unmatched response in dccm.logs

Vernon Schryver vjs@calcite.rhyolite.com
Sat May 29 18:36:13 UTC 2004

> From: jim@carroll.com

> I've been studying our DCC installation, and have a question regarding
> client operation.  We're running dcc-dccd-1.2.39.  On our client machines,
> we run dccm with -d.  I've discovered this message in our syslog files.
> May 29 11:21:01 mrinse2 dccm[88790]: unmatched response from
>,6277 h=0x5f72/0x5f72 p=0x15ad6/0x15ad6 r=0x511e07/0x511e94 t=0x3

> It appears the op_nums.r do not match between the request and the response.
> I'm not quite clear on what this means.  Can anyone shed some light on what
> this might mean?

The DCC client-server protocol uses UDP/IP with retransmissions that
use exponential backoff starting from a timeout related to current
average round trip time (RTT).  Each request and matching response
include unique numbers that allow the server to recognize retransmissions
and the client to match responses with requests.  The client code does
not create a new socket for each operation, but reuses a socket.

Those messages happen only with -d because they are not strictly errors
but happen with scenarios like this.

  - the client sends a request for mail message #1
  - the client waits for a while and retransmits the request
  - the response to the first request arrives and is processed
  - the client sends a request for mail message #2
  - the response to the retransmission for message #1 arrives at the socket
  - the client notices that the transaction ID and other values
     do not match.  If debugging is turned on, the client code
     logs an "unmatched response" message.

Vernon Schryver    vjs@rhyolite.com

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