Olivier Langlois's blog

In the book Internetworking with TCP/IP volume 2 - Design, implementation, and internals at the chapter 13 (TCP: Output processing), section 13.16 Other TCP Procedures, the source code of the function tcpiss() called to obtain the Initial Send Sequence (ISS) is presented. The function works by initializing a static variable with a clock counter value and then increment that value to TCPINCR whose value is 904.

No explanation is provided about the 904 value and there is even an exercise asking the reader to contact the coauthor David Stevens by e-mail to know why they are using 904. I first tried to find the answer on the net before contacting them but the information was unavailable.

So here is the answer of Douglas E. Comer, coauthor of the book, on the topic for the next fellow looking for the same answer:

It's an inside joke. The TCP standard says to choose a value other than 1.
My co-author, Dave Stevens, and I tossed a coin, and we used his birthday (September 4th). There are a couple of other fun items in the book (hint: look in the index).

Regards,
Doug Comer

After reading Mr. Comer reply, I have checked the index and there is a list of well-known constants such as the world renowned 1.3.6.1.2.1 !