Topic domain
Networking
Packet movement from link layer to application.
- Articles
- 80
- Clusters
- 8
- Foundational
- 24
- Intermediate
- 32
- Advanced
- 24
Link Layer
10Ethernet, framing, and local delivery.
- Ethernet Frame Layout and the 64-Byte Minimum
- MAC Address Structure and How a NIC Filters Group Traffic
- The Frame Check Sequence: CRC-32 and the Errors It Misses
- MAC Learning and How a Bridge Builds Its Forwarding Table
- Spanning Tree Port States and the Cost of a Blocked Link
- ARP and the Neighbour Cache Lifecycle
Addressing, forwarding, and route selection.
- The IPv4 Header Fields That Forwarding Rewrites
- Longest-Prefix Match and the Structures That Implement It
- CIDR Aggregation and the Reasons It Cannot Fully Summarize
- How a Router Decides a Destination Is On-Link
- Fragmentation, the MTU Floor, and Path MTU Discovery Black Holes
- ICMP Errors as the Only Feedback Channel for Forwarding
Reliability, flow control, and congestion.
- Sequence Numbers and the Sliding Window Arithmetic
- Retransmission Timeout Estimation and Karn's Algorithm
- Advertised Windows and Zero-Window Probing
- Connection State Transitions and the TIME_WAIT Hold
- SACK Blocks and the Retransmission Queue
- The Effective Window: Congestion Window, Receive Window, and Buffers
Signals, fairness, and queue management.
- Ack Clocks and the Congestion Window: How a Sender Infers Available Capacity
- Additive Increase, Multiplicative Decrease, and the Fairness of the Sawtooth
- Slow Start: Exponential Probing and the Initial Window Problem
- Duplicate Acknowledgements and How a Sender Distinguishes Loss from Reordering
- Queueing Delay as a Congestion Signal and the Base RTT Problem
- Tail Drop, Buffer Bloat, and How Much Queue a Link Actually Needs
Latency budgets, buffering, and measurement.
- Decomposing Path Latency into Propagation, Serialization, and Queueing
- Why Queueing Delay Rises Sharply Near Link Utilization
- Standing Queues and the Buffer Sizing Trade-Off
- Serialization Delay and the Cost of Small Packets
- Counting Round Trips in Connection Setup
- What ICMP Echo and TTL Probing Actually Measure
Radio behavior, medium access, and mobility.
- The Hidden Terminal Problem and Why 802.11 Reserves the Medium
- DCF Backoff and the Contention Window Growth Rule
- 802.11 Address Fields and the Four-Address Distribution System
- OFDM Subcarrier Spacing, Guard Intervals, and Channel Width
- Rate Adaptation and How a Station Chooses Its MCS
- A-MPDU Aggregation and Block Acknowledgement
Overlays, tunnels, and software dataplanes.
- The Overlay and Underlay Split in Virtual Networks
- VXLAN Header Layout and the Scope of the VNI
- Tunnel Encapsulation Overhead and Path MTU Discovery
- Flood and Learn Versus a BGP EVPN Control Plane
- Overlay Endpoint Resolution and Its Caches
- UDP Source Port Entropy and Underlay ECMP
Diagnosis, configuration, and failure recovery.
- Interface Configuration Order: Link State, Address Assignment, and Route Installation
- Where a Packet Capture Sits Determines What It Shows
- Interface Drop Counters and What Each One Actually Attributes
- Neighbour Cache Staleness and Peers That Become Unreachable
- Confirmed Commit and Automatic Rollback for Remote Configuration Changes
- Correlating Logs Across Devices When Clocks Disagree
All Networking articles
- Oct 22, 2026 Tunnel Encapsulation Overhead and Path MTU Discovery How many bytes each overlay scheme adds, why ICMP-based path MTU discovery breaks across a tunnel, and the options for keeping an inner frame inside the underlay MTU.
- Oct 7, 2026 OFDMA Scheduling and Multi-User Transmission in Wi-Fi How a 20 MHz channel is split into resource units, how a trigger frame coordinates simultaneous uplinks, and what the scheduling overhead costs.
- Sep 18, 2026 Multipath TCP and the Data-Level Sequence Space How MPTCP separates a 64-bit connection-level byte counter from the 32-bit per-subflow counters, why the mapping needs a signal option, and what reordering costs.
