CS 3640: Introduction to Networks and Their Applications Fall 2018, Lecture 4: Packet switching performance metrics Instructor: Rishab Nithyanand Teaching Assistant: Md. Kowsar Hossain 1
You should… • Be checking Piazza regularly for announcements. • Have found your groupmates for assignment 1. • Have downloaded and gone over assignment 1. • Know and understand: • The three Internet design principles. • Encapsulation. • The components of the Internet. • Circuit- vs. packet- switched networks. 2
Rules of engagement • There should be no gaps in seating. • Ask and answer questions. • I’d like to know who you are and remember your name. • Avoid use of electronics in class (except when I ask for it). • Collaborate and be helpful. • When working in teams: “Seek to understand before being understood.” • End-of-class reading: http://www.slate.com/articles/technology/technology/2014/01/programme r_privilege_as_an_asian_male_computer_science_major_everyone_gave.html 3
Recap: Circuit switching vs. Packet switching
This week in class 1. 2. 3. Recap: Design Circuit & packet Performance & principles switching delays 5
How do we assess the performance of a packet-switched network? • Delay • How long does it take a packet to get to its destination? • Loss • What fraction of packets that are sent end up getting dropped? • Throughput • At what rate is data being received by the destination?
How do we assess the performance of a packet-switched network? • Delay • How long does it take a packet to get to its destination? • Which entities can impact this? • Switches/routers and links. S S S
The impact of switches on delays in packet-switched networks • Switches: What do they do? • They get packets and do some work (error checking, etc.) & then figure out which link the packet should go on. • Processing delay: At what rate can a switch figure out the right link? [“ d proc ”] • They convert packets into bits and write these bits to the link. • Queuing delay: How long does a packet wait in the buffer before it gets processed? [ “ d queue ”] • We say that the network is “congested” when the queueing delays are high.
The impact of links on delays in packet-switched networks • Links: What do they do? • They convert packets to link signals & then physically move bits from one end to the other. • Transmission delay: How many bits can be put on the link per second? [ d trans ] • Propagation delay: How long do bits take to reach the other end of the link? [ d prop ]
End-to-end delay of a packet-switched network • End-to-end delay is the sum of all the delays added by each switch/link between the source & destination. 𝑜 • (𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 d e2e = 𝑢𝑝𝑢𝑏𝑚 𝑡𝑥𝑗𝑢𝑑ℎ 𝑒𝑓𝑚𝑏𝑧𝑡 + 𝑢𝑝𝑢𝑏𝑚 𝑚𝑗𝑜𝑙 𝑒𝑓𝑚𝑏𝑧𝑡 = σ 𝑗=1 𝑟𝑣𝑓𝑣𝑓 ) Here, d i is the delay associated with the i th of n switches/links. •
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Transmission delay of a link • How many bits can be put on the link per second? • A: How much can you spend?
