Application Layer Chapt 7 1

The Application Layer

• The web: HTTP, HTML, XML, I-mode, WAP2, lynx, DNS

• Mail

• Multimedia

• ftp, telnet, finger

• Security at host

Application Layer Chapt 7 2

DNS (Domain Name System)

• Hierarchical, domain-based naming scheme• Distributed database maintained on name servers

– Provides IP (dotted numbered) addresses for host name

• Application program calls “resolver” which contacts name server via UDP packet– (Unless address is cached)– UDP packet returned with IP address (or error message)

• Application program sends TCP connection request to supplied IP address (note: process # is not included)

Application Layer Chapt 7 3

DNS Name Space

• Hierarchical domain– Top level

• Countries (besides US), generic in US– Generic may be second level in other countries

» Includes com, edu, gov, mil, org, (biz, …)

» Top level assigned by ICANN (Internet Corporation for Assigned Names and Numbers)

– Countries include jp (Japan), in (India), nl (Netherlands), us (default; can be omitted)

Application Layer Chapt 7 4

More about domain names

• Case insensitive (although UNIX path may not be)

• Each domain allocates subdomains– fdu.edu

• alpha.fdu.edu

• zeus.fdu.edu

Application Layer Chapt 7 5

Each Domain Server maintains:

• Resource record table – Fields:

• Domain_name; TTL; class; type; value– Domain_name (e.g., fdu.edu)– TTL: used to limit caching time– Class: (IN for Internet; others)– Type

» A (for each domain – specifies one or more IP addresses)» SOA- for top record of that domain (contains email of

admin, zone id, flags)» MX – identifies mail server » NS- identifies domain’s name server

– Value: data for type field

Application Layer Chapt 7 6

• How to find information from the DNS database

• http://www.dnsstuff.com

information has been limited over the years

Database – DNS lookup

Application Layer Chapt 7 7

Zoned Name Space:

• Database is partitioned among name servers– Partitioning is up to domains– Depends on how many name servers it will

have active

Application Layer Chapt 7 8

Sample query1) Resolver (or proxy server) queries local name

servers– Local name servers can supply “authoritative record” of

records in its domain– May return cached record of remote domain

• TTL field to limit this

2) Else resolver queries Name Server of top domain• These addresses are “well known”; redundant servers• Each name server knows addresses of its subdomains

– Authoritative record returned» Cached for later use

• (Alternatively, address of name server is returned for host query)

Application Layer Chapt 7 9

Name Servers

• At different levels (assume addresses are not cached)– Flits.cs.vu.nl to linda.cs.yale.edu

• Query to its local name server cs.vu.nl– Query to edu name server

» (known to local name server)» Query to yale.edu domain server query to the cs department at yale.edu address returned» Address returned

– Address returned

• Address returned

• Alternatively, each server returns address for next query

Application Layer Chapt 7 10

DNS Vulnerabilities

• "How Vulnerable Is the 'Net?"Network World (04/18/05); Duffy, Jim

“The Internet is vulnerable to hacker-induced disruption, but some points are more prone to attack than others, according to experts. The DNS core infrastructure has been significantly strengthened since a major denial-of-service attack hit the system in October 2002; at that time, DNS servers were located at 13 sites in four countries, but now the DNS infrastructure comprises between 50 and 100 machines in 80 locations spread across 34 countries. But the highly distributed nature of the DNS infrastructure also makes it more difficult to defend, argues independent security consultant Stephen Cobb, who says the Internet could be disabled for as many as 10 days if attackers had inside knowledge.”

Application Layer Chapt 7 11

Application – electronic mail

• Netiquette– Smileys and emoticons

• RFC 822 won over the CCITT (ITU) X.400 recommendation for standardizing e-mail

• Email system is layered– User agent (user interface – ex: IMAP, POP)

• Today these have GUIs– Message transfer agent or daemon (implementation for

sending email – ex: SMTP)– Message formatting protocol (ex: MIME, RFC 822)

• Mailbox is created to buffer incoming mail• Mailing lists allow group mail

Application Layer Chapt 7 12

Mail services

• Composition (compose)– Services such as automatically inserting sender’s email

to reply; attaching document in forward– Transfer (send)– Reporting

• Confirmation of non-delivery (NAK) and possible delivery

– Displaying • Special viewer may be used for word, adobe, etc.• Formatting

– Disposition – discard, saving, mailboxes

Mail layers

• Message transfer protocol– Ex: SMTP

• Companion protocol– Ex: RFC 822 and MIME

• User Interface– Ex: Post Office Protocol, Internet Message

Access Protocol• Similar to SMTP exchange following

Application Layer Chapt 7 13

RFC822

• Messages are 2 parts: header and body, both in ASCII text (although today the ASCII can be an encoded version of something else.)– Header: <CRLF> for each header line

• Required fields -- To: Subject: Date: From:, etc.

• Extended in 1993, 1996 by MIME to allow body to contain different types of data

Application Layer Chapt 7 14

Application Layer Chapt 7 15

MIME

• Multipurpose Internet Mail Extensions– Uses RFC 822 format, but adds header:

• MIME version # (if missing, MIME is not in use)• Content-Description• Content-ID• Identification of Encoding (if any)

– Base64 for binary code (encodes each 24 binary bits into a valid ASCII pattern)

• Identification of types and format of data:– Text/ plain or richtext -- Image/ gif, jpeg -- Audio/ basic, MP3– Video/MPEG – Application/msword -- Multipart/mixed

Application Layer Chapt 7 16

SMTP

• Simple Mail Transfer Protocol– What if mail is on ISP?

