We Have Learned Last Time CSci4211: Weekly Summary1 Weekly Summary Part I.

We Have Learned Last Time

CSci4211: Weekly Summary

1

Weekly SummaryPart I

CSci4211: Weekly Summary 2

What We Learned Last Time (Sep 10) What is a network? What is a computer/data network?

Compare w/ diff. networks (telephone, postal office, transportation networks, …) “bolts-&-nuts” view vs. service perspective

Network is a shared resource! ways to share (“multiplex”) resources? TDMA, FDMA, CDMA,

… Packet switching vs. circuit switching:

packets, packet switching and statistical multiplexing• For “bursty” data applications• store-&-forward

delay, losses and congestion vs. call blocking New: four types of delays

propagation, transmission, processing & queueing delays

Architecture: layering & hourglass different technologies, “boxes” (routers, switches), & apps Protocols and Interfaces (API)

CSci4211: Introduction 3

Switching & Multiplexing• Network is a shared resource

– Provide services for many people at same time– Carry bits/information for many people at same time

• How do we do it? – Switching: how to deliver information from point A to

point B?– Multiplexing: how to share resources among many

users

Think about postal service and telephone system!

Switching and multiplexing are closely related!


Switching/Multiplexing Strategies

• Circuit switching– set up a dedicated route (“circuit”) first – carry all bits of a “conversation” on one circuit

• original telephone network• Analogy: railroads and trains/subways

• Packet switching– divide information into small chunks (“packets”)– each packet delivered independently – “store-and-forward” packets

• Internet (also Postal Service, but they don’t tear your mail into pieces

first!)• Analogy: highways and cars

• Pros and Cons? - think taking subways vs. driving cars, during off-peak vs. rush hours!

Analogy: railroad and train


http://www.answers.com/main/Record2?a=NR&url=http://commons.wikimedia.org/wiki/Image:Railway%20turnout%20labelled.jpg

Analogy: Highway and cars

CSci4211: Introduction

6

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 code division

Trivia Q:You must have heard of the term “CDMA” (think the company Qualcom, for which it is most associated with), what does “CD” in CDMA stands for?


7

Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:


8

Networks with Circuit Switchinge.g., conventional (fixed-line) telephone

networks

End-end resources reserved for “call”

• link bandwidth, switch capacity

• dedicated resources: no sharing

• circuit-like (guaranteed) performance

• call setup required



Circuit Switched Networks• All resources (e.g. communication links)

needed by a call dedicated to that call for its duration– Example: telephone network– Call blocking when all resources are used

Numerical example• How long does it take to send a file of

640,000 bits from host A to host B over a circuit-switched network?– All links are 1.536 Mbps– Each link uses TDM with 24 slots/sec– 500 msec to establish end-to-end circuit

Let’s work it out!

10.5 seconds


Packet SwitchingEach end-end “data

stream” divided into packets

• users 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 Packets may suffer delay or

losses!

Bandwidth division into “pieces”

Dedicated allocationResource reservation

12CSci4211: Introduction


Statistical Multiplexing

• Time division, but on demand rather than fixed• Reschedule link on a per-packet basis• Packets from different sources interleaved on the link• Buffer packets that are contending for the link• Buffer buildup is called congestion• This is packet switching, used in computer networks

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

14CSci4211: Introduction

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 = ?

R R RL

more on delay later …

15 sec


15

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?

M

Nn

nMn ppn

M

1

1


16


Circuit Switching vs Packet SwitchingItem Circuit-

switchedPacket-switched

Dedicated “copper” path Yes No

Bandwidth available Fixed Dynamic

Potentially wasted bandwidth Yes No (not really!)

Store-and-forward transmission No Yes

Each packet/bit always follows the same route

Yes Not necessarily

Call setup Required Not Needed

When can congestion occur At setup time On every packet

Effect of congestion Call blocking Queuing delay

Four sources of packet delay

1. nodal processing: • check bit errors• determine output link

A

B

propagation

transmission

nodalprocessing queueing

2. queueing• time waiting at output

link for transmission • depends on congestion

level of router

CSci4211: Introduction18


Delay in packet-switched networks

3. Transmission delay:• R=link bandwidth

(bps)• L=packet length

(bits)• time to send bits into

link = L/R

4. Propagation delay:• d = length of physical

link• s = propagation speed

in medium (~2x108 m/sec)

• propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

Note: s and R are very different quantitites!

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



Statistical Multiplexing and Queueing

A

B

C10 MbsEthernet

1.5 Mbs

45 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link


Queueing delay (revisited)

• R=link bandwidth (bps)

• L=packet length (bits)

• a=average packet arrival ratetraffic 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!

Queueing delay and Packet loss

• Queue (aka buffer) preceding link in buffer has finite capacity

• When packet arrives to full queue, packet is dropped (aka lost)

• lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all


“Real” Internet delays and routes

• What do “real” Internet delay & loss look like? • Traceroute program: provides delay

measurement from source to router along end-end Internet path towards destination. For all i:– sends three packets that will reach router i on path

towards destination– router i will return packets to sender– sender times interval between transmission and

reply.

3 probes

3 probes

3 probes


“Real” Internet delays and routes

Let’s Traceroute to www.bbc.com


Throughput• throughput: rate (bits/time unit) at

which bits transferred between sender/receiver– instantaneous: rate at given point in time– average: rate over longer period of time

server, withfile of F bits

to send to client

link capacity

Rs bits/sec

link capacity

Rc bits/sec pipe that can carry

fluid at rate

Rs bits/sec)

pipe that can carryfluid at rate

Rc bits/sec)

server sends bits

(fluid) into pipe


Throughput (cont’d)

• Rs < Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

Rs > Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

link on end-end path that constrains end-end throughput

bottleneck link


Throughput: Internet scenario

10 connections (fairly) share backbone bottleneck link R

bits/sec

Rs

Rs

Rs

Rc

Rc

Rc

R

• per-connection end-end throughput: min(Rc,Rs,R/10)

• in practice: Rc or Rs is often bottleneck



Introduction (cont’d) Key network functions: -- naming, addressing, routing & forwarding

networks are distributed & complex systems!

