TTCP recommendations for large-delay, large-bandwidth networks
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This recommendation identifies one "bug" and one implementation recommendation
regarding the ttcp in large-delay, large-bandwidth networks, also known as long
fat networks (LFN).

Wrap-Around Bug
-----------------------

The identified "bug" only occurs for transfers greater that 2^32 bytes
(4,294,967,296 bytes).  At this point, the variable, nbytes, must be classified
as a 64 bit integer or there will be an integer wrap-around problem and ttcp
will report an incorrect throughput.  Most users of ttcp will never see this
problem.  However, for those experimenting with wide area network with links at
622 Mbits/sec or greater, the problem may surface.

Bug fix for integer wrap-around problem:

Original code:  unsigned long nbytes;
Fix:            unsigned long long nbytes;

Implementation Recommendation
-----------------------------------------  

The implementation recommendation concerns the reporting of ttcp throughput
measurement.  Currently, most, if not all, versions of ttcp report throughput
in kilobits, kilobytes, megabits or megabytes per second in which kilo is
defined as 1024 instead of 1000 and mega is defined as 1000*1024 instead of
1,000,000 or 1024*1024.  This causes a 2.4% mismatch between actual throughput
readings on telecommunication instrumentation such as ATM test equipment and the
measurements reported via ttcp.

Table 1 illustrates this example.  A Hewlett Packard ATM tester was used to
capture ATM PDUs (protocol data units) as was tcpdump and ttcp.  All methods
provided roughly the same calculated throughput in megabits per second - here a
megabit is defined as 1,000,000 bits.  However, ttcp reported a throughput of
125 megabits per second.  The difference is 2.4 percent and is a result of the
use of 1000 * 1024 instead of 1,000,000 when defining a megabit.

                                    TABLE 1
*******************************************************************************
                           Data    Data Sent             Calc    Report Correct
                  Time    per PDU   in Time     Tput     Tput      Tput   Tput 
         #PDU's   (Sec)    (bytes)   (MB/s)    (MB/s)    (Mb/s)   (Mb/s) (Mb/s)
HP BSTS   639   0.3640385   9140    5840460   16.0435   128.3482
TCPDUMP   639   0.3641900   9140    5840460   16.0368   128.2948
   TTCP   640   0.3655740   9140    5849600   16.0011   128.0091    125   128
*******************************************************************************

The following source code excerpt is typical of ttcp and was from nttcp
available at ftp://ftp.arl.mil/pub/ttcp/nttcp.c.  Note that KB (kilobytes) is
defined as 1024 bytes not 1000 bytes and Mb (megabits) is defined as 1000
kilobytes where kilobytes is defined as 1024 bytes.  In order to have ttcp
results match those of telecommunication instrumentation equipment, the
following changes should be made: 1024 should be changed to 1000 for the KB/sec
and KB/cpu calculations and the 128000 should be changed to 125000 for the
Mb/sec calculation.  Otherwise, an adjustment of 128/125 should be multiplied
to the ttcp reported throughput measurement in order for them to match actual
measurement.

Example:  throughput = [nbytes(bytes) / realt(sec)] * 8(bits/byte) * 
                       [1 (Mbit) / 1,000,000 (bits)]

                     = [nbytes(bytes) / realt(sec) / 125000(bytes/Mbit) 

/*
 *      T T C P . C
 *
 * Test TCP connection.  Makes a connection on port 5001
 * and transfers fabricated buffers or data copied from stdin.
 *
 * Usable on 4.2, 4.3, and 4.1a systems by defining one of
 * BSD42 BSD43 (BSD41a)
 * Machines using System V with BSD sockets should define SYSV.
 *
 * Modified for operation under 4.2BSD, 18 Dec 84
 *      T.C. Slattery, USNA
 * Minor improvements, Mike Muuss and Terry Slattery, 16-Oct-85.
 *
 * Distribution Status -
 *      Public Domain.  Distribution Unlimited.
 */

"ttcp%s: %ld bytes in %.2f real seconds = %.2f KB/sec = %.4f Mb/s\n",
                trans?"-t":"-r",
                nbytes, realt, ((double)nbytes)/realt/1024,
                                ((double)nbytes)/realt/128000 );
        if (verbose) {
            fprintf(stdout,
                "ttcp%s: %ld bytes in %.2f CPU seconds = %.2f KB/cpu sec\n",
                trans?"-t":"-r",
                nbytes, cput, ((double)nbytes)/cput/1024 );
                
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