Application Layer

End of Saga

Content

• Domain Name Service• Electronic Mail• World Wide Web

DNS

• What is DNS?• What services does it provide?• How does it operate?• Message format• Types of messages

What is DNS?

• DNS is a host name to IP address translation service

• DNS is– a distributed database implemented in a hierarchy

of name servers– an application level protocol for message

exchange between clients and servers

Why DNS?

• It is easier to remember a host name than it is to remember an IP address.

• An name has more meaning to a user than a 4 byte number.

• Applications such as FTP, HTTP, email, etc., all require the user to input a destination.

• The user generally enters a host name.• The application takes the host name supplied by the

user and forwards it to DNS for translation to an IP address.

DNS Services

• Besides the address translation service, DNS also provides the following services:– Host aliasing: a host with a complicated name can

have one or more aliases that are simpler to remember , e.g., relay1.west-coast.media.com -> media.com. The longer name is the canonical hostname, the shorter the alias hostname.

DNS Services (cont’d)

– Mail server aliasing: same as above, aliases can exist for long canonical host names.

– Load Balancing: a set of servers can have one name mapped onto several machines. DNS provides the full list of names to the end user’s application which generally takes the first one in the list. DNS rotates the names on the list.

How does it work?

• DNS works by exchanging messages between client and server machines.

• A client application will pass the destination host name to the DNS process (in Unix referred to as the gethostbyname() routine) to get the IP address.

• The application then sits and waits for the response to return.

DNS

Why not centralize DNS?• single point of failure• traffic volume• distant centralized database• maintenance

doesn’t scale!

Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1st approx:• client queries a root server to find com DNS server• client queries com DNS server to get amazon.com DNS server• client queries amazon.com DNS server to get IP address for

www.amazon.com

DNS: Root name servers• contacted by local name server that can not resolve name• root name server:

– contacts authoritative name server if name mapping not known– gets mapping– returns mapping to local name server

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)

TLD and Authoritative Servers

• Top-level domain (TLD) servers:– responsible for com, org, net, edu, etc, and all top-level

country domains uk, fr, ca, jp.– Network Solutions maintains servers for com TLD– Educause for edu TLD

• Authoritative DNS servers: – organization’s DNS servers, providing authoritative

hostname to IP mappings for organization’s servers (e.g., Web, mail).

– can be maintained by organization or service provider

Local Name Server

• does not strictly belong to hierarchy• each ISP (residential ISP, company, university)

has one.– also called “default name server”

• when host makes DNS query, query is sent to its local DNS server– acts as proxy, forwards query into hierarchy

DNS Queries

• Recursive:– The client machine sends a request to the local name

server, which, if it does not find the address in its database, sends a request to the root name server, which, in turn, will route the query to an intermediate or authoritative name server. Note that the root name server can contain some hostname to IP address mappings. The intermediate name server always knows who the authoritative name server is.

DNS Queries (cont’d)

• Iterative:– The local server queries the root server. If address

not in its database, will have the name/address of an intermediate or authoritative name server and forward that information to the local name server so that it can directly communicate with the intermediate or authoritative name server. This is to prevent the overloading of the root servers that handle millions of requests.

DNS: caching and updating records

• once (any) name server learns mapping, it caches mapping– cache entries timeout (disappear) after some time– TLD servers typically cached in local name servers

• Thus root name servers not often visited

• update/notify mechanisms under design by IETF– RFC 2136– http://www.ietf.org/html.charters/dnsind-charter.html

Operation of DNS

• DNS uses caching to increase the speed with which it does the translation.

• The DNS data is stored in the database in the form of resource records (RR). The RRs are directly inserted in the DNS messages.

• The RRs are a 4 tuple that consist of: {name, value, type, TTL}.

Resource RecordsDNS: distributed db storing resource records (RR)

o Type=NSo name is domain (eg., foo.com)

o value is hostname of authoritative name server for this domain

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

o Type=Ao name is hostnameo value is IP address

o Type=CNAMEo name is alias name for some

“canonical” (the real) name, eg., www.ibm.com is really servereast.backup2.ibm.com

o value is canonical name

o Type=MXo value is name of mailserver

associated with name

Summary

• DNS provides a mechanism for maintaining the user friendliness of the Internet by hiding some of the operational details.

• DNS servers have to be created manually. Recently an update protocol was introduced that allows DNS to exchange data for additions and deletions.

E - Mail

• Electronic mail is among the most widely available application services. Each user, who intends to participate in email communication, is assigned a mailbox, where out-going and incoming messages are buffered, allowing the transfer to take place in the background.

Electronic Mail

Three major components:

user agents mail reader composing, editing,

reading mail messages mail servers

mailbox contains incoming messages for user

message queue of outgoing (to be sent) mail messages

simple mail transfer protocol: SMTP to send email messages

between mail servers

user mailbox

outgoing message queue

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 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

Scenario: Alice sends message to Bob1) Alice uses UA to compose message and “to”

bob@someschool.edu2) Alice’s UA sends message to her mail server; message

placed in message queue3) Client side of SMTP opens TCP connection with Bob’s

mail server4) SMTP client sends Alice’s message over the TCP

connection5) Bob’s mail server places the message in Bob’s mailbox6) Bob invokes his user agent to read message

useragent

mailserver

mailserver user

agent

1

2 3 4 56

Mail message format

SMTP: protocol for exchanging email msgsRFC 822: standard for text message format:

header lines, e.g., To: From: Subject:different from SMTP commands!

body the “message”, ASCII characters only

header

body

blankline

Message format: multimedia extensions

MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content

type

From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

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

Most Common Scenario

Web based E-mail

Post Office Protocol - 3

POP3 and IMAP

More about POP3• Previous example uses “download and delete” mode.• Bob cannot re-read e-mail if he changes client• “Download-and-keep”: copies of messages on different clients• POP3 is stateless across sessionsIMAP• Keep all messages in one place: the server• Allows user to organize messages in folders• IMAP keeps user state across sessions:

– names of folders and mappings between message IDs and folder name

WWW & HTTP

Web and HTTPFirst some jargon• 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• Each object is addressable by a URL• Example URL:

www.someschool.edu/someDept/pic.gif

host name path name

WWW Architecture

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

URLs – Uniform Resource Locaters

Some common URLs.

HTTP Command

Static Web Pages

Dynamic Pages

Dynamic Pages