1 Polytechnic UniversityM. Veeraraghavan
Review of networking concepts
Prof. Malathi Veeraraghavan
University of Virginia
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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)
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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
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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
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Switches
• Connect multiple links and route traffic from one link to another
End-userequipment
End-userequipment
End-userequipment
End-userequipment
Switch
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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?
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Summarized addresses
• What are summarized addresses?
• Why summarize addresses?
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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
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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
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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)
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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Internetworking
• An internet– A path that traverses multiple networks
possibly ones using different networking techniques
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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
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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
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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
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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
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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
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Summary
• Reviewed networking concepts