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cjp
Asked: 2012-01-12 11:43:51 +0800 CST

What's the difference between Send-Q and S-BCNT in FreeBSD netstat -anx?

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What's the exact meaning of Send Buffer byte count in the extended netstat statistics, and why is it always bigger than Send-Q?

In other words, what's counted against the S-BCNT that's not counted against the Send-Q?

Is S-BCNT a better number to use to figure the cost to the system of a particular connection?

freebsd tcp netstat

1 Answers

  1. Best Answer

    You can find definition along with comments at sys/sys/sockbuf.h:

        u_int   sb_cc;      /* (c/d) actual chars in buffer */
    u_int   sb_mbcnt;   /* (c/d) chars of mbufs used */

    BSDs are using structures called mbuf/mbuf_cluster for network data. They used everywhere from driver code to socket layer. Even if packet had 1 byte of payload it will consume 256 (mbuf) bytes in mbuf. If packet size (with all frame/packet headers) >= 256 it will consume 256 (mbuf) + 2048 (mbuf_cluster) bytes. Hence the difference between sb_cc and sb_mbcnt.

    To answer your question: S-BCNT is more correct value to use.

    PS. For more information consult to TCP/IP Illustrated Volume 2 - W. Richard Stevens and Garry R. Wright, Chapter 16 - Socket I/O, Section 16.3 - Socket Buffers, p.476

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