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Discuss: Need to send a 1000KB packet over a 1, 10, and 100 Gbps link adapter. What is the transmission delay over each one? 8000 Kbits = 8 x 10^6 bits 100 Gbps = 100 x 10^9 bits/sec 10 Gbps = 10 x 10^9 bits/sec 1 Gbps = 10^9 bits/sec Transmission delay for a 1000KB packet: @100Gbps: 8x10 -5 sec @10 Gbps: 8x10 -4 sec @1 Gbps: 8x10 -3 sec Transmission delay = data size/transmission rate of link interface
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Propagation delay of a link • How long does it take to move one bit from one end of the link to the other? • Depends on how long the link is. • Depends on the material used by the link. Since links are usually optic fiber, they propagate bits at 2x10 8 mps . • • This is the speed of light in glass. If we could cost-effectively send bits in vacuum, this would be 3x10 8 mps. •
Speed of light in glass = 2x10 8 mps 𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Discuss: What is the propagation delay of a 100m optic fiber link? Time to travel 100 meters @ 2x10 8 mps = 100m/2x10 8 mps= .5x10 -6 sec • Propagation delay = length of link/propagation speed of link
Propagation delay = length of link/propagation speed of link Transmission delay = data size/transmission rate of link interface 𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Discuss: Assume we have no congestion (no queueing delay) and an infinitely fast processor on the switch (no processing delay). Link propagation speed: 2x10 8 mps • Link length: 20x10 3 m • • Two different networks. When should you invest in a faster link interface card? • Scenario 1: • Link adapter: 1 Gbps, Data: 1 GB? What is d trans ? What is d prop ? d trans = 1x8x10 9 /1x10 9 = 8s, d prop = 20x10 3 /2x10 8 = 10 -4 s, • • d e2e = 8+10 -4 s • d trans is dominant. Investing in a faster link interface card is a good idea. • Scenario 2: • Link adapter: 1 Gbps, Data: 100 B? What is d trans ? What is d prop ? d trans = 8x10 2 /1x10 9 = 8x10 -7 s, d prop = 20x10 3 /2x10 8 = 10 -4 s, • • d e2e = (8x10 -7 ) + 10 -4 s • d prop is dominant. Investing in a faster link interface card a waste.
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Processing delay of a switch • How long does a switch take to error check and figure out the next destination? • Depends on the switch CPU. • CPUs are typically multi-core 2-3GHz. • Depends on the per-packet operations required. • Operations per packet usually require O(10 3 ) CPU cycles.
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Discuss: What is the per packet processing delay of a 3 GHz switch using 3x10 3 cycles per packet? Time to process a packet = 3x10 3 cpp/3x10 9 cps = 10 -6 s • • Currently, this is never the bottleneck. CPUs are way faster than networks. • Adding too much functionality at the network layer could make it one, however. Processing delay = Cycles per packet/CPU clock speed
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Queueing delay of a switch • How long does a packet need to be buffered before the link can handle it? • Depends on packet arrival rate. • How many packets are already queued? • Bursty traffic is likely to have a longer time in the buffer. • Depends on packet dispatch rate. • How fast can things be removed from the buffer? Depends on d trans . • A lot messier to calculate. • Queueing theory: A whole research area with 100s of PhD dissertations.
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Queueing delay of a switch: A simple deterministic model • Part of the complication is that d queue changes over time and different even for packets arriving at the same time. • At time t, here is what we do know:
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Queueing delay of a switch: A simple deterministic model • What does this look like? b 0 +b 1 b 0 x 0 x 1 x 2
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Queueing delay of a switch: A simple deterministic model • What does this look like? b 0 +b 1 ′ = 𝑐 0 𝑦 0 𝑠 ′ = 𝑦 1 + 𝑐 1 b 0 𝑦 1 𝑠 x 0 x 1 x 2 x' 0 x’ 1
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Queueing delay of a switch: A simple deterministic model • What does this look like? b 0 +b 1 b 0 x 0 x 1 x 2 x' 0 x’ 1
𝑒 𝑗 𝑢𝑠𝑏𝑜𝑡 + 𝑒 𝑗 𝑞𝑠𝑝𝑞 + 𝑒 𝑗 𝑞𝑠𝑝𝑑 + 𝑒 𝑗 𝑟𝑣𝑓𝑣𝑓 • Queueing delay of a switch: A simple deterministic model • Queue statistic: Average queue occupancy over time. • How full is the queue, on average? This is the average vertical distance between A and D. • Scenario: At the start of every second, 100 bits arrive to a queue at rate 1000 bits/second. The transmission rate is 500 bits/second. • What is the maximum queue occupancy (required buffer size to not have packet loss)? • What is the average occupancy over time? 𝟔𝟏 + 𝟔𝟏−𝒋 𝒋 𝟔𝟏 σ 𝒋=𝟐 𝟔𝟏 = 𝟑𝟔𝒄𝒋𝒖𝒕 𝟑
Recommend
More recommend
