networking - etsn01 advanced telecommunications

NetworkingETSN01 Advanced Telecommunications

Emma Fitzgerald2016

Introduction Networking stack Physical layer Data link layer Network layer Transport layer Application layer

Introduction

Networking stack

Physical layer

Data link layer

Network layer

Transport layer

Application layer

Lund University Slide 2 of 57


An example: Booking a hotel

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HBP: Hotel Booking Protocol

What data needs to be exchanged to book a hotel room?



What language should we use to convey the HBP data?



How can we get the HBP data to the hotel in Sydney?



What would happen if we wanted to book a plane ticketinstead? Or used a different language? Or sent an email instead ofcalling?

Would this affect the other layers?



Introduction

Networking stack

Physical layer

Data link layer

Network layer

Transport layer

Application layer



Networking is like a cake...

Image: sweets.seriouseats.com



The networking stack

Just like our hotel booking example, communicating over anetwork uses independent layers



The networking stack

Not all layers are present in all parts of the network.



Introduction

Networking stack

Physical layer

Data link layer

Network layer

Transport layer

Application layer



The physical layer

The physical layer consists of the physical connection between twodevices and the modulation and coding to represent data as asignal on that connection.

Examples: Radio, optic fibre, twisted-pair cable

Images: Wikimedia commons



Introduction

Networking stack

Physical layer

Data link layer

Network layer

Transport layer

Application layer



The data link layer

The data link layer is responsible for getting data between twodevices that have a physical layer connection between them. It isalso known as the Medium Access Control (MAC) layer.

It controls when each device should use the link, and may alsoinclude error correction and retransmission.

Examples: Ethernet, Point to Point Protocol (PPP), 802.11(WiFi), Bluetooth




Introduction

Networking stack

Physical layer

Data link layer

Network layer

Transport layer

Application layer



The network layer

The network layer is responsible for end-to-end data delivery, thatis, between two devices (hosts) anywhere on the network.

It is responsible for routing and forwarding.

Examples: Internet Protocol (IP), Internet Control MessageProtocol (ICMP), routing protocols




Routing and Forwarding

Routing is the process of selecting the best end-to-end paththrough a network.

Forwarding is the process of selecting the next hop for a packet.



Circuit Switching vs. Packet Switching

In circuit switching, dedicated resources are allocated end-to-endfor a particular flow of data. No other flows may use thoseresources.

In packet switching, no resources are allocated to a flow, and eachflow’s data is broken up into discrete packets. Each packet isrouted and forwarded independently.



Circuit Switching

Image: computernetworkingsimplified.com



Packet Switching

Image: computernetworkingsimplified.com



Dijkstra’s Algorithm

• Algorithm to find the shortest path through a graph• Dijkstra invented it in about 20 minutes without the aid of

pen or paper to demonstrate the capabilities of the ARMACcomputer

• Traverses through a graph, starting with an initial node, anditeratively updates estimates of the shortest paths to all othernodes

• Can either traverse the entire graph, or stop when we reachsome target node

Image: Wikimedia commons



Dijkstra’s Algorithm

1. Assign tentative distance estimate to each node: 0 for theinitial node, ∞ for all other nodes

2. Mark all nodes except the initial node as unvisited. Set initialnode as current node.

3. For the current node, iterate through its unvisited neighboursand calculate their distance. Compare to the current distancevalue and take whichever is smaller as the new value.

4. Mark the current node as visited.

5. If we have reached the target node or there are no moreunvisited nodes with distance <∞, stop. Otherwise, set theunvisited node with the smallest distance as the new currentnode and go back to step 3.



The Bellman-Ford Algorithm

• In Dijkstra’s algorithm, edge weights must be non-negative.

• The Bellman-Ford algorithm is also used to compute theshortest path.

• Slower than Dijkstra’s but works even with negative weightsand in cases where we do not have complete information atthe start.

• Dijkstra’s algorithm starts at the source and builds up theshortest path step-by-step. In the Bellman-Ford algorithm,first we find all shortest paths with one edge, then two edges,and so on until we have covered the whole graph.



The Bellman-Ford Algorithm

1. Set the distance for the source node to 0 and for all othernodes to ∞. Set the predecessor of all nodes to null.

2. Iterate through each edge (u, v). If the distance to u, plus theedge weight, is less than the current distance to v, we havediscovered a shorter path to v. Set the distance to v to thisnew value and the predecessor of v to u.

3. Repeat step 2 N − 1 times, where N is the number of nodesin the graph. (N − 1 is the maximum length of a non-cyclicpath.)

4. For each edge (u, v) in the graph, if the distance to u plus theedge weight is less than the distance to v, the graph containsa negative-weight cycle. Terminate.

5. Otherwise, we have found the shortest paths to each nodefrom the source.



Static and Dynamic Routing

• Static routing: manually configured tables• Dynamic routing

• Distance vector routing• Link state routing

• Each node has a routing table that contains the cost to eachdestination and the next hop for each.

Image: technet.microsoft.com



Distance Vector Routing

• Uses the Bellman-Ford algorithm

• Nodes begin only with knowledge of their immediateneighbours and the costs to reach them.

• Nodes then send this information (the routing table)to theirneighbours.

• If a neighbour sends us a route that is shorter than one wealready have, update our table to reflect this.

• After updating, send the new table to our neighbours.

• If a node goes down, discard any lines in the routing tablethat have it as the next hop and follow the above to find anew route.



Link State Routing

• Uses Dijkstra’s algorithm

• Each node floods the network with the list of nodes it canconnect to and the costs to them.

