Loading in 5 sec....

Buffer Sizing for Congested Internet Links Amogh Dhamdhere, Hao Jiang and Constantinos DovrolisPowerPoint Presentation

Buffer Sizing for Congested Internet Links Amogh Dhamdhere, Hao Jiang and Constantinos Dovrolis

- 68 Views
- Uploaded on

Download Presentation
## PowerPoint Slideshow about ' Buffer Sizing for Congested Internet Links Amogh Dhamdhere, Hao Jiang and Constantinos Dovrolis' - fiona-rios

**An Image/Link below is provided (as is) to download presentation**

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript

Outline

Outline

Buffer Sizing for Congested Internet Links

Amogh Dhamdhere, Hao Jiang and Constantinos Dovrolis

(amogh,hjiang,dovrolis)@cc.gatech.edu

Networking and Telecommunications Group,

College of Computing,

Georgia Tech.

Outline

- Motivation and related work
- Objectives and traffic model
- The utilization constraint alone
- Utilization and loss rate constraints
- Parameter estimation and simulation results

Amogh Dhamdhere

IEEE Infocom 2005

Motivation

- Router buffers are important in packet networks
- Absorb rate variations of incoming traffic
- Prevent packet losses during traffic bursts

- Increasing buffer space increases the utilization of the link and decreases the loss rate
- Increasing buffer also increases queuing delays !
- So smaller buffers are desirable

- Fundamental Question: What is the minimum buffer requirement to satisfy constraints on the utilization, loss rate and queuing delay ?

Amogh Dhamdhere

IEEE Infocom 2005

Rules of Thumb

- Some router vendors suggest 500ms of buffering.
- Why 500ms ?

- Bandwidth Delay Product rule: Capacity of link times the “typical” RTT (B = CT)
- Which RTT should we use ?
- Many TCP flows with different RTTs ?
- How do different types of flows (large vs small) affect the buffer requirement ?

- Several variants of this rule
- e.g. Capacity times link delay

Amogh Dhamdhere

IEEE Infocom 2005

Related Work

- Approaches based on queuing models e.g. M/M/1/k
- TCP is not open-loop. TCP flows are reactive
- Modeling Internet traffic is difficult

- “Stanford” model (Appenzeller et al. Sigcomm 2004)
- Buffer requirement for full utilization decreases with square root of N
- Did not consider the loss rate at the link
- Assumed that flows are completely desynchronized
- Applicable when the number of flows is large

- Morris (1997 and 2000)
- Buffer proportional to the number of flows (B = 6*N)
- Considered all flows active at the link

Amogh Dhamdhere

IEEE Infocom 2005

Outline

- Motivation and related work
- Objectives and traffic model
- The utilization constraint alone
- Utilization and loss rate constraints
- Parameter estimation and simulation results

Amogh Dhamdhere

IEEE Infocom 2005

Our Objectives

- Full utilization:
- The average utilization of the link should be at least % when the offered load is sufficiently high

- Maximum loss rate:
- The loss rate p should not exceed , typically 1-2% for a saturated link

- Minimum queuing delays:
- High queuing delay causes higher transfer latencies and jitter
- Also increases cost and power consumption
- Should satisfy utilization and loss rate constraints with minimumamount of buffering possible

- All of these objectives may not be feasible !

Amogh Dhamdhere

IEEE Infocom 2005

Traffic Classes

- Locally Bottlenecked Persistent (LBP) TCP flows
- Large TCP flows limited by losses at the target link
- Loss rate p is equal to the loss rate at the target link

- Remotely Bottlenecked Persistent (RBP) TCP flows
- Large TCP flows limited by losses at target link and other links
- Loss rate is greater than loss rate at target link

- Window Limited Persistent TCP flows
- Large TCP flows, throughput limited by the advertised window

- Short TCP flows and non-TCP traffic

Amogh Dhamdhere

IEEE Infocom 2005

Assumption

- Key Assumption: LBP flows account for most of the traffic at the target link (80-90 %)
- In this case, we can ignore the buffering requirement of non-LBP flows
- non-LBP flows also contribute to the utilization and loss rate at the target link
- Contribution is small if fraction of non-LBP traffic is small

