Application Layer Protocols

Introduction to Layer 7 Protocols

1. HTTP (Hypertext Transfer Protocol)

The most widely used protocol on the web for request-response communication.

Pattern: Synchronous request/response

Transport: TCP

Stateless: Yes

Use Cases: Web APIs (REST), websites, microservices communication

Protocols on top: REST, GraphQL, gRPC (over HTTP/2)

2. WebSocket

A full-duplex communication protocol over a single, long-lived TCP connection.

Pattern: Bidirectional streaming

Transport: TCP (via HTTP handshake)

Stateful: Yes

Use Cases: Real-time apps like chat, gaming, live dashboards

Compared to HTTP: Persistent and allows server push

3. gRPC

A high-performance RPC framework using HTTP/2 and Protocol Buffers.

Pattern: Client-server, supports streaming and unary calls

Transport: HTTP/2

Efficient: Binary data (Protobuf)

Use Cases: Internal service communication, mobile/backend APIs

4. MQTT (Message Queuing Telemetry Transport)

A lightweight Pub/Sub protocol optimized for unreliable or low-bandwidth networks.

Pattern: Publish-Subscribe

Transport: TCP/IP

Features: QoS levels, retained messages, last will

Use Cases: IoT devices, telemetry, sensors

5. AMQP (Advanced Message Queuing Protocol)

A binary, application-layer protocol for message-oriented middleware.

Pattern: Supports both Point-to-Point and Pub/Sub

Transport: TCP

Features: Queues, topics, routing, transactions

Used By: RabbitMQ, Apache Qpid

6. SMTP (Simple Mail Transfer Protocol)

Protocol for sending email between servers.

Pattern: Push (store-and-forward)

Transport: TCP (port 25/587)

Use Cases: Email services

7. XMPP (Extensible Messaging and Presence Protocol)

An XML-based messaging protocol.

Pattern: Publish-Subscribe, Chat

Transport: TCP

Use Cases: Instant messaging, presence systems (e.g., Jabber)

Summary

Protocol	Pattern	Transport	Default Port	Common Use Case
HTTP	Request/Response	TCP	80/443	Web, APIs
WebSocket	Full-duplex	TCP	80/443	Real-time apps
SMTP	Push	TCP	25/587	Sending email
IMAP	Pull	TCP	143/993	Reading email
POP3	Pull (delete local)	TCP	110/995	Reading email
SSH	Remote shell	TCP	22	Secure remote access
FTP	File Transfer	TCP	21	File uploads/downloads
DNS	Lookup	UDP/TCP	53	Domain resolution
SNMP	Monitoring	UDP	161	Network management
Telnet	Remote shell	TCP	23	Legacy access
gRPC	RPC	HTTP/2	50051	Microservices, APIs
MQTT	Pub/Sub	TCP	1883/8883	IoT messaging
AMQP	P2P + Pub/Sub	TCP	5672/5671	Enterprise messaging
XMPP	Messaging + Pub/Sub	TCP	5222	Chat systems
CoAP	REST + Pub/Sub	UDP	5683	Lightweight IoT