[LARTC] problem with prio qdisc and TOS

[LARTC] problem with prio qdisc and TOS

[Andreas Wright]

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Hello ,

I have presented my problem before also.I have linux box with 2 interfaces eth0(incoming) and eth1(outgoing).

 If I just attach a PRIO qdisc on eth1(outgoing) will I be able to prioritise packets based on the TOS value.The TOS value is set by a test application.At very high data rates will the data stream with lower priority TOS value be starved because it cannot send out of packets from its band on eth1 interface?.

Does something be set in linux kernel so that kernel sets priority for packets  according to TOS value.I guess this priorty will be used by PRIO qdisc(by default) to decide which band the packet belongs.

Please clarify this by giving your inputs.

Regards,

Andreas.




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Re: [LARTC] problem with prio qdisc and TOS

[Chris Wilson]

Hi Andreas,

I hope I have this right, based on my limited understanding of Linux QoS. 
If I'm wrong then I hope someone will correct me =)

>  If I just attach a PRIO qdisc on eth1(outgoing) will I be able to
> prioritise packets based on the TOS value.

You don't even need to do that, the default qdisc is pfifo_fast which also 
prioritises by TOS flags.

> The TOS value is set by a test application.

It can also be overridded by a Netfilter rule, if you want to force or 
change priority for certain types of packets.

> At very high data rates will the data stream with lower
> priority TOS value be starved because it cannot send out of packets from
> its band on eth1 interface?.

Yes, if there is not enough capacity to send on eth1 then lower-priority 
packets will be starved, will queue up until the queue is full and then 
get dropped.

> Does something be set in linux kernel so that kernel sets priority for
> packets according to TOS value.I guess this priorty will be used by PRIO
> qdisc(by default) to decide which band the packet belongs.

Yes, it happens by default, with the pfifo_fast qdisc.

Cheers, Chris.
-- 
   ___ __     _
 / __// / ,__(_)_  | Chris Wilson -- UNIX Firewall Lead Developer |
/ (_ / ,\/ _/ /_ \ | NetServers.co.uk http://www.netservers.co.uk |
\ _//_/_/_//_/___/ | 21 Signet Court, Cambridge, UK. 01223 576516 |

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Re: [LARTC] problem with prio qdisc and TOS

[Andreas Wright]

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hello all,
As I had mentioned in the previous mail I was trying to prioritise outgoing packets on eth1 interface based on TOS value.I am testing this by sending packets from 2 IP sources (with 2 different TOS values) at very high data rate sending packets alternatively.On the outgoing interface I am using a sniffer to see if the lower priority source is getting starved.
But the sniffer capture shows that packets from both sources arrive alternatively as they are sent.I tried using default qdisc is pfifo_fast  and PRIO also.
Please anyone tell me what could be the problem.
Regards
Andreas
 Chris Wilson <chris@netservers.co.uk> wrote:Hi Andreas,

I hope I have this right, based on my limited understanding of Linux QoS. 
If I'm wrong then I hope someone will correct me =)

> If I just attach a PRIO qdisc on eth1(outgoing) will I be able to
> prioritise packets based on the TOS value.

You don't even need to do that, the default qdisc is pfifo_fast which also 
prioritises by TOS flags.

> The TOS value is set by a test application.

It can also be overridded by a Netfilter rule, if you want to force or 
change priority for certain types of packets.

> At very high data rates will the data stream with lower
> priority TOS value be starved because it cannot send out of packets from
> its band on eth1 interface?.

Yes, if there is not enough capacity to send on eth1 then lower-priority 
packets will be starved, will queue up until the queue is full and then 
get dropped.

> Does something be set in linux kernel so that kernel sets priority for
> packets according to TOS value.I guess this priorty will be used by PRIO
> qdisc(by default) to decide which band the packet belongs.

Yes, it happens by default, with the pfifo_fast qdisc.

Cheers, Chris.
-- 
___ __ _
/ __// / ,__(_)_ | Chris Wilson -- UNIX Firewall Lead Developer |
/ (_ / ,\/ _/ /_ \ | NetServers.co.uk http://www.netservers.co.uk |
\ _//_/_/_//_/___/ | 21 Signet Court, Cambridge, UK. 01223 576516 |




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