eecs 122: introduction to computer networks network service and applications

1Katz, Stoica F04

EECS 122: Introduction to Computer Networks

Network Service and Applications

Computer Science Division

Department of Electrical Engineering and Computer Sciences

University of California, Berkeley

Berkeley, CA 94720-1776

2Katz, Stoica F04

Overview

• Taxonomy of Communication Networks• Services and Applications

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Communication networks can be classified based on the way in which the nodes exchange information:

Taxonomy of Communication Networks

Communication Network

SwitchedCommunication

Network

BroadcastCommunication

Network

Circuit-Switched


Packet-Switched


Datagram Network

Virtual Circuit Network

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Broadcast Communication Networks- Information transmitted by any node is received by every

other node in the network• Examples: usually in LANs (Ethernet, WiFi)

- Problem: coordinate the access of all nodes to the shared communication medium (Multiple Access Problem)

Switched Communication Networks- Information transmitted to a sub-set of designated nodes

• Examples: WANs (Telephony Network, Internet)- Problem: how to forward information to intended node(s)?

• Done by special nodes (e.g., routers, switches) executing routing protocols

Broadcast vs. Switched Communication Networks

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Network


Network

Circuit-Switched


Packet-Switched


Datagram Network


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Circuit Switching

Three phases1. circuit establishment

2. data transfer

3. circuit termination

If circuit not available: “Busy signal” Examples

- Telephone networks

- ISDN (Integrated Services Digital Networks)

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Telegraph Network

Alexander Graham Bell- 1876: Demonstrates the telephone at US Centenary

Exhibition in Philadelphia

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Telephone Network

Almon Brown Strowger (1839 - 1902)- 1889: Invents the “girl-less, cuss-less” telephone

system, aka the mechanical switching system

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Timing in Circuit Switching

DATA

Circuit Establishment

Data Transmission

Circuit Termination

Host 1 Host 2Node 1 Node 2

propagation delay between Host 1 and Node 1

propagation delay between Host 2 and Node 1

processing delay at Node 1

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Circuit Switching

Node (switch) in a circuit switching network

incoming links outgoing linksNode

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Circuit Switching: Multiplexing/Demultiplexing

Time divided in frames and frames into slots Relative slot position inside a frame determines to which

conversation data belongs- E.g., slot 0 belongs to red conversation

Requires synchronization between sender and receiver—surprisingly difficult!

In case of non-permanent conversations- Needs to dynamically bind a slot to a conservation- How to do this?

If a conversation does not use its circuit the capacity is lost!

Frames

0 1 2 3 4 5 0 1 2 3 4 5Slots =

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Network


Network

Circuit-Switched


Packet-Switched


Datagram Network


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Packet Switching

Data sent as formatted bit-sequences (Packets) Packets have following structure:

• Header and Trailer carry control information (e.g., destination address, check sum)

Each packet traverses the network from node to node along some path (Routing)

At each node the entire packet is received, stored briefly, and then forwarded to the next node (Store-and-Forward Networks)

Typically no capacity is allocated for packets

Header Data Trailer

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Packet Switching

Node in a packet switching network

incoming links outgoing linksNode

Memory

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Packet Switching: Multiplexing/Demultiplexing

Data from any conversation can be transmitted at any given time

- Single conversation can use the entire link capacity if it is alone

How to tell them apart?- Use meta-data (header) to describe data

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Network


Network

Circuit-Switched


Packet-Switched


Datagram Network


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Datagram Packet Switching

Each packet is independently switched- Each packet header contains destination address

No resources are pre-allocated (reserved) in advance

Example: IP networks

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Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Timing of Datagram Packet Switching

Packet 1

Packet 2

Packet 3

processing

delay of Packet 1 at Node 2

Host 1 Host 2Node

1Node

2

propagationdelay betweenHost 1 and Node 2

transmission time of Packet 1at Host 1

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Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

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Network


Network

Circuit-Switched


Packet-Switched


Datagram Network


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Virtual-Circuit Packet Switching

Hybrid of circuit switching and packet switching- Data is transmitted as packets

- All packets from one packet stream are sent along a pre-established path (=virtual circuit)

Guarantees in-sequence delivery of packets However, packets from different virtual circuits

may be interleaved Example: ATM networks

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Virtual-Circuit Packet Switching

Communication with virtual circuits takes place in three phases 1. VC establishment

2. data transfer

3. VC disconnect

Note: packet headers don’t need to contain the full destination address of the packet