- Our model is applicable in links where this assumption holds
- Edge links and links in access networks are candidates

Amogh Dhamdhere

IEEE Infocom 2005

Outline

- Motivation and related work
- Objectives and traffic model
- The utilization constraint alone
- Utilization and loss rate constraints
- Parameter estimation and simulation results

Amogh Dhamdhere

IEEE Infocom 2005

TCP Window Dynamics

- Saw-tooth behavior of TCP
- Padhye (1998)
- TCP throughput can be approximated by
- Average window size is independent of RTT
- Valid when loss rate is small

Amogh Dhamdhere

IEEE Infocom 2005

Util. Constraint - Multiple TCP Flows

- heterogeneous LBP flows with RTTs
- Consider initially the worst-case scenario: Global Loss Synchronization.
- All flows decrease windows simultaneously in response to losses.
- We derive that
- As a bandwidth-delay product
- Where is the harmonic mean of the RTTs

Amogh Dhamdhere

IEEE Infocom 2005

Util. Constraint - Multiple TCP Flows

- is called the effective RTT of the flows
- Influenced more by smaller values

- Intuition:
- Flows with smaller RTTs have larger portion of their window in the bottleneck buffer
- Hence have larger influence on the required buffer
- Flows with large RTTs have larger portion of their window “on the wire”

- Practical Implication:
- A few connections with very large RTTs cannot significantly influence the buffer requirement, as long as most flows have small RTTs

Amogh Dhamdhere

IEEE Infocom 2005

Partial Synchronization Model

- In practice, flows are not completely synchronized
- Loss Burst Length: Number of packets lost by flows during a congestion event
- Empirical observation: Loss burst length increases almost linearlywith i.e.
- A simple probabilistic argument gives us,
- Partial loss synchronization reduces the buffer requirement.

Amogh Dhamdhere

IEEE Infocom 2005

Validation

- ns2 simulations.
- Heterogeneous flows,
%

- Partial synchronization model accurately predicts the buffer requirement.
- Deterministic model overestimates the buffer requirement !

Amogh Dhamdhere

IEEE Infocom 2005

Outline

- Motivation and related work
- Objectives and traffic model
- The utilization constraint alone
- Utilization and loss rate constraint
- Parameter estimation and simulation results

Amogh Dhamdhere

IEEE Infocom 2005

Utilization and Loss Rate

- End-user perceived service is poor when the loss rate is more than 5-10%
- Particularly for short and interactive flows
- Results by Morris (1997)
- High variability in the completion times of short transfers
- Some “unlucky” flows suffer repeated losses and timeouts

- The buffer size controls the loss rate
- Upper bound the loss rate to . Assume is 1%

Amogh Dhamdhere

IEEE Infocom 2005

Relation between loss rate and N

- homogeneous LBP flows at the target link. Link capacity C, flow RTTs T
- Assume that the flows saturate the link and their throughput is given by
- p is proportional to the square of
- Hence to maintain loss rate at less than
- But this requires admission control
- Such schemes not deployed yet

Amogh Dhamdhere

IEEE Infocom 2005

Flow Proportional Queueing

- First proposed by Morris (2000)
- Don’t limit
- Increase RTTs to decrease loss rate

- Increase RTT by increasing buffer, which increases queuing delay
- Solving for B gives
- Where
- Practically, packets for , and packets for

Amogh Dhamdhere

IEEE Infocom 2005

Flow Proportional Queueing (contd.)

