lecture 14: midterm review

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Lecture 14:

Midterm Review

Slides adapted from:Computer Networks: A Systems Approach (Peterson and Davis)

Computer Networking: A Top Down Approach Featuring the Internet (Kurose and Ross)

ITCS 6166/8166 091Spring 2007

Jamie PaytonDepartment of Computer Science

University of North Carolina at Charlotte

February 21, 2007

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A hodge podge of topics

• General overview to put things in perspective….

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What’s the Internet?A “nuts and bolts” view

• millions of connected computing devices: hosts = end systems

• running network apps• communication links

– fiber, copper, radio, satellite– transmission rate =

bandwidth

• routers: forward packets (chunks of data)

local ISP

companynetwork

regional ISP

router workstation

servermobile

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Network Structure

• network edge: applications and hosts

• network core: – routers

– network of networks

• access networks, physical media: communication links

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Network Edge• end systems (hosts):

– run application programs– e.g. Web, email– at “edge of network”

• client/server model– client host requests, receives

service from always-on server– e.g. Web browser/server; email

client/server

• peer-peer model:– minimal (or no) use of dedicated

servers– e.g. Skype, BitTorrent, KaZaA

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The Network Edge Connection-oriented Service

Goal: data transfer between end systems

• handshaking: setup (prepare for) data transfer ahead of time– Hello, hello back human

protocol

– set up “state” in two communicating hosts

• TCP - Transmission Control Protocol – Internet’s connection-

oriented service

TCP service [RFC 793]

• reliable, in-order byte-stream data transfer– loss: acknowledgements

and retransmissions

• flow control: – sender won’t overwhelm

receiver

• congestion control: – senders “slow down sending

rate” when network congested

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The Network Edge

Connectionless service

Goal: data transfer between end systems– same as before!

• UDP - User Datagram Protocol [RFC 768]: – connectionless – unreliable data transfer– no flow control– no congestion control

App’s using TCP: • HTTP (Web), FTP (file

transfer), Telnet (remote login), SMTP (email)

App’s using UDP:• streaming media,

teleconferencing, DNS, Internet telephony

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The Network Core

• mesh of interconnected routers

• the fundamental question: how is data transferred through net?– circuit switching:

dedicated circuit per call: telephone net

– packet-switching: data sent thru net in discrete “chunks”

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The Network Core

Circuit Switching

End-end resources reserved for “call”

• link bandwidth, switch capacity

• dedicated resources: no sharing

• circuit-like (guaranteed) performance

• call setup required

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The Network Core

Circuit Switchingnetwork resources

(e.g., bandwidth) divided into “pieces”

• pieces allocated to calls

• resource piece idle if not used by owning call (no sharing)

• dividing link bandwidth into “pieces”– frequency division– time division

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Circuit Switching

FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

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The Network Core

Packet Switching

each end-end data stream divided into packets

• user A, B packets share network resources

• each packet uses full link bandwidth

• resources used as needed

resource contention: • aggregate resource

demand can exceed amount available

• congestion: packets queue, wait for link use

• store and forward: packets move one hop at a time– Node receives complete

packet before forwarding

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Packet Switching

Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing.

TDM: each host gets same slot in revolving TDM frame

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

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Packet-switching

Store-and-Forward

• Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps

• Entire packet must arrive at router before it can be transmitted on next link: store and forward

• delay = 3L/R (assuming zero propagation delay)

Example:• L = 7.5 Mbits• R = 1.5 Mbps• delay = 15 sec

R R RL

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Packet switching versus circuit switching

• 1 Mb/s link• each user:

– 100 kb/s when “active”

– active 10% of time

• circuit-switching: – 10 users

• packet switching: – with 35 users, probability >

10 active less than .0004

Packet switching allows more users to use network!

N users

1 Mbps link

Q: how did we get value 0.0004?See notes from lecture!

