TCP Sequence numbers related problem

Question

While opening a TCP connection , the initial sequence number is to be derived using ToD clock that keeps running even when the host is down. The low order 32 bits of the counter of the ToD clock is to be used for the initial seq no. The clock counters increments once per milli second. The maximium packet lifetime is given to be 64s.

Which one of the following choices is closest to the minimum permissible rate at which seq no used for packet of a connection can increase?

A).015 /s

B) .064/s

C).134/s

D).327/s

Can anyone explain the solution and theory related to this problem in detail?

Answer 1

The initial sequence number (ISN) is supposed to be a random 32bit number. Using the lower bits of ToD value is assuming that this is random enough.

The sequence number increases by one for every payload byte sent over the network. This makes the rate of increase dependent upon the throughput of the connection, the loss rate, and the amount of data being transmitted. It has nothing to do with the "maximum packet lifetime".

If this question is referring to the TCP PAWS option's Timestamp (TS) values. This still doesn't really make sense because the rate in increment of the timestamp value is left up to the implementer. If the clock rate increase is given as the TS increment time, then that defines the maximum throughput before duplicates TS's occur inside a single window's worth of data. In the case of one millesecond, the maximum throughput is 8Tbps.

In short, this question doesn't make any sense to me. This question looks like it was written by a NNES, which could be the source of the confusion.