INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 DCCP-NAT Encapsulation Internet Draft T. Phelan Document: draft-phelan-dccp-natencap-03.txt Sonus Networks Expires: May 2010 November 18, 2009 Intended status: Proposed Standard Datagram Congestion Control Protocol (DCCP) Encapsulation for NAT Traversal (DCCP-NAT) Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. 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 23, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. Phelan Expires - May 2010 [Page 1]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document specifies an alternative encapsulation of the Datagram Congestion Control Protocol (DCCP), referred to as DCCP-NAT. This encapsulation will allow DCCP to be carried through the current generation of Network Address Translation (NAT) middleboxes without modification of those middleboxes. Phelan Expires - May 2010 [Page 2]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 Table of Contents 1. Introduction...................................................4 2. Terminology....................................................4 3. DCCP-NAT.......................................................4 3.1 UDP Header.................................................5 3.2 DCCP-NAT Generic Header....................................6 3.2.1 DCCP-RAW Checksum Field...............................6 3.3 Partial Checksum Extension Header..........................7 3.4 Minimum Checksum Coverage Feature..........................7 3.5 Other DCCP Headers and Options.............................8 3.6 Service Codes and the DCCP Port Registry...................8 4. DCCP-NAT and Higher-Layer Protocols............................8 5. Signaling the Use of DCCP-NAT..................................9 5.1 SDP for RTP over DCCP......................................9 6. Security Considerations.......................................10 7. IANA Considerations...........................................10 8. References....................................................10 8.1 Normative References......................................10 9. Author's Address..............................................11 Phelan Expires - May 2010 [Page 3]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 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. In order for the [RFC4340] encapsulation to pass through Network Address Translation (NAT) devices, these devices must be updated to recognize and properly modify DCCP. This is the long-term objective for DCCP, and work is underway to specify the necessary operations. However, in the short term it would be useful to have an encapsulation for DCCP that would be compatible with NAT devices conforming to [RFC4787]. This document specifies that encapsulation, which is referred to as DCCP-NAT. For convenience, the [RFC4340] encapsulation is referred to as DCCP-RAW. 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-NAT The basic approach here is to insert a UDP ([RFC768]) "shim" layer between the IP header and a DCCP packet with a modified generic header (modified to eliminate redundancies between UDP and DCCP). 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-NAT 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 all combinations of DCCP-RAW and DCCP-NAT encapsulations with IPv4 and IPv6. Phelan Expires - May 2010 [Page 4]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 The basic format of a DCCP-NAT packet is: +-----------------------------------+ | IP Header (IPv4 or IPv6) | Variable length +-----------------------------------+ | UDP Header | 8 bytes +-----------------------------------+ | DCCP-NAT Generic Header | 12 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 [RFC768]: 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-NAT, 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-NAT 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-NAT is the DCCP-NAT datagram). For DCCP-NAT, when the UDP Checksum is non-zero, Length MUST be at least the size of the UDP header (8 bytes) plus the minimum size of a DCCP-NAT header (12 bytes), for a total minimum value of 20 bytes. When the UDP Checksum is zero, the DCCP-NAT header MUST also contain a Partial Checksum Extension Header, therefore the minimum DCCP-NAT header is 16 bytes and the total minimum is 24 bytes. Received packets with a UDP Length of less than the applicable minimum length MUST be ignored. Phelan Expires - May 2010 [Page 5]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 Checksum: 16 bits This field is the Internet checksum of a network-layer pseudoheader and the entire UDP packet. For DCCP-NAT, a packet with a checksum field equal to 0 that does not contain a Partial Checksum Extension Header, or contains an invalid Partial Checksum Extension Header MUST be ignored as incorrect checksum. 3.2 DCCP-NAT Generic Header Unlike the DCCP-RAW generic header, the DCCP-NAT generic header takes only one form; it does not support short sequence numbers. Its format is as follows: 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 | Type | Sequence Number (high bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . Sequence Number (low bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ All DCCP-NAT generic header fields function as specified in [RFC4340]. 3.2.1 DCCP-RAW Checksum Field For DCCP-NAT, the function of the DCCP-RAW generic header field Checksum is performed by the UDP Checksum field. If the UDP Checksum field in a received packet is non-zero and is invalid, that packet MUST be ignored as per the invalid checksum procedures of DCCP-RAW (i.e., the options in the packet MUST NOT be processed). If the UDP Length field in a received packet is less than the length of the UDP header plus the entire DCCP-NAT header (including the generic header, Partial Checksum Extension Header if present, type- specific fields and options), or the UDP Length field is greater than the length of the packet from the beginning of the UDP header to the end of the packet, that packet MUST also be ignored as per the invalid checksum procedures. If the UDP Checksum field is zero, then the Partial Checksum Extension Header procedures apply. See section 3.3 for more information. Phelan Expires - May 2010 [Page 6]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 3.3 Partial Checksum Extension Header If the UDP Checksum field is zero, the DCCP-NAT generic header MUST be immediately followed by a Partial Checksum Extension Header. Additional type-specific header fields and DCCP Options would then follow the Partial Checksum Extension Header. If the UDP