Anybody normally send packets with unused options?


John Romkey (romkey@BORAX.LCS.MIT.EDU)
Fri, 28 Mar 86 22:55:46 est


   Date: Thu, 27 Mar 86 19:21:55 PST
   From: Murray.pa@Xerox.COM

        ...

   The question is, has somebody else already sliced the cake some other
   way? For example, is some system normally sending a few empty option
   bytes to trick the alignment into comming out clean in their
   environment?

   Does anybody have any data on how often non-empty options are used? John
   and I watched some traffic arriving at XEROX.COM, and we didn't see any.

PC/IP (PC/TCP, whatever it's called today) has explicit routines for
allocating "IP" and "UDP" packets. The routines take two arguments: a
packet size and an IP options length. The UDP alloc routine calls the
IP alloc routine, which gets a packet buffer and builds a partial IP
header in it, setting the IP header size field and saves space for the
options. IP and UDP based protocols use macros to find the address to
start storing data in the packets; these macros take the options field
into consideration.

The headers get finished up when you call the appropriate send routines.

That code is based on some Version 6 Unix code done locally a long
time back, maybe even on some ancient BCPL code.
                                        - john romkey
                                          ftp software



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