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List:       freebsd-hackers
Subject:    Re: changing the ToS in IP Header
From:       "Ashish Kulkarni" <ashish () symonds ! net>
Date:       2003-05-27 21:12:16
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Kenjiro Cho <kjc@csl.sony.co.jp> wrote:
> Tell your ISP not to use 0x02; it violates the standard.
> You may modify the upper 6 bits for an arbitrary value, though.

Well, my cable ISP provides a 3rd party Windows-only client that enables
forwarding of packets only when logged in. I've reverse engineering that
protocol sucessfully, and it requires the TOS to be 0x02 ...  I imagine
this TOS is only valid uptil the gateway, after which a different TOS is
used. Anyway, all this is unofficial so I can't really go to my ISP and
mention it ;-)
> The lower 2 bits of the (now deprecated) TOS field are officially
> assigned to ECN (RFC3168).  0x02, ECT(0), is used to indicate that the
> sender is ECN-capable.
>
> ALTQ supports diffserv and is capable of rewriting the upper 6 bits of
> the TOS field.
> http://www.csl.sony.co.jp/~kjc/software.html

I have one question: as you said, I can use ALTQ to reset all the upper 6
bits to zero, so as to get the pattern 0x02 (using the kernel patch by
Terry Lambert). Will this have any implications for its working and for
ECN? I need this because several protocols (ssh, etc) use a modified TOS,
which I want to be reset to default value.

Thanks,
Ashish




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