lecture 17: final review€¦ · cse 123: lecture 17: final review 2 3 final mechanics bulk of the...

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CSE 123: Computer Networks

Chris Kanich

Lecture 17:

Final Review

Last class!!!

Overview

Signaling

Framing

Error detection

Reliable transmission

Flow control

Bridging/Switching

Congestion control

Routing

QoS

Wireless

2 CSE 123: Lecture 17: Final Review

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Final Mechanics

Bulk of the final covers material after midterm

Routing, QoS, Wireless

Some material on signaing, framing, transport, etc.

MAC, ARQ, TCP, IP

Based upon lecture material, homeworks, and project

May be a question regarding the projects

Closed book, one page of notes

Expect similar style to midterm, just longer

CSE 123: Lecture 17: Final Review

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TCP/IP Protocol Stack

HTTP

TCP

I

P

Ethernet interface

HTTP

TCP

I

P

Ethernet interface

I

P

I

P

Ethernet interface

Ethernet interface

SONET interface

SONET interface

host host

router router

CSE 123: Lecture 17: Final Review 4

Application Layer

Transport Layer

Network Layer

Link Layer

Signals and Channels

A signal is some form of energy (light, voltage, etc)

Varies with time (on/off, high/low, etc.)

Can be continuous or discrete

We assume it is periodic with a fixed frequency

A channel is a physical medium that conveys energy

Any real channel will distort the input signal as it does so

How it distorts the signal depends on the signal


Channel Properties

Bandwidth-limited

Range of frequencies the channel will transmit

Means the channel is slow to react to change in signal

Power attenuates over distance

Signal gets softer (harder to “hear”) the further it travels

Different frequencies have different response (distortion)

Background noise or interference

May add or subtract from original signal

Different physical characteristics

Point-to-point vs. shared media

Very different price points to deploy


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Signaling

Digital modulation

FSK, ASK, PSK

Dealing with noise

Shannon’s law

Sampling at the receiver

Intersymbol Inteference: Nyquist Limit

Synchronous vs. Asynchronous coding

Clock recovery

NRZ, Manchester, 4B/5B


(Data) Link Layer

Framing

Break stream of bits up into discrete chunks

Error handling

Detect and/or correct errors in received frames

Media access

Arbitrate which nodes can send frames at any point in time

Not always necessary; e.g. point-to-point duplex links

Multiplexing

Determine appropriate destination for a given frame

Also not always required; again, point-to-point


Framing

Framing determines when payload starts/stops

Lots of different ways to do it, various efficiencies

Sentinel-based framing requires stuffing

Increases the size of the packet

Alternatives include fixed size frames


Payload Header Trailer

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Error Detection

Error handling through redundancy

Adding extra bits to the frame

Hamming Distance

When we can detect

When we can correct

Checksum

Cyclic Remainder Check (CRC)

Reliable Transmission

Automatic Repeat Request (ARQ)

Acknowledgements (ACKs) and timeouts

Stop-and-Wait

Sliding Window

Forward Error Correction


Sliding Window

Single mechanism that supports:

Multiple outstanding packets

Reliable delivery

In-order delivery

Flow control

At the core of all modern ARQ protocols

Go-Back-N is a special case

Receive window size of one


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Media Access Control

Methods to share physical media: multiple access

Fixed partitioning

Random access

Channelizing mechanisms

Contention-based mechanisms

Aloha

Ethernet


Partitioning Visualization

FDMA

TDMA

CDMA

po

wer

po

wer

pow

er

Courtesy Takashi Inoue


Aloha transmits even if another host is transmitting

Thus guaranteeing a collision

Instead, listen first to make sure channel is idle

Useful only if channel is frequently idle

Why?

