Networking

OSI Model

• Open Systems Interconnection Model

Layers

Data Units

• Frame
  • Structure: Frame header (e.g. Ethernet) + (Network header + Transport header + data) + Frame footer
  • Source and destination MAC addresses, length, checksum
• Packet
  • Structure: Network header (e.g. IP) + (Transport header + data)
  • Version (IPv4/IPv6), source and destination IP addresses, length flags, TTL, protocol, checksum
• Segment
  • Structure: Transport header (e.g. TCP) + data
  • Source and destination ports, sequence number, ack number, data offsets, flags, checksum

IP

• Internet Protocol
• No concept of connection.
• Packets are passed from one computer to the next, until reaching the destination.
• No delivery guarantee, no receiving ack.
• Sometimes multiple copies of the same packet are passes, taking different paths, and thus arriving at different times.
• Designed to be able to route around connectivity problems.

TCP

• Transmission Control Protocol
• Built on-top of IP.
• Connection-based.
• Once a connection has been made between two parties, sending data between the two is much like writing to a file on one side and reading from a file on the other.
• Reliable and ordered, i.e., arrival and ordering are guaranteed.
• Takes care of splitting your data info packets and sending those across the network, so you can write bytes as a stream of data.
• Makes sure it doesn't send data too fast for the Internet connection to handle (flow control).
• Hides all complexities of packets and unreliability.
• Sends an ack for every packet received.
• Queues up data until there's enough to send as a packet.
• TCP tends to induce packet loss for UDP packets whenever they share a bottleneck node (same LAN/WAN).

UDP

• User Datagram Protocol
• Built on-top of IP, very thin layer over it.
• Unreliable protocol, usually around 1-5% packet loss.
• No guarantee of ordering.
• Minimizes transmission delay.
• Send a packet to destination IP address and port; the packet will get passed from computer to computer and arrive at destination or get lost.
• Receiver simply listens on specific port and gets notified when a packet arrives, with the sender address:port, and packet size.
• One guarantee over IP – a packet will either arrive as a whole (all of it) at destination or not at all (no partial delivery).
• You need to manually break your data up into packets and send them.
• You need to make sure you don't send data too fast for your Internet connection to handle.
• Good for when you want data to get as quickly as possible from client to server without having to wait for lost data to be resent, usually real-time data.
• Examples: real-time gaming, metrics reporting, video/audio streaming.