TCP level.
  Three message protocols.

TCP - Transmission Control Protocol packet
  Generated at session level (by the application or system calls).
 
  Source and destination ports - used to distinguish which application
  or service is being communicated with on a particular machine.

  Sequence number - offset from beginning of whole payload. Used to 
    rebuild the whole block of data being transfered. 

  A variety of control/condition flags.

  Data being transmitted.

  Check sum - Usually crc. Was packet delivered uncorrupted.

  * Error checking - has all the data for the session or activity been 
    delivered, is it re-assembled in the right order, and there is no
    duplication of packets. 

  * Source and target systems negotiate a "connection" before actual 
    transmission of data begins.

UDP - User Datagram Protocol packet
  A simpler version of a TCP packet - single unit of communication. Often
    used for communicating control information.

  Source and destination ports, Size, checksum and data.

  Classified as stateless. The packet is simply sent. If the target does not 
    respond, send assumes packet lost.

  May be used where time sensitive communication occurs. Better to loose one 
    of number of sequential packets then to delay data stream attempting to 
    recover a lost packet. 

  Used for DNS (domain name service), DHCP (Dynamic Host Configuration Protocol), 
    SNMP (Simple Network Management Protocol), RIP (Routing Information Protocol), etc.
    Communications where request and response are fairly simple and short.

ICMP - Internet Control Message Protocol.
  Used to send errors or confirm existence of other end of connection (ping).   

  Consists of a set of predefined flags and some additional data.
  
  Also classified as stateless.

  Not sent by application, but used by the OS and routers, i.e  processes handling TCP/IP.