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.
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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.
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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.
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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.
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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].
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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
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Work: +1-708-576-2753 Work: +1-408-526-8186
E-mail: solomon@comm.mot.com E-mail: tli@cisco.com
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