RFC 687

RFC 687: IMP/Host and Host/IMP Protocol Change 🎀

Okay bestie, let's start with the title block. This is RFC 687 💖 from the Network Working Group, titled IMP/Host and Host/IMP Protocol Change. It was written by David C. Walden (WALDEN@BBN), NIC #32654, and dated June 1975 🌸. The document doesn't give a category or status, and it doesn't say what it obsoletes or updates, so there's nothing to report there ✨.

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So the vibe here is a glow-up for the IMP/host and host/IMP protocol 💅. The big goal is letting hosts be addressed on more than 63 IMPs, and it's giving "fix the limits, don't rebuild everything" energy 🦋. Lots of big changes are being discussed across the whole world-wide packet-switching community, but the team can't do massive fundamental changes on the timeline they need. Like, there are already almost 60 IMPs 😭. Each of the characteristics is covered below, and you need to know Section 3 of BBN Report 1822 first 🧚. And yes, the expanded protocol is backwards compatible 🫶.

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1. Expanded Leader Size ✨

The leader is growing from two to five 16-bit words 🌈. That gives room for field expansions and new fields, so it's basically a closet upgrade 👑.

2. Expanded Address Field 🦋

The address field goes to 24 bits: 16 bits of IMP address and 8 bits of host address 💜. The team says this is more than enough for any foreseeable ARPA Network growth, which is a very confident claim and I respect it 💅.

3. New Message Length Field 🍓

A brand-new field lets the source host optionally tell the IMP subnetwork the message length, in bits 🎀. The IMP subnetwork might use that info, when it's available, to make better use of network buffer storage. The destination host might use it too, for its own buffer storage 🧚. This field is 13 bits wide.

4. Expanded Handling Type Field 🌸

The handling type field, which currently tells priority and non-priority message streams apart, gets expanded to eight bits 🔥. That opens up the possibility of several parallel message streams between pairs of hosts, each with different handling. Examples include priority, non-priority, varying numbers of packets per message (more on that in item 5), unordered messages (the current type-3 messages), and a stream that needs guaranteed capacity ⭐. Just a heads up: only some of these will be available in the near term 🙈.

5. Source Host Control of Packets per Message 💖

The source host will be able to set up a message stream that uses a given number of packets per multi-packet message, like two packets per message or five 🦋. Right now the IMP network needs eight packet-buffers for reassembly, but after this change it won't have to, so these messages may get better service 💅. This is a big win for users who need both low delay and high throughput 🫶.

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6. Unordered (type-3) Message Change 🎀

Unordered messages get flagged by a handling type instead of a message type, like they are right now 🧚. This fits better with the need to check the host access control capabilities of every message ✨. Heads up though: it's a slight backward incompatibility for the three or so hosts that currently use type-3 messages in their research 😭.

7. Change in Format of Fake Host Addresses 💜

The For/From IMP bit is getting eliminated 👑. The fake host addresses will just be the four highest host numbers. For example, IMP Teletype will be host 252 🌈.

8. Addition of a Parameter to the IMP to Host NOP 🍓

The IMP to host NOP gets a new parameter that specifies the address of the host, meaning both the IMP number and the host number 🌸.

9. Backward Compatibility 🫶

The old and new formats will run in parallel in the IMPs for the foreseeable future, so host software can phase over gradually 🎀. A host can tell its IMP whether to use the old or new format, and it can even switch back and forth between the two for debugging 🔥. The mode gets set by choosing a format in the host to IMP NOP message, and the IMP follows whichever mode the latest host to IMP NOP message it received used ✨. A host can pick either format without knowing much about the other one, since it only has to discard the other format's messages if they show up 💅. When the IMP starts up, it sends several NOP messages of each type so the hosts can make their choice 🧚.

Here's the catch: a host that doesn't implement the new format can't address hosts on IMPs with an IMP number greater than 63 😭. Still, wherever possible, the IMPs will do the conversion needed so that old-format and new-format hosts can talk to each other, in both directions 💖. Last bit: it'll be possible to convert the leader format from old to new, or new to old, without knowing the message type 🦋.

10. (Not Present) 🙈

This document goes from item 9 straight to item 11, so there's no item 10 in the original. Keeping the numbering as written, iykyk 💀.

11. Non-blocking Host Interface ⭐

A mechanism lets the IMP refuse a message from a host without blocking the host interface 🌈. The IMP can gather the resources it needs to send the refused message, then ask the host to resend it 👑. The host can also ask to be able to send a message and be notified when that's possible, without the message actually being sent and refused first 🫶.

12. Maximum Message Length 🌸

The maximum number of data bits in a message may go down by a few bits 🎀.

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So the team is working out the details of an implementation plan for these changes to the IMP software 🔥. They'll share an implementation schedule and other necessary info, like format details, well ahead of time, so hosts that want the new protocol can implement it in time once it's available 💖✨.

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