RFC 663

Network Working Group                         Rajendra K. Kanodia
Request for Comments #663                        MIT, Project MAC
NIC #31387                                      November 29, 1974



The current  Host-to-Host  protocol  does  not  provide  for  the
following three aspects of network communication:

     1. detection of messages lost in the transmission path
     2. detection of errors in the data
     3. procedures for recovery in the event of lost messages or
        data errors.

In this memo we propose an extension to the Host-to-Host protocol
that  will  allow  detection  of  lost  messages  and  an orderly
recovery from this situation.  If Host-to-Host protocol  were  to
be  amended  to  allow for detection of errors in the data, it is
expected that the recovery procedures proposed here  will  apply.

With  the  present  protocol,  it  may  some times be possible to
detect loss of messages in the transmission path.  However, often
a lost message (especially one on a control link) simply  results
in  an  inconsistent state of a network connection.  One frequent
(and frustrating) symptom of a message loss on a control link has
been the "lost allocate" problem which results in  a  "paralyzed"
connection.   The  NCP (Network Control Program) at the receiving
site  believes  that  sender  has  sufficient  allocation  for  a
connection,  whereas the NCP of the sending host believes that it
has no allocation (due to either loss of or error  in  a  message
that  contained  the  allocate  command).  The result is that the
sending  site  can  not  transmit  any  more  messages  over  the
connection.   This  problem  was  reported in the NWG-RFC #467 by
Burchfiel and Tomlinson.  They also proposed an extension to  the
Host-to-Host  protocol  which allows for resynchronization of the
connection status.  Their proposed solution was opposed by  Edwin
Meyer  (NWG-RFC  #492)  and  Wayne Hathaway (NWG-RFC #512) on the
grounds that it tended to mask  the  basic  problem  of  loss  of
messages  and  they  suggested  that  the  fundamental problem of
message loss should be solved rather than its  symptoms.   As  an
alternative  to  the  solution  proposed  in  NWG-RFC #467, Wayne
Hathaway suggested that Host-to-Host  protocol  header  could  be
extended  to include a "Sequence Control Byte" to allow detection
of lost messages.  At about the same time Jon Postel suggested  a
similar  scheme  using  message numbers (NWG-RFC #516).  A little
later David Walden proposed that four unused bits of the  message
sequence  number  (in  the IMP leader) be utilized for sequencing

                    - 1 -

messages (NWG-RFC #534).  His scheme is similar to those proposed
by Postel and  Hathaway;   however  it  has  the  advantage  that
Host-to-Host protocol mechanisms can be tied into the IMP-to-Host
protocol mechanisms.

The  protocol  extension  proposed here uses the four bits of the
message sequence number in the message leader  for  detection  of
lost  messages.  However, to facilitate recovery, it uses another
eight bit field (presently unused) in the 72 bit  header  of  the
regular  messages.   In the next section of this paper we discuss
some of the basic ideas underlying our protocol.  In  section  3,
we  provide  a  description of the protocol.  It is our intention
that section 3 be a self-contained and  complete  description  of
the protocol.


The  purpose  of this section is to provide a gentle introduction
to the central ideas on which this protocol  is  based.   Roughly
speaking,   our   protocol   can  be  divided  into  three  major
components.   First  is  the  mechanism  for  detecting  loss  of
messages.   Second  is  the  exchange  of information between the
sender and the receiver in the event  of  a  message  loss.   For
reasons  that  will soon become obvious, we have termed this area
as "Exchange of Control Messages".  The third  component  of  our
protocol  is  the  method of retransmission of lost messages.  In
this section, we have reversed the order of  discussion  for  the
second  and third components, because the mechanisms for exchange
of  control  messages  depend  heavily  upon  the  retransmission

A  careful  reader  will find that several minor issues have been
left unresolved in this section.  He (or  she)  should   remember
that this section is not intended to be a complete description of
the  protocol.   Hopefully, we have resolved most of these issues
in the formal description of the protocol provided in the section


The 32 bit Host-to-IMP and IMP-to-Host leaders contain a  12  bit
message-id  in  bit  positions  17 to 28 (BBN Report #1822).  The
Host-to-Host protocol (NIC 8246) uses 8 bits  of  the  message-id
(bit  positions 17 to 24) as a link number.  The remaining 4 bits
of the message-id (bits 25 to 28) are presently unused.  For  the
purposes  of  the  protocol to be presented here, we define these

