[Spread-users] experience with using multicast

[Jonathan Stanton]

Hi,

On Fri, Feb 20, 2004 at 07:40:31AM -0600, Greg Shebert wrote: 
[...]
> i have several nodes on different subnets on a lan that i am trying to
> group together using multicast based groups...
> 
> things seem to work for a short time and then stop... then, tons of
> multicast traffic starts flying around my network but nothing is being
> propogated to my spread client apps...
> 
> the relevant section of my config looks like this:
> 
> 
> Spread_Segment  225.10.2.1:4803 {
>         ops1                    10.2.17.12
>         alservice1              192.92.12.51
>         krserv1                 10.5.0.12
>         alserv1                 192.92.12.14
> }
[...]

One possible issue is that Spread sets the TTL on multicast packets to 1. 
Meaning that they will not pass a ip-multicast router, as they can only go 
one 'hop'. That is coded on purpose as Spread's use of multicasst is 
really to improve efficiency on LANs. 

>From your email, I'm not sure if the hosts (ops1, alservice1, etc) are 
located on separate lans with a router between them, or if they are ona 
switched lan with multiple address ranges. i.e. will a packet sent to 
225.10.2.1 have to go through a multicast router or is it disseminated as 
an Ethernet multicast packet by the switch.

If you want to use multicast through a router, you will need to change the 
line in data_link.c that sets "ttl_val = 1" to set it to a larger value 
(if you only need one router hop, then change it to "2") You do not 
necessarily want to change it to a larger value then you need as then your 
multicast packets could be routed onto the broader mbone (multicast 
enabled Internet) if your site has that type of multicast enabled.

You might also want to make sure that ip multicast packets are reaching 
all of hte hosts correctly outside of spread. The spsend and sprecv test 
programs that are included in the spread source (but not built by default) 
can be used to test that by sending to the multicast address and running 
receivers on the hosts. 

Cheers,

Jonathan



-- 
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Jonathan R. Stanton         jonathan at cs.jhu.edu
Dept. of Computer Science   
Johns Hopkins University    
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