Transcript Document

Review of networking concepts
Prof. Malathi Veeraraghavan
University of Virginia
M. Veeraraghavan
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Polytechnic University
Outline
• Review of basic concepts in networking
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–
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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)
– Types of communication links
•
•
•
•
Twisted pair
Coaxial cable
Optical fiber
Wireless links
End-user
equipment
End-user
equipment
– 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-user
equipment
End-user
equipment
Switch
End-user
equipment
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End-user
equipment
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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 ( N  1)
2
• N links – since we only need one link from
an endpoint to a switch
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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
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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-user
equipment
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Switch
End-user
equipment
Switch
End-user
equipment
Switch
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Different types of networks
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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
Switching modes
Networking modes
Connectionless
Packet-switching
IP, SS7
MPLS
IP + RSVP
ATM, X.25
Telephone network,
SONET/SDH, WDM
Circuit-switching
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Connection-oriented
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Types of data 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
Consuming end
Live
Sending end
Live
Stored
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Stored
Interactive/
Live streaming
Recording
Stored streaming
File transfers
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Types of data 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
Consuming end
Live
Sending end
Live
Stored
M. Veeraraghavan
Stored
Interactive/
Live streaming
Recording
Stored streaming
File transfers
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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
M. Veeraraghavan
Packet-switched CO networks
Short transfers
(e.g. short email)
Connectionless
networks
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Ideal networks
Long transfers
(e.g. large image,
audio, video or data)
Circuit-switched
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
End-user
equipment
Network
Web server
(Usually runs
on fixed hosts)
Web clients
Network
Network
End-user
equipment
Email-sending clients
(outlook,
messenger)
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Outgoing
email servers
(pop, imap)
Network
Incoming
email servers
(smtp)
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End-user
equipment
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Email-receiving
clients
(outlook, messenger)
Protocol Stacks
AL
AL
TL
TCP/UDP
NL
NL
NL
IP
DLL
DLL DLL
DLL DLL
DLL
PHY
PHY PHY
PHY PHY
PHY
Switch
Switch
Endpoint
Endpoint
•
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
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Example protocols
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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
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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
Dest.
Next hop
B
B
II
Host A
I
Dest.
Next hop
III-*
IV
Routing
protocol
Routing
protocol
Routing
protocol
IV
Dest.
Next hop
III-*
III
Host B
III
V
Routing tables
• Routing protocols exchange
topology/loading/reachability information
• Routes to destinations are precomputed and stored in
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routing tables
Signaling protocol for NL connection setup in
a PS CO network - Phase 2
Connection
setup (B)
II
d/L1
b
a
Host A
IN
Port /Label
I
c
Connection
setup
b
a
a/L1
OUT
Port/Label
c/L2
Connection
setup
IV
III
d
IN
Port /Label
b
Host B
c
V
d
Connection
setup
OUT
Port/Label
a/L2
•
b/L3
a
c
IN
Port /Label
OUT
Port/Label
c/L1
Virtual circuit
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
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Signaling protocol for NL connection setup in
a CS CO network - Phase 2
Connection
setup (B)
Host A
II
d/2
b
a
I
IN
OUT
Port /Timeslot Port/Timeslot
c
a
Connection
setup
b
Connection
setup
a
a/1
c/2
IV
III
d
c
IN
OUT
Port /Timeslot Port/Timeslot
b/1
b
Host B
c
V
d
Connection
setup
IN
OUT
Port /Timeslot Port/Timeslot
a/2
c/2
Circuit
• 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/
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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
Dest.
Next hop
B
B
II
Host A
I
Dest.
Next hop
B
II
M. Veeraraghavan
Host B
III
SYN
ACK
SYN
V
IV
Dest.
