Re: [Q] About block/nonblock TCP Sending in ns2

[John Heidemann <johnh@ISI.EDU>]

On Thu, 09 Sep 1999 14:37:11 CDT, he@expert.cc.purdue.edu wrote: 
>John:
>
>	Thank you for your answer. I agree with you NS2
>dont have threads. But I think in the TCP layor, it is
>in fact has a "infinate buffer" which means whenever appliication
>layor "send()" message to Tcp layor, it will simply "buffer"
>the data in the buffer (of course, only moving pointer, no
>real data copying needed). The following is a little analysis,
>hope to get some wisdom from you:
>
>-------------------Ns 2 Tcp Application buffer-----------------------
>
> I've searched through some related code in ns2.  It looks like their Tcp
>application doesnt implement a application buffer for packets sending
>(equals to the buffer for socket in real OS). What they do is roughly like
>this: 
>		(refer to "tcp-full.cc")
>       1) Whenever application wants to send n bytes just advance a
>pointer in TCP protocol called "curseq_" for n bytes
>
>        2) Compute the highest byte could be sent by: 
>
>        int win = window() * maxseg_;  // window() in pkts
>
>        int topwin = curseq_; // 1 seq number past the last byte we can send
>        if ((topwin > highest_ack_ + win) || infinite_send_) 
>                topwin = highest_ack_ + win; 
>
>        3) Try to send out all bytes below "topwin", the link layer is
>said to do some buffering, but that is obvious not the "buffer" we meant. 
>               Based on above, it seems that if the application layor
>sending much faster than the link layor can adopt, the pointer "curseq_"
>will increase much faster than the "highest_ack_ + win", and the
>difference between curseq_ and "highest_ack_+win" is equal to the socket
>buffer size in real world. If we define this difference as AppBufSize,
>then AppBufSize can go infinately large in the simulator (considering that
>in simulation, they don't really buffer packet content, so this
>implementation is obvious the easist, just advance a pointer)  which means
>the Application buffer is infinately large. 
>	So, NS2 doesnt need to "block" message sending call at all
>since it has an infinate buffer conceptually and can buffer the 
>message sent anyway no matter how fast upper layor inject the messages.


You're correct, ns doens't implement socket buffers.  (It is designed
to provide a very simple TCP implementation, not a complete simulation
of all aspects of a given real OS.)

It does have the TCP sender and congestion windows which will limit
the connection, though.

   -John Heidemann