Internet Engineering Task Force C. Perkins INTERNET DRAFT IBM 25 October 1995 Minimal Encapsulation within IP draft-ietf-mobileip-minenc-01.txt Status of This Memo This document is a submission by the Mobile-IP Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the mobile-ip@tadpole.com mailing list. Distribution of this memo is unlimited. This document is an Internet-Draft. 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 not appropriate to use Internet Drafts as reference material, or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the internet-drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Abstract This document specifies a method by which an IP datagram may be encapsulated (carried as payload) within an IP datagram, without incurring all the overhead of using a standard IP header. Encapsulation is suggested as a means to effect "re-addressing" datagrams (i.e, delivering them to an intermediate destination other than that specified in the IP destination field) for any of a variety of reasons, but particularly those useful for adherence to the mobile-IP specification. Perkins Expires 25 April 1996 [Page i] Internet Draft Minimal Encapsulation for IP 25 October 1995 1. Introduction This document specifies a method by which an IP datagram may be encapsulated (carried as payload) within an IP datagram, without incurring all the overhead of using a standard IP header, as specified in [5], Encapsulation is suggested as a means to effect "re-addressing" datagrams -- that is, delivering them to an intermediate destination other than that specified in the IP destination field. The process of encapsulation and decapsulation a datagram is frequently referred to as "tunneling" the datagram, and the encapsulator and decapsulator are then considered to be the the "endpoints" of the tunnel. 2. Motivation The mobile-IP working group has specified the use of encapsulation as a way to deliver packets from a mobile host's "home network" to an agent which can deliver packets to the mobile host by conventional means [1]. The use of encapsulation may also be desirable whenever the source (or an intermediate router) of an IP datagram must influence the route by which a datagram is to be delivered to its ultimate destination. Other possible applications include preferential billing, choice of routes with selected security attributes, and general policy routing. See [5] for a discussion concerning the advantages of encapsulation versus source routing. Since using IP headers to encapsulate IP datagrams requires the unwarranted duplication of several fields within the inner IP header, it is possible to save some additional space by specifying a new encapsulation mechanism that eliminates the duplication. The scheme outlined in this protocol specification comes from the mobile-IP working group (in earlier Internet Drafts), and is similar to that which had been outlined in [3]. 3. Minimal Encapsulation A minimal forwarding header is defined for datagrams which are not fragmented prior to encapsulating. Use of this encapsulating method is optional. Minimal encapsulation must not be used when an original datagram is already fragmented, since there is no room in the inner header to store fragmentation information. Perkins Expires 25 April 1996 [Page 1] Internet Draft Minimal Encapsulation for IP 25 October 1995 The minimal encapsulation process produces a datagram structured as shown below; the IP header of the original datagram is modified, then followed by the minimal forwarding header, followed by the unmodified IP payload of the original datagram. +---------------------------+ +---------------------------+ | IP Header | | Modified IP Header | +---------------------------+ ====> +---------------------------+ | | | Minimal Forwarding Header | | IP Payload | +---------------------------+ | | | | +---------------------------+ | IP Payload | | | +---------------------------+ Encapsulation is performed as follows. The protocol field in the IP header is replaced by protocol number 55 for the minimal encapsulation protocol. The destination field in the IP header is replaced by the care-of address of the mobile node. If the encapsulating agent is not the original source of the datagram, the source field in the IP header is replaced by the IP address of the encapsulating agent. When decapsulating a datagram, the fields in the forwarding header are restored to the IP header, and the forwarding header is removed from the datagram. The format of the minimal forwarding 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol |S| reserved | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Original Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : Original Source Address : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Protocol Copied from the protocol field in the original IP header. Perkins Expires 25 April 1996 [Page 2] Internet Draft Minimal Encapsulation for IP 25 October 1995 S Source field present bit, which indicates whether the Original Source Address field is present. 0 not present. 1 present. reserved Sent as zero; ignored on reception. Header Checksum The 16-bit one's complement of the one's complement sum of the encapsulation header. For computing the checksum, the checksum field is set to 0. Original Destination Address Copied from the destination field in the original IP header. Original Source Address Copied from the source field in the original IP header. Present only if the S-bit is set. The encapsulating agent is free to use existing IP mechanisms appropriate for delivery of the encapsulated payload to the tunnel endpoint. In particular, this means that use of IP options and fragmentation are allowed, unless the "Don't Fragment" bit is set in the inner IP header. This is required so that hosts employing Path MTU discovery [4] can obtain the information they seek. 4. ICMP messages from within the tunnel ICMP messages are to be handled as specified in [5], including the maintenance of soft state. 5. Security Considerations Security considerations are not addressed in this document, but are generally thought to be similar to those outlined in [5]. Perkins Expires 25 April 1996 [Page 3] Internet Draft Minimal Encapsulation for IP 25 October 1995 6. Acknowledgements The text for most of section 3 was taken from the mobile-IP draft [2]. References [1] IETF Mobile-IP Working Group. IPv4 Mobility Support. ietf-draft-mobileip-protocol-12.txt - work in progress, September 1995. [2] IETF Mobile-IP Working Group. IPv4 Mobility Support. ietf-draft-mobileip-protocol-10.txt -- outdated draft, May 1995. [3] David B. Johnson. Scalable and Robust Internetwork Routing for Mobile Hosts. In Proceedings of the 14th International Conference on Distributed Computing Systems, pages 2--11, June 1994. [4] J. Mogul and S. Deering. Path MTU Discovery. RFC 1191, November 1990. [5] C. Perkins. IP Encapsulation within IP. Internet Draft -- work in progress, October 1995. Author's Address Questions about this memo can be directed to: Charles Perkins Room J1-A25 T. J. Watson Research Center IBM Corporation 30 Saw Mill River Rd. Hawthorne, NY 10532 Work: +1-914-784-7350 Fax: +1-914-784-7007 E-mail: perk@watson.ibm.com The working group can be contacted via the current chairs: Jim Solomon Tony Li Motorola, Inc. cisco systems 1301 E. Algonquin Rd. 170 W. Tasman Dr. Schaumburg, IL 60196 San Jose, CA 95134 Perkins Expires 25 April 1996 [Page 4] Internet Draft Minimal Encapsulation for IP 25 October 1995 Work: +1-708-576-2753 Work: +1-408-526-8186 E-mail: solomon@comm.mot.com E-mail: tli@cisco.com Perkins Expires 25 April 1996 [Page 5]