                    - 2 -

four bits to be  the  message  sequence  number  (MSN  in  short)
associated  with  the link.  Thus message-id consists of an eight
bit link number and a four bit message sequence number.  The four
bit MSN provides a sixteen element sequence number for each link.
A network connection has a sending host (referred to as  "sender"
henceforth),   a   receiving   host   (referred  to  as  receiver
henceforth), and a link on which messages  are  transmitted.   In
our  protocol  the  sender starts communication with the value of
MSN set to one (i.e. the first message on any link has one in its
MSN field.)  For the next message on the same link the  value  of
MSN  is  increased  by one.  When the value of MSN becomes 15 the
next value chosen is one.  This results in the following sequence
1, 2, ...., 13, 14, 15, 1, 2, ...., etc.  The receiver can detect
loss  of  messages  by  examining  this  sequence.    Each   hole
corresponds  to  a  lost  message.   Notice  that  the  detection
mechanism will fail if a sequence of exactly 15 messages were  to
be   lost.   For  the  time  being,  we  shall  assume  that  the
probability of loosing a  sequence  of  exactly  15  messages  is
negligible.   However,  we  shall later provide a status exchange
mechanism (Section 2.6) that can be used to prevent this failure.

Notice that in the sequence described above we have  omitted  the
value  zero.   Following  a  suggestion made by Hathaway (NWG-RFC
#512) and Walden (NWG-RFC #534) the new protocol uses  the  value
zero  to  indicate to the receiving host that the sending host is
not using message sequence numbers.   We,  in  fact,  extend  the
meaning  associated  with  the  MSN  value zero to imply that the
sending host has not implemented the detection and error recovery
protocol being proposed here.


The discussion above brings us  to  the  issue  of  compatibility
between  the  present  and  the new protocols.  Let us define the
hosts with the present protocol to be type A and the  hosts  with
the new protocol to be type B.  We have three situations:

     1. Type  A  communicating  with  type  A:   there   is   no
        difference from the present situation.
     2. Type A communicating with type B: from  the  zero  value
        MSNs  in  messages  sent  by the type A host, the type B
        host can detect the fact that the other host is a type A
        host.  Therefore  the  type  B  host  can  simulate  the
        behaviour of a type A host in its communication with the
        other  host,  and  the type A host will not be confused.
        As we will see later  that  this  simulation  is  really
        simple and can be easily applied selectively.
     3. Type B host communicating with type B:  Both  hosts  can
        detect the fact that the other host is a type B host and

                    - 3 -

        use  the  message detection and error recovery protocol.

There is one difficulty here that we have not yet resolved.  When
starting communication how does a type B host  know  whether  the
other  host is type A or type B?  This difficulty can be resolved
by assuming that a type A host will not be confused by a non-zero
MSN in the messages that it receives.   This  assumption  is  not
unreasonable   because  a  type  A  host  can  easily  meet  this
requirement by making a  very  simple  change  to  its  NCP  (the
Network  Control  Program),  if  it does not already satisfy this
requirement.  Another assumption that is crucial to our protocol,
is that the type A hosts always set the  MSN  field  of  messages
(they send out) to zero.  As of this writing, the author believes
that   no  hosts  are  using  the  MSN  field  and  therefore  no
compatibility problem should arise.


Before getting down to the details of  the  actual  protocol,  we
will  attempt here to explain the essential ideas underlying this
protocol  by  considering  a   somewhat   simplified   situation.
Consider  a  logical  communication  channel  X, which has at its
disposal  an  inexhaustible  supply  of  physical   communication
channels  C(1),  C(2),  C(3),  ........,  etc.  (See footnote #1)
Channel X is  to  be  used  for  transmission  of  messages.   In
addition  to  carrying  the  data, these messages contain (1) the
channel name X, and (2) a Message Sequence Number (MSN).  Let  us
denote  the  sender  on  this channel by S and the receiver by R.
Let us also assume that at the start of the communication, R  and
S  are  synchronized  such that R is prepared to receive messages
for logical channel X  on the physical  channel  C(1)  and  S  is
prepared for sending these messages on C(1).  S starts by pumping
a  sequence  of  messages  M(1),  M(2), M(3), ........, M(n) into
channel C(1).  Since these messages contain sequence  numbers,  R
is  able to detect loss of messages on the channel C(1).  Suppose
now that R discovers that message number K (where K <n) was  lost
in  the  transmission  path.   Let  us further assume that having

(1) One method of recovery may be to let the  receiver  save  all
properly  received  messages and require the sender to retransmit
only those messages that were lost.   This  method  requires  the
receiver  to have the ability to reassemble the messages to build
the data stream.  A second method of recovery may be to abort and
restart  the  transmission  at  the  error  point.   This  method
requires  that  the receiving host be able to distinguish between
legitimate messages and messages to be ignored.   For  simplicity
we  have  chosen the second method and an inexhaustible supply of
physical  channels  serves  to  provide  the  distinction   among

