3rd Edition: Chapter 3

3rd Edition: Chapter 3

Chapter 5 TCP Control Flow Networking CS 3470, Section 1 TCP Control Flow TCP connections include

3-way handshake to start connection Transfer of data with ACKs Connection tear-down SequenceNum field contains the sequence number for the first byte of data carried in segment

2 TCP Connection Establishment Both the client and server have to open a connection Server must start first to be able to accept incoming connections

Performs a passive open Client can then connect to the server Performs an active open 3

TCP Connection Establishment Both parties (client and server) must agree on the starting sequence numbers Each side has its own set of sequence numbers

that start at some random number Sequence numbers are exchanged during the TCP three-way handshake algorithm, which establishes a TCP connection between two hosts 4 TCP Three-Way Handshake Algorithm

Acknowledgment field identifies the next sequence number expected, implicitly acknowledging all earlier sequence numbers TCP State-Transition Diagram Some arcs are not shown

Most of states that involve sending segments also schedule a timeout for the ACK response If ACK not received, retransmit If after several tries the ACK does not arrive, TCP gives up and returns to CLOSED state 6

7 TCP State-Transition Diagram When closing connection Connection in TIME_WAIT state cannot move to

CLOSED until it has waited 120 seconds Prevents delayed FINs from accidentally tearing down new connections 8 Introduction to Project 4 Model TCP state-transition diagram in your

own program Will help you make sense of network traffic for last project 9 Closing a Connection Reaching agreement: two approaches

Abort: send close msg to peer, delete state info Graceful: send close msg, but before deleting state Wait for peer to acknowledge close()

Problem solved? Can I decide to close, knowing that What if close() message lost? Other entity also agreed to close and knows that I will close

Can two armies coordinate their attacks If communication is unreliable? 10 The Byzantine Generals Problem Two generals are on the tops of two mountain.

The Byzantine Generals Problem They communicate only through messengers (pigeon network?) The Byzantine Generals Problem

They need to coordinate the attack The Byzantine Generals Problem If they attack at the same time, they win The Byzantine Generals Problem

If they attack at different times, they will die The Byzantine Generals Problem Question: can they guarantee a synchronized attack? The Byzantine Generals Problem

11 am OK? The Byzantine Generals Problem Sounds good. The Byzantine Generals Problem I got your Sounds

good. The Byzantine Generals Problem What if the last message is lost? great! The Byzantine Generals Problem

What if the last message is lost? Does the other general know I got his Sounds good message? great! The Byzantine Generals Problem

Over an unreliable network, we cannot guarantee network synchronization Rats. The Byzantine Generals Problem

It can be proven that no protocol exists that works to solve this problem. Three-way handshake will not work, so does not four-way handshake. What do we do? The Byzantine Generals Problem

It can be proven that no protocol exists that works to solve this problem. Three-way handshake will not work, so does not four-way handshake. What do we do?

Three- or four-way handshake is normally used to release connections Then give up Connection Release 6-14, a, b Four protocol scenarios for releasing a connection. (a) Normal case of a three-way handshake. (b) final ACK lost.

25 Connection Release 6-14, c,d (c) Response lost. (d) Response lost and subsequent DRs lost. 26

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