[sctp-dev] Basic Questions

[Christopher Hegarty - Sun Microsystems Ireland]

Hi David,

comments inline...

David Ryan wrote:
> Hi,
> 
> I have recently downloaded the Client/Server example from Chris 
> Hegarty's Blog.  I've been playing around with it and getting a feel for 
> SCTP.  My environment is Java 7 b59 running on Open Solaris 2009.06  (in 
> VMware Fusion on OSX).
Great that you are playing around with this. I really appreciate the 
feedback/questions. Interesting environment, good to see that you are on 
  09.06

> I changed the example code to send different sized packets and found 
> that the maximum size of data returned by receive was 16,000 bytes.  Is 
> this the exact same size of the underlying network packet?  I'm assuming 
> it is and that the packet size is large because it didn't actually go 
> over a network.
I assume you are sending messages larger that 16,000 bytes but the 
receive is return an incomplete message, i.e. isComplete returns false, 
right?

The packet size does indeed look very large for a single PDU. I would 
suspect that several PDU's are available in the input buffer and being 
combined before returning to the user. Can you see the MTU of interface 
that you are sending/receiving on, 'ifconfig -a'?

The API supports SCTP_DISABLE_FRAGMENTS socket option to disable the 
implementation from fragmenting messages. On your platform I suspect 
that this option is true by default. Can you check the value of this? 
Maybe you could set it to false to determine the exact PMTU, if it is 
not actually 16,000 which is may be.

> Is it possible to discover the PMTU size from the API so that a protocol 
> can be optimized for SCTP?
It is not currently possible to retrieve the PMTU through the API, but 
it should possible to implement an implementation specific socket option 
to support this, and I could add it to the API if necessary.

Are you sure you want to do this? The reason I ask is that the PMTU may 
be different for the different routes on an association with multiple 
interfaces. Typical applications will not directly control which 
destination address to use when sending, so trying to size your messages 
to an specific size based on the PMTU may not be the best approach. Or 
do you have a different scenario?

> Also, great to see SCTP on Java.  Disapointing its not on all platforms 
> yet, but I'm sure that will be fixed in good time.
Yes, hopefully over time other platforms/ports can be supported.

-Chris.

> 
> Regards,
> David.