Design recommendation wanted.

[Yannick Turgeon]

Hello all, 

I'm implementing a protocol to communicate with a server through serial port
(Yes Peter and Alex it's me again!). This means looking for lost packet,
checksum error, connection failure. 

Whitout error, the normal course of things is:
(C = Client - the one I'm programming, S = Server)

Type #1 command:
C: Send a packet to ask something
S: Reply with the final answer
C: Acknowledge reception

Type #2 command:
C: Send a packet to ask something
S: Reply with part of the answer
C: Acknowledge reception of the partial answer
S: Reply with the following part of the answer
C: Acknowledge reception of the partial answer
...
S: Confirm the end of the answer
C: Acknowledge reception

Added to that, the server can send me an error packet due to my last sent
packet being lost or checksum error or packet wrongly construct or
impossibility to answer my "correctly constructed and received question".
It includs in this error whether or not I should resend the last packet.
Even more, there is a fixed maximum number of tentative to resent a packet.

So my question is a general design one: 
Anybody has any hint how to handle in a well structured way that much
possible errors and situation? How?

Say I got 20 commands I can ask to server (which is near the real
situation).

I tried to make a base Command class that has a function that looks like:

def getReply(expected):
  try:
    packet = serial.readline()          #raise TimeoutError()
    packet_type, data = decode(packet)  #raise FormatPacketError()
                                        #      ChecksumError()
                                        #      PacketNumberError()
    if packet_type == ERROR:
        raise ServerReplyError()
    if packet_type <> expected:
        raise UnexpectedPacketError()
    c = PACKETS[packet_type](data)  #PACKETS is a dict containing Classes
                                    #raise DataFormatError()
  expect TimeoutError:
    blabla
  expect FormatPacketError:
    blabla
  expect ChecksumError:
    blabla
  expect PacketNumberError:
    blabla
  expect UnexpectedPacketError:
    blabla
  expect ServerReplyError:
    blabla
  expect DataFormatError:
    blabla


With this solution, I will get lost very soon. I feel that I would need a
kind of State-Managing system/class that would keep track of the command
processing in general. But I'm not grasping it. I'm conscious that by not
knowing the total situation, it's difficult for anybody to help but any
suggestion, recommendation would be much appreciated.

Yannick