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networking

Harsh Kapadia's Computer Networking knowledge base.

SMTP: Simple Mail Transfer Protocol
OSI Layer: Application Layer
Port: 25 (default, insecure), 2525 (insecure), 587 (secure with TLS) & 465 (deprecated, secure with TLS)
TCP/UDP: TCP
Only used to send e-mails
- from sender to sender’s mail server
- from sender’s mail server to receiver’s mail server
It is a ‘Push Protocol’ as it is used to send data.
Uses TCP, so it is a connection oriented protocol.

POP3: Post Office Protocol 3
OSI Layer: Application Layer
Port: 110 (insecure) & 995 (secure with TLS)
TCP/UDP: TCP
Only used to receive e-mails from receivers’s mail server to receiver.
It is a ‘Pull Protocol’ as it is used to receive data.
Uses TCP, so it is a connection oriented protocol.
Does not provide synchronization of e-mails and folders.
One e-mail delivered to one of receiver’s devices, the e-mail is deleted from the server. (It can be configured to prevent this though.)

IMAP: Internet Message Access Protocol
OSI Layer: Application Layer
Port: 143 (insecure) & 993 (secure with TLS)
TCP/UDP: TCP
Only used to receive e-mails from receivers’s mail server to receiver.
It is a ‘Pull Protocol’ as it is used to receive data.
Uses TCP, so it is a connection oriented protocol.
Provides synchronization of e-mails and folders.
The e-mail is kept on the server and local copies are cached on devices as well.

MIME: Multipurpose Internet Mail Extensions
Supplementary/add-on protocol to the e-mail transfer protocols.
e-mail transfer protocols can inherently only transfer normal text (NVT 7-bit ASCII) data, but it uses the MIME to extend its capabilities.
MIME allows the user to send audio, video, image and other types of files via e-mail. It also helps support different languages like German, French, etc. that don’t follow the 7-bit ASCII format.
Converts files to the NVT 7-bit format for sending them and parses them back to their original format on the other end.
Although MIME was created for SMTP, it can be used with POP3, IMAP and HTTP as well.
MIME headers
- Mime-Version: 1.1
- Content-Type: media-type/media-subtype (Eg: text/html, multipart/form-data, image/png, video/mp4, text/css, audio/mp3, etc.)
- Content-Transfer-Encoding (Values: 7bit, 8bit, base64, etc.)
- Content-Id
- Content-Description

An e-mail MIME header :point_down:

S/MIME: Secure/Multipurpose Internet Mail Extensions
Provides security (authentication and encryption) for e-mails.
Main functions
- Digital signature (authentication and non-repudiation)
- Encryption (integrity and confidentiality)

NOTE:

Instructions in the e-mail assignment.

Read about PGP in cryptography.md.