[p2p-hackers] How reliable is UDP?

Paul Campbell paul at ref.nmedia.net
Wed Dec 29 01:53:06 UTC 2004


On Thu, Dec 23, 2004 at 08:10:55AM +0100, David E. Meier wrote:
> Hi p2p-hackers,
> 
> I know that UDP traffic is not reliable by design. However, I wonder how
> reliable UDP packets reach their destination. Does someone on the list
> have experience with UDP packet loss? Is this usually in the range of a
> percent or more or less? Or is it even a common case that all packets
> arrive? Dave.

Pretty darned reliable.

Remember...the base IP protocol goal is to deliver packets. Routers use
packet dropping to signal congestion to the TCP protocol. So other than
actual packet losses (which are so minimal that currently TCP ASSUMES
all packet losses are congestion notifications), the only losses are due to
the congestion signalling protocol. In reality, they could actually eventually
be due to real congestion, but that would never happen since you of course
implemented congestion control in your UDP protocol, right? :)

It is a serious problem for wireless protocols since in the wireless
environment, packet loss is a real issue. Hence things like the "TULIP"
protocol have been developed to encapsulate TCP/IP at either the TCP or
IP layer and implement their own ACK protocols (and sometimes ACK spoofing)
specifically to cover up for standard TCP protocol weaknesses of assuming
packet loss is always due to packet loss so that standard implementations
don't get confused and give artificially low throughputs as a result.




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