Scalability & Stability of the Internet Infrastructure Farnam Jahanian Department of EECS University of Michigan.
Download ReportTranscript Scalability & Stability of the Internet Infrastructure Farnam Jahanian Department of EECS University of Michigan.
Scalability & Stability of the Internet Infrastructure
Farnam Jahanian Department of EECS University of Michigan
Context
•Routers •Name Servers •Critical Services •Protocol Scrubbers •Replication schemes •Countermeasures
Active Response Capabilities Network Infrastructure
•Network Attacks •Operational Faults •S/H Failures
Anomalous Network Events LIGHTHOUSE: Survivable Network Infrastructure
•Event Aggregation •Data Mining
Analysis Engines Coarse and Fine Grained Measurement Tools
•Netflow Statistics •Windmill Probes Joint projects between U. Michigan & Merit Network
Motivation
Increasing reliance of financial and national utility infrastructures on interconnected IP-based networks
Explosive growth in both size and topological complexity of the underlying communication infrastructure
Reliance on off-the-self infrastructure & shrink-wrapped code
Network infrastructure is vulnerable:
– inherent instability and transient oscillations – delayed convergence and long failover – coordinated denial of service attacks on network resources – hardware and software failures – operational faults and misconfigurations
Imminent Collapse of the Internet
Collapse of the Internet
?
Now
Internet Growth Explosive growth in both size and topological complexity
Internet end-system growth Traffic volume & characteristics Infrastructure topological evolution
Infrastructure Topological Evolution Between 1995-1999:
Decentralization: from a single backbone network to a
conglomeration of 100s of backbone and 1000s ISP.
Loss of hierarchy and abstraction: from strict hierarchical
network to increasingly a full-mesh interconnection.
Significant bandwidth increase: from signle T3 (45MB) circuit
and T1 (1MB) links to multiple OC48 (1.2GB) circuits and OC12 (622MB) lines between nodes.
Internet Evolution: NSFNet
Hello/EGP
NSFNet Backbone
Hello/EGP Hello/EGP
Regional Campus Campus Regional Campus Regional Campus
Hierarchical network with a single central backbone
Internet Evolution: Today
AS1 AS2 C2 C1 AS4 AS3 C3 C4
Full-mesh interconnection of ISP backbones and customers
Impact of Instability & Failures
–
Increased end-to-end Loss/Latency
–
Increased delay in convergence & network reachability
–
Backbone infrastructure CPU/Memory requirements
–
Backbone “route flap storms”
–
Network management complexity
Background: Internet Architecture
BGP BGP BGP
Background: Internet Routing
Two major categories
– Inter-domain (BGP between autonomous systems) – Intra-domain (OSPF, ISIS, IGRP inside an AS)
BGP
– Incremental: announcements and withdraws – Updates include policy (e.g. MED, ASPath) – Maintain multiple possible routes
Background: BGP Routing Protocol
BGP is an incremental protocol that sends update information only upon changes in network topology or routing policy.
Two forms of messages:
announcements: New network accessible Prefer another route to network destination withdrawals: Destination network is no longer accessible
Routing policies vs. shortest number of hops
Background: Internet Core
Networks aggregated into CIDR (Classless Inter-Domain Routing) prefixes
Prefix represents a set of destination IP addresses
At Internet “core” all routers maintain paths to “default free” routes
Originally 5 major Internet Exchange Points (IXPs)
In 1996, approximately 30,000 default-free routes
Roadmap
Study of stability of routing in the Internet backbone
– Transient oscillations, pathological redundant updates – congestion collapse and correlation to network usage – SIGCOMM’97 and INFOCOMM’99
Study of route availability and failover rates
– long-term availability of Internet backbone routes – Case study of regional provider – FTCS’99
Study of convergence behavior of routing protocols
– Injection of route changes into the Internet backbone – Impact of convergence delay on end-to-end path – 18-month study & ongoing
Internet Exchange Points Deployed probes machines at five public exchange points Collected all routing updates at IXPs over four year period
Internet Routing Instability Results
Number of BGP routing updates exchanged per day in the Internet core is orders of magnitude larger than expected.
Most routing information is dominated by pathological, or redundant updates, which do not directly reflect changes in routing policy or topology.
