rfc:ien:ien42
IEN: 42 Winston Edmond Section: 2.2.2.5.1 BBN
12 June 1978
SATNET Information
Addresses
Definitions
The internet gateway is that part of the PDP-11 code which passes
messages between networks. It is a Host on two different nets.
The speech host is a different piece of PDP-11 code.
The internal gateway is part of the SIMP code which performs
limited gateway functions. It is a Host both on SATNET and one
other network.
List of Hosts on SATNET.
At Etam:
The BBN gateway is ARPANET Host 3 on IMP 40 and is SATNET Host
61 (75 octal).
The PDP-11 speech Host is SATNET Host 31.
There is an internal gateway, ARPANET Host 4 on IMP 39 (SDAC)
and SATNET Host 32.
At Goonhilly:
The UCL gateway is ARPANET Host 3 on IMP 42 (LONDON), and is
SATNET Host 60 (74).
The PDP-11 speech Host is SATNET Host 32.
At Tanum:
The NDRE gateway is ARPANET Host 3 on IMP 41 (NORSAR), and is
SATNET Host 38 (46).
The PDP-11 speech host is SATNET Host 33.
At Clarksburg:
The PDP-11 internet gateway is SATNET Host 39.
The PDP-11 speech Host is SATNET Host 34.
There is an internal gateway, RCCNET (Net 3) Host 1 on IMP 51
(63), and SATNET Host 30 (36).
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In summary then:
SATNET ! ETAM ! Goonhilly ! Tanum ! Clarksburg ! —————–!————!————!————!————! Internet Gateway ! 61 (75) ! 60 (74) ! 38 (46) ! 39 (47) ! Speech Host ! 31 (37) ! 32 (40) ! 33 (41) ! 34 (42) ! Internal Gateway ! 21 (25) ! - ! - ! 30 (36) !
The notation "xx (yy)" indicates the Host number where xx is
the decimal value and yy is the octal value.
ARPANET ! ETAM ! Goonhilly ! Tanum ! Clarksburg ! —————–!————!————!————!————! Internet Gateway ! 40-3 ! 42-3 ! 41-3 ! - ! Speech Host ! - ! - ! - ! - ! Internal Gateway ! 39-4 ! - ! - ! RCC-51-1 !
The notation "xx-y" means IMP xx and Host y on that IMP
(both expressed in decimal).
There are also some permanently assigned group names which may be used to broadcast a message. These group names are valid as Destination addresses, but are not valid as Source addresses.
Group ID Rate Members
---------- ---- -------
128. (200) 16K E G T C
129. (201) 64K E G T
130. (202) 16K E G C
131. (203) 16K E T C
132. (204) 16K G T C
133. (205) 64K E G
134. (206) 64K E T
135. (207) 16K E C
136. (210) 64K G T
137. (211) 16K G C
138. (212) 16K T C
140. (214) 16K Etam and Clarksburg, internal gateways
For groups 128 to 138, it is the PDP-11 Host that is the member of
the group at the specified location. For example, group ID 134
sends a message to the PDP-11 at Etam and the PDP-11 at Tanum. The
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IEN: 42 12 June 78 SATNET Information
PDP-11 will determine which of its internal parts is to process
the message.
Note that when the Host-SIMP protocol arrives, the message will be delivered to the destination with the group ID in the destination field.
Buffer Size
The current size of the buffer used for satellite channel communication is 425 (octal) words. Of this, 400 (256.) words are for data. "Data" is considered to begin immediately following the three word Host-SIMP leader, and continues to the last word of the message. The Internet-Header, if any, is counted as part of the data area. For those who think in terms of bits, 256. words = 4096 bits.
Timestamps
Below is a description of each timestamp implemented in SIMP 3. The order is the order in which a message would ordinarily be stamped.
1. VDH interface (Input from Host to SIMP)
(A) Implementation
When the hardware detects the end of a packet, it initiates an
interrupt at level 2 (0 is highest priority, 7 is lowest).
Very shortly after the interrupt happens, local time (10us
clock) is read, and the time is saved with the incoming packet.
