Network Working Group A. McKenzie Request for Comments: 215 BBN NIC #7545 30 August 1971 Categories: C.2, D.1, D.3, G.1 Updates: none Obsoletes: none
The Terminal IMP Implementation
By early December there will be six Terminal IMPs incorporated
Most of the choices made during protocol implementation on the
It should be remembered that the Terminal IMP is designed to
The remainder of this note describes, and attempts to justify,
A) Deviations from Protocols
1) The Terminal IMP does not guarantee correct response
to ECO commands. If some Host A sends a control
message containing ECOs to the Terminal IMP, and the message arrives at a time when
a) the Terminal IMP has a free buffer and
b) the control link from the Terminal IMP to Host A is not blocked
then the Terminal IMP will generate a correct ERP for
each ECO. In all other cases the ECO commands will
be discarded. (All control messages sent by the
Terminal IMP begin with a NOP control command, so if Host A sends a control message consisting of 60 ECO commands, the Terminal IMP will answer (if at all) with a 121-byte message -- 1 NOP and 60 ERPs.)
The reason for this method of implementation is that
to guarantee correct response to ECO in all cases
requires an infinite amount of storage. For
example, suppose Host A sends control messages, each containing an ECO command, to Host B at the rate of one per second, but that Host A accepts messages from the network as slowly as possible (one every 39
seconds, say). Then Host B has only three choices which do not violate protocol:
a) Declare itself dead to the network (i.e., turn
off its Ready line), thereby denying all its
users use of the network.
c) Implement "infinite" storage for buffering
Since it is clear that none of the "legal" solutions
are possible, we have decided to do no buffering,
which should (we guess) satisfy the protocol well
over 99% of the time.
2) The Terminal IMP does not guarantee to issue CLS
commands in response to "unsolicited" RFCs. There are currently several ways to "solicit" an RFC, as follows:
a) A terminal user can tell the Terminal IMP to
perform the ICP to the TELNET Logger at some
foreign Host. This action "solicits" the RFCs defined by the ICP.
b) A terminal user can send an RFC to any particular Host and socket he chooses. This "solicits" a matching RFC.
c) A terminal user can set his own receive socket
"wild." This action "solicits" an STR from
anyone to his socket. Similarly, the user can set his send socket "wild" to "solicit" an RTS.
If the Terminal IMP receives a "solicited" RFC it
handles it in accordance with the protocol. If the Terminal IMP receives a control message containing one or more "unsolicited" RFCs it will either issue CLS commands or ignore the RFCs according to the
criteria described above for answering ECOs (and for the same reasons). Further, if the Terminal IMP
does issue a CLS in response to an unsolicited RFC it will not wait for a matching CLS before
considering the sockets involved to be free for other use.
3) After issuing a CLS for a connection, the Terminal
IMP will not wait forever for a matching CLS.
There are two cases:
b) The Terminal IMP has sent a CLS for an
established connection (matching RFCs exchanged)
In either of these cases the Terminal IMP will wait for a matching CLS for a "reasonable" time (probably 30 seconds to one minute) and will then "forget" the connection. After the connection is forgotten, the Terminal IMP will consider both sockets involved to be free for other use.
Because of program size and table size restrictions,
the Terminal IMP assigns socket numbers to a terminal
as a direct function of the physical address of the
terminal. Thus (given this socket assignment scheme)
the failure of some foreign Host to answer a CLS
could permanently "hang" a terminal. It might be
argued that the Terminal IMP could issue a RST to the offending Host, but this would also break the
connections of other terminal users who might be
performing useful work with that Host.
4) The Terminal IMP ignores all RET commands. The
Terminal IMP cannot buffer very much input from the network to a given terminal due to core size
limitations. Accordingly, the Terminal IMP allocates only one message and a very small number of bits
(currently 120 bits; eventually some number in the range 8-4000, based on the terminal's speed) on each connection for which the Terminal IMP is the
receiver. Given such small allocations, the Terminal IMP attempts to keep the usable bandwidth as high as possible by sending a new allocation, which brings the total allocation up to the maximum amount, each time that:
a) one of the two buffers assigned to the terminal is empty, and
b) the allocations are below the maxima.
