FAQ

How can a receiver detect sequence number gaps when sequence numbers are implicit for application messages?

First, consider the cases where sequence numbers are explicit in FIXP:

• Every datagram over UDP
• At every heartbeat interval, which will likely be much more rapid than FIX hearbeats in the past. (NASDAQ is not FIXP but similar in principle - their heartbeat interval in in the realm of 100 ms.)
• When context switching on a multiplexed transport
• When switching from retransmission to real-time on a recoverable flow

So, it is only implicit without a context switch or idle period over TCP. But recall that TCP guarantees in-sequence delivery of packets to an application. That is why FIXP was designed to use implicit sequencing in that case -- it avoids duplication of effort at two layers of the stack.

If the transport delivers messages in sequence but the application drops them, that is an application layer problem and cannot be solved the session layer.