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Packet switching versus circuit switching

• Great for bursty data– resource sharing– simpler, no call setup

• Excessive congestion: packet delay and loss– protocols needed for reliable data transfer,

congestion control• Q: How to provide circuit-like behavior?

– bandwidth guarantees needed for audio/video apps– still an unsolved problem (chapter 7)

Is packet switching a “slam dunk winner?”

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Delays in Packet-Switched Networks

• Transmission delay– Sending of bits onto the wire– Depends on link bandwidth

• Propagation delay– Propagation of bits inside the wire– Depends on medium

• Processing delay– Handling of bits on receiving end– Depends on processor and memory speed

• Queuing delay– Wait time due to statistical multiplexing– Depends on network load and scheduling algorithm

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Understanding Packet Delay• Transmission delay:

– R=link bandwidth (bps)– L=packet length (bits)– time to send bits into

link = L/R

A

B

propagation

transmission

nodalprocessing queueing

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Understanding Packet Delay

A

B

propagation

transmission


• Propagation delay:– d = length of physical link– s = propagation speed in medium (~2x108

m/sec)– propagation delay = d/s

Note: s and R are very different quantities!

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• Processing delay:– B = Check bit errors– O = Determine

output link– Delay = b + o

A

B

propagation

transmission


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• Queuing delay:– Time waiting at output

link for transmission – Depends on

congestion level of router

A

B

propagation

transmission


• Complex topic– Varies from packet to

packet!• Must use statistical

measures to estimate queuing delay

– Thousands of research papers written on queuing delay

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Overview of Queuing Delay

• R=link bandwidth (bps)

• L=packet length (bits)

• a=average packet arrival rate

traffic intensity = La/R• La/R ~ 0: average queueing delay small• La/R -> 1: delays become large• La/R > 1: more “work” arriving than can

be serviced, average delay infinite!

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Caravan analogy

• Analogy– Car ~ bit– Caravan ~ packet– Toll booth ~ host

• 12 sec to take money from each car

– Highway ~ connection• Speed limit is 100 km/hr

toll booth

toll booth

ten-car caravan

100 km

100 km

• Q: How long until caravan is lined up before 2nd toll booth?

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Nodal Delay

• dproc = processing delay– typically a few microsecs or less

• dqueue = queuing delay– depends on congestion

• dtrans = transmission delay– = L/R, significant for low-speed links

• dprop = propagation delay– a few microsecs to hundreds of msecs

proptransqueueprocnodal ddddd

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Packet Loss

• Queue (aka buffer) has finite capacity

• When packet arrives to full queue, packet is dropped (i.e., lost)

• Lost packet may be:– Retransmitted by previous node– Retransmitted by source end system– Not retransmitted at all

• We’ll discuss techniques for dealing with lost packets later in the semester

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Network Protocol Layers

• Network functionality is organized into layers– Each layer implements a service

• Layer actions are encapsulated• Each layer relies on services

provided by layer below

• Benefits of layered approach– Modularity

• Simplifies maintenance, updating

– Explicit structure • Allows identification, relationship of

system pieces

application

transport

network

link

physical

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Internet Protocol Stack

• application: network applications– FTP, SMTP, HTTP– application-layer messages

• transport: data transfer– TCP, UDP– segments

• network: routing data from source to destination– IP, routing protocols– datagrams

• link: data transfer between neighboring network elements– PPP, Ethernet– frames

• physical: bits “on the wire”

application

transport

network

link

physical

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sourceapplicatio

ntransportnetwork

linkphysical

HtHn M

segment Ht

datagram

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

Encapsulationmessage M

Ht M

Hn

frame

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Change Gears…

• Now we’ll talk about stuff at the application layer– See lecture slides for application layer!

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Change Gears…

• Now we’ll talk about stuff at the transport layer– UDP – TCP

• See lecture slides for TCP and congestion control!

