[Click] Question about cooperating elementclasses

[Bobby Longpocket]

I like the idea of moving packet queuing into ARPQuerier proper.

There may be reasons that packets need to stay on the same path whether or not they hit address resolution.  If nothing else, it makes packet flow easier to follow.

To make sure the 'right' ARPQuerier is awakened to send pending packets, just use external click logic.  If the existing configuration doesn't ensure that the sender of a query will receive the response, then just Tee() the ARP input to all the related ARPQueriers.

I like the idea on a more superficial level because I don't like that something called "ARPTable" has gotten into the packet storage business.

--- On Thu, 2/17/11, Eddie Kohler <kohler at cs.ucla.edu> wrote:

> From: Eddie Kohler <kohler at cs.ucla.edu>
> Subject: Re: [Click] Question about cooperating elementclasses
> To: "Philip Prindeville" <philipp_subx at redfish-solutions.com>
> Cc: click at pdos.csail.mit.edu
> Date: Thursday, February 17, 2011, 2:36 PM
> But in this case, right, it doesn't
> actually matter whether we map packets to 
> the "correct" ARPQuerier?  We don't expect many
> packets to get dropped, or 
> even queued; it's not super performance sensitive. 
> It'd definitely be easier 
> to emit packets to SOME Querier than it would be to emit
> them to the ENQUEUING 
> ARPQuerier.
> 
> Eddie
> 
> 
> On 02/17/2011 01:28 PM, Philip Prindeville wrote:
> > Yes, busted.
> >
> > It's a RouterBricks thing.
> >
> > And yes, you guessed exactly right.
> >
> >
> > On 2/17/11 11:32 AM, Eddie Kohler wrote:
> >> Hey Philip,
> >>
> >> Thanks for your patience...
> >>
> >> It is certainly possible that ARPQuerier should do
> the queueing instead of
> >> ARPTable.
> >>
> >> The "logic" for for the current behavior is as
> follows: ARP responses are
> >> sensitive to a given NIC, so if multiple
> ARPQueriers share an ARPTable, it's
> >> OK that all queued packets are emitted from
> *WHICHEVER* ARPQuerier happens
> >> to receive the response.
> >>
> >> I don't completely buy this "logic". But could you
> explain more about your
> >> application, to help me understand the ideal final
> behavior? Why do you want
> >> to share ARPTables? Is this a sort of a
> RouterBricks thing, where you have
> >> multiple ARPQueriers on different threads, writing
> to multiple output queues
> >> on THE SAME output NIC?
> >>
> >> Eddie
> >>
> >>
> >> On 02/12/2011 11:22 AM, Philip Prindeville wrote:
> >>> I wanted to make a change to ARPTable which
> would give it the ability to
> >>> invoke a hook back to ARPQuerier when it
> deletes queued up packages, but
> >>> wasn't sure if there's a clean way to do
> this.
> >>>
> >>> In C, I'd just set a pointer to a callback
> function, but this isn't C.
> >>>
> >>> So what's the best way to do it? Keep in mind
> that the ARPTable might have
> >>> been
> >>>
> >>> Also, it's the case that the same ARPTable
> instance might be shared amongst
> >>> several ARPQuerier instances (because we're in
> a multi-core environment
> >>> with several threads running on the same
> NIC).
> >>>
> >>> Oh, one other question about ARPTable vs.
> ARPQuerier interactions... if I
> >>> have that same situation where I'm running
> multithreaded, and a response
> >>> packet gets delivered to one ARPQuerier,
> thereby updating the ARPTable...
> >>> will the queued up packets in each instance of
> the ARPQueriers get updated
> >>> and unblock the packets?
> >>>
> >>> Or are the queued packets actually held by the
> ARPTable (and therefore in a
> >>> single place)?
> >>>
> >>> Looking at how ARPQuerier::handle_ip() and
> ARPQuerier::handle_response()
> >>> interact, it looks like the packets are all
> held by the ARPTable, and when
> >>> the queue gets drained, all drained by the
> same instance... so any affinity
> >>> of individual packets to threads is lost.
> >>>
> >>> Packet *cached_packet;
> >>> arpt->insert(ipa, ena,&cached_packet);
> >>>
> >>> while (cached_packet) {
> >>> Packet *next = cached_packet->next();
> >>> handle_ip(cached_packet, true);
> >>> cached_packet = next;
> >>> }
> >>>
> >>>
> >>> Correct?
> >>>
> >>> Thanks.
> >>>
> >>> -Philip
> >
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