00 - Fundamentals

ucla | CS 118 | 2024-04-02 08:26

Basic Definitions

hosts - endpoints
Network apps - apps that communicate across the network using data packets
routers - packet switching to correct routes
bandwidth - transmission rate
internet - a network of networks of ISPs, enterprises, cloud services, and hosts
protocols - rules of data transmission; HTTP, TCP, UDP, IP, etc.
internet protocol standards - standards usually set by other orgs: IEEE, RFC (request for comments), W3C (WWW consortium)

all network apps have no insight into lower layers, they generate data and assume the network can connect any hosts
don’t care how data is transmitted, just pushes the data
application communication is usually where this happens: e.g. browser on client sends HTTP request to apache on server

doesn’t actually deliver data, but from an application layer viewpoint, transport layer delivers requests
The transport layer has no percepts on the network layer of which ISP its going through or router etc. just the type of data so it knows what protocol to use: TCP, UDP

packet forwarding between endpoints through routers/ISPs to other ISPs until it reaches the requested endpoints
IP is used to determine which network devices to packet forward to until it reaches the destination
IP only knows the next neighbor (i.e. connected router) but has no percepts on any future “hops”
IP can also prevent packet loss and circular routing and other issues (even without knowing which ISPs/routers its been through by the IP header - hop count, etc.)

Routers will probabilistically delay certain packet routings to serve into a packet buffer (used to work on a reservation system like restaurants)
this means every packet has some probability of being delayed
when the buffer is full, packets are dropped

3 basic measures
- throughput - bits/sec
- loss rate - % of packets lost
- delay - sec, msec packets require for round-trip
link bandwidth - throughput across 2 endpoints directly connected on aa single link layer

wired link loss - due to transmission errors or congestion on the wire
wireless link loss - due to transmission rate (bandwidth), host mobility (number of hosts connected on the link)

4 sources
- node processing - error checking, tag checking (usually inconsequential with todays technology)
- transmission delay - length/rate : R=link bandwidth (bits/sec), L=packet length (bits)
- queuing delay - # packets in queue $\times$ transmission time of each packet
- propagation delay - distance/sec : d=physical link length, s=propagation speed in medium (near light, ~2e8 m/sec)

entire packet must arrive at router before it can be transmitted to next hop, so we need to store the bits received so far