Re: Suggestions for custom application-layer protocol?

From: Grant Edwards (grante_at_visi.com)
Date: 05/26/05

  • Next message: Grant Edwards: "Re: Suggestions for custom application-layer protocol?" 
    Date: Thu, 26 May 2005 18:22:18 -0000

    On 2005-05-26, Walter Roberson <roberson@ibd.nrc-cnrc.gc.ca> wrote:

    >:For simplicity of implementation, I'd recommend implementing a message
    >:layer over the TCP socket - something as simple as a two byte binary header
    >:to indicate the length of the message to follow.
    >
    >:This means you can then read a whole message - text based or otherwise,
    >:and the process it accordingly. You don't need to mess about with parsing
    >:lines and stuff like that.
    >
    > You still need to "mess about" if your data might not fit in a single
    > packet.

    And even if it does.

    > Recall that when you read() from a socket you are not certain
    > to get all of the data unless the data length is no greater
    > than your system's atomicity limits [PIPE_BUF]. The POSIX
    > minimum value for PIPE_BUF is 512 -- so in the general case a
    > write() of more than 512 bytes may require multiple read()
    > statements.

    The data may have passed through a bridge or router that may
    have broken it up even further. It's a bad idea to make any
    assumptions about any correspondance between write() and read()
    calls on a TCP connection. It's just a byte stream: you
    oughtn't depend on any sort of datagram behavior whatsoever.

    A 500 byte "message" may have been sent using a single call to
    write(), but the receiver had better be prepared to call read()
    anywhere from 1 to 500 times. 

    -- 
Grant Edwards                   grante             Yow!  Two with FLUFFO,
                                  at               hold th' BEETS...side of
                               visi.com            SOYETTES!
  • Next message: Grant Edwards: "Re: Suggestions for custom application-layer protocol?"

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