[LARTC] Clamping tcp mss values to decrease latency

[LARTC] Clamping tcp mss values to decrease latency

[Ross Skaliotis]

Hello,

I have an interesting question. I can't find a solution anywhere to this
problem.

I have an adsl connection which is able to send a full data packet in
about 150ms. I want to cut this time down to decrease latency while
serving files, so I thought I'd try and reduce the packet size for the
offending upload streams (my webserver). The trick is that I also want all
other tcp connections to have the normal full packet size. Enter
MSS-clamping. My pppoe client for linux can do mss clamping on every
packet that goes out and come in. Not enough control. There is an option
to do mss-clamping in iptables, however this only works in the forward
table. I can't match and control outgoing web connections, just
connections getting routed through the machine. Any ideas?

-Ross Skaliotis

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Re: [LARTC] Clamping tcp mss values to decrease latency

[Martin Devera]

Hi,

MSS can be clamped per route. See ip tool and LARTC howto
for syntax. I'm using it with great success.
devik

> MSS-clamping. My pppoe client for linux can do mss clamping on every
> packet that goes out and come in. Not enough control. There is an option
> to do mss-clamping in iptables, however this only works in the forward
> table. I can't match and control outgoing web connections, just
> connections getting routed through the machine. Any ideas?

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Re: [LARTC] Clamping tcp mss values to decrease latency

[bert hubert]

On Mon, Feb 25, 2002 at 12:13:53AM -0500, Ross Skaliotis wrote:
> Hello,
> 
> I have an interesting question. I can't find a solution anywhere to this
> problem.
> 
> I have an adsl connection which is able to send a full data packet in
> about 150ms. I want to cut this time down to decrease latency while
> serving files, so I thought I'd try and reduce the packet size for the
> offending upload streams (my webserver). The trick is that I also want all
> other tcp connections to have the normal full packet size. Enter
> MSS-clamping. My pppoe client for linux can do mss clamping on every
> packet that goes out and come in. Not enough control. There is an option
> to do mss-clamping in iptables, however this only works in the forward
> table. I can't match and control outgoing web connections, just

Control and outgoing can be done with regular MTU settings. Just lower the
MTU size of your ppp interface and the kernel will set a lower MSS
automatically.

You can also set MTU per route.

Regards,

bert

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Re: [LARTC] Clamping tcp mss values to decrease latency

[Martin Devera]

> > other tcp connections to have the normal full packet size. Enter
> > MSS-clamping. My pppoe client for linux can do mss clamping on every
> > packet that goes out and come in. Not enough control. There is an option
> > to do mss-clamping in iptables, however this only works in the forward
> > table. I can't match and control outgoing web connections, just
> 
> Control and outgoing can be done with regular MTU settings. Just lower the
> MTU size of your ppp interface and the kernel will set a lower MSS
> automatically.

Be VERY careful when doing it. There is too many websites blocking
MTU discovery protocol and you will be no longer able to reach them.
These system maintainers should be shot. :-\
Setting max MSS in route is better way ..

> You can also set MTU per route.

you probably wanted to say MSS here ?
 
devik

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Re: [LARTC] Clamping tcp mss values to decrease latency

[bert hubert]

On Mon, Feb 25, 2002 at 12:58:54PM +0100, Martin Devera wrote:
> > > other tcp connections to have the normal full packet size. Enter
> > > MSS-clamping. My pppoe client for linux can do mss clamping on every
> > > packet that goes out and come in. Not enough control. There is an option
> > > to do mss-clamping in iptables, however this only works in the forward
> > > table. I can't match and control outgoing web connections, just
> > 
> > Control and outgoing can be done with regular MTU settings. Just lower the
> > MTU size of your ppp interface and the kernel will set a lower MSS
> > automatically.
> 
> Be VERY careful when doing it. There is too many websites blocking
> MTU discovery protocol and you will be no longer able to reach them.
> These system maintainers should be shot. :-\
> Setting max MSS in route is better way ..

Not for locally generated packets - if you lower MTU, MSS will get set too.

Regards,

bert

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http://www.tk                              the dot in .tk
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Re: [LARTC] Clamping tcp mss values to decrease latency

[Martin Devera]

> > Be VERY careful when doing it. There is too many websites blocking
> > MTU discovery protocol and you will be no longer able to reach them.
> > These system maintainers should be shot. :-\
> > Setting max MSS in route is better way ..
> 
> Not for locally generated packets - if you lower MTU, MSS will get set too.

At my production server when I set MTU to 640 (I hoped in better link
utilization) I suddenly was not able to reach some webs from it. I
discovered that the are sending me 1500 byte packets with DF set and
they didn't pay atttention to my MTU. It is obvious problem in MTU
discovery.

You are roght that MSS is set to MTU but it is bad luck it you have
small MSS but you can't reach some hosts.

MSS setting for routes works well for me even for localy generated
flows. Is there some problem I don't see ?

regards, devik

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Re: [LARTC] Clamping tcp mss values to decrease latency

[Ross Skaliotis]

Yes, but is this able to sort streams by tcp port?

-Ross

On Mon, 25 Feb 2002, Martin Devera wrote:

> Hi,
>
> MSS can be clamped per route. See ip tool and LARTC howto
> for syntax. I'm using it with great success.
> devik
>
> > MSS-clamping. My pppoe client for linux can do mss clamping on every
> > packet that goes out and come in. Not enough control. There is an option
> > to do mss-clamping in iptables, however this only works in the forward
> > table. I can't match and control outgoing web connections, just
> > connections getting routed through the machine. Any ideas?
>
>

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Re: [LARTC] Clamping tcp mss values to decrease latency

[Martin Devera]

Hmm .. IIRC the fwmark can be used as routing
key. It might do the trick. Am I right Bert ?

> Yes, but is this able to sort streams by tcp port?
> 
> > MSS can be clamped per route. See ip tool and LARTC howto
> > for syntax. I'm using it with great success.
> > devik

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Re: [LARTC] Clamping tcp mss values to decrease latency

[bert hubert]

On Mon, Feb 25, 2002 at 02:23:48PM +0100, Martin Devera wrote:

> At my production server when I set MTU to 640 (I hoped in better link
> utilization) I suddenly was not able to reach some webs from it. I
> discovered that the are sending me 1500 byte packets with DF set and
> they didn't pay atttention to my MTU. It is obvious problem in MTU
> discovery.
> 
> You are roght that MSS is set to MTU but it is bad luck it you have
> small MSS but you can't reach some hosts.
> 
> MSS setting for routes works well for me even for localy generated
> flows. Is there some problem I don't see ?

No ;-)

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Re: [LARTC] Clamping tcp mss values to decrease latency

[Martin Devera]

> > discovery.
> > 
> > You are roght that MSS is set to MTU but it is bad luck it you have
> > small MSS but you can't reach some hosts.
> > 
> > MSS setting for routes works well for me even for localy generated
> > flows. Is there some problem I don't see ?
> 
> No ;-)

So short reply to my long mail ;-)) You are probably really short
of your tea ;)
By the way I finally read your man pages - good work. Once I finish
new htb I could write FIXME parts.
devik

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Re: [LARTC] Clamping tcp mss values to decrease latency

[bert hubert]

On Mon, Feb 25, 2002 at 02:44:56PM +0100, Martin Devera wrote:
> Hmm .. IIRC the fwmark can be used as routing
> key. It might do the trick. Am I right Bert ?

I think it might! 

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