What’s so special about the Internet? -- Internet Architecture: layering & hourglass

different technologies, “boxes” (routers, switches), & apps

Protocols and Interfaces (API)

What today’s Internet looks like? economics & policies

What may go wrong? bit errors, packet losses,, node failures, software bugs, app

crashes. ….. and Attacks !!!


What’s so special about the Internet?

• Internet is based on the notion of “packet switching”– enables statistical multiplexing– better utilization of network resources for transfer of

“bursty” data traffic • Internet’s key organizational/architectural principle:

“smart” end systems + “dumb” networks– architecture: functional division & function placement– hourglass Internet architecture: enables diverse

applications and accommodates evolving technologies– “dumb” network (core): simple packet-switched, store-

forward, connectionless “datagram” service, with core functions: global addressing, routing & forwarding

– “smart” end systems/edges: servers, PCs, mobile devices, …; diverse and ever-emerging new applications!


Internet Hourglass Architecture

WiFi, Bluetooth,Docsis, gMPLS, DWDM/fiber, …,3G/4G cellular, ….

p2p file sharing, skype, YouTube, Netflix, Cloud Computing

bitTorrent, DHT, SIP, DASH, ….

enabling diverse applications & new types of end devices

accommodating evolving & new technologies

netw

ork

core

netw

ork

edge

/end

hos

ts

Internet Protocol Stack• application: supporting network

applications– FTP, SMTP, HTTP, DASH, …

• transport: process-process data transfer– TCP, UDP

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

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

• physical: bits “on the wire”

application

transport

network

link

physical


32


Layered Architecture

• Layering simplifies the architecture of complex system

• Layer N relies on services from layer N-1 to provide a service to layer N+1

• Interfaces define the services offered

• Service required from a lower layer is independent of its implementation– Layer N change doesn’t affect

other layers– Information/complexity hiding– Similar to object oriented

methodology


Protocols and Services• Protocols are used to implement services

– Peering entities in layer N provide service by communicating with each other using the service provided by layer N-1

• Logical vs physical communication

What’s a protocol?human protocols:• “what’s the time?”• “I have a question”• introductions

network protocols:• machines rather

than humans• all communication

activity in Internet governed by protocols (why this concept is so important!!!)


35


Protocol Packets• Protocol data units (PDUs):

– packets exchanged between peer entities• Service data units (SDUs):

– packets handed to a layer by an upper layer• Data at one layer is encapsulated in packet at a lower

layer– Envelope within envelope: PDU = SDU + (optional)

header or trailer

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



Introduction (cont’d) Key network functions: -- naming, addressing, routing & forwarding

networks are distributed & complex systems!

What’s so special about the Internet? -- Internet Architecture: layering & hourglass

different technologies, “boxes” (routers, switches), & apps

Protocols and Interfaces (API)

What today’s Internet looks like? -- economics & policies

What may go wrong? bit errors, packet losses,, node failures, software bugs, app

crashes. ….. and Attacks !!!


Internet Structure

LANs

International lines

Regional or local ISP local ISPs

company university

National or tier-1 ISP

National or tier-1 ISP

IXPsor private peering

Regional ISPs

company

access via WiFi hotspots

Internet: “networks of networks”!

Home users

Internet eXcangePoints

Home users

Internet structure: network of networks

• Roughly hierarchical• At center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T,

L3, Cable and Wireless), national/international coverage– treat each other as equals

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

IXP

Tier-1 providers also interconnect at Internet Exchange Point


Tier-1 ISP: e.g., Sprint

…

to/from customers

peering

to/from backbone

….

………

POP: point-of-presence



• “Tier-2” ISPs: smaller (often regional) ISPs– Connect to one or more tier-1 ISPs, possibly other tier-2

ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

IXP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is

customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at IXP



• “Tier-3” ISPs and local ISPs – last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

IXP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet



• a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

IXP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

traceroute www.cnn.com


Routing & forwarding:how do packets gofrom A to B?

B

A

Map of Internet


Fundamental Issues in NetworkingNetwork is a shared resource


• Switching and Multiplexing – How to share resources among multiple users, and

transfer data from one node to another node• Naming and Addressing

– How to find name/address of the party (or parties) you would like to communicate with

– Address: byte-string that identifies a node• unicast, multicast and broadcast addresses

• Routing and Switching/Forwarding: – process of determining how to send packets towards

the destination based on its address: finding out neighbors, building routing tables

– transferring data from source to destination


Fundamental Problems in Networking …

Or what can go wrong?• Bit-level errors: due to electrical interferences• “Frame-level” errors: media access delay or

frame collision due to contention/collision/interference

• Packet-level errors: packet delay or loss due to network congestion/buffer overflow

• Out of order delivery: packets may takes different paths

• Link/node failures: cable is cut or system crash


Fundamental Problems in Networking

What can be done?• Add redundancy to detect and correct

erroneous packets• Acknowledge received packets and retransmit

lost packets• Assign sequence numbers and reorder packets

at the receiver• Sense link/node failures and route around

failed links/nodesGoal: to fill the gap between what applications

expect and what underlying technology provides

What’s the Internet: Recap

• protocols control sending, receiving of messages– e.g., TCP, IP, HTTP, FTP,

PPP• Internet: “network of

networks”– loosely hierarchical– public Internet versus

private intranet• Internet standards

– RFC: Request for comments

– IETF: Internet Engineering Task Force

– IEEE

local ISP

companynetwork

regional ISP

router workstation

servermobile



Fundamental Issues in NetworkingNetwork is a shared resource


• Switching and Multiplexing – How to share resources among multiple users, and

transfer data from one node to another node• Naming and Addressing

– How to find name/address of the party (or parties) you would like to communicate with

– Address: byte-string that identifies a node• unicast, multicast and broadcast addresses

