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Re: Java on Tiger and UDP

Unfortunately I tried this. Using the getters OS X says the limits are around 64k, even though it lets through less than 8k. Using the setters has no effect, the result is still less than 8k. :-(

Nothing seems to increase this size.

Jamie

On 27/07/2005, at 6:53 PM, Dan Creswell wrote:

From TCP/IP Illustrated (W Richard Stevens):

"Theoretically, the maximum size of an IP datagram is 65536 bytes,
imposed by the 16-bit total length field in the IP header. With an IP
header of 20 bytes and a UDP header of 8 bytes this leaves a maximum of
65507 bytes of user data in a UDP datagram. Most implementations
however provide less than this maximum."

He goes on to explain that there can be kernel limits or bugs that limit
the size of the datagram which is possible in this case but Stevens also
talks about API level restrictions:

"Most systems provide a default of just over 8192 bytes for the maximum
size of a UDP datagram that can be read or written."

This default is controlled by the default buffer sizes which are set via
DatagramSocket.setReceiveBufferSize() and
DatagramSocket.setSendBufferSize(). There are also get equivalents.
I'd suggest that you modify your program to invoke the get methods and
see what the values are. If the values are less than your preferred
packet size, try invoking the set methods and see if that makes the
difference.

Hope that helps,

Dan.

Jamie Curmi wrote:


On 27/07/2005, at 5:04 PM, Moises Lejter wrote:


Jamie Curmi wrote:



On Panther, you could send UDP packets > 16356 bytes from Java, and
receive them without issue.
On Tiger, any UDP packets > 16356 bytes are dropped. You just
never receive them.



I thought the max UDP packet size was limited to something like 8K
(system buffer size)... A google search found a reference to a page
in the MS Windows 2003 doc that claims that a UDP packet *must* be
less than 16384, and another discussion that suggests that whatever
the max size is has to include header info, so your actual payload
will be about 20 bytes less... Elsewhere someone says that the max
UDP packet size might have to be smaller than the smallest MTU on any
devices along the way, or the packet might not make it through...

Bottom line, it seems to have to be small... :-)



Have tested the code on Linux and Windows XP. Both can go all the way
up to close to 64k.

That is quite a difference between Panther and Tiger. Panther acts the
same as all other platforms - but Tiger has an 8k limit. And no
apparent way to change it.

:-(

Jamie

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References: 
 >Java on Tiger and UDP (From: Jamie Curmi <email@hidden>)
 >Re: Java on Tiger and UDP (From: Moises Lejter <email@hidden>)
 >Re: Java on Tiger and UDP (From: Jamie Curmi <email@hidden>)
 >Re: Java on Tiger and UDP (From: Dan Creswell <email@hidden>)



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