1. Applications Outline Email (SMTP) Web (HTTP) Network management (SNMP)
Name Resolution (DNS)
Multimedia (SIP/H.323)

2. Overview Client-server model
Client sends a request
Server responds with a reply
Protocols specify the set of requests/responses, with companion protocols specifying data format
SMTP with RFC 822 and MIME
HTTP with HTML
SNMP with MIB
Programs vs. protocols
Different browsers use the sample protocol (HTTP), etc.

3. Email: Message Format RFC 822
Based on ASCII text
Message has two parts
Header: Date, From, Subject, To, etc
Body
MIME (Multipurpose Internet Mail Extension)
Extend RFC 822 to carry formatted documents (MS Word), image, audio, video, etc.
Header lines + Message Body
Header lines: MIME-Version, Content-Description, Content-Type, Content-Transfer-Encoding, etc.
Can be Multipart, each having a different data type

4. Email: Message Format Data encoding (Content-Transfer-Encoding)
Text: 7bit ASCII
Arbitrary binary
Base64 encoding: Encode 3 bytes binary into 4 bytes of ASCII (a 6b subset)
Date: Fri, 03 Nov :09:
From: Hotline
Subject: NMSU Hotline -- Nov. 3, 2006
To: NMSU Hotline
Message-id:
MIME-version: 1.0
X-MIMEOLE: Produced By Microsoft MimeOLE V
X-Mailer: Microsoft Outlook Express
Content-type: text/plain; format=flowed; charset=iso ; reply-type=original
Content-transfer-encoding: 8BIT

5. Email: Message Transfer
Building blocks
Mail reader
MS Outlook
Mail daemon
variants of sendmail
background process that sends/receives mails through SMTP over TCP
Mail gateway
on the way between sender and receiver
store and forward mail, can retry for days until successful
Why not send mail from sender’s daemon to the receivers’ directly
receiver has the freedom to read mail at different hosts
receiver’s host is turned off

6. Email: Message Transfer

7. Email: Message Transfer
SMTP: a request and reply protocol, client posts a command, server returns a code
telnet mail.nmsu.edu 25
HELO nmsu.edu % client identifies its domain name
250 OK luminis- .nmsu.edu % server echoes back
MAIL
250
RCPT
DATA
354 Start mail input
Blah blah blah
<CRLF>.<CRLF> % signals the end.
QUIT
221 Closing connection

8. Mail Reader
If mail reader resides at the same host as the mailbox (daemon), it can read mail directly from the mailbox
If not, two main options:
POP (Post Office Protocol)
IMAP (Message Access Protocol)

9. Web: Three Key Components
URL: Uniform Resource Locator
Specifying protocol, server, resource
e.g.,
HTML: HyperText Markup Language
In ASCII text
Format text, reference resources (images, etc)/hyperlinks
Interpreted by Web browsers when rendering a page
HTTP: HyperText Transfer Protocol

10. HTTP
A request-response protocol for transferring text/image/video, etc.
A message consists of
Start line <CRLF> (request/response)
Message header <CRLF><CRLF> (parameters of the request/response)
Message body <CRLF>
Example:
Telnet 80
GET % client start line
HTTP/ Accepted (or 404 Not Found) % server start line
Message body is the HTML text, nontext (images) encoded in MIME

11. HTTP Request Operations
Description
OPTIONS
Request information about available options
GET
Retrieve document identified in URL
HEAD
Retrieve meta-information about document identified in URL
POST
Give information (e.g., annotation) to server
PUT
Store document under specified URL
DELETE
Delete specified URL
TRACE
Loopback request message
CONNECT
For use by proxies

12. HTTP Response Codes Code Type Example Reasons 1xx Informational
Request received, continuing process
2xx
Success
Action successfully received, understood, and accepted
3xx
Redirection
Further action must be taken to complete the request
4xx
Client error
Request contains bad syntax or cannot be fulfilled
5xx
Server error
Server failed to fulfill an apparently valid request

13. HTTP Versions HTTP 1.0 requires one TCP connection for each data item
HTTP 1.1 allows persistent connection, where multiple data items use the same TCP connection
Advantages: less connection setup overhead, higher throughput (just one slow-start)
Problem: less certain how long the server should keep the connection alive, need to properly choose time-out period.
HTTP/2 optimized for increased speed.

14. Web: Caching Caching can be done at
Client’s browser
Local site (proxy)
Routers close to client in the ISP network
Cache lifetime: can be determined by the Expires header field of the web page
Benefits
Client: less delay; server: less load

15. Network Management: SNMP
A request-response protocol over UDP
Request: get/set variables in MIB
MIB (management information base)
Use ASN.1 as data presentation format
Include variables such as
System: node location, up time, host name
Interfaces: physical address, #packets sent/received
ARP table
IP: routing table, #packets forwarded/dropped, etc.
TCP: #open conn/resets/timeouts/
UDP: #UDP pact sent/received.

