1. Computer Networks 364 Protocols
John Morris
Computer Science/Electrical Engineering
University of Auckland
URL:

2. Protocols - HTTP HyperText Transfer Protocol (HTTP)
WWW application layer protocol
Client: browser (Netscape, Opera, that other one, … )
Server: a web server (source of Web pages - Apache, … )
Defines the language used by clients to request web pages
RFC 2616 (HTTP/1.1) [ RFC 1945 (HTTP/1.0) ]
Default port is 80
RFC = Request for Comment
Now managed by the Internet Engineering Task Force (IETF) Over 2000 RFCs
Standards for the Internet

3. Protocols - HTTP HyperText Transfer Protocol (HTTP)
Web pages consist of a number of objects
Basic page
Embedded images, etc
Each object is fetched from the server in a single session
Open TCP connection
GET message from client
Response from server with object
Close connection
HTTP is stateless
Server does not keep track of state of session with client
Each request/response pair is independent of any other
Suitable for information serving only applications
Transaction oriented applications
eg database update
generally require some state to be maintained
HTTP makes it difficult to implement ‘safe’ transaction based systems
but
Cookies provide a simple mechanism for maintaining state
Stateless protocols are simpler!

4. Protocols - HTTP HyperText Transfer Protocol (HTTP)
Web pages consist of a number of objects
...
Each object is fetched from the server in a single session
Open TCP connection
GET message from client
Response from server with object
Close connection
Obviously rather inefficient
TCP connection establishment is expensive
Persistent connections
TCP connection is left open for subsequent requests
Further efficiency from pipelining
Send additional requests before first response received
Allows browser to do useful work while server is fetching objects
Parsing to discover embedded objects,
Formatting and displaying pages, etc

5. } HTTP example GET /somedir/page.html HTTP/1.1
Method
URL
Version
GET /somedir/page.html HTTP/1.1
Host:
Connection: close
User-agent: Mozilla/4.0
Accept-language:fr
(extra carriage return, line feed)
Request line }
Header lines
Methods: GET POST HEAD

6. HTTP request messages
General form

7. HTTP response HTTP/1.1 200 OK Connection: close
Version
Status code
Status message
Status line
HTTP/ OK
Connection: close
Date: Thu, 06 Aug :00:15 GMT
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun :23:24 GMT
Content-Length: 6821
Content-Type: text/html
(data data data data data . . .)
Header lines
Entity body

8. HTTP response - general format

9. HTTP response - common status codes
* 200 OK: Request succeeded and the information is returned in the response.
* 301 Moved Permanently: Requested object has been permanently moved; new URL is specified in Location: header of the response message. The client software will automatically retrieve the new URL.
* 400 Bad Request: A generic error code indicating that the request could not be understood by the server.
* 404 Not Found: The requested document does not exist on this server.
* 505 HTTP Version Not Supported: The requested HTTP protocol version is not supported by the server.

10. Try out http (client side) for yourself
1. Telnet to your favorite Web server:
telnet 80
Opens TCP connection to port 80
(default http server port) at
Anything typed in sent
to port 80 at
2. Type in a GET http request:
By typing this in (hit carriage
return twice), you send
this minimal (but complete)
GET request to http server
GET /~ross/index.html HTTP/1.0
3. Look at response message sent by http server!

11. User-server interaction: authentication
Authentication : control access to server content
authorization credentials: typically name, password
stateless: client must present authorization in each request
authorization: header line in each request
if no authorization: header, server refuses access, sends
WWW authenticate:
header line in response
client
server
usual http request msg
401: authorization req.
WWW authenticate:
usual http request msg
+ Authorization: <cred>
usual http response msg
usual http request msg
+ Authorization: <cred>
time
usual http response msg

12. Cookies: keeping “state”
client
server
Server-generated #
rembered by server
Later used for:
authentication
remembering
user preferences,
previous choices
Server sends “cookie” to client in response msg
Set-cookie:
Client presents cookie in later requests
cookie:
usual http request msg
usual http response +
Set-cookie: #
usual http request msg
cookie: #
cookie-
spectific
action
usual http response msg
usual http request msg
cookie: #
cookie-
spectific
action
usual http response msg

13. Conditional GET: client-side caching
server
Goal
Don’t send object if client has up-to-date cached version
Client
Specify date of cached copy in http request
If-modified-since: <date>
Server
Response has no object if cached copy is up-to-date:
HTTP/ Not Modified
http request message
If-modified-since: <date>
object
not
modified
http response
HTTP/ Not Modified
http request message
If-modified-since: <date>
object
modified
http response
HTTP/ OK <data>

14. Web Caches (proxy server)
Goal
Satisfy client request without involving original server
User sets browser
Web accesses via web cache
Client sends all http requests to web cache
Object in web cache
web cache returns object
else
web cache requests object from original server,
returns object to client
origin
server
Proxy
server
http request
http request
client
http response
http response
http request
http response
client
origin
server

