A Measurement Study on the Impact of Routing Events on

End-to-End Internet Path Performance

Feng Wang1 , Zhuoqing Morley Mao2

Jia Wang3, Lixin Gao1, Randy Bush4

1University of Massachusetts, Amherst2University of Michigan3AT&T Labs-Research4Internet Initiative Japan

Presentation modified with permissionPresenter: Young-Rae Kim

Date: Feb. 24, 2009

Table of Contents Background Motivation Open Question Our Work Methodology How Routing Failure Occur Summary Conclusion R-BGP Appendix

Background : Border Gate Protocol(BGP) The Border Gateway Protocol(BGP) is the core

routing protocol of the Internet. It maintains a table of IP networks or ‘prefixes’ which designate network reachability among autonomous systems(AS).

Most Internet users do not use BGP directly. However, most ISP must use BGP to establish routing between one another.

Background : Border Gateway Protocol (BGP) Beacons BGP Beacons are for research purposes to

improve our understanding of BGP dynamics. A BGP Beacon is an unused prefix which has a

well-defined schedule for announcement and withdrawal.

Given the known schedule of announcements and withdrawals, we can study the dynamics of BGP using publicly available BGP update data.

Background : MRAI timer MRAI (Minimum Route Advertisement Interval)

timer is specified in BGP. This timer acts to rate-limit updates, on a per-destination basis.

BGP(BGP-4) suggests values of 30s and 5s for this interval for external BGP(eBGP) and internal BGP(iBGP) respectively.

The MRAI serves to suppress messages which BGP would otherwise send out to describe transitory states, and so allow BGP to converge with significantly fewer messages sent.

Background : Internet Control Message Protocol (ICMP) Chiefly used by networked computers’ OS to

send error messages (i.e. indicating that a requested service is not available or that host or router could not be reached.)

It differs in purpose from TCP/UDP in that it is typically not used to send and receive data between end systems.

ICMP can be used directly by user using ping and trace routes.

Motivation Real-time services have made high availability of end-to-

end Internet paths of paramount importance. – low packet loss rate, low delay, high network availability, and

fast reaction time

Internet path failures are widespread [Labovitz:98, Markopoulou:04,Feamster:03].

– can last as long as 10 minutes

Degraded end-to-end path performance is correlated with routing dynamics.

Open Questions

How routing changes result in degraded end-to-end path performance?

What kinds of routing dynamics cause the degraded end-to-end performance?

How factors such as topological properties, or routing policies affect performance degradation?

Our Work

Study end-to-end performance under realistic topologies.

Investigate several metrics to characterize the end-to-end loss, delay, and out-of-order packets.

Characterize the kinds of routing changes that impact end-to-end path performance.

Analyze the impact of topology, routing policies, MRAI timer and iBGP configurations on end-to-end path performance.

Methodology A multi-homed prefix

– BGP Beacon prefix: 192.83.230.0/24

Controlled Routing Changes – Failover events: Beacon changes from the state of having both

providers to the state of having only a single provider.

– Recovery events: Beacon changes from the state of having a single provider for connectivity to the state of having both providers.

Provider 1

Beacon

Provider 2 Provider 1 Provider 2 Provider 1 Provider 2

Beacon Beacon

Failover event Recovery event

Active Probing From 37 PlanetLab hosts to the Beacon host (a host

within the Beacon prefix)– Back-to-back traceroutes– Back-to-back pings– UDP probing (50msec interval)

Data plane performance metricsInternet

Provider 2

Beacon host

Provider 1

host Bhost A

host C

metricsActive probing

traceroute ping UDP probing

Pack loss

Delay

Out-of-order

Packet Loss Loss burst: consecutive UDP probing packets lost

during a routing change event.

Failover Recovery

Correlating Packet Loss with Routing Failures ICMP replies

– temporary loss of reachability (!N or !H) – forwarding loops (exceeded TTL)

Routing failures– temporary loss of reachability and transient routing loops

Correlate loss bursts with ICMP messages – time window [-1 sec, 1 sec]

Underestimate the number of loss bursts due to routing failures – missing ICMP packets.

