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Last Weeks CCIE™ Challenge Question
In X.25, when the network permits only a window size of 2, there is a
way to increase the effective window size to a specific
destination.
Answer
Explanation
Cisco has a parameter x25 nvc n, which lets you work
around this limitation when throughput is a major concern. Increasing
the NVC count above 1 allows you to create multiple virtual circuits
to the same destination. The effect is quite similar to a multilink
frame layer, in which the effective window size becomes
n times w.
RFC 1356 points out that this method may have unexpected and
undesirable characteristics: "Another method of increasing
performance is to open multiple virtual circuits to the same
destination, specifying the same CUD. Like packet size, this is not
always the best method. However, opening multiple circuits also
competes more effectively for the physical path, by taking more
shares of the available bandwidth. While this may be desirable to the
user of the encapsulation, it may be somewhat less desirable to the
other users of the path. Opening multiple circuits may also cause
datagram sequencing and reordering problems in end systems with
limited buffering (e.g., at the TCP level, receiving segments out of
order, when a single circuit would have delivered them in order).
This will only affect performance, not correctness of operation.
Opening multiple circuits may also increase the cost of delivering
datagrams across a public data network."
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