Review of networking concepts

1 Polytechnic UniversityM. Veeraraghavan

Review of networking concepts

Prof. Malathi Veeraraghavan

University of Virginia


Outline

• Review of basic concepts in networking– Prerequisite: A first course on networking– Communication links and switches– Types of networks– Shared links: media access control (MAC)


What is a communication network?

• Simplest “network” – Single link between two pieces of end-user

equipment (e.g., PC, telephone)

End-userequipment

End-userequipment

– Types of communication links• Twisted pair

• Coaxial cable

• Optical fiber

• Wireless links– Radio frequencies

– Infra-red frequencies


What is needed to send data on communication links?

• Error control– Error detection:

• Parity checks, Checksum, Cyclic Redundancy Code (CRC)

– Error correction:• ARQ (Automatic Repeat reQuest) • FEC (Forward Error Correction)

• Flow control: handles rate mismatch between sender and receiver– x-ON/x-OFF– Window based flow control– Rate based flow control


Switches

• Connect multiple links and route traffic from one link to another

End-userequipment

End-userequipment

End-userequipment

End-userequipment

Switch


Why use a switch?

• If there are N endpoints (end-user equipment), then how many links are needed for full mesh connectivity?

• How many physical links are needed if these endpoints are connected through a switch?


Answers

• Number of direct links needed to connect N nodes is

• N links – since we only need one link from an endpoint to a switch

2)1( NN


Cost of using a switch?

• Switch cost

• Can all endpoints have full connectivity at all times to all other endpoints?– Yes, with multiplexing on the links


Concept of multiplexing

• Time division multiplexing– Allows data from different sessions to be

combined at different times on to the same line– How many DS0s in a T1?

• Wavelength division multiplexing– Difference between FDM (Frequency Division

Multiplexing) and WDM?– Relation between frequency and wavelength


Answers

• 24 DS0s in a T1

• Term WDM is the same as FDM at optical frequencies – see EM spectrum chart

• Speed of light c = f: wavelength; f: frequency


Transceiver rate

• Rate of transmission and reception at endpoints and the switch– Needs to be sufficient for “full mesh”

connectivity “all the time”– e.g., if DS0s used between endpoints in full

mesh network, then T1s can be used in 25 endpoint network with a switch for full mesh connectivity


Types of switches

• Circuit switches: Position-based switching– Switch consults a table to determine output port on which to send

data bits based on their arriving position• “Position”: Interface (space), time slot and/or wavelength

– Space division switch: switch based on input interface– Time division switching: interface + time slot– Wavelength division switching: interface + wavelength– No buffers

• Packet switches: Label-based switching– Switch consults a table to determine output port on which to send the

packet based on value of label (in packet header)– Label could be changed on outgoing port or could stay the same– Have buffers to hold packets


Switch designs

• See lectures on circuit switching and packet switching in Course on Data Networks

• Compare unfolded view of a CS with that of a PS

• See relevance of queueing theory to delays of calls or packets through switches


Network of switches

• Expand 1-switch network to a multi-switch network

• Why not build one gigantic switch?– Scalability limitations

End-userequipment

End-userequipment

Switch

Switch End-userequipment

Switch


Different types of networks

• A network is defined by its “switching mode” and its “networking mode”

• Circuit switching vs. packet switching– Circuit-switching: switching based on position (space, time, ) of arriving bits

– Packet-switching: switching based on information in packet headers

• Connectionless vs. connection-oriented networking:– CL: Packets routed based on address information in headers

– CO: Connection set up (resources reserved) prior to data transfer

Packet-switching

Circuit-switching

Switching modesConnectionless Connection-oriented

Networking modes

ATM, X.25IP, SS7MPLS

IP + RSVP

Telephone network, SONET/SDH, WDM


Types of data transfers

Sending end

Consuming endLive Stored

Live

Stored

Interactive/Live streaming

Recording

Stored streaming File transfers

An application could consist of different types of data transfers

— An http session has an interactive component, but could also have a non-real-time transfer


Matching applications & networks

Data transfers

Non-real-time(stored at sender and receiver ends)

Real-time(consumed or sent live)

Interactive (two-way)(consumed and sent live)

e.g. telephony, telnet, ftp, http

Streaming (one-way)(consumed live;

sent from live or stored source)e.g. radio/TV broadcasts

Recording (one-way)(stored at receiver end;

sent from live source); e.g. Replay

Short transfers(e.g. short email)

Long transfers(e.g. large image,

audio, video or data)

