[This message was posted by Erik Scholander of  <[email protected]> to the 
"General Q/A" discussion forum at http://fixprotocol.org/discuss/22. You can 
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A question pertaining to a similar area (in FIX 5.0 SP1). What is the official 
way for the acceptor to respond to a malformed Logon request? (with no reset 
flag set or anything like that).

A malformed Logon can be because of 2 reasons:

1. Logon has too low sequence number.
2. Logon has too high NextExpectedSequenceNumber. (requesting future messages)

According to the FIX 4.4 vol 2 p 33 the response to a Logon with a too low 
sequence number is to send a Logout. According to the more recent fixt spec a 
message with incorrect sequence number (except Sequence Reset - reset) should 
never be acted upon.

So what to do? as I see it there are 4 possible actions:

1. disconnect. Not very helpful to the initiator (how do distinguish this from 
a validation error for example).
2. send a Logout explaining reasons (is this allowed according to FIX? And 
there is also a problem with that the Logon reply sequence number will probably 
not be in accordance with the initiators next expected sequence number so it 
will probably not be acted upon anyways).
3. send a Reject, same problems as with sending a Logout.
4. send a FAST Alert. Not very nice to mix these layers up, and this message 
will probably be eaten by the FAST tunnel at many vendors any ways.

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