アプリケーション層プロトコル

Why your inbox looks the same on every device

• Open your email on a laptop, then on a phone. The same messages are there, and the ones you have read are marked read on both.
• That is not obvious. For twenty years the normal thing was for a mail program to download everything and delete it from the server, so whichever device fetched the mail first was the only one that had it.
• Both behaviours are still in use, and choosing between them is choosing between two protocols with the same job and opposite designs.
• This lesson is the application layer 应用层 protocols the syllabus names: web, file transfer, the three mail protocols, and peer-to-peer.

The web: HTTP and HTTPS

• HTTP 超文本传输协议 is how a browser fetches web pages and their resources from a server. It runs over TCP, normally on port 80.
• HTTPS 安全超文本传输协议 is HTTP carried inside TLS 传输层安全, so the traffic is encrypted, on port 443.
• What HTTPS adds: an eavesdropper on the path sees only ciphertext, the server proves its identity with a certificate, and any tampering is detected. Use it whenever the data is private, which today means always.

File transfer: FTP

• FTP 文件传输协议 transfers files between a client and a server: upload, download, list, rename and delete.
• It is designed for whole files, where HTTP is designed for documents and resources a browser renders.
• Plain FTP sends its credentials in the clear, which is why secured variants replaced it for anything sensitive.

Email: SMTP, POP3 and IMAP

• SMTP 简单邮件传输协议 sends mail: from the client to its outgoing server, and from server to server across the internet. It is the push half of email.
• POP3 邮局协议3 downloads mail to one device and usually deletes it from the server. The mail lives on that device.
• IMAP 互联网邮件访问协议 leaves mail on the server and synchronises the state, folders, read flags, deletions, across every device. The mail lives on the server.

Every one of these protocols ends up inside frames like this one

Worked example: choose a mail protocol

Peer-to-peer: BitTorrent

• BitTorrent 比特流 is a peer-to-peer 对等网络 protocol: there is no single server holding the file for everyone.
• The file is split into pieces. A tracker helps peers find each other, and each peer then downloads pieces from many other peers in parallel while uploading the pieces it already has.
• The effect: the more popular a file, the more peers there are to serve it, so the load spreads instead of concentrating. A single server would need bandwidth proportional to the number of downloaders.

Every downloader is also an uploader

Worked example: why peer-to-peer for a large release

The protocols at a glance

Marks that slip away

You've got it