CS 3640: Introduction to Networks and Their Applications Fall 2018, - - PowerPoint PPT Presentation

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CS 3640: Introduction to Networks and Their Applications

Fall 2018, Lecture 3: Switching Instructor: Rishab Nithyanand Teaching Assistant: Md. Kowsar Hossain

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Rules of engagement: Recap

• There should be no gaps in seating.
• Ask and answer questions.
• Avoid use of electronics in class.
• Collaborate and be helpful.

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You should…

• Have found the course on Piazza
• http://piazza.com/uiowa/fall2018/cs3640
• Have found and explored the class website
• https://www.rishabn.com/cs3640-f18
• Have Python and Jupyter notebooks ready to go
• Assignment 1 releasing this week (groups will be announced on Piazza)
• Know and understand the three Internet design principles

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This week in class

1.

Recap: Design principles

2. 3.

Circuit & packet switching Performance & delays

Recap: Layering Provide protocols for applications to use (HTTP, email, etc.) Provide (reliable) end-to-end delivery Provide best-effort global delivery Put bits on the medium

Each layer only provides a service to the layer above it. Make the Internet modular and layered.

Recap: The end-to-end principle

• Functionality should be implemented at the lower layer if and
• nly if:
• It can be implemented completely
• It can be implemented correctly
• It is not made redundant by a higher layer
• We can make exceptions for performance optimizations.
• As long as it does not add a burden to applications that do not need the

functionality.

• This is a principle, not a rule.
• Violations exist on the Internet today.
• Examples: Firewalls, proxies, and NATs. (We’ll get to them later in the term).

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Recap: Fate-sharing

• Store network state only in the entities that rely on this state.
• This way critical state information is lost only when the entity that relies on

it fails.

• Simpler: Trust entities with a task only if they benefit from

the successful completion of that task.

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Recap: Building blocks of the Internet

• End-hosts (all layers)
• Users of the Internet
• Can be clients or servers
• Access networks (network + link layer)
• Local Internet Service Providers (ISPs)
• Facilitate end-user access to the network core.
• Network core (network + link layer)
• Regional and global ISPs
• Establish connections with access networks around the world through connections

with other networks

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Recap: Encapsulation and packets

Data Application Header Transport Data Header Network Header Data Header Link Data Header Header

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This week in class

1.

Recap: Design principles

2. 3.

Circuit & packet switching Performance & delays

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Recap: Access networks

• Access networks connect end-hosts to the Internet infrastructure.
• This is what your Internet Service Provider (ISP) does. They may provide

cable, DSL, dial-up, or satellite access networks.

Local ISP “A” Local ISP “B” Local ISP “C” Home network Enterprise network Mobile network

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Access networks: What do they look like?

• Who owns them?
• A lot of the time, you know them primarily as phone or cable companies.
• Why?
• All three technologies use the same infrastructure (and it already exists)!
• This is why you get “package deals”.

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Access networks: Cable/DSL access technology

Shared vs. point-to-point access

• Cable: shared access
• DSL: point-to-point access

Reasons: historical. Impact: Misleading advertising of cable download speeds.

Recap: The network core

• This is the core infrastructure of the Internet
• The core is where your local ISP connects to other “larger” ISPs. The larger ISPs do the
• same. Eventually, we end up with a large hierarchical network of networks – the Internet.

Local ISP Local ISP Local ISP Home network Enterprise network Mobile network Regional ISP Regional ISP Global ISP Global ISP

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The network core: What should it look like?

• This is where the magic happens.
• Thousands of routers that enable scalability and connect billions
• f end-points with each other.

Discuss: which one is better?

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The network core: What should it look like? + Cheap. O(n) links

• Capacity (1/n)
• Resilience (limit: 1 link)

+ High resilience (limit: n-1) + Capacity (1)

• Expensive! O(n2) links.

+ Cheap. O(n) links

• Capacity (1/n)
• Resilience (limit: 1 link)

Factors to consider

• Resilience (#links that need to fail to fracture the network),
• Cost (not too many links), and
• Capacity (not too few links).

