On 12/3/2013 10:47 AM, Pedro Roque Marques wrote: > Joe, > > On Dec 3, 2013, at 7:54 AM, Joe Touch <[email protected]> wrote: > >> >> >> On 12/3/2013 12:55 AM, Pedro Roque Marques wrote: >>> Is the fundamental IP behavior that you are referring to the TTL decrement >>> between two addresses on the same "subnet" ? >>> I believe you are wrong: to my knowledge proxy ARP is compliant with Host >>> and Router requirement RFCs. >> >> For IPv4, RFC1812 says that all 1's "limited broadcast" >> (255.255.255.255) MUST NOT be forwarded. RFC2644 updates that RFC to say >> that even subnet-directed broadcasts MUST NOT be forwarded (as >> configured by default). >> >> So by default, a router would never forward the broadcast for which the >> proxy ARP would be an appropriate response. > > yes. > >> Which would mean that the >> proxy would work only if already populated at the router; > > I don't follow. Lets assume i've a device that is an IP compliant > router which has interface A and interface B. > The quote above means that it should not forward a packet address to > 255.255.255.255 between link A and link B (or vice-versa). This has > nothing no relationship to whether this device is capable of generating > ARP requests or proxy ARP replies.
ARP requests would never be forwarded across such a device. This means that proxy ARP responses would either have to be manually configured or learned by the actions of some other party. There's no opportunity to have the proxy ARP be a short-cut for a relayed ARP - which limits the cases where proxy ARP can be used. >> there would be >> no means to forward the initial request if the cache were empty. So >> proxy ARP would work if manually configured, but would be unreliable if >> automatically populated if used on a router that spans two groups of >> hosts on the same subnet. > > Proxy ARP is unrelated to subnet broadcasts. See the example above; not always. Joe