• POP3, IMPS define delivery to final hop – Allows ISP to filter email (SPAM), forward mail, return canned message

– Tanenbaum (in his previous text) suggested you try:• On the alpha and on windows (command mode)• Enable telnet by program and features/turn windows features on or

off/ check telnet/client– telnet <your ISP’s mail server’s name> <port #>– Try 25 for port# - that is SMTP port

» Such as: telnet mail.fdu.edu 25– when you get connection, type

» HELP– You’ll get list of commands that are supported– QUIT to exit– Note: you are creating a connection – Norton may not like that

SMTP example from Computer Networks: A Systems Approach

 By Larry L. Peterson, Bruce S. Davie rom

Bob at Princeton emails Alice & Tom at CiscoHELO cs.princeton.edu

250 Hello daemon@mail.cs.princton.edu [128.112.169.24]

MAIL FROM: <Bob.princeton.edu>

250 OK

RCPT TO: <Alice@cisco.com>

250 OK

RCPT TO: <Tom@cisco.com>

550 OK No such user here

DATA

354 ...message data… done.

<CRLF>

250 OK

QUIT

221 Closing Transaction

Application Layer Chapt 7 17

• ----Original Message-----From: MAILER-DAEMON@fdu.edu [mailto:MAILER-DAEMON@fdu.edu] Sent: Tuesday, November 23, 2010 2:53 PMTo: levine@fdu.eduSubject: Undeliverable mail: <no subject>

•  

• Failed to deliver to 'levine@fdu.edu'

• Message is not submitted using approved methods. Please use proper MX records and mail client settings as applicable to FDU policy.

• Try this on a Linux system with a mail agent 

Application Layer Chapt 7 18

WWW and the Internet

• WWW (or WWWn if a server farm is used) is not the same thing as the Internet– Set of server pages and web protocols

• The Internet is a set of routers, switches, links, hosts, and Internet software/firmware– The Internet preceded the WEB– Hosts linked together by the Internet transferred

information (perhaps with anonymous ftp sites)

Application Layer Chapt 7 19

HTTP• Cooperating clients and servers that link

through URLs (hypertext links)

• Most people use GUIs (images; videos)

• HTTP’s message types include:– GET– PUT– CONNECT (for proxies)– HEAD (to obtain metadata)

Application Layer Chapt 7 20

HTTP codes

• 202 accepted

• 404 not found

• 301 moved permanently

• 5xx server error

Application Layer Chapt 7 21

SNMP (Network Management)• Protocol that allows distributed nodes to

collect and possibly disseminate state information

• Request/replies => GET and SET

• MIB (Management Information Base)– System parameters; interfaces/ adaptors, ARP,

IP (routing tables, lost packets, etc.), TCP connection information, UDP traffic, control protocols

Application Layer Chapt 7 22

Application Layer Chapt 7 23

Multimedia Applications

• Continuous• User interaction• Typically audio & video (text, haptics,

olfaction,etc.)• Possibly multi-stream input

– Multiple inputs

• Possibly teleconferencing – Synchronizing input streams

Application Layer Chapt 7 24

Audio

• Acoustic (pressure) wave to input device (ear)• Human can input in range between 20Hz –20kHz

– Distinguish about 1M distinct levels– Sensitive to changes in a few msec (jitter)

• Telephone lines limit to 4 kHz band– 8000 samples/sec sufficient (Nyquist)– 128-256 levels introduce quantizing noise– PCM 56-64kbps requirement– Less with compression (typically differential value is

transmitted)

Application Layer Chapt 7 25

Audio- CDs

• 44100 samples/sec – 22kHz band• 16 bit samples, linear - 65k levels• 705.6 kbps, 1.4Mbps for stereo needed

– Without compression

• MIDI standard for Music synthesizing– Standard for connector, cable, etc.– Encodes type of instrument– Event (foot pedal, key)

• Parameters (note value, amplitude, length)

Application Layer Chapt 7 26

Analog Video

• Black and white 2-dimensional analog– 525 levels (digital)– Diagonal trace; horizontal, vertical retrace– Transmits intensities (analog)– 25-30 frames/second

• Humans need about 60 frames/sec to eliminate jitter• Interlacing of lines displayed• No buffer

• 4-6MHz needed with guard bands• Analog HDTV has 1000 scan lines

Application Layer Chapt 7 27

Digital Video

• Pixels of 8 bits for black & white intensity• 3 * 8 or 24 bits for R/G/B

• CIF digital television format– 360 x 288 pixels sent every 30th of a second

• In black & white ( 8 bits) – 25Mbps

• CCIR digital television format– 720 x 576 every 30th of a second

• HDTV – 1280 x 720 every 60th of a second

Application Layer Chapt 7 28

Digital audio and video

• Buffers store data to smooth jitter– Repeat frame or interpolate value for lost frame

• Compression algorithms– Differentials are sent– Some frames are more “equal” than others– Routers have to ensure that first (I) frame in a

sequence of differentials is not discarded