[Udpcast] Re: udpcast

Alain Knaff alain at knaff.lu
Wed Dec 11 11:28:46 CET 2002

Le Mar 10 Déc 2002 23:45, vous avez écrit :
> > What do you mean by "server"? The sender, or just the DHCP server?
> The sender.  The disk images will be stored on a machine one one subnet
> and all the receivers will be on another subnet.


> > 1. If you mean the sender of data, you must be aware that multicast
> > works best within the same subnet. It is possible to get it to work in
> > different subnets by using the --ttl version on both the sender and
> > the receivers. This sets the "time to live" on the multicast packets,
> > allowing the to traverse routers. However, this only works if the
> > router supports multicast routing, but most unfortunately don't :-(
> Our router does support multicast, it is a Cisco 6500.  I worked out the
> problem.  The address range is a reserved, non forwardable
> range.  I had to chose and address outside this range for the routers to
> forward the packets. I used the address
> I also came across some other problems.  Sometimes, all the receivers
> timeout on the sender.  I increased the timeout but that did not help.
> I think what is happening is the receiver misses a packet and sends a
> notification to the sender.  That packet gets lost though and both
> sender and receiver are then left waiting for the other to make a move.

The udpcast server sends packets in batches ("slices") of up to 1024
packets. When a slice is transmitted, the sender "asks" the receivers
to send their confirmations. Receivers reply either with a
confirmation that everything has been received (good), or with a
bitmap of missing packets.
Missing packets are then transmitted, and confirmation is again asked
of those receivers which weren't ok the first time around.
If a receiver does not reply at all, the sender will re-ask for
confirmation every couple of seconds, until all confirmations are in.
If a receiver has not replied for about a minute, the sender drops
that receiver from its list, in order to be able to finish the
transmission with the others.

> If I limit the bit rate, this problem is less frequent so I think the
> router has bandwidth restrictions and once they are exceeded it drops
> packets.  This would not be a problem though if the session did not
> timeout.

Unfortunately for the moment the timeout duration is hardwired, and is
200. This is specified in line 921 of senddata.c:

 if(rexmitSlice->rxmitId > 200)

You can change it there to a higher value. In a future version, this
will become configurable.

> If this is the case, how hard do you think it would be for the receiver
> to retransmit whatever it needs to get the transmission going again, if
> say it has not got a reply from the sender in so many milliseconds?
> John.

Obviously, after a while, the router is dropping all the packets in
one direction (at least) after a while. After such an occurrence, no
matter what the receiver sends will not unblock the situation.

In order to debug this, you can try running the command-line variant
of udpcast on an server and  (already installed) receiver(s), and run
tcdpump along with it to check where the packets are dropped.


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