                    - 4 -

discovered loss of a message, R can communicate this fact to S by
sending an appropriate control message on another logical channel
that is explicitly reserved for transmission of control  messages
from  R to S.  This channel, named Y, is assumed to be completely

We now provide a rather  simplistic  recovery  protocol  for  the
scenario sketched above. Having detected the loss of message M(K)
on channel X, R takes the following series of actions:

      1- R stops reading messages on C(1),
      2- R discards those messages that were received on C1  and
are  placed after M(K) in the logical message sequence,
      3- R prepares itself to read messages M(K), M(K+1), .....,
etc.  on the physical channel C(2),
  and 4- R sends a control message to S on  control  channel  Y,
which  will  inform  S  to the effect that there was an
error on logical channel X while using physical channel
C(1) in message number K.

When S receives this control message on Y, it takes the following

      1- S stops sending messages on C(1),
  and 2- begins  transmission  of  messages  starting  with  the
sequence number K, on the physical channel C(2).

This  resynchronization protocol is executed every time R detects
an error.  If physical channel C(CN) was being used at  the  time
of  the  error,  then the next channel to be used is C(CN+1).  We
can define a "receiver synchronization state"  for the channel X,
as the triplet R(C, CN, MSN), where C is the name of the group of
physical channels, CN is the number of the  physical  channel  in
use, and MSN is the number of message expected. (See footnote #1)
We can specify a message received on a given C-channel as M(MSN).
When R receives the message M(R.MSN) on the channel C(R.CN),  the
synch-state  changes  from  R(C,  CN,  MSN)  to  R(C, CN, MSN+1).
However if M.MSN for the message received is greater  than  R.MSN
then  a  message  has been lost, and R changes the synch-state to
R(C, CN+1,  MSN).   What  really  happens  may  be  described  as
follows:  upon  detection  of  error  in  a logical channel X, we
merely discard the physical channel that was in use at  the  time
of  error, and restart communication on a new physical channel at
the point where break occurred.

(1) Notice that we have prefixed this triplet  by  the  letter  R
(for  the  receiver.)   We  will  prefix  other similarly defined
quantities by different letters.  For example M can be  used  for
messages.   This  notation  permits  us to write expressions like
M.MSN = R.MSN, where M.MSN stands for the message sequence number
of the message.

                    - 5 -

This scheme provides a reliable transmission path X, even  though
the physical channels involved are unreliable.  In this scheme we
have  assumed  that  (1)  a  completely  reliable  channel  Y  is
available for exchange of control messages, and (2) that there is
a large supply of physical channels  available for use of X.   In
the  paragraphs that follow we shall revise our protocol to use a
single physical channel and  then  apply  this  protocol  to  the
channel Y in such a way that Y would become "self-correcting."

Now  suppose  that channel X has only one physical channel (named
X') available for its use rather than the inexhaustible supply of
physical channels.  Our protocol would still work,  if  we  could
somehow simulate the effect of a large number of C-channels using
the  single  channel X'.  One method of providing this simulation
is to include in each message the name of the C-channel on  which
it  is  being  sent,  and  send  it on X'.  Now the receiver must
examine each message received on X' to determine the C-channel on
which this message was sent.  Our protocol still works except for
one minor difference,  namely,  the  receiver  must  now  discard
messages  corresponding  to C-channels that are no longer in use,
whereas in the previous system the  C-channels  no  longer  being
used  were  simply  discarded.  To be sure, X' can be multiplexed
among only a finite number of C-channels; however, we can provide
a sufficiently large number of C-channels so that during the life
time of the logical channel X, the probability of exhausting  the
supply  of  C-channels would be very low.  And even if we were to
exhaust the supply of C-channels, we could recycle them  just  as
we recycle the message sequence numbers.

A  physical  message received on X' can now be characterized by a
pair of C-channel number and a message sequence number, as  M(CN,
MSN).  The receiver synchronization state becomes a triplet R(X',
CN,  MSN).   This  state  tells  us  that R is ready to receive a
message for X on the physical channel X'  and  for  this  message
M.CN  should be equal to R.CN and M.MSN should be equal to R.MSN.
All messages with M.CN less than R.CN will be  ignored.   If  for
the  next  message received on X', M.CN = R.CN and M.MSN = R.MSN,
then R changes the synch state to R(X', CN, MSN+1).   If  M.CN  =
R.CN  but  M.MSN  >  R.MSN  then a message has been lost and  the
synch-state R(X', CN, MSN) changes to R(X', CN+1,  MSN).   Notice
that  we  have  not  yet said anything about the situation M.CN >
R.CN.  We will later describe a scheme for  using  this  case  to
provide for error correction on the control channel itself.


