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An Approach to Alleviate Link Overload as Observed on an IP Backbone

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An Approach to Alleviate Link Overload as Observed on an IP Backbone

Tuesday, April 1st

Infocom 2003

Sundar Iyer1,2, Supratik Bhattacharrya2, Nina Taft2, Christophe Diot2

1Stanford University, 2ATL SprintLabs

- Introduction
- Pathology of link overload
- Alleviate overload - deflection routing
- Performance analysis

- Overload: More than 50% utilization

- IP backbones are
- Overprovisioned low average utilization
- Have multiple paths

- Routing algorithms
- balance load across multiple shortest paths
- should reduce the likelihood of overload

- Shortest path routing
- puts load on a small set of equal cost shortest paths
- causes unequal use of link capacity

- Unpredictable traffic
- Short term load fluctuations e.g. hotspots

- Failure
- Link failures, fiber cuts, network maintenance

- Hard to predict all factors apriori

- Operators upgrade persistently overloaded links
- Peaks in link utilization cannot increase average utilization

- Severe link overload causes packet drops
- Interactive, real-time applications make it mandatory to overcome overload

- Introduction
- Pathology of link overload
- Alleviate overload - deflection routing
- Performance analysis

- Measurement of data from the Sprint backbone
- Analyzed 138 backbone links for 9 months
- SNMP link utilization data polled every 5 minutes
- The link utilization is an exponentially weighted moving average (EWMA)
- Measurements under-estimate overload
- Short term fluctuations are missed

Maximum Load

- Observation 1: There is always some overloaded link

Non-Overloaded links

Overloaded links

- Observation 2: Most of the links are not overloaded

- Observation 3: Two types — Persistent

and temporary overload

Periods of link overload

- Observation 4: Often just 1-2 links are simultaneously overloaded

Link Utilizations

- Observation 5: Link failures cause temporary overload

- Observation 6: Fiber cuts cause severe overload

- Introduction
- Pathology of link overload
- Alleviate overload - deflection routing
- Performance analysis

Allow normal network operation most of the time

- Previous techniques
- useful for long term overload
- change normal functioning of the network
- useful when overload is common

- We observe that link overload
- is relatively rare (0.1% of the time on any link)
- are typically caused due to link failures/maintenance
- lasts for minutes-hours on average
- occurs on maximum of 1-2 links simultaneously
- can be easily overcome by deflecting packets

- Problem:
- How can we design a simple, stateless, loop-free deflection algorithm to overcome link overload?

- Theorem 1: (sufficiency)
- Any deflection algorithm which deflects packets with “strictly decreasing cost” is loop-free

- A packet is forwarded from node s to d according to the strictly decreasing cost criteria as follows
- If shortest path not overloaded
Forward the packet on the shortest path with cost C

- If link to neighboring node n is not overloaded
Forward the packet to n if n’s cost to d is C

- Else
Forward the packet on the shortest path

- If shortest path not overloaded

Yes

No

- Loop-free deflection routing:

- we do not consider the cost of reaching the deflection node

20

15

30

Router: n1

10

10

Router: s

Router: d

Router: n2

20

25

Router: n3

- Shortest path routing:
- forward packet on the shortest path
- the sequence of costs to a destination is strictly decreasing

- Problem:
- Can we always find loop-free deflection paths according to the strictly decreasing cost criteria?

- Theorem 2: (sufficiency)
- A network with redundant equal length paths always has a loop-free deflection path if the link weights are in a ratio 1 + 1/(d-1), where d is the diameter of the network

- Intuition:
- All link weights are in the range [Wmin ,Wminx]
- the minimum cost of the shortest path is dWmin
- the maximum cost of the deflection path is (d-1)Wminx
- (d-1)Wminx dWmin x 1 + 1/(d-1)

- Criteria for Theorem 2
- Need equal length shortest paths between any two nodes
- Weights need to be within a bounded ratio “1 + 1/(d-1)”
- The diameter d of the network should be small

ANA-1

SJ-1

NYC-1

CHI-1

RTP-1

FW-1

CHI-4

ANA-4

NYC-4

RTP-4

SJ-4

FW-4

SJ-2

RTP-2

NYC-2

CHI-2

FW-2

ANA-2

ANA-3

RTP-3

CHI-3

NYC-3

SJ-3

FW-3

PoP

New York

PoP

San Jose

PoP

Chicago

Small diameter, d=3

Redundant equal length paths are guaranteed

PoP

Anaheim

PoP

RTP

PoP

Fort-Worth

Large inter-POP weights are within ratio

Perfect Mesh in PoPs

SJ-1

NYC-1

RTP-1

FW-1

NYC-4

RTP-4

SJ-4

FW-4

SJ-2

NYC-2

RTP-2

FW-2

Redundant equal length paths not guaranteed

RTP-3

FW-3

NYC-3

SJ-3

Small (wmax) Intra-POP Weights

CHI-1

CHI-2

CHI-4

CHI-3

Diameter is larger

ANA-1

ANA-2

ANA-4

ANA-3

Large Inter-POP Weights

- Inter-PoP Network: PoPs as a single ‘logical node’
+ All criteria for theorem 2 are satisfied

- The complete network
- Equal length redundant paths does not exist

- Diameter of the network is not small

- Maximum intra-PoP link weight wmax is unrelated and very small compared to inter-PoP link weights

- Problem
- Cannot satisfy theorem 2 for the complete network

Inter-

PoP

Intra-

PoP

- A packet is forwarded from node s to d as follows, where wgain = wmax
- If shortest path not overloaded
Forward the packet on the shortest path (with cost C)

- If link to neighboring node n is not overloaded
Forward the packet to n if n’s cost to d is C –wgain

- Else if link to (intra-PoP) node n’ is not overloaded
Forward the packet if its cost to d is C +wmax

- Forward the packet on the shortest path

- If shortest path not overloaded

- Theorem 3:
- The practical deflection routing algorithm has no inter-PoP loops

- Comments
- The sequence of costs strictly decreases across PoPs
- This is in keeping with the idea of ‘PoPs’

- Link failures
- The algorithm is extended by setting wgain = (n-1)wmax

- Introduction
- Pathology of link overload
- Alleviate overload - deflection routing
- Performance analysis

- Simulation parameters
- 14 node inter-PoP network and 4-5 node intra-PoP network
- Estimated traffic matrix with gravity models & link measurements
- Deflection threshold was set to 45%
- Deflection based on fast EWMA
- Simulations for link failures and fiber cuts

- Deflection routing decreases the maximum load amongst all links in the backbone

- Deflection routing algorithm
- Based on practical considerations and overload pathology
- Exploits backbone architecture, meshed topology
- Mandates a condition on weights which is not too restrictive
- Is loop-free across PoPs

- Note
- Needs a redundant backbone network with equal-length paths
- Useful when average utilization is low

- Future Work
- Stability needs to be investigated