TCP Three Way Handshake

layer4protocol-theorytcptransport-protocol

I have the following query about the tcp handshake.

To establish a connection the server must send a packet with SYN flag to 1
Then the server replies with a SYN,ACK flag to 1 , and afterwards the host (client) replies with ACK flag to 1 and SYN 0.

But to terminate a connection does the host send first termination or the server? Also if its the server is does the server send ACK , FIN to 1 and host replies with ACK 1 and FIN 0?

Best Answer

TCP does not have clients or servers, that is an application concept. TCP creates peer connections. From the application perspective, either the client or the server could request a connection with the other party, depending on how the application is architected. A server application provides a service to a client application. TCP neither knows nor cares which is a server or client.

In such a peering arrangement, either party can request disconnection. That really depends on how the application or service works. Unfortunately, questions about applications and protocols above OSI layer-4 are off-topic here.

For more information about how TCP operates, you can refer to RFC 793, Transmission Control Protocol, and there is Section 2.7. Connection Establishment and Clearing.

Related Solutions

Tcp – How to understand the sequence number of segments in TCP termination process in TCP/IP Illustrated, Volume 1: The Protocols (2nd Edition)

It is clearly spelled out in RFC 793 Transmission Control Protocol, Section 3.5 Closing a Connection:

      TCP A                                                TCP B

  1.  ESTABLISHED                                          ESTABLISHED

  2.  (Close)
      FIN-WAIT-1  --> <SEQ=100><ACK=300><CTL=FIN,ACK>  --> CLOSE-WAIT

  3.  FIN-WAIT-2  <-- <SEQ=300><ACK=101><CTL=ACK>      <-- CLOSE-WAIT

  4.                                                       (Close)
      TIME-WAIT   <-- <SEQ=300><ACK=101><CTL=FIN,ACK>  <-- LAST-ACK

  5.  TIME-WAIT   --> <SEQ=101><ACK=301><CTL=ACK>      --> CLOSED

  6.  (2 MSL)
      CLOSED

                         Normal Close Sequence

                               Figure 13.

The RFC is the definition of TCP, and textbooks may have errors (it happens all the time). It is a good idea to also study the RFCs of any protocols which you are learning.

Tcp – Calculating 3 way TCP Handshake Duration

It's called 3-way handshake, so it is transmitted three times: SYN -> SYN/ACK -> ACK. The minimum time required is ~~two~~ 1.5 times the round-trip time (RTT).

(Each side sees a 1x RTT delay for the handshake to happen while the server is one transmission delay / .5 RTT behind the client - assuming equal transmission delay for both directions.)

In your capture, the only transmission time is between SYN and SYN/ACK. The successive ACK is the reaction to the received SYN/ACK. The socket is established on this side and the local node fires away with GET.

On the client side, the sequence is

SYN sent
longer delay (RTT + remote stack overhead) - 0.020003
SYN/ACK received
very small delay (local stack overhead) - 0.000077
final ACK is sent, socket is open
very small delay (local stack & application overhead) - 0.000223
GET sent

On the listener (server) side, this would look like

SYN received
very small delay (local stack overhead)
SYN/ACK sent
longer delay (RTT + remote stack overhead)
final ACK is received, socket is open
very small delay (remote stack & application overhead)
GET is received

Best Answer

Related Solutions

Tcp – How to understand the sequence number of segments in TCP termination process in TCP/IP Illustrated, Volume 1: The Protocols (2nd Edition)

Tcp – Calculating 3 way TCP Handshake Duration

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