CSci4211: Weekly Summary1 What We Learned Last Time (Sept 9) What is a Computer Network/Internet? services components and their functions Compare & Contrast:

CSci4211: Weekly Summary

1

What We Learned Last Time (Sept 9)

• What is a Computer Network/Internet? • services• components and their functions

Compare & Contrast: Telephone Networks & Postal Service• Packet Switching vs. Circuit Switching

• statistical multiplexing• compare and contrast: TDM, FDM

• Fundamental Issues and Problems in Networking• Layered Architecture and Protocol

CSci4211: Key Notes 2

Summary of Introduction 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

Packet switching vs. circuit switching: packets, packet switching & statistical multiplexing delay, losses and congestion vs. call blocking

Key functions: naming, addressing, routing & forwarding Networks are distributed & complex systems!

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

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

….. and Attacks !!! What today’s Internet looks like? economics & policies


Stuff We Didn’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 Delay, Loss, and Throughput

various delays: transmission delay, propagation delay, processing delay, queuing delay, ..


A Simplified Illustration of Internet


5

Announcements (Sep 23)

• Project #1: out today, and Due Friday Oct 8 (11:59 pm).

Please start working on it as soon as possible!

• Written Homework Assignment #1 will be posted by Sep 30, due Thu Oct 14 (6:30pm)

• P.S.: Prof. Zhang will be on a oversea trip from Sep 25-Oct 12. No office hours during this period (may still have intermittent Internet access).


What We Learned Last Time (Sept 16)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

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

Domain Name System (DNS)

download“wireshark” software!


Highlight of Today’s Lecture (Sept 23)

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?• especially, if the other party is behind NAT

Unstructured P2P Application Examples: BitTorrent Gaaza, Skype, …

Distributed Hashing Tables (DHT) Java Socket Programming Project #1 Overview


8

Announcement (Sep 16)

• Please sign up to the class google group!

• Advanced Notice: Programming Project # 1 will be posted next week!

• Please start reading about (Java) socket programming in the textbook, and try the examples!


9

What We Learned Last Time (Sept 12)

• Application Layer• application requirements & transport services• application programming paradigms

• client-server vs. peer to peer• Case Study 1: Web & HTTP

• web client vs. web server (TCP, port 80)• HTTP: request & reply

• Case Study 2: Email• SMTP: “interactive” • Mail Access Protocols: POP, IMAP

• DNS: why do we need DNS?• DNS server architecture & query process


10

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


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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


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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


13

Announcement & Reminder (Oct 3)

• Homework Assignment #2: Due Oct 17• check the class website

Reminder:• Start working on your project #1, if you haven’t yet!

http://www.itlabs.umn.edu/classes/Fall-2002/csci4211/


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What We Learned Last Time (Sept 19 & Sept 26)

• 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)

• simply reliable data transfer protocol • connection set-up: 3-way handshake

• BSD Socket Programming• create socket, bind socket, close socket• connect, listen accept• read/write, sendto/recvfrom, …


15

Reminders (Oct 10)

• Homework Assignment #2: Due Oct 17• Project #1: Due Oct 17

• Mid-Term: Oct 24 (in two weeks !!!)• Start reviewing the material you have learned so far:

•basic network concepts•application layer•transport layer

http://www.itlabs.umn.edu/classes/Fall-2002/csci4211/homeworks/two/hw2.html


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What We Learned Last Time (Oct 3)

• TCP Connection Management•Review TCP 3-way handshake connection setup• Closing a connection: key issues and how TCP does it

• More efficient reliable data transfer protocols• What’s the problem with Stop&Wait protocol?• 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

• round trip time estimation and TCP timeoutInterval


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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


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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


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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 window

Receiver:• 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.


20

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


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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


22

GBN in Action


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Selective Repeat: Sliding Windows


24

Selective Repeat in Action


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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?


26

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?


27

Reminders (Oct 17)

• Homework Assignment #2: Due today midnight• Project #1: Due today midnight

• Mid-Term: Oct 24 (7:30 – 9pm next Thursday !!!)

• 5 “big questions”• cover the material up to Oct 10 (inclusive)

•basic network concepts•application layer (exclude BSD/Java socket APL)

•transport layer •network layer (IP addressing, VC vs. Datagram)


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• Finishing up TCP and Transport Layer • 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


29

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


30

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


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TCP Congestion Avoidance

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

Congestion Avoidance


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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


33

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.


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What We Learned Last Time (Oct 10) …

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

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

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

• Network Service Models• Virtual Circuits: how it works?

• VC set-up & VCI translation table• data forwarding

• Datagram: how it works?


35

Routing & Forwarding:Logical View of a Router

A

ED

CB

F

22

13

1

1

2

53

5


36

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


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“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


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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


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Virtual Circuit vs. Datagram• 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: – 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


40

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


41

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


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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


43

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


44


45

Midterm & Other Tidbits (Oct 28)

• Midterm Exams:• graded• statistics:

• Max. 90, Median 87, Mean 73, Min. 40• talk to us if you have questions, especially if you didn’t do well

• Project 2 extended to Wed Nov 3, 11:59pm

• please check “Important Dates” for upcoming deadlines!


46

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


47

Stop & Wait: Performance AnalysisExample:

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!


48

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!


49

Questions?


50

Today’s Topics (Oct 28)• IP Addressing and IP datagram service model:

• What’s an IP address?• IP addressing scheme:

• class-based vs. classless (CIDR) •How do you get one? Manually config., DHCP• How does an ISP get their address blocks?

• IP datagram forwarding: host vs. routers • longest prefix matching

• IP datagram format, IP fragmentation and re-assembly; ICMP

• Router architecture: a quick overview• If time permits, overview of routing


51


52

Announcement & Reminder (Nov 21)

• Hw #3 due today at midnight• Hw #4 is posted on class website

• Project II “milestone” report (optional)• email TA ([email protected]) what you have done, and what still need to be done; issues & problems you are facing, etc.

mailto:[email protected]


53


• Network Layer Routing• basic issues

• Two Basic Routing Algorithms• Link State vs. Distance Vector• How does LS work?

•Today:•DV•Routing in Internet

•RIP, OSPF and BGP


54

What We Learned Last Time (Nov 14)

• 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?

• Media Access Control •Shared media: issues and difficulties•Taxonomy of MAC protocols• Random access control:

•ALOHA and Slotted ALOHA


55

Announcement & Reminder (Dec 5)

• Project II due mid-night today•sign up for demo slots if haven’t done so

• Hw #4 due mid-night next Tuesday (Dec 10)

• Next Thursday’s class is last class of semester!

• We’ll finish up whatever leftover today• Review material covered in entire course, focusing on material after midterm• Go over a sample final exam (will posted on web)


56

What We Learned Last Time (Nov 21)

• Random Access Control:•CSMA: nonpersistent vs. p-persistent• CSMA/CD•What are they? How do they work?

• Adaptive Controlled Access:• token passing vs. polling

• Ethernet•MAC: 1-persistent CSMA/CD, binary random backoff• collision domain, bit-time and min. frame size • 10BaseT, 100 BaseT (Fast Ethernet), Gigabit Ethernet

• Bridge: what does a bridge do?• transparent (“plug & play”)• bridge forwarding table: self-learning

Documents

CSci4211: Weekly Summary1 What We Learned Last Time (Sept 9) What is a Computer Network/Internet? services components and their functions Compare & Contrast: