Username / Password : Request For Comments

RFC Number : 91

Title : Proposed User-User Protocol.

Network Working Group George H. Mealy
Request for Comments: 91 Harvard University
December 27, 1970



There are many good reasons, and maybe one or two bad ones, for making
it appear that communication over the Network is only a special case of
input/output -- at least as far as user programming is concerned. Thus,
for instance, the Harvard approach toward implementing the HOST-HOST
protocol and Network Control Program treats each link as a 'logical
device' in PDP-10 terminology. Setting up a connection is similar to
local device assignment, and communication over a link will make use of
the standard system input/output UUO's. This makes it possible to sue
existing programs in conjunction with the Network without modification
-- at least if other PDP-10's are being dealt with.

This takes us only so far, however. The notion of a 'logical device'
does not exist on the PDP-10; it does on the IBM 360 (I am speaking here
at the level of the operating system -- user program interface).
Furthermore, in the absence of a Network standard requiring fixed
representations for integers, reals, etc. (which I would oppose), any
pair of user processes must arrive at a local agreement, and one or both
must assume the burden of data conversion where necessary. Any standard
protocol should allow such agreements to be given expression and should
accommodate at least the minimum of control information that will allow
such agreements to function in practice. Finally, we must note that the
IMP-IMP and HOST-HOST protocols do not provide for a check that an
action requested by a user process is actually accomplished by the other
processes; this type of issue has always been regarded as subject to
treatment at the USER-USER protocol level.

This proposal is intended to face the above three types of issue only to
a certain extent. I can best explain that extent by stating the
criteria I would use to judge any USER-USER protocol proposal:

1. The notion of a (logical) record
should be present, and the notion of a message should be
suppressed. (To a FORTRAN programmer, that which is
written using one WRITE statement with no accompanying
FORMAT is a record; to an OS/360 machine language
programmer, PUT writes a record).

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2. It should be possible to so implement the
protocol in HOST systems and/or library routines that now
existing user programs can access files anywhere in the
Network without program modification. (Initially, at
least, this ability must be restricted to HOST systems of
the same type).

3. The protocol should be implementable (not
necessarily implemented) in any HOST system at the SVC or
UUO level. Specific knowledge of the characteristics of
the other HOST involved should be unnecessary.

It should be noted that the above imply that some user programs must be
aware of the nature of the other HOST -- at least in each case where the
second criterion fails. As we make progress in (or give up on) the cases
where the failure now occurs, the burden of accommodating system
differences will shift toward implementation in protocols (i.e., the
HOST systems) or, by default, in user programs.

Quite clearly, any proposal initiated today should be suspect as to the
extent to which it 'solves' ultimate problems. How ambitious to be is
strictly a matter of taste. At this stage, I prefer to try something
which I believe can be used by all of us (and, hence, is worth doing),
goes a reasonable distance towards solving our short-range problems, is
easy to do, and offers hope of viability in the long range view. In the
following, I intend to describe the proposal itself with, I hope, proper
motivational arguments for its pieces. I will then sketch the specific
implementation we at Harvard are making for the PDP-10 and describe how
we intend to apply it in the specific case of storage of files on other
PDP-10's in the Network.


The following protocol is intended to apply to the data bits in messages
between the end of the marking bits and the beginning of the padding

The present IMP-IMP and HOST-HOST protocols are unaffected by this

The general principle is that each segment (this is not a technical
term) of data is preceded by control information specifying its nature
and extent. The basic scheme has been evolved from that used in the SOS
buffering system (see the papers in JACM, April 1959 and especially that
by O.R. Mock).

Our point of view is that a link is a carrier of information.

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Information is carried in segments of a fixed maximum length called

That this is so is an accident, from the user's point of view; when he
wishes to transmit a contiguous stream of data, he will in general,
segment it in a different (from the IMP-IMP or HOST-HOST protocol view)
manner -- we will call his segment a record. It should be clear that
this is entirely analogous between the notion of (physical) block and
(logical) record. On the side, file storage systems also make use of
control and status information; we will also.

At the USER-USER protocol level, all information transmitted over the
link is a sequence of flags followed by (possibly null) data blocks.

The general format will be:


The OPERATION field is always present and is four bits long. The COUNT
field, when present, gives the number of data bytes following in the
data block. The bytes size is set be the last preceding SIZE flag (in
most cases). The byte may be between zero and 255 bits long (Yes,
Virginia, zero is zero even when you have a System/360). The OPERATION
field and the COUNT field (when present) are called the flag and the
data bytes (when present) the data block. Flags followed by data blocks
(even when null due to a zero count) are called block flags, and other
flags are called whyte* flags.

It is to be noted that, since the SIZE flag sets the byte size for the
following blocks, byte size may be set at that 'natural' for the sending
or for the receiving HOST, depending on local agreement between the
sending and receiving processes. It is specifically required that a SIZE
flag appear in each message prior to any block flag (except the ASCII
flag); the SIZE flag may be introduced on a default basis by the
routine(s) implementing the protocol and is intended partially as a
means of detecting certain classes of error.

The COUNT field is 8 bits in length (except in the EOM flag, where it is
16 bits long). The flags are as follows:

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Whyte Flags:

0 - NUL No operation (consider next flag)
1 - RS Record Separator (end of record)
2 - GS Group Separator (end of group)
3 - FS File Separator (end of file)
4 - ESC Escape to local convention for flags
5 - (reserved for later assignment)
6 - EOM N End of Message (M is total bit count)
7 - SIZE N Byte size is N bits
8 - IGNORE N Ignore following data bits

Block Flags:

9 - SYS N N bytes of data for receiving HOST system
10 - CONTROL N N bytes of control data follow
11 - STATUS N N bytes of status data follow
12 - LABEL N N bytes of identification data follow
13 - KEY N N bytes of key data follow
14 - ASCII N N (8-bit) bytes of ASCII data follow
15 - BLOCK N N bytes of data follow

I have already mentioned the requirement for SIZE. Absence of the SIZE
floag in any message containing block flags (except ASCII) is a definite
error. EOM is partially another error-checking device and partially a
device for bypassing the padding conundrum. A user program should never
see EOM on input; the user may write an EOM to force transmission. EOM
delimits the end of the useful information in the message and restates
the total number of bits in the message, starting with the first bit
following the marking and ending with the last bit of the EOM count
field, to check possible loss of information. This is a check against
errors in the IMP-HOST electrical interface and in the HOST mushyware.
EOM must appear at the end of each messager, unless ESC has apeared.

ESC is intended as a (hopefully) unused [a]scape hatch, for nonuse by
those installations and/or applications wishing to avoid using more than
four bits of the USER-USER protocol on any link. For instance, it may be
desired to use a link as a bit stream, ignoring even message boundaries.
If and when anarchists can achieve local agreement, more power to them!

NUL and IGNORE are intedned to be space fillers, in case it is helpful
to make the first bit of the subsequent data block occur on a convenient
address boundary. (An especially helpful HOST interrupt routine might
even paste a combination of NUL and IGNORE over the marking bits when
receiving a message -- in which case, their bit count should be
transmitted on to the GET rountines to correct the EOM bit count check).
The separator operations introduce the notions of logical record, group,
and file. Specifically, there is no requirement that a record be

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contained entirely within a message or that only a single record be
contained in a message! In addition, there is no requirement that only
one file be transmitted during a connection. For instance, a user might
wish to use a link to transmit a collection of rountines, and then do
something else with the link. By local agreement, then, a single routine
might consist of a cumber of records forming a group, the whole
collection might form a file, and the link might remain connected after
the FS flag is received.

The interpretation of the various block flags is similarly open to local
agreement. The two flags intended to convey pure data are ASCII and
BLOCK; the difference between them is only (as far as the protocol is
concerned) that the byte size is implicit for ASCII (8 bits) and
explicit for BLOCK (the count field of the next preceding SIZE flag).
Beyond this, however, the semantic content of the block following ASCII
is governed by the current standards for ASCII; EBCDIC information may
not be transmitted in an ASCII block!!

CONTROL and STATUS are intended for communication of control information
between user processes, and the interpretation of their accompanying
data blocks is open to local agreement. Generically, CONTROL means 'try
to do the following' and STATUS means 'but I feel this way, doctor.' A
CONTROL flag will prompt a returned STATUS flag, sooner or later, or
never. LABEL is intended for use in identifying the following unit(s) of
data, at the file or group level. Again, the specific interpretation is
a matter of local agreement. KEY is intended to mimic the notion of
address or key -- this is at the record, data item, or even physical
storage block level. For the familiar with PDP-10 system and/or OS/360,
the following parallels are offered for guidance:

USER-USER protocol OS/360 PDP-10

LABEL DSCB File retrieval information

The '?' notations above indicate lack of a very direct parallel. It is
worth noting that the OS/360 GET and PUT have direct parallels in any
implementation of the USER-USER protocol that embodies the notion of
record; our implementation of the protocol will lead to introduction of
this notion for all PDP-10 input/output involving disc and tape storage,
as well as IMP communication.