Ideal networks

Connectionlessnetworks Circuit-switched

networks

Packet-switched CO networks


Congestion control

• What is it?– The purpose of a network is to allow sharing of

resources– This means if demand is high, there could be

competition for resources from multiple users– What are network resources:

• Link capacity (bandwidth)

• Switch buffer space (only in packet switches)


Congestion control

• In CO networks– Congestion control: mostly preventive

– Connection Admission Control (CAC)• Check availability of bandwidth and buffer resources before

admitting a connection

• CS CO networks: congestion will not occur once circuits are admitted

• PS CO networks: congestion can occur after connection is admitted if connection admission is based on statistical multiplexing

– Have some supplemental reactive congestion control scheme


Congestion control

• In CL networks– Have packet switches detect congestion and

send reactive messages asking sender to slow down

– e.g., datagram routers in SS7 networks send such messages; SRP (Spatial Reuse Protocol) switches in 802.17 MANs send such messages

– IP routers implement Explicit Congestion Notification (ECN) procedures


End-to-end path

• Transport protocols– Ensure reliable transfer across a communication

path consisting of many links (“zero” loss)– OR ensure delay-controlled path across a

communication path consisting of many links– Error control and flow control– Delay control (e.g., RTP)– Congestion control and connection control –

special in TCP


Applications

• Most Internet applications are client-server based

Web serverEnd-userequipment

Network

Web clients(Usually runson fixed hosts)

Network Network

End-userequipment

Email-sending clients(outlook, messenger)

Outgoingemail servers(pop, imap)

Network

End-userequipment

Email-receiving clients(outlook, messenger)

Incomingemail servers

(smtp)


Protocol Stacks

• OSI model: two more layers between AL and TL– Session layer and presentation layer

• PHY: Physical; DLL: Data Link Layer; NL: Network Layer; TL: Transport Layer; AL: Application Layer

DLL

NL

TL

AL

PHY

Endpoint Switch

NL

DLL

PHY PHY

DLL DLL

IP

TCP/UDP

AL

PHY

EndpointSwitch

NL

DLL

PHY PHY

DLL


Example protocols

• AL protocols: http, smtp, ftp, PCM voice

• TL protocols: TCP, UDP, RTP, AAL

• NL protocols: IP, ATM

• DLL protocols: PPP, HDLC

• PHY protocols: DS0, DS1

• Ethernet: PHY+DLL+NL


Functions of protocol layers

• PHY: sends bits across a link• DLL: error control and flow control on a

link• NL: switching (routing), multiplexing,

congestion control• TL: error control and flow control on an

end-to-end basis• AL: Functions specific to the application


Congestion control and connection control in TCP

• IP routers did not implement ECN until recently– TCP performs congestion control

– Senses whether network switches (routers) are congested or not

– Adjusts rate accordingly

– Slow start and congestion avoidance

• Concept of a “connection” at the TL– End hosts maintain state information regarding a TCP connection

to track sequence numbers and ACKs

– Connection open (SYN) and close (FIN) procedures

– Contrast with a “connection” at the NL, where each switch maintains state about the connection


User plane, control plane, and management plane

• Management plane: consists of all the protocols needed to “configure” data tables for the operation of the network– For example, protocols for routing data dissemination (distributed or

centralized)– Other functions: performance, fault mgmt., accounting, security

• Control plane: – Connection control protocols

• in CO networks, this includes connection setup at each switch (connections at the network layer)

• in CL networks, this includes connection setup only at the endpoints (connections at the transport layer, if the TL protocol is reliable)

– Call control protocols

• User plane: protocols for the actual flow of data


Routing protocol in all three types of networks - Phase 1

Host AHost B

I

IV V

III

II

Routing protocol

Routing protocol

Routing protocol

Dest. Next hop

III-* IVDest. Next hop

III-* III

Dest. Next hop

B B

Routing tables

• Routing protocols exchange topology/loading/reachability information

• Routes to destinations are precomputed and stored in routing tables


Signaling protocol for NL connection setup in a PS CO network - Phase 2

• Connection setup consists of each switch on the path– Route lookup for next hop node to reach destination

– CAC (Connection Admission Control) for buffer and BW

– Writing the input/output label mapping tables and programming the scheduler

Host AHost B

I

IV V

III

II

Connection setup

Connection setup

a

b

c

a

bc

d

d c

a

b

INPort /Label

OUTPort/Label

a/L1 c/L2IN

Port /LabelOUT

Port/Label

a/L2 c/L1

INPort /Label

OUTPort/Label

d/L1 b/L3Connection setup (B)