- Intuition:
- packets per flow, either in buffer (B term) or “on the wire” ( term)

- Differences with Morris’ FPQ scheme
- Morris did not take into account the term
- Set arbitrarily to 6 packets
- Applied the rule for allflows active at the link

- Increasing RTTs may violate delay constraint
- In that case, choose the minimum buffer that can satisfy utilization and loss constraints

Amogh Dhamdhere

IEEE Infocom 2005

Integrated Model

- Separate results for utilization and loss rate constraints
- Satisfy the most stringent of the two requirements
- B for utilization decreases with , while B for loss rate increases with
- : Crossover point
- Called the BSCL formula

Amogh Dhamdhere

IEEE Infocom 2005

Integrated Model - Validation

- Simulations using ns2.
- Heterogeneous flows, varied from 1 to 200.
- Utilization % and loss constraint %

Utilization constraint

Loss rate constraint

Amogh Dhamdhere

IEEE Infocom 2005

- Motivation and related work
- Objectives and traffic model
- The utilization constraint alone
- Utilization and loss rate constraints
- Parameter estimation and simulation results

Amogh Dhamdhere

IEEE Infocom 2005

Parameter Estimation

- Flow Classification:
- Zhang et al. (2002): Classify TCP flows based on rate limiting factors

- Number of LBP flows:
- LBP flows: all rate reductions due to packet losses at target link
- RBP flows: Some rate reductions due to losses elsewhere

- Effective RTT:
- Jiang et al. (2002): Passive algorithm to measure TCP Round Trip Times from packet traces

- Loss Synchronization:
- Measure loss burst length from trace or use approximation

Amogh Dhamdhere

IEEE Infocom 2005

Evaluation - Setup

- ns2 simulations.
- Multi-level tree topology with wide range of RTTs (20ms to 550ms).
- Target link capacity 50Mbps.
- varied from 1 to 400.
- 20 RBP flows, 10 window limited flows.
- Mice flows with average size 14 packets, exponential inter-arrivals.
- Non-LBP traffic (R) is varied between 5% and 20% of C.

Amogh Dhamdhere

IEEE Infocom 2005

Results – Loss Rate

- BSCL can bound loss rate close to the target, if R is less than 10%.
- Accuracy decreases as fraction of non-LBP traffic increases.
- Stanford model and the rule of thumb cannot bound loss rate.

Amogh Dhamdhere

IEEE Infocom 2005

Results - Utilization

- For a large number of flows, all three schemes achieve full utilization.
- For smaller number of flows, BSCL sometimes leads to underutilization.
- Due to the probabilistic nature of loss synchronization.

Amogh Dhamdhere

IEEE Infocom 2005

Summary

- Derived a buffer sizing formula (BSCL) for congested links, taking into account both utilization and loss rate of the target link.
- Applicable for links in which 80-90% of the traffic comes from large locally bottlenecked TCP flows.
- Account for the effects of heterogeneous RTTs and partial loss synchronization.
- Validated the results through simulations.

Amogh Dhamdhere

IEEE Infocom 2005

Parameter estimation -

- Distinguishing between LBP and RBP flows:
- Intuition: For a LBP flow, rate reduction should be preceded by a loss at the target link.
- For RBP flows, rate reduction will not always be accompanied by a loss at the target link (due to losses in other links).

Amogh Dhamdhere

IEEE Infocom 2005

Why is Buffer Size Important ?

- Router buffer size affects:
- Utilization of the link.
- Loss rate of the link.
- Fairness among TCP connections.

- Results by Morris (1997):
- A very small buffer can lead to underutilization.
- Loss rate increases as the square of N.

Amogh Dhamdhere

IEEE Infocom 2005

Partial Synchronization Model (contd.)

- Consider a congestion event with the average loss-burst length .
- A simple probabilistic argument gives us,
- Remarks:
- For global loss synchronization, and the buffer requirement becomes B = CT.
- Partial loss synchronization reduces the buffer requirement.
- For heterogeneous connections, replace T with the effective RTT.

Amogh Dhamdhere

IEEE Infocom 2005

- Motivation and related work
- Objectives and traffic model
- The utilization constraint alone
- Utilization and loss rate constraints
- Parameter estimation and simulation results

Amogh Dhamdhere

IEEE Infocom 2005

Results - Loss Rate

- BSCL can bound loss rate close to the target, if R is less than 10%.
- Accuracy decreases as fraction of non-LBP traffic increases.
- Stanford model and the rule of thumb cannot bound loss rate.

Amogh Dhamdhere

IEEE Infocom 2005

Download Presentation

Connecting to Server..