15. Why Web Caching? Assume origin Cache is “close” to client servers
eg in same network
Smaller response time
Cache “closer” to client
Decrease traffic to distant servers
Link out of local network often bottleneck
Cache works on locality of reference principle
Recently used objects more likely to be needed again
Temporal locality
Keep them ‘closer’
Processor caches use the same principle (+ spatial locality!)
origin
servers
public
Internet
1.5 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache

16. FTP: File Transfer Protocol
user
interface
client
file transfer
FTP
server 21
user
at host
local file
system
remote file
system
Transfer file(s) to/from remote host
Client/Server model
Client: side that initiates transfer (either to/from remote)
Server: remote host
RFC 959
ftp server: port 21

17. FTP: Separate Control and Data Connections
FTP client contacts server at port 21
Specifies TCP as transport protocol
Two parallel TCP connections opened:
Control connection
Exchange commands, responses between client and server
Out of band control
Data connection
File data to / from server
FTP server maintains state
Current directory
Earlier authentication
FTP
client
server
TCP control connection
port 21
TCP data connection
port 20

18. FTP Commands and Responses
Sample commands:
Sent as ASCII text over control channel
USER username
PASS password
LIST
Return list of files in current directory
RETR filename
Retrieves (gets) file
STOR filename
Stores (puts) file onto remote host
Sample return codes
Status Code and Phrase (as in HTTP)
331 Username OK, password required
125 data connection already open; transfer starting
425 Can’t open data connection
452 Error writing file

19. Electronic Mail SMTP Three major components: User agents Mail servers
user mailbox
outgoing
message queue
user
agent
mail
server
SMTP
Three major components:
User agents
Mail servers
Simple Mail Transfer Protocol: SMTP
User Agent
Mail reader
Composing, editing, reading mail messages
Examples
Eudora, Outlook, elm, Netscape Messenger
Outgoing, incoming messages stored on server

20. eMail: Mail servers SMTP Mail Servers
user
agent
mail
server
SMTP
Mail Servers
Mailbox contains incoming messages (yet to be read) for user
Message queue of outgoing (to be sent) mail messages
SMTP protocol
Used between mail servers to send messages
Client: sending mail server
Server: receiving mail server

21. eMail: SMTP Messages must be in 7-bit ASCII RFC 821
First published: 1982
Uses TCP to reliably transfer message
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
Legacy of 1982
Binary data must be encoded before transfer

22. Sample SMTP interaction
S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr, pleased to meet you
C: MAIL FROM:
S: 250 Sender ok
C: RCPT TO:
S: 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
This is why 7-bit ASCII is required!

23. Exercise: make a simple Java mail sender
Try SMTP yourself
telnet servername 25
See 220 reply from server
Enter
HELO
MAIL FROM
RCPT TO
DATA
QUIT
HELP
commands
You can send
Using telnet to send commands yourself
By writing a simple program to do it for you!
Exercise: make a simple Java mail sender
Able to send messages directly from programs

24. SMTP: Final Words Comparison with HTTP: Uses persistent connections
Requires message (header & body) to be in 7-bit ASCII
Certain character strings not permitted in message
Example
CRLF.CRLF
Thus message has to be encoded
Usually
base-64 or
quoted printable
Server uses CRLF.CRLF to determine end of message
Comparison with HTTP:
HTTP: pull
push
Command and response, interaction and status codes
All ASCII in both
HTTP
Each object encapsulated in its own response message
SMTP
Multiple objects sent in multipart message

25. Mail message format header body blank line RFC 821
SMTP protocol for exchanging messages
RFC 822
Text message format
Header lines, e.g.
To:
From:
Subject:
Different from SMTP commands!
Defines semantics (interpretation) also
Body
The “message”
ASCII characters only!
header
blank
line
body

26. Message format: Multimedia extensions
RFC 822 format OK for text messages
Inefficient for multimedia
Multipurpose Internet Mail Extensions (MIME)
RFC 2045, 2056
Additional lines in message header declare MIME content type
MIME version
From:
To:
Subject: Picture of yummy crepe
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
......base64 encoded data
Method used
to encode data
Multimedia data
type, subtype,
parameters
Encoded data

27. MIME types Content-Type: type/subtype; parameters
Text
Subtypes: plain, html, ...
Image
Subtypes: jpeg, gif
Audio
Subtypes
basic
8-bit -law encoded
32kadpcm
32 Kbps coding (RFC 1911)
Video
Subtypes: mpeg, quicktime
Application
Other data that must be processed by reader before becoming “viewable”
Subtypes
msword
octet-stream
Arbitrary binary data