Connectionsetup

Virtual circuit


Signaling protocol for NL connection setup in a CS CO network - Phase 2

• Connection setup consists of each switch on the path– Route lookup for next hop node to reach destination– CAC (Connection Admission Control) for BW (note: no buffers)– Writing the port/timeslot/mapping table

Host AHost B

I

IV V

III

II

Connection setup

Connection setup

a

b

c

a

bc

d

d c

a

b

INPort /Timeslot

OUTPort/Timeslot

a/1 c/2IN

Port /TimeslotOUT

Port/Timeslot

a/2 c/2

INPort /Timeslot

OUTPort/Timeslot

d/2 b/1Connection setup (B)

Connectionsetup

Circuit


TL connection setup in a CL PS network - Phase 2

• Notion of transport layer connections– Exchange initial sequence numbers end-to-end to allow for ARQ

(Automatic Repeat reQuest) based error correction, i.e., retransmissions in case of errors

Host AHost B

I

IV V

III

II

Dest. Next hop

B IIDest. Next hop

B III

Dest. Next hop

B B

Routing tables

SYNSYN

ACK


User-plane packet forwarding in a PS CO network - Phase 3

• Labels are VPI/VCIs in ATM• Labels are translated from link-to-link

Host AHost B

I

IV V

III

II

a

b

c

a

bc

d

d c

a

b

L2

L1

L1

L3

INPort /Label

OUTPort/Label

a/L1 c/L2


User-plane actions in a circuit-switched network - Phase 3

• Bits arriving at switch I on time slot 1 on port a are switched to time slot 2 of port c

Host AHost B

I

IV V

III

II

a

b

c

a

bc

d

d c

a

b

1 2

1 2

1 21 2

OUTPort/Timeslot

INPort /Timeslot

a/1 c/2


User-plane packet forwarding in a CL PS network - Phase 3

• Packet headers carry destination host address (unchanged as it passes hop by hop)

• Each CL packet switch does a route lookup to determine the outgoing port/next hop node

Host AHost B

I

IV V

III

II

a

b

c

a

bc

d

d c

a

b

B

B

B

B


Addressing

• Where are endpoint addresses used:– In CL PS networks, endpoint addresses are

carried in packet headers– In CO networks, be it PS or CS, endpoint

addresses are carried in connection setup messages


Summarized addresses

• What are summarized addresses?

• Why summarize addresses?


Summarized addresses

• What are summarized addresses?– An address that represents a group of endpoint

addresses– e.g., all 212 numbers, 128.238 IP addresses

• Why summarize addresses?– Reduces routing table sizes – hold one entry for a

summarized address instead of a large number of individual addresses

– Reduces routing message lengths that convey reachability information


Examples of signaling protocols

• SS7 (Signaling System No. 7) network (with its SS7 protocol stack) carries signaling messages to set up and release circuits in a telephone network


Examples of routing protocols

• In an Ethernet network– Spanning tree algorithm and address learning

• In the Internet:– Link-state routing protocols, such as Open Path

Shortest First (OSPF)

– Distance-vector based routing protocols, such as Routing Information Protocol (RIP)

• In telephone networks:– Real-Time Network Routing (RTNR)


Examples of addressing schemes

• Internet– 4-byte IP addresses

• Telephone networks– 8-byte E.164 address (telephone number)

• ATM networks– 20-byte ATM End System Address (AESA)


Broadcast links

• Wireless

• Copper: ethernet hubs

• Optical fiber: Passive star couplers

Ethernet hubor

WDM Passive Star Coupler

Blind broadcast

Ethernet switch(packet switch)

Dest: A

A


MAC protocols

• Medium Access Control (MAC) protocols are used in broadcast links to allow a node to access medium and send information

• As if “switch” is in endpoints

• Wasteful of resources because all endpoints receive all packets

End-userequipment

A

End-userequipment

B End-userequipment

C

To B

To B

B’s MAC layer checks destination address todetermine whether the packet should be “switched” to

the application or dropped

C’s MAC layer checks destination address todetermine whether the packet should be “switched” to

the application or dropped


Consider wireless links

• Naturally broadcast medium– One transmitter sends data; multiple receivers

can receive the signal and obtain the data– Need a MAC (Medium Access Control)

protocol to share the “naturally broadcast” wireless medium

Endpoint

Endpoint

Endpoint


Shared links in wired domain

• Distance limitation between farthest hosts – Shannon’s capacity; SNR; attenuation

outbound

inbound

Multipoint drops: potential interference on inbound line – polling; e.g. multidrop telephone lines

Hub or opticalpassive star coupler

Host HostHost

Hubs/Optical passive star couplers: any data received on one line is broadcast to all other lines


