IMP/Host and Host/IMP Protocol change (RFC0704)
Original Publication Date: 1975-Sep-01
Included in the Prior Art Database: 2019-Feb-15
Internet Society Requests For Comment (RFCs)
Network Working Group Paul J. Santos, Jr. (BBN) Request for Comments 704 Sept 1975 NIC #33490
IMP/Host and Host/IMP Protocol Change
This note is a revision of RFC 687 and sketches the design of an expansion to the IMP/host and host/IMP protocol which will include among other things the possibility of addressing hosts on more than 63 IMPs. Our intention in this expansion is to correct certain existing limits without fundamental changes in the philosophy of the IMP/host protocol; i.e., while many issues which would represent fundamental changes to the IMP/host protocol are presently under discussion in the world-wide packet-switching community, we are not able to undertake massive fundamental changes on a time scale compatible with the short term needs for network improvement (e.g., already there are 62 IMPs).
The following paragraphs cover each of the major characteristics of the expanded protocol. A knowledge of Section 3 of BBN Report 1822 is assumed. As is discussed below, the expanded protocol is backwards compatible.
1. Expanded Leader Size. The leader will be expanded from two to six 16-bit words. This will provide space for necessary field expansions and additions. The expansion of the IMP/host (host/IMP) leader to 96 bits from 32 causes word-boundary problems for some hosts. To be able to deliver messages between two hosts of which one is using the old protocol and the other the new, without shifting the data in the IMP words, it is necessary that the data (i.e. the first bit of the host/host leader) start at an even multiple of 8-bit bytes from the beginning of the entire message. On the other hand, each host prefers (in fact requires, if no shifting is to be performed by the host) that the combined host/IMP (IMP/host) and host/host leaders occupy some integral number of machine words (defined as the smallest sequence of bits that can be independently accessed by the host/IMP interface). With a total host/IMP (IMP/host) and host/host leader of 136 bits, only machines with 8-, 16-, 32-, and 64-bit words will find the leader size suitable. To simplify things for machines with other word lengths, a provision of the protocol permits each host to tell its IMP a number of 16-bit padding words to be inserted between the host/IMP (IMP/host) and host/host leaders. This padding will be stripped off during host-to-IMP processing by the IMP, and added in during IMP-to-host processing. Thus, for instance, 24-bit machines can specify one 16-bit word of padding, and 10- and 36-bit machines can specify five 16-bit words.
2. Expanded Address field. The address field will be expanded to 32 bits, 16 bits of IMP address, 0 bits of host address, and 8 bits for (future) network address. This expansion is adequate for any forseeable ARPA Network growth.
3. New Message Length Field. A new field will be added which will allow the source host to optionally specify the message length (in bits) to the IMP subnetwork. The IMP subnetwork may be able to use thi...