16. Name Service: DNS DNS (domain name system) Terminology
Translate a name ( into an IP address ( ).
Names are meaningfully structured, easily to remember
Terminology
Name space: a collection of names, which can be
Flat: not structured
Hierarchical: e.g. URL
Name server resolves a name to its value (address)

17. Domain Naming System Hierarchy edu com gov mil org net uk fr princeton
■ ■ ■
mit
cisco
■ ■ ■
yahoo
nasa
■ ■ ■
nsf
arpa
■ ■ ■
navy
acm
■ ■ ■
ieee cs ee
physics
ux01
ux04

18. Name Servers Partition hierarchy into zones
edu
com
princeton
■ ■ ■
mit cs ee
ux01
ux04
physics
cisco
yahoo
nasa
nsf
arpa
navy
acm
ieee
gov
mil
org
net uk fr
Root
name server
Each zone implemented by two or more name servers (for redundancy)
Princeton
Cisco
■ ■ ■
name server
name server CS
■ ■ ■ EE
name server
name server

19. Resource Records
Each name server maintains a collection of resource records
(Name, Value, Type, Class, TTL)
Name/Value: not necessarily host names to IP addresses
Type
A: name to ip address mapping
NS: value gives domain name for host running name server that knows how to resolve names within specified domain (pointer).
CNAME: value gives canonical name for particular host (alias).
MX: value gives domain name for host running mail server that accepts messages for specified domain (alias for ).
Class
allow other entities to define types, currently just IN (internet)
TTL
how long the resource record is valid

20. Root Server (princeton.edu, cit.princeton.edu, NS, IN)
(cit.princeton.edu, , A, IN)
(cisco.com, thumper.cisco.com, NS, IN)
(thumper.cisco.com, , A, IN) …

21. Leave level server lists A records for all hosts
Princeton Server
(cs.princeton.edu, optima.cs.princeton.edu, NS, IN)
(optima.cs.princeton.edu, , A, IN)
(ee.princeton.edu, helios.ee.princeton.edu, NS, IN)
(helios.ee.princeton.edu, , A, IN)
(jupiter.physics.princeton.edu, , A, IN)
(saturn.physics.princeton.edu, , A, IN)
(mars.physics.princeton.edu, , A, IN)
(venus.physics.princeton.edu, , A, IN)
Leave level server lists A records for all hosts

22. CS Server (cs.princeton.edu, optima.cs.princeton.edu, MX, IN)
(cheltenham.cs.princeton.edu, , A, IN)
(che.cs.princeton.edu, cheltenham.cs.princeton.edu, CNAME, IN)
(optima.cs.princeton.edu, , A, IN)
(opt.cs.princeton.edu, optima.cs.princeton.edu, CNAME, IN)
(baskerville.cs.princeton.edu, , A, IN)
(bas.cs.princeton.edu, baskerville.cs.princeton.edu, CNAME, IN)

23. Name Resolution Strategies Local server forward iterative recursive
knows root server (host knows the local server)
site-wide cache

24. Web Services
Web services are those provided to applications over the network
An order fulfillment application provide service to an order application without manual intervention
Two examples
SOAP
a framework and software toolkits for generating protocols customized to each network application
REST
based on HTTP, regarding a web service as a web resource identified by a URL, accessed by HTTP

25. Multimedia Applications
Building blocks
Resource provision/QoS: RSVP
Data transport: RTP/RTCP
Session control: SIP, H.323

26. Session Control Protocols
IETF protocols
SDP (session description protocol)
SAP (session announcement protocol)
SIP (session initiation protocol)
SCCP (simple conference control protocol)
Usage examples
Webcasting a training session (SDP+SAP)
Making a VOIP call (SDP+SIP)

27. SDP SDP conveys the following information
Name, purpose of the session
Start, end time
Media types (audio, video, etc.)
Multicast address, transport protocol, port number, encoding
SDP messages are in ASCII text, each line formated as <type> = <value>

28. SIP
SIP a request-reply protocol, similar to , and provides the following functions
Locating user and determining user availability
Different from (no availability requirement)
Determining user’s choice of media, encoding scheme, etc
Wide range of devices: PC, laptop, PDA, etc.
Setting up a session and related parameters (e.g. #port)
Managing the session (e.g., transfer session/call forwarding, modify parameters)

29. SIP
Sessions are facilitated by SIP proxies, similar to agents in Mobile IP
Example
Sender (bruce) sends a message to his local proxy, which is forwarded to receiver’s proxy
SIP:
Receiver’s proxy forwards the message to one or more devices (IP addresses) of the receiver’ choice
Sending a message to multiple devices can be parallel or serial (if no answer from PC, send to cell phone)
Proxy maintains a user location database, which users update by sending registration messages

30. SIP

31. SIP Telphone (one hop): Connect Ringing OK(pick up) Media
Hung up 
Invite and OK can contain SDP message to negotiate parameter

32. H.323
A ITU multimedia over IP standard, can somewhat interoperate with IETF standards
Companion protocols
H.255 for call control, like SIP
H.245 for call property description, like SDP
Gatekeeper
Translating among address formats, admission control, locating a gateway
Gateway
Connect a H.323 network to telephone network (PSTN), translating signaling and media between the two

33. H.323