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

[Matt Watson]

Why are you trying to change the ToS from 46 (0x2e) Expedited for the RTP/RTCP packets to 16 (0x10)?

I mean... these values really only need to be meaningful to yourself, your switches, your routers etc.... however

ToS 46 (0x2e) is the "standard" value for RTP / RTCP as it is basically the highest priority value you can assign to something... ToS 46 is basically suppose to indicate that it trumps all other traffic and should be send before anything else (Which is a good thing for the RTP traffic)

The SIP Signalling traffic is a little less important and its standard ToS value is 26 (0x1a).

You also don;t need to use IPTables to set these values... Asterisk will do it for you as long as you have installed libcaps (I believe its required for it).

And I don;t know what phones you are using... but your phones are probably also setting these values for you.... I know the Aastra phones have QoS/ToS settings under Options -> Network -> Type of Service

--
Matt
________________________________________
From: asterisk-users-bounces at lists.digium.com [asterisk-users-bounces at lists.digium.com] On Behalf Of Vikas [topgun9 at gmail.com]
Sent: Wednesday, May 07, 2008 7:59 AM
To: Asterisk Users Mailing List - Non-Commercial Discussion
Subject: [asterisk-users] Setting the TOS using IPtables screws up the DSCP     field

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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