TOS field in IP packet


C. Philip Wood (cpw%sneezy@LANL.GOV)
Thu, 1 Sep 88 16:29:49 MDT


I'm curious about a packet that shows up every so often on the LANL Internet.

The ones I caught were ICMP ECHO requests. What was curious about
them was the Type of Service field.

In one case the TOS field was '11111110' which maps to

        Precedence == Network Control
        Delay == Low
        Throughput == High
        Reliablility == High
        Reserved bit 6 == on

Now, I can understand how a particular implimentation would like to
have low delay, high throughput and high reliability - wouldn't we
all. I also understand that setting more than two of these is
considered excessive.

However, as I understand it TOS is not something that is actually offered
by vendors now or understood to well by the Internet in general.

I next, asked the owner of the machine to do a few more echo's, The TOS
field varied. The machine is a SUN with IPC board running 3.5.

Another interesting fact is that the VMS system running Wollengong
TCP/IP software responded with ROUTINE precedence. I tried the same thing
with a BSD4.3 system and it also returned
the packet, but with the same TOS bits. In either case I would think
the responder should drop the packet because of the bit set in the reserved
field.

I assume that some coder forgot to clear the TOS field.

One questions are: Are those reserved bits still reserved and supposed
to be zero? And if the are reserved and non-zero, should we drop the packet.

Phil Wood, cpw@lanl.gov



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