If nodes can detect collisions, abort! Requires a minimum frame size (“acquiring the medium”)

Requires a full duplex channel

Binary exponential back-off balances delay w/load


Carrier Sense (CSMA) + CD

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Transport Layer

Provides process naming/demultiplexing

Port numbers

Two main protocols in use on the Internet

User Datagram Protocol (UDP)

» Unreliable, datagram service

Transport Control Protocol (TCP)

» Reliable byte-stream

» Requires connection establishment/three-way handshake


Transmission Control Protocol

Reliable bi-directional bytestream between processes

Uses a sliding window protocol for efficient transfer

Connection-oriented

Conversation between two endpoints with beginning and end

Flow control

Prevents sender from over-running receiver buffers

Congestion control

Prevents sender from over-running network capacity


How fast should a sending host transmit data?

Not to fast, not to slow, just right…

Should not be faster than the sender’s share

Bandwidth allocation

Should not be faster than the network can process

Congestion control

Congestion control & bandwidth allocation are

separate ideas, but frequently combined


Congestion Control

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Window-based congestion control

Unified congestion control and flow control mechanism

rwin: advertised flow control window from receiver

cwnd: congestion control window

» Estimate of how much outstanding data network can deliver in a

round-trip time

Sender can only send MIN(rwin,cwnd) at any time

Complicated with slow start and fast recovery

Ramp up quickly

Avoid backing all the way down on isolated loss events


TCP’s Algorithm

Hubs/Repeaters

Physical layer device

One “port” for each LAN

Repeat received bits on one port out all other ports

LAN1 Hub LAN2

LA

N3


Store and forward device

Data-link layer device

Buffers entire packet and then rebroadcasts it on other ports

Creates separate collision domains

Uses CSMA/CD for access to each LAN (acts like a host)

Improves throughput

Some bridges can learn topology

Spanning Tree Algorithm


Bridges

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internetworking

Switching still only moves frames on common link layer

MAC addresses are unique but flat

Routers forward packets from source to destination

May cross many separate networks along the way

All packets use a common Internet Protocol

Any underlying data link protocol

Any higher layer transport protocol


Routers

A router is a store-and-forward device

Routers are connected to multiple networks

On each network, looks just like another host

A lot like a switch, except at the network layer

Must be explicitly addressed by incoming frames

Not at all like a switch, which is transparent

Removes link-layer header, parses IP header

Looks up next hop, forwards on appropriate network

Each router need only get one step closer to destination


IP Addresses

32-bits in an IPv4 address

Dotted decimal format a.b.c.d

Each represent 8 bits of address

Hierarchical: Network part and host part

E.g. IP address 128.54.70.238

128.54 refers to the UCSD campus network

70.238 refers to the host ieng6.ucsd.edu

Subnetting/CIDR aggregation

Network mask/prefix


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Layers of Identifiers

Host name (e.g., www.ucsd.edu)

Used by humans to specify host of interest

Unique, selected by host administrator

Hierarchical, variable-length string of alphanumeric carachters

IP address (e.g., 128.54.70.238)

Used by routers to forward packets

Unique, topologically meaningful locator

Hierarchical namespace of 32 bits

MAC address (e.g., 58:B0:35:F2:3C:D9)

Used by network adaptors to identify interesting frames

Unique, hard-coded identifier burned into network adaptor

Flat name space (of 48 bits in Ethernet)


Naming Protocols

Domain Name System Distributed, hierarchical database

Distributed collection of servers

Caching to improve performance

IP to MAC Address mapping

Dynamic Host Configuration Protocol (DHCP)

Address Resolution Protocol (ARP)


How to choose best path?

Defining “best” can be slippery

How to scale to millions of users?

Minimize control messages and routing table size

How to adapt to failures or changes?