So  far  we  have  discussed  two  schemes  for the detection and
retransmission aspects of  the  lost-message  problem.   In  this

                    - 6 -

section, we discuss methods by which the receiver communicates to
the sender the fact of loss of messages.

We continue with the scenario developed in the above section with
a small change.  For the purposes of the discussion that is about
to  follow  we  shall  assume that there are actually two perfect
channels available for exchange of control messages.  One channel
from S to R named S->R, and the other from R  to  S  named  R->S.
The  purpose  of S->R will become clear in a moment.  In order to
let R communicate the fact of loss of messages to S, We provide a
control message called L__o_s_t__M_e_s_s_a_g_e__f_r_o_m__R_e_c_e_i_v_e_r (LMR) which  is
of  the  following  form: LMR(X, CN, MSN), where X is the name of
the channel, CN is the new  C-channel  number,  and  MSN  is  the
message  sequence  number  of the lost message.  If more than one
message has been lost, then R uses the MSN of the  first  message
only.  When S receives this message, it can restart communication
at  the  point  where  the  break  occurred  using  the C-channel
specified  by  the  LMR   message.    This   will   restore   the
communication  path  X.   What  happens  if  S  can  not  restore
communication at the break point because it  does  not  have  the
relevant  messages  any more?  This issue can be solved in one of
the two ways: either let the protocol specify a fixed rule that S
will be required to close the connection, or the  protocol  could
allow  S  and R (and may be the users on whose behalf S and R are
communicating on X) to negotiate the action to be taken.  For the
protocol to be presented here, we have taken the approach that  S
may, at its option, either close the connection or negotiate with
R.   What  action  a specific host takes can either be built into
its NCP or determined dynamically.  Those hosts that do not  have
very  powerful  machines will probably chose the static option of
closing the  connection;   other  hosts  may  make  the  decision
depending upon the circumstances.  For example, a host may decide
that  loss  of  messages  is not acceptable during file transfers
whereas  loss of a single message can  be  ignored  for  terminal
output  to  interactive  users.   A  host might even let the user
processes decide  the  course  of  action  to  be  taken.   If  S
determines  that it can not retransmit lost messages, it may want
to let R decide what action is to be taken.   If  S  so  decides,
then  it  may  communicate  this  fact  to  R  by  transmitting a
_L_o_s_t__M_e_s_s_a_g_e__f_r_o_m__S_e_n_d_e_r  (LMS)  control  message  to  R  on  the
channel  S->R.   An LMS message is of the following form:  LMS(X,
CN, MSN, COUNT), where X is the name of the channel,  CN  is  the
name  of  the C-channel obtained from the LMR message, MSN is the
message sequence number of the first message in the  sequence  of
lost  messages,  and  COUNT  is  the  number  of  messages in the
sequence.  S resets its own synch-state for connection X to  S(X,
CN,  MSN+COUNT).   When  S  has  informed  R  of its inability to
retransmit lost messages, the burden of the decision falls on  R,
and  S  simply  awaits R's reply before taking any action for the
channel X.  When R receives the LMS, it may  either  decide  that
loss  is  unacceptable and close the connection, or it may decide
to let S continue.  In the later case R informs S of its decision

                    - 7 -

to continue by transmitting  a  L__o_s_s__o_f__M_e_s_s_a_g_e__A_c_c_e_p_t_a_b_l_e  (LMA)
control  message to S.  Upon receiving the LMA control message, S
resumes transmission on X.  To avoid the possibility of errors in
exchange  of  control  messages,  the  LMA  control  message   is
specified  to  be  an  exact  replica of the LMS control message,
except for the message code which determines  whether  a  control
message is LMA or LMS or something else.

In  general,  a  sending host has only a limited amount of memory
available for storing messages for possible retransmission later.
In section 2.6 we provide a status exchange mechanism that can be
used to determine when to discard these messages.


We continue our discussion with the  scenario  developed  in  the
previous  section.  The receiver R may detect loss of messages on
control channels by examining the message sequence numbers on the
messages.  When R detects that a message has  been  lost  on  the
channel   S->R,  it  (R)  may  transmit  an  LMR  to  S  on  R->S
communicating the fact of loss of messages.  When S receives  the
LMR  for  the  control  link,  it must either retransmit the lost
messages,  or  "close"  the  connection.  (In  the   context   of
Host-to-Host protocol, closing the network connection for control
link  implies exchange of reset commands, which has the effect of
reinitializing all communication between R and S.)   For  control
links,  S  does  not  have  the  option  of sending an LMS to the
receiver.  If S can not retransmit  the  lost  messages  then  it
aborts  the  output  queue  (if  any)  for  the channel S->R, and
inserts a Reset command at the break point.  Essentially, we  are
specifying  that  if  S  can not retransmit lost control messages
then the error would be considered irrecoverable and fatal.   All
user  communication  between  S  and  R  is  broken  and  must be
restarted from the beginning.

