Internet Experiment Note 108

                      Internet User Queues

                       William W. Plummer

                  Bolt Beranek and Newman, Inc.
                        50 Moulton Street
                      Cambridge MA   02138

                          19 June 1979

                      Internet User Queues
William W. Plummer                                   19 June 1979

The Internet JSYSes provide a facility for user programs to  send
and  receive  Internet packets.  This set of calls is parallel to
the "IMP special queue" calls, which dealt only with the ARPANET.

In order to use the Internet calls a program must first assign an
Internet Queue using ASNIQ (JSYS 756).  The  arguments  for  this
JSYS  are  a  set  of  mask  and  value words that determine what
messages may be sent and which of the incoming messages  will  be
delivered  to  the owning job.  A successful ASNIQ returns with a
handle (a small number, short-hand for the queue),  in  T1  which
can  then  be  fed  to  SNDIN  and  RCVIN (SeND InterNet, ReCeiVe

The arguments for SNDIN  are  an  Internet  Queue  Handle  and  a
pointer  to  a  buffer  of data in the caller's space.  Word-0 of
this buffer is the size of the buffer, including this count word.
Words 1 through 5 must have a Version 4 Internet Header (see last
page).  Various fields of the header are  checked  for  legality,
the  source  host word is filled in, the Internet header checksum
inserted.  Then a network  interface  is  selected  and  a  local
leader for that network is constructed in the packet.  The packet
is then queued for output on that interface and SNDIN returns.

Basically, users have no control over which interface is selected
for packet output and no knowledge of which interface an incoming
packet  arrived on.  This is analogous to the ARPANET where users
have no control over which IMPs a message actually goes  through.
There  is  an  exception  when  a  system is equipped with a "Raw
Packet Interface" (RPI) which is used  to  connect  to  a  remote
gateway  or  a encryption device (such as a BCR).  Bit zero in T1
to ASNIQ will flag the queue as using the  RPI  rather  than  the
normal interfaces.

RCVIN  takes  the  same arguments, an Internet Queue Handle and a
buffer pointer.  Normally, if no messages are already waiting  on
the  queue, RCVIN waits.  This wait can be defeated by turning on
the "don't wait" control bit which forces an error return  if  no
messages  are  waiting.   When  a  message  is  available,  it is
delivered to the user- supplied  buffer.   The  number  of  words
filled  is  set  into the left half of word-0 (the count word) of
the buffer.  Ordinarily this can be ignored, but if  the  message
was  too big for the buffer, this tells how much space would have
been required.

When finished, the queue handle can be released by  using  RELIQ.
All  handles  owned by the job may be released by supplying -1 as
the argument.

                              - 1 -

                      Internet User Queues
William W. Plummer                                   19 June 1979

All Internet JSYS calls use only the left 32 bits of  each  word.
This  is  true  of  both  the  ASNIQ  argument  block and of data

Messages  left  waiting  on  an  input  queue  for  a  long  time
(currently  30  seconds)  will  be deleted.  Also, a queue cannot
hold very many messages (32, today).  Flooding an Internet  input
queue to a slow receiver will result in dropped messages.

Messages  addressed  to  the  sending  host  will  ordinarily  be
delivered without  sending  over  any  network  at  all  and  are
reasonably fast.

User  programs have no control over which network a given message
will be sent out over.  This decision will be made  by  the  host
gateway module on the basis of routing information supplied to it
by  other  Internet  gateways.   This  means that all networks to
which  a  host  is  connected  must  go  down   before   Internet
communications  will  be completely stifled, and even then, forks
within a job will  be  able  to  communicate  due  to  the  local
delivery mechanism.

Programs  using Internet messages must be aware that messages are
not necessarily delivered in the order in which they  were  sent,
some  messages  may  be  dropped and others duplicated.  Some may
traverse a broadcast network and be clobbered by  other  packets,
lightning,  flaky  intermediate gateways, etc.  Thus, some higher
level protocol is needed in most cases.  TCP is one example,  but
there are others (the "Datagram Protocol", XNET, etc.).

