Internet%20Routing%20(COS%20598A)%20Today:%20Intradomain%20Routing%20Convergence

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Internet Routing (COS 598A)Today Intradomain
Routing Convergence

• Jennifer Rexford
• http//www.cs.princeton.edu/jrex/teaching/spring2
  005
• Tuesdays/Thursdays 1100am-1220pm

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General Course Stuff

• Tuesday March 1
• Lecture by Larry Peterson on PlanetLab
• No assignment for written reviews of papers
• Ill be attending a Computing Research
  Association (CRA) Board of Directors meeting in
  D.C.
• Course projects
• Good to start thinking up a project idea
• Make an appointment to chat with me
• Written report due on Deans Date, Tue May 10
• Oral presentations during exam period
• No formal leading a class discussion item

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Splitting Convergence into Two Lectures

• One class is just not enough
• Today on intradomain routing
• Next Thursday on interdomain routing
• Impact on reading you already did
• One paper intradomain, one on interdomain
• Though class will focus just on intradomain
• Impact on reading for Thursday March 3
• One paper intradomain, one on interdomain
• Though class will focus just on interdomain
• Push off the next topic to following class

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Outline

• Routing-protocol convergence
• Steps in reacting to a link failure
• Effects on the data packets
• Intradomain routing protocols
• Steps in protocol convergence
• Implementation overhead and timers
• Operational practices to reduce convergence
• Multiple shortest paths between routers
• Costing out during planned maintenance

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Routing Convergence

• The only constant is change
• Equipment failures, or new deployment
• Routing-protocol configuration changes
• Planned maintenance on the network
• Routing protocols adapt
• Detect the change
• Propagate messages
• Compute new routes
• Update the forwarding tables

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Converging After a Failure

• Failure detection
• Router recognizes an incident link has failed
• Failure notification
• Router informs other routers about the change
• Path re-computation
• Routers compute new paths avoiding the link
• Forwarding-table update
• Routers update their forwarding tables
• Data traffic starts to flow over the new path

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Bad Things Happen During Convergence

• Transient inconsistencies
• Routers have different views of the network
• Forwarding decisions may be inconsistent
• Effects on data traffic
• Black-hole packet loss
• Loops packets going in circles
• Delay packets going on very long paths
• Out-of-order new packets arrive before old ones
• Want to minimize convergence delay
• and especially the effects on the data traffic

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Example Black-hole Causing Packet Loss

• Router forwarding to dead link
• Doesnt know (yet) that the link is dead
• Or, hasnt computed a new forwarding entry

d
s
Fortunately, IP only promises best effort
delivery!
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Example Forwarding Loop

• Set of routers disagree
• One router acting on old information
• Another router acting on new information

s
d
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Intradomain Routing Convergence
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Interior Gateway Protocols (IGPs)

• Routers running OSPF or IS-IS
• Flood link-state advertisements (LSAs)
• Compute shortest paths from link weights
• Determine next hop to other routers

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Knowing a Link is Dead Heart-Beats

• Periodic hello packets (hello_interval, 10sec)
• Timeout if not received (dead_interval, 40 sec)
• Declare failure and flood the info to others
• Small values lead to faster detection, but also
• Higher bandwidth consumption for hellos
• False detection during congestion interval
• False detection if router CPU falls a little
  behind

hello
hello
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Knowing the Link is Dead Interface Support

• Smart interface hardware
• Detects loss of connectivity at lower layer
• Interrupts the router CPU about the failure
• Common in Packet Over SONET technology
• E.g. Sprint paper sees delays less than 100 msec
• But
• Some media dont support it (e.g., Ethernet, ATM)
• so, you often need heartbeats anyway
• Also, want heartbeats to detect failures the
  hardware cannot detect on its own

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Flooding the Link-State Advertisement

• After detecting the failure
• Router sends LSA out each link
• Each router does the same
• and so on
• Flooding delay
• (CPU delay at each hop) (diameter of the
  network)

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Computing the Shortest Paths

• Each router re-computes
• Shortest-path tree rooted at this router
• Determine next-hop to every other router

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Reducing the Computational Overhead

• Good system
• Fast processor
• High-speed memory
• Good algorithms
• Traditional approach computes from scratch
• Incremental algorithms compute only the changes
• Especially nice if only one edge changes
• Pre-computation
• Pre-compute effects of certain failure scenarios
• E.g., all single-link or single-router failures

