Transport Layer

Natawut Nupairoj, Ph.D.

Department of Computer Engineering

Chulalongkorn University

Outline

Overview. Transport Layer Functions. UDP and TCP protocols.

Overview

Core of OSI and TCP/IP protocols. Interface between lower-level and upper-level

protocols (even application layers). Provide network-independent interface

Application-to-application delivery.

Transport Layer

Network-Independent Interface

Example: TCP Layer

Transport Layer Functions

Application-to-application delivery. Addressing

For not just end-to-end, but for application-to-application.

Service access point. Some transports: reliable layer over

unreliable network layer.

Service Access Point: Port

Well-Known Port Addresses

Port 80 – HTTP (web server). Port 25 – SMTP (mail server). Port 23 – Telnet. Port 22 – SSH. Port 110 – POP3.

Reliable Delivery

Error control Corrupted packets.

Sequence control Delay packets. Out-of-order delivery. Transmission speed mismatches.

Loss control Lost packets.

Duplicate control Duplicate packets.

Providing Reliability

Assume unreliable network layer. Use flow control – variable-size sliding

window protocol Acknowledgement. Window size is dynamic and adjustable in real-

time.

UDP: User Datagram Protocol

Connectionless transport protocol. Simple and efficient:

no connection setup. no flow control.

Unreliable. Discard when error occurs.

UDP Datagram Format

TCP: Transmission Control Protocol

Features Connection-oriented. Reliable. Full duplex. Assume unreliable network protocol. Flow control and congestion control. Three-way handshake for connection establishment. Application sends/receives data as “stream”.

TCP Segment Format

TCP Flow Control

Variable-size sliding window protocol: Sequence number: the first byte being sent. Acknowledgement: the next byte expected. Window size: control the transmission speed.

0 = receive all, but want to more data.

Out of order segments ? Depending on the receiver: Kept or discard.

TCP Sliding Window Protocol

TCP Sliding Window Protocol

TCP Congestion Control

Congestion - 2 causes: fast network, low-capacity receiver. slow network, high-capacity receiver.

Using window size. Sender maintains 2 windows:

receiving window. congestion window. The effective window is the minimum of the two w

indow.

TCP Congestion Window

After connection is established: congestion window size = one maximum segment

s (negotiate during connection). 64K threshold.

Sender sends one maximum segment. If the ack comes back before timeout, double the congestion window size.

TCP Congestion Window

Keep sending until: reach the threshold: increase the window size by

one. reach the receiving window size: no more increasi

ng. timeout: set threshold = half of the current conges

tion window size and start sending from one maximum segment again.