7 Rate Limiting and Traffic Shaping Flashcards

1
Q

Data (Source Classification)

A

bursty, periodic, regular

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2
Q

Audio (Source Classification)

A

continuous, periodic

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3
Q

Video (Source Classification)

A

continuous, bursty (compression), periodic

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4
Q

CBR (a type of class of sources)

A

Constant Bit Rate source

In a constant bit rate source of traffic, traffic arrives at regular intervals, and packets are typically the same size as they arrive, resulting in a constant bit rate of arrival.

Audio is an example of a constant bit rate source

When we shape CBR traffic, we shape it according to a peak rate.

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5
Q

VBR (a type of class of sources

A

Variable Bit Rate source

Many other sources of traffic are VBR

Video and Data are often variable bit rate

VBR is shaped according to both an average rate and a peak rate where the average rate might actually be a small fraction of the peak rate.

At certain times, peak rate might well exceed the average rate.

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6
Q

“Leaky Bucket” (Traffic Shaping)

A

Each flow has its own bucket

Data arrives in a bucket of size beta and drains from the backet at rate rho.

The parameter rho controls the average rate.

Data can arrive faster or slower into the bucket but it cannot drain at a rate faster than rho.

Therefore the maximum average rate that traffic can be sent is the smooth rate, rho.

Size of the bucket controls the maximum burst size that a sender can send for a particular flow.

At times, the sender might be able to send at a faster rate, as long as the total size of the burst does not exceed the size of the bucket or does not overflow the bucket.

The leaky bucket allows flows to periodically burst, and the regulator at the bottom of the leaky bucket ensures that the average rate does not exceed the drain rate of the drain rate of the bucket.

Setting a larger bucket size can accommodate a larger burst size

Setting a larger value of rho can accommodate or enable a faster packet rate

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7
Q

(r,T) Traffic Shaping

A

Traffic divided into T-bit frames

Flow can inject less than or equal to r bits in any T-bit frame

A flow that obeys this rule has what is known as an rT smooth traffic shape

One cannot send a packet that’s larger than r bits long.

Unless T is very long, the maximum packet size may be very small

Range of behaviors is typically limited to fixed rate flows

Variable flows have to request data rates that are equal to the peak rate, this is wasteful

Flow can send a certain number of bits every time unit instead of 1 packet every time unit(like leaky bucket)

When flow exceeds a particular rate, the excess packets in that flow are given a lower priority.

If network is heavily loaded or congested, the packets from a flow that exceeds the rate may be preferentially dropped.

Priorities may be assigned at the sender or at the network.

At the sender, the application may mark its own packets since the sender knows which ones are lower or higher in priority.

In the network, the routers may mark packets with a lower priority, which is called policing

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8
Q

Shaping Bursty Traffic Patterns

A

Sometimes we may want to shape bursty traffic patterns allowing for bursts to be sent on the network, but still ensuring that the flow does not exceed some average rate.

For this we might use what is called a token bucket

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9
Q

Token Bucket

A

permits burstiness, but bounds it
no discard or priority

difficulty with policing

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10
Q

Leaky Bucket

A

smooths bursty traffic

priority policies

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11
Q

Apply policing to token buckets

A

Use a Composite Shaper, which is to combine a Token Bucket Shaper with a Leaky bucket

This allows for good policing, confirming that the flow’s data rate does not exceed the average data rate allowed by the smooth Leaky Bucket rate is easy.

But the implementation is more complex since each flow now requires two counters and two timers (one counter and one timer for each bucket)

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12
Q

1 byte = ?

A

8 bits

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13
Q

8 bits = ?

A

1 byte

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14
Q

Power Boost

A

A traffic shaping mechanism that was first deployed in commercial broadband networks in June 2006 by Comcast

Allows a subscriber to send at higher rate for a brief period of time

targets spare capacity for users who do not put sustained load on the network

2 types of power boost

capped
if the rate at which the user can achieve during this burst window is set to not exceed a particular rate, then we say that the policy is capped PowerBoost

uncapped
The configuration is simple.

PowerBoost bucket size, the max amount of traffic that can be sent that exceeds the sustained rate.

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15
Q

Power Boost has effect on the latency as well as the loss rate

A

Shaping traffic < or equal to Rsus can prevent this buffering

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16
Q

Buffer Bloat

A

Packets are waiting in buffer, higher delays

Delay is data in buffer divided by Rsus

Very bad for time-critical apps (voice and video)

Buffer bloat shows up in home routers, hosts, home APs, and switches/routers

Solutions:

  1. smaller buffers
  2. shaping
17
Q

Passive Measurement (one way of seeing what traffic is being sent on network)

A

collection of packets, flow stats that are already on the network

18
Q

Active Measurement (one way of seeing what traffic is being sent on network)

A

inject additional traffic to measure various characteristics

tools include ping, trace route

19
Q

Why Measure traffic on the network?

A

Billing

Security

20
Q

How to measure (passively)?

A

SNMP (Simple Network Management Protocol)
-“Management Information Base” (MIB)
Supported essentially on all networking equipment, but it is course and you cannot express complex queries on the data. It is course because we can’t ask specific questions such as how much traffic has been sent by a particular host or a particular flow

Packet Monitoring
Whereby monitors can see full packet contents or at least headers

Flow Monitoring
Where a monitor may see specific stats about individual flows in the network

21
Q

Packet Monitoring

A

Full packet contents (or packet headers)
Example: tcpdump, ethereal, wireshark

Provides lots of detail, can see timing information and information in the packet headers

Disadvantage is that it has high overhead, hard to keep up with high speed links and often requires a separate monitoring device

22
Q

Flow Monitoring

A

Monitors record stats per-flow

  • next-hop IP
  • src/dst AS & prefix

Sampling
-flow stats based on samples of packets

+ less overhead
- more coarse, no packets/payloads

Typically packets are grouped together into flows if they occur close together in time

A flow is something that shares a common:
src &amp; dst IP
src &amp; dst port
protocol type
TOS byte
interface
23
Q

Which monitoring can provide number of bytes in each flow?

A

Both Packet and Flow

24
Q

Which monitoring can provide packet headers and timing information (packet-level)?

A

Packet