- Sep 16, 2026 Active Queue Management and Latency Under Load How tail-drop buffers convert delay into loss, what controlled-delay algorithms change about the signal a sender receives, and why their tuning constants are what they are.
- Sep 11, 2026 MAC Learning and How a Bridge Builds Its Forwarding Table A transparent bridge learns source addresses, forwards unknown destinations out every port, and ages entries out; the failure modes of each step.
- Sep 6, 2026 Priority and Fair Queueing for Mixed Traffic How strict priority starves low classes, how deficit round robin allocates bandwidth by packet size, and how the two combine on an access link.
- Sep 1, 2026 Policy Routing: Keying the Lookup on More Than the Destination How multiple routing tables and a rule list restore source-based and interface-based selection, what the extra lookup costs, and why asymmetric paths become routine.
- Jul 11, 2026 Correlating Logs Across Devices When Clocks Disagree How clock offset and resolution error bound the accuracy of a merged timeline, and how to order events from independent devices without trusting their timestamps.
- Jun 27, 2026 Spatial Reuse, OBSS Interference, and BSS Coloring Why overlapping cells defer to each other even when they could transmit, what the preamble carries to fix it, and how a colour is chosen without coordination.
- Jun 19, 2026 Flood and Learn Versus a BGP EVPN Control Plane How a tunnel endpoint discovers the remote endpoint for a MAC address by flooding and learning from the reply, what that costs at scale, and how a routing protocol replaces the guesswork.
- Jun 7, 2026 Overlay Endpoint Resolution and Its Caches How a host maps an inner destination to the tunnel endpoint that hosts it, what the per-flow and per-prefix caches hold, and why a stale mapping is worse than a missing one.
- May 11, 2026 The Effective Window: Congestion Window, Receive Window, and Buffers Why three independent limits govern how much a TCP sender may have outstanding, how the congestion window grows and collapses, and which limit binds in practice.
- Apr 29, 2026 Why Queueing Delay Rises Sharply Near Link Utilization How the shape of the M/M/1 waiting-time curve explains the knee in latency-versus-load graphs, and why utilisation targets are set where they are.
- Mar 21, 2026 Interface Configuration Order: Link State, Address Assignment, and Route Installation Why an interface can hold a valid address and still be unreachable, how duplicate address detection suspends usability, and which piece of state to inspect first.
- Mar 17, 2026 Timestamps, RTTM, and Protection Against Wrapped Sequences How an echoed timestamp lets a sender time a retransmitted segment, why PAWS rejects segments by timestamp rather than sequence number, and what the option costs per segment.
- Mar 13, 2026 Fast BSS Transition and the Key Hierarchy That Makes Roaming Quick Why reassociating to a new access point normally costs a full authentication and key exchange, and how pre-derived keys cut that to two messages.
- Feb 27, 2026 The IPv4 Header Fields That Forwarding Rewrites Which of the twenty header bytes a router must read, which it decrements or recomputes in place, and where the header checksum stops protecting anything.
- Feb 5, 2026 ICMP Errors as the Only Feedback Channel for Forwarding Which errors a router is allowed to generate, what it must quote back, and why rate limiting and filtering turn a diagnostic protocol into a silent failure mode.
- Jan 27, 2026 Advertised Windows and Zero-Window Probing What the receive window field promises, how the receiver computes the value it advertises, and how a sender recovers when the peer advertises zero and then forgets to reopen.
- Jan 26, 2026 Tail Drop, Buffer Bloat, and How Much Queue a Link Actually Needs A drop-tail queue signals congestion only by overflowing, and the delay it adds while full is the latency every other flow experiences. The bandwidth-delay product gives a floor, not an optimum.
- Jan 24, 2026 Interface Drop Counters and What Each One Actually Attributes How kernels classify discarded packets into drop, error, and overrun counters, and why a clean counter set can coexist with a link that loses traffic.
- Jan 18, 2026 Link Aggregation and the Hash That Chooses a Member How LACP bundles links, why the distributor hashes rather than round-robins, and the ordering guarantee that constrains every possible choice.
