improving datacenter performance and robustness with multipath tcp

IMPROVING DATACENTER PERFORMANCE AND ROBUSTNESS WITHMULTIPATH TCP

Costin Raiciu†, Sebastien Barre‡, Christopher Pluntke†,Adam Greenhalgh†, Damon Wischik†, Mark Handley†

Before Start 我想要先用我的話說一次這篇 Paper想做的事情 :

這邊 Paper的重點在於， single path用在 TCP上面的時代已經過去了，multipath TCP才是主流。 Multipath TCP又主要有三種 topologies:

fattree, VL2, Bcube。這篇 paper又分析了在不同情況下multipath確實比 single path優秀的地方。還有一些影響

performance的因素的分析。最後是有沒有其他可能再發展的 topology

OUTLINE Introduction Data center networking

Topology. Routing. Path Selection. Congestion Control.

Multipath TCP in summary MPTCP in data center

Examples of Benefits Analysis

Influence of Topology Number of Subflows Influence of the Traffic Matrix

Evolving topologies with MPTCP Analysis Discussion about DHFT

Summary

Introduction Traditional single path TCP has many

significant problems, ill-suited for such network. (intra-data center traffic.)

Ex. Bottleneck, can’t scale up, congestion.

We propose using Multipath TCP as a replacement for TCP in such data centers.

Introduction Traditional data center topology.

…

Introduction 4 components to a data center

architecture: 1. physical topology 2. routing 3. path selection 4. congestion control of traffic on the

selected path.







Summary

Data center networking---Topology

Traditional data center topology.

…


That has many problems and can be improved a lot like we have discussed before.

So, fat-tree is one kind of solutions.

Fat Tree Topology [Fares et al., 2008; Clos, 1953]

K=4Aggregation

Switches

K Pods with K Switches

each

Racks of servers


And we also have VL2 and Bcube these two kinds of solutions.

BCube Topology [Guo et al, 2009]

BCube (4,1)

VL2 Topology [Greenberg et al, 2009, Clos topology]

10Gbps

20 hosts

10Gbps …

Data center networking---Routing

We cannot expect the host itself to know which of these paths is the least loaded, so the routing system must spread traffic across these paths.

Use randomized load balancing, where each flow is assigned a random path from the set of possible paths.

Data center networking---Path Selection

randomized load balancing有他一定的問題，就是有一定的機率某些 path會有超大流量、同時會有一些 path沒什麼 (極低或零 )流量。 The use of a centralized flow

scheduler has been proposed.

Multipath TCP: Congestion Control [NSDI, 2011]

Data center networking---Congestion Control

Our hypothesis is that given sufficiently many randomly chosen paths, MPTCP will find at least one good unloaded path, and move most of its traffic that way. In so doing it will relieve congestion on links that got more than their fair share of ECMP balanced flows.

Multipath TCP in summary By making the window increase depend

on the total window size, subflows that have large windows increase faster than subflows with small windows.





Influence of TopologyNumber of Subflows Influence of the Traffic Matrix


Summary

MPTCP in data center Three main benefits: 1. Better aggregate throughput 2. Better fairness 3. Better robustness

MPTCP in data center Depend on 4 requirements: 1. The congestion control scheme used. 2. The physical topology. 3. The traffic matrix generated by the

applications. 4. The level of load in the network.

Examples of Benefits Throughput: Using MPTCP in a FatTree network, we

can see the more subflows we have, the more throughput we have.

The reason is simple, n flows share one link, each flow need to achieves 1/n capacity.

Examples of Benefits

Examples of Benefits Fairness: Every host’s throughput is shown ranked

in order of increasing throughput. Is is clear that not only did the utilization improve with MPTCP, but also the fairness improved.

Examples of Benefits 這是一個累積百分比的圖，可以看出 single

TCP有些 path perform 真的很差 (很閒的意思 )，而MPTCP則大幅改善此情況。

Analysis Then, we have questions:

How many subflows are needed? How does the topology affect results? How does the traffic matrix affect results?

Analysis We need to view in these aspects to

answer the question:


Influence of Topology We compare VL2 and Bcube.

Influence of Topology Why Bcube is relative low？ I guess, just guess, is because in Bcube,

pathes between two hosts are not in same length which lead to worse flows management.

Number of Subflows We were surprised that eight subflows

were needed to achieve 90% throughput.

Why eight, and on what does this depend?

Number of Subflows Results:

Influence of the Traffic Matrix







Summary

Evolving topologies with MPTCP In single homed topologies: Hosts links are often bottlenecked. ToR switches failure also lead to crash

tens of hosts.

Evolving topologies with MPTCP 以 FatTree為例子

Evolving topologies with MPTCP So, how do we improved？ There are two solutions: perfect switch

and dual-homed FatTree.

Evolving topologies with MPTCP Perfect switch: Serves as a good control experiment,

giving an upper bound on what any network core might provide using single links to the hosts.

Evolving topologies with MPTCP Dual-homed FatTree (DHFT): is to the host and four connect the links

between the two layers of switches. If we remove one port per host from the

core and use it to connect the second interface on each server, the network requires the same number of switch ports.

Analysis QUESTION: Will perfect switch really improve

performance？

Analysis Testing throughput over MPTCP, perfect

switch TCP, normal TCP.

Analysis QUESTION: Will perfect switch really improve

performance？ ANSWER: Yes, it is.

Analysis QUESTION: Dual-homed FatTree really work？ Does it have any problem？When may

troubles occur？

Analysis Testing relative throughput over MPTCP,

TCP DHFT, TCP perfect switch.

Analysis QUESTION: Dual-homed FatTree really work？ Does it have any problem？When may

troubles occur？ ANSWER: Yes, it works only when load is low. When

load is high, throughput will go down near to TCP.

Discussion about DHFT DHFT’s worst case performance is 75% and best

case is around 200%. 在流量不穩定的情況下，DHFT + MPTCP有可能會有較差的表現產生。 Beyond performance, DHFT improves

robustness: any lowerpod switch failure does not cut-off an entire rack of servers.

DHFT is not optimal by any measure, but it shows that we can create topologies with better performance if we assume MPTCP is the transport protocol.







Summary

Summary “One flow, one path” thinking has

constrained datacenter design Collisions, unfairness, limited utilization

Multipath transport enables resource pooling in datacenter networks: Improves throughput Improves fairness Improves robustness

Reference conferences.sigcomm.org/sigcomm/

2011/slides/s266.ppt

Q & (A)

improving datacenter performance and robustness with multipath tcp

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