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Process-to-Process Communication

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1. The Role of the Transport Layer

While the Data Link Layer is responsible for Node-to-Node (Hop-to-Hop) delivery and the Network Layer is responsible for Host-to-Host (Source-to-Destination) delivery, the Transport Layer is responsible for Process-to-Process Delivery.

A host computer runs dozens of network applications simultaneously (web browser, email client, music stream, chat application). Receiving a packet at the host interface is insufficient—the operating system must deliver the payload to the exact running program (process) that initiated the connection!

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2. Port Numbers and Address Ranges

Processes are addressed via 16-bit unsigned integers called Port Numbers, providing $2^{16} = 65{,}536$ possible ports ($0$ to $65{,}535$).

The Internet Assigned Numbers Authority (IANA) divides the port spectrum into three ranges:

1. Well-Known Ports ($0 - 1023$):
   • Reserved for universal standard server applications. Requires root/administrator privileges to bind.
   • Examples:
     • Port 20 / 21: FTP (File Transfer Protocol)
     • Port 22: SSH (Secure Shell)
     • Port 25: SMTP (Simple Mail Transfer Protocol)
     • Port 53: DNS (Domain Name System)
     • Port 67 / 68: DHCP (Server / Client)
     • Port 80: HTTP (World Wide Web)
     • Port 443: HTTPS (HTTP Secure over TLS)
2. Registered Ports ($1024 - 49151$):
   • Registered with IANA by software vendors to avoid port collisions (e.g., Port 3306 for MySQL, Port 5432 for PostgreSQL, Port 8080 for HTTP Alternate).
3. Dynamic / Private / Ephemeral Ports ($49152 - 65535$):
   • Short-lived, temporary port numbers assigned automatically by the local operating system kernel to client applications when they initiate an outbound connection. Reclaimed when the socket closes.

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3. The Socket Address

An IP address alone only identifies a physical computer. A port number alone only identifies a process on a machine.

The combination of an IP Address and a Port Number is called a Socket Address:

Every end-to-end transport communication is uniquely identified across the entire planet by a Socket Pair:

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4. Multiplexing and Demultiplexing

Connectionless Demultiplexing (UDP):

• A UDP socket is identified solely by a two-tuple:

• All incoming UDP datagrams directed to that destination port are delivered to the same socket, regardless of which source IP or source port they originated from.

Connection-Oriented Demultiplexing (TCP):

• A TCP socket is identified by a strict four-tuple:

• When a client connects to a web server on Port 80, the server OS creates a dedicated socket specifically for that four-tuple, allowing a single server port (Port 80) to handle thousands of concurrent client connections without mixing up data streams!

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