- Jan 1, 2026 The BGP Best-Path Order and Why Its Steps Are Ordered That Way The comparison sequence a router applies to routes for one prefix, which attributes are local to an autonomous system, and how the tiebreak steps create hot-potato routing.
- Dec 31, 2025 Airtime Fairness and Why One Slow Client Halves a Cell How per-station throughput diverges from per-station airtime, how frame duration is computed from rate and size, and what a scheduler can do about it.
- Dec 21, 2025 Spanning Tree Port States and the Cost of a Blocked Link BPDU exchange elects a root, each bridge picks a least-cost port, and the resulting states determine which ports forward; convergence delays are the price.
- Dec 19, 2025 Neighbour Cache Staleness and Peers That Become Unreachable How ARP and Neighbor Discovery entries age through reachable, stale, and probe states, and why a stale entry keeps sending traffic to an address that has moved.
- Dec 6, 2025 XDP and eBPF in the Receive Path Where an XDP program runs relative to the socket layer, what the three attach modes give up, and which packet-processing decisions can be made before a socket buffer exists.
- Dec 5, 2025 One-Way Delay and the Clock Offset Problem Why a one-way delay measurement needs synchronised clocks, how a constant offset biases the result, and how two-way exchanges bound the error.
- Dec 4, 2025 Duplicate Acknowledgements and How a Sender Distinguishes Loss from Reordering Three duplicate ACKs are a heuristic for one lost segment, not a proof. Fast retransmit and fast recovery act on it, and the ambiguity with packet reordering shapes the whole design.
- Dec 1, 2025 DCTCP and the Proportional Response to Congestion Marks A single ECN mark carries one bit, but the fraction of marked packets carries the severity of congestion. DCTCP converts that fraction into a window reduction sized to match.
- Nov 24, 2025 Measuring Latency Under Load Without Fooling Yourself How to design a delay measurement that separates fixed path delay from queueing, which load patterns expose bufferbloat, and why idle-path numbers mislead.
- Nov 18, 2025 Counting Round Trips in Connection Setup How handshakes, name resolution, and security negotiation add serial round trips before the first byte of payload, and which of them can be overlapped.
- Nov 17, 2025 Model-Based Congestion Control: Delivery Rate, Minimum RTT, and Pacing Estimating the bottleneck rate and the propagation delay separately lets a sender set its window from the product of the two instead of probing for loss. The estimators are the whole design.
- Nov 15, 2025 ARP and the Neighbour Cache Lifecycle How an IPv4 address becomes a MAC address, why the cache entry has states rather than an expiry, and what happens to queued packets during resolution.
- Nov 3, 2025 Link MTU, Jumbo Frames, and Where the Path Breaks Why 1500 bytes became the standard payload, what changes when both ends agree on more, and how MTU mismatches appear as silent large-packet loss.
- Nov 3, 2025 RTT Fairness: Why Shorter Paths Win and How Algorithms Compensate Two flows on the same bottleneck with different round trips do not get equal shares, and the bias is a direct consequence of the control law. The fixes trade that bias against stability.
- Oct 21, 2025 Queueing Delay as a Congestion Signal and the Base RTT Problem A queue that is filling raises round-trip time before it drops anything, so delay can warn earlier than loss. Turning that warning into a control signal requires a stable estimate of the delay without queues.
- Oct 17, 2025 Active Queue Management and Explicit Congestion Notification Random early detection marks or drops before the buffer fills, and ECN lets the receiver echo the mark instead of losing a packet. What the queue can signal and what the sender must do with it.
- Oct 16, 2025 Reading a BGP Session Failure From Its State and Its Last Error How the session states narrow a failure to transport, capability negotiation, or policy, and why a session that reaches Established can still exchange no routes.
- Oct 9, 2025 Connection State Transitions and the TIME_WAIT Hold Why closing a TCP connection takes four segments rather than two, what TIME_WAIT protects against, and why the state exists on the side that closed first.
- Oct 7, 2025 Flow Sampling Error When Deciding Whether a Link Is Full How sampled flow records estimate volume, why small conversations disappear entirely, and how to tell sampling noise from a real change in traffic mix.
- Oct 4, 2025 A-MPDU Aggregation and Block Acknowledgement How 802.11n packs many frames into one transmission, why each subframe keeps its own check sequence, and what the block ack bitmap window buys.