In the above paragraph, we considered the situation  in  which  R
was  able to detect the loss of messages.  It is however possible
that it is the last message transmitted on S->R that is lost.  In
this  case,  R will not be aware of the loss.  In this situation,
recovery can  be  initiated  only  if  S  can  either  positively
determine  or  simply  suspect  that a message has been lost.  In
general, after having transmitted a control message, S  would  be
expecting  some  sort  of  response  from  R.  For  example, if S
transmits a Request-for-Connection (RFC) control message to R,  S
expects  R  to reply either with a Close (CLS) command or another
RFC.  If, after an appropriate time  interval has elapsed  and  S
has  received  no reply from R, our protocol specifies that S may
retransmit the control message.  In retransmitting,  S  must  use

                    - 8 -

the same C-channel and MSN values that were used for the original
message.    Since  R  can,  now,  receive  duplicate  copies,  we
stipulate that if R receives a duplicate of the message  that  it
has already received, it may simply ignore the duplicate.


There  are  two problems that have not yet been solved.  First, a
sending host will usually have only a limited  amount  of  buffer
space   in   which  it  can  save  messages  for  possible  later
retransmission.  So far, we have not provided any method by which
a  host  may  positively  determine  whether  the  receiver   has
correctly received a certain message or not.  Thus it has no firm
basis  on  which  it  may  decide to release the space used up by
messages that have been already transmitted.  The second  problem
is  created  by  our recycling the message sequence numbers.  For
the MSN mechanism to work correctly, it is essential that at  any
given  instant  of  time there be no more than 15 messages in the
transmission path.  If there were more than 15 messages, some  of
these  messages  would have same MSN and LRN, and if one of these
messages were to be lost, it would be impossible to identify  the
lost  message.   However,  the second problem should not arise in
the ARPA Network, since the IMP sub-network will not  allow  more
than  eight  outstanding  messages between any host pair (NWG-RFC

We can solve both these problems by a simple yet highly  flexible
scheme.  In this scheme, there are two new control messages.  One
of these, called R__e_q_u_e_s_t S__t_a_t_u_s _f_r_o_m S__e_n_d_e_r (RSS), can be used by
the  sending  host to query the receiver regarding the receiver's
synch-state.  The receiver can supply  its  copies  of  C-channel
number  and  MSN for a transmission path by sending a S__t_a_t_u_s _f_r_o_m
R__e_c_e_i_v_e_r (SFR) control message over the control channel.  An  SFR
provides  positive  acknowledgement;  differing  with  the  usual
acknowledgement schemes in  only  that  here  acknowledgement  is
provided  only upon demand.  Upon receiving SFR, the sender knows
exactly which messages have been properly delivered, and  it  may
free  the  buffer  space used by these messages.  The RSS and SFR
scheme can also be used to ensure that there  are  no  more  than
fifteen messages in the transmission path at any given time.

                    - 9 -


This  protocol  is  proposed  as an amendment to the Host-to-Host
Protocol for the purpose of letting  hosts  detect  the  loss  of
messages  in  the  network. It also provides  recovery procedures
from such losses.  This protocol is divided into two parts.  Part
1 states the compatibility requirements.  Part 2 states  the  new
protocol and must be implemented by hosts that desire to have the
ability  to  recover  from  loss of messages in the network.  The
reader  will  find  many  comments  interspersed  throughout  the
description of this protocol.  These comments are not part of the
protocol and are provided solely for the purpose of improving the
reader's understanding of this protocol.

The  terminology used in this protocol is similar to that used in
the Host-to-Host protocol.  We speak of  a  "network  connection"
between  a pair of hosts, called the "receiver" and the "sender."
A network connection is described by a pair  of  socket  numbers,
and  once  established, a network connection is associated with a
link (which is described by a link number.)  A network connection
is a logical communication path and the link assigned to it is  a
physical  communication  path.   In  addition to links associated
with the network connections, there are "control-links"  for  the
transmission  of  "control  commands."   When  we  use  the  term
"connection" it may refer to either a network connection  or  the
link  assigned  to  it;  the context decides which one.  The term
"links" encompasses the connection-associated-links  as  well  as
control-links.   Notice  that  a  receiver  of  a  connection may
transmit control commands regarding this connection.


3.1.1 HOST TYPE "A"

Those hosts that have not adopted the part  2  of  this  protocol
amendment will be known as type A hosts.

(Comment: All existing hosts are type A hosts.)

3.1.2 HOST TYPE "B"

Those  hosts,  that  adopt  the part 2 of this protocol amendment
will be known as type B hosts.

                   - 10 -


A four bit number associated with regular messages and  contained
in  the  bits  25  through  28 of the Host-to-IMP and IMP-to-Host
leaders for regular messages [BBN Report No. 1822].  This  number
is  used  by  the  type  B hosts to detect loss of messages  on a
given link.  Type A hosts always set the MSN  (for  the  messages
they  send out) to zero.  When in use by a type B host, the first
message on a link (after the connection has been established) has
the MSN value of one.  For each successive message on this  link,
the MSN value is increased by one until it reaches a value of 15.
The next message has the MSN value of one.

(Comments:  Type  B  hosts  will  use the MSN mechanism only when
communicating with other type B  hosts.  If  a  type  B  host  is
communicating   with  a  type  A  host,  the  type  B  host  will
essentially simulate the behaviour of a type A host and use  zero
MSN values for this communication.)


The  Link Resynch Number is an eight bit number associated with a
link and used for resynchronizing the communication.   For  links
associated  with  a  network  connection (i.e. user links), it is
intially set to zero.  For control links, it is set  to  zero  at
the  time  of  the  Network Control Program (NCP) initialization.
For a given link, its current LRN value is copied  into  the  LRN
field  of  all messages sent out on the link.  The LRN values may
be manipulated by type B hosts in accordance  with  the  protocol
described later.

(Comments:   Our  protocol  specifies  that for all communication
involving a type A host, the LRN value  will  never  change  from
zero.   Since the LRN field is presently unused, all hosts set it
to zero even though they do not explicitly recognize  this  field
as an LRN field.  This guarantees compatibility.)


An  eight bit field in the bits 33 through 40 of the Host-to-Host
message header.

                   - 11 -



|     I      I      I     I
I LMR | link | LRN  | MSN I

The LMR command is sent by a receiving host to  let  the  sending
host  know  that  one  or  more messages have been lost.  The MSN
field specifies the message sequence number of the first  message
lost.   The  LRN  field  specifies the new LRN value that must be
used if and when communication is restored.

(Comments:  As will be seen later, the LMR command also  has  the
effect of resetting the bit and message allocation in the sending
host   to   zero.   In  order  to  permit  a  sender  to  restore
communication, an LMR command will be usually accompanied with an
allocate command.  However notice  that  these  comments  do  not
apply  to  the  control  links  because  there  is  no allocation
mechanism for the control links.)


I        I       I        I         I       I
I  LMS   I Link  I  LRN   I  MSN    I COUNT I

This command is sent by a sender host in reply to an LMR  command
if it (the sender) can not retransmit the lost messages specified
by  the LMR command.  The purpose of this command is to query the
receiver whether or not  the  loss  of  messages  is  acceptable.
After  sending  this command, the sender waits for a reply before
transmitting any messages over the link involved.   This  command
may  not  be  sent for control links.  The LRN and MSN values are
same as those specified by the LMR command.  COUNT specifies  the
number of messages lost.


This  command  is  identical  to  the  LMS command accept for the
command code.  Upon receipt of an LMS  command,  a  receiver  may

                   - 12 -

send  this command to the sender to let the sender know that loss
of messages is acceptable.  All fields in this command are set to
corresponding values in the LMS command.

3.2.4  CLS2 - CLOSE2

I       I              I                 I       I      I
I CLS2  I  my socket   I your socket     I  LRN  I MSN  I

The CLS2 command is similar to the current CLS command except for
the LRN and MSN fields included in the  new  command.   Both  the
receiving and sending hosts copy their values of LRN and MSN into
the CLS2 command.  Upon receiving a CLS2 command, a host compares
the LRN and MSN values contained in the CLS2 command with its own
values  for  the  connection  involved.   If  these values do not
match, then an  error  has  occurred  and  a  host  may  initiate
recovery procedures.

(Comments:   The  purpose  of  this command is to ensure that the
last message  transmitted  on  a  connection  has  been  received


I        I             I             I
I  ECLS  I my socket   I  your socketI

The  ECLS  command  is similar to the current CLS command.  It is
used  for   closing   connections   which   have   sufferred   an
irrecoverable loss of messages.

