Host-IMP interface
A 1969 sketch of the two programs that move a host computer's messages to and from its IMP.
Deloche, G., "Host-IMP interface", RFC 7, DOI 10.17487/RFC0007, May 1969, <https://www.rfc-editor.org/info/rfc7>.
@misc{rfc7,
series = {Request for Comments},
number = 7,
howpublished = {RFC 7},
publisher = {RFC Editor},
doi = {10.17487/RFC0007},
url = {https://www.rfc-editor.org/info/rfc7},
author = {G. Deloche},
title = {{Host-IMP interface}},
pagetotal = 7,
year = 1969,
month = may,
} RFC text
Contents
Network Working Group G. Deloche
Request for Comment: 7 University of California at Los Angeles
NIC: 4693 May 1969
Host-Imp Interface
G. Deloche --> Prof. J. Estrin
Prof. L. Kleinrock
Prof. B Bussel
D. Mandell
S. Crocker
L. Bonamy
Object: Arpa Network - Specification Outlines for Host-IMP (HI)
Interface Programs.
Outline
I. Introduction
II. Scope of the software organization.
II-1 Network program
II-2 Handler program
III. Questions
[The original of RFC 7 was hand-written, and only partially illegible
copies exist. RFC 7 was later typed int NLS by the Augmentation
Research Center (ARC) at SRI. The following is the best
reconstruction we could do. RFC Editor.]
I. Introduction
This paper is concerned with the preliminary software design of the
Host IMP interface. Its main purpose is on the one hand to define
functions that will be implemented, and on the other hand to provide
a base for discussions and ...(unreadable).
This study is based upon a study of the BBN Report No. 763.
II. Scope of the software organization.
The system is based upon two main programs: the Handler program that
drives the channel hardware unit, and the Network program which
carries out the user's transmission requests.
As the communication is full duplex, each of these programs can be
viewed as divided into two parts: one is concerned with the output
data, the other with the input. (See Fig. 1)
These two programs exchange data through a pool of buffers, and
logical information through an interface table.
In the following we only focus on the output part of each program
(See Fig. 2). The input part would be very similar.
II-1. Network program.
II-1-1. Multiplex function.
This program multiplexes the outgoing messages (and distributes the
incoming messages). The multiplexing consists in stacking up all the
user's (or caller, or party) requests and filling up the pool of
buffers so as to keep the handler busy emitting.
Multiplexing (and distribution) is based on the link identification
numbers. (Link = logical connection between two users). The
multiplexing problem is closely related to the interface between a
user's program and the network program, that is in
fact...(unreadable) operating system (See below: Questions).
II-1-2. Output message processing.
When a user's program wants to send out text it should indicate the
following information (through a macro, or as call parameters): text
location, text length in bytes, and destination.
Using these data the Network program:
* prepares a 16 bit Host heading (1 bit: trace, 2 bits: spares, 8
bits: link identification no., 5 bits: destination host)
* inserts a 16 bits marking between the header and the text so as
to start the text at a word boundary. This marking consists of
a one preceding the first bit of the text and, in turn,
preceded by fifteen zeros to fill up the gap.
* checks the length of the user's text - if it exceeds 1006 bytes
+- -+
|8080 (max host message length) - 32 (heading + marking)|
|-------------------------------------------------------|
| 8 (byte = 8 bits) |
+- -+
the program breaks down the text into a sequence of messages whose
maximum length is 1006 bytes - Each of these messages is preceded by
a heading as explained above.
Remark: in that case one of the heading space bits could be used for
indicating that several messages belong to the same text.
* _transcodes_ the EBCDIC characters constituting the messages
into ASCII characters.
* _fills_ the buffers of the pool with the content of the
messages.
* _updates_ the content of the interface table and moves the
filling pointers (see below).
II-2. Handler program.
This program is initiated either by the network program, or by the
I/O interrupt.
This program will be very short. It will be coded in master mode
(privileged instructions) and should be integrated in the I/O
supervisor of the operating system.
This program:
* _controls_ the channel hardware unit. It initiates the
emission, eventually provides data chaining between the
buffers, tests the different device status upon receiving an
interrupt.
* _empties_ the buffers that are filled up by the network
program.
* _explores_ and _updates_ the interface table (see below).
* can eventually insure a control transmission procedure with the
IMP (See Questions).
II-3 Buffers and Interface Table.
II-3-1 Buffers.
They should be large enough for containing the maximum host message
text + heading and marking (1006 + 4 = 1010 bytes).
Consequently the buffer size could be chosen equal to 256 words (1024
bytes). As for the buffer number it will determine the link
utilization frequency -
II-3-2 Interface table.
It is through this table that the network program informs the handler
with the location and length of the emitting data.
This table could be a ring table with 2 pointers: one for filling,
the other for extracting. They are respectively updated by the
network and the handler program.
[Length] of the message
contained
in the buffer
^
|
+--------------+----------+
| Buffer addr. | nb bytes |
+--------------+----------+
| | | <== Filling pointer
+--------------+----------+ |
| | V
| |
// //
| |
| |
| |
+--------------+----------+
| Buffer addr. | nb bytes | <== eEtracting pointer
+--------------+----------+ |
| Buffer addr. | nb bytes | V
+--------------+----------+
III. Questions.
III-1. Why is there not a simple control procedure between the HOST and
the IMP? What happens if a message, issued from the HOST,
reaches the IMP with an error due to the transmission?
From the BBN specifications it appears that this error will be
transmitted as far [as] the receiving HOST.