- Aug 25, 2025 Route Preference, Metric, and the Split Between RIB and FIB How competing routes to one prefix are ranked, why administrative distance is not a metric, and what the control plane must do before the forwarding plane can use a table.
- Aug 16, 2025 CIDR Aggregation and the Reasons It Cannot Fully Summarize How prefix length replaced address classes, which blocks can be merged into one announcement, and why holes and multi-homing leave the table larger than the address plan suggests.
- Aug 14, 2025 Longest-Prefix Match and the Structures That Implement It Why a routing table cannot be a hash map, how a binary trie answers the longest-prefix query, and what multibit tries and TCAM buy in exchange for memory.
- Jul 22, 2025 The Overlay and Underlay Split in Virtual Networks How a virtual network carries its own addresses inside an ordinary routed network, what the mapping between the two layers costs, and where the separation leaks.
- Jul 18, 2025 Link Aggregation Member Selection and the Mismatches That Stall It How frames are hashed onto bundle members, why a single flow cannot exceed one link, and which LACP negotiation failures leave a bundle passing some traffic.
- Jul 17, 2025 SYN Cookies and the Cost of Half-Open Connections How a server avoids storing state for every handshake attempt by encoding the connection parameters into the initial sequence number, and what the encoding gives up.
- Jul 16, 2025 Fragmentation, the MTU Floor, and Path MTU Discovery Black Holes How a datagram is split into pieces that a smaller link can carry, why the reassembly rules create ambiguity, and the specific way discovery fails when ICMP is filtered.
- Jun 26, 2025 OFDM Subcarrier Spacing, Guard Intervals, and Channel Width Why a wide Wi-Fi channel is divided into many narrow subcarriers, what the cyclic prefix protects against, and how spacing changed in later amendments.
- Jun 15, 2025 How a Router Decides a Destination Is On-Link The difference between a route that names a neighbour and one that names a gateway, how the link-layer address is resolved, and what happens when resolution fails.
- Jun 5, 2025 SACK Blocks and the Retransmission Queue How selective acknowledgement turns a cumulative ACK into a map of received ranges, why the sender keeps a scoreboard, and what the option length limit costs.
- May 25, 2025 Match-Action Pipelines and Flow Caching in Software Switches How a packet is classified by a sequence of table lookups, why the first packet of every flow takes a slow path, and how a wildcard cache shortens the common case without breaking correctness.
- Mar 5, 2025 Draining a Router for Maintenance Without Dropping In-Flight Traffic How to withdraw a router from the forwarding path by changing what the control plane advertises, how long convergence takes, and why the drain interval must exceed it.
- Jan 21, 2025 Additive Increase, Multiplicative Decrease, and the Fairness of the Sawtooth Why the increase is additive and the decrease is multiplicative, how the resulting sawtooth converts loss rate into throughput, and where the convergence argument stops holding.
- Dec 27, 2024 UDP Source Port Entropy and Underlay ECMP Why a tunnel collapses many flows onto one underlay path, how deriving the outer source port from an inner-header hash restores path diversity, and when the hash runs out of entropy.
- Nov 22, 2024 Serialization Delay and the Cost of Small Packets Why shrinking packets reduces queueing delay but raises packet rate, header overhead, and per-packet forwarding cost, and where the crossover sits.
- Nov 7, 2024 The Hidden Terminal Problem and Why 802.11 Reserves the Medium Why listening before transmitting fails on radio, how RTS/CTS and the duration field build a virtual carrier sense, and what the reservation costs per frame.
- Nov 4, 2024 VXLAN Header Layout and the Scope of the VNI A byte-level reading of the eight-byte VXLAN header, why the tenant identifier is twenty-four bits, and what the reserved fields constrain.
- Oct 18, 2024 MAC Address Structure and How a NIC Filters Group Traffic The bit layout of a 48-bit MAC address, why the I/G and U/L bits sit in the first transmitted byte, and the receive-filter modes a NIC applies before any driver code runs.
- Sep 22, 2024 Slow Start: Exponential Probing and the Initial Window Problem A new connection has no estimate of path capacity, so it doubles its window each round trip until a threshold or a loss. The cost is a burst that a small buffer cannot absorb.
