CS4/MScComputer Networking

Lecture 2: Layered Network Architectures

Network API

Application API

2

Layered Network Architectures

• Networks are very complex systems

• Need a overall plan, an architecture

• How does a comp. scientist deal with complexity?

Divide and conquer + Abstraction. The challenge is to provide a useful service to other components and be efficient to implement in the underlying system.

• Abstraction leads to layering, if you do it a few times

• Layering provides modularity (think OO programming):–“Upgrade” a layer without affecting the others

–Add a parallel functionality in a layer, an alternative service to the layer’s “user”

3

• Service: manner in which information is transferred– Information: stream, block

– “Quality”: lossless, delay guarantees, ..

• Protocol: set of agreed rules that accomplishes a task/service– Corresponds to a layer

• Interface: interaction with other entities– Service interface – with higher level entities

– Peer interface – with equivalent entity at remote computer

Layering, Services, Protocols and Interfaces

Application programs

Process-to-process channels

Host-to-host connectivity

Hardware

Host 1 Host 2

Serviceinterface

Peer-to-peerinterface

High-levelobject

High-levelobject

Protocol Protocol

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Layer services, encapsulation

n+1entity

n-SAP

n+1entity

n-SAP

n entity n entity

n-SDU

n-SDU

n-SDU

H

H n-SDU

n-PDU

Service interface

Peer interface

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• Segmentation – SDU too large– Sequencing, out-of-order delivery,…

• Blocking – SDU too small– Merge SDU before sending off a PDU

• Multiplexing – multiple n+1 level entities active at any one time

Effect of external factors to services

n+1entity

n+1entity

n+1entity

n+1entity

n entity n entity

n-SDUn-SDU

n-SDUH

H n-SDUn-PDU

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The OSI 7-layer network architecture

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

NetworkLayer

Application

Data LinkLayer

PhysicalLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

Application

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• Evolved from Arpanet and other packet networks in 1983

• Communication across multiple diverse networks (internetworking)

• Assumes minimum service from underlying network, so that it can operate over any network

– Basic packet exchange is connectionless unreliable

• Provides two common services to applications– TCP: reliable connection oriented byte stream

– UDP: best-effort (no promises!) connectionless block data

Overview of the Internet architecture

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• Application layer covers top three OSI layers

• A sub-network is treated as a link

• Clear separation of internet layer from technology-dependent network interface layer

• Layering is not strictly enforced

The Internet architecture and protocols

TCP UDP

IPNetwork

Application

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• Provides a standard set of functions that can be called by applications

• Bi-directional

• Berkeley Socket is the most common API

• Variations depending on the operating system

Application Programming Interface

Host A Host B

port number port number

• Application references a socket through a descriptor• Socket bound to a port number

Application 1

Socket

Socketinterface

User

Kernel

Application 2

User

Kernel

Underlying communication

protocols

Underlying communication

protocols

Communications network

Socket

Socketinterface

The Application Layer

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Network Applications

• Examine a popular network application: Web–Client-server architecture

–The underlying protocol: HTTP

• and a commonly used internet service: DNS

• Purpose:–Learn how some common applications work

–See some protocols in action

• More applications and details in Ch2 of Kurose-Ross

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HTTP overview

HTTP: hypertext transfer protocol

• Web’s application layer protocol

• client/server model

– client: browser that requests, receives, “displays” Web objects

– server: Web server sends objects in response to requests

• HTTP 1.0: RFC 1945

• HTTP 1.1: RFC 2068

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response

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HTTP overview (continued)

Uses TCP:• client initiates TCP connection

(creates socket) to server, port 80

• server accepts TCP connection from client

• HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)

• TCP connection closed

HTTP is “stateless”• server maintains no

information about past client requests

Protocols that maintain “state” are complex!

• past history (state) must be maintained

• if server/client crashes, their views of “state” may be inconsistent, must be reconciled

aside

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HTTP connections

Nonpersistent HTTP• At most one object is sent over a TCP connection.

• HTTP/1.0 uses nonpersistent HTTP

Persistent HTTP• Multiple objects can be sent over single TCP connection between

client and server.

• HTTP/1.1 uses persistent connections in default mode

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Response times

Definition of RTT: time to send a small packet to travel from client to server and back.

