UHACC Presentation: The TCP Protocol
                       (Presentation discussion points)
                        by Chuck Geigner, 13 Oct 2005


## The TCP protocol is written up in RFC's 793 
(http://www.rfc-editor.org/rfc/rfc792.txt)and 1122
It defines the rules for sturdy, synchronous data communications between two 
hosts (on top of IP). 
TCP 
  + guarantees packets are reassembled in the correct order at the terminus 
  + detects and retransmits lost packets
  + performs error checking on packets

## Sockets (sys/socket.h)
  outgoing: use connect()
  incoming: bind() to a local address and port #, and
            listen() for incoming connection, and
	    accept() incoming connection requests
  2 types:
    stream   (SOCK_STREAM, TCP)
    datagram (SOCK_DGRAM, UDP - doesn't connect() topic for another day :))

  C sockets programming HOWTO
  http://beej.us/guide/bgnet/output/htmlsingle/bgnet.html

## TCP flags and how the connection is established/torn down:

Handshake:
  SYN - request for new connection, initial sequence number, maximum segment 
        size in bytes and receive buffer size, also in bytes
  SYN/ACK - acknowledgement + suggested TCP sequence number
  ACK - TCP sequence number(+1) acknowledgement (acknowledge data)


Data transfer:
  PSH - push data
  URG - urgent datagram (see below)

Terminate:
  FIN - final, close connection
  RST - reset connection

##Features:

+ Time to Live (TTL): There is a TTL field in each packet which is 
  decremented at each hop until the packet reaches its destination or the 
  count reaches zero, whichever occurs first. 
  Once the TTL reaches zero, it will notify the source host via the Time 
  Exceeded Message (0 - TTL exceeded in transit; 1 - fragment reassembly 
  time exceeded). 

+ Header Checksum : The checksum is used to determine the integrity of the 
  sent packet. 
  According to the RFC, the method is: "The 16 bit one's complement of the 
  one's complement sum of all 16 bit words in the header.  For computing 
  the checksum, the checksum field should be zero."

+ Parameter Problem Message: If a gateway or destination host detects a 
  problem with a parameter in a datagram header that causes it to reject 
  it, it can return a pointer to the errored header portion.

+ Source Quench: Source quench requests will be sent from gateways that 
  run out of buffer space and discard packtes before they can send them on 
  to the next hop. 
  The origin will reduce the outgoing packet rate until it receives no more 
  source quench messages.
  Once the origin ceases to receive source quench messages, it will then 
  automatically increase the rate gradually until it begins to receive them 
  again.
  This mechanism allows a live transmission to automatically adapt to 
  changing network conditions and achieve maximum transfer rate between 
  endpoints.


+ TCP supports urgent data. Flag for this is MSG_OOB
  Urgent data is used to signal the receiver that some important message is 
  part of the data stream and that it should be processed as soon as 
  possible.

EOF