• Routing and Switching/Forwarding: – process of determining how to send packets towards

the destination based on its address: finding out neighbors, building routing tables

– transferring data from source to destination


Fundamental Problems in Networking …

Or what can go wrong?• Bit-level errors: due to electrical interferences• “Frame-level” errors: media access delay or

frame collision due to contention/collision/interference

• Packet-level errors: packet delay or loss due to network congestion/buffer overflow

• Out of order delivery: packets may takes different paths

• Link/node failures: cable is cut or system crash


Fundamental Problems in Networking

What can be done?• Add redundancy to detect and correct

erroneous packets• Acknowledge received packets and retransmit

lost packets• Assign sequence numbers and reorder packets

at the receiver• Sense link/node failures and route around

failed links/nodesGoal: to fill the gap between what applications

expect and what underlying technology provides


Internet Summary• Computer networks/Internet use packet

switching• Layered architecture for handling complexity &

attaining maintainability– Key notions: protocols, services and interfaces

• Internet is based on TCP/IP protocol suite– Networks of networks!– Shared, distributed and complex system in global

scale– No centralized authority

• Fundamental issues in networking– addressing/naming – routing/forwarding– error/flow/congestion control, media access control


The Internet Network layer

routingtable

Routing protocols• path selection• RIP, OSPF, BGP

IP protocol• addressing conventions• packet handling conventions

ICMP protocol• error reporting• router “signaling”

Transport layer: TCP, UDP

Data Link layer (Ethernet, WiFi, PPP, …)

Physical Layer (fiber optics, radio, …)

Networklayer

Internet Protocol Stack• application: supporting network

applications– FTP, SMTP, HTTP, DASH, …

• transport: process-process data transfer– TCP, UDP

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

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

• physical: bits “on the wire”

application

transport

network

link

physical


55


Stuff We Won’t Talk About or Elaborated on

Various Physical Media twisted pairs, co-axial cables, fiber optics, radio,

satellite, etc. Access Networks

DSL, Cable Modem, Fiber to the Curb/Home, … ISPs and Internet Backbones


57


58

Announcements & Reminders (Sep 18) Written Homework Assignment #1 due next Friday Oct 10 11:59pm (submit via moodle) please download wireshark! Project #1: will be out today, and Due Friday Oct 3 11:59 pm (submit via moodle)

Please start working on it as soon as possible!

Next Thursday (Sep 25): TAs will• talk about Java/Python Socket Programming

& APIs• Programming project #1• “hands-on” exercises via wireshark• “jeopardy” game!


59

What We Learned Last Time (Sept 11)• Review what is a Computer Network/Internet?

• Packet Switching vs. Circuit Switching• statistical multiplexing

• “Hourglass” Internet Architecture • layering and layered architecture: pros & cons? • notions of protocols, services & interfaces

• protocols: specify formats, rules & actions for “peer” communications

• Four components of packet delays:• propagation delay, transmission delay, processing

delay, and queueing delay • What Internet looks like today? “hierarchical”

structures• notions of “tiers” (tier-1, tier-2, …) of ISPs


What We Learned Last Time (Sept 11) …

Applications and Application Layer Protocols Basics of Building Applications: a networking perspective application processes and inter-process communications API: socket overview “Addressing” processes (“whom is the other party is”)

IP addresses and port numbers What transport services to use?

TCP and UDP Application Structures

client-server - data centers, cloud services peer-to-peer

Case studies: applications/application protocols world wide web and HTTP: transaction-oriented app protocol email and SMTP (& POP, IMAP): session-based app protocol

download“wireshark” software!


61

Web and HTTP Summary

GET /index.html HTTP/1.0 HTTP/1.0200 Document followsContent-type: text/htmlContent-length: 2090 -- blank line --HTML text of the Web page

Client Server

Transaction-oriented (request/reply), use TCP, port 80


62

Email SummaryAlice

Messagetransfer agent(MTA)

Messageuser agent(MUA)

outgoing mail queue

Bob Messagetransfer agent(MTA)

Messageuser agent(MUA)

user mailbox

client

server

SMTP over TCP(RFC 821)

port 25POP3 (RFC 1225)/ IMAP (RFC 1064) for accessing mail

SMTP

Questions?


63


Highlight of Today’s Lecture (Sept 18)

Domain Names & Domain Name System Peer-to-Peer programming paradigm

key problem in p2p programming paradigm?• how to find the other party? What’re the IP of and port

# used by the other party? Unstructured P2P Application Examples:

Napster, Gnutella, KaZaa, BitTorrent, Skype Distributed Hashing Tables (DHT) (very briefly)

A Quick Intro to Multimedia Networking YouTube and Netflix Overview


65

CSci4211: Application Layer 66

Internet Domain Names• Hierarchical: anywhere

from two to possibly infinity

• Examples: afer.cs.umn.edu, lupus.fokus.gmd.de– edu, de: organization type

or country (a “domain”)– umn, fokus: organization

administering the “sub-domain”

– cs, fokus: organization administering the host

– afer, lupus: host name (have IP address)

. (root)

. com . edu. uk

yahoo.comumn.edu

cs.umn.eduitlabs.umn.edu

afer.cs.umn.eduwww.yahoo.com


67

DNS example

Root name server:• may not know

authoritative name server

• may know intermediate name server: who to contact to find authoritative name server

requesting hosthomeboy.aol.com

afer.cs.umn.edu

root name server

local name serverdns.aol.com

1

23

4 5

6

authoritative name serverdns.cs.umn.edu

intermediate name serverdns.umn.edu.

7

8


What We Learned Last Time (Sept 26) …

Peer-to-Peer Applications: Napster, Gnutella, .. A Quick Primer on Multimedia Networking Overview of YouTube and Netflix Architectures


69

Reminders (Oct 2)

• Project #1: Due Tomorrow (Friday Oct 3 11:59 pm).

• Written Homework Assignment #1 due next Friday (Oct 10 11:59pm)


Highlight of Today’s Lecture (Oct 2)

Transport Layer: UDP and TCP Quickly go over UDP: connectionless service

header format: src/dst port #’s; checksum Focus on: TCP – connection-oriented service

header format: Seq. #, Ack#, flags, etc.

