CN · Short notes
20 short notes in syllabus order, about 176 minutes to read end to end.
A computer network has to solve many separate problems at once: put bits on a wire, find the boundaries of a frame, detect damaged frames, share a medium, find a path across many networks, deliver data reliably between two programs, and give those programs names and conventions.
A network is a mesh of links and switches, and switching is the way data from one link is moved onto the next on its way to the destination. There are two fundamentally different answers.
Link and path performance is described by a few quantities: how long a packet takes to cross a link or a path (delay or latency), how many bits per second actually arrive (throughput), and how many bits are "in flight" on a path at once (the bandwidth-delay product).
Noise, interference and faulty hardware flip bits. A receiver that cannot tell a damaged frame from a good one will pass garbage upward. Error detection adds redundant bits computed from the data; the receiver recomputes them and compares.
The cyclic redundancy check is the error-detecting code used in the trailer of almost every link-layer frame: Ethernet's 32-bit frame check sequence (FCS) is a CRC, and so are the checks in Wi-Fi, HDLC and PPP.
When many stations share one broadcast channel (a coaxial cable, a hub, a radio frequency), two frames sent at overlapping times destroy each other: a collision. The medium access control (MAC) sublayer of the data link layer decides who may transmit when.
Ethernet is the dominant wired local area network technology. Its original form was a shared bus (later a hub) on which all stations contended with CSMA/CD (Medium Access Control).
A router forwards each packet by looking up its destination in a forwarding table; routing is the process that fills that table. Routing algorithms come in two families. In link-state routing every router learns the whole topology and runs Dijkstra's algorithm itself (Link-State Routing).
In link-state (LS) routing every router learns the complete topology of its routing domain and computes shortest paths itself.
IPv4 is the network-layer protocol of the Internet. It carries datagrams from a source host to a destination host across many networks, hop by hop, with no connection set-up and no guarantee of delivery, order or freedom from duplication: a best-effort service.
Classful addressing gave every network a fixed size: 2^24, 2^16 or 2^8 addresses. An organisation needing 2000 addresses had to take a class B block of 65,536 or several class C blocks, and every class C block was a separate entry in the Internet's routing tables.
Every link has a maximum transmission unit (MTU): the largest IP datagram, header included, that one link-layer frame can carry. Ethernet's is 1500 bytes; other links have larger or smaller ones. A datagram crossing several links may meet one whose MTU is smaller than the datagram.
A home or office network typically has many hosts but one public IPv4 address.0/16), which are not routed on the public Internet and may be reused by every organisation.
Error detection tells a receiver that a frame is damaged; it does not get the data through. Automatic repeat request (ARQ) protocols add acknowledgements, timers and retransmission so that every frame is eventually delivered once and in order.
IP delivers datagrams between hosts, unreliably. The transport layer turns this into communication between processes, identified by 16-bit port numbers, and optionally adds reliability.
A TCP receiver places arriving bytes in a receive buffer, from which the application reads them when it chooses. If the sender sends faster than the application reads, the buffer fills and further bytes would be lost.
Flow control stops a sender from overrunning its receiver.
An application reaches TCP and UDP through the operating system's socket interface, which first appeared in Berkeley UNIX and is now on nearly every system. A socket is an endpoint, named by a file descriptor, through which a process sends and receives data.
com; IP needs addresses. The Domain Name System maps between them, and also tells mail systems where to deliver mail and gives aliases. It is a distributed, hierarchical database: no server holds all names. The namespace is a tree.
The HyperText Transfer Protocol carries the Web. A client (browser) sends a request naming a resource and a method; the server sends back a response with a status code, header fields and usually a body.1 runs over TCP, by default on port 80 (443 for HTTPS, HTTP over TLS).
