ODP: [lwip-devel] Fast ARP for a "mini" system

[Małowidzki , Marek]

Dear Jacob,
 
thank you for your feedback.
 
> An Ethernet bridge maps MAC -> interface. I use a fixed size cache table
> with the number of entries a power of 2. To look up a MAC address I
> calculate two hash values: an initial entry number, and a stride. My
> lookup function is as follows: (C++, sorry)

C++ is OK :) I was thinking about a hash table but the implementation would be 
more complex (this is an important factor as the IP stack is just one of 
multiple development tasks for us). For example, if we wanted to implement 
repeating ARP queries (and, freeing resources, i.e., buffered packets), we 
would need to keep an additional list (of pending entries). Now, in case when 
we have to remove an last-recent-used entry (the table is full), we would also 
need to update somehow the ARP pending queries list (or, guard somehow against 
the existing, obsolete entries).

Our device has just 2 interfaces, it is a kind of a simple router. We can 
afford memory overhead for 2 ARP tables. The least significant byte of an IP 
address would give us a "perfect hash" value - a unique one and of a value from 
a known range, unlike MAC addresses in your example, which are far more complex 
to handle. Our task is simpler here and thus the idea of using the simplest 
data structure, an fixed-size array.

Marek

<<winmail.dat>>