[asterisk-users] Setting the TOS using IPtables screws up the DSCP field

[Guilherme Loch Waltrick Góes]

This happens because the TOS and DSCP are the same field. TOS is the first
implementation of QoS on the IP header, DSCP is it's evolution and uses the
same field on the IP header, you can use only one of the two at the same
time.
Best Regards,

On Wed, May 7, 2008 at 8:59 AM, Vikas <topgun9 at gmail.com> wrote:

> Concise summary: When I set the TOS to Minimize-Delay the DSCP field
> in the packet changes from "Expedited Forwarding" to "Unknown"
>
> Here are the details:
>
> Scenario 1: IpTables is not used to set the TOS
>
> This is what the packet looks like using wireshark:
> Internet Protocol, Src: 59.93.192.xx (59.93.192.xx), Dst:
> 64.62.134.yyy (64.62.134.yyy)
>    Version: 4
>    Header length: 20 bytes
>    Differentiated Services Field: 0xb8 (DSCP 0x2e: Expedited
> Forwarding; ECN: 0x00)
>        1011 10.. = Differentiated Services Codepoint: Expedited
> Forwarding (0x2e)
>        .... ..0. = ECN-Capable Transport (ECT): 0
>        .... ...0 = ECN-CE: 0
>
>
> Scenario 2: IpTables is used to set the TOS
>
> Output of $/etc/rc.d/init.d/iptables status
> Table: mangle
> Chain PREROUTING (policy ACCEPT)
> num  target     prot opt source               destination
>
> Chain INPUT (policy ACCEPT)
> num  target     prot opt source               destination
>
> Chain FORWARD (policy ACCEPT)
> num  target     prot opt source               destination
> 1    TOS        udp  --  0.0.0.0/0            0.0.0.0/0           udp
> dpts:5060:5069 TOS set 0x10
> 2    TOS        tcp  --  0.0.0.0/0            0.0.0.0/0           tcp
> dpts:5060:5069 TOS set 0x10
> 3    TOS        udp  --  0.0.0.0/0            0.0.0.0/0           udp
> dpts:10000:20000 TOS set 0x10
>
> This is what the packet looks like using wireshark:
> Internet Protocol, Src: 59.93.192.xx (59.93.192.xx), Dst:
> 64.62.134.xxx (64.62.134.xxx)
>    Version: 4
>    Header length: 20 bytes
>    Differentiated Services Field: 0xb0 (DSCP 0x2c: Unknown DSCP; ECN:
> 0x00)
>        1011 00.. = Differentiated Services Codepoint: Unknown (0x2c)
>        .... ..0. = ECN-Capable Transport (ECT): 0
>        .... ...0 = ECN-CE: 0
>
>
> I have no idea what am I doing wrong.
>
> Here is some reference reading I did:
> http://www.tucny.com/dscp&tos
>
> Any pointers in the right direction will be very much appreciated.
>
> Thanks for your time,
>
> Sysadmin
> http://www.debtconsolidationcare.com
> Internets First get out of debt community
>
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