Instability and redundant updates exhibit a specific periodicity of 30 and 60 seconds.
Instability and redundant updates show a surprising correlation to network usage and exhibit corresponding daily and weekly cyclic trends.
Instability Results (Continued)
Instability is not dominated by a small set of autonomous systems or routes.
Instability is not disproportionately dominated by prefixes of specific lengths, i.e. independent of aggregation.
Discounting policy fluctuation and pathological behavior, there remains a significant level of Internet forwarding instability.
Details: SIGCOMM’97 & INFOCOMM’99
Growth in Routing State Linear growth in routing table
Initial Findings
(SIGCOMM’97)
Up to 60 million routes! BGP updates/day for only 30,000 default-free
– On avg. 2-6 Million withdraws per day (mostly duplicates) – e.g., ISP A had 259 routes but withdrew 2.4 million routes
All state changes well distributed across prefix lengths, autonomous systems
Unexpected frequency components
– 30 second inter-arrival time between updates – Daily/weekly components
More Initial Observations
Most routing updates pathological ( millions!
)
– Some due to misconfiguration Private networks Host routes Multicast routes – Majority duplicate updates Duplicate withdraws (WWDup > 99.99%) Duplicate announcements (AADup)
BGP Updates
30 Second Frequency Components 1997
Origins of Pathological Updates
(INFOCOM99)
Majority stem from two router software implementation issues:
– stateless BGP withdraws – non-transitive attribute filtering
Frequency due to non-jittered router timers
– lack of precise specification
Others sources of pathologies:
– BGP/IBGP misconfiguration – Still others DSU/CSU oscillation – And still others due distance-vector algorithm
After Initial Publication of Results
One popular vendor validated our conjectures and released updated software in 1997
– Software rapidly deployed by ISPs – Stateful BGP reduced updates by orders of magnitude – Addition of random intervals to timers diminished frequency components
BGP Announcements and Withdraws
NANOG presentation ISP Geeks Release Mainline Release
Frequency Components
1997 1998
BGP Failures -- Congestion Collapse
(BGP Frequency)
A Short Story Sigcomm '97 findings were puzzling: Bandwidth Utilization
Instability Hypothesis:
Congestion causes underlying TCP to backoff
BGP-level timers expire, causing termination
Border Gateway Protocol (BGP)
MCI Sprint
Interdomain protocol between Autonomous Systems Routing peers exchange reachability information incrementally BGP uses TCP as the transport protocol between peer routers
BGP Congestion Collapse Hypothesis
Congestion causes underlying TCP to backoff BGP-level timers expire, causing termination Interaction between BGP and TCP leads to router congestion collapse High bandwidth utilization BGP Instability Validated using Windmill tool (SIGCOMM98)
What about Failures?
Some state changes due to policy changes & network failures
Cannot distinguish between policy, intra-domain and inter domain failures
Methodology:
– Measure long-term rate of failure for Internet backbone routes – Case study of regional provider
Internet Infrastructure Failures
(FTCS99)
Internet significantly less reliable and available than PSTN telephone network.
After a network becomes unreachable, in most cases, it takes longer than 5 mins before it is reachable again.
Even for transient oscillations, convergence of backbone routing states may be in the order of mins!
Route failover (re-routing of traffic to a given network) occurs on average of once every three days or more.
A small fraction of network paths contribute disproportionately to number of long-term outages
Definitions
Route Failure:
Prefix destination unavailable for 30 or more minutes
Route Repair:
A failed route becomes available
Route Failover:
A route replaced with one associated with a different path
Route Failures:
How long before a network is unreachable?
Route Repairs:
How long before a network is reachable again?
Failover:
How long before traffic is re-routed?
Conventional Wisdom on Convergence
Internet is highly redundant
– Just reroute around in a few milliseconds
Routing protocol convergence takes only a few
????
“Bad news travels fast”
– Fast withdraw propagation valid goal
Not True!
BGP has great convergence properties
– Path vector solved the convergence and counting to infinity (looping) problems
All my customers are multi-homed, triple-homed
– Convergence --
what, me worry ?