The packet is then passed down to a lower processing level.
A little while later, the lower level will take the packet. If
it should turn out that this packet was the last packet of a
message (and if the message was not previously rejected by the
Host-SIMP protocol) then the local time will be converted to
global time and written as the value of the timestamp.
This stamp is called level 3.
(B) Comments
If the message is more than one VDH packet long, the earlier
packets may have already been copied over into chunk buffers by
the time the last packet arrives. This means the stamp ignores
the time the SIMP has already spent on the message. This time
will show up as a general processing delay. If the VDH line is
continuously receiving messages, the intermessage delay time
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may be used to estimate the time spent. If the message fits
entirely in one packet (SIMP 3.1) the problem vanishes.
It would not be too difficult to timestamp the message with the
arrival time of the first packet, if this should prove
preferable.
2. Satellite channel output
(A) Implementation
The time that the message is to be sent out is known in advance
by the chosen protocol. That time is used for the timestamp.
It is the time when the satellite channel will be turned on for
the sending of the message.
This stamp is called level 0.
(B) Notes.
The message is timestamped only the first time it is sent.
Retransmissions will not cause additional stamping.
3. Satellite channel input.
(A) Implementation
When the first sixteen data bits of an incoming message have
been received from the satellite channel, the hardware records
the time. This time is later noted and kept with the incoming
message.
If the message is from the leader, global time will be updated.
This means the relationship between local and global time may
change! This change will cause the local SIMP's global time to
be brought into line with that of the leader, if it was not
already.
The hardware receive time of the message is then converted to
global time, and this value is used for the timestamp.
This stamp is called level 0.
(B) Notes
To figure out when the packet has fully arrived in the SIMP,
one must add the packet transmission time to the declared
receive time. In the case of a mixed rate incoming message,
this calculation may be moderately complex.
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4. VDH transmit side -- SIMP to Host.
(A) Implementation
When the message is removed from the Host output queue, global
time is computed and written as the value of the timestamp.
This stamp is called level 3.
(B) Notes
The following events occur after the timestamp time:
1) Host-SIMP to 1822 leader conversion;
2) Copying message from chunk buffer to VDH packets;
3) Sending packets across VDH line.
Items (2) and (3) occur in parallel. Processing for the next
message may be begun when the last packet of the previous
message has been queued (software) for eventual output.
5. Message Received
(A) Implementation
When a message is received from the satellite channel, it is in
a contiguous buffer. If the message is not destined for a Host
on this SIMP, it is thrown away. If kept, it is passed to a
lower interrupt level. There the message is copied into chunk
buffers and some processing of poda header info occurs. The
chunk buffer message is then taken off the queue by the
background code, the current time in global time is computed,
and this value is used for the timestamp. It is timestamp level
2.
After the message is stamped, the message's checksum will be
computed. The message is discarded if the checksum test fails.
An ACK will then be created, if one is wanted, to acknowledge
the correct receipt of the message. Finally, the message will
be handed to Delivery, in the Host protocol module.
(B) Notes
This timestamp may be used to estimate:
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1) when the SIMP has finally decided to accept the message;
2) when the ACK for the message is entered;
3) when the message is given to the HPM.
Host Status Measurements
There is a cryptic printout in the Host Status line printed by MON25. Perhaps this will clarify what's happening.
"PS=1/45" means "packets sent". Specifically, the SIMP VDH sent 45 total data packets, 1 of which was a retransmission, during the last reporting interval (approx. one minute intervals).
"PR=2/67" means "packets received". During the last reporting interval, the SIMP VDH received 67 good packets other than Hello/I-heard-You packets. Two packets arrived with hardware checksum errors, and it is unknown what type of packet these might have been originally.
"HI=67/66" means "Hello/I-Heard-You". These packets are required by the VDH protocol. They are used to determine that the VDH connection works well enough to send real data. They are also used to detect the other host going down in the absence of other traffic. In the example, during the last reporting interval, the reporting SIMP received 67 Hellos and received 66 I-Heard-You's.
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