CODEWITHSUNDEEP
javanetwork-programmingubuntuoperating-systemudp
Kurtt.Lin
Kurtt.Lin

Reputation: 289

why does this Java programme cause UDP packet loss?

I'm running experiments on my machine A and B, both with Ubuntu Server 11.04 installed. A and B are connected to the same 1000M/bps switch.

A is the sender:

while (total<=10,000) send_udp_datagramPacket(new byte[100]) to B

B is the receiver:

while(true) receive()

But finally I got less than 10,000 (about 9960) at B. Why is this happening? Where did the lost packets go? Were they not actually sent to the wire to the switch? Or the switch lost them? Or they indeed got to B, but B's OS discarded them? Or they reached to Java, but Java threw them away because of a full buffer?

Any reply would be appreciated.

Upvotes: 0

Views: 1266

Answers (3)

Chad Schouggins
Chad Schouggins

Reputation: 3521

Remember, UDP does not provide reliable communication, it is intended for situations in which data loss is acceptable (streaming media for instance). Chances are good that this is a buffer overflow (my guess, don't rely on it) but the point is that if this data loss is not acceptable, use TCP instead.

If this is just for the sake of experimentation, try adding a delay (Thread.sleep()) in the loop and increase it until you get the maximum received packets.

EDIT: As mentioned in a comment, the sleep() is NOT a fix and WILL eventually loose packets...that's just UDP.

Upvotes: 5

user207421
user207421

Reputation: 310850

Why does this Java programme cause UDP packet loss?

The question is ill-formed. Neither Java nor your program causes UDP packet loss. UDP causes UDP packet loss. There is no guarantee that any UDP packet will ever arrive. See RFC 768.

Upvotes: 1

Peter Lawrey
Peter Lawrey

Reputation: 533442

But finally I got less than 10,000 (about 9960) at B. Why is this happening?

UDP is a lossy protocol. Even if you got 10,000 in this test you would still have to code for the possibility that some packets will be lost. They can also be fragmented (if larger than 532 bytes) and/or arrive out of order.

Where did the lost package go?

They were dropped.

Were they not actually sent to the wire to the switch?

They can be dropped just about anywhere. I don't believe Java has any logic for dropping packets (but this to is not guaranteed in all implementations) It could be dropped by the OS, the network adapter, corrupted on the wire, dropped by the switch.

Or the switch lost them?

It will do this if the packet arrived corrupt in some way or a buffer filled.

Or they indeed got to B, but B's OS discarded them?

Yes, or A's OS could have discarded them.

Or they reached to Java, but Java threw them away because of a full buffer?

Java doesn't have its own buffers. It uses the underlying buffers from the OS. But the packets could be lost at this stage.

Note: No matter how much you decrease the packet loss, you must always allow for some loss.

Upvotes: 2

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