18-Month Study of Convergence Behavior
Instrument the Internet
– Inject routes into geographically and topologically diverse provider BGP peering sessions (Japan, Michigan, US Exchange Points, Canada, UK) – Periodically fail and change these routes (i.e. send withdraws or new attributes) – Time events using ICMP ping and NTP synchronized BGP “routeviews” monitoring machines – Wait 18 months… (50,000 routing events)
Passive & Active Measurement Infrastructure Fault Injection Server
Stub AS ICMP Echos
Upstream ISP2 ISP3
Internet
BGP
Stub AS
BGP ISP4 ISP5 BGP RouteViews Data Collection Probe ISP6 Upstream ISP1
Terminology
Tdown:
A previously available route is withdrawn. This is a route failure.
Tup :
previously unavailable route is announced as available.
This is a route repair.
Tshort:
A route is replaced with another route having a shorter path. This is a route failover.
Tlong:
A route is replaced by another route with a longer path.
This is a route failover.
Avg. number of messages generated by each ISP following a routing update event
3.5
3 2.5
2 1.5
1 Japan Verio Michnet CANet Tdown Tlong Tup Tshort •
Tdown and Tlong generated more messages than Tup and Tshort
•
Significant variation among ISPs within each category of message
Withdraw Convergence (Tdown) After a BGP route is withdrawn, barring other failures, how long does it take Internet routing tables to reach steady-state?
Withdraw Convergence 100 90 80 70 60 50 40 30 20 10 0 0 20 40 60 80 100 120
Seconds Until Convergence
140 160
Convergence delay after a Tdown
per.japan
per.canet
per.michnet
per.verio
Withdraw Convergence
Different providers exhibit different behavior
70% of withdraws from most ISPs take more than a minute
For ISP in Canada, 20% withdraws took more than three minutes to converge
Observed latencies of up to 10 mins for certain events
No correlation between convergence latency and geography or topological (except for MichNet)
Failovers and Repairs What are the relative convergence latencies for failovers and repairs?
Does bad news (withdraws) travel faster?
Failures, Failovers and Repairs
100 90 80 70 60 Tup 40 Tshort
Bad News Does Not Travel Fast!
Tdow n 30 20 10 0 0 20 40 60 80 100 120
Seconds Until Convergence
140 160
Failures, Failovers and Repairs
Bad news does not travel fast… Repairs (Tup) exhibit similar convergence properties as long
short path failover Failures (Tdown) and short
long failovers also similar
– Slower than Tup (e.g. a repair) – 60% take longer than two minutes – Failover times degrade the
greater
the degree of multi homing!
End2End Connectivity Impact of delayed convergence on E2E connectivity?
After a failover, how long before my site is reachable?
– Modified ICMP pings sent once a second – Source IP address block of pseudo-AS – 100 randomly chosen web sites from cache logs
Impact of Convergence Delay on End-to-End Path
60 50 40 30 20 Fault Tlong Tshort 10 0 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5
One Minute Bins Before and After Fault
6 7 8 9
Avg. packet loss to 100 web sites (1 min bins in the ten mins preceding and following a routing update)
What is Happening?
Non-deterministic ordering of BGP update messages leads to
– Transient oscillations – Each change in FIB adds delay (CPU, BGP bundling timer) – At extreme, convergence triggers BGP dampening
BGP Bad News
Given best current routing practices, inter-domain BGP convergence times degrade
exponentially
with increase in the degree of interconnectivity for a given route … and the degree of inter-connectivity (multi-homing, transit, etc) is increasing
Internet vs. Telephone Network
Packet-switched vs. circuit-switched
No explicit reservation on the Internet
Fault-tolerant switches in telephone networks
Significantly shorter development, testing and deployment cycle in the Internet world
Reliability vs. time-to-market
Relative degree of operational experience
Small number of telecommunication companies vs. a conglomeration of thousands of ISPs
Growing reliance on the Internet for commerce, healthcare, education, ...
Challenges Facing Today’s Internet are Bandwidth and Latency The Next Challenge Jeopardizing the Explosive Growth of the Web is AVAILABILITY .
Acknowledgements
Michigan Students & Merit Staff:
Abha Ahuja, Mukesh Agrawal, Paul Howell, Craig Labovitz, Rob Malan, Matt Smart, David Watson
Sponsors:
National Science Foundation, DARPA, Intel, IBM, HP