In that case must an HOST-HOST control procedure be provided?
III-2. Where will the special channel hardware unit be connected
(MIOP/SIOP)?
How will this device be notified of an outgoing message end in order
to start the padding?
(The program will provide to the MIOP SIOP the number of bytes of the
outgoing message, and will receive back an interrupt when the last
byte is sent out. Is it that signal which will be also sent to the
special device?)
Vice versa how does the Handler know the length of the incoming
message? From the contents of the previous one or should this
program always ready to receive a message of maximum length? (Then
an interrupt should be triggered when the real end is detected by the
hardware).
III-3. When does the Gordo documentation will be available in order to
design the user-network program interface. What are the
mechanisms for program initiations, transferring parameters from
one program to another, etc...
HOST (Sigma 7) <--|/|--> Outside world
Users |/|
| |/|
| |/| Special Standard
V |/| | |
+--------------+ +------------+ |/| V | V
o----| | | | |/| +------+------+
| Output |-->| Output |-->|/|-->| | |-->IMP
. . .| | | | |/| | |
| | | | |/| | | |
o----| (Multiplex) | | | |/| | |
|_ _ _ _ _ _ _ | |_ _ _ _ _ _ | |/| | _ _ _| _ _ _|
o----| | | | |/| | | |
| Input |<--| Input |<--|/|<--| |<--IMP
. . .| | | | |/| | | |
| | | | |/| | |
o----|(Distribution)| | | |/| +------|------+
| | | | |/| HARDWARE
+--------------+ +------------+ |/| Interface
NETWORK HANDLER |/|
Program Program |/|
|/| (Fig. 1)
+------------------+
| | | |
| | | interface |
| | | |
| |
| ====> Data |
| |
| ----- Logical |
| information|
+------------------+
+ - - - - - - - - - - - - - -+
| |
interface table
| +----------+ | | |
| | +---->| |<----+ | |
| | | | +----------+ | | interrupt | |
| | ^ | | +----<-------| |
| | | | +- -+ | | | | |
| | | V | _________ | V V V | |
o----| |-+ +---------+-+ | | || +-+--+--+---+ | |
| | \ |+---------+| | |_________|| |+---------+| | |
| | \|| || | | || || | |
| | +| || | - - - - - -| || || | |
| | || || | | || || | |
- - - - - -|| NETWORK ||=>- _________ -=>|| HANDLER ||=======>| |
| | +| Progr. || | | || || Progr. || | |
| | /|| || | |_________|| || || | |
| | / |+---------+| | | |+---------+| | |
o----| |-+ +-----------+ | _________ | +----+------+ | |
^ | | | | || | | |
| | | | |_________|| +--->---------->| |
| ^ +- ^ -+ Commands | |
Users | | | |
| Pool of buffers | |
User's ^
Interface |
Hardware
Interface
(Fig. 2)
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Bob German & Lorrie Shiota 1/02 ]
AI ELI5
On this page
§1Two Programs and Shared Tables
RFC 7 outlines the software design for the Host-IMP interface, based on a study of BBN Report No. 763. The system rests on two main programs: the Handler program, which drives the channel hardware unit, and the Network program, which carries out the user's transmission requests.
Because communication is full duplex, each program has an output part and an input part. The two programs exchange data through a pool of buffers and logical information through an interface table. The RFC only works out the output side and says the input side would be very similar.
Two helpers share a table and a pile of boxes. One packs what people want to send. The other carries the boxes to the network.
§2What the Network Program Does to Outgoing Text
The Network program multiplexes outgoing messages and distributes incoming ones, using link identification numbers (a link is a logical connection between two users). A user's program supplies the text location, the text length in bytes and the destination.
The Network program then prepares a 16-bit host heading: 1 trace bit, 2 spare bits, an 8-bit link number and a 5-bit destination host. It inserts a 16-bit marking, a one preceded by fifteen zeros, so the text starts on a word boundary. If the text exceeds 1006 bytes (8080 bits less 32 bits of heading and marking, divided by 8) it is broken into several messages, each with its own heading. It also transcodes EBCDIC characters to ASCII, fills the buffers and updates the interface table.
Before mailing, the helper writes an address label, adds a little padding so the letter starts neatly, and cuts a long letter into several envelopes if it is too big.
§3Handler, Buffers and Interface Table
The Handler program is started by the Network program or by an I/O interrupt. It is expected to be very short, coded in master mode (privileged instructions) and integrated in the I/O supervisor of the operating system. It controls the channel hardware, empties the buffers the Network program has filled, and reads and updates the interface table.
Each buffer must hold the largest host message text plus heading and marking (1006 + 4 = 1010 bytes), so the RFC suggests 256 words (1024 bytes). The interface table could be a ring with two pointers, one for filling (moved by the Network program) and one for extracting (moved by the Handler). Each entry holds a buffer address and a byte count.
The carrier only follows the table: it picks up the next box, takes it to the mailbox, and crosses it off the list.
§4Open Questions
The outline closes with questions. Why is there no simple control procedure between the host and the IMP, and what happens if a message reaches the IMP with a transmission error? From the BBN specifications the error appears to be passed on to the receiving host, so the author asks whether a host-to-host control procedure is needed.
It also asks where the special channel hardware unit will be attached and how the Handler learns the length of an incoming message. A last question asks when documentation will be available to design the interface between user programs and the Network program.
The author ends by asking the group what to do when a letter gets smudged on the way, and where the mailbox should be bolted on.