Node and link failures, plus message loss


Routing

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Source routing

Complete path listed in packet

Virtual circuits

Set up path out-of-band and store path identifier in routers

Local path identifier in packet

Destination-based forwarding

Router looks up address in forwarding table

Forwarding table contains (address, next-hop) tuples


Forwarding Options

Routing within a network/organization A single administrative domain

The administrator can set edge costs

Overall goals Provide intra-network connectivity

Adapt quickly to failures or topology changes

Optimize use of network resources

Non-goals Extreme scalability

Lying, and/or disagreements about edge costs


Intra-domain Routing

Static

Type in the right answers and hope they are always true

…So far

Link state

Tell everyone what you know about your neighbors

Dijkstra’s Algorithm

Distance vector

Tell your neighbors when you know about everyone

Bellman-Ford Algorithm


Basic Routing Approaches

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Graph algorithm for single-source shortest path tree

S {}

Q <remaining nodes keyed by distance>

While Q != {}

u extract-min(Q)

S S plus {u}

for each node v adjacent to u

“relax” the cost of v

u is done


Dijkstra’s Shortest Path

Define distances at each node X

dx(y) = cost of least-cost path from X to Y

Update distances based on neighbors

dx(y) = min {c(x,v) + dv(y)} over all neighbors V

3

2

2

1

1

4

1

4

5

3

u

v

w

x

y

z

s

t du(z) = min{c(u,v) + dv(z),

c(u,w) + dw(z)}

Bellman-Ford Algorithm


Message complexity

LS: with n nodes, E links,

O(nE) messages sent

DV: exchange between

neighbors only

Speed of Convergence

LS: relatively fast

DV: convergence time varies

May be routing loops

Count-to-infinity problem

Robustness: what happens

if router malfunctions?

LS:

Node can advertise incorrect

link cost

Each node computes only its

own table

DV: Node can advertise incorrect

path cost

Each node’s table used by

others (error propagates)

Link-state vs. Distance-vector


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Border routers summarize and

advertise internal routes to

external neighbors and vice-

versa

Border routers apply policy

Internal routers can use notion of

default routes

Core is default-free; routers must

have a route to all networks in

the world

R1

Autonomous system 1

R2

R3

Autonomous system 2R4

R5 R6

AS1

AS2

Border router

Border router

Inter-domain Routing


Neighboring ASes have business contracts How much traffic to carry

Which destinations to reach

How much money to pay

Common business relationships Customer-provider

» E.g., Princeton is a customer of USLEC

» E.g., MIT is a customer of Level3

Peer-peer

» E.g., UUNET is a peer of Sprint

» E.g., Harvard is a peer of Harvard Business School

Business Relationships


Extension of distance-vector routing

Support flexible routing policies

Avoid count-to-infinity problem

Key idea: advertise the entire path

Distance vector: send distance metric per destination

Path vector: send the entire path for each destination

3 2 1

d

“d: path (2,1)” “d: path (1)”

data traffic data traffic

Path-vector Routing


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Implement inter-domain routing on the Internet

Used and abused to model business practices

Default decision process for route selection

Highest local pref, shortest AS path, lowest MED, prefer

eBGP over iBGP, lowest IGP cost, router id

Many policies built on default decision process, but…

Possible to create arbitrary policies in principal

» Any criteria: BGP attributes, source address, prime number of

bytes in message,

Border Gateway Protocol


QoS


Routers manage their own resources

Buffer management may entail marking/dropping

Scheduling discipline determines outgoing packet order

Token bucket and RED

Mechanisms to control traffic flowing through routers

Networks can provide quality of service

Combines per-router traffic policing with network signalling

IntServ and DiffServ are contrasting approaches

Wireless


New challenges

Hidden terminal problem

Asymmetric ranges

Half-duplex radios (can’t do CD)

802.11/WiFi

Common technology for local-area wireless

Uses CSMA/CA

RTS/CTS and NAV for virtual carrier sense

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For next time…

There is no next time!

Study hard, sleep well, don’t be late!

8 AM, 3 hr exam!

It’s been a pleasure teaching you all!

Feel free to contact if you have questions/need advice

If you haven’t already, please complete your CAPE!


lecture 17: final review€¦ · cse 123: lecture 17: final review 2 3 final mechanics bulk of the...

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