This file contains technical documentation of the MIT Serial Line
Framing Protocol, as implemented in the Merit Computer Network.
It was retyped from the original information from MIT on 11/10/87.
Appendix I: Serial line Framing Protocol
(This description is from a very old file written when we
were first implementing the protocol. Details are still
accurate, though.)
This is preliminary documentation on the serial line
protocol used between the IBM PCs and the PC Gateway. The
protocol has two levels: the low-level protocol (LLP) and the
local net protocol. The low level protocol wraps a packet and
delivers it to the PC Gateway. The local net protocol tells the
PC Gateway what kind of packet it is. Currently there two types
of packets: Internet and Address Request. When the PC Gateway
receives an Internet packet, its action is to forward the
internet packet to a process which checks the packet for
validity and then sends it to the net. On receipt of an Address
Request packet, the PC Gateway sends an Address Request packet
back to the PC with four bytes of data (the PC's internet
address) in the body of the packet.
Neither the LLP nor the local net protocol provide for
prioritized transmissions, checksums or complex line control.
They are merely a simple way to get packets to and from
machines.
The local net protocol consists of a four byte leader. For
an internet packet, this leader is: 2,1,0.0. For an Address
Request, the leader is 2,3,0.0. There is no data in the Address
Request packet sent from the PC; it is only the four byte local
net header wrapped in the serial line protocol.
The receipt of any packet with a local net header that does
not identify the packet as either internet or address request is
an error and the receipt of such a packet should be logged and
the packet discarded.
LLP consists of four bytes with special meanings when
received over the serial line. These are: ESC, REQ, ACK, and
END.
A packet is enclosed in a REQ and an END. When a PC wishes
to send a packet it first sends a REQ to the PC Gateway. It
then waits a suitable length of time to receive an ACK from the
PC Gateway. If no ACK byte is received, the PC Gateway is
assumed to be unable to handle the packet right now and a
timeout is said to have occurred. The PC may either retry, wait
or return an error.
After the PC receives the ACK signal, it may begin sending
the packet. The first four bytes of the packet should be the
local net header and an error will occur if they are not valid.
When the PC has completed sending the packet, it should send an
END byte to the Gateway. The PC gateway will then consider the
packet and act upon it.
1 2
The PC Gateway goes through a similar process when it sends
a packet to a PC; only the roles are reversed. It is an error
for the PC Gateway to send a PC its address if the PC has not
requested its address from the Gateway.
If a machine should receive a REQ embedded in a packet,
this indicates that the END signal was dropped somewhere. The
receiving machine should drop the packet it was receiving and
begin to receive a new one. ACKs may be mixed inside packets to
allow immediate response to REQs. The receipt of an ACK by a
machine should be duly logged (and perhaps appropriately ignored
if the PC doesn't have an outstanding request).
The final code, ESC, is used to allow the characters whose
codes are used by ESC, REQ, ACK and END to appear in packets.
Receiving an ESC indicates that the next byte should be looked
at to produce the correct character. Here is a table of the
codes for the signals and the ESC sequences to produce the data
whose codes they use.
A simple way to unstuff the bytes is to add the character
following an ESC to the ESC to get the correct code and then put
it in the packet as data. It is an error to have any character
>3 follow an ESC.
If a machine receives any character other than a REQ or an
ACK when it is not in the process of receiving a packet, it
should discard that character.
Low-level protocol specification: IBM to Gateway, Gateway
to IBM.
The following ASCII codes are used as flags in the manner
specified:
242 - Prefix code for sending data codes which are set
aside for protocol use.
243 - Request to send (REQ).
244 - Acknowledge (ACK).
245 - End of packet (END).
A typical data transfer occurs as follows:
IBM wants to send packet to Gateway: It sends REQ and waits for ACK.
Gateway is ready to receive packet: It sends ACK
IBM sends packet to Gateway followed by END.
The packet itself is encoded so that REQ, ACK, and END never
appear in the text. This is done by performing the following
substitutions:
1 3
Note that transfers can occur in both directions
simultaneously. However the ACK signal may be embedded within a
data packet and must be explicitly removed:
IBM wants to send packet to Gateway: It sends REQ and waits for ACK.
Gateway is ready to receive packet: It sends ACK.
IBM starts sending packet.
Gateway wants to send packet to IBM: It sends REQ and waits for ACK.
IBM is ready to receive packet: It sends ACK.
IBM continues sending its packet, while Gateway sends a packet to
IBM.
Timeouts may occur if the wait between a REQ and an ACK is
too long or no packet characters are transmitted for too long a
time. In both cases no recovery action is undertaken: the other
system is assumed to have crashed.
Receipt of protocol codes within a data packet has the
following consequences:
REQ - End portion of data packet being sent has been lost.
ACK - Should be removed from input packet and its presence
logged for use by the process which is sending
characters.
END - Packet has been completely sent.
