Re: [Boost-users] [asio] What happens when socket receive bufferoverflows?

Subject: Re: [Boost-users] [asio] What happens when socket receive bufferoverflows?
From: Cliff Green (cliffg_at_[hidden])
Date: 2010-04-09 13:50:31

Not sure if anyone replied yet or not ...

>> ... protocol that puts a prefix on each message indicating the number its bytes number. 

Yes, a fairly common design (I've done a variation on it at least a dozen times).

>> ... errors related to reading a prefix that would indicate a strange number of bytes incoming, a number that was far greater than the max size possible of message ...

Also pretty common problem. I'd bet a major body part there's a bug in the sending (or receiving) side of the app. Something has dropped some bytes somewhere, or lost track of where it is in the "message framing".

>>...  So i reduced the socket buffer size to 1K, and the application crashed with a lot less network load than before. ... At the moment, i am almost sure that the problem is the socket receive buffer which overflows and the bytes are discarded, creating a "gap" in the byte stream which causes an inconsistent state on the application. 

As mentioned, you're probably on the mark as to why you're crashing. But not as to the underlying cause.

You're focusing on an OS or TCP stack problem, rather than your code. The TCP protocol has "flow control" built in, and if the receiving side fills up or is congested, it will tell the sending side to quit sending. If packets get dropped while the flow control is "kicking in", the packets will (eventually) get re-transmitted. (On a side note, this type of congestion can be a system-level problem, with certain use cases.)

>> ... What should happen when the socket receive buffer overflows? It silently discards the bytes or what?

TCP never "silently discards" bytes (UDP will, obviously). There's a bug in your code (including the way you're using Asio).

Asio streambufs or iostreams can leave characters in the buffer, and your logic must account for these.

If you're still having the problem, reduce your code to the smallest example that will reproduce the crash, and post it to the mailing list (there's also an Asio mailing list you can use).

Cliff