• Every node builds up a picture of the entire network, then canuse Dijkstra’s algorithm to determine the shortest path toeach destination.

• The routing table is then constructed based on the computedshortest paths.



Path Vector Routing

• Both distance vector and link state routing are only usedwithin a single autonomous system (typically a network run bya single provider).

• These protocols do not scale well to large networks such asthe internet.

• Between autonomous systems, we use a variant of distancevector routing called path vector routing. Each autonomoussystem has a speaker node.

• Only speaker nodes communicate across the AS boundary,and exchange information about which destinations they canreach and the paths to them.



IP Addresses

On the internet, routing destinations are described by InternetProtocol addresses.

32 bits for IPv4, 128 bits for IPv6(e.g. 2001:db8:0:1234:0:567:8:1)

Image: www.hotspotshield.com



CIDR

• IP addresses are hierarchical. Each address belongs to ssubnet.

• Originally, the subnet part of the address had a fixed lengthdepending on the address class.

• With the introduction of classless inter-domain routing(CIDR), the subnet could be any length.

• Subnet addresses are written with a suffix indicating thenumber of bits in the subnet, e.g. 192.168.2.0/24 (IPv4) or2001:db8::/32 (IPv6).

• Destinations in a router’s routing and forwarding tables maybe full IP addresses or subnets.

• A destination IP address will then be matched to the mostspecific destination in the table when making forwardingdecisions.



Introduction

Networking stack

Physical layer

Data link layer

Network layer

Transport layer

Application layer



The transport layer

The transport layer exists only in the end hosts and is responsiblefor connection-oriented communication, multiplexing different dataflows, reliable data delivery, flow control, and congestion control.

Not all transport layer protocols provide all of these functions.

Examples: Transmission Control Protocol (TCP), User DatagramProtocol (UDP)



Connection-oriented vs. connectionless communication

Connection-oriented communication means that a connection isestablished between the hosts before any data is transferred.

The connection must be maintained throughout thecommunication.

In connectionless communication, hosts can send data anytimewithout a connection.



Connection-oriented vs. connectionless communication

Is the postal service connection-oriented or connectionless?

What about telephones?

What other examples can you think of?



Multiplexing

A single host may have many different flows of data. These maybe from different applications or different instances of the sameapplication.

In both TCP and UDP, flows are given unique port numbers.

Some of these are standard for particular applications, e.g. port 80for HTTP (web), port 25 for SMTP (email)

The transport protocol uses the port number to deliver data to thecorrect application.


www.tcpipguide.com


Reliable data delivery

Data can easily get lost along the way between end hosts. Thiscan happen at any layer.

What are some ways data might get lost at the physical layer?

At the data link layer?

At the network layer?

How can we get reliable data delivery over an unreliablechannel?



Flow control

A receiving host may have limited buffer space for incomingmessages and takes time to process each message.

Flow control refers to signalling between the sender and receiver toensure the sender does not send data faster than the receiver canprocess it.



Congestion control

Congestion in a network occurs when there is too much data beingsent and the network is unable to deliver it all.

This can result in data loss or long delays.

Congestion control refers to mechanisms for detecting andreducing congestion.

What are some ways end hosts could detect or reducecongestion?



UDP

UDP is the simplest transport protocol. It performs multiplexingand error checking, but nothing else.

When a host wants to send data to another host, it just sends it. Ifthe data gets lost, too bad.



TCP

TCP is connection-oriented, and includes all the bells and whistles:

• Multiplexing

• Reliable data delivery

• Error detection

• Ordered data delivery

• Flow control

• Congestion control



TCP connections

TCP uses a three-way handshake to establish a connection.

Because this handshake is asymmetrical, there is a differencebetween servers and clients.

A server must have a port open, listening for clientconnections




TCP connections

A four-way handshake is used to close a connection. Each host canclose its side of the connection independently.

Image: Wikimedia commonsLund University Slide 45 of 57


Data delivery

TCP uses sequence numbers to make sure data is complete and inorder when it is delivered.

It also includes error detection, and segments with errors areretransmitted.



Flow control

Flow control in TCP uses a sliding window mechanism.


Image: slidingwindowmorda.blogspot.se


Congestion control

In TCP, lost data is considered a sign of congestion and the sendershould reduce its rate.

The sender has a congestion window, which refers to the maximumnumber of unacknowledged segments that may be in transit at atime.



Congestion control

A TCP connection begins in slow start, where the congestionwindow is doubled every round trip.

When a threshold is reached, it changes to congestion avoidance,where the congestion window increases by 1 maximum segmentsize each time.

When packet loss is detected, the congestion window is halved.


Image: cnp3book.info.ucl.ac.be

Image: www.isi.edu


Introduction

Networking stack

Physical layer

Data link layer

Network layer

Transport layer

Application layer



The application layer

The application layer is where data is actually generated andconsumed. It is the layer that interfaces with the user. It includesthe World Wide Web, email, instant messaging, video streaming,and any other application that communicates over the Internet.

Examples: Hypertext Transfer Protocol (HTTP), Simple MailTransfer Protocol (SMTP), Extensible Messaging and PresenceProtocol (XMPP), Skype

Images: Wikimedia commons, texascatny.comLund University Slide 55 of 57


Headers and Footers

Each layer of the networking stack add a header and possibly afooter.

The process of adding headers and footers is called encapsulationand removing them again at the other end is called decapsulation.

Information in headers and footers includes source and destinationaddresses, checksums, packet size, and protocol identifiers.

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networking - etsn01 advanced telecommunications

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