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Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Timing of Virtual-Circuit Packet Switching

Packet 1

Packet 2

Packet 3

Host 1 Host 2Node

1Node

2

propagation delay between Host 1 and Node 1VC

establishment

VCtermination

Datatransfer

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Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

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Packet-Switching vs. Circuit-Switching

Most important advantage of packet-switching over circuit switching: Exploitation of statistical multiplexing:

- Efficient bandwidth usage; ratio between peek and average rate is 3:1 for audio, and 15:1 for data traffic

However, packet-switching must handle congestion:

- More complex routers- Harder to provide good network services (e.g., delay

and bandwidth guarantees) In practice they are combined:

- IP over SONET, IP over Frame Relay

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Overview

• Taxonomy of Communication Networks Services and Applications

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The Internet Protocol (IP)

Problem:- Many different network technologies- e.g., Ethernet, Token Ring, ATM, Frame Relay, etc.- How can you hook them together?

• n x n translations?

IP was invented to glue them together- n translations- Minimal requirements (datagram)

The Internet is founded on IP- “IP over everything”

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Addressing

Every Internet host has an IP address- e.g., 67.114.133.15

Packets include destination address- Network is responsible for routing packet to address

Host-view:

Source Destination

Network

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IP-centric View

Host A

Host BHost E

Host D

Host C

Router 1 Router 2

Router 3

Router 4

Router 5

Router 6 Router 7

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Physical View

A big mess!

Every “link” could be a whole network of ATM, frame relay, ethernet, DSL, etc.

Beauty of IP: you can ignore these different network technologies

In many networks, IP is used only at the edge

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Back to IP

Host A

Host BHost E

Host D

Host C

Router 1 Router 2

Router 3

Router 4

Router 5

Router 6 Router 7

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Routing

Routers have “routing tables”- Tables mapping each destination with an outgoing link- Requires that routing table is highly compressible!- Implications for address assignment, mobility, etc.

Routing decisions made packet-by-packet- Routers keep no connection state

Question: Why have the network do routing?- Why not the hosts?- Compare delivery-by-hand to FedEx

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Internet Service

• “Best-Effort” service- No guarantees about packet delivery

- Hosts must cope with loss and delay

Why this service model?- Why not guarantee no loss and low delay?

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Domain Name Service (DNS)

Humans/applications use machine names- e.g., www.cs.berkeley.edu

Network (IP) uses IP addresses- e.g., 67.114.112.23

DNS translates between the two- An overlay service in its own right- Global distribution of name-to-IP address mappings—a

kind of content distribution system as well- Unsung hero of the Internet

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File Transfer (FTP, SCP, etc.)

Get file from soup.cs.berkeley.edu

DNS

soup.cs.berkeley.eduGet address for soup.cs.berkeley.edu 67.132.22.5

Get file

file

Your PC

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Email

Email message exchange is similar to previous example, except

Exchange is between mail servers DNS gives name of mail server for domain

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Web

Get www.icir.org/file.html

DNS

www.icir.orgGet address for

www.icir.org 67.132.22.5

Get file.html

file.html

Your PC Proxy

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Caching

Caches can be visible or transparent

Visible: - Client is configured to ask cache

Transparent: - Cache intercepts packet on its way to web server

- Example of a “application-aware middlebox”

- Violates purity of architecture, but are prevalent...

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Content Distribution Network (CDN)

How to get closest copy of replicated content?

CDNs have mirror servers distributed globally

CDN customers allow CDN to run their DNS

“Smart” DNS server returns results based on requester’s IP address

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Gnutella (P2P)

Overlay Network

Dashed lines are “virtual links”

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Gnutella (cont’d)

User asks for file (by metadata)

Each host sends request to its “neighbors” in overlay network

Responses sent back to original requesting node

Many variations on P2P file sharing.....

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Overlay Networks

Create a set of “virtual links” between hosts

Communication between neighbors on overlay is done by IP

But the overlay can use different routing, or application-specific processing, at overlays nodes

IP is often overlay on circuit-switched network- App-specific networks increasingly overlaid on IP

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Architecture

The assignment of tasks and knowledge

Who does what, and where is the state kept?

How they do it: algorithms and implementation

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Internet Architecture

Routers do routing, and almost nothing else- No application-specific functions

Hosts do all application-specific processing

Allowed wide variety of applications to flourish on Internet

eecs 122: introduction to computer networks network service and applications

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