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Re: Precedence field value in ICMP Error Messages with LINUX Kernels 2.2.x & 2.4
From: "Mikhail Evstiounin" <evstiounin () telerama com>
Date: Thu, 19 Oct 2000 22:06:05 -0400


----- Original Message -----
From: "Ofir Arkin" <ofir () itcon-ltd com>

This is a corrected post for the post I have sent on 14.10.2000
Titled "TOS Field value in ICMP Error Messages with LINUX
Kernels 2.2.x & 2.4".
---------------------------------------------------------------

Each IP Datagram has an 8-bit field called the “TOS Byte”,
which represents the IP support for prioritization and
Type-of-Service handling.


Ofir, take a look at RFC2474 - this is a proposed standard (from
December of 1998 -- we will see in December if it becomes a complete
standard). It completely redefines ToS field in IPv4. And it's a
native field for IPv6.

The “TOS Byte” consists of three fields.

The “Precedence field”, which is 3-bit long, is intended to
prioritize the IP Datagram. It has eight levels of prioritization.


Higher priority traffic should be sent before lower priority
traffic.

The second field, 4 bits long, is the “Type-of-Service” field.
It is intended to describe how the network should make tradeoffs
between throughput, delay, reliability, and cost in routing an
IP Datagram.

The last field, the “MBZ” (most be zero), is unused and most be
zero. Routers and hosts ignore this last field. This field is 1
bit long.




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