[Xorp-users] PIM/SM: Missing Join and Register-Stop from RP

Holger Kummert Holger.Kummert at sophos.com
Fri Mar 14 10:30:24 PDT 2014



Am 13.03.2014 18:56, schrieb Williams, Mark:
>  From what I understand, JOINs aren't send towards the source, they are sent to the RP.

This counts only for downstream routers.

>
> Sources don't know where receivers are; they just forward the
> traffic  at layer two to their nearest multicast neighbor/router. It's up to the
> neighbor/router and upstream RP to build the tree.

That's true. But between RP and S (the router where the actual source of 
the multicast stream is connected to) there is a small SPT built up.
The setting is like

    sender --- S --- RP --- another MC-Router --- receiver

 > The register-stop
> message occurs once the source/tree is built.

No, there are many cases where a Register-Stop is sent from RP to S.
E.g. when RP has no Joins received from downstream (= no one is 
interested in the multicast data) and there are Register-Messages
coming from S to RP. Then RP has to save this information, discard
all the Register messages and send an Register-Stop back to S
(doesn't happen).

When later a receiver Joins (through the MC-Router) at RP, RP should
Join itself to S (what also doesn't happen ...).

 >      I think this is the normal
 > function of the RP, and how it works regardless of implementation.
> Behaves the same way in Alcatel-Lucent and Cisco.

Yes, that's how it should work like. But unfortunately XORP doesn't.

Currently I'm going through the source code and look for an improvement.


Holger


>
> -Mark Williams
>
> -----Original Message-----
> From: xorp-users-bounces at xorp.org [mailto:xorp-users-bounces at xorp.org] On Behalf Of Ben Greear
> Sent: Thursday, March 13, 2014 10:45 AM
> To: Holger Kummert
> Cc: xorp-users at xorp.org
> Subject: Re: [Xorp-users] PIM/SM: Missing Join and Register-Stop from RP
>
> On 03/13/2014 10:31 AM, Holger Kummert wrote:
>>
>> Hello,
>>
>> I do not understand XORP's behavior when running PIM/SM and Register
>> messages (that encapsulate multicast traffic) are received by the RP
>> from S.
>> What I am really missing is the Join msg towards S in order to avoid
>> the encapsulating overhead. Therefore the Register-Stop (in order to
>> stop sending the Register msgs) is also not sent by RP.
>>
>> Shouldn't both messages be sent according to RFC 4601, ch. 3.2?
>>
>> The only situation where I could force the Register-Stop to be sent
>> was by enabling and setting 'switch-to-spt-threshold' to '0'.
>> This sending is independent of any Join'ed receivers and happens
>> immediately after the receive of the first Register message.
>> But this is obviously not the intended usage.
>>
>> Anyhow under no circumstances any Join messages are sent by RP to S.
>>
>> Could someone give more insight on this?
>
> I'm afraid I never knew much about multicast, and what I did learn while hacking on xorp mcast logic I have mostly forgotten.
>
> But, if you figure out a patch that makes xorp work better, please post it and I will try to review it.
>
> Thanks,
> Ben
>
>>
>>
>> Thanks in advance,
>> Holger
>>




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