TCP Connection Management connection set-up: three-way handshake connection tear-down: “two-army” problem

Reliable Data Transfer the simple Stop-&-Wait (or Alternate Bit) protocol efficiency of protocols: issue w/ Stop-&-Wait Sliding window protocols: Go-Back-N & Selective

Repeat


71


72

Reminders (Oct 9)

Homework Written Assignment #1: Due Friday Oct 10 (11:59 pm).

• Programming Project #2: posted next Thursday, due Monday Nov 3, 11:59pm

• Take-Home Quiz I: hand out Wed Oct 22 (or Tuesday evening Oct 21), due Friday Oct 24!• will provide you with a sample quiz to work on early

next week


What We Learned Last Time (Oct 2)Transport Layer

• multiplexing and de-multiplexing

UDP: connectionless transport service • src/dst port no.’s, checksum

TCP: connection-oriented, reliable service • seq #, ack #, special “flags” (SYN, ACK, FIN, RST) • connection set-up: 3-way handshake• closing a connection: graceful wait

Reliable Data Transfer• simply reliable data transfer protocol : stop & wait

• efficiency of protocols: issue w/ stop-&-wait; pipelining

• sliding window protocols: Go-Back-N & Selective Repeat


74

What We Learned Last Time (Sep 26) ..

TCP: Some Key Issues • How to deal with “lost” packets?

• how to detect a packet is “lost”?• need to set a timer (“retransmission timer”)• what value shall we choose to set the timer?

• what to do with “lost” packets?• retransmit the “lost” packet

• What problem packet retransmission create?• (potential) duplicate packets

• e.g., when a “lost” packet is not actually lost, but takes longer to get delivered, after timer times out

• how to recognize duplicate packets?• sequence # (how many bits shall we use?)

• How to design a simple “reliable” transfer protocol?• stop-&-wait protocol (or alternative bit protocol)

• TCP connection management: set-up & tear-down

75

Question:

a. why “3-way”, not “2-way”?

b. What initial sequence # should client (and server) use?

c. What kind of “state” client and server need to maintain?

TCP 3-Way Hand-Shake

client

SYN, seq=x

server

SYN+ACK, seq=y, ack=x

ACK, seq=x, ack=y

initiate connection

connectionestablished

connection established

SYNreceived

(1st data segment)


76

Connection Setup Error Scenarios

• Importance of (unique) initial seq. no.?– When receiving SYN, how does server know it’s a new

connection request?– When receiving SYN+ACK, how does client know it’s a

legitimate, i.e., a response to its SYN request?

• Dealing with old duplicate (aka “ghost”) packets from old connections (or from malicious users)– If not careful: “TCP Hijacking”

• How to choose unique initial seq. no.?– randomly choose a number (and add to last syn# used)

• Other security concern:– “SYN Flood” -- denial-of-service attack


77

Connection Setup Error Scenarios

• Lost (control) packets– What happen if SYN lost? client vs. server actions– What happen if SYN+ACK lost? client vs. server

actions– What happen if ACK lost? client vs. server actions

• Duplicate (control) packets– What does server do if duplicate SYN received?– What does client do if duplicate SYN+ACK received?– What does server do if duplicate ACK received?


78

3-Way Handshake: Finite State Machine

Client FSM?

info (“state”) maintained at client?

Server FSM?

closed

Upper layer: initiate connection

?

?

sent SYN w/ initial seq =x

SYNsent

connestab’ed

?

?

SYN+ACK received

sent ACK

?

?

?

?



79

Client wants to close connection:

Step 1: client end system sends TCP FIN control segment to server

TCP: Closing ConnectionRemember TCP duplex connection!

client server

FIN

serverclosing

ACK

halfclosed

FINclientclosin

g

halfclosed

Step 2: server receives FIN, replies with ACK. half closed

Server finishes sending data, also ready to close:

Step 4: server sends FIN.

Step 3: client receives FIN.

half closed, wait for server to close


80

Step 5: client receives FIN, replies with ACK.– Enters “timed wait” - will

respond with ACK to received FINs

TCP: Closing Connection (revised)client

FIN

server

ACK

FIN

clientclosin

ghalf

closed

server

closing

halfclose

d

Two Army Problem!

Step 6: server, receives ACK. connection fully closed

full closed

fullclose

d

ACKStep 7: client, timer expires, connection fully closed

tim

ed w

ait ACK FINX timeout

81

TCP Connection Management FSM

TCP clientlifecycle

TCP client lifecycle


82

TCP Connection Management FSM

TCP serverlifecycle

TCP server lifecycle


83

Socket: Conceptual Viewsocket()


84

BSD Socket Programming (connectionless)


85

BSD Socket Programming Flows (connection-oriented)



What We Learned Last Time (Oct 2) …TCP: connection-oriented, reliable (in-order) data delivery -- connection management

Reliable Data Transfer Protocols Stop-&-Wait: a simple reliable data transfer protocol Efficiency of “Stop-&-Wait” reliable transfer protocol More Efficient Reliable Transfer Protocols

Sliding Window-based Go-Back-N Reliable Transfer Protocol Selective Repeat Key Issues: window size & “state maintenance” (to

keep sender & receiver in sync)

87

Simple Reliable Data Transfer Protocol

“Stop-and-Wait” Protocol – also called Alternating Bit Protocol

• Sender: – i) send data segment (n bytes) w/ seq =x

• buffer data segment, set timer, retransmit if time out – ii) wait for ACK w/ack = x+n; if received, set x:=x+n, go

to i) • retransmit if ACK w/ “incorrect” ack no. received

• Receiver:– i) expect data segment w/ seq =x; if received, send ACK

w/ ack=x+n, set x:=x+n, go to i) • if data segment w/ “incorrect” seq no received, discard data

segment, and retransmit ACK.


