ELEG 651 and CPEG 419

ELEG 651 and CPEG 419Lecture 2 – Chapter 2

The Application Layer

Goals of this Chapter

• To understand common application protocols work– Web (http)– Email (smtp)– FTP– DNS– P2P– IM

• To understand how the design alternatives for application design– A network application runs on many hosts, it is a distributed

application– This chapter discusses several designs of distributed

applications

Road Map

• Application basics• Web• Email• FTP• DNS• P2P

– Graph theory– State diagrams– P2P design

• IM

Road Map



• IM

Creating a network app

write programs that– run on (different) end

systems– communicate over network– e.g., web server software

communicates with browser software

No need to write software for network-core devices– Network-core devices do not

run user applications – applications on end systems

allows for rapid app development, propagation

application

transportnetworkdata linkphysical

application


application


An App-layer protocol defines

• Types of messages exchanged, – e.g., request, response

• Message syntax:– what fields in messages &

how fields are delineated

• Message semantics – meaning of information in

fields

• Rules for when and how processes send & respond to messages

Public-domain protocols:• defined in RFCs• allows for

interoperability• e.g., HTTP, SMTP

Proprietary protocols:• e.g., Skype

Ports

• An application is identified by the hosts IP address, transport protocols, and port– E.g., A web server has a

particular IP address, listens with TCP on port 80.

– A web browser on a host will connect a request a file from the web server. The browser is identified by the host’s IP address and a TCP port.

host

TCP

0

45674568

216-1

UDP

0

216-1

host(web server)

TCP

0

80

216-1

UDP

0

216-1

What transport service does an app need?

Data reliability• some apps (e.g., audio) can

tolerate some loss

• other apps (e.g., file transfer, telnet) require 100% reliable data transfer

Timing• some apps (e.g., Internet

telephony, interactive games) require low delay to be “effective”

Throughput• some apps (e.g., multimedia)

require minimum amount of throughput to be “useful” (i.e., in order for the user to gain utility)

• other apps (“elastic apps”) make use of whatever throughput they get

Security• Encryption, data integrity, …

Transport service requirements of common apps

Application

file transfere-mail

Web documentsreal-time audio/video

stored audio/videointeractive gamesinstant messaging

Data loss Throughput Time Sensitive

Internet transport protocols services

TCP service:• connection-oriented: setup

required between client and server processes

• reliable transport between sending and receiving process

• flow control: sender won’t overwhelm receiver

• congestion control: throttle sender when network overloaded

• does not provide: timing, minimum throughput guarantees, security

UDP service:• unreliable data transfer

between sending and receiving process

• does not provide: reliability, flow control, congestion control, timing, throughput guarantee, or security

• Does not require connection set-up

• Packets can be sent at any rate desired (but this might be cause considerable congestion)

Internet apps: application, transport protocols

Application

e-mailremote terminal access

Web file transfer

streaming multimedia

Internet telephony

Applicationlayer protocol

SMTP [RFC 2821]Telnet [RFC 854]HTTP [RFC 2616]FTP [RFC 959]HTTP (eg Youtube), RTP [RFC 1889]SIP, RTP, proprietary(e.g., Skype)

Underlyingtransport protocol

Road Map



• IM

Web and HTTP• Web page consists of objects• Object can be HTML file, JPEG image, Java applet, audio file,…• Web page consists of base HTML-file which includes several

referenced objects• The browser first requests the base file• The base file species text and URLs of objects• The browser requests these objects, where ever they are (not

always on the same server)• HTTP is used to request the base file and all the other files• Note, that HTTP can be used for other applications besides web• Each object is addressable by a URL• Example URL:

www.someschool.edu/someDept/pic.gif

host name path name

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

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response

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

HTTP connections

Nonpersistent HTTP• At most one object is

sent over a TCP connection.

Persistent HTTP• Multiple objects can

be sent over single TCP connection between client and server.

Nonpersistent HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index

1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80

2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index

1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client

3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket

time

(contains text, references to 10

jpeg images)

5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects

6. Steps 1-5 repeated for each of 10 jpeg objects

4. HTTP server closes TCP connection.

initiate TCPconnection

time time

Non-Persistent HTTP: Response time

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

Response time:

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

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

• as little as one RTT for all the referenced objects

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.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message

HTTP request message: general format

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)

header lines

data, e.g., requestedHTML file

HTTP response status codes

200 OK– request succeeded, requested object later in this message

301 Moved Permanently– requested object moved, new location specified later in this message

(Location:)

400 Bad Request– request message not understood by server

404 Not Found– requested document not found on this server

505 HTTP Version Not Supported

In first line in server->client response message.

