gen_udp looses datagrams

Lars G J Carlsson etxlg@REDACTED
Fri May 19 11:58:34 CEST 2000


This is the first time I stick my neck out on this list, i.e. PLEASE
cut me some slack if this turns out to be a really silly comment.

In view of the fact that it is always the first packet (out of two)
that is lost, could it be that the host that you are trying to reach
happens to NOT be in the ARP table initially?
If you know that the hosts are in continuous communication already, please
stop reading now, the rest maybe correct (or NOT!) but it is certainly 
irrelevant for you.

If the ARP table does not have an entry for where the datagram should go it
will issue an ARP broadcast to resolve it, however, I think it is not
regulated in standards what to do with the packet that triggered the ARPOP_REQ.
It could be buffered awaiting succesfull resolution, in which case the question
becomes; how many datagrams will be buffered before the initial one is discarded.
>From some reading (RFC826 ?) it would seem that it is legal to just throw away a 
datagram if no link destination is immidiately available.
It doesn't help telling me what platform you are on since I don't really know how
any of them actually does this, sorry.


> From: "Vance Shipley" <vances@REDACTED>
> To: "Luke Gorrie" <luke@REDACTED>
> Cc: <erlang-questions@REDACTED>
> Subject: RE: gen_udp looses datagrams
> Date: Thu, 18 May 2000 12:41:43 -0400
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> Luke writes:
> }  "Vance Shipley" <vances@REDACTED> writes:
> }  
> }  > We're finding that when we have an active UDP socket
> }  > we can, and do, loose incoming datagrams.  At times
> }  > we will receive only the latter of two packets sent
> }  > immediately after one another.  Why would this be?
> }  
> }  Datagrams are unreliable by nature. They're really low-level, just IP
> }  packets annotated with port numbers afaik - they're not guaranteed to
> }  arrive, and may arrive out of order. For reliability you need to come
> }  up with some sort of scheme to compensate for lost or reordered
> }  packets, or use TCP.
> While I completely understand this I very much doubt that it is the
> issue here.  The communication is across a LAN and the other observation
> is that it is only the first packet which is lost on every occasion 
> we have seen.
> 	-Vance

Lars Carlsson

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