Run a mock round

TCP congestion control keeps the sender from overwhelming the network. The sender maintains a congestion window, cwnd, and sends at most the minimum of cwnd and the receiver's advertised window.

Classic four phases:

• Slow start: cwnd starts small and doubles each round trip until the slow-start threshold.
• Congestion avoidance: cwnd grows by roughly one segment per round trip, additive increase.
• Fast retransmit: three duplicate ACKs imply one lost segment, so resend it without waiting for a timeout.
• Fast recovery: halve cwnd and continue.

cwnd ^        /|  AIMD sawtooth
     |      /  |
     |   /     |\__
     +-----------------> time

On timeout cwnd drops to one and slow start restarts. This additive-increase, multiplicative-decrease behaviour is why TCP is fair and stable. Modern algorithms differ: CUBIC uses a cubic growth function, and BBR models bandwidth and round-trip time instead of treating loss as the only signal.

DNS maps names to records, most importantly A and AAAA addresses, plus CNAME, MX, TXT and NS. A lookup can walk several servers, but caching means most lookups stop early.

Resolution steps:

1. Check the local hosts file and OS resolver cache.
2. Query the configured recursive resolver, often via the ISP or a public service.
3. If not cached, the resolver asks a root server, which points to the top-level domain servers.
4. The TLD server points to the domain's authoritative name servers.
5. The authoritative server returns the record; the resolver caches it for its TTL and answers the client.

Queries use UDP port 53, falling back to TCP for large responses, and DoT or DoH when encryption is wanted.

client -> recursive resolver -> root -> .com -> ns.example.com

Caching and TTLs reduce load and latency. Debug with dig +trace or nslookup, and remember that propagation delays are really cache expiry.

Network Address Translation rewrites IP addresses and often ports as packets cross between a private network and the public Internet. It exists mainly because IPv4 addresses are scarce.

A home router has one public address and gives private addresses such as 192.168.1.x to devices. When an inside host opens a connection, the router replaces the source address and port with its public address and a chosen port, and records the mapping. Replies matching that mapping are translated back.

10.0.0.5:52000 -> 203.0.113.7:40001 -> server
mapping table keeps the reverse path

Types include static NAT, dynamic NAT and PAT or masquerading, which multiplexes many hosts over one address. NAT conserves addresses and hides internal topology, but it breaks end-to-end connectivity, complicates peer-to-peer and burdens the device with state. It also motivates STUN, TURN and hole punching. IPv6 removes the need.

The browser parses the URL, then finds the server's IP: it checks its cache, the OS cache, /etc/hosts, then asks a resolver, which recurses from the root servers through the top-level domain to the authoritative name server. Meanwhile a TCP connection is opened to port 443 with the three-way handshake.

For HTTPS, a TLS handshake follows: the client sends supported ciphers and a random value, the server returns its certificate, they verify it against trusted CAs, agree on keys via ECDHE and switch to encrypted records. Modern TLS 1.3 does this in one round trip.

The browser then sends an HTTP request with method, path, headers and cookies. The server may hit a load balancer, a cache and application code, and returns a status and body. The browser parses HTML, builds the DOM, fetches subresources, builds the CSSOM, runs layout and paint, and executes JavaScript. All of these stages are visible in browser developer tools.

The OSI model is a seven-layer reference model: Physical, Data Link, Network, Transport, Session, Presentation and Application. The TCP/IP model as taught is four layers: Link, Internet, Transport and Application.

OSI                     TCP/IP
7 Application  }        Application (HTTP, DNS, TLS)
6 Presentation }   ->
5 Session      }        Transport (TCP, UDP)
4 Transport             Internet (IP, ICMP)
3 Network               Link (Ethernet, Wi-Fi, ARP)
2 Data Link
1 Physical

OSI is a conceptual framework that names what each layer should do and helps when reasoning about problems and protocols. TCP/IP is the model actually implemented by the Internet, and it merges the upper layers and leaves some OSI concerns, such as encryption, to the application or to TLS. In troubleshooting people still speak in OSI layers: a layer 2 problem means a switching or MAC issue, while layer 7 means the application payload.

A content delivery network caches content on edge servers close to users, reducing latency and offloading the origin.

How it works: DNS or anycast routes the user to a nearby edge. On a cache miss the edge fetches from the origin or from a parent tier, stores the object according to its cache headers, and serves subsequent requests locally. TTLs, Cache-Control, ETag revalidation and surrogate keys control freshness and purging.

user -> nearest edge -> (miss) -> parent/origin

Benefits are lower round-trip time, less origin bandwidth, resilience to spikes, and absorption of some DDoS traffic. Design considerations include choosing cacheable content, correct cache keys that account for query strings and cookies, an invalidation strategy, and consistency trade-offs. Static assets are easy; dynamic content needs careful rules, edge compute or short TTLs. Always measure cache hit ratio, origin offload and time to first byte.

TCP and UDP are transport-layer protocols with opposite trade-offs.

TCP is connection-oriented. It sets up a connection with a three-way handshake, numbers every segment, acknowledges delivery, retransmits lost data, orders bytes and applies flow and congestion control. It is used when correctness matters: HTTP, SMTP, SSH and database connections.

UDP is connectionless. It sends independent datagrams with no handshake, acknowledgements or ordering, so delivery is best-effort. It is used when low latency matters more than reliability: DNS, DHCP, VoIP, video streaming and online games. QUIC adds reliability and encryption on top of UDP in user space.

TCP: SYN, SYN-ACK, ACK -> ordered, reliable byte stream
UDP: datagram -> fire and forget

TCP's guarantees cost latency and header overhead; UDP is cheaper but application code must handle loss, duplication and reordering when required.

TCP opens a connection with a three-way handshake so both sides agree on initial sequence numbers and confirm they can send and receive.

1. Client sends SYN with its initial sequence number and options such as MSS and window scaling.
2. Server replies SYN-ACK: it acknowledges the client's sequence number and sends its own SYN.
3. Client sends ACK acknowledging the server's sequence number; the connection is established.

Client          Server
  SYN seq=x  ->
             <- SYN-ACK seq=y ack=x+1
  ACK ack=y+1 ->

Two steps would not be enough: each side needs to know the other received its SYN, and the exchange prevents stale duplicates from an old connection being mistaken for a new one. Sequence numbers are randomised to make blind spoofing hard.

Teardown takes four steps, FIN, ACK, FIN, ACK, because each direction closes independently, and the side that closes last waits in TIME_WAIT to absorb delayed segments.

Switching and routing both forward packets, but at different layers and scopes.

A switch works at layer 2 using MAC addresses within a single broadcast domain, the LAN. It learns which MAC is reachable on which port by inspecting source addresses and forwards frames only to the correct port, flooding unknown destinations. VLANs segment one physical switch into logical LANs.

A router works at layer 3 using IP addresses and moves packets between networks. It consults a routing table built from directly connected networks, static routes and dynamic protocols such as OSPF, EIGRP or BGP, and picks the longest matching prefix. It also decrements the TTL and can perform NAT.

host -> switch (MAC) -> router (IP) -> internet

Routers break broadcast domains and connect dissimilar networks; switches are faster and cheaper but confined to the local segment. Layer 3 switches combine both functions.