If  a  particular  protocol  is implemented in the monitor (e.g.,
TCP) and that protocol module is turned on, no messages  of  that
protocol  type  will  be  passed  to  users  via Internet queues.
Assigning a queue will still be possible,  but  no  traffic  will
reach the user unless that protocol module is disabled.

Not  all  Internet protocols have ports.  If you are implementing
one which does not, be  sure  that  .IQPTM  in  the  ASNIQ  block
contains  a  zero.  If the protocol uses ports, they are expected
to be in the  first  two  16-bit  bytes  following  the  Internet
header,  source  port  first  and  destination  port second.  The
location of this word is found by  adding  the  contents  of  the
Internet  data  offset field to the address of the zeroth word of
the Internet header.

                              - 2 -

                      Internet User Queues
William W. Plummer                                   19 June 1979

An additional feature of the Internet Queue Mechanism is that  it
(optionally)  provides  an  logical  host  capability.   That is,
incoming messages can be demultiplexed on the basis  of  bits  in
the  Internet  Destination  Host  field.   Exactly which bits are
considered logical host specifier  bits  depends  on  the  naming
conventions  in  force in the network (area) to which the host in
question is connected.  For ARPANET hosts, the middle 8-bit  byte
of  the 24-bit Internet Host is considered to be the logical host

ASNIQ takes a specification of what numbers are to be used in the
logical host bits of the source address  for  packets  which  are
sent  and received.  When a packet is sent, the logical host bits
are obtained  from  the  user's  packet  and  combined  with  the
Internet  Host  Identifier  for  the  host being used, the result
being  inserted  into  the  Source  Host  field  of  the  packet.
Incoming packets are accepted by the Internet Layer if they match
the Internet Host Identifier (Name) when the logical host bits of
the destination address have been set to zero.

It  is  illegal to assign an Internet Queue such that the Logical
host mask includes any bits which are not  part  of  the  logical
host field.  ASNIQ will give an error return in this case.

                              - 3 -

                      Internet User Queues
William W. Plummer                                   19 June 1979

Assign Internet Queue JSYS

;T1/    Pointer to queue descriptor block
;       (Bit-0: use Raw Packet Interface)
;T2/    (Unused, must be 0)
;T3/    (Unused, must be 0)
;       ASNIQ           (JSYS 756)
;Ret+1:  Failed.  Error code in T1.
;Ret+2: Success.  Internet queue handle in T1.

Format of a queue descriptor block:

        Word offset     Name    Contents

             0          .IQPRV  Internet protocol number in
                                bit 24-31.  Others should be 0.

             1          .IQFHV  Internet foreign host value word in
                                bits 0-31.

             2          .IQSHV  Internet source host value word in
                                bits 0-31.  Used for logical host selection.

             3          .IQPTV  Internet port value word.  Local port
                                value in bits 0-15, foreign port in 16-31.

             4          .IQPRM  Mask word corresponding to .IQPRV.

             5          .IQFHM  Mask word corresponding to .IQFHV

             6          .IQSHM  Mask word corresponding to .IQSHV

             7          .IQPTM  Mask word corresponding to .IQPTV
                                (Use 0 for portless protocols)

The  mask  words specify those bit positions where an exact match
is required.  Thus, one can make  .IQFHM  contain 0 in  order  to
talk  to  all Internet hosts.  Or by making say the low 3 bits of
the local port mask word be 0, one owns eight ports.   Note  that
an error will result unless the current QDB differs in the masked
in  bits from all other Internet queues which are assigned at the
instant the ASNIQ is issued.

Possible errors:

600737,NTWZX1,NET WIZARD capability  required
600740,ASNSX1,All Internet queues in use

                              - 4 -

                      Internet User Queues
William W. Plummer                                   19 June 1979

600741,ASNSX2,Conflict with some other job (# in AC2)

                              - 5 -

                      Internet User Queues
William W. Plummer                                   19 June 1979

Release Internet Queue JSYS

;T1/    An Internet Queue Handle or -1 for all
;T2/    (Not used.  Must be 0)
;T3/    (Not used.  Must be 0)
;       RELIQ   (JSYS 757)
;Ret+1:  Failure.  Error code in T1.
;Ret+2: Success.

