Computer Networks Exam 1

What is a protocol?

Defines the format and order of messages exchanged between two or more communicating entities, as well as the actions taken upon the transmission and/or receipt of a message or other event

What does a protocol govern?

all activity in the Internet that involves two or more communicating remote entities

Message Exchange and Actions

Protocols specify the precise sequence of messages sent and received, along with the corresponding actions taken by the communicating entities

What happens if entities run different protocols?

They can’t interoperate, and no useful work can be accomplished

Hardware-implemented protocols control…

the flow of bits on a wire

Congestion-control protocols manage…

packet transmission rates

Router protocols determine…

packet paths

What happens when you request a web page?

Your computer sends a connection request to the web server and waits for a reply. After the server returns a connection reply, your computer sends a “GET” message for the page, and the server then sends the Web page back.

Types of application-layer protocols

HTTP and SMTP

Protocol Stack

when the protocols of various layer are taken together

End Systems (Hosts)

the computing devices connected to the internet

Why are they called “end systems”?

they reside at the edge of the Internet

Why are they called “hosts”?

they run application programs

Categories of end systems

Clients and Servers

End systems are connected by…

a network of communication links and packet switches

What are communication links made of?

Various media, including coaxial cable, copper wire, optical fiber, and radio spectrum

Packet Switches

devices that take incoming packets from one communication link and forward them onto an outgoing link

The internet can be broadly divided into…

The Network Edge and The Network Core

Network Edge

consists of end systems and the access networks that connect them to the internet

Access Networks

physically connect an end system to its first router (“edge router”)

Common types of access networks

Digital Subscriber Line (DSL), Cable Internet Access, Fiber to the Home (FTTH), Ethernet, WiFi (IEEE 802.11), Wide-Area Wireless Access (3G, LTE 4G, 5G)

Network Core

mesh of packet switches and communication links that interconnects the Internet’s end systems

Packets

smaller chunks that messages are broken into

Packet Switching

packets are transmitted independently through the network

How is packet switching different from circuit switching?

No link resources are reserved in advance, and packets may experience delays due to congestion.

ISPs (Internet Service Providers)

what end systems access the internet through

Points of Presence (PoPs)

locations within a provider’s network where customer ISPs can connect

Multi-homing

ISPs may connect to two or more provider ISPs to enhance reliability

Internet Exchange Points (IXPs)

meeting points where multiple ISPs can peer together

The 5 layers of the Protocol Stack

Application, Transport, Network, Link, and Physical layer

Application Layer

where network applications and their protocols reside

Transport Layer

transports application-layer messages between application processes running on different hosts

Two main Internet transport protocols

TCP and UDP

Network Layer

responsible for moving “datagrams” from one host to another

Link Layer

moves “framers” from one network element to an adjacent one over a single communication link

Physical Layer

moves individual bits within a frame across the physical transmission medium

If you wanted to send information to a friend over this network, does this information travel directly to them via their address?

No. The information is broken down into smaller pieces and routed through a series of intermediate devices.

What is an end system, or a host?

An end system, or host, is a computing device connected to the Internet.

Clients

initiate communication sessions by contacting server processes

Servers

generally more powerful machines that store and distribute content like Web pages and videos, and relay e-mail

When a packet of information travels from application layer down to the physical layer, it will accumulate headers. Broadly speaking, in what order are the headers attached/detached from the message when the message passes down from the application layer to the physical layer?

The packet of information undergoes a process called encapsulation

• Application Layer: application creates original message (M)

• Transport Layer: receives the application-layer and adds its own transport-layer header (Ht). Combined unit is called a segment (Ht + M)

• Network Layer: receives segment from transport layer and adds its own network-layer header (Hn) to create a datagram (Hn + Ht + M)

• Link Layer: receives datagram from the network layer and adds its own link-layer header (HI) and often a trailer (Tr) to create a frame (HI + Hn + Ht + M + Tr)

• Physical Layer: the physical layer takes the frame and converts it into individual bits for transmission across the physical medium

Packet sniffer – what is this?

a tool designed for observing the messages exchanged between executing protocol entities

Packet sniffer — broadly speaking what kinds of information can you view with it?

• Sensitive Information: passwords, social security numbers, etc.

• Contents of various protocol fields

• Application-layer data

• HTTP message formats: details from web browser

• HTTP authentication and security

• DNS queries and replies

• UDP header fields and checksum calculations

• TCP segment details

• ICMP and SNMP messages

What are some ways that functionality like this could be used maliciously?

1. Theft of Sensitive Information

2. Reconnaissance for Further Attacks

3. Impersonation

4. Woman-in-the-Middle and Reply Attacks

5. Switching Poisoning in Switched LANs

HTTP (HyperText Transfer Protocol)

defines how Web clients request Web pages from servers and how those servers transfer Web pages back to clients

SMTP (Simple Mail Transfer Protocol)

primary application-layer protocol for Internet electronic mail

What is a socket?

a software interface that serves as the boundary between the application layer and the transport layer within a host (also referred to as the Application Programming Interface (API))

Domain Name System (DNS)

the Internet’s directory service that translates human-friendly hostnames into numerical IP addresses

Why is DNS important in network communication?

It bridges human and machine identification

Without DNS, is it possible to connect to other end systems? What is required?

Yes, it’s possible. To connect to another end system without DNS, the sending process must directly specify the following two pieces of information to identify the receiving process on the destination host.