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
NCP, ICP, and TELNET:
The Terminal IMP Implementation
By early December there will be six Terminal IMPs incorporated
into the network, with additional Terminal IMPs scheduled for delivery
at a rate of about one per month thereafter. For this reason the
implementation of network protocols (and deviations from them) may be of
interest to the network community. This note describes the choices made
by the Terminal IMP system programmers where choices are permitted by
the protocols, and documents some instances of non-compliance with
protocols.
Most of the choices made during protocol implementation on the
Terminal IMP were influenced strongly by storage limitations. The
Terminal IMP has no bulk storage for buffering, and has only 8K of 16-
bit words available for both device I/O buffers and program. The
program must drive up to 64 terminals which generally will include a
variety of terminal types with differing code sets and communication
protocols (e.g., the IBM 2741 terminals). In addition, the Terminal IMP
must include a rudimentary language processor which allows a terminal
user to specify parameters affecting his network connections. Since the
Terminal IMP exists only to provide access to the network for 64
terminals, it must be prepared to maintain 128 (simplex) network
connections at any time; thus each word stored in the NCP tables on a
per-connection basis consumes a significant portion of the Terminal IMP
memory.
It should be remembered that the Terminal IMP is designed to
provide access to the network for its users, not to provide service to
the rest of the network. Thus the Terminal IMP does not contain
programs to perform the "server" portion of the ICP; in fact, it does
not have a "logger" socket.
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The Terminal IMP program currently implements only the NCP, the
ICP, and the TELNET protocol since these are of immediate interest to
the sites with Terminal IMPs. It is anticipated that portions of the
data transfer protocol will be implemented in the future; the portions
to be implemented are not yet clearly defined, but will probably include
the infinite bit stream (first) and the "transparent" mode (later).
Developments in the area of data transmission protocol will be
documented in the future.
The remainder of this note describes, and attempts to justify,
deviations from the official protocols and other design choices of
interest. Although written in the present tense, there are some
additional known instances of deviation from protocol which will be
corrected in the near future.
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.
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b) Refuse to accept messages from the network
faster than the slowest possible foreign Host
(i.e., about one every 39 seconds). If Host B is
a Terminal IMP, this is almost certainly slow
enough to soon reach a steady state of no users.
c) Implement "infinite" storage for buffering
messages.
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:
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a) The Terminal IMP has sent an RFC, grown tired of
waiting for a matching RFC, and therefore issued
a CLS
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
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spontaneous RET, it would probably issue a second RET
as soon as it received the ALL. This would be likely
to lead to an infinite (and very rapid) RET-ALL loop
between the two machines, chewing up a considerable
portion of the Terminal IMP's bandwidth. Since the
Terminal IMP can't "afford" to communicate with such
a Host, it ignores all RETs.
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
very useful.
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
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socket of the user-specified foreign Host and
simultaneously to set the terminal user's send and
receive sockets in a state where each will accept
any RFC from the specified Host. The 32-bit socket
number transmitted over the logger connection is
ignored, and the first RTS and STR addressing the
user's sockets will be accepted (and answered with
matching RFCs).
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.
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b) If a control command opcode greater than 13 is
found during the processing of a control message
then the remainder of the control message 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 ]
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RFC, FYI, BCP