Classification of MAC protocols

MAC protocols

Fixed-assignment schemes

Random-access schemes

Demand assignment schemesCircuit-switched

(e.g., FDMA, TDMA)

Connectionlesspacket-switched(e.g., Ethernet,802.11)

Connection-orientedpacket-switched(e.g., CDMA, polling)Channelization


Shared link as a LAN:relation between MAC protocols and LANs

• A shared link allows multiple end stations to hear a transmission from any station

• No node is serving as a “forwarding engine” for packets in a controlled fashion– hubs, passive star couplers, ring adapters, taps blindly

send data UNLIKE switches, routers, bridges

• This shared link concept works well as a local area network– if too large a network – with many hosts – each host

will get a small percentage of bandwidth


Shared links as “access” links

• Two reasons for using shared links on the access segment– individual endpoints (hosts/phones) generate

small quantities of data traffic– Costs should be kept low for end users

• Consequence: access links are often shared

• MAC protocols in the upstream direction


Shared link in the presence of basestations/APs?

• Is it still one shared link if basestations/APs forward data between two endpoints that cannot “hear” each other

– No, basestations/APs become forwarding engines, i.e., switches

– If a cell phone under one basestation calls another cell phone under the same basestation and the basestation allocates frequencies for both ends and forwards data bits

• Not different from a circuit switch forwarding bits received on one DS0 to another DS0

– Same thing when an AP uses destination addresses to rebroadcast data – it acts as a packet switch


Compare TDMA on an access link with TDM on an inter-switch link

• Similar in concept: sharing resources on one link among many users

• Difference: – Multiple senders on access link

– One sender in each direction on inter-switch link

Basestation

Endpoint EndpointEndpoint

Timeslot 1 Timeslot 2 Timeslot 3

Circuitswitch

CircuitswitchT1 line

carrying24 different DS0s(phone calls)


Internetworking

• An internet– A path that traverses multiple networks

possibly ones using different networking techniques


Simplest network – one link

Endpoint Endpoint

Endpoint Endpoint

EndpointEndpoint

Switch Switch

One network – same type of switches – link rates can be different

Single networks

Endpoint

Endpoint

Endpoint

A shared link:often used tocreate a LAN


Endpoint

Endpoint

Switch Switch

Network 1

Endpoint

Endpoint

Switch Switch

Network 2IP router

An internetwork

The Internet approach to internetworking

• Have all endpoints speak the IP (Internet Protocol) in addition to their own network protocols

• For loss-sensitive applications: run TCP, an end-to-end transport protocol, irrespective of whether

– both ends are within the same network– the two ends are on different networks

• IP routers are connectionless packet switches – they forward IP packets from one network to another based on the destination IP

address carried in the IP header and information stored in their routing tables

Network 3


Network 1 Network 2

T1

Inter-T:TCP

Inter-N: IP

A

N1

L1

P1

N1

L3

P3

T1

N2

L4

P4

T2

Inter-N: IP

IP router routerrouter

N1

L2

P2 P3

N1

L1 L2

P1 P2

N2

L5 L6

P5 P6

N2

L4 L5

P4 P5

T2

Inter-T:TCP

Inter-N: IP

A

N2

L6

P6

Switch Switch Switch Switch

Endpoint Endpoint

L3

Protocol stacks in the Internet


Today’s most common networksin the Internet

• Ethernet within enterprises using a combination of– shared-medium Ethernet LANs with hubs, or– with Ethernet switches – which are connectionless

packet switches

• PDH/SONET networks in the MAN and WAN domains– Routers are interconnected by T1, T3, OC3 connections

that are set up through a PDH/SONET circuit-switched network

– PPP, Point-to-Point Protocol, is executed on these circuits


Need Internet address and Network address

Host A

Host B

Switch1

Switch2

Ethernet 1

Host C

Host D

Switch3

Switch4

Ethernet 2

IP router

Internetwork

Host E

Host F

Switch Switch

Ethernet 3

Host A sends a packet to Host C:- Places Host C’s IP address in IP header- To get through Ethernet 1, it needs Ethernet address of IP router’s

interface 1- Switch 1 and Switch 2 forward packets based on destination

Ethernet address of IP router’s interface 1- IP router forwards packet to port 2 to reach Host C (based on IP

level routing data using destination IP address of host C)- IP router needs Ethernet address of Host C to send the packet

through Ethernet 2- Switch 3 and 4 forward packets based on destination Ethernet

address of Host C

1 2

3


Summary

• Reviewed networking concepts

Review of networking concepts

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