From: Joseph M. Newcomer on
Also note that the illlusion that UDP doesn't drop packets is just that; even on a private
subnet, the network stack itself is free to drop packets, and will, under heavy load
conditions. I've seen it happen.

On Thu, 22 Oct 2009 11:41:20 -0500, TomChapman <TomChapman12(a)> wrote:

>Stephen Myers > wrote:
>> TomChapman wrote:
>>> By a closed network I mean: The customer has several computers that
>>> talk to each other through a single router. There are no router
>>> connections except for this small cluster of computers. It is closed
>>> to the outside world. There is no connection, for instance, to the
>>> companies normal office network. This is done for security reasons.
>>> And also for control. The company that developed this system is in
>>> total control of what is being sent on the interconnecting network.
>>> There is no outside influence.
>>> My client has three of these clusters.
>>> To gain access to the data I will plug into each router. You may say I
>>> am breaking their "closed network security", but that is what they
>>> want me to do.
>>> I have ZERO control over the fact that the data is being provided
>>> using UDP. That has already been determined. One of the servers in
>>> each of the networks sends out these UDP packets that several of the
>>> other computers in the cluster utilize. My program will simply be a
>>> new receiver of the same data that is already being sent. What is also
>>> different is that my computer will connect to all three clusters and
>>> receive data from all three.
>>> The customer tells me... When UDP is used in a small closed network
>>> where there is only one router and only one route between computers
>>> and when all of the computers are within a few feet of each other
>>> using reliable cabling, that UDP serves their purpose perfectly.
>>> They assure me that I will receive every message. They will be in
>>> proper order. And nothing will be duplicated.
>>> Joseph M. Newcomer wrote:
>>>> On Thu, 22 Oct 2009 09:41:37 -0500, TomChapman
>>>> <TomChapman12(a)> wrote:
>>>>> I am using Visual C++ version 6.
>>>>> My next project is to write an application that receives data from
>>>>> three UDP data streams. Each data stream will originate from a
>>>>> different computer. Each of the three computers will be in its own
>>>>> closed network.
>>>> ****
>>>> What is a "closed network"? ****
>>>>> In addition, my program also needs to act as a TCP server on the
>>>>> regular (open) network at the customer site. (I will also write a
>>>>> small TCP client application that will connect to my TCP server
>>>>> program.)
>>>>> I have worked with TCP many times, but never UDP. I have never
>>>>> worked with closed networks. I have questions. Please help me.
>>>> ****
>>>> UDP is generally considered a Really Bad Idea Most Of The Time. It
>>>> has severe
>>>> limitations, including the fact that messages may be dropped
>>>> arbitrarily, truncated
>>>> silently, you can receive two copies of the same message, and
>>>> messages can be received in
>>>> a different order than the order in which they were sent.
>>>> I fail to see why "open" or "closed" (whatever they might mean!) have
>>>> any relevance.
>>>> Either you get the message or you don't, and how it gets there is not
>>>> your problem.
>>>> ****
>>>>> I assume the computer for my application will need 4 network
>>>>> connections. One to each closed network and one to the regular network.
>>>>> Question: Is this correct? If my 4 network card plan is not correct.
>>>>> Please tell me how to implement this situation.
>>>> ****
>>>> If a "closed" network means that it has to have a separate path to
>>>> your computer, then
>>>> yes, you need a connection. This is part of the network topology
>>>> problem, not one of
>>>> programming. All networks need a path to your computer; how they get
>>>> it is not a
>>>> programming issue.
>>>> ****
>>>>> Several years ago I wrote my networking classes which are based on
>>>>> MFC CAsyncSocket. When I open a socket to receive one UDP socket
>>>>> stream do I somehow have to stipulate which network to use? Or does
>>>>> this magically work?
>>>> ****
>>>> Try it. Note that UDP is *not* a "stream" protocol, but a *message*
>>>> protocol, and the
>>>> concept of streams does not exist. A message is sent. I might get
>>>> there, it might not,
>>>> you will never know. You will not know if someone has sent you a
>>>> message if it does not
>>>> arrive. The message may be truncated to a maximum of 536 bytes
>>>> (including headers). ****
>>>>> When I open my TCP server socket, how do I stipulate which network
>>>>> is to be used?
>>>> ****
>>>> As far as I know, this is all magic handled in the network stacks.
>>>> Note that if the
>>>> "closed" networks have disjoint IP address ranges, you might be able
>>>> to specify a
>>>> restricted range of IP addresses to use. ****
>>>>> I have looked at CAsyncSocket and I see nothing about network
>>>>> connection selection. How will this work?
>>>> ****
>>>> The network stacks are supposed to sort this mess out. WinSock does
>>>> not support any
>>>> concept of knowing what "adapter" is being used.
>>>> joe
>>>> ****
>>>> Joseph M. Newcomer [MVP]
>>>> email: newcomer(a)
>>>> Web:
>>>> MVP Tips:
>> I'd be worried that the routers are not configured to be on separate
>> subnets. If that's the case you've got major headaches trying to talk
>> to the networks from a single machine. The TCP/IP stack won't have
>> anyway of determining the NIC to use for a particular message or
>> connection.
>> Steve
>I also thought about this issue. I will find the answer out from my
>customer. I hope that each cluster is not using some standard IP scheme
>where each cluster's IPs are identical.
>Thank you,
Joseph M. Newcomer [MVP]
email: newcomer(a)
MVP Tips: