RE: [lwip-devel] Suggestion re ARP

[Zschocke, Florian]

Mountifield, Tony wrote on Wednesday, April 28, 2004 12:54 PM:

> My suggestion is to have a #if option to control whether or
> not LWIP updates its ARP table from 3rd-party requests it sees.
> Comments? 

This is the algorithm as laid out in RFC 826
(ftp://ftp.rfc-editor.org/in-notes/std/std37.txt):

-----------------------------------------
?Do I have the hardware type in ar$hrd?
Yes: (almost definitely)
  [optionally check the hardware length ar$hln]
  ?Do I speak the protocol in ar$pro?
  Yes:
    [optionally check the protocol length ar$pln]
    Merge_flag := false
    If the pair <protocol type, sender protocol address> is
        already in my translation table, update the sender
        hardware address field of the entry with the new
        information in the packet and set Merge_flag to true. 
    ?Am I the target protocol address?
    Yes:
      If Merge_flag is false, add the triplet <protocol type,
          sender protocol address, sender hardware address> to
          the translation table.
      ?Is the opcode ares_op$REQUEST?  (NOW look at the opcode!!)
      Yes:
        Swap hardware and protocol fields, putting the local
            hardware and protocol addresses in the sender fields.
        Set the ar$op field to ares_op$REPLY
        Send the packet to the (new) target hardware address on
            the same hardware on which the request was received.
------------------------------------------

As you can see, updates for existing entries are done for any ARP packet
received but new entries are only merged into the table when addressing the
host itself.

Florian