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The Network as Seen by the Hosts

Before going on to discuss operating system communication protocol, some definitions are needed.

A HOST is a computer system which is a part of the network,

An IMP (Interface Message Processor) is a Honeywell DDP-516 computer which interfaces with up to four HOSTs at a particular site, and allows HOSTs access into the network. The configuration of the initial four-HOST network is given in figure 1. The IMPs from a store-and-forward communications network. A companion paper in these proceedings covers the IMPs in some detail. [3]

A message is a bit stream less than 8096 bits long which is given to an IMP by a HOST for transmission to another HOST. The first 32 bits of the message are the leader. The leader contains the following information:

  • (a) HOST
  • (b) Message Type
  • (c) Flags
  • (d) Link Number

When a message is transmitted from a HOST to its IMP, the HOST field of the leader names the receiving HOST. When the message arrives at the receiving HOST, the HOST field names the sending HOST.

Only two message types are of concern in this paper. Regular messages are generated by a HOST and sent to its IMP for transmission to a foreign HOST. The other message type of interest is a RFNM (Request-for-Next-Message). RFNM's are explained in conjunction with links.

The flag field of the leader controls special cases not of concern here.

The link number identifies over which of 256 logical paths (links) between the sending HOST and the receiving HOST the message will be sent. Each link is unidirectional and is controlled by the network so that no more than one message at a time may be sent over it. This control is implemented using RFNM messages. After a sending HOST has sent a message to a receiving HOST over a particular link, the sending HOST is prohibited from sending another message over that same link until the sending HOST receives a RFMN. The RFNM is generated by the IMP connected to the receiving HOST, and the RFNM is sent back to the sending HOST after the message has entered the receiving HOST. It is important to remember that there are 356 links in each direction and that no relationship among these is imposed by the network.

The purpose of the link and RFMN mechanism is to prohibit individual users from overloading an IMP or a HOST. Implicit in this purpose is the assumption that a user does not use multiple links to achieve a wide band, and to a large extent the HOST-HOST protocol cooperates with this assumption. An even more basic assumption, of course, is that the network's load comes from some users transmitting sequences of messages rather than many users transmitting single messages coincidently.

In order to delimit the length of the message, and to make it easier for HOSTs of differing word lengths to communicate, the following formatting procedure is used. When a HOST prepares a message for output, it creates a 32-bit leader. Following the leader is a binary string, called marking, consisting of an arbitrary number of zeros, followed by one. Marking makes is possible for the sending HOST to synchronize the beginning of the text message with its word boundaries. When the last bit of a message has entered an IMP, the hardware interface between the IMP and HOST appends a one followed by enough zeros to make the message length a multiple of 16 bits. These appended bits are called padding. Except for the marking and padding, no limitations are placed on the text of a message. Figure 2 shows a typical message sent by a 24-bit machine.