[sctp-dev] Socket Options Issues

[Danny]

Hi Chris, Bruce and Alan.

I just subscribed to this list and therefore had to make a new subjects 
in order to add to the conversation about the Send/Receive Buffer size 
settings.

I had the exact same problem and have seen the solution proposed by 
Chris to solve it in b81. However i am a little worried about the 
dividing by 2 solution.


I use lksctp too on Fedora and indeed they double the size today. But 
what if they change strategy or if another protocol provider is used 
that does not double the size.
I have build a work-around in may class constructors that has a solution 
that may be robust over time and that i would like to share (possibly 
you are already doing this but that's behind my knowledge).

I retrieve the buffer size (for both send and receive buffers) and back 
them up in a variable. I then set the size to a well known value (eg 
4096) and retrieve it back.
I then initialize a variable with the result of the division of the 
retrieved size divided by the well know set size. I call that my buffer 
multiplier.
Finally i a then set the initial buffer size back from my backup 
variables multiplied with the buffer multipliers (which for lksctp are 
0.5 today).
The multiplier can now be used to correct the retrieved buffer sizes by 
a simple multiplification.

The back-draw of my solution is that i have no means to make the change 
inside getOption() but you do.

I do not agree that in SCTP this buffer size should only be a hint. I 
can understand it for TCP but in SCTP one must be able to optimize 
fragmentation.
Consistency is important, the returned value must be reliable to 
indicate how many bytes the buffer can hold, while it is less important 
to know that a buffer
from, let's say, 2048 bytes approximately consumes 4096 bytes of memory. 
By adding to the documentation that programmers should keep in mind that 
the
buffer size must roughly be multiplied by 2 to get the consumed memory 
they should be all set.

It was during the implementation of SCTP fragmentation that i discovered 
the buffer size problem in my case. In order to stay with the topic i 
will post a second post about this
and a minor problem with using SCTP with the Selector in separate post's.

Greetz,
Danny