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MSS and EnablePMTUDiscovery.

Last response: in Windows 2000/NT
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Anonymous
October 22, 2004 9:35:02 AM

Archived from groups: microsoft.public.windowsnt.protocol.tcpip (More info?)

Hello,

We have modified registry entry EnablePMTUDiscovery to value 0 (disabled) to
get a MTU of 576 in all interfaces.

But when this machine sends a SYN TCP packet, the value sent is 1460 not
536. Where is the problem?

thacks.
Anonymous
October 24, 2004 2:18:45 AM

Archived from groups: microsoft.public.windowsnt.protocol.tcpip (More info?)

Did you try that after reboot , just hint ?
Arkady

"eltiosanti" <eltiosanti@discussions.microsoft.com> wrote in message
news:02D269D5-C62D-4D26-8DED-B944B1FDAEFD@microsoft.com...
> Hello,
>
> We have modified registry entry EnablePMTUDiscovery to value 0 (disabled)
to
> get a MTU of 576 in all interfaces.
>
> But when this machine sends a SYN TCP packet, the value sent is 1460 not
> 536. Where is the problem?
>
> thacks.
Anonymous
October 26, 2004 7:55:01 AM

Archived from groups: microsoft.public.windowsnt.protocol.tcpip (More info?)

Hi,
Yes, of course, and we tried with entry "RouterMTU" too. Both are working
wrong (NT, 2000).

Thanks.

"Arkady Frenkel" wrote:

> Did you try that after reboot , just hint ?
> Arkady
>
> "eltiosanti" <eltiosanti@discussions.microsoft.com> wrote in message
> news:02D269D5-C62D-4D26-8DED-B944B1FDAEFD@microsoft.com...
> > Hello,
> >
> > We have modified registry entry EnablePMTUDiscovery to value 0 (disabled)
> to
> > get a MTU of 576 in all interfaces.
> >
> > But when this machine sends a SYN TCP packet, the value sent is 1460 not
> > 536. Where is the problem?
> >
> > thacks.
>
>
>
October 20, 2008 2:27:29 PM

the mss you see in the tcp syn always is derived from the local mtu of the network adapter.
path mtu discovery has nothing to do with that.

if you disable path mtu discovery, the WINDOWS system will send packets with a max size of 570 (or something around that). the mss is ignored for that system.

if path mtu discovery is enabled, the system sends packets with the lower of the 2 mss values it gets from tcp syn.
if a packed has to be dropped in between, the dropping system sends back an icmp unreachable (hopefully) with the mtu size it supports. from then on the connection will use this value as the max packetsize.

now the problem with pmtud is if one system in between the communication drops a packet but does not send icmp unreachables. in that case the communication is broken.
unless you enable pmtu blackhole router detection.
pmtubhdetect does wait for packets to be retransmitted, and after the max retransmit count it removes the dont-fragment bit from the packet. if this is successful, the system reduces the maximum packetsize for that connection and so on.

so the most effective method for high speed communication on reliable links is pmtud with pmtubhdetect.

if you dont want efficiency and can live with small packet sizes and a lot of overhead for high volume data, you can disable pmtud :-)
!