CSci4211: Transport Layer: Part II

88

• Can’t keep the pipe full– Utilization is low when bandwidth-delay product (R x RTT)is large!

Sender Receiver

data (L bytes)

ACK

first packet bit transmitted, t = 0

RTT

first packet bit arrives

ACK arrives, send next packet, t =

RTT + L / R

Problem with Stop & Wait Protocol


89

Stop & Wait: Performance Analysis

Example: 1 Gbps connection, 15 ms end-end prop. delay, data segment size: 1 KB = 8Kb

– U sender: utilization, i.e., fraction of time sender busy sending– 1KB data segment every 30 msec (round trip time) --> 0.027% x 1 Gbps = 33kB/sec throughput over 1 Gbps link

00027.0008.30

008.

*/

/

LRRTT

L

RLRTT

RLsenderU

ms 008.0s108

b/s 10

kb 8

bps) rate,ion (transmiss

bits)in length (packet

6

9transmit

R

LT

Moral of story: network protocol limits use of physical resources!


90

Pipelined ProtocolsPipelining: sender allows multiple, “in-flight”,

yet-to-be-acknowledged data segments– range of sequence numbers must be increased– buffering at sender and/or receiver

• Two generic forms of pipelined protocols: Go-Back-N and Selective Repeat


91

Pipelining: Increased Utilization

first packet bit transmitted, t = 0

sender receiver

RTT

last bit transmitted, t = L / R

first packet bit arriveslast packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

last bit of 2nd packet arrives, send ACKlast bit of 3rd packet arrives, send ACK

U sender

= .024

30.008 = 0.0008

microseconds

3 * L / R

RTT + L / R =

Increase utilizationby a factor of 3!



92

Reliable Data Transfer Protocols Reliable Data Transfer Protocols• basic mechanisms: seq. no, ACK, timer, retransmission• Simplest protocol: Stop-&-Wait

• To ensure correct operations of the protocol: at least 1-bit (0 or 1) needed for seq. no. (why?)

• More efficient reliable data transfer protocols• What’s the problem with Stop & Wait protocol?• Sliding window protocols: Go-Back-N and Selective Repeat

• concept of ”sliding window”• sender algorithm:

• when to retransmit, when to send new packets? when to move window forward?

• receiver algorithm: • when/what to acknowledge? when to move window

forward? when to buffer packets, and when to pass to upper layer?

• relationship between window size & seq. no. space


93

Go-Back-N: Basic IdeasSender:

• Packets transmitted continually (when available) without waiting for ACK, up to N outstanding, unACK’ed packets

• A logically different timer associated with each “in-flight” (i.e., unACK’ed) packet

• timeout(n): retransmit pkt n and all higher seq # pkts in windowReceiver:

• ACK packet if corrected received and in-order, pass to higher layer, NACK or ignore corrupted or out-of-order packets

• “cumulative” ACK: if multiple packets received corrected and in-order, send only one ACK with ack= next expected seq no.


94

Go-Back-N: Sliding Windows Sender:• “window” of up to N, consecutive unack’ed pkts allowed• send_base: first sent but unACKed pkt, move forward when ACK’ed

Receiver:• rcv_base: keep track of next expected seq no, move forward

when next in-order (i.e., w/ expected seq no) pkt received

may be received (and can be buffered, but not ACK’ed)

expected, not received yet

rcv_base


95

GBN in Action


96

Selective Repeat

• As in Go-Back-N– Packet sent when available up to window limit

• Unlike Go-Back-N– Out-of-order (but otherwise correct) is ACKed– Receiver: buffer out-of-order pkts, no “cumulative”

ACKs– Sender: on timeout of packet k, retransmit just pkt k

• Comments– Can require more receiver buffering than Go-Back-N– More complicated buffer management by both sides– Save bandwidth

• no need to retransmit correctly received packets


97

Selective Repeat: Sliding Windows


98

Selective Repeat in Action


99

Seqno Space and Window Size

• How big the sliding window can be?– MAXSEQNO: number of available sequence

numbers– Under Go-Back-N?

• MAXSEQNO will not work, why?– What about Selective-Repeat?


100

Selective Repeat: Dilemma

Example: • seq #’s: 0, 1, 2, 3• window size=3

• receiver sees no difference in two scenarios!

• incorrectly passes duplicate data as new in (a)

Q: what relationship between seq # size and window size?


Highlight of Remainder of Today’s Lecture (Oct 9)

We’ve spent a lot of recap what we have learned last time

• TCP connection management• Reliable data transfer protocols

Back to TCP: TCP primarily uses Go-Back-N

TCP Round-Trip-Time (RTT) Estimation TCP Flow Congestion

• receive-advertised window TCP Congestion Control

two phases: slow start & congestion avoidance


102


103

Announcement & Reminder (Oct 16)

• Programming Project #2: posted on the class website, due Monday Nov 3, 11:59pm• please take a look at it – if have questions, send email

to [email protected]• Please start working on it soon

• Take-Home Quiz I: hand out Wed Oct 22 (or Tuesday evening Oct 21), due Friday Oct 24!• sample quiz sent to the mailing list early this week• sample solution will be sent out tomorrow


104

What We Learned Last Time Before

Transport Layer Summary• Connectionless (UDP) vs. Connection-Oriented

(TCP) • port numbers & checksum

TCP: connection-oriented, reliable data transfer protocol -- key functions• connection management (set-up & tear-down)• reliable data transfer• flow control • Congestion control

• TCP Connection Management• TCP 3-way handshake connection setup

• why 3-way? why unique initial sequence numbers?• closing a connection: key issues and how TCP does it

• key issues: lost/delayed packets, old “ghost” packets


105

What We Have Learned Last Time• Review reliable data transfer protocols:

• stop-&-wait, & efficiency of protocols • Go-back-N, selective repeat: window size & seq. no. space

• round trip time estimation and TCP timeoutInterval• TCP Flow Control: avoid overwhelm receiver

• RecvWin: receiver advertises available buffer space• Congestion Control: avoid overwhelm network routers

• two approaches: end-to-end implicit vs. router-assisted

• TCP congestion control: end-to-end implicit• issue 1: how to determine network is congested• issue 2: how to adjust sending rate to avoid congestion• TCP congestion control mechanisms

• two parameters: ConWin, threshold• slow start • congestion avoidance: AIMD, duplicate ACKs,

timeout• fast retransmit/fast recovery


106

TCP Flow Control: How It Works

(Suppose TCP receiver discards out-of-order segments)

• spare room in buffer= RcvWindow= RcvBuffer-[LastByteRcvd -

LastByteRead]

• Rcvr advertises spare room by including value of RcvWindow in segments

• Sender limits unACKed data to RcvWindow– guarantees receive buffer

doesn’t overflow


107

TCP Slowstart

• exponential increase (per RTT) in window size (not so slow!)

