TDS protocol

- simple request-reply protocol

- request and reply blocks are divided into fragments of length,
agreed between the client and server

- initial fragment size is 512 bytes

- fragment structure:

       H   E   A   D   E   R          |    D A T A
  +-------+--------+----------+-------+---------------------+
  | flag1 | flag 2 | fragment | flag3 | data                |
  |       |        | length   |       |                     |
  +-------+--------+----------+-------+---------------------+
    1       1        2          4       fragment length - 8

flag1:
  client->server, login packet: 0x02
  client->server, work:         0x01
  server->client:               0x04

flag2:
  last fragment:                0x01
  others:                       0x00

fragment length:
  network byte order, data len + fragment header len

flag3:
  ?
