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Subject: Re: Precedence field value in ICMP Error Messages with LINUX Kernels 2.2.x & 2.4
From: Mikhail Evstiounin (evstiounintelerama.com)
Date: Thu Oct 19 2000 - 21:06:05 CDT

----- Original Message -----
From: "Ofir Arkin" <ofiritcon-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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