settings of the CsCov field), but, since the UDP checksum field always covers the entire packet, not the function. 3.4. Explicit Congestion Notification DCCP-UDP implementations SHOULD follow the procedures of DCCP-STD section 12 by setting the ECN fields in the IP Headers of outgoing packets and examining the values received in the ECN fields of incoming packets. Implementations that do not support ECN MUST follow the procedures in DCCP-STD section 12.1 for implementations that are not ECN capable. 3.5. Path Maximum Transmission Unit Discovery DCCP-UDP implementations SHOULD follow DCCP-STD section 14 with regard to maximum packet size and Path Maximum Transmission Unit Discovery (PMTUD). 3.6. Service Codes and the DCCP Port Registry There is one Service Code registry and one DCCP port registration that apply to all combinations of encapsulation and IP version. A DCCP Service Code specifies an application using DCCP regardless of the combination of DCCP encapsulation and IP version. An application MAY choose not to support some combinations of encapsulation and IP version, but its Service Code will remain registered for those combinations and MUST NOT be used by other applications. An application SHOULD NOT register different Service Codes for different combinations of encapsulation and IP version. See [RFC5595] for more information on DCCP Service Codes Similarly, a port registration is applicable to all combinations of encapsulation and IP version. Again, an application MAY choose not to support some combinations of encapsulation and IP version on its registered port, although the port will remain registered for those combinations. Applications SHOULD NOT register different ports just for the purpose of using different encapsulation combinations. Phelan Expires February 27, 2011 [Page 6] Internet-Draft DCCP-UDP Encapsulation August 2010 4. DCCP-UDP and Higher-Layer Protocols In general, the encapsulation of a higher-layer protocol within DCCP SHOULD be the same in both DCCP-STD and DCCP-UDP. At this time, encapsulations of DTLS over DCCP, defined in [RFC5238] and RTP over DCCP, defined in [RFC5762], have been already defined. The encapsulations of those protocols in DCCP-UDP SHALL be the same as specified in those documents. Higher-layer protocols that require different encapsulations for different DCCP modes MUST justify the reasons for the difference and MUST specify the encapsulations for both DCCP-STD and DCCP-UDP. If a document does not specify different encapsulations for DCCP-STD and DCCP-UDP, the specified encapsulation SHALL apply to both DCCP-STD and DCCP-UDP. 5. Signaling the Use of DCCP-UDP Applications often signal transport connection parameters through outside means, such as the Session Description Protocol (SDP). Applications that define such methods for DCCP MUST define how the DCCP encapsulation is chosen, and MUST allow either type of encapsulation to be signaled. 5.1. SDP for RTP over DCCP [RFC5762] defines SDP extensions for signaling RTP over DCCP connections. Since it predates this document, it does not define a method for determining the DCCP encapsulation type. This document updates [RFC5762] to add a method for determining the DCCP encapsulation type. A new SDP attribute "dccp-encap" is defined for signaling the DCCP encapsulation according to the following ABNF [RFC5234]: dccp-encap-attr = %x61 "=dccp-in-udp" [":" udp-port-num] udp-port-num = *DIGIT where *DIGIT is as defined in [RFC5234]. The presence of "a=dccp-in-udp" in an SDP offer indicates that the offerer is listening for DCCP-UDP connections on the indicated UDP port (if udp-port-num is included) or on the IANA allocated port for the DCCP-UDP service if no port is included. The absence of "a=dccp-in-udp" in an SDP offer indicates that the Phelan Expires February 27, 2011 [Page 7] Internet-Draft DCCP-UDP Encapsulation August 2010 offerer is listening for DCCP-STD connections. The presence of "a=dccp-in-udp" conveys no information about whether or not the offerer is listening for DCCP-STD connections. 6. Security Considerations DCCP-UDP provides all of the security risk-mitigation measures present in DCCP-STD, and also all of the security risks. The purpose of DCCP-UDP is to allow DCCP to pass through NAT devices, and therefore it exposes DCCP to the risks associated with passing through NAT devices. It does not create any new risks with regard to NAT devices. DCCP-UDP may also allow DCCP applications to pass through existing firewall devices, if the administrators of the devices so choose. The option is a binary one however; either allow all DCCP applications or allow none. Proper control of DCCP application-by- application will require enhancements to firewalls. 7. IANA Considerations A port allocation request will be placed with IANA for the dccp-udp service port in UDP. The following new SDP attribute ("att-field") is to be registered: Contact name: Tom Phelan Attribute name: dccp-in-udp Long-form attribute name in English: DCCP in UDP Encapsulation Type of attribute: Media level Subject to charset attribute? No Purpose of the attribute: See this document section Section 5.1 Allowed attribute values: See this document section Section 5.1 8. References [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. Phelan Expires February 27, 2011 [Page 8] Internet-Draft DCCP-UDP Encapsulation August 2010 [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. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP)", RFC 5238, May 2008. [RFC5595] Fairhurst, G., "The Datagram Congestion Control Protocol (DCCP) Service Codes", RFC 5595, September 2009. [RFC5596] Fairhurst, G., "Datagram Congestion Control Protocol (DCCP) Simultaneous-Open Technique to Facilitate NAT/ Middlebox Traversal", RFC 5596, September 2009. [RFC5597] Denis-Courmont, R., "Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol", BCP 150, RFC 5597, September 2009. [RFC5762] Perkins, C., "RTP and the Datagram Congestion Control Protocol (DCCP)", RFC 5762, April 2010. Author's Address Tom Phelan Sonus Networks 7 Technology Dr. Westford, MA 01886 US Phone: +1 978 614 8456 Email: tphelan@sonusnet.com Phelan Expires February 27, 2011 [Page 9]