A few sample codes:

Trying out HTTP (client side) for yourself

1. Telnet to your favorite Web server:

Opens TCP connection to port 80(default HTTP server port) at cis.poly.edu.Anything typed in sent to port 80 at cis.poly.edu

telnet cis.poly.edu 80

2. Type in a GET HTTP request:

GET /~ross/ HTTP/1.1Host: cis.poly.edu

By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server

3. Look at response message sent by HTTP server!

Wireshark (ethereal)

• Wireshark captures all packets that pass through the hosts interface• To run Wireshark , libpcap (linux) or winpcap (windows) must be installed. It

comes with wireshark package• Then, run wireshark• Select Capture• Find the active interface

– E.g., mot generic dialup, nor vnp, nor packet scheduler, but wireless …. With IP address

– Then select prepare– Let’s watch TCP packets on port 80

• Next to capture filter, enter TCP port 80– Select update in realtime and autoscroll– Might need to enable or disable “capture in promiscuous mode”– Press start– Press close

• Load www.eecis.udel.edu page in browser• Press stop in Wireshark • Find http request to 128.4.40.10.

– Right click and select follow TCP stream

Web caches (proxy server)

• user sets browser: Web accesses via cache

• browser sends all HTTP requests to cache– object in cache: cache

returns object – else cache requests

object from origin server, then returns object to client

Goal: reduce network utilization by satisfying client request without involving origin server

client

Proxyserver

client

HTTP request

HTTP response

HTTP request HTTP request

origin server

origin server

HTTP response HTTP response

More about Web caching

• cache acts as both client and server

• typically cache is installed by ISP (university, company, residential ISP)

Why Web caching?• reduce response time for

client request• reduce traffic on an

institution’s access link.• Internet dense with

caches: enables “poor” content providers to effectively deliver content (but so does P2P file sharing)

Caching example

Assumptions• average object size = 100,000 bits• avg. request rate from institution’s

browsers to origin servers = 15/sec

• delay from institutional router to any origin server and back to router = 2 sec

Consequences• utilization on LAN = 15%• utilization on access link = 100%• total delay = Internet delay + access

delay + LAN delay

= 2 sec + minutes + milliseconds

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

Caching example (cont)

possible solution• increase bandwidth of access

link to, say, 10 Mbps

consequence• utilization on LAN = 15%

• utilization on access link = 15%

• Total delay = Internet delay + access delay + LAN delay

= 2 sec + msecs + msecs

• often a costly upgrade

originservers

public Internet


10 Mbps access link

institutionalcache

Caching example (cont)

possible solution: install cache

• suppose hit rate is 0.4

consequence• 40% requests will be satisfied

almost immediately• 60% requests satisfied by origin

server• utilization of access link reduced

to 60%, resulting in negligible delays (say 10 msec)

• total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs

originservers

public Internet


1.5 Mbps access link

institutionalcache

Conditional GET

• Goal: don’t send object if cache has up-to-date cached version

• cache: specify date of cached copy in HTTP requestIf-modified-since:

<date>

• server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not

Modified

cache server

HTTP request msgIf-modified-since:

<date>

HTTP responseHTTP/1.0

304 Not Modified

object not

modified

HTTP request msgIf-modified-since:

<date>

HTTP responseHTTP/1.0 200 OK

<data>

object modified

Road Map

• Application basics• Web• FTP• Email• DNS• P2P


• IM

FTP: the file transfer protocol

• transfer file to/from remote host• client/server model

– client: side that initiates transfer (either to/from remote)– server: remote host

• ftp: RFC 959• ftp server: listens on port 21

file transfer FTPserver

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host

FTP is weird: separate control and data connections• FTP client contacts FTP server at port 21,

TCP is transport protocol• client authorized over control connection

– This is done in “clear text” (i.e., unencrypted)– So if some one if sniffing packets, your

password might be learned.– Sniffing packets is difficult on ethernet,

encrypted wifi, and DSL, but is possible on cable modems

• client browses remote directory by sending commands over control connection.

• Data is transferred over different connections. Two approaches

– Active– Passive

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20

• Active– The client opens a TCP socket with

on some port (port number >1024)– The client sends the server the port– The server connects to the client’s

port where the servers source port is 20

• Active mode is a problem for firewalls

– If my desktop is not a server, if should not receive any requests for connections.

– But FTP servers will make such a requests

FTP Passive mode

• When a file is to be transferred, the server opens a port (number>1024 and not 20)

• The server sends this port number information over the command connection

• The client connects to the servers over this port.

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionhigh port

• Drawback of passive– Some enterprises (companies) like to

control which applications are used• E.g., web browsing is ok, but skype is

not

– One way to do this is to block out going connections based on the port.

– However, this will cause FTP to fail, unless the device that blocks connections is smart

Road Map



• IM

Email Protocol Design• Basic assumption: weak user agents and strong mail servers

– The user wants to send the mail and leave– The user wants to get the mail– The user may come and go whenever (e.g., roaming laptop)– It should be possible to send mail to a user even if neither user is online at the same time.– We conclude that there must be a middle man/mail server.

• Servers are not that strong: The protocol must be as robust as possible to servers being offline – No single server – why

• Single point of failure• The server would have to be too big (congestion)

– We conclude that there should be many mail servers

• Two types of hosts– Users– Mail servers

• Each user has a mail box in its mail server– Users retrieve mail from their mail server at there convenience

• Users give mail to their mail servers to deliver the mail• Mail servers communicate with

– The users that have mail boxes in the server– Other mail servers

useragent

mailserver

mailserver user

agent

Email Protocol Design• Two types of hosts

– Users– Mail servers




useragent

mailserver

mailserver user

agent

User composes mail and sends it to its mail server (or a mail server that will send mail for it)

Mail server finds the destination mail server and attempts to send the mail

Destination user requests emails from mailbox

Destination server gives mails to user

Email Protocol Design• Two types of hosts

– Users– Mail servers




useragent

mailserver

mailserver user

agent

User composes mail and sends it to its mail server (or a mail server that will send mail for it)

Mail server finds the destination mail server and attempts to send the mail

Destination user requests emails from mailbox

Destination server gives mails to user

SMTP SMTP POP3IMAP…

Electronic Mail: Details

Three major components: • user agents

• mail servers

• simple mail transfer protocol: SMTP

User Agent

• a.k.a. “mail reader”

• composing, editing, reading mail messages

• e.g., Eudora, Outlook, elm, Mozilla Thunderbird

• Put outgoing on server (with SMTP)

• Get incoming messages from server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Electronic Mail: mail servers

Mail Servers • mailbox contains incoming

messages for user

• message queue of outgoing (to be sent) mail messages

• SMTP protocol between mail servers to send email messages

– client: sending mail server

– “server”: receiving mail server

• Reliable: several attempts and provide notification if delivery fails

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Electronic Mail: SMTP [RFC 2821]

• uses TCP to reliably transfer email message from client to server, port 25

• direct transfer: sending server to receiving server• Emails are pushed to servers (but users pull messages from

servers)• three phases of transfer

– handshaking (greeting)– transfer of messages– closure

• command/response interaction– commands: ASCII text– response: status code and phrase

• messages must be in 7-bit ASCII– Makes it difficult to send attachments

Scenario: Alice sends message to Bob

1) Alice uses UA to compose message and “to” [email protected]

2) Alice’s UA sends message to her mail server; message placed in message queue

3) Client side of SMTP opens TCP connection with Bob’s mail server

4) SMTP client sends Alice’s message over the TCP connection

5) Bob’s mail server places the message in Bob’s mailbox

6) Bob invokes his user agent to read message

useragent

mailserver

mailserver user

agent

1

2 3 4 56

Sample SMTP interaction

S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

Client connects to server

Try SMTP interaction for yourself:

• telnet mail.eecis.udel.edu 25• see 220 reply from server

• enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands

above lets you send email without using email client (reader)

SMTP: final words

• SMTP uses persistent connections

• SMTP requires message (header & body) to be in 7-bit ASCII

• SMTP server uses CRLF.CRLF to determine end of message

Comparison with HTTP:

• HTTP: pull• SMTP: push

• both have ASCII command/response interaction, status codes

• HTTP: each object encapsulated in its own response msg

• SMTP: multiple objects sent in multipart msg

Mail access

• POP3 and IMAP are two protocols for access mail on a mail server

• Web-based mail works differently, the web mail server and the mail server can be integrated, so that there is no user agent.

Mail access protocols

• SMTP: delivery/storage to receiver’s server• Mail access protocol: retrieval from server

– POP: Post Office Protocol [RFC 1939]• authorization (agent <-->server) and download

– IMAP: Internet Mail Access Protocol [RFC 1730]• more features (more complex)• manipulation of stored msgs on server

– HTTP: gmail, Hotmail, Yahoo! Mail, etc.

useragent

sender’s mail server

useragent

SMTP SMTP accessprotocol

receiver’s mail server

Road Map



• IM

DNS – domain name system

• Change names, like www.yahoo.com into IP address.• Services provided by DNS

– Name to address translation– Host aliasing

• A host relay1.west-coast.yahoo.com could have two aliases, yahoo.com and www.yahoo.com.

• In this case, the canonical hostname is relay1.west-coast.yahoo.com. • DNS can provide canonical host names

– Mail server aliasing• When a mail server wants to send a mail to [email protected], it does not send

it to www.udel.edu, but to mail.udel.edu. Or maybe udmail.udel.edu. DNS can translate udel.edu to mail.udel.edu

– (Cheap) Load distribution • Cnn.com has several servers.• DNS will respond with all address, • but it will reorder the addresses every time.• If the client uses the first address listed, then each client will use different

servers. • Content distribution networks (CDN) are better ways of load balancing

http://www.yahoo.com/

mailto:[email protected]

DNS - structure

• Centralized DNS?– Pros– Cons

1.

2.

3.

4.

5.

• Instead, a distributed hierarchical database is used.

Domain Hierarchy

edu com gov mil org net uk in

UD upenn yahoo cisco whitehouse nasa navy arpa acm

eecis art

bohacek_pc1 bohacek_pc10

Administrative Zones in the Domain Hierarchy

edu comgov mil org net uk in

UD upenn yahoo ciscowhitehouse nasa navy arpa acm

eecis art

bohacek_pc1 bohacek_pc10

root

It is possible that .edu and .gov are administered togetherNote that UD administered art but not eecisSome times a single service provider will administer the domains for a large number of .coms

Root servers

• Each layer in the hierarchy knows about the domain names below it• The highest level is the root.

– There are 13 root “servers”

– Each of these servers is actually several servers, and some of the machines that comprise a server are distributed geographically.

13 root name servers worldwide

b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)

i Autonomica, Stockholm (plus 28 other locations)

k RIPE London (also 16 other locations)

m WIDE Tokyo (also Seoul, Paris, SF)

a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)

overview

• Top-level domain (TLD) servers– There are around 200 top-level domains– These include com, edu, mil, info, in, uk, cn, – Currently,

• network solutions maintains the TLD servers for com

• Educause maintains the TLD servers for edu

– The root servers know the addresses and names of all top level servers

• Organizations have a hierarchy of DNS servers

DNS queries

• Suppose a host needs the IP address of bohacek-pc1.eecis.udel.edu• If this IP address is not in cache, the host asks its local DNS server.• If the DNS server does not have it in cache, it checks if is had the IP address of the

DNS server of eecis.udel.edu in cache• If not, it checks if IP address of the dns server of udel.edu in cache• If not, it check if it has the IP address of the top-level domain server of edu in cache• It not, it asks the root server for the IP address of the edu TLD server

– The DNS server always has the IP address of the root servers• The local DNS server asks the edu TLD server for address of bohack-

pc1.eecis.udel.edu. • The TLD server does not know that IP address, but instead gives the IP address of

the dns server for UD• The local DNS server asks the UD dns server for the address of bohack-

pc1.eecis.udel.edu.• The UD dns server does not know the address, but instead returns the address of the

eecis dns server.• The local DNS server asks the eecis dns server for the address of bohacek-

pc1.eecis.udel.edu• Eecis dns server replies with the address.• This address is returned to the host that orginally asked the question.

DNS Queries

Browser wants to show www. eecis.udel.edu

Browser needs the IP address of www. eecis.udel.edu

Host asks local DNS server for IP address of www. eecis.udel.edu

• Local DNS server checks if it has the IP address of www.eecis.udel.edu in cache.

• If not, it checks if is had the IP address of the DNS server of eecis.udel.edu in cache

• If not, it checks if IP address of the dns server of udel.edu in cache

• If not, it check if it has the IP address of the top-level domain server of edu in cache

• .if not, …..

What is the IP address of www.eecis.udel.edu?

Root server (IP address are always known)

Root server does not know. Instead, it responds with dns server that might, specifically, the TLD server for .edu

What is the ip address of www.eecis.udel.edu?

TLD server for .edu

TLD server does not know. Instead replies with the name and IP address of the UD DNS server


UD dns server does not know. Instead it replies with the name and IP address of the eecis dns server.


It is 128.4.1.2

It is 128.4.1.2

http://www.eecis.udel.edu/

Browser wants to show

www.eecis.udel.edu

DNS Queries

Browser needs the IP address of

www.eecis.udel.edu Host asks local DNS server for IP

address of www.eecis.udel.ed

u

1. Local DNS server checks if it has the IP address of www.eecis.udel.edu in cache.

2. If not, it checks if is had the IP address of the DNS server of eecis.udel.edu in cache

3. If not, it checks if it has the IP address of the DNS server of udel.edu in cache

4. If not, it checks if it has the IP address of the top-level domain server of edu in cache

5. .if not, …..


