Flow control parameter negotiation
Description of Facility
The following facilities allow non-default flow control parameters to be selected.
See throughput issues
Packet Size
Facility code | packet size
from called DTE |
packet size
to called DTE |
43 hex |
+---------------+
| 43 | 01000010 | packet size .............................#values are log to
base 2 of
Window Size
Facility code | window size | window size |
44 hex | from called DTE | to called DTE |
| 43 | 01000010 | packet size .............................#
+--------------------------------------------------------------------------------------------+
| X.2 LEVEL 3 PER CALL FACILITY | X.2 (1984) | UK(a) | UK(b) |interworking|
| OPTIONS | REF CLASS | now | now |R5a|R5b| R6 | |
+-------------------------------------+-------------+--------+-----+---+---+----+------------|
| Flow control parameter negotiation..| 2.5 E |
|
|
| Packet sizes 2048 and 4096..........| | . |
| |
+--------------------------------------------------------------------------------------------+
The ability to negotiate packet and window size is a subscription option (see
flow control parameter negotiation subscription option).
UK(a) - supported. In order to use this facility both ends of the call must
subscribe to parameter negotiation, otherwise the values will be negotiated
down to their default values.
UK(b) - supported. Packet and Window size is set independently at both ends
of the call. The network will perform the appropriate concatenation and splitting
of packets as required. If a packet has been split the M-bit will be set, except
for the final packet.
Since UK(b) uses local significance and 128 byte packets internally within the network, an increase in Packet or window size may NOT increase the throughput, in fact this assembling and disassembling of packets may actually slow down throughput.
¬es& = yTemporary note for reviewers
UK(a) - If very high packet and window sizes are used and data is sent at
a high rate, ie with only 1 or 2 flags between frames. This may cause the network
node to crash. (Kevin Joseph has the figures).
UK(b) - High packet and window sizes should be discouraged since they do not improve throughput and they require a large amount of memory for the slot, this could reduce the number of ports that can be built on an engine, or cause the engine to go into congestion or crash.
&UKa& = yUKa support
UKa does not support packet sizes of 2048 and 4096.
Release 4 will negotiate packet sizes of 2048 and 4096 down to 1024, current
release will clear the call if these values are requested.
Public Network always operates with the same packet and window sizes at each end of the
network.
Action of network depends on whether the port is enhanced:
If enhanced:
* All incoming calls will contain packet and window facility fields
If not enhanced:
* All incoming calls will be negotiated to 2 and 128 by network, facility fields not passed to destination DTE.
* Call requests will be cleared if it contains flow control facility fields requesting values other than 2 and 128.
RD 0004 part 8:1.C (PICS proforma Public Network1984)
FR1 - Flow control parameter negotiation ... M = Mandatory
FS1 - Flow control parameter negotiation ... M = Mandatory
V9s - Packet sizes supported sending...
16,32,64,128,256,512,1024,2048,4096
V9r - Packet sizes supported receiving ...
16,32,64,128,256,512,1024,2048,4096
V10s - window sizes supported sending...
1-7 (1-127 if modulo 128 support)
V10r - window sizes supported receiving .
1-7 (1-127 if modulo 128 support)
Therefore release 4 is not compatible with ONA for packet size 2048 and 4096.