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If I know the MULTICS terminology, I could extend the set of parallels
above with more precision. Although my terminology has been drawn from
systems with explicit input/output imperatives, I wish to emphasize that
this setup in intended to handle control and data communication in
general; MULTICS is a system in which the classical distinction between
external and internal storage is blurred (from the user's point of view)
in a manner I wish it blurred in the USER-USER protocol. I offer SYS
with only slight trepidation. The general notion is that one should be
able to communicate directly with a foreign HOST rather than via a
foreign user process as its intermediary, SYS is like a UUO or SVC, but
for the foreign HOST's consumption rather than my HOST's. From the
HOST's point of view, the problem in implementation is in establishing a
process context record unconnected with any local user process. This,
however, is strongly associated with our current LOGON conundrum. On the
PDP-10, for instance, users are more or less identified with local
teletype lines, and any link is not one of those! Hence, subterfuge is
necessary to let a foreign user log on. OS/360 is as (actually, more)
perverse in its own way.

The process of logging a foreign process onto my local system is not
(except possibly for MULTICS) a simple matter of having a special (!!)
user job present which is responsible for doing it. When and if
anything else is possible, the HOST must provide a system instruction
(UUO or SVC or whatever) that gives the requisite information
establishing a process independent in all senses of the process that
made the request. Otherwise, self- protection mechanisms which are
reasonable for any system will make us all much more interdependent that
we wish. To do this, there must exist in every system a UUO/SVC that
does the right thing (ATTACH, but forget me). If this is true, then the
LOGON process over the Network is tantamount to issuance of a foreign
UUO/SVC by another node in the Network. I see no reasonable way around
this. If that is the case, then SYS N is the kind of flag to use to
convey the requisite data. If that is so, then it is only reasonable to
let SYS convey a request for any OS instruction at the user program-
operating system interface level!

The practical questions of implementation are something else! In the
case of the PDP-10, I can pretty well see how to turn a SYS into either
a LOGON request to execute a monitor command or UUO (would that they
were the same) as the case might be. OS/360 is more sophisticaed,
unfortunately. MULTICS might make it. Naytheless, I hope that is clear
that what we want to do, which is what the protocol should reflect, is
quite a different question from that of how it is to be done in the
context of a specific HOST system. What we want to do is, in general,
rather independent of the system we are dealing with as far as the
protocol is concerned, and we should not fail to introduce general
notions into the protocol just because we are uncertain as to how they
may have to be translated into particular implementation practice.

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Although the following can be implemented as either a set of user
routines or imbedded in the monitor as UUO's (our first implementation
will be the former), the latter version will be used for descriptive
purposes. The UUO's would be:

PUTF CH, E Put flag
PUTD CH, E Put data
PUT CH, E Put record
GETFD CH, E Get flag or data
GET CH, E Get record

In the above, 'CH' is the logical channel number. The customary OPEN or
INIT UUO is used to open the channel. Standard format user buffers are
assigned. However, the ring and buffer headers will be used in a
nonstandard way, so that data mode 12 is assigned for used with Network
buffering and file status bit 31 must be on for input. (Any of the
devices DSK, DTA, MTA, or IMP can be used in this mode.)

In the Harvard NCP and HOST-HOST protocol implementation, user buffers
do not correspond directly to messages. On output, each user buffer will
be formatted into a message; on input, a message may become one or two
user buffer loads (128 word buffers are used in order to make maximum
use of the facilities of the disk services routines).


This UUO places a flag into the output buffer. The effective address is
the location of a word:

XWD operation, count

In the case of block flags, the count is ignored, since it will be
computed from the number of bytes actually placed in the buffer before
the next use of PUTF. PUTF and PUTD will insert EOM flags automatically
as each buffer becomes full; if data bytes are currently being placed in
the buffer by PUTD, it will also insert an EOM flag after computing the
count for the previous block flag in the buffer and place a new block
flag of the same type at the beginning of the next buffer, after
inserting a SIZE flag stating the then current byte size.


This UUO places data into the output buffer. The effective address is
the location of the data byte (if the byte size is less than 36) or of
the next 36 bit word of data to be placed in the buffer. In the first
case, the byte is assumed to be in the low order part of the word

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addresses. In the second case, the data word containing the final bits
of the byte contains them in the high order part of the word, and the
next data byte starts a new word in PDP-10 storage. Thus, for a byte
size of 64, two .ne 5 entries to PUTD would be used per byte
transmitted, the first containing 36 bits and the second containing 28
bits, left- justified. This strategy allows maximum use of the PDP-10
byte handling instructions.


