Ross Callon (rcallon@PARK-STREET.BBN.COM)
Wed, 9 Sep 87 15:17:04 EDT
In the ISO IP, the DEC bit is located in an options field. In particular,
the "Quality of Service Maintenance" option contains three possible classes
of encodings, one of which is called "globally unique". One bit of this
globally unique QOS option is the congestion experience bit.
The ISO spec explicitly specifies which functions must be implemented,
and which are optionally implemented. Support for the QOS option is not
required. Thus not only is the presence of the DEC bit optional, but even
if it is there gateways are not required to look at it. In addition,
"Congestion Notification" is also optional. Even if the source uses
the globally unique QOS option, and the gateways update the DEC bit
appropriately, it is not required (at least by IS 8473) that the
destination will notify the Transport Implementation.
Clearly, optional implementation of the congestion bit is a problem,
especially since if some end systems pay attention to it and others ignore
it the "good guys" will end up suffering. This may be overcome by requiring
use of the DEC bit in implementers agreements (such as produced by the COS).
The placement of the bit in an option field means that if you want to use it
for all packets you are requiring the gateways to actually parse the options
fields for every packet. This increases the processing demand on gateways.
This may be okay for low bandwidth internets, but gets more undesirable for
higher bandwidths (what constitutes higher bandwidths depends upon how much
processing power you are willing to put in your gateways).
I don't think the DEC bit has actually been used in an operational
environment (does anyone out there know differently?). What we see in the
Arpanet seems to be a relatively large number of short and/or discontinuous
flows such as electronic mail and telnet traffic. Transport based flow
control methods may be more appropriate for a smaller number of longer term
flows. The combination of slow start (which is part of the way that the
DEC bit is to be used) plus regulating the growth rate of flows on each
connection, will prevent short term connections from "jumping in" with a
large increment of traffic all at once. However, for very short lived
connections, this may be nearly equivalent to simply using a fixed very
small window size. This approach also assumes that all users (or at least
the users associated with the overwhelming majority of network traffic)
will use a window based transport protocol. All of this suggests that the
DEC bit is more an interesting research approach for use in some common
environments, rather than a proven technique.
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