liminfoTCP/IP Reference
Free reference guide: TCP/IP Reference
About TCP/IP Reference
The TCP/IP Reference is a searchable cheat sheet covering the core protocols and concepts of internet networking. It organizes 25 entries into six categories: TCP (handshake, termination, header structure, flags, window, congestion control), UDP (header, use cases), IP (IPv4 header, address classes, subnet masks, TTL, IPv6, OSI model), ICMP (ping, error messages, traceroute), DNS (query process, record types, lookup tools), and HTTP (methods, status codes, HTTP/2, HTTP/3, HTTPS/TLS).
Each entry provides a concise description plus a detailed example showing packet formats, header field sizes, protocol behaviors, and command-line tools. The reference is designed for network engineers, system administrators, DevOps professionals, and computer science students studying networking fundamentals.
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Key Features
- TCP connection lifecycle: 3-way handshake (SYN/SYN-ACK/ACK) and 4-way termination with TIME_WAIT
- TCP header field breakdown: source/dest port, sequence/ack numbers, flags, window size, checksum
- TCP congestion control algorithms: slow start, congestion avoidance, fast retransmit, fast recovery
- IPv4 header structure, address classes (A/B/C), subnet mask CIDR notation, and IPv6 format
- DNS resolution process and record types: A, AAAA, CNAME, MX, TXT, NS, SOA, PTR
- HTTP methods (GET/POST/PUT/PATCH/DELETE), status code ranges (1xx-5xx), and HTTP/2 multiplexing
- HTTPS/TLS 1.3 handshake flow: ClientHello, ServerHello, key exchange, symmetric encryption
- OSI 7-layer model mapped to real protocols: Application through Physical layer
Frequently Asked Questions
What does the TCP 3-way handshake entry explain?
It details the three-step process for establishing a TCP connection: the client sends a SYN with an initial sequence number x, the server responds with SYN-ACK (its own sequence y and acknowledgment x+1), and the client completes with ACK (ack=y+1). It also covers the 4-way termination with FIN/ACK exchanges and the TIME_WAIT state lasting 2 times the Maximum Segment Lifetime.
How are TCP congestion control algorithms covered?
The reference explains four key algorithms: Slow Start where the congestion window (cwnd) grows exponentially, Congestion Avoidance where cwnd grows linearly, Fast Retransmit triggered after receiving 3 duplicate ACKs, and Fast Recovery where cwnd is halved instead of reset. These form the basis of TCP Reno and similar implementations.
What IPv4 and IPv6 information is included?
For IPv4, the reference covers the full header structure (20+ bytes), address classes A through C with their CIDR ranges, private IP ranges (10.x, 172.16-31.x, 192.168.x), subnet mask notation from /24 to /28, and TTL behavior. For IPv6, it shows the 128-bit address format, abbreviation rules, loopback (::1), and link-local (fe80::) addresses.
What DNS information does this reference provide?
Three DNS entries are included: the full query resolution process (local cache, /etc/hosts, recursive server, root/TLD/authoritative chain), eight major record types (A, AAAA, CNAME, MX, TXT, NS, SOA, PTR), and command-line lookup tools (nslookup, dig with specific server and record type, host).
How are HTTP methods and status codes organized?
The HTTP category covers seven request methods (GET, POST, PUT, PATCH, DELETE, HEAD, OPTIONS) with their semantic meanings, five status code ranges (1xx Informational through 5xx Server Error) with common examples like 200, 301, 404, and 500, plus separate entries for HTTP/2 features (multiplexing, HPACK, server push) and HTTP/3/QUIC (0-RTT, no HOL blocking).
Does this reference explain the TLS handshake?
Yes. The HTTPS/TLS entry walks through the handshake: ClientHello sends supported cipher suites, ServerHello responds with the selected cipher and certificate, key exchange occurs (typically ECDHE), and symmetric-key encrypted communication begins. It also notes that TLS 1.3 reduces this to a 1-RTT handshake.
Is the OSI 7-layer model included?
Yes. The OSI reference maps all seven layers from Physical (cables, signals) through Application (HTTP, FTP, DNS), with specific protocol examples at each layer including Ethernet and ARP at Data Link, IP and ICMP at Network, TCP and UDP at Transport, and TLS at Presentation.
Who is this TCP/IP reference designed for?
It is designed for network engineers, system administrators, DevOps professionals, and students studying for certifications like CCNA, CompTIA Network+, or university networking courses. The concise format with protocol header diagrams and command-line examples makes it useful for both study and on-the-job reference.