What does the application want to send?
Bytes, stream, request?
Applications generate data without needing to know every physical hop.
Side 55
Computer
Networks
A study of how data moves between machines. Computer networks turn messages into packets, packets into link transmissions, and local connections into routed end-to-end communication.
packet→link→route→transport→application
06network layers
05routing ideas
05reliability questions
55Side
Networks divide messages into transferable units.
Packetization allows many flows to share links while keeping forwarding decisions local.
How is end-to-end data divided?
Transport layer.
Transport protocols add ports, sequencing and reliability behavior.
What network-level unit is routed?
IP header + payload.
Packets carry source/destination addressing and move independently through routers.
How does one local link carry the packet?
Link-layer encapsulation.
Frames change as packets cross different local networks.
How does the receiver reconstruct data?
Ordering + integrity.
Higher layers rebuild the application’s intended data stream or message.
Links turn bits into physical signals.
Different media trade bandwidth, latency, range, mobility and interference differently.
Local wired networking.
Frames move across switched local networks with high throughput and low latency.
Light through optical media.
Fiber provides high bandwidth over long distances with low signal loss.
Shared radio medium.
Wireless stations contend for airtime and experience interference and variable signal quality.
Wide-area radio access.
Cellular systems coordinate spectrum, handoff and shared infrastructure for mobility.
Time to traverse the path.
Propagation, serialization, queueing and processing all contribute delay.
Maximum data rate.
High bandwidth does not eliminate latency, and available throughput may be lower than nominal link rate.
Routers decide where packets go next.
Routing protocols build forwarding information from local and network-wide knowledge.
Address
Identify network location.
IP addresses support hierarchical routing across interconnected networks.
Forward
Choose next hop.
Routers match packet destinations against forwarding tables.
Metric
Compare candidate paths.
Cost can reflect hop count, policy, bandwidth or other routing attributes.
Converge
Adapt after topology changes.
Routing systems need time to learn failures and settle on new paths.
Policy
Routing is not only shortest path.
Between autonomous systems, business and security policy influence which routes are accepted and advertised.
Transport protocols create end-to-end behavior.
The network can deliver packets independently; transport protocols decide whether applications see reliable streams, datagrams or controlled congestion.
| Feature | TCP | UDP | Question |
|---|---|---|---|
| Connection | Connection-oriented | Connectionless | Does the application need session state? |
| Reliability | Retransmits lost data | No built-in retransmission | Who handles loss? |
| Ordering | Ordered byte stream | Independent datagrams | Does order matter? |
| Congestion control | Built in | Application responsibility | How is network overload managed? |
| Overhead | Higher | Lower | What trade-off between control and simplicity? |
Reliable transport does not make the network lossless.
It detects missing data and recovers through retransmission and control.
Applications build protocols on top of transport.
Naming, requests, responses and state live above the packet-routing substrate.
Name → address.
The Domain Name System distributes hierarchical naming across many servers and caches.
Request / response for web resources.
HTTP defines application semantics while transport handles delivery.
Authenticate and encrypt sessions.
TLS protects application data against eavesdropping and tampering when configured correctly.
Store-and-forward messaging.
Different protocols handle submission, transfer and mailbox retrieval.
Deliver time-sensitive media.
Applications buffer and adapt quality to changing throughput and delay.
Machine-to-machine application contract.
Network APIs layer data formats and operations over transport protocols.
Networks are designed around partial failure.
Links drop packets, routes change, names expire and endpoints disappear; resilient communication depends on detection, redundancy and bounded trust.
Checksums
Detect corruption in transmitted data.
Retransmission
Recover from lost packets when reliability is required.
Redundant paths
Allow routing around failed links or nodes.
Timeouts
Prevent applications from waiting indefinitely on failed peers.
Authentication
Verify identity before trusting a peer or endpoint.
Segmentation
Limit reach and blast radius by separating networks and privileges.
A website is slow but bandwidth is high.
Inspect latency, DNS resolution, connection setup, server processing, packet loss and application round trips rather than assuming the bottleneck is link capacity.
One network path fails.
Routing can select an alternate path if topology and policy provide one, but reconvergence takes time and may temporarily drop packets.
A connection is encrypted.
Encryption protects data in transit; it does not prove the endpoint itself is trustworthy or secure.
Computer Networking: A Top-Down ApproachKurose & Ross · protocol layers
Computer NetworksAndrew Tanenbaum · broad foundation
TCP/IP IllustratedStevens · protocol behavior
High Performance Browser NetworkingIlya Grigorik · latency and web transport