Cisco Systems CCNA 2 manual TCP Operation, TCP operation, Synchronization or three-way handshake

10.1 TCP Operation

Essential Labs:	10.1.6
Optional Labs:	None
Core TIs:	10.1.6
Optional TIs:	10.1.1 – 10.1.5, and 10.1.7

Course-Level Claim: Students can describe the operation of the major transport layer protocols and the interaction and transportation of application layer data.

Certification-Level Claim: Students can evaluate the TCP/IP communication process and its associated protocols.

Hands-on skills: none

10.1.1 TCP operation

IP addresses allow for the routing of packets within and between networks. However, it makes no guarantee about delivery. The transport layer is responsible for the reliability of data flow. This is accomplished through the use of sliding windows and sequencing numbers along with a synchronization process that ensures communication. Have the students come up with an analogy. One excellent example is a student who studies a foreign language for one year, visits the country where the language is used, and asks everyone to repeat their words for reliability and speak slowly for flow control.

10.1.2 Synchronization or three-way handshake

TCP is connection-oriented. Prior to data transmission two hosts go through a synchronization process to establish a virtual connection. This process ensures that both sides are ready for data and allows for the devices to determine the initial sequence numbers. This process is a three-way handshake. Sequence numbers (SYN) and the role they play will be discussed in detail in a later section. At this point it is important for students to understand that sequence numbers are used to initiate communication between two devices. The sequence numbers give each host a way to acknowledge the SYN bits so that the receiver knows the sender responds to the proper connection request. This is done with bits in the TCP header. These bits are called flags. The two flags involved are Sequence numbers (SYN) and Acknowledge numbers (ACK). These flags are used to synchronize the SYN and ACK numbers between the hosts. This will initialize the new session.

The three-way handshake is a three-step process that establishes the virtual connection between two devices:

1.The source host initiates a connection. The host sends a packet with the SYN bit set and indicates an initial sequence number of x with a bit in the header set to indicate a connection request.

2.The destination host receives the packet, records the sequence number of x, replies with an acknowledgment of x + 1, and includes its own initial sequence number of y. It also sets the SYN bit to indicate the start of the return conversation.

112 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 10 Copyright ♥ 2004, Cisco Systems, Inc.