OTish: using short-term TCP connections to send to multiple slaves

alister alister.nospam.ware at ntlworld.com
Mon Nov 17 11:46:49 CET 2014


On Mon, 17 Nov 2014 08:56:43 +1100, Chris Angelico wrote:

> On Mon, Nov 17, 2014 at 4:21 AM, Roy Smith <roy at panix.com> wrote:
>> In article <mailman.15887.1416150791.18130.python-list at python.org>,
>>  Chris Angelico <rosuav at gmail.com> wrote:
>>
>>> UDP for anything more than your network's MTU is inefficient
>>
>> Why do you say it's inefficient?  Sure, the UDP datagram will get
>> fragmented and re-assembled at the other end, but it's not like TCP
>> would do any better.  One way or another, your data is going to be
>> transmitted in packet that fit into the MTU.
> 
> Sorry, is less efficient. I was responding to the point about 64KB being
> way too big for UDP, when really it's perfectly possible. It's common to
> keep UDP packet sizes as low as possible to take advantage of
> non-fragmentation efficiency (hence the DNS TCP switchover at 512
> bytes), but I've sent some huge UDP packets around.
> 
>>> plus you'd need to roll your own acknowledgement system so you know
>>> when the client got the data, at which point you're basically
>>> recreating TCP.
>>
>> That I certainly agree with.  UDP should be used for fire-and-forget,
>> where its not critical that every bit of data gets through.  A classic
>> example is high-volume logging.  Once you start thinking about any kind
>> of acknowledgements, you should just switch to TCP.  The alternative is
>> that you will slowly end up reinventing TCP (and doing a bad job of
>> it).
> 
> Another good use of UDP is where it's not 100% critical that *every*
> update get to *every* client instantly, but a client can "catch up" when
> it gets the next notification. (Since clients can be down, you'd need
> some kind of catch-up mechanic anyway.) If the data involved can fit
> inside a single UDP packet, this is trivially easy: just overwrite with
> the new state. Otherwise, some kind of update counter works too ("hmm, I
> got update 5, this one's update 7, I guess I'd better catch up"). If
> most clients collect most UDP updates, but occasionally they establish a
> TCP connection to do the full catch-up, you can get some handy
> conveniences.
> 
>>> NAT is the most common cause of breakage, but any router does have the
>>> power to monitor and drop connections on any basis it likes. (I can't
>>> imagine any reason a non-NAT router would want to prevent connections
>>> from going idle, but it could be done.)
>>
>> It's common in corporate networks for routers to forcibly close idle
>> connections (i.e. inject RST packets).  Often, the IT security guys
>> think idle connections represent some kind of threat (and good luck
>> trying to argue with them).
> 
> Yeah... but I can't imagine any reason for those connections to be a
> threat. I've heard of it happening but I cannot fathom the reasoning
> behind it. (You're also suggesting a much nicer approach than the
> worst-case I was talking about: with RST injection, at least one end
> will be immediately aware of the closure. If the router just starts
> dropping packets, much harder.)
> 
> ChrisA

I have recently been caught by this issues with call logging software for 
a PBX I maintain.

TCP sessions can get closed for a multitude of reasons. they are commonly 
closed by the TCP/IP stack because they have been idle too long & the 
resources are needed elsewhere. ideally a close message should be sent 
but this does not always happen (& is perfectly within specification)

if a sending devise finds that its connection is no longer open it should 
re-initiate the connection with a three way handshake.

Certain versions of the PBX firmware did not re initiate the connection 
but just kept retrying indefinitely meaning that the call logger did not 
get the data until the PBX output was stopped & restarted.

(a small python script to emulate things helped me prove what was 
happening :-) )
 



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