• loss event: timeout (Tahoe TCP) and/or or three duplicate ACKs (Reno TCP)

initialize: Congwin = 1for (each segment ACKed) Congwin++until (loss event OR CongWin > threshold)

Slowstart algorithm Host A

one segment

RT

T

Host B

time

two segments

four segments


108

TCP Congestion Avoidance

/* slowstart is over */ /* Congwin > threshold */Until (loss event) { every W segments ACKed: Congwin++ }Threshold: = Congwin/2Congwin = 1perform slowstart

Congestion Avoidance


109

TCP Congestion Control: Recap

• end-end control (no network assistance)

• sender limits transmission: LastByteSent-LastByteAcked CongWin• Roughly,

• CongWin is dynamic, function of perceived network congestion

How does sender perceive congestion?

• loss event = timeout or 3 duplicate ACKs

• TCP sender reduces rate (CongWin) after loss event

three mechanisms:– AIMD– slow start– conservative after

timeout events

rate = CongWin

RTT Bytes/sec


110

TCP Congestion Control: Recap (cont’d)

• When CongWin is below threshold, sender in slow-start phase, window grows exponentially.

• When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly.

• When a triple duplicate ACKs occurs, threshold set to CongWin/2, and CongWin set to threshold.

• When timeout occurs, threshold set to CongWin/2, and CongWin is set to 1 MSS.


111

Highlight of Today’s Lecture (Oct 16)• Network Layer !

• basic functions: addressing, routing & forwarding• IP addresses and IP prefixes:

• 32 bits, dot notation• two-level hierarchy: network part & host part

• how to determine network part?• “classful” addressing scheme• “classless” addressing scheme: CIDR

• Where do you get an IP address?• manual config., or via DHCP -- how does it

work?• Where do you get an IP prefix?• Network Service Model: IP Datagram service

• destination-based, hop-by-hop, forwarding• forwarding (routing) table • table look-up: longest prefix matching


112

Routing & Forwarding:Logical View of a Router

A

ED

CB

F

22

13

1

1

2

53

5


113


114

Reminders (Oct 24)• Take-Home Quiz I: due tomorrow 5:30pm • please work individually, no discussion!

• We will answer any clarifying questions in the beginning of today’s lecture

• After you have finished your Take-Home Quiz I, please start working on your Project #2

• come to TAs’ office hours if you need help!

• Made a few changes and updates to the lecture notes (csci4211-network-part1.ppt), please re-download them!


115

What We Learned Last Time (Oct 17) …

• Network Layer• basic functions: addressing, routing & forwarding• IP addresses and IP prefixes:

• 32 bits, dot notation• two-level hierarchy: network part & host part

• how to determine network part?• “classful” addressing scheme• “classless” addressing scheme: CIDR

• Where do you get an IP address?• manual config., or via DHCP -- how does it

work?• Where do you get an IP prefix?• Network Service Model: IP Datagram service

• destination-based, hop-by-hop, forwarding• forwarding (routing) table • table look-up: longest prefix matching


116

IP Addressing: Network vs. Host

• Two-level hierarchy – network part (high order

bits)– host part (low order bits)

• What’s a network ? (from IP address perspective)

– device interfaces with same network part of IP address

– can physically reach each other without intervening router

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

multi-accessLAN

point-to-point link


117

“Classful” IP Addressing

32 bits

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

77 15 23 31

• Disadvantage: inefficient use of address space, address space exhaustion

• e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network


118

Classless Addressing: CIDR CIDR: Classless InterDomain Routing• Network portion of address is of arbitrary

length• Addresses allocated in contiguous blocks

– Number of addresses assigned always power of 2• Address format: a.b.c.d/x

– x is number of bits in network portion of address

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23


119

Datagram vs. Virtual Circuit • Objective of both: move packets through routers from

source to destination• Datagram Model:

– Routing: determine next hop to each destination a priori

– Forwarding: destination address in packet header, used at each hop to look up for next hop

• routes may change during “session”– analogy: driving, asking directions at every gas station,

or based on the road signs at every turn • Virtual Circuit Model (will be studied today!):

– Routing: determine a path from source to each destination

– “Call” Set-up: fixed path (“virtual circuit”) set up at “call” setup time, remains fixed thru “call”

– Data Forwarding: each packet carries “tag” or “label” (virtual circuit id, VCI), which determines next hop

– routers maintain ”per-call” state


120

Datagram Networks: the Internet model

• no call setup at network layer• routers: no state about end-to-end connections

– no network-level concept of “connection”• packets forwarded using destination host

address– packets between same source-dest pair may take

different paths, when intermediate routes change!

application

transportnetworkdata linkphysical

application


1. Send data 2. Receive data

CSci4211: Network Layer: Part I

121

IP Forwarding Table4 billion possible entries! (in reality, far less, but can still have millions of “routes”)

forwarding table entry format destination network next-hop (IP address) link interface (1st IP address , network mask ) 11001000 00010111 00010000 00000000, 200.23.16.1 0 11111111 11111111 11111000 00000000

11001000 00010111 00011000 00000000, - (direct) 1 11111111 11111111 11111111 00000000

11001000 00010111 00011001 00000000, 200.23.25.6 2 11111111 11111111 11111000 00000000

otherwise 128.30.0.1 3

Network Layer4-122

Route aggregation: Shrinking the forwarding table

“Send me anythingwith addresses beginning 200.23.0.0/20”

200.23.2.0/23

200.23.4.0/23

200.23.14.0/23

UMN

Organization 0

CSE DepartmentInternet

Organization 1

200.23.6.0/23Organization 2

...