(Comments: A connection may be closed in any one of the following
three ways:

      1. A connection which has not yet been opened  succesfully
may  be  closed  by  the  CLS command.  All connections
involving type A hosts must be  closed  using  the  CLS
      2. Connections between type B hosts may  be  closed  using
CLS2  command.   A connection is considered closed only
if matching CLS2 commands have been  exchanged  between

                   - 13 -

the sender and the receiver.
      3. Those connections  between  type  B  hosts,  that  have
sufferred  an  irrecoverable  loss of messages, must be
closed by the ECLS command.)


I      I        I

A sending host may issue an RSS command to  find  out  about  the
status  of transmission on a particular connection or the control


I       I        I
I RSR   I LINK   |

A receiver can issue an RSR command to find out about the  status
of  transmission  on a particular connection or the control link.


I        I        I        I       I
I SFR    I  LINK  I  LRN   I MSN   I

A receiving host may issue this  command  to  inform  the  sender
about  the  state of a particular connection or the control link.


                   - 14 -

I        I        I        I       I
I SFS    I  LINK  I  LRN   I MSN   I

A sending host may issue this  command  to  inform  the  receiver
about  the  state of a particular connection or the control link.


3.3.1 PART ONE

All type A hosts must use zero MSN and LRN values on the messages
sent out by them.  When communicating with a host of  type  A,  a
type B host must simulate the behaviour of type A host.

(Comments:   Notice  that  this  simulation is not complicated at
all.  All that  is  required  is  that   hosts  that  adopt  this
protocol  must  not use it when communicating with the hosts that
have not adopted it.)

3.3.2 PART TWO

This part of the protocol is stated as a set of rules which  must
be  observed  by  all  type B hosts when communicating with other

    (1). A type B sending host must use message sequence numbers
on all regular messages that it sends to other  type  B
hosts  as  specified  in  the definition of the message
sequence numbers (Section 3.1.3).
    (2). A type B sending host must use link resynch numbers  on
all  regular  messages  that  it  sends to other type B
hosts as specified in the definition  of  link  resynch
number (Section 3.1.4).
    (3). A sending host may retransmit a message if it  suspects
that  the  message  may  have  been lost in the network
during previous transmission.
    (4). A sending host may issue an RSS command to the receiver
to determine the state of transmission on any link.
    (5). A sending host must use the ECLS  command  to  close  a
connection, if the connection is being closed due to an

                   - 15 -

irrecoverable  transmission  error.  Otherwise, it must

    (1). A receiver host will maintain LRN and  MSN  values  for
each link on which it receives messages.  Initial value
of  LRN  will be zero, and initial value of MSN will be
one.   For  each  receive  link,  the  host  should  be
prepared  to  receive a message with LRN and MSN values
specified by its tables.  When the  host  has  received
the  expected  message  on a given link, it will change
its table MSN value as specified in the  definition  of
    (2). On a given link, if a host receives a message  with  an
LRN  value  smaller than the one in use, it will ignore
the message.
    (3). If a host receives a duplicate message  (same  LRN  and
MSN values), it will ignore the duplicate.
    (4). A host will examine  the  MSN  values  on  all  regular
messages  that  it receives to detect loss of messages.
If on any link, one or more messages are found missing,
it will concern itself with only the first message lost
and take the following series of action:

       1. Increase its own LRN value for this  link  by
       2. Send an LMR command to the sending host  with
          LRN  field set to the new value and MSN field
          set to  the  sequence  number  of  the  first
          message lost.
       3. Realizing that LMR  command  will  cause  the
          allocation  to be reset to zero, it will send
          more allocation. This is  not  applicable  to
          the control links.

However,  if  a  host  does  not  want  to initiate the
recovery procedures, it may simply close the connection
by an ECLS command.
    (5). A receiver host may  issue the RSR command to determine
the state of transmission on any link.
    (6). If a connection is being closed due to an irrecoverable
error, a receiving host  must  use  the  ECLS  command.
Otherwise it must use the CLS2 command.

