1. Part II
Application Layer

2. Web caches (proxy server)
client
origin
server
client
origin
server
Application Layer

3. More about Web caching
why Web caching?
Application Layer

4. If-modified-since: <date> If-modified-since: <date>
Conditional GET
client
server
Goal: don’t send object if cache has up-to-date cached version
HTTP request msg
If-modified-since: <date>
object
not
modified
before
<date>
HTTP response
HTTP/1.0
304 Not Modified
HTTP request msg
If-modified-since: <date>
object
modified
after
<date>
HTTP response
HTTP/ OK
<data>
Application Layer

5. HTTP
Response response protocol but how can the server push updates to the client
Example instant messages
Does the client need to constantly resquest/ poll
Do you have a message for me
Answers full duplex websockets
Application Layer

6. WebSocket's Full duplex communication
Allows the webserver to send content without first being requested
Kind of new standardized in RFC 6455 in 2011
Application Layer

7. Request a socket
GET /chat HTTP/1.1 Host: server.example.com Upgrade: websocket Connection: Upgrade Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ== Origin: Sec-WebSocket-Protocol: chat, superchat Sec-WebSocket-Version: 13
Application Layer

8. Servrer Response
HTTP/ Switching Protocols Upgrade: websocket Connection: Upgrade Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo= Sec-WebSocket-Protocol: chat
dGhlIHNhbXBsZSBub25jZQ== + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
SHA1 Hash
Base 64 encoding
s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Application Layer

9. Subsequent Messages
Don’t follow frames/packets don’t follow standard framing:
There are 16 frame times 6 are use and 10 are reverse for the future.
Binary
Text
etc
Application Layer

10. JavaScript example of WebSocket
Application Layer

11. JavaScript webrequest
Application Layer

12. Chapter 2: outline 2.1 principles of network applications
app architectures
app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP
2.8 socket programming with TCP
Application Layer

13. FTP: the file transfer protocol
user
interface
client
FTP
server
user
at host
remote file
system
local file
system
ftp: RFC
ftp server: port 21
Application Layer

14. Application Layer

15. FTP: separate control, data connections
TCP control connection,
server port 21
TCP data connection,
server port 20
FTP
client
FTP
server
Application Layer

16. FTP commands, responses
sample commands:
sample return codes
Application Layer

17. Chapter 2: outline 2.1 principles of network applications
app architectures
app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP
2.8 socket programming with TCP
Application Layer

18. A Story: The Inventor of email
Still Alive
Shiva Ayyadurai
Application Layer

19. Electronic mail User Agent Three major components: outgoing
user mailbox
outgoing
message queue
Three major components:
User Agent
mail
server
user
agent
Application Layer

20. Electronic mail: mail servers
mailbox
message queue
SMTP protocol
user
agent
mail
server
user
agent
SMTP
mail
server
user
agent
SMTP
SMTP
user
agent
mail
server
user
agent
user
agent
Application Layer

21. Electronic Mail: SMTP [RFC 2821]
1 ) 2) 3) 4)
Application Layer

22. Scenario: Alice sends message to Bob
user
agent
user
agent
mail
server
mail
server
Alice’s mail server
Bob’s mail server
Application Layer

23. 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
Application Layer

24. SMTP: final words
comparison with HTTP & FTP:
Application Layer

25. Mail message format SMTP: header RFC 822: : header lines, e.g.,
different
Body:
header
blank
line
body
Application Layer

26. Mail access protocols SMTP: delivery/storage to receiver’s server
user
agent
SMTP
SMTP
user
agent
sender’s mail
server
(e.g., POP,
IMAP)
receiver’s mail
server
SMTP: delivery/storage to receiver’s server
mail access protocol: retrieval from server
POP:
IMAP:
HTTP:
Application Layer

27. POP3 protocol authorization phase transaction phase, client: C: list
S: +OK POP3 server ready
C: user bob
S: +OK
C: pass hungry
S: +OK user successfully logged on
authorization phase
client commands: : .
server responses
transaction phase, client:
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
Application Layer

28. Application Layer Protocols IOT
MQTT:
Message Queuing Telemetry Transport
Application layer protocol implement ontop of TCP/IP
Why not UDP
Small footprint
Andy Stanford-Clark
First version of the protocol in 1999 (IBM)
Application Layer

29. Why is MQTT interesting
This maybe one of the protocols that power the internet of things
Amazon has added MQTT support AWS and freeRTOS
Application Layer

30. How does MQTT work Topics
There is no need to configure a topic, publishing on it is enough.
Topics are treated as a hierarchy
Temperature/roof
Application Layer