TCP Three Way Handshake

ESTABLISHING A CONNECTION

The “three way handshake” is essentially a unidirectional attempt to establish the connection, i.e. there is an initiator and a responder.

The TCP’s should however be able to establish the connection even if

a simultaneous attempt is made by both TCP’s to establish the

connection. Simultaneous attempts are treated like “collisions” in

“Aloha” systems and these conflicts are resolved into unidirectional

attempts to establish the connection. This scheme was adopted because

(i) Connections will normally have a passive and an active end,

and so the mechanism should in most cases be as simple as

possible.

(ii) It is easy to implement as special cases do not have to be

accounted for.

The example below indicates what a three way handshake between TCP’s

• A and B looks like
• 
  A                                                      B
  
  –>          <SEQ x><SYN>              –>
  
  
  
  <–    <SEQ y><SYN, ACK x+l>  <–
  
  
  
  –> <SEQ x+1><ACK y+l><DATA BYTES> –>

The receiver of a “SYN” is able to determine whether the “SYN” was

real (and not an old duplicate) when a positive “ACK” is returned for

the receiver’s “SYN,ACK” in response to the “SYN”. The sender of a

“SYN” gets verification on receipt of a “SYN,ACK” whose “ACK” part

references the sequence number proposed in the original “SYN” [pun

intended]. If the TCP is in the state where it is waiting for a

response to its SYN, but gets a SYN instead, then it always thinks

this is a collision then it forgets that it had sent a SYN. The TCP will try to

establish the connection again after some time, unless it has to

respond to an arriving SYN. Even if the wait times in the two TCPs

are the same, the varying delays in network transmission will usually

be adequate to avoid a collision on the next cycle of attempts to

send SYN.

The State of TCP When Establishing a Connection

S1 S2 R are three state of TCP while establishing a connection

S1 = SYN sent

S2 =  My SYN verified

R = SYN received

if S1 Value is set 1 than means Syn has been sent. Same with S2 and R.

The example below indicates State of TCP in three way handshake between TCP’s

                      A                                                                            B

                      ------------                                                           -----------

                      S1 S2 R                                                              S1 S2 R

                      0 0 0                                                                     0 0 0

 (M1)            1 0 0 --> <SEQ x><SYN>                                     …

(M2)            0 0 0 <-- <SEQ y><SYN )                                <-- 1 0 0

(M1)            B returns no SYN sent                                   --> 0 0 0

(M1)           1 0 0 --> <SEQ z><SYN> *                              --> 0 0 1

(M3)           1 1 1 <-- <SEQ y+1><SYN,ACK z+1>             <-- 1 0 1

(M4)           1 1 1 --> <SEQ z+1><ACK y+1><DATA>       --> 1 1 1

Note: "..." means that a message does not arrive, but is delayed

in the network. State changes are upon arrival or upon departure

of a given message.