This JSYS releases ownership of an Internet queue so  that  other
jobs  can  assign  it.   Note that neither the RESET JSYS nor the
EXEC RESET command execute RELIQs.   Thus,   once  the  queue  is
assigned, it stays assigned until the job logs out.

Possible errors:

600742,SQX1,Internet queue handle out of range
600743,SQX2,Internet queue not assigned

                              - 6 -

                      Internet User Queues
William W. Plummer                                   19 June 1979

Send an Internet Message JSYS

;T1/    Internet Queue Handle
;T2/    Pointer to buffer containing message
;T3/    (Not used. Must be 0.)
;       SNDIN   (JSYS 754)
;Ret+1:  Failure.  Error code in T1.
;Ret+2: Success.

The buffer must contain a word count in the right half of word-0,
a  valid  Internet header in words 1 through 5, and possibly some
data in words 6 and following.  If port filtering is  being  used
(.IQPTM was non-zero for ASNIQ), the ports must be located in the
word  following the Internet header.  The address of this word is
found by adding the address of word-1 in the buffer to the number
in the Internet data offset field.

The monitor fills in the source host field in the packet and also
the Internet header checksum.  The rest of the header is what  is
supplied by the user.

Possible errors:

600742,SQX1,Internet queue handle out of range
600743,SQX2,Internet queue not assigned
600732,SNDIX1,Invalid message size
600733,SNDIX2,Insufficient system resources (No buffers available)
600735,SNDIX4,Invalid header value for this queue
                (Includes Internet Packet Length too big, Data offset
                too small, filtering on ports but packet length says
                packet does not contain a port word, and header does
                not fit the ASNIQ arguments).

                              - 7 -

                      Internet User Queues
William W. Plummer                                   19 June 1979

Receive Internet Message JSYS

;T1/    Flags,,Internet Queue Handle
;               Bit-0: On to cause fail return instead of waiting
;T2/    Pointer to buffer where message will be put
;T3/    (Not used.  Must be 0.)
;       RCVIN   (JSYS 755)
;Ret+1:  Failure.  Error code in T1.
;Ret+2: Success.

Each RCVIN gets one message from the named queue.  These messages
match the values in the queue descriptor block when masked by the
mask  words  in  the  block.  The number of words filled plus one
(counting the count word) is placed in the left half of word-0 of
the buffer.  If the message was too  big  as  determined  by  the
Internet  data length field, as much as will fit in the buffer is
transferred and an error return given.  No  retry  for  the  same
message is possible.

Possible errors:

600742,SQX1,Internet queue handle out of range
600743,SQX2,Internet queue not assigned
600732,SNDIX1,Invalid message size

                              - 8 -

                      Internet User Queues
William W. Plummer                                   19 June 1979

; The following structure definitions have been excerpted from
; the file INPAR.MAC which is used in building TENEX and TOPS20
; monitors with Internet code.  Those who are unfamiliar with the
; structure facility should refer to SYS:MACSYM.MAC.  Basically,
; DEFSTR(Name,Location,Bit,Size)  defines a structure called Name.
; This field is held in the word (possibly an index) at Location.
; The right-hand bit number is Bit and the field is Size bits wide.
; A typical reference to the data offset field might be:   LOAD T1,PIDO,(PKT)
; where PKT is an index register which points at the zero-th word
; of the structure.
; Word offsets:

 .IPKVR==0              ; Word with version, type of service, etc
 .IPKSG==1              ; Word with segmentation info
 .IPKPR==2              ; Word with time to live, checksum, protocol
 .IPKSH==3              ; Word with source host
 .IPKDH==4              ; Word with destination host

  ; Overlays for the above:
  ; Overlays for the above:
  ; Unused bit
  ; Overlays for above:
  ; Overlays for above:

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