# Q2(a) · Webmail and POP3 protocol journey

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2024-ai-final/q2a

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2024-ai-final--q2a/

Exact historical variant. Current-syllabus questions solved; excluded topics and source limitations stated.

### Q2(a). Alice sends email from a web-based account; Bob accesses mail through POP3. Explain all application protocols in the journey. For each, state push or pull, TCP or UDP, and port numbers. [10 marks]

### Answer

|  |  |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| **Alice browser** | → | **HTTP(S) webmail service** | → | **SMTP between mail servers** | → | **Bob mailbox** | → | **POP3 client** |

| Stage | Protocol and role | Transport / conventional ports |
| --- | --- | --- |
| Browser submits message | HTTP POST over HTTPS; application uploads/pushes the message | HTTPS:TCP443; historical HTTP:TCP80 |
| Sender mail server→recipient mail server | SMTP pushes the mail to recipient domain | TCP25 for server relay; client submission commonly587 |
| Bob retrieves/synchronizes | POP3 pulls messages for download | TCP110 (POP3) /995 with implicit TLS |

DNS may first locate the recipient domain’s MX record and mail-host IP (typicallyUDP53, withTCP53 when needed). HTTP is a request/response protocol rather than a pure push-only protocol; classify this particular POST as submission. The named IMAP/POP3 stage is historical email context; SMTP is the explicitly listed current application topic.