Constants
DISCONNECT = 1
  Transport layer generic messages
IGNORE = 2
UNIMPLEMENTED = 3
DEBUG = 4
SERVICE_REQUEST = 5
SERVICE_ACCEPT = 6
KEXINIT = 20
  Algorithm negotiation messages
NEWKEYS = 21
KEXDH_INIT = 30
  Key exchange method specific messages
KEXDH_REPLY = 31