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Re: Tos in IP header

On Tue, 19 Feb 2002, Daniel Pittman wrote:

> On Tue, 19 Feb 2002, Marc REYNES wrote:

> TOS is a TCP concept, not an IP concept. This implies that all protocols
> except TCP, such as UDP, IGMP, ICMP, etc, have not got those bits.[1]
> As such, you /can't/ set the TOS bits on a UDP packet.

Are you sure about this?  RFC 791 (Internet Protocol) defines bytes 2-3 
(zero-indexed) of the IP header as type-of-service: the first three 
bits govern precedence, the next three delay, throughput, and 
reliability, and the final two are reserved.


Since TOS is defined in the IP header, any packet that uses IP for 
network services can have TOS set.  You may well be correct that the 
precedence bits are ignored by most equipment today--I've never made
use of them.


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