[Click] arpquerier.{hh|cc}

[Eddie Kohler]

Hi David,

It sounds like you're looking at an old version of the code!  I'd  
suggest you download the anonymous CVS version of Click and have a  
look at the new, even brain-break-ier ARPQuerier.

> Hi,
> I need to understand perfectly how the arpquerier element works.
> I know there is an "ARPEntry* _map[256]" to save ip-eth  
> translations and packets not even translateds...
> I know there is a lock to protect access to the map...
> I know there is a timeout to check, poll, ... old map entries...
> And in short, I know how it works well.
>
> However, I have two specific doubts in relation to its implementation:
> 1. Why is the _lock variable necessary?
> I know that it is necessary to acquire and release write/read  
> permission before write/read the map, but it is necessary?
> Each instance of arpquerier has its own map, so, there is no  
> interference between different instances of arpquerier.
> So, is it to protect the case in that click runs in multithreading  
> and the two elements that can call the push function (the two input  
> ports) make it at the same time?

Exactly.


> 2. In "ARPQuerier::handle_ip(Packet* p)" function, more or less,  
> there are three cases:
> a) the destination ip address of packet p is in the map and the  
> corresponding ARPEntry is ok (has the correct translation).
> b) the destination ip address of packet p is in the map but the  
> corresponding ARPEntry is not ok.
> c) the destination ip address of packet p is not in the map.
> In case c), handle_ip will make a new ARPEntry and will send the  
> corresponding ARPRequest with send_query_for(...).
> In case b), handle_ip will add the packet to the corresponding  
> ARPEntry and will send a send_query_for(...).
> And in case a), there are two possibilities depending on the  
> polling flag of the corresponding ARPEntry:
> a.1) polling==0 => handle_ip will add the ethernet header to the  
> packet and will send the packet.
> a.2) polling==1 => handle_ip will add the ethernet header to the  
> packet, will send the packet and moreover will send the  
> corresponding ARPRequest with send_query_for(...).
>
> Is the case a.2) that I don't understand:
> Why if polling==1, handle_ip will send a send_query_for(...)? The  
> map has the translation, why ask for it?

Because entries expire (after 30 seconds).  We need a new response to  
keep the mapping refreshed.

> I know that the expire_hook function (scheduled by a Timer) is the  
> responsible of set polling=1 when an ARPEntry is "old".
> But why polling and not deleting?
> Or, is like a margin of security? Is to avoid lose ARPEntries when  
> the corresponding node is even in the network? Is it necessary?

Not perhaps necessary, but useful for the reason you specify (to  
avoid losing entries).

Eddie


>
> Do you understand me?
> Thank you very very much!
>
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