Re: so_rerror

[Roy Marples <roy%marples.name@localhost>]

On 05/11/2018 23:10, Havard Eidnes wrote:
> > Whether it arrived at the kernel by UDP or carrier pigeon and
> > could not be delivered for reason X, we should not be discarding
> > this silently.
>
> You can not rely on the implemented error reporting to report all lost
> packets sent via UDP.  That's the nature of the UDP protocol, which is
> just a tiny demux layer on top of IP, as IP packets can be discarded
> in a queue (e.g. as part of executing random early discard as a queue
> management scheme) anywhere on the path between the two participating
> hosts.

But that's they key point isn't it? Two participating hosts.


My argument is that if I create a client/server setup using UDP on one 
host only, I would expect the receiver to log an error if it could not 
receive something the sender claimed it sent because at this point it's 
all self contained.

Protocols such as TCP and UDP are designed for on the wire, but in this 
instance there is no wire.

> Therefore, to my mind it makes no sense to insist that a particular
> type of packet loss event (receive buffer overflow) *must* always be
> reported, as there are other packet loss events which will never be
> reported to the receiver.

With more than one host I agree entirely, but for a single host I disagree.

> I'm with Christos: if you want some sort of reliability and still
> insists on using UDP, you need a protocol layer above UDP, with
> sequencing, retransmissions, performance enhancements with large
> windows, flow control, fairness, etc., so ... perhaps the insistence
> on UDP isn't well-thought-out and instead TCP begins to make more
> sense as a choice?

This is the thing I don't understand though.
I'm not trying to make anything more reliable.
I'm advocating that we should report things that are unreliable - we got 
something but could not deliver it.
How the unreliable is solved I leave as an exercise to the reader, but 
we should at least empower them with knowledge that it happened so they 
can try to fix it if they so want.

Roy