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src/articles/computer_networking.md
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src/articles/computer_networking.md
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This is a work in progress article I'm using to build up my understanding of computer networking.
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# Standards #
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## [OSI model](https://en.wikipedia.org/wiki/OSI_model) ##
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The Open Systems Interconnection model is a basis for standards development in systems interconnection.
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### Physical Layer ###
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Converts digital bits into electrical, radio or optical signals
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* NIC: Network Interface Controller
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* Ethernet hub
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* Network switch
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* Physical transmission media
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Standards at this layer include Bluetooth, Ethernet, USB
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### Data link layer ###
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* Medium Access Control Address (MAC) https://en.wikipedia.org/wiki/MAC_address
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* Logical link control (LLC)
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### Others ###
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* network
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* transport
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* session
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* presentation
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* application
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## [IEEE 802](https://en.wikipedia.org/wiki/IEEE_802) ##
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Family of standards for local area networks (LANs) and other types.
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# Terms #
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# Internet Protocol Suite #
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* RFC 1122
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* RFC 1123
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## Link Layer ##
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* Address Resolution Procotol (ARP) https://en.wikipedia.org/wiki/Address_Resolution_Protocol
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## Transport Layer ##
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* TCP
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* UDP
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* QUIC
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210
src/articles/linux_internals.md
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This article is my attempt to explain Linux internals to myself and act as a structured way of keeping notes.
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Linux is an operating system kernel - which is distinct from the user-space where user applications will run. The kernel directly interfaces with hardware via its firmware and associated drivers. It also manages resources for sharing by user applications. Other open-source kernels that may be of interest are those based on BSD and the GNU Hurd microkernel.
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The [Linux kernel](https://www.kernel.org/doc/html/v4.13/index.html) can be interacted with or controlled from user space by means of a File based API or a C API which aim to be POSIX compatible.
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The types of hardware interfaced in the kernel include:
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* graphics
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* audio
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* memory
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* cpu
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* gpu
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* network
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* keyboard and mouse
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* storage and i/o peripherals
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with the kernel handling control messages including power messages for associated devices.
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As part of its resource management the kernel includes:
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* filesystems and virtual file management
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* memory management functionality
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* process scheduling and interprocess communication
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* networking
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* security
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* media support
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In the user-space of operating systems that use the Linux kernel (will controversially refer to as Linux distros for brevity) some foundational elements and applications are:
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* an init system - often `systemd` and previously collections of 'sysvinit scripts' or niche alternatives like `shepherd`.
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* system daemons for administrative services
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* a graphics system - oten `wayland` now replacing `x org`
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* an audo or multimedia system - `pipewire` is becoming a common replacement for `pulseaudio` and others
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* a terminal emulator and shell
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* network interface management, such as ethernet or wifi
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* a package manager
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* a bootloader
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# APIs #
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https://en.wikipedia.org/wiki/Linux_kernel_interfaces#Linux_API
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The kernel tries to follow the [Portable Operating System Interface (POSIX)](https://en.wikipedia.org/wiki/POSIX) and [Single Unix Specification](https://en.wikipedia.org/wiki/Single_UNIX_Specification) where applicable.
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## File Based ##
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Device drivers are interacted with in directories:
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* `/dev` https://en.wikipedia.org/wiki/Device_file#DEVFS
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* `/sys`
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Processes are interacted with in:
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* `/proc`
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* `/proc/sys`
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## System calls and similar ##
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https://en.wikipedia.org/wiki/System_call
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`ioctl` (input/output control) is a system call for device specific io operations.
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* `sysctl` (system control)
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* `ioctl` (io control)
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* `fcntl` (file control)
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Other communication mechanisms include [netlink](https://en.wikipedia.org/wiki/Netlink) sockets which allow IPC between both kernel and userspace programs. It is designed to be a more flexible successor to `ioctl`.
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# User Space #
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## Init Systems ##
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### Systemd ###
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Important utilities:
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* `systemctl`
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* `journalctl`
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* `notify`
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* `loginctl`
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* `systemd-boot`
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Important daemons:
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* `systemd`
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* `journald`
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* `resolved`
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* `networkd`
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* `logind`
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* `user-session`
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* `udevd`
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Important libraries:
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* `libnotify`
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* `libudev`
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## Devices ##
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Devices are managed in user space with the `udev` utility - which has the following parts:
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* libudev which can be used as a library for device info
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* udevd daemon for managing the `/dev` virtual file hierarchy
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* the `udevadm` command line utility for admin and diagnostics
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## Window management and device input events ##
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Wayland and X are the two most common display servers on Linux systems. They both follow a client-server approach, where the latter in Wayland can be part of the compositor.
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Display servers can react to device input events via the `libinput` library, which in turn uses `libevdev` to handle evdev `ioctls` from the kernel.
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## Network ##
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`iptables` allows configuration of IP packet filter ruls in the Linux kernel firewal, which are implemented as netfiler modules.
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[avahi](https://en.wikipedia.org/wiki/Avahi_(software)) is a zero-configuration networking implementation (allows network service use by freshly networked computers or peripherals) including multicast DNS and DNS service discovery. Apples' Bonjour and Systemd's `systemd-resolved` are other implementations.
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The Desktop-Bus (DBus) is a user space middleware allowing communication between multiple processes (e.g. IPC).
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## Other ##
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* avahi
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* dbus
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* udisks
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* cgroups
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* autofs
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* kdbus
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* Plokit (Policy Kit) https://en.wikipedia.org/wiki/Polkit
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* Pluggable Authentication Module (PAM) https://en.wikipedia.org/wiki/Pluggable_Authentication_Module
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* Name Service Switch (NSS) https://en.wikipedia.org/wiki/Name_Service_Switch
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* procfs https://en.wikipedia.org/wiki/Procfs
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* sysfs https://en.wikipedia.org/wiki/Sysfs
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# Kernel Space #
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## Input events ##
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* evdev
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* `/dev/input`
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## Filesystems ##
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* ext4
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* btrfs
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* xfs
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* jfs
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* fat32
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* FUSE
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## Storage ##
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* SCSI
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* libATA
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## Virtualization ##
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* KVM
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* Xen
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## Process Management ##
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* `clone(2)` and `clone3(2)`
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* `futex(7)` and `futex(2)`
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* Completely Fair Scheduler
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* https://en.wikipedia.org/wiki/Earliest_eligible_virtual_deadline_first_scheduling
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* Native Posix Thread Library (NPTL) is kernel side of pthreads
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## Security ##
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* Linux Security Modules
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* SELinux
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* AppArmor
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* POSIX ACLs
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## Memory ##
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* DMA buffers
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## Audio ##
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* Advanced Linux Sound Architecture (ALSA)
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## Graphics ##
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* Direct Rendering Manager (DRM)
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* Kernel Mode Setting (KMS)
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## Network ##
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* New API
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* mac80211
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* [Netfiler](https://en.wikipedia.org/wiki/Netfilter)
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## Power and Control ##
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* ACPI https://en.wikipedia.org/wiki/ACPI
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