Thus, if a spontaneous RET were received, the
reasonable thing for the Terminal IMP to do would be to immediately issue a new ALL. However, if a
foreign Host had some reason for issuing a first
5) The Terminal IMP ignores all GVB commands.
Implementation of GVB appears to require an
unreasonable number of instructions and, at the
moment at least, no Host appears to use the GVB
command. If we were to implement GVB we would always RET all of both allocations and this doesn't seem
6) The Terminal IMP does not handle a total bit-
allocation greater than 65,534 (2^16-2) correctly. If the bit-allocation is ever raised above 65,534 the Terminal IMP will treat the allocation as infinite. This treatment allows the Terminal IMP to store the bit allocation for each connection in a single word, and to avoid double precision addition and
subtraction. Our reasons for this decision are:
a) A saving of more than 100 words of memory which
would be required for allocation tables and for
double precision addition/subtraction routines.
b) Our experience, which indicates that very few
Hosts (probably one at most) ever raise their
total bit allocation above 65,534 bits.
c) Our expectation that any Host which ever raises
its bit allocation above 65,534 probably would be
willing to issue an infinite bit allocation if
one were provided by the protocol. Once the bit
allocation is greater than about 16,000, the
message allocation (which the Terminal IMP
handles correctly) is a more powerful method of
controlling network loading of a Host system than
bit allocation. We believe that Hosts which have
loading problems will recognize this.
7) The Terminal IMP ignores the "32-bit number" in the
ICP. When the Terminal IMP (the "user site")
initiates the Initial Connection Protocol the actual procedure is to send the required RTS to the logger
The ICP allows the foreign Host to transmit the RFCs
involving Terminal IMP sockets "U+2" and "U+3" at
any time after receipt of the RFC to the (foreign
Host's) logger socket. In particular, the RFCs may arrive at the Terminal IMP before the 32-bit
number. In the case of a "normal" foreign Host, the first incoming RFCs for sockets U+2 and U+3 will come from the sockets indicated by the 32-bit number, so it doesn't matter if the number is ignored. In the case of a pathologic foreign Host, a potentially
infinite number of "wrong" RFCs involving U+2 and
U+3 may arrive at the Terminal IMP before the 32-bit number is sent. The Terminal IMP would be required to store this stream of RFCs pending arrival of the 32-bit number, then issue CLS commands for all
"wrong" RFCs. However, the Terminal IMP does not
have infinite storage available for this purpose (it is also doubtful that a terminal user really wants to converse with a pathologic foreign Host) so the
Terminal IMP assumes that the foreign Host is
"normal" and ignores the 32-bit number.
B) Other Design Choices Related to Protocol
1) The Terminal IMP ignores incoming ERR commands and does not output ERR commands.
2) The Terminal IMP assumes that incoming messages have the format and contents demanded by the relevant protocols. For example, the byte size of incoming TELNET messages is assumed to be 8. The major checks which the Terminal IMP does make are:
a) If an incoming control message has a byte count greater than 120 then it is discarded.
c) If an incoming data message has a byte count indicating that the bit allocation for the connection is exceeded (based on the assumed byte size) then the message is discarded.
3) If one control message contains several RST commands only one RRP is transmitted. If several control messages, each containing RST commands, arrive "close together" only one RST is returned. [The actual implementation is to set a bit each time a RST is found (in "foreground") and to reset the bit when a RRP is sent (in "background").]
4) Socket numbers are preassigned based on the hardware "physical address" (in the terminal multiplexing device) of the terminal. The high order 16 bits of the socket number give the device number (in the range 0-63) and the low order bits are normally 2 or 3 depending on the socket's gender (zero is also used during ICP). [We would be pleased to see socket number length reduced to 16 bits; in that case the high order 8 bits would be mapped to the device and the low order 8 bits would contain 2 or 3.]
5) During ICP, with the Terminal IMP as the user site, the Terminal IMP follows the "Listen" option rather than the "Init" option (as described at the top of page 3, NIC #7170). In other words, the Terminal IMP does not issue the RFCs involving sockets U+2 and U+3 except in response to incoming RFCs involving those sockets. In this context, we will mention that the "deadlock" mentioned in NWG-RFC #202 does not exist, since the ICP does not give the server the "Listen" option (see NIC #7170, page 2).
[ This RFC was put into machine readable form for entry ] [ into the online RFC archives by Randy Dunlap 5/97 ]