You (the sender) want to email Sakib (the receiver). Think of your email as a parcel. The internet is the postal system. We’ll walk through the 5-layer TCP/IP model and watch this parcel travel.
Layers:
- Application
- Transport
- Network
- Data Link
- Physical
1) Application Layer “Write & Prepare the Parcel”
What happens:
- You type the email and attach files.
- The email client (MUA) prepares an SMTP message.
- Attachments are wrapped using MIME (and often compressed or base64-encoded).
- TLS (encryption) is planned so nobody can read the email on the wire.
Standard practices
- Keep attachments small; many providers limit to ~20–25 MB. Prefer cloud links for big files.
- Use UTF-8 for text.
- Turn on TLS (STARTTLS or implicit TLS) so the path is encrypted.
Quick mental hook: “Write it, wrap it, lock it (TLS).”
2) Transport Layer “Cut Into Chunks & Number Them”
What happens:
- The SMTP message may be bigger than what the network can send at once.
- TCP splits it into segments (chunks), numbers each, and ensures delivery with ACKs.
- A 3-way handshake (SYN, SYN-ACK, ACK) sets up the session.
- Client uses an ephemeral port; server listens on a well-known port.
Ports (common)
- Submission: 587 (SMTP with STARTTLS)
- SMTP server to server: 25
- IMAP over TLS: 993, POP3 over TLS: 995
Standard practices
- Submit mail on 587 with authentication and STARTTLS.
- Don’t disable Nagle/algorithms etc. without reason; defaults are usually fine.
- If sending very large data often, consider links instead of huge attachments.
Quick mental hook: “Cut, number, handshake.”
3) Network Layer “Put IP Addresses On Every Chunk”
What happens:
- Each TCP segment is wrapped in an IP packet with source IP and destination IP.
- Routers use the destination IP to forward packets hop by hop to Sakib’s mail server.
- TTL (time to live) prevents endless looping.
Standard practices
- Make sure DNS and MX records for your domain are correct (delivery depends on it).
- Your client is often behind NAT; your router translates private IP → public IP.
Quick mental hook: “Address every page with IP.”
4) Data Link Layer “Wrap For The Local Hop (MAC & Frame)”
What happens:
- On each local link (your Wi-Fi/Ethernet, your ISP’s next hop), IP packets are wrapped into frames with MAC addresses.
- ARP resolves “which MAC is next?” for local delivery.
- Each hop unwraps/re-wraps frames as the packet moves through.
Standard practices
- On unstable Wi-Fi, expect retries. For important sends, prefer a stable link (Ethernet).
- Keep drivers/firmware updated for fewer link-level drops.
Quick mental hook: “Local delivery uses MAC.”
5) Physical Layer “Turn Frames Into Signals”
What happens:
- Frames become electrical/optical/radio signals (Ethernet, fiber, Wi-Fi).
- Bits travel over cables/air between devices.
Standard practices
- Use good cables/APs. Poor cables and congested Wi-Fi = packet loss → TCP retransmissions → slow email sends.
- For offices, wire critical machines.
Quick mental hook: “Bits ride wires or waves.”
On Sakib’s Side “Unwrap & Reassemble”
Reverse happens:
- Physical → Data Link → Network → Transport → Application.
- TCP reorders segments, fills gaps, and hands the complete message to SMTP on the server.
- The Mail Delivery Agent stores it in Sakib’s mailbox.
- Sakib opens her client (IMAP/POP3) and reads it.
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