[Cerowrt-devel] Fwd: [PATCH] ag71xx: Added support for baby-jumbo packets.

dpreed at reed.com dpreed at reed.com
Sat Jun 23 15:29:02 EDT 2012

I find it curious that packet aggregation is done in 802.11n standards, but that the packets are limited to 1540.   I bet that limit may not be there in all Atheros NICs, and maybe not in this one.  Will investigate now that I'm curious.

-----Original Message-----
From: "Robert Bradley" <robert.bradley1 at gmail.com>
Sent: Friday, June 22, 2012 4:41pm
To: cerowrt-devel at lists.bufferbloat.net
Subject: Re: [Cerowrt-devel] Fwd: [PATCH] ag71xx: Added support for baby-jumbo packets.

On 22/06/12 21:11, dpreed at reed.com wrote:
> Good step.  Why not allow 9K byte jumbos when one's packets traverse a path that is internal to the local area, and all the 1 GigE links support 9K?

All the posts I could see claim that the System-on-Chip NIC (Atheros 
AR7161) cannot handle packets greater than 1540 octets, so routing 9k 
packets from wired->wireless or wired->WAN would not be an option.  If I 
understood Dave's previous post correctly, though, the built-in switch 
allows jumbo packets, so wired->wired internal traffic should work fine 
with 9K packets already.

The only issue with that setup is that with mismatched MTUs, it might be 
impossible to communicate with the router and the 9k-friendly nodes at 
the same time.  Thankfully, on Linux at least, you can set per-route 
MTUs (http://lartc.org/howto/lartc.cookbook.mtu-discovery.html), so it 
might be possible to exploit that to do what you want.  Maybe something 
like this would work?

ip route add default via mtu 1500
ip route add dev eth0 mtu 1500
ip route add dev eth0 mtu 9000

Robert Bradley
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