Checksum field is non-zero the Partial Checksum Extension Header MUST NOT be included. The format of the header is as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum Coverage | PChecksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The fields are defined as follows: Checksum Coverage: 16 bits This is the number of bytes of user data that are covered by the partial checksum. It MUST NOT be greater than the entire length of the user data (from the end of the DCCP-NAT header, including options, to the end of the packet). Packets whose Checksum Coverage fields are greater than the length of the user data MUST be ignored as incorrect checksum. PChecksum: 16 bits This is the Internet checksum of the DCCP-NAT header plus Checksum Coverage bytes of the user data. Using the TCP/IP checksum algorithm, the PChecksum field is first set to zero. If the Checksum Coverage field is odd, the data to be summed is extended by one byte set to zero. This byte does not overwrite the corresponding byte in the DCCP-NAT packet, and is not transmitted. The PChecksum field is then set to the one's complement of the one's complement sum of the sixteen-bit words covered (DCCP header plus Checksum Coverage bytes of user data plus one zero byte if Checksum Coverage is odd). Note that PChecksum does not include an IP pseudoheader. Packets with invalid PChecksum fields MUST be ignored as incorrect checksum. 3.4 Minimum Checksum Coverage Feature The Minimum Checksum Coverage Feature lets a DCCP endpoint determine whether its peer is willing to accept packets with partial checksum coverage. It takes values from 0 to 15. For DCCP-NAT the feature values are interpreted as follows: o Minimum Checksum Coverage = 0, the peer will not accept packets with partial checksum. All UDP Checksum fields should be non- zero and the Partial Checksum Extension Header is never included. Phelan Expires - May 2010 [Page 7]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 o Minimum Checksum Coverage > 0, the peer will accept packets with partial checksum as long as the Checksum Coverage field is at least (Minimum Checksum Coverage - 1)*4. As in DCCP-RAW, peers may refuse to process packets with unacceptable Checksum Coverage. Such packets SHOULD be reported using Data Dropped options with Drop Code 0, Protocol Constraints. 3.5 Other DCCP Headers and Options All type-specific DCCP headers are as in DCCP-RAW, except that the short sequence number version of the acknowledgement header is not supported. All option and feature encodings are as in DCCP-RAW. 3.6 Service Codes and the DCCP Port Registry There is one Service Code registry and one DCCP port registry and they 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. 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. Since the port registry supports multiple applications registering the same port (as long as the Service Codes are different), other applications MAY register on the same port, but those registrations are also applicable to all combinations of encapsulation and IP version. 4. DCCP-NAT and Higher-Layer Protocols In general, the encapsulation of a higher-layer protocol within DCCP SHOULD be the same in both DCCP-RAW and DCCP-NAT. At this time, encapsulations of DTLS over DCCP, defined in [RFC5238] and RTP over DCCP, defined in [RTP-DCCP], have been already defined. The encapsulations of those protocols in DCCP-NAT 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-RAW and DCCP-NAT. If a document does not specify different encapsulations for DCCP-RAW and Phelan Expires - May 2010 [Page 8]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 DCCP-NAT, the specified encapsulation SHALL apply to both DCCP-RAW and DCCP-NAT. 5. Signaling the Use of DCCP-NAT 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 [RTP-DCCP] 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 [RTP-DCCP] 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-encap:" dccp-encap-type dccp-encap-type = dccp-raw-encap / dccp-nat-encap dccp-raw-encap = "dccp-raw" dccp-nat-encap = "dccp-nat" [":" udp-port-num] udp-port-num = *DIGIT where *DIGIT is as defined in [RFC5234]. For example: o To specify a connection that will use DCCP-RAW encapsulation use: a=dccp-encap:dccp-raw o To specify a connection that will use DCCP-NAT encapsulation, with the DCCP-NAT service running on the default UDP port for DCCP-NAT defined in section 7 use: a=dccp-encap:dccp-nat o To specify a connection that will use DCCP-NAT encapsulation, with the DCCP-NAT service running UDP port 50,000 use: a=dccp-encap=dccp-nat:50000 The absence of an "a=dccp-encap" attribute SHALL be interpreted as "a=dccp-encap:dccp-raw". Phelan Expires - May 2010 [Page 9]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 Note that the port number in the media description (m=) always specifies the DCCP port number. 6. Security Considerations DCCP-NAT provides all of the security risk-mitigation measures present in DCCP-RAW, and also all of the security risks, except those associated with short sequence numbers (since DCCP-NAT does not support that feature). The purpose of DCCP-NAT 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-NAT 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 has been placed with IANA for the dccp-nat service port in UDP. The following new SDP attribute ("att-field") is to be registered: Contact name: Tom Phelan <tphelan@sonusnet.com> Attribute name: dccp-encap Long-form attribute name in English: DCCP encapsulation type Type of attribute: Media level Subject to charset attribute? No Purpose of the attribute: See this document section 5.1 Allowed attribute values: See this document section 5.1 8. References 8.1 Normative References [RFC4340] Kohler, E., Handley, M., Floyd, S., "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006. Phelan Expires - May 2010 [Page 10]
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009 [RFC768] Postel, J., "User Datagram Protocol", RFC 768, August 1980. [RFC4787] Audet, F., Jennings, C., "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", RFC 4787, January 2007. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [RTP-DCCP] Perkins, C., "RTP and the Datagram Congestion Control Protocol (DCCP)", draft-ietf-dccp-rtp-07.txt, June 2007. [RFC5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP)", RFC 5238, May 2008. [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 5234, October 2005. 9. Author's Address Tom Phelan Sonus Networks 7 Technology Park Dr. Westford, MA USA 01886 Phone: 978-614-8456 Email: tphelan@sonusnet.com Phelan Expires - May 2010 [Page 11]