An Exampleplanet02.csc.ncsu.edu experiences packet loss on July 30, 2005

Loss Bursts due to Routing Failures Failover events: 76% packets lost Recovery events: 26% packets lost

Failover Recovery

How Routing Failures Occur (Failover)?

R1

Beacon

R4 R5

R6

R2 R3Provider 1 Provider 2Peer link

0

02 0

001 0

0

0

Prefer-customer routing policy: routes received from a provider’s customers are always preferred over those received from its peers.

AS 0

Customer link

How Routing Failures Occur (Failover)? (contd.)

R1

Beacon

R4 R5

R6

R2 R3

Provider 1 Provider 2

Peer link

002 0

001 0 0

0

R7 R9Provider 32 01 0

1 01 02 0

No-valley routing policy: peers do not transit traffic from one peer to another.

AS 0

Peer link

R8

How Routing Failures Occur? (Recovery)

R1R2 R4

R3

0Beacon

path (0) Path (0)

Withdraw (2 0)

5. R1 regains its connection to the Beacon

1. Path 0 R3 recovery.

2. R3 sends the path to R2

3. R2 sends a withdrawal

to R14. R3 sends the recovery path to R1

iBGP constraint: a route received from an iBGP router cannot be transited to another iBGP router

Provider 1

Provider 2

AS 0

Summary During failover and recovery events

– Routing changes impact packet loss significantly.– Multiple loss bursts are observed in 60% of events.– Routing changes can lead to long packet round-trip delays and

reordering.

Loss bursts explained by routing failures last longer than those unidentified ones.

Loss bursts caused by forwarding loops last longer than those caused by loop-free routing failures.

Conclusions During failover and recovery events

– routing failures contribute to end-to-end packet loss significantly.

Routing policies, iBGP configuration and MRAI timer values play a major role in causing packet loss during routing events.

Degraded end-to-end performance can be experienced by a diverse set of hosts when there is a routing change.

Accommodate routing redundancy may eliminate majority of identified path failures.

Resilient Border Gate Protocol (R-BGP)

The End

Thanks!

Location of Lost Bursts (Failover events)

Location of the first lost bursts caused by routing failures.

From ISP 2’s BGP updates:– Routing failures do occur and are not visible from ICMP messages due to

short duration.

From another AS’s BGP updates, and Oregon RouteView– Routing failures are cascaded to other ASes.

Class ISP 1 ISP 2 Other tier1 Non tier-1

Failover 1 92% 0 5% 3%

Failover 2 0 9% 73% 18%

Location of Lost Bursts (Recovery events)

Location of the first lost bursts caused by routing failures.

BGP updates from ISP 2– 12 withdrawals over 724 recovery events

Class ISP 1 ISP 2 Other tier1 Non tier-1

Failover 1 90% N/A 0% 10%

Failover 2 N/A 0% 59% 41%

Representativeness

Connectivity of Destination Prefixes– SS: Single-homed prefixes via a single upstream link– SM: Single-homed prefixes via multiple upstream links– MS: Multi-homed prefixes via a single upstream link– MM: Multi-homed prefixes via multiple upstream links

Routing tables from one tier-1 ISP on January 15, 2006

class SS SM MS MMpercentage 48% 6% 29% 17%

Representativeness (contd.)

Multi-homed destination prefixes

ISP 2 ISP 3

ISP 1

destination

Customer link Customer link

Peer link

Representativeness (contd.)

Multi-homed destination prefixes with multi-upstream links

ISP 2

ISP 1

ISP 1 ISP 2

Loss Burst Length

loss burst length can be as long as 480 packets for failover events, and 180 packets for recovery events

Loss burst length

Failover events Recovery events

Multiple Loss Bursts

Multiple loss bursts after the injection of a withdrawal message or an announcement.

Failover Recovery

Methodology Evaluation Our measurement is not significantly biased by ICMP

blocking– The number of ICMP messages in the absence of routing

change (0.6%).

– ICMP messages from 68 ASes, and 53% of them belong to 10 tier-1 ASes.

– 52% of ISP1’s routers, and 95% of ISP2’s routers generate ICMP messages.