Messages in folders keeps user state across sessions

messages in folders
• keeps user state across
sessions:
– names of folders and
mappings between
message IDs and folder
name

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ASCII Code
• Basic E‐mail message contents are sent using
ASCII codes
– American Standard Code for Information
Interchange – defined by AT&T in 1963.
• Originally 7 bits per character.
– Apps add ‘0’ bit as 8 th bit so it uses 1 byte per
character.
• To send anything outside ASCII codes requires
“Transfer Encoding” method that converts
non‐ASCII to ASCII.
Printable ASCII Characters
Hex Char Hex Char Hex Char Hex Char Hex Char Hex Char
20 30 0 40 @ 50 P 60 ` 70 p
21 ! 31 1 41 A 51 Q 61 a 71 q
22 " 32 2 42 B 52 R 62 b 72 r
23 # 33 3 43 C 53 S 63 c 73 s
24 $ 34 4 44 D 54 T 64 d 74 t
25 % 35 5 45 E 55 U 65 e 75 u
26 & 36 6 46 F 56 V 66 f 76 v
27 ' 37 7 47 G 57 W 67 g 77 w
28 ( 38 8 48 H 58 X 68 h 78 x
29 ) 39 9 49 I 59 Y 69 i 79 y
2A * 3A : 4A J 5A Z 6A j 7A z
2B + 3B ; 4B K 5B [ 6B k 7B {
2C , 3C < 4C L 5C \ 6C l 7C |
2D ‐ 3D = 4D M 5D ] 6D m 7D }
2E . 3E > 4E N 5E ^ 6E n 7E ~
2F / 3F ? 4F O 5F _ 6F o 7F

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E‐mail Addressing
• Format: <local part> @ <DNS name>
– The <DNS name> (example: ‘gmail.com’) is looked up as
an MX record on DNS server.
– The <local part> is passed to the SMTP server to identify a
particular mailbox on that server. This is typically a user
name on the e‐mail system.
How does HTTP work?

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Common Application Layer Protocols
Hypertext Transfer Protocol / Markup Language
Example
URL:
• First, the browser interprets the three parts of the URL:
– 1. http (the protocol or scheme)
– 2. (the server name)
– 3. index.html (the specific file name requested)
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Hypertext Transfer Protocol / Markup Language
URL:
• Browser checks with a DNS server to convert
into a numeric address
• Using the HTTP protocol requirements , sends a GET
request to the server and asks for the file index.html
• Server sends the HTML code for this web page
• Browser deciphers the HTML code and formats the page
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HTTP and HTTPS
• HTTP developed to
publish and retrieve HTML pages
• Used for data transfer
• Specifies a request/response protocol
• Three common message types are GET,
POST, and PUT
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HTTP message formats
• HTTP message formats are very similar to SMTP formats:
– Client requests consist of 4‐letter command keyword followed
by URL, options and data.
– Server responses consist of 3‐digit response code followed by
options and data
• More in RFC 2616 (HTTP 1.1)
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Brief history
HTTP/1.1
•RFC 2068: Jan 1997
•RFC 2616: Jun3 1999
SPDY
•Started circa 2009
•Multiple versions
•Influenced HTTP/2
•Deprecated 2015
•Chrome no longer
supports it
HTTP/2
•Built on SPDY
framework
• RFC 7540
•Standardized in 2015
Example: HTTP protocol
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Keyword Argument(s) Example
GET
Dow nload file from server