[OpenIndiana-discuss] exceeding MTU?

[Richard Elling]

On Mar 9, 2012, at 9:23 AM, Martin Frost wrote:

> I've been seeing timeouts in the middle of transfers while doing
> 'zfs send ... | ssh ... zfs receive ...' between subnets here.
> 
> In investigating with tcpdump, I found that packets are being sent
> with lengths bigger than the supposed MTU of 1500, and I would like
> to prevent that, as it may be the cause of the timeouts.
> 
> Do I need to explicit use '-mtu 1500' in ifconfig?  If so, how do I
> make the system use it automatically when it brings the net up?
> 
>    823;hostfs/host/tcpdump.3#| ifconfig -a
>    lo0: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> mtu 8232 index 1
> 	    inet 127.0.0.1 netmask ff000000 
>    igb0: flags=1004843<UP,BROADCAST,RUNNING,MULTICAST,DHCP,IPv4> mtu 1500 index 6
> 	    inet XX.XX.XX.XX netmask fffffe00 broadcast 171.64.41.255
> 	    ether aa:bb:cc:dd:ee:ff
>    lo0: flags=2002000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv6,VIRTUAL> mtu 8252 index 1
> 	    inet6 ::1/128 
>    igb0: flags=20002004841<UP,RUNNING,MULTICAST,DHCP,IPv6> mtu 1500 index 6
> 	    inet6 XXXX::XXX:XXXX:XXXX:XXXX/10 
> 	    ether aa:bb:cc:dd:ee:ff
> 
> Lots of packets show this length:
> 
>    03:08:30.135800 IP hostfs.Stanford.EDU.41861 >
>    hostbk.Stanford.EDU.ssh: Flags [P.], ack 3805, win 64436, options
>    [nop,nop,TS val 236770530 ecr 193082465], length 16432

AIUI, that is an IP packet, not an Ethernet frame.  MTU applies to Ethernet frames.
 -- richard

> 
> Some earlier batches of packets had lengths of 3400, then 4344, then
> 16432, then 2896, then 7240.
> 
> I assume it's doing adaptive sizing, but my real question is why are
> any of these packets bigger than the 1500 MTU shown by ifconfig for
> igb0 and how do I prevent use of packets longer than 1500?
> 
> Thanks!
> Martin
> 
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--
DTrace Conference, April 3, 2012, http://wiki.smartos.org/display/DOC/dtrace.conf
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