This UUO places a whole logical record in the output buffer(s). The
effective address is that of a word:

IOWD count, location

A PUTF UUO must have been used to output the proper SIZE flag.
Thereafter, each use of PUT will output a BLOCK flag,*

simulate a number of calls to PUTD using the IOWD to discover the
location and size of the user data area, and then output a RS flag to
indicate end of record.

In the case of byte size of less than 36 bits, PUT will use the ILDB
instruction to pick up bytes to be output by PUTD. Hence, the standard
PDP-10 byte handling format is used, and the count part of the IOWD is
the total byte count, not word count.

The above UUO'S have both an error return and a normal return.


The calling sequence for this UUO is:

error return
whyte flag return
block flag return
data return

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The effective address is the location at which the flag or data will be
returned. The flag is returned in the same format as for PUTF and the
data in the same format as for PUTD. Certain flags (NUL, IGNORE, and
EOM) will be handled entirely with the UUO and will not be reported to
the user. SYS should eventually be handled this way, but initially will
be handled by the user.


The calling sequence for this UUO is:

error return
end of file return
end of group return
normal return

GET transmits the next logical record to the user, using GETFD together
with an IOWD in the same format as for PUT. If the IOWD count runs out
before end of record, the remainder of the record will be skipped. In
any case, the updated IOWD will be returned at the effective address of
the UUO in order to inform the user how much data was transmitted or


Assume that I have a link connected to another PDP-10 and a user process
there that is listening. In order to get that process to send me a file,
the sequence of flags that might be transmitted can be represented as
follows, where the UUO'S executed by me are in the left margin, the
flags are indented, and the commentary opposite them indicates the
nature of the data block transmitted:

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CONTROL Data with OPEN parameters, requesting OPEN
LABEL File identification data for LOOKUP
EOM Forces message to be transmitted

STATUS Status returned by OPEN
SIZE Byte size to be used
LABEL File retrieval information

CONTROL Data requesting INPUT from file
EOM Forces request to be transmitted

STATUS Status bits returned by INPUT
GET Logical record (one file buffer load)
(loop back to second PUTF, above, for other records)

Finally, the status information returned by the second GETF indicates
end of file, and I wind up with the sequence:

CONTROL Data requesting a CLOSE
EOM Forces transmission

STATUS Status bits returned by CLOSE

In the case I am getting a file, the main loop looks like:

CONTROL Data requesting OUTPUT

PUT Logical record (one file buffer load) PUTF
EOM Forces transmission

STATUS Status bits returned by OUTPUT

The use of both the record and the flag transmission UUO's is worth
noting, as well as the use of the EOM flag to force transmission of a
message when switching between input and output over the link. PUT and
GET UUO's are clearly required above for transmission of the CONTROL and
LABEL data; I suppressed them for the sake of clarity.

For this application, the handshaking nature of the transmission of
CONTROL and STATUS flags are mandatory. While the protocol would permit

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transmission of a complete file without the handshaking, it would be an
all or nothing proposition - a single error would necessitate doing it
all over again, presuming that the receiving process did not end up in a
complete tangle.


The PDP-10 space required to implement the above protocol is about 400
instructions, divided equally between the input and the output side.
Enough experimental coding has been done to confirm the feasibility of
this basic strategy, taken together with experience with implementation
and use of the SOS buffering system.

The above does not touch the question of LOGON protocol, except
indirectly. My belief is that it can be accommodated in the framework of
this proposal, but I have not tested this theory as yet. As indicated
further above, I would be tempted to handle the matter with the SYS
flag, given that SYS data is interpreted directly by the system (in our
system, we would use the RUN UUO to run the LOGON CUSP, which would, in
turn handshake using ASCII data over the link). In this way, I think we
might be able to dispense with the notion of dedicated sockets and the
reconnection morass.

One other point that needs thought is the question of how to handle the
interrupt on link facility. Should it have any direct relation to the
GET/PUT UUO's, or be handled on the side? I am inclined to think that it
should be treated qua interrupt of the user process, quite independently
of the matter of data transmission over the link. Some of our current
work on the PDP-10 monitor would lend itself rather easily to
implementation as a true interrupt.


1. A message is that string of bits between any two HOST-HOST headers.
2. In memory of an attractive, but nonspelling, SDC secretary who
could not distinguish between black and white, at least during 1957
and in manuscript form.
3. PUTF may be used to ouput the block flag, if a different from
BLOCK is required.

[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Colin Barrett 9/97 ]

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