Response time:

• one RTT to initiate TCP connection

• one RTT for HTTP request and first few bytes of HTTP response to return

• file transmission time

total = 2RTT+transmit time

time to transmit file

initiate TCPconnection

RTTrequestfile

RTT

filereceived

time time

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Persistent HTTP

Nonpersistent HTTP issues:• requires 2 RTTs per object

• OS overhead for each TCP connection

• browsers often open parallel TCP connections to fetch referenced objects

Persistent HTTP• server leaves connection

open after sending response

• subsequent HTTP messages between same client/server sent over open connection

Persistent without pipelining:• client issues new request only

when previous response has been received

• one RTT for each referenced object

Persistent with pipelining:• default in HTTP/1.1

• client sends requests as soon as it encounters a referenced object

• as little as one RTT for all the referenced objects

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HTTP request message

• Two types of HTTP messages: request, response

• HTTP request message:– ASCII (human-readable format)

GET /somedir/page.html HTTP/1.1Host: www.someschool.eduUser-agent: Mozilla/4.0Connection: close Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

headerlines

Carriage return, line feed

indicates end of message

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Uploading form input

Post method:• Web page often includes form input

• Input is uploaded to server in body of request message

URL method:• Uses GET method

• Input is uploaded in URL field of request line:www.google.com/search?monkeys&banana

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HTTP response message

HTTP/1.1 200 OK Connection closeDate: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html

data data data data data ...

status line(protocol

status codestatus phrase)

headerlines

data, e.g., requestedHTML file

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Cookies: keeping “state”

client serverusual http request msg

usual http response +set-cookie: 1678

usual http request msgcookie: 1678

usual http response msg

usual http request msgcookie: 1678

usual http response msg

cookie-specificaction

cookie-spectificaction

servercreates ID

1678 for user

entry in backend

database

access

access

Cookie file

amazon: 1678ebay: 8734

Cookie file

ebay: 8734

Cookie file

amazon: 1678ebay: 8734

one week later:

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DNS: Domain Name System

Internet hosts, routers:• IP address (32 bit) - used by

computers, network equipment

• “name”, e.g., www.yahoo.com -used by humans

How to map between IP addresses and name ?

Domain Name System:• distributed database

implemented in hierarchy of many name servers

• application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation)– note: core Internet function,

implemented as application-layer protocol

– complexity at network’s “edge”

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DNS

Why not centralize DNS?• single point of failure

• traffic volume

• distant centralized database

• maintenance

doesn’t scale!

DNS services• Hostname to IP address

translation

• Host aliasing– Canonical and alias names

• Mail server aliasing

• Load distribution– Replicated Web servers: set

of IP addresses for one canonical name

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Root DNS Servers

com DNS servers org DNS servers uk DNS servers

ac.ukDNS servers

co.ukDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.

Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail).– Can be maintained by organization or service provider

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Local Name Server

• Does not strictly belong to hierarchy

• Each ISP (residential ISP, company, university) has one.– Also called “default name server”

• When a host makes a DNS query, query is sent to its local DNS server– Acts as a proxy, forwards query into hierarchy.

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requesting hostmars.ed.ac.uk

crete.uch.gr

root DNS server

local DNS serverdns.ed.ac.uk

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4

5

61

authoritative DNS serverdns.uch.gr

78

TLD DNS server

Example

Host mars.inf.ed.ac.uk wants IP address for crete.csd.uch.gr

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Speeding up DNS: caching and updating records

• Once (any) name server learns mapping, it caches the information– TLD servers typically cached in local name servers

» Thus root name servers not often visited

– cache entries timeout (disappear) after some time

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DNS records

DNS: distributed db storing resource records (RR)

• Type=NS– name is domain (e.g.

foo.com)– value is hostname of

authoritative name server for this domain

RR format: (name, value, type, ttl)

• Type=A– name is hostname

– value is IP address

• Type=CNAME– name is alias name for some

“canonical” (the real) namewww.ibm.com is reallyservereast.backup2.ibm.com

– value is canonical name

• Type=MX– value is name of mailserver

associated with name

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Inserting records into DNS

• Example: just created startup “Network Utopia”

• Register name networkuptopia.com at a registrar– Need to provide registrar with names and IP addresses of your

authoritative name server (primary and secondary)

– Registrar inserts two RRs into the com TLD server:

(networkutopia.com, dns1.networkutopia.com, NS)

(dns1.networkutopia.com, 212.212.212.1, A)

• Put in authoritative server Type A record for www.networkuptopia.com and Type MX record for networkutopia.com

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• Kurose, Ross– 2.2, 2.5

– also read 2.3, 2.4, 2.6 for other applications/services

• Leon-Garcia, Widjaja– 2.1, 2.5

Reading

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• Kurose, Ross– 1.7, 2.1 2.7, 2.8

• Leon-Garcia, Widjaja– Chapter 2, 5.1

• Tanenbaum– Chapter 1

• Stallings:– Chapters 1, 2

• Peterson & Davie– Sections 1.2-1.3

Reading