28. MIME: Multipart Type Arbitrary ASCII string which defines boundaries
From:
To:
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain
Dear Bob,
Please find a picture of a crepe.
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
......base64 encoded data
Arbitrary
ASCII string
which defines
boundaries
of a part

29. Mail Access Protocols SMTP SMTP POP3 or IMAP
user
agent
user
agent
sender’s mail
server
receiver’s mail
server
SMTP: delivery to and storage on receiver’s server
Mail access protocol: retrieval from server
POP: Post Office Protocol [RFC 1939]
Simple, limited functions
Authorization (agent  server) and download
IMAP: Internet Mail Access Protocol [RFC 2060]
More features (more complex)
Manipulation of stored messages on server
Set up, search folders, etc
HTTP: Hotmail , Yahoo! Mail, etc

30. POP3 Protocol C: list Authorization phase Transaction phaseclient:
S: +OK POP3 server ready
C: user alice
S: +OK
C: pass hungry
S: +OK user successfully logged on
Authorization phase
Client commands:
user: declare username
pass: password
Server responses
+OK
-ERR
Transaction phaseclient:
list: list message numbers
retr: retrieve message by number
dele: delete
quit
C: list
S: 1 498
S: 2 912
S: .
C: retr 1
S: <message 1 contents>
C: dele 1
C: retr 2
C: dele 2
C: quit
S: +OK POP3 server signing off

31. DNS: Domain Name System
People: Many identifiers:
IRD #, name, passport #
Internet hosts, routers:
IP address (32 bit)
Used for in datagrams
“name” eg gaia.cs.umass.edu
Used by humans (with some exceptions!)
? 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”

32. DNS name servers No server has all name  IP address mappings
Why not centralize DNS?
Single point of failure
Congestion
Traffic volume on central server
Distance
Time to reach centralized database
Maintenance
Doesn’t scale!
No server has all name  IP address mappings
Local name servers:
Each ISP, company has local (default) name server
DNS query first goes to local name server
Authoritative name server:
For a host: stores that host’s IP address, name
Can perform name/address translation for that host’s name

33. 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
b USC-ISI Marina del Rey, CA
l ICANN Marina del Rey, CA
e NASA Mt View, CA
f Internet Software C. Palo Alto, CA
i NORDUnet Stockholm
k RIPE London
m WIDE Tokyo
a NSI Herndon, VA
c PSInet Herndon, VA
d U Maryland College Park, MD
g DISA Vienna, VA
h ARL Aberdeen, MD
j NSI (TBD) Herndon, VA
13 root name servers worldwide

34. authorititive name server
Simple DNS example
root name server
Host surf.eurecom.fr wants IP address of gaia.cs.umass.edu
1. Contacts its local DNS server, dns.eurecom.fr
2. dns.eurecom.fr contacts root name server, if necessary
3. Root name server contacts authoritative name server, dns.umass.edu, if necessary 2 4 3 5
authorititive name server
dns.umass.edu
local name server
dns.eurecom.fr 1 6
requesting host
surf.eurecom.fr
gaia.cs.umass.edu

35. DNS example Root name server: root name server
May not know authoritative name server
May know intermediate name server: who to contact to find authoritative name server 2 6 7 3
local name server
dns.eurecom.fr
intermediate name server
dns.umass.edu 4 5 1 8
authoritative name server
dns.cs.umass.edu
requesting host
surf.eurecom.fr
gaia.cs.umass.edu

36. DNS: iterated queries Recursive query: Iterated query:
root name server
Recursive query:
Puts burden of name resolution on contacted name server
Heavy load?
Iterated query:
Contacted server replies with name of server to contact
“I don’t know this name, but ask this server”
iterated query 2 3 4 7
local name server
dns.eurecom.fr
intermediate name server
dns.umass.edu 5 6 1 8
authoritative name server
dns.cs.umass.edu
requesting host
surf.eurecom.fr
gaia.cs.umass.edu

37. DNS: Caching and updating records
Once (any) name server learns mapping, it caches mapping
Cache entries timeout (disappear) after some time
Update/notify mechanisms being designed by IETF
RFC 2136

38. RR format: (name, value, type, ttl)
DNS records
DNS: Distributed database storing resource records (RR)
RR format: (name, value, type, ttl)
Type=A
name is hostname
value is IP address
Type=CNAME
name is alias name for some “cannonical” (real) name
is really servereast.backup2.ibm.com
value is cannonical name
Type=NS
name is domain (eg foo.com)
value is IP address of authoritative name server for this domain
Type=MX
value is name of mailserver associated with name

39. DNS protocol, messages DNS protocol query and reply messages
both with same message format
Message header
Identification
16 bit # for query, reply to query uses same #
Flags
query or reply
recursion desired
recursion available
reply is authoritative

40. DNS protocol, messages Name, type fields for a query RRs in reponse
to query
Records for
authoritative servers
Additional “helpful”
info that may be used