...Port 1

Port 0

Port 7


Network Layer4-123

Route aggregation with more specific routes

UMN-2 has a more specific route to CSE department

“Send me anythingwith addresses beginning 200.23.0.0/20”

200.23.2.0/23

200.23.14.0/23

UMN

Organization 0

CSE DepartmentInternet

UMN-FAST “Send me anythingwith addresses beginning 200.23.14.0/23”

200.23.4.0/23Organization 2

...

...


124

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?


124


125

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 interface 0

interface 1

But not interface 2, the 3rd entry is also a match, but shorter!


126

Announcements & Reminders (Oct 31)• Homework Assignment #2 posted

Due due Friday Nov 22 11:59pm • 7 big problems + 1 bonus problem

• Project #2 : due Monday 11:59pm • if you have troubles w/ the project, ask TAs for help!

• Re-scheduling Thursday Dec 12’s optional lecture

• Dec 12 is “study/reading” day, can’t have class!• Reschedule to either Mon (Dec 9, preferred), Tue (Dec

10) or Wed (Dec 11) at the same time (6:30pm to 9pm)

• the lecture will be in Mech Eng. Building• topics of lecture: catch-up, wrap up & summary, final

review, Q&A, etc. • set up a doodle poll, please let us which day works

best


127

Take Home Quiz 1 Stats & Other Tidbits

• Take-Home Quiz I: graded (rather leniently)!• statistics: Max. 100, Median 93, Mean 88.5, Min. 45.5

• Please feel free to talk to us if you have questions, especially if you didn’t do well

• We’ll contact the students who did below 60, also reach out to students who are below 70.

• please check “Important Dates” for upcoming deadlines!


128

What We Learned Last Time (Oct 24)• Review IP addressing & datagram forwarding

• network prefix, dest-based forwarding, longest prefix matching

• Virtual Circuit• connection-oriented network service

• between hosts (or ingress/egress routers)• need to connect set-up (“signaling”) before any data can

be transmitted• connection set-up (signaling) process:

• given a route/path, select (locally unique, outgoing) VCI, & set up VCI translation table at each router

• data forwarding: • based on VCI carried in packets, perform VCI translation

• router architecture: interconnect fabric, in/output processing • IP datagram format: TTL, datagram id, offset, flags, …• MTU & IP fragmentation & reassembly


129

Virtual Circuit: Signaling Protocols

• used to setup, maintain teardown VC• used in ATM, frame-relay, X.25• used in part of today’s Internet: Multi-Protocol Label

Switching (MPLS) operated at “layer 2+1/2” (between data link layer and network layer) for “traffic engineering” purpose

application


application


1. Initiate call 2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data


130

During data packet forwarding phase, input VCI is used to look up the table, and is “swapped” w/ output VCI (VCI translation, or “label swapping”)

VCI translation table (aka “forwarding table”), built at call set-up phase

1

2

13

1

2 2

1

four “calls” going thru the router, each entry corresponding one call

green call

purple call

blue call

orange call


131

Virtual Circuit: Example

0

13

2

0

1 3

2

0

13

2

511

4

7

Router 3

Host B

Router 2

Host A

Router 1

Router 4

“call” from host A to host B along path: host A router 1 router 2 router 3 host B

• each router along path maintains an entry for the call in its VCI translation table

• the entries piece together a “logical connection” for the call

• Exercise: write down the VCI translation table entry for the call at each router


132

IP Datagram Format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgs fragment offset

upper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

• 20 bytes of TCP• 20 bytes of IP• = 40 bytes +

app layer overhead


133

IP Fragmentation and Reassembly: Exp

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example• 4000 byte datagram• MTU = 1500 bytes

• offset in the second fragment:

185x8=1480 (why not 1500 bytes =length?)• offset in the third

fragment: 370x8=2960

Except for last fragment, IP fragment payload size (i.e., excluding IP header) must be multiple of 8!


134

Announcements & Reminders (Nov 7)• Project #2: due this past Wednesday 11:59pm • But if you haven’t finished it yet, please continue

working on it & submit ASAP

• Please start working on your homework assignment #2

• Project # 3 description posted on the class website

• Please take a look at it, and TAs will discuss the project in more details on Nov 14’s class

• Re-scheduling Thursday Dec 12’s optional lecture

• please answer the Doodle poll if u haven’t done it yet• based on current poll, mostly likely will be held on Mon

Dec 6 at the same time (6:30pm to 9pm)• may hold another one on Tue, or video-tape the lecture?


135

What We Learned Last Time (Oct 31)• ICMP; briefly, NAT & IPv6• Network Layer Routing

• basic issues: exchange routing info & compute routes• Two Basic (Distributed) Routing Algorithms

• Link State vs. Distance Vector• How does Link State (LS) work?• How does Distance Vector (DV) work?

• Today:• Issue with DV: county-to-infinity & possible hacks• Routing in Internet

• Intra-domain routing protocols: RIP, OSPF • Inter-domain routing protocol: BGP

• Data Link Layer:• MAC address & relation w/ IP: ARP• local area networks & bridging/switching

algorithm

CSci4211: Network Layer: Part II

136

Link State Algorithm

• Basic idea: Distribute to all routers– Topology of the network

• Cost of each link in the network

• Each router independently computes optimal paths– From itself to every destination– Routes are guaranteed to be loop free if

• Each router sees the same cost for each link• Uses the same algorithm to compute the best

path


137

Topology Dissemination

• Each router creates a set of link state packets (LSPs)– Describing its links to neighbors– LSP contains

• Router id, neighbor’s id, and cost to its neighbor

• Copies of LSPs are distributed to all routers– Using controlled flooding

• Each router maintains a topology database– Database containing all LSPs


138

A

ED

CB

F

2

2

13

1

1

2

53

5

Topology Database: Example

link state database


139

Constructing Routing Table:Dijkstra’s Algorithm

• Given the network topology– How to compute shortest path to each destination?

