Simple Message Framing Sample for TCP Socket

[收藏]General things to take into consideration while doing socket programming.

verygis — Fri, 20 Jun 2008 17:40:39 GMT

Ihadtorecentlywriteaclient-serversocketapplicationandsoIwasmullingoverallthatneeds...

re: Simple Message Framing Sample for TCP Socket

JonCole — Mon, 30 Oct 2006 19:56:32 GMT

Sean,

Yes, you are correct that I needed to put that line after the Socket.Receive inside the while loop. Also, I noticed I was using the wrong offset into the data buffer for that first read of the message body. I have fixed both problems in the sample code above.

Thanks for taking the time to point this out,
Jon

re: Simple Message Framing Sample for TCP Socket

Sean Beebe — Mon, 30 Oct 2006 00:49:07 GMT

Hi Jon,

Thanks for the example. It was very helpful for me.

Also, I think I may have discovered a bug. In the second while loop of ReadMessage shouldn't totalread += currentread come after the socket.Receive.

Thanks,

Sean

Simple Message Framing Sample for TCP Socket - Part 2 (Asynchronous)

Jon's WebLog — Tue, 25 Apr 2006 23:18:47 GMT

As a follow up to my last post (http://blogs.msdn.com/joncole/archive/2006/03/20/555721.aspx) I decided...

re: Simple Message Framing Sample for TCP Socket

valkyrie — Fri, 21 Apr 2006 06:02:18 GMT

Thank you. That was the most suscint framing of this particular conversation I have ever seen.

Thank you again.

Regards.

re: Simple Message Framing Sample for TCP Socket

Denis — Tue, 11 Apr 2006 16:57:59 GMT

While Alan is correct that this is one scenario is possible with Send(), Send() it makes it appear as if this is deterministic when it is in fact not. If you send two buffers (B1 and B2) the following are some of the possible scenarios:

1. B1 is sent as one segment and B2 is sent as another over the wire.
2. B1 and B2 are coalesced into one segment that is sent over the wire
3. Part of B1 is sent in one segment, and the remaining bits are sent along with B2 in another segment.
4. B1 and part of B2 are sent in one segment followed by the rest of B2 in another segment.

Many factors come into play. If the Nagle algorithm is disabled, the behavior will be different. If the connection is waiting for an ACK, buffers will likely be coalesced. etc...

The important points are:

1. As Alan stated, the Send(header), Send(payload) is probably not a good idea.
2. TCP is a stream protocol. Message boundaries are not guaranteed.

re: Simple Message Framing Sample for TCP Socket

JonCole — Tue, 11 Apr 2006 08:09:12 GMT

Alan, Good feedback. I considered writing the code as you mention to eliminate the duplicate calls to Send(), but I purposely made the decision to leave the duplicate calls in to emphasize the fact that the message size is sent as a "prefix" to the actual message. I will add a note to the post to encourage people to read the comments you have posted.

re: Simple Message Framing Sample for TCP Socket

JonCole — Tue, 11 Apr 2006 08:00:51 GMT

Ioan, You are correct. I definitely had a type-o in my explanation and I have fixed the mistake.

re: Simple Message Framing Sample for TCP Socket

Alan McFarlane — Mon, 10 Apr 2006 16:19:57 GMT

I've seen very many people misuenderstanding this and have answered many myself and agree with you diagnosis. However your send side code provokes bad behaviour, almost guaranteeing that your readers' next question is "How do I turn off Nagle? As the performance sucks..."

By doing send(), send(), the stack will send the first chunk (the length), wait for TCP ACK before sending the second chunk (the data). Thus breaking another of the cardinal TCP programmer rules, "Send *all* related data in one send()". See e.g. http://msdn.microsoft.com/library/en-us/dnwxp/html/comperfnetapppt1.asp?frame=true and http://msdn.microsoft.com/library/en-us/winsock/winsock/tcp_ip_specific_issues_2.asp?frame=true

Perhaps you can cover such subjects in subsequent postings. :-)

Alan

re: Simple Message Framing Sample for TCP Socket

Ioan Herisanu — Sat, 08 Apr 2006 19:59:14 GMT

If you send "Hello World" and receive "he" + "llo world" then you have not a TCP socket. For TCP one should have received "He" + "llo World". Please, pay attention when working with these,as some novices will understand really bad things