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Changing Gears…

• Now we’ll talk about stuff specifically at the network layer– IP addressing– Forwarding and routing

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IP Addressing: introduction• IP address: 32-bit

identifier for host, router interface

• interface: connection between host/router and physical link– router’s typically have

multiple interfaces– host typically has one

interface– IP addresses

associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

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Subnets• IP address:

– subnet part (high order bits)

– host part (low order bits)

• What’s a subnet ?– device interfaces

with same subnet part of IP address

– can physically reach each other without intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet

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Subnets 223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe• To determine the

subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.

Subnet mask: /24

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IP addressing: CIDRCIDR: Classless InterDomain Routing

– subnet portion of address of arbitrary length– address format: a.b.c.d/x, where x is # bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

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NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

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Network Layer Overview:

Forwarding and Routing

• Forwarding: move packets from router’s input to appropriate router output

• Routing: determine route taken by packets from source to dest.

– routing algorithms

analogy:

• routing: process of planning trip from source to destination

• forwarding: process of getting through single interchange

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1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding table

header value output link

0100010101111001

3221

Network Layer Overview:

Forwarding and Routing

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Forwarding table Destination Address Range Link Interface

11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111

otherwise 3

4 billion possible entries

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Longest prefix matching

Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?

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Internet inter-AS routing: BGP

• BGP (Border Gateway Protocol): the de facto standard

• BGP provides each AS a means to:1. Obtain subnet reachability information from

neighboring ASs.2. Propagate reachability information to all AS-internal

routers.3. Determine “good” routes to subnets based on

reachability information and policy.

• allows subnet to advertise its existence to rest of Internet: “I am here”

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BGP basics• Pairs of routers (BGP peers) exchange routing info

over semi-permanent TCP connections: BGP sessions– BGP sessions need not correspond to physical links.

• When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix.– AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

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Distributing reachability info• With eBGP session between 3a and 1c, AS3 sends prefix

reachability info to AS1.• 1c can then use iBGP do distribute this new prefix reach info to

all routers in AS1• 1b can then re-advertise new reachability info to AS2 over 1b-

to-2a eBGP session• When router learns of new prefix, creates entry for prefix in its

forwarding table.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

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Path attributes & BGP routes

• When advertising a prefix, advert includes BGP attributes. – prefix + attributes = “route”

• Two important attributes:– AS-PATH: contains ASs through which prefix advertisement has

passed: AS 67 AS 17 – NEXT-HOP: Indicates specific internal-AS router to next-hop

AS. (There may be multiple links from current AS to next-hop-AS.)

• When gateway router receives route advertisement, uses import policy to accept/decline.

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BGP route selection

• Router may learn about more than 1 route to some prefix. Router must select route.

• Elimination rules:1. Local preference value attribute: policy

decision

2. Shortest AS-PATH

3. Closest NEXT-HOP router: hot potato routing

4. Additional criteria

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BGP messages

• BGP messages exchanged using TCP• BGP messages:

– OPEN: opens TCP connection to peer and authenticates sender

– UPDATE: advertises new path (or withdraws old)– KEEPALIVE keeps connection alive in absence of

UPDATES; also ACKs OPEN request– NOTIFICATION: reports errors in previous msg; also

used to close connection

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BGP routing policy

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

• A,B,C are provider networks• X,W,Y are customer (of provider networks)• X is dual-homed: attached to two networks

– X does not want to route from B via X to C– .. so X will not advertise to B a route to C

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BGP routing policy (2)

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

• A advertises to B the path AW • B advertises to X the path BAW • Should B advertise to C the path BAW?

– No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

– B wants to force C to route to w via A– B wants to route only to/from its customers!

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Why different Intra- and Inter-AS routing ? Policy: • Inter-AS: admin wants control over how its traffic routed,

who routes through its net. • Intra-AS: single admin, so no policy decisions needed

Scale:• hierarchical routing saves table size, reduced update

traffic

Performance: • Intra-AS: can focus on performance• Inter-AS: policy may dominate over performance

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Summary

• Routing!!!

• Next time– Midterm

• After break– More routing!!!

lecture 14: midterm review

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