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The network core: Finding a compromise with switches

Routers vs. switches

• Routers operate at the network layer (they make routing decisions)
• Switches operate at the link layer (they put packets on the right wire)

We use these terms interchangeably in the network core. Why? Modern Internet backbone switches come with network-layer functionality. S S S Fully connected switches.

• High resilience.

Adding another layer above routers.

• O(m2) links.
• (m/n) average capacity.

m (#switches) << n (#networks/routers)

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Switching: How do we share a common link? S S S

Method 1: Make a reservation

• Reserve the maximum bandwidth you will

need ahead of time.

• Reservation-based sharing.

Method 2: Just hope for the best

• Just send packets when you have to.
• On-demand sharing.

Discuss: Which is better?

Think about the end-to-end principle

Src1 Src2 Dst2 Dst1

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Switching: How do we share a common link?

Method 1: Make a reservation

• Reserve the maximum bandwidth you will

need ahead of time.

• Reservation-based sharing.

Method 2: Just hope for the best

• Just send packets when you have to.
• On-demand sharing.

Scenario:

• Bandwidth of link: 30 Mbps.
• Src1 needs 15 Mbps at peak and Src2 needs 15 Mbps at peak. They peak at

different times.

• Both sources can send at maximum rate regardless of method.

15 Mbps 15 Mbps 15 Mbps 15 Mbps

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Switching: How do we share a common link?

Method 1: Make a reservation

• If we allow peak rate reservation, we turn away Src2.
• If we allow equal reservation, we underserve both

sources (12.5Mbps capacity/15Mbps source). Results in lag (reliable transport) or high packet loss (unreliable transport).

Scenario:

• Bandwidth of link: 25 Mbps.
• Src1 needs 15 Mbps at peak and Src2 needs 15 Mbps at peak. They peak at

different times.

15 Mbps 15 Mbps 15 Mbps 15 Mbps

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Switching: How do we share a common link?

Method 2: Hope for the best

• Everything works out just fine!

Scenario:

• Bandwidth of link: 25 Mbps.
• Src1 needs 15 Mbps at peak and Src2 needs 15 Mbps at peak. They peak at

different times.

15 Mbps 15 Mbps 15 Mbps 15 Mbps

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Switching: How do we share a common link?

Which is better?

• Bursty applications prefer on-demand sharing.
• (peak rate/avg rate) is high, less predictable.
• Smooth applications prefer reservation-based sharing.
• (peak rate/avg rate) is low, more predictable.
• Discuss: Which one is better for the Web? Which one is better for phones?

15 Mbps 15 Mbps 15 Mbps 15 Mbps 15 Mbps 15 Mbps 15 Mbps 15 Mbps

Switching: How is reservation-based sharing implemented?

Circuit Switching.

• Phase I: Circuit request and establishment.
• Each switch reserves the requested bandwidth and forwards the request to the next one.
• Confirmation sent to source back via reserved bandwidth.
• Phase II: Data transfer.
• Phase III: Circuit teardown.

Switching: How is reservation-based sharing implemented?

Circuit Switching.

• Phase I: Circuit request and establishment.
• Phase II: Data transfer.
• Data transfer happens via established circuit.
• Phase III: Circuit teardown.

Switching: How is reservation-based sharing implemented?

Circuit Switching.

• Phase I: Circuit request and establishment.
• Phase II: Data transfer.
• Phase III: Circuit teardown.
• A teardown request is sent via the circuit.
• After the confirmation is sent from the destination, each switch deletes the reservation.

Circuit switching: What does a “reservation” look like?

• Time division multiplexing
• Splitting time between each source.
• Frequency division multiplexing
• Split the link frequencies between each source.

S1 S2 S3 S1 S2 S3 S1 S2 S3 Where have you seen frequency division multiplexing before? Wi-Fi routers. 2.4GHz vs 5GHz. Trade-offs: The higher the frequency, the lower the range. The higher the frequency, the higher the bandwidth (more data/source or more sources). But if you have only a few devices, do you really need to spend more $$$ for a 5GHz router?

Performance and efficiency of circuit switching S S

Performance and efficiency of circuit switching S S

Performance and efficiency of circuit switching S S

Performance and efficiency of circuit switching: Why bursty traffic is bad S S

Performance and efficiency of circuit switching: The minimum cost (~2 RTTs) is high S S

Performance and efficiency of circuit switching: The cost of failure is high S1 S2 S3

Summary: Circuit switching

• Circuit switching is how reservation-based link sharing is

implemented.