- Aug 21, 2024 Ack Clocks and the Congestion Window: How a Sender Infers Available Capacity Why a TCP sender measures the path with returning acknowledgements instead of a timer, how the congestion window combines with the receive window, and what a stalled clock does to the estimate.
- Aug 12, 2024 Standing Queues and the Buffer Sizing Trade-Off Why deep buffers turn into permanent added latency, how the bufferbloat diagnosis differs from ordinary congestion, and what the bandwidth-delay product says about how much buffer is needed.
- Jul 26, 2024 Network Namespaces and veth Pairs as Virtual Routers How a Linux network namespace holds an independent forwarding table, how veth pairs connect two of them, and what a packet crossing between namespaces actually costs.
- Jul 25, 2024 IPv6 Forwarding, Extension Header Chains, and Scoped Addresses How a fixed forty-byte header pushes options into a chain a router must walk, why scopes are part of the address, and which rules a forwarder applies differently than in IPv4.
- Jul 6, 2024 Retransmission Timeout Estimation and Karn's Algorithm Why the retransmission timer is computed from a smoothed round-trip estimate plus a variance term, and why timing a retransmitted segment corrupts the estimate.
- May 24, 2024 Ethernet Frame Layout and the 64-Byte Minimum The field-by-field layout of an Ethernet II frame, why the payload floor is 46 bytes, and how a receiver distinguishes padding from real data.
- Mar 1, 2024 Nagle's Algorithm and the Delayed Acknowledgement Deadlock Why coalescing small writes and batching acknowledgements interact badly, how the 40 ms delay arises, and which socket options disable each mechanism.
- Dec 25, 2023 What ICMP Echo and TTL Probing Actually Measure The difference between the forward and return paths a traceroute samples, why hop times are often wrong, and what rate limiting on the control plane does to the numbers.
- Dec 6, 2023 The Frame Check Sequence: CRC-32 and the Errors It Misses How the four-byte Ethernet trailer is computed, why CRC-32 catches every burst up to 32 bits, and which corruption patterns pass it undetected.
- Oct 21, 2023 Inside the NIC Receive Path: Descriptors, Rings, and NAPI A frame arriving at a network interface moves through a DMA ring, a descriptor, an interrupt, and a poll loop; where each step can drop it.
- Oct 5, 2023 802.11 Address Fields and the Four-Address Distribution System Why a Wi-Fi frame carries up to four MAC addresses, how the ToDS and FromDS bits select which is which, and what breaks when a bridge is inserted.
- Sep 2, 2023 802.1Q Tagging and the Four-Byte Insertion Where the VLAN tag sits in a frame, what PCP and DEI carry, how untagged and priority-tagged frames are told apart, and what a trunk port must do.
- Aug 20, 2023 SRv6 Segment Lists as a Source-Routed Overlay How a 128-bit IPv6 address doubles as a forwarding instruction, what the segment list in the routing header encodes, and why path state moves from the core to the edge.
- May 1, 2023 Confirmed Commit and Automatic Rollback for Remote Configuration Changes How a candidate datastore plus a confirm timer bounds the damage from a configuration change that severs the connection used to make it, and where the mechanism fails.
- Mar 7, 2023 Rate Adaptation and How a Station Chooses Its MCS How a transmitter infers channel quality it cannot measure, why the retry count is a noisy signal, and what the probe-then-fallback strategies cost.
- Jan 30, 2023 Decomposing Path Latency into Propagation, Serialization, and Queueing The four terms that sum to the delay across a network path, how each scales with distance, link rate, and load, and which ones a sender can actually influence.
- Jun 24, 2022 DCF Backoff and the Contention Window Growth Rule How a station decides when to transmit after the medium goes idle, why the backoff is counted in slots rather than microseconds, and how the window doubles after loss.
- May 1, 2022 Where a Packet Capture Sits Determines What It Shows How capture points on hosts, bridges, and taps differ in what they observe, and why segmentation offload and checksum offload make a local capture disagree with the wire.
- Dec 15, 2021 Sequence Numbers and the Sliding Window Arithmetic How a 32-bit byte counter replaces per-byte bookkeeping, why the window variables are indices into a modular sequence space, and where the arithmetic breaks.