On Fri, 5 Dec 2008, Alan Bateman wrote:
For the specific problem at hand then I assume that running with
-Djava.net.preferIPv4Stack=true fixes the problem.
Correct, though that option seems a bit of a misnomer, because it
essentially completely disables IPv6 in Java.
More generally, it can be hard to develop a portable multicasting
application. Typically, one binds to the wildcard address and then joins the
multicast group. This is usually preferable because it is not possible to
receive multicast datagrams on some systems when bound to a specific local
address or a multicast address.
Yes, this is often sufficient with very simple apps, or where you run
only one app on a host. But unfortunately often life is a bit more
complex :-(
The issues with interference that you allude
to on Linux are indeed a pain. I'm surprised there isn't a parameter or
option to configure this behavior (Michael might want to jump in here as he
was researching this specific topic a few months ago). 4701650 also touches
on this topic.
You may already be aware of the Linux interference issues in depth,
but let me add a couple of references for the record:
Linux netdev posting where in the followups there is extensive has
discussion why it's implemented this way (different than in windows
for example), and why it isn't changing:
http://marc.info/?l=linux-netdev&m=121579002105636&w=2
Bug #6579726 describes this "cross traffic" API issue on Linux. This
has resulted in a number of bug/doc reports and workarounds in some
Java programs:
http://www.jboss.org/community/docs/DOC-11710
http://www.jboss.org/community/docs/DOC-9469
https://jira.jboss.org/jira/browse/JGRP-777
As regards the suggestion to choose the protocol family at creation time -
some of that is already in place for jdk7 as it was needed to add multicast
support to NIO (and specifically because it is not portable to join IPv4
multicast groups with an IPv6 socket). When creating a DatagramChannel you
can specify the protocol family and this should correspond to the address
type of the multicast groups that you join. Classic networking needs a bit of
work to allow mixing of IPv4 and IPv6 sockets in the same VM. Chris or
Michael might want to say more about that.
It's great to hear there will be improvements in this area.
--
Pekka Savola "You each name yourselves king, yet the
Netcore Oy kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings
