Review for Midterm - School of Computing and Information...

Review for Midterm

Introduction 1-2

Chapter 1Introduction

Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith RossAddison-WesleyMarch 2012

Slides adopted from original ones provided by the textbook authors.

Introduction

Chapter 1: roadmap1.1 what is the Internet?1.2 network edge

§ end systems, access networks, links1.3 network core

§ packet switching, circuit switching, network structure1.4 delay, loss, throughput in networks1.5 protocol layers, service models1.6 networks under attack: security1.7 history

1-3

Introduction 1-4

Access networks

v DSL: several Mbps, dedicated accessv Cable: tens of Mbps, shared access v Ethernet: up to Gbps, shared access v Wireless: WIFI/cellular, shared access

Keep in mind: v bandwidth (bits per second) of access network?v shared or dedicated?

Introduction 1-5

Physical Media

v guided media § Twisted pair: Ethernet§ Coax: cable networks§ Fiber: optical networks

v unguided media § terrestrial microwave§ LAN (e.g., Wifi)§ wide-area (e.g., cellular)§ satellite

Introduction


§ end systems, access networks, links

1.3 network core§ circuit switching, packet switching, network structure

1.4 delay, loss, throughput in networks1.5 protocol layers, service models1.6 networks under attack: security1.7 history

1-6

Introduction

Circuit switchingend-end resources allocated

to, reserved for “call”between source & dest:

v link bandwidth, switch capacityv dedicated resources: no sharingv circuit-like (guaranteed)

performancev call setup required

v P3

1-7

Introduction 1-8

Network Core: Packet Switching

each end-end data stream divided into packets

v user A, B packets sharenetwork resources

v each packet uses full link bandwidth

v resources used as needed

resource contention: v aggregate resource demand

can exceed amount availablev congestion: packets queue,

wait for link usev store and forward: packets

move one hop at a time

P5 P8

Introduction




1-9

Introduction 1-10

Nodal delay

v dproc = processing delay§ typically a few microsecs or less

v dqueue = queuing delay§ depends on congestion

v dtrans = transmission delay§ = L/R, significant for low-speed links

v dprop = propagation delay§ a few microsecs to hundreds of msecs

proptransqueueprocnodal ddddd +++=

P10

Introduction 1-11

Other Performance Metrics

v Packet loss: due to buffer overflowv Throughput: data transmission rate, constrained by

bottleneck link

Introduction




1-12

Introduction

Internet protocol stackv application: supporting network

applications§ FTP, SMTP, HTTP

v transport: process-process data transfer§ TCP, UDP

v network: routing of datagrams from source to destination§ IP, routing protocols

v link: data transfer between neighboring network elements§ Ethernet, 802.11 (WiFi), PPP

v physical: bits “on the wire”

application

transport

network

link

physical

1-13

Application Layer 2-14

Chapter 2Application Layer

Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith RossAddison-WesleyMarch 2012



Chapter 2: outline

2.1 Principles of network applications

2.2 Web and HTTP2.3 FTP 2.4 Electronic mail

§ SMTP, POP3, IMAP2.5 DNS

2.6 P2P applications2.7 Socket programming

with UDP and TCP

16

Application architectures

v Client-server§ Always-on server, intermittently connected client.§ Servers are bottlenecks.

v Peer-to-peer (P2P)§ Peers intermittently connected. § Highly scalable but difficult to manage.

Application Layer 2-


Internet transport protocols services

TCP service:v reliable transport between

sending and receiving process

v flow control: sender won’t overwhelm receiver

v congestion control: throttle sender when network overloaded

v does not provide: timing, minimum throughput guarantee, security

v connection-oriented: setup required between client and server processes

UDP service:v unreliable data transfer

between sending and receiving process

v does not provide:reliability, flow control, congestion control, timing, throughput guarantee, security, orconnection setup,


Chapter 2: outline

2.1 principles of network applications§ app architectures§ app requirements

2.2 Web and HTTP (P4 P7 P8 P9)

2.3 FTP

2.4 electronic mail§ SMTP, POP3, IMAP

2.5 DNS2.6 P2P applications (P22

P26)

Basic info and techniques of each protocol. No programming questions.

Transport Layer 3-19

Chapter 3Transport Layer

Computer Networking: A Top Down Approach

6th edition Jim Kurose, Keith Ross

Addison-WesleyMarch 2012



Chapter 3 outline

3.1 transport-layer services

3.2 multiplexing and demultiplexing

3.3 connectionless transport: UDP

3.4 principles of reliable data transfer

3.5 connection-oriented transport: TCP§ segment structure§ reliable data transfer§ flow control§ connection management

3.6 principles of congestion control

3.7 TCP congestion control


Transport vs. network layer

v network layer: logical communication between hostsv transport layer: logical communication between

processes § relies on, enhances, network layer services

v two transport-layer protocols§ reliable, in-order delivery: TCP§ unreliable, unordered delivery: UDP


Chapter 3 outline









Demultiplexing

v UDP socket identified by 2-tuple:§ dest IP address§ dest port number

v TCP socket identified by 4-tuple: § source IP address§ source port number§ dest IP address§ dest port number


Chapter 3 outline









UDP: User Datagram Protocol [RFC 768]

v “best effort” service, UDP segments may be:§ lost§ delivered out-of-order

to appv connectionless:

§ no handshaking between UDP sender, receiver

§ each UDP segment handled independently of others

source port # dest port #

32 bits

applicationdata

(payload)

UDP segment format

length checksum


Internet checksum (P4)

1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1

1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1

wraparound

sumchecksum

v Goal: detect “errors” (e.g., flipped bits) in transmitted segment

v When adding numbers, a carryout from the most significant bit needs to be added to the result

v checksum: 1s compliment of sum


Chapter 3 outline









Reliable Data Transfer

v TCP requires reliable data transfer, but IP is not reliable.

v Versions of rdt (P14)§ rdt1.0: over a reliable channel§ rdt2.0: channel with bit errors -> checksum, ACK/NAK,

retransmission§ rdt2.1: with corrupted ACK/NAKs -> retransmission,

sequence #§ rdt2.2: without NAKs -> ACK retransmission§ rdt3.0: channels with packet loss -> timer


Pipelined protocols (P23, P24)v increased utilization:v go-Back-N

§ always send ACK for correctly-received pkt with highest in-order seq #

§ timeout(n): retransmit pkt n and all higher seq # pkts in window

§ seq # size = window size + 1v selective repeat

§ receiver individually acknowledges all correctly received pkts§ timeout(n): sender only resends pkts for which ACK not

received§ seq # size = window size * 2

U sender =

n L / R RR RTT + L / R


Chapter 3 outline









TCP segment structure

source port # dest port #

32 bits

applicationdata

(variable length)

sequence numberacknowledgement number

receive window

Urg data pointerchecksumFSRPAUhead

lennot

used

options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)

Maximum segment size (MSS)

v MSS: maximum bytes of TCP payloadv Sequence #: byte-stream # of first byte in

segmentv E.g. file size 500,000 bytes, MSS 1,000 bytes



TCP seq. #’s and ACKs (P27)

Seq. #’s:§ byte stream “number” of first byte in segment’s data

ACKs:§ seq # of next byte expected from other side§ cumulative ACK

Setting the time out§ TimeoutInterval = EstimatedRTT + 4*DevRTT§ EstimatedRTT = (1- α)*EstimatedRTT + α*SampleRTT§ DevRTT = (1-β)*DevRTT + β*|SampleRTT-EstimatedRTT|


Chapter 3 outline









TCP sender events:data rcvd from app:v create segment with

seq #v seq # is byte-stream

number of first data byte in segment

v start timer if not already running § think of timer as for

oldest unacked segment

§ expiration interval: TimeOutInterval

timeout:v retransmit segment

that caused timeoutv restart timerack rcvd:v if ack acknowledges

previously unacked segments§ update ACK status§ start timer if there are

still unacked segments§ triple duplicate ACKs:

retransmit


TCP receiver events

event at receiver

arrival of in-order segment withexpected seq #. All data up toexpected seq # already ACKed

arrival of in-order segment withexpected seq #. One other segment has ACK pending

arrival of out-of-order segmenthigher-than-expect seq. # .Gap detected

arrival of segment that partially or completely fills gap

TCP receiver action

delayed ACK. Wait up to 500msfor next segment. If no next segment,send ACK

immediately send single cumulative ACK, ACKing both in-order segments

immediately send duplicate ACK,indicating seq. # of next expected byte

immediate send ACK, provided thatsegment starts at lower end of gap


Chapter 3 outline









TCP flow control

buffered data

free buffer spacerwnd

RcvBuffer

TCP segment payloads

to application processv receiver “advertises” free

buffer space by including rwnd value in TCP header of receiver-to-sender segments§ RcvBuffer size set via

socket options (typical default is 4096 bytes)

§ many operating systems autoadjust RcvBuffer

v sender limits amount of unacked (“in-flight”) data to receiver’s rwnd value

v guarantees receive buffer will not overflow

receiver-side buffering


Chapter 3 outline









TCP 3-way handshake

SYNbit=1, Seq=x

choose init seq num, xsend TCP SYN msg

ESTAB

SYNbit=1, Seq=yACKbit=1; ACKnum=x+1

choose init seq num, ysend TCP SYNACKmsg, acking SYN

ACKbit=1, ACKnum=y+1

received SYNACK(x) indicates server is live;send ACK for SYNACK;

this segment may contain client-to-server data received ACK(y)

indicates client is live

SYNSENT

ESTAB

SYN RCVD

client state

LISTENserver state

LISTEN


FIN_WAIT_2

CLOSE_WAIT

FINbit=1, seq=y

ACKbit=1; ACKnum=y+1

ACKbit=1; ACKnum=x+1wait for server

close

can stillsend data

can no longersend data

LAST_ACK

CLOSED

TIMED_WAIT

timed wait for 2*max

segment lifetime

CLOSED

TCP: closing a connection

FIN_WAIT_1 FINbit=1, seq=xcan no longersend but canreceive data

clientSocket.close()

client state server state

ESTABESTAB


Chapter 3 outline









Approaches towards congestion control

two broad approaches towards congestion control:

end-end congestion control:

v no explicit feedback from network

v congestion inferred from end-system observed loss, delay

v approach taken by TCP

network-assisted congestion control:

v routers provide feedback to end systems§ single bit indicating

congestion (SNA, DECbit, TCP/IP ECN, ATM)

§explicit rate for sender to send at


Chapter 3 outline









Summary: TCP Congestion Control (P40, P46)

v when cwnd < ssthresh, sender in slow-start phase, window grows exponentially.

v when cwnd >= ssthresh, sender is in congestion-avoidance phase, window grows linearly.

v when triple duplicate ACK occurs, ssthresh set to cwnd/2, cwnd set to ssthresh+3

v when timeout occurs, ssthresh set to cwnd/2, cwnd set to 1 MSS.


fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K

TCP connection 1

bottleneckrouter

capacity R

TCP Fairness

TCP connection 2

Chapter 4Network Layer

Computer Networking: A Top Down Approach

6th edition Jim Kurose, Keith Ross

Addison-WesleyMarch 2012

Network Layer 4-47


Network Layer 4-48

Chapter 4: network layer

chapter goals:v understand principles behind network layer

services:§ network layer service models§ forwarding versus routing§ how a router works§ routing (path selection)§ broadcast, multicast

v instantiation, implementation in the Internet

Network Layer 4-49

4.1 introduction4.2 virtual circuit and

datagram networks4.3 what’s inside a router4.4 IP: Internet Protocol

§ datagram format§ IPv4 addressing§ ICMP§ IPv6

4.5 routing algorithms§ link state§ distance vector§ hierarchical routing

4.6 routing in the Internet§ RIP§ OSPF§ BGP

4.7 broadcast and multicast routing

Chapter 4: outline

Network Layer 4-50

Network layerv transport segment from

sending to receiving host v on sending side

encapsulates segments into datagrams

v on receiving side, delivers segments to transport layer

v network layer protocols in every host, router

v router examines header fields in all IP datagrams passing through it

applicationtransportnetworkdata linkphysical


networkdata linkphysical network

data linkphysical

networkdata linkphysical







networkdata linkphysicalnetwork

data linkphysical

Network Layer 4-51

Two key network-layer functions

v forwarding: move packets from router’s input to appropriate router output

v routing: determine route taken by packets from source to dest.

§ routing algorithms

analogy:

v routing: process of planning trip from source to dest

v forwarding: process of getting through single interchange

Network Layer 4-52

R3

A

B

C

R1

R2

R4 D

E

FR5

R5FR3ER3D

Next HopDestination

R5FR4ER4D

Next HopDestinationInterplay between routing and forwarding

Network Layer 4-53

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

routing algorithm determinesend-end-path through network

forwarding table determineslocal forwarding at this router

Network Layer 4-54

Connection setup

v 3rd important function in some network architectures:§ ATM, frame relay, X.25

v before datagrams flow, two end hosts andintervening routers establish virtual connection§ routers get involved

v network vs transport layer connection service:§ network: between two hosts (may also involve intervening

routers in case of VCs)§ transport: between two processes

Network Layer 4-55

Network service modelQ: What service model for “channel” transporting datagrams from sender to receiver?

example services for individual datagrams:

v guaranteed deliveryv guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams:

v in-order datagram delivery

v guaranteed minimum bandwidth to flow

v restrictions on changes in inter-packet spacing

Network Layer 4-56

Network layer service models:

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

Network Layer 4-57

4.1 introduction4.2 virtual circuit and

datagram networks4.3 what’s inside a router4.4 IP: Internet Protocol

§ datagram format§ IPv4 addressing§ ICMP§ IPv6

4.5 routing algorithms§ link state§ distance vector§ hierarchical routing

4.6 routing in the Internet§ RIP§ OSPF§ BGP

4.7 broadcast and multicast routing

Chapter 4: outline

Network Layer 4-58

Connection, connection-less service

v datagram network provides network-layer connectionless service

v virtual-circuit network provides network-layer connection service

v analogous to TCP/UDP connecton-oriented / connectionless transport-layer services, but:§ service: host-to-host§ no choice: network provides one or the other§ implementation: in network core

Network Layer 4-59

Virtual circuits

v call setup, teardown for each call before data can flowv each packet carries VC identifier (not destination host

address)v every router on source-dest path maintains “state” for

each passing connectionv link, router resources (bandwidth, buffers) may be

allocated to VC (dedicated resources = predictable service)

“source-to-dest path behaves much like telephone circuit”§ performance-wise§ network actions along source-to-dest path

Network Layer 4-60

VC implementation

a VC consists of:1. path from source to destination2. VC numbers, one number for each link along path3. entries in forwarding tables in routers along path

v packet belonging to VC carries VC number (rather than dest address)

v VC number can be changed on each link.§ new VC number comes from forwarding table

Network Layer 4-61

VC forwarding table12 22 32

1 23

VC numberinterfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …

forwarding table innorthwest router:

VC routers maintain connection state information!

Network Layer 4-62


Virtual circuits: signaling protocols

v used to setup, maintain teardown VCv used in ATM, frame-relay, X.25v not used in today’s Internet

1. initiate call 2. incoming call3. accept call4. call connected

5. data flow begins 6. receive dataapplicationtransportnetworkdata linkphysical

Network Layer 4-63

Datagram networksv no call setup at network layerv routers: no state about end-to-end connections

§ no network-level concept of “connection”v packets forwarded using destination host address

1. send datagrams



2. receive datagrams

Network Layer 4-64

1

23

Datagram forwarding table

IP destination address in arriving packet’s header

routing algorithm

local forwarding tabledest address output link

address-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses, so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-65

Destination Address Range

11001000 00010111 00010000 00000000through11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q: but what happens if ranges don’t divide up so nicely?

Datagram forwarding table

Network Layer 4-66

Longest prefix matching

Destination Address Range

11001000 00010111 00010*** *********

11001000 00010111 00011000 *********

11001000 00010111 00011*** *********

otherwise

DA: 11001000 00010111 00011000 10101010

examples:DA: 11001000 00010111 00010110 10100001 which interface?

which interface?

longest prefix matching

Link interface

0

1

2

3

when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.

Example

v Consider a datagram network using 8-bit host addresses. Suppose a router uses longest prefix matching and has the following forwarding table:

v For each of the interfaces, give the associated range of destination host addresses and the number of addresses in the range.

Network Layer 4-67

Prefix Match Interface00 001 1011 2

Review for Midterm - School of Computing and Information...

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