Transport Layer Natawut Nupairoj, Ph.D. Department of Computer Engineering Chulalongkorn University

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.