TCP Specification


To avoid confusion we must prevent segments from one incarnation of a connection from being used while the same sequence numbers may still be present in the network from an earlier incarnation.

We want to assure this, even if a TCP crashes and loses all knowledge of the sequence numbers it has been using.

When new connections are created, an initial sequence number (ISN) generator is employed which selects a new 32 bit ISN.

The generator is bound to a (possibly fictitious) 32 bit clock whose low order bit is incremented roughly every 4microseconds.

Thus, the ISN cycles approximately every 4.55 hours.

Since we assume that segments will stay in the network no more than the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55 hours we can reasonably assume that ISN's will be unique.