• Reserves peak resources end-to-end (from source to destination).
• All packets take the same route from source to destination.
• What does it handle well?
• Smooth and predictable traffic.
• Good for old phone systems.
• What makes it unsuitable for the Internet?
• Very inefficient for bursty traffic.
• Failure cost is high. (switch/link failure = circuit failure)

Switching: How is on-demand link sharing implemented?

Packet switching

• Treat each packet independently of the others.
• In contrast, circuit switching treated a collection of packets as one “flow”.

15 Mbps 15 Mbps 15 Mbps 15 Mbps

Switching: How is on-demand link sharing implemented?

Packet switching

• You don’t need to establish a circuit to get a packet to its destination.
• You just need to put the packet on the right wire (forward it). Recall: Each packet has a

destination address on it.

• Discuss: What happens if the traffic is too much for a link?
• Use a buffer.
• Size of the buffer determines how much burstiness you can handle.
• Full buffer = dropped packets.

15 Mbps 15 Mbps 15 Mbps 15 Mbps

Packet switching: The cost of failure

Scenario: Link failure during transmission. Discuss: What happens when a link or switch fails?

S S S Src1 Src2 Dst2 Dst1

Packet switching: The cost of failure

What happens when a link or switch fails?

Packets just get routed around the failed link or switch.

S S S Src1 Src2 Dst2 Dst1

Summary: Packet switching

• Packet switching is how on-demand link sharing is implemented.
• Treat packets independently.
• Packets can take many different routes to get to their destination.
• What does it handle well?
• Failures and bursty traffic. This makes it great for the Internet!
• What are its limitations?
• Needs to buffer packets if there more coming in than can be put on the link.

Small buffers can result in lost packets.

Circuit switching vs. Packet switching

Packet-switched networks: How we assess performance

• Discuss: How do we measure the performance of a packet-

switched network? What factors impact these metrics?

• Delay (time to reach the destination), loss (how many packets are dropped),

and throughput (how fast is the destination getting data).

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Announcements

• Office hours
• Me: W 9-10 am [MLH 201L]
• This week: W 11-12.
• Md. Kowsar Hossain: M 1:30-2:30 pm [MLH 101N]
• Assignments
• Groups announced on Piazza.
• Assignment 1 releasing tonight [2 days early]! Check the class website after 8 pm.
• Iowa mid-term elections!
• They matter! Frustrated with tuition increases?
• Iowa executive office (Gov, Treasurer, etc.) + 4 national house + 25 state senate + all state house

seats are up for election!

• Registration deadline: Oct 27, 2018.
• Election day: Nov 6, 2018 (in-class office hours, no new material).
• Get registered! Go vote!

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Assignment 1

• Available online tonight!
• 2 days early to help coordinate your groups and find each other.
• Worth 13% of final grade.
• Due: 9/13 @ 23:59. Late policy applies.

Assignment 1 Groups Group ID Group Hawk IDs 1 ['kdzhou', 'okueter', 'lburden', 'tnlowry', 'xxing2'] 2 ['mcagley', 'yzheng19', 'tsimonson', 'mfmrphy'] 3 ['dstutz', 'msmith3', 'sklemm', 'ymann'] 4 ['jmagri', 'zzhang103', 'uupadhyay', 'atran4'] 5 ['jglowacki', 'ppeterschmidt', 'kzhang24', 'tgoodmn'] 6 ['zhenbsong', 'bzhang22', 'ywang391', 'cweiske'] 7 ['xiaosong', 'bchoskins', 'jpflint', 'hpen'] 8 ['apatrck', 'xchen117', 'rdong6', 'weigui'] 9 ['jdhatch', 'susmerano', 'jpthiede', 'yitzhou'] 10 ['zluo1', 'godkin', 'nsonalkar', 'nicgoh'] 11 ['jstoltz', 'lye1', 'shangwchen', 'ywang455'] 12 ['hrunning', 'apizzimenti', 'yonghfan', 'jblue'] 13 ['gongyzhou', 'trjns', 'awestemeier', 'gmich']