Re: getsockopt/setsockopt with SO_RCVBUF and SO_SNDBUF "non-standard"behaviour

From: Rick Jones
Date: Wed Jul 18 2012 - 13:32:33 EST

On 07/18/2012 09:11 AM, Eric Dumazet wrote:

That the way it's done on linux since day 0

You can probably find a lot of pages on the web explaining the

If your application handles UDP frames, what SO_RCVBUF should count ?

If its the amount of payload bytes, you could have a pathological
situation where an attacker sends 1-byte UDP frames fast enough and
could consume a lot of kernel memory.

Each frame consumes a fair amount of kernel memory (between 512 bytes
and 8 Kbytes depending on the driver).

So linux says : If user expect to receive XXXX bytes, set a limit of
_kernel_ memory used to store these bytes, and use an estimation of 100%
of overhead. That is : allow 2*XXXX bytes to be allocated for socket
receive buffers.

Expanding on/rewording that, in a setsockopt() call SO_RCVBUF specifies the data bytes and gets doubled to become the kernel/overhead byte limit. Unless the doubling would be greater than net.core.rmem_max, in which case the limit becomes net.core.rmem_max.

But on getsockopt() SO_RCVBUF is always the kernel/overhead byte limit.

In one call it is fish. In the other it is fowl.

Other stacks appear to keep their kernel/overhead limit quiet, keeping SO_RCVBUF an expression of a data limit in both setsockopt() and getsockopt(). With those stacks, there is I suppose the possible source of confusion when/if someone tests the queuing to a socket, sends "high overhead" packets and doesn't get to SO_RCVBUF worth of data though I don't recall encountering that in my "pre-linux" time.

The sometimes fish, sometimes fowl version (along with the auto tuning when one doesn't make setsockopt() calls) gave me fits in netperf for years until I finally relented and split the socket buffer size variables into three - what netperf's user requested via the command line, what it was right after the socket was created, and what it was at the end of the data phase of the test.

rick jones
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