TCP Connection establishment

The "three-way handshake" is the procedure used to establish a connection. This procedure normally is initiated by one TCP and responded to by another TCP. The procedure also works if two TCP simultaneously initiate the procedure. When simultaneous attempt occurs, each TCP receives a "SYN" segment which carries no acknowledgment after it has sent a "SYN". Of course, the arrival of an old duplicate "SYN" segment can potentially make it appear, to the recipient, that a simultaneous connection initiation is in progress. Proper use of "reset" segments can disambiguate these cases.

The   three-way   handshake    reduces   the   possibility   of   false    connections.   It   is   the implementation of a trade-off between memory and messages to provide information for this checking.

The  simplest  three-way  handshake  is  shown  in  figure  below.  The  figures  should  be interpreted in the following way. Each line is numbered for reference purposes. Right arrows (-->) indicate departure of a TCP segment from TCP A to TCP B, or arrival of a segment at B from A. Left arrows (<--), indicate the reverse. Ellipsis (...) indicates a segment which is still in the network (delayed). TCP states represent the state AFTER the departure or arrival of the segment (whose contents are shown in the center of each line). Segment contents are shown in abbreviated form, with sequence number, control flags, and ACK field. Other fields such as window, addresses, lengths, and text have been left out in the interest of clarity.

  TCP A                                                      TCP B

1.  CLOSED                                               LISTEN

2.  SYN-SENT    --> <SEQ=100><CTL=SYN>               --> SYN-RECEIVED

3.  ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN- RECEIVED

4.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK>      --> ESTABLISHED

5.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED

Basic three-Way Handshake for Connection Synchronisation

In line two of above diag. TCP A begins by sending a SYN segment indicating that it will use sequence numbers opening with sequence number hundred. In line three, TCP B sends a SYN and acknowledges  the  SYN  it  getting  from  TCP  A.  Here we have to note that  the  acknowledgment  field indicates TCP B is now expecting to hear sequence one hundred one, acknowledging the SYN which occupied sequence hundred.

At line four TCP A reply with an drain segment containing an ACK for TCP B's SYN; and in line five, TCP A sends some data. Note that the sequence number of the section in line five is the same as in line four since the ACK does not occupy sequence number space (if it did, we would wind up ACKing ACK's!).

concurrent initiation is only to some extent more difficult, as is shown in diag. below. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to ESTABLISHED.

TCP(transmission control protocol) A                                                          TCP B

  1.  CLOSED                                                                                        LISTEN

  2.  SYN-SENT    --> <SEQ=100><CTL=SYN>                                      -->   SYN RECEIVED

  3.  ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK>  «--     SYN RECEIVED

  4.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK>           -->   ESTABLISHED

  5.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED

Fundamental three-Way Handshake for Connection Synchronization

 Problem regarding two-way handshake

The one and only real problem with a two-way handshake is that duplicate packets(message) from a previous connection( which has been closed) among the 2 nodes might still be floating on the network. After a SYN has been sent to the responder, it may receive a carbon copy packet of a previous connection and it would regard it as a packet from the present connection which would be unwanted. Once more spoofing is another issue of concern if a 2 way handshake is used.let there is a node C which sends connection request to B saying that it is A.Now B sends an ACK to A which it discards & asks B to close connection. Among these 2 events C can send a lot of packets(message) which will be delievered to the application..

The 1st 2 figures show how a 3 way handshake deals with problems of duplicate or delayed connection requests and duplicate or delayed connection acknowledgements in the network.The three diag. highlights the problem of spoofing associated with a 2 way handshake.

Some Conventions

1. The ACK contains 'x+1' if the sequence number received is 'x'.

2. If 'ISN' is the sequence number of the connection packet then first data packet has the seq number 'ISN+1'

3. Seq no. are thirty two bit.They are byte seq number(each byte has a seq no.).Through a packet first seq number and length of the packet is sent.

4. Acknowledgements are cumulative.

5. Acknowledgements have a seq number of their own but with a length zero. So the next data packet have the sequence number same as ACK.

Connection Establishment

• The sender sends a SYN packet with sequence number say 'x'.
• The receiver on getting SYN packet responds with SYN packet with sequence no. 'y' and ACK with seq number 'x+1'

• On receiving both SYN and ACK packet, the sender replys with ACK packet with seq number 'y+1'
• The receiver when gets ACK packet, initiates the link.

Connection Release

• The initiator sends a FIN with the current seq and acknowledgement no.
• The responder on getting this informs the application program that it will receive no more data and sends an acknowledgement of the packet. The connection is now closed from 1 side.

• Now the responder will follow analogous steps to close the connection from its side.

Once this is done the connection will be completely closed.

TCP connection is a duplex connection. That means there is no difference among 2 sides once the connection is established. 

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