[lwip-users] ARP lookups and UDP servers

[Michael Tharp]

Hi all,

I have an embedded application that is a NTP server (UDP based). Due to 
the timing sensitivity of the application I want to avoid doing an ARP 
lookup just to reply to a single query packet. I see there is an 
ETHARP_TRUST_IP_MAC option that will automatically save the MAC from 
incoming packets into the ARP table, however there are two things that 
keep it from helping much.  Firstly it calls etharp_update_arp_entry 
with the ETHARP_FLAG_FIND_ONLY option which gives up if there isn't 
already a free entry. This means that if there are more clients than ARP 
table slots, the table fills up and it starts making ARP queries again. 
Secondly, even if I change that to ETHARP_FLAG_TRY_HARD which will 
displace the oldest entry, it only works for on-subnet clients. Distant 
clients will force a lookup of the gateway's IP each time. Of course a 
bigger ARP table will help, but I can only increase it so much and I 
want to be able to support subnets with arbitrarily many hosts on them, 
so relying on that is not practical.

Ideally I think I would simply like to take the original packet and flip 
it around so both layer 2 and layer 3 source and destination addresses 
are exchanged, and no lookup is needed. Is there any existing facility 
that can help with this? I can figure out how to manually munge the IP 
headers and send the reply as a raw layer 2 frame but it would be great 
if a function already existed to do this. Maybe a udp_reply() function 
that takes a pbuf as input.

Also this optimization needs to eventually happen for IPv6 as well. I 
haven't looked yet to see whether IPv6 has the same issue but I assume 
it has to, but in ND rather than ARP of course.

Misc details: using NO_SYS=1 (so raw API only), 32KB RAM total, hardware 
checksumming is on.