Week 2 - Performance & Layering Flashcards
How do packets travel?
They travel in hops to reach the destination, there’s four sources of packet delay at each hop (nodal processing, queueing, transmission, propagation).
Communication link lore.
It’s bi-directional, point-to-point connection, could be active or idle. It has propagation delay (time it takes bits to go from sender to receiver).
Link latency - propagation delay, determined by the length of the link.
Link throughput - constant bitrate (bps), determined by the technology of the link.
What 2 jobs do routers have?
Their distributed routing algorithms and packet forwarding.
What is Queueing Delay and Loss?
Packets arriving at a router are places ona finite-sized queue. Waiting in the queue introduces delay. If the queue is full, new packets are dropped.
What is a total noday delay?
Nodal processing, queueing, propagation and transmission delay. Together, these delays accumulate to give a total nodal delay. Nodal delay is the delay at a single router.
What is nodal processing delay?
Checking bit errors, choosing output link, it’s typically less than a msec and negligible.
Basically, the time that a node spends processing a packet.
What is queueing delay?
It is time waiting at output link for transmission, depends on congestion level of the outbound link.
What is transmission delay?
formula
Transmission delay is the amount of time required for transmitting all of the packet’s bits into the link.
dt = packet size (bits) / link bandwith (bps)
or L/R => packet length / transmission rate
What is propagation delay?
formula
Propagation delay is the time required to propagate from the beginning of the link to the router. The bit propagates at the propagation speed of the link, which depends on the physical medium of the link. It is the distance between two routers (d) divided by the propagation speed (s).
the length of the link / speed of light (2x10^8 m/s) = d/s
midterm question
What are the layers involved in accessing a Web Page?
HTTP: get web pages, images, data and post data to servers (URLs, caching, redirects, proxies, cookies).
TLS: encrypt traffic (optional, but today universal)
TCP: byte-streams, it handles limited packet size, dropped and reordered packets.
IP: forward packets across multiple hops, best-effort approach to route packets to their destination IP address
then Ethernet/WiFi: link sharing with multiple devices, locally
What is a throughput?
How many bits per second are going through the network.
Average throughtput - F (file size) / T (time)
It is limited by the link with the slowest throughput along the path - the bottleneck link. You cannot pump data faster than the rate of the slowest link. Instantaneous throughput can be bottleneck link rate, but average throughput may be lower.
What is the formula of queueing delay?
If packets arrive at L bits/sec and the link rate is R bits/sec, then as long as L<R, there will be no queueing.
alfa - average packet arrival rate
L - packet length (in bits)
R - link bandwidth (bit transmission rate)
L*alfa/R > 1 - average delay infinite
What is traceroute?
Traceroute is a program used to measure end-to-end delay in a computer network by sending special packets to a destination and recording the time it takes for each packet to return. The program sends multiple packets, each marked with a number, towards the destination, and when routers receive these packets, they send back messages with their names and addresses.
