[openib-general] [ANNOUNCE] Contribute RDS (Reliable DatagramSockets) to OpenIB

[Grant Grundler]

On Fri, Nov 04, 2005 at 10:19:52AM -0800, Grant Grundler wrote:
...
> o The comments in "Recv operations" talk about "backpressure".
>   Is this another way of saying the driver should drop packets once
>   the "fairness threshold" is exceeded?

Ranjot's slideset answered this question (I think):
| o Slow receiver ports are stalled at sender side
|   - combination of activity (LRU) and memory utilization used
|     to detect slow receivers
|   - sendmsg() to stalled destination port returns
|     EWOULDBLOCK, application can retry
|   - recvmsg() on a stalled port un-stalls it

I'm having trouble reconciling previous "connectionless" and
"transperent to user space" comments this this slide.
Especially the "EWOULDBLOCK" return code.

If a reciever can cause a sender to stall, it implies the packets
will get dropped on the send side. This is a subtle change
in behavior that I don't think any UDP application can assume.
But I'm no networking protocol expert...

thanks,
grant