Next hop
B
III
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Routing tables
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User-plane packet forwarding in a PS
CO network - Phase 3
II
L1
b
Host A
IN
Port /Label
a/L1
a
I
c
L1
III
c
d
b
a
OUT
Port/Label
c/L2
L3
a
IV
d
b
Host B
c
V
L2
• Labels are VPI/VCIs in ATM
• Labels are translated from link-to-link
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User-plane actions in a circuit-switched
network - Phase 3
II
1
2
b
Host A
a
I
1
a
1
c
2
b
d
c
a
IV
III
d
2
b
Host B
c
V
IN
OUT
Port /Timeslot Port/Timeslot
a/1
c/2
1
2
• Bits arriving at switch I on time slot 1 on
port a are switched to time slot 2 of port c
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User-plane packet forwarding in a CL PS
network - Phase 3
II
b
Host A
a
I
c
B
B
a
B
III
c
d
b
a
IV
d
b
Host B
c
V
B
• 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
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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
Dest: A
Ethernet hub
or
WDM Passive Star Coupler
A
Ethernet switch
(packet switch)
Blind broadcast
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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
B’s MAC layer checks destination address to
determine whether the packet should be “switched” to
the application or dropped
End-user
equipment
A
To B
M. Veeraraghavan
C’s MAC layer checks destination address to
determine whether the packet should be “switched” to
the application or dropped
End-user
equipment
B
To B
End-user
equipment
C
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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
outbound
Hub or optical
passive star coupler
inbound
Host
Multipoint drops: potential
interference on inbound line –
polling; e.g. multidrop telephone
lines
Host
Host
Hubs/Optical passive star couplers: any
data received on one line is broadcast to
all other lines
• Distance limitation between farthest hosts –
Shannon’s capacity; SNR; attenuation
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Classification of
MAC protocols
MAC protocols
Fixed-assignment
schemes
Random-access
schemes
Circuit-switched
(e.g., FDMA,
TDMA)
Connectionless
packet-switched
(e.g., Ethernet,
802.11)
Channelization
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Demand
assignment
schemes
Connection-oriented
packet-switched
(e.g., CDMA, polling)
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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
Basestation
Circuit
switch
Endpoint
Endpoint
Endpoint
Timeslot 1
Timeslot 2
Timeslot 3
Circuit
switch
T1 line
carrying
24 different DS0s
(phone calls)
• 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
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Internetworking
• An internet
– A path that traverses multiple networks
possibly ones using different networking
techniques
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Single networks
A shared link:
often used to
create a LAN
Simplest network – one link
Endpoint
Endpoint
Endpoint
Endpoint
Endpoint
One network – same type of switches – link rates can be different
Endpoint
Endpoint
Switch
Switch
Endpoint
Endpoint
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The Internet approach to
internetworking
An internetwork
Endpoint
Endpoint
Switch
Endpoint
•
Switch
Endpoint
IP router
Network 1
•
Switch
Switch
Network 3
Network 2
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
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University
– they forward IP packets from one network
to another based onPolytechnic
the destination
IP
address carried in the IP header and information stored in their routing tables
Protocol stacks in the Internet
E n dpoin t
E n dpoin t
A
A
Inter-T : T C P
IP router
routerrou ter
Inter -N : IP
Inter -N : IP
T1
Sw itch
N1
Sw itch
N1
N1
T1
T2
N1
N2
Inter-T : T C P
Inter -N : IP
Sw itch
Sw itch
N2
N2
T2
N2
L1
L1
L2
L2
L3
L3
L4
L4
L5
L5
L6
L6
P1
P1
P2
P2
P3
P3
P4
P4
P5
P5
P6
P6
N etw ork 2
N etw ork 1
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Today’s most common networks
in 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
Internetwork
IP router
Host A
Switch
1
Switch
2
1
Host B
2
Switch
3
Switch
4
Host D
3
Ethernet 1
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
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- Switch
and 4 forward packets based on destination Ethernet address
of Host C
Host C
Ethernet 2
Host E
Switch
Switch
Host F
Ethernet 3
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Summary
• Reviewed networking concepts
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