                   - 16 -

    (1). RSR command: the sender must transmit a SFS command  to
the receiver for the link involved.
    (2). ECLS command: The sender must cease transmission, if it
has  not  already done so, and issue an ECLS command if
it has  not  already  issues  either  a  ECLS  or  CLS2
    (3) CLS2 command: The sender must compare the  LRN  and  MSN
values  of  the CLS2 command with its own values of the
LRN and MSN for the connection involved.  If  an  error
is  indicated,  it may either close the connection with
an ECLS, or initiate recovery action  as  specified  in
the section
    (4). LMR command for  a  connection  (i.e.,  not  a  control
link):  The  sender may follow any one of the following
three courses of action:

       1. Close the connection with an ECLS command.
       2. Set the allocations for the link involved  to
          zero,  set  LRN  to that specified in the LMR
          command, and  restart  communication  at  the
          point of break.
       3. Set the allocations for the link involved  to
          zero,  set  the  LRN to that specified in the
          LMR command, and send an LMS command  to  the
          receiver  host informing him that one or more
          of   the   lost   messages   can    not    be
          retransmitted.  After sending an LMS command,
          a  sending  host  must  not transmit any more
          messages  on  the  link  involved  until  and
          unless  it  receives  an LMA command from the
          receiver host.

(Comments:  As  we  have  mentioned  before  (Section  2.3),  the
decision  regarding which course of action to follow depends upon
a number of factors.  For example, if a host has implemented only
the detection of lost messages aspect of  our  protocol  (and  no
recovery),  then  it  will  chose the first option of closing the

    (5). LMR for a control link: The sender may take one of  the
following two actions:

       1. Set the LRN to  that  specified  in  the  LMR
          command  and  begin  retransmission  of  lost
       2. Set the LRN to  that  specified  by  the  LMR
          command,  and  insert  a Reset command at the
          break point.

                   - 17 -

(Comment:  If a sending host can not retransmit messages lost  on
a   control   link,   then   this   protocol  requires  that  all
communication between the two host be broken, and  reinitialized.
We do not explicitly specify the actions that are associated with
the  exchange  of Reset commands.  These actions are specified by
the Host-to-Host protocol.)

    (6). LMA command:  When  a  sending  host  receives  an  LMA
command  matching  an  LMS command previously issued by
it, it may resume transmission.

(Comments: The protocol does not require the sending host to take
any specific action with regard to a SFR. However, a sending host
may use the information contained in the  SFR  command  regarding
the  state of transmission.  From a SFR command a sender host may
determine what messages have been received properly.  The  sender
may   use  this  information  to  cleanup  its  buffer  space  or
retransmit messages that it might suspect are lost.) RECEIVING HOST'S RESPONSE TO CONTROL MESSAGES

    (1). RSS command: A receiver is obligated to transmit a  SFR
to the sender for the link involved.

    (2). ECLS command:  The receiver must close  the  connection
by  issuing  an ECLS command if it has not already done

    (3). CLS2 command: A receiver must compare the LRN  and  MSN
values  of  the  command  with  its  own values for the
connection involved.  If an error is indicated, it  may
either  close  the  connection  by  an  ECLS command or
initiate recovery procedures as  specified  in  section

    (4). LMS command: The receiver may take one of the following
two courses of action:

     (1). Close the connection  specified  by  the  LMS
          command, by issuing an ECLS command.
     (2). Set the  link  involved  to  be  prepared  to
          receive  messages  starting with the sequence
          number MSN + COUNT, where MSN and  COUNT  are
          those   specified   by   the   LMS   command.
          (Comment: This action implies  that  receiver
          is  willing  to  accept  the loss of messages
          specified by the LMS command.)

(Comments: The protocol does not require the receiver to take any
specific action with regard to a SFS command. However a  receiver

                   - 18 -

host may use the information  contained in it.)


The  design  of  this  protocol  has been governed by three major
principles.  First, we believe that to be useful within the  ARPA
Network,  any  new  protocol must be compatible with the existing
protocols, so that each host can make the transition to  the  new
protocol at its own pace and without large investment.  Secondly,
the  protocol  should  tie  into  the  recovery  mechanism of the
IMP-to-Host Protocol.  The price we pay for this is the small MSN
field and a   message oriented protocol rather than a byte stream
oriented protocol.  The third consideration has been flexibility.
While this protocol guarantees detection of  lost  messages,  the
philosophy  behind  the recovery procedures is that a host should
have several options, each option providing a different degree of
sophistication.  A host can implement a recovery  procedure  that
is  most  suitable  for  its  needs  and  the capabilities of its
machine.  Even though two hosts may  have  implemented  different
recovery  procedures,  they  can communicate with each other in a
compatible way.  In a network of independent machines  of  widely
varying  capabilities and requirements, this seems to be the only
way of implementing such a protocol.  Even though  this  protocol
provides  a  variety  of  options in a given error situation, the
choice of a specific action must be  consistent  with  the  basic
requirements  of  the  communication  path.  For example, partial
recovery is not  acceptable  during  file  transfers.   We  fully
expect   the  File  Transfer  Protocol  to  specify  that  if  an
irrecoverable error occurs, the file transfer must be aborted.

                   - 19 -