• Some notation– X: source node– N: set of nodes to which shortest paths are known so

far• N is initially empty

– D(V): cost of known shortest path from source X– C(U,V): cost of link U to V

• C(U,V) = if not neighbors

CSci4211: Network Layer: Part II 140

Algorithm (at Node X)

• Initialization– N = {X}– For all nodes V

• If V adjacent to X, D(V) = C(X,V) else D(V) =

• Loop– Find U not in N such that D(U) is smallest– Add U into set N– Update D(V) for all V not in N

• D(V) = min{D(V), D(U) + C(U,V)}– Until all nodes in N


Dijkstra’s Algorithm: Example

Step012345

start NA

ADADE

ADEBADEBC

ADEBCF

D(B),p(B)2,A2,A2,A

D(C),p(C)5,A4,D3,E3,E

D(D),p(D)1,A

D(E),p(E)infinity

2,D

D(F),p(F)infinityinfinity

4,E4,E4,E

A

ED

CB

F

2

2

13

1

1

2

53

5


142

A

ED

CB

F

2

2

13

1

1

2

5

3

5

Routing Table Computation

dest next

B BC DD DE DF D


143

Distance Vector Routing

• A router tells neighbors its distance to every router– Communication between neighbors only

• Based on Bellman-Ford algorithm– Computes “shortest paths”

• Each router maintains a distance table– A row for each possible destination– A column for each neighbor

• DX(Y,Z) : distance from X to Y via Z• Exchanges distance vector with neighbors

– Distance vector: current least cost to each destination


144

Distance Table: Example

A

E D

CB7

8

1

2

1

2

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

des

tin

atio

n


145

Distance Vector Routing Algorithm

iterative:• continues until no

nodes exchange info.• self-terminating: no

“signal” to stop

asynchronous:• nodes need not

exchange info/iterate in lock step!

distributed:• each node talks only

with directly-attached neighbors

Distance Table data structure

• each node has its own• row for each possible destination• column for each directly-attached

neighbor to node• example: in node X, for dest. Y via

neighbor Z:

D (Y,Z)X

distance from X toY, via Z as next hop

c(X,Z) + min {D (Y,w)}Z

w

=

=


Distance Vector Routing: Overview

Iterative, asynchronous: each iteration caused by:

• local link cost change • message from neighbor:

its least cost path change from neighbor

Distributed:• each node notifies

neighbors only when its least cost path to any destination changes– neighbors then notify

their neighbors if necessary

wait for (change in local link cost or msg from neighbor)

recompute distance table

if least cost path to any

dest has changed, notify neighbors

Each node:


147

Distance Vector Algorithm: Example

X Z12

7

Y

D (Y,Z)X

c(X,Z) + min {D (Y,w)}w=

= 7+1 = 8

Z

D (Z,Y)X

c(X,Y) + min {D (Z,w)}w=

= 2+1 = 3

Y


Distance Vector Algorithm: Example

X Z12

7

Y


149

Announcement & Reminder (Nov 21)• Project #3 due tomorrow (Friday)

• Hw #2 due: Monday Nov 25 11:59pm

• Take-Home Quiz II: email to you Mon evening Nov 25, due Mon Dec 2

• similar to (& easier than) Hw #2

• Extra office hours: Tuesday Nov 26: 2:30-4pm

Two Optional Lectures (Mon Dec 9 & Tue Dec 10)

• 6:30pm - 9pm; Venue: TBA (ME 108?)


150

What We Learned Last Times (Nov 7 & 14)• Wrap up basic routing algorithms

• count-to-infinity problem in DV• Routing in the Internet

• two-level hierarchy; notion of AS• intra-domain: RIP, OSPF, IS-IS • inter-domain: BGP, path-vector, policy-based

• AS relationship: provider-customer; peering• routing policies: e.g., prefer customer routes

• Data Link Layer• basic functions: framing, access control, etc.

• MAC addresses and ARP• What is MAC address? How it differs from IP address• Why do we need ARP? How does ARP work?• TAs on Nov 14: layer-2 switches & layer-2

forwarding• switch “self-learning” algorithm


151


152


153

Announcement & Reminder (Dec 5)• Project #4 due tomorrow (Friday)

• Optional Lectures (Mon Dec 9 & Tue Dec 10)• 6:30pm - 9pm; Venue: Mech Eng. Bld: ME 18 • same class to accommodate your schedule• wrap up, review, & Q&A, etc.

• Extra office hours: • Tuesday Dec 17: 2:30-4:00pm• Thursday Dec 19: 4:00-6:00pm

• Final Exam: • Thursday Dec 19: 6:30pm to 8:30pm, venue:

here!• similar to Quiz II, but comprehensive (cover

everything)• open-book, open-note, open-Internet


154


155


156


157


158

What We Learned Last Time (Nov 21)• Review & Finish up (layer-2) switches/bridges

• how does a switch build its switching table? -- self-learning algo: use src MAC; forward using dst MAC• loops in topology and “broadcast storm” -- spanning tree algorithm: rooted at a particular switch

• Media Access Control: shared media -- issues and difficulties ?

• Taxonomy of MAC protocols• Random access control:

• ALOHA and Slotted ALOHA: collision? efficiency?• CSMA: listen before talking:

-- non-persistent vs. p-persistent• CSMA/CD: collision detection – listen while talking

-- maximum time to detect collision?• Ethernet and its Evolution: 10-base, 100-base, Gigabit,

… -- why minimum frame size constraint?• “Taking Turns” Protocols: example – Token Ring• Protocol Efficiency: Heavy vs. light loads? Other Issues?


159

Questions?

CSci4211: Key Notes 160

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