Question

What could be good list of failure scenaros for testing a reliable UDP layer? I have thought of the below cases:

Drop Data packets
Drop ACK, NAK Packets
Send packets in out of sequence.
Drop intial hand shaking packets
Drop close / shutdown packets
Duplicate packets

Please help in identifying other cases that reliable UDP needs to handle?

Answer 1

The list you ve given sounds pretty good. Also think about:

Very delayed packets (where most packets come through fine, but one or two are delayed by several minutes);
Very delayed duplicates (where the original came through quickly, but the duplicate arrived after several minutes delay);
Silent dropping of all packets above a certain size (both unidirectional and bidirectional cases);
Highly variable delays;
Sequence number wrapping tests.

Answer 2

Have you tried intentionally corrupting packets in transit?

Also, have you considered a scenario where only one-way communication is possible? In this case, the sending host thinks that the send failed, but the receiving end successfully processes the message. For instance:

host A sends a message to host B
B successfully receives message and replies with ACK
ACK gets dropped in the network
A waits for timeout and re-sends message (repeats steps 1-3)
host A exceeds retry count and thinks the send failed, but host B has in fact processed the message

Answer 3

I have thought UDP is a connectionless and unreliable protocol and that is does not require and specific transport handshake between hosts. And hence there is no such thing as a reliable UDP protocol.

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