6 Transport Layer Computer Networks Tutun Juhana Telecommunication Engineering School of Electrical Engineering & Informatics Institut Teknologi Bandung Computer Networks

Transport Layer Services

Process-to-Process Communication

Addressing: Port Numbers To define the processes, we need identifiers called port numbers In the TCP/IP protocol suite, the port numbers are integers between 0 and 65,535

In client-server paradigm: Client program defines itself ephemeral port number  Short lived  recommended to be greater than 1,023 Server process must use well-known port numbers

ICANN Ranges

Socket Addresses Socket address is the combination of an IP address and a port number

Encapsulation and Decapsulation

Multiplexing and Demultiplexing

Flow Control

Although flow control can be implemented in several ways, one of the solutions is normally to use two buffers. One at the sending transport layer and the other at the receiving transport layer

Error Control Detect and discard corrupted packets. Keep track of lost and discarded packets and resend them. Recognize duplicate packets and discard them. Buffer out-of-order packets until the missing packets arrive.

Sequence Numbers

We can use both positive and negative signals as error control Acknowledgment We can use both positive and negative signals as error control The receiver side can send an acknowledgement (ACK) for each or a collection of packets that have arrived safe and sound can simply discard the corrupted packets The sender can detect lost packets if it uses a timer When a packet is sent, the sender starts a timer; when the timer expires, if an ACK does not arrive before the timer expires, the sender resends the packet. Duplicate packets can be silently discarded by the receiver. Out-of order packets can be either discarded (to be treated as lost packets by the sender), or stored until the missing ones arrives.

Combination of Flow and Error Control

Congestion Control Congestion control refers to the mechanisms and techniques to control the congestion and keep the load below the capacity

Open-Loop Congestion Control Retransmission Policy Window Policy Acknowledgment Policy

Closed-Loop Congestion Control Closed-loop congestion control mechanisms try to alleviate congestion after it happens

Connectionless and Connection-Oriented Services

TRANSPORT-LAYER PROTOCOLS

Simple Protocol

Stop-and-Wait Protocol

Efficiency The Stop-and-Wait protocol is very inefficient if our channel is thick and long By thick, we mean that our channel has a large bandwidth (high data rate) by long, we mean the round-trip delay is long The product of the above two is called the bandwidth-delay product We can think of the channel as a pipe. The bandwidth-delay product then is the volume of the pipe in bits The pipe is always there. If we do not use it, we are inefficient. The bandwidth-delay product is a measure of the number of bits a sender can transmit through the system while waiting for an acknowledgment from the receiver.

Pipelining In networking and in other areas, a task is often begun before the previous task has ended  This is known as pipelining There is no pipelining in the Stop-and-Wait protocol because a sender must wait for a packet to reach the destination and be acknowledged before the next packet can be sent

Go-Back-N Protocol

Timers Resending packets Although there can be a timer for each packet that is sent, in our protocol we use only one. The reason is that the timer for the first outstanding packet always expires first. We resend all outstanding packets when this timer expires. Resending packets When the timer expires, the sender resends all outstanding packets

Selective-Repeat Protocol

Bidirectional Protocols: Piggybacking