Root server (IP addresses are always known)

Root server does not know. Instead, it responds with name and address of a

server that might, specifically, the TLD server

for .eduWhat is the IP address of

www.eecis.udel.edu?TLD server for .edu

TLD server does not know. Instead replies with the name and IP address of

the UD DNS server


UD DNS server does not know. Instead it replies with the name and IP address of the eecis dns server.


It is 128.4.1.2

It is 128.4.1.2

UD DNS server

eecis DNS server



www.eecis.udel.edu

DNS Queries




u


2. If yes, then return it

It is 128.4.1.2



www.eecis.udel.edu

DNS Queries




u



3. If yes, query it…


It is 128.4.1.2

It is 128.4.1.2

eecis DNS server



www.eecis.udel.edu

DNS Queries




u





5. .if so, then query it…


TLD server for .edu

TLD server does not know. Instead replies with the name and IP address of

the UD DNS server


UD DNS server does not know. Instead it replies with the name and IP address of the eecis dns server.


It is 128.4.1.2

It is 128.4.1.2

UD DNS server

eecis DNS server


Attack on DNS

• Hackers have tried to bring down DNS by performing a DoS on the root servers– DoS – denial of service. Sends more

packets or requests for service than the server can accommodate. Resulting in poor service for normal users.

• This failed because– There are many very strong root servers and have

firewalls/filters• The attacks used ICMP ping packets• DNS requests would have been more effective

– It is rare that a root server is needed• Usually only the TLD server is needed• Or only a domain server.

DNS Message Details

• DNS Record– (Name, Value, Type, Class, TTL)– If Type = A

• Name is the host name• Value is the IP address of the host

– If Type = NS• Name is a domain name• Value is the name of the DNS server for the domain• E.g., (udel.edu, dns.udel.edu, NS, …, …)

– Type = MX• Name is the domain name• Value is the name of the mail server for the domain• E.g., (udel.edu, mail.udel.edu, MX, …, …)

– Type = CName• Name is a host name• Value is the canonical name of the host• E.g., (www.yahoo.com, relay-east.yahoo.com, CName, …, …)

– TTL is the time to live, so DNS caches can be timed out– Class is no longer used, it is set as IN

http://www.yahoo.com/

DNS query

• (Name, Type, Class)

• (UDel.edu, MX, IN)– Please provide the name of the UD’s mail

server

• (mail.UDel.edu, A, IN)– Please provide the IP address for mail.udel.edu

DNS message format

DNS protocol : query and reply messages, both with same message format

msg header• identification: 16 bit #

for query, reply to query uses same #

• flags:– query or reply– recursion desired – recursion available– reply is authoritative

DNS message format

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used


www.eecis.udel.edu


www.eecis.udel.edu

DNS Queries





5. .if not, …..

Root server (IP addresses are always known)

TLD server for .edu

UD DNS server

eecis DNS server

1 00 0

(www.eecis.udel.edu, A,IN)

1 00 0


0 00 4

(edu, edu-serverA.net, NS, IN)

(edu-serverA.net, 124.5.1.1, A, IN)

(edu, edu-serverB.net, NS, IN)

(edu-serverB.net, 124.5.1.2, A, IN)

1 00 0


0 00 4

(udel.edu, dns2.udel.edu, NS, IN)

(udel.edu, dns2.udel.edu, 128.178.2.2, A, IN)

(udel.edu, dns1.udel.edu, NS, IN)

(dns1.udel.edu, 128.173.2.1, A, IN)

1 00 0


0 00 4

(eecis.udel.edu, dns1.eecis.udel.edu, NS, IN)

(dns1.eecis.udel.edu, 128.4.1.10, A, IN)

(eecis.udel.edu, dns2.udel.edu, NS, IN)

(dns2.udel.edu, 128.4.1.11, A, IN)

1 00 0


0 10 0

(www.eecis.udel.edu, 128.4.1.1, A, IN)

0 10 0

(www.eecis.udel.edu, 128.4.1.1, A, IN)


DNS Flags

• The DNS header has a query ID– The query has this ID and the server copies this ID

into the response

• Flag indicating query or answer• Flag indicating whether the server is the

authoritative server for the answer (as oppose to a cached answer)

• A recursive desired flag indicating that the host/server would like the server to perform the recursive DNS lookup

• A recursive available flag indicating whether the server is available to to the recursive lookup

DNS

• Which transport protocol should DNS use?

• Why?

Documents

ELEG 651 and CPEG 419