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Updating the Forwarding Table

• Forwarding table
• Map destination prefix to outgoing link(s)
• Copy of table on each interface card
• Highly optimized for fast lookups
• Updating the forwarding table
• Computing the new forwarding table
• Making updates to the copy of the line card
• Important source of delay
• Sprint end-to-end study around 1 second
• ATT router-level study 100 msec 300 msec

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All Together Looking Inside the Router
LSA Processing
Route Processor (CPU)
OSPF Process
LSA Flooding
Topology View
SPF Calculation
FIB Update
FIB
Forwarding
Forwarding
Switching Fabric
Interface card
Interface card
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Significance of Protocol Timers

• Hello and dead intervals
• Failure-detection delay vs. false diagnosis
• Pacing the link-state advertisements
• Combining LSAs vs. longer convergence delay
• Some routers wait till after re-running Dijkstra!
• Delaying start of shortest-path computation
• Reducing computations vs. convergence delay
• Especially useful if failure affects multiple
  links

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Operational Practices
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Reducing the Effects of Convergence

• Long convergence delay is bad
• Transient problems with loss and delay
• Disruptive for VoIP and online gaming
• Solution 1 better equipment
• Interfaces that detect failures automatically
• Cranking down the values of the timers
• Faster CPUs and path-computation algorithms
• Solution 2 network design and operation
• Improve forwarding-plane convergence
• Improve convergence during maintenance

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Equal-Cost Multi-Path (ECMP)

• Multiple shortest paths
• Router can compute multiple shortest paths
• Forwarding table has multiple outgoing links
• Router splits traffic evenly over the links

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ECMP Reduces Forwarding-Plane Convergence

• Suppose one of the outgoing link fails
• Incident router detects the failure
• Quick recomputation of paths without this link
• Local forwarding table updated to use other link
• Other routers have no forwarding-table change!!!

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Only red router changes its forwarding table!
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Exploiting This Observation in Traffic Engineering

• Traffic engineering
• Given a topology and a traffic matrix
• set link weights to control the flow of traffic
• to minimize some objective function
• Bias toward solutions with ties
• Penalize solutions with just one shortest path
• Favor solutions that lead to multiple paths
• even if the link loads are a little less
  balanced
• Applied in some traffic-engineering tools
• Demand from ISPs buying the tools
• with customers demanding fast convergence

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Examples of Planned Failures

• Upgrades
• Changing link to higher capacity
• Loading new operating system on a router
• Swapping out an old interface card
• Maintenance
• Fixing a flaky optical amplifier
• Configuration changes that require a reboot
• Cable intrusions
• Construction activities near a fiber

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Planned Events Happen Often

• Sprint study
• Maintenance window
• From 10pm to 6am EST, covering east to west
• Period of low network traffic, so less congestion
• Not much business-critical traffic
• Responsible for 50 of intradomain failures
• Significance
• Planned events should be easier to handle
• The operator knows the failure(s) will happen
• but, how to tell the routing protocol?
• or, how to prepare the network in advance?

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Costing Out of Equipment

• Increase cost of link to high value
• Triggers immediate flooding of LSAs
• Leads to new shortest paths avoiding the link
• While the link still exists to forward during
  convergence
• Then, can safely disconnect the link
• New flooding of LSAs, but no influence on
  forwarding

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Bigger Picture

• Learn about a planned event
• E.g., replace optical amplifier
• Map the event to the IP equipment
• E.g., find link(s) that traverse the amplifier
• Increase the weight on each link
• Slowly, perhaps one at a time to reduce overhead
• Disable the equipment
• Disconnect amplifier and replace with new one
• Reintroduce the links into the network
• Slowly, change one link weight at a time

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Even Bigger Picture

• What if maintenance would cause congestion?
• Reducing the capacity of the network
• Link weights not optimized to new topology
• Compute weight changes to make
• Re-optimize the setting of the link weights
• based on the soon-to-be new topology
• Then, do the maintenance
• Cost out the IP links
• Fix/upgrade the equipment
• Cost in the IP links
• Then, go back to the old weight setting

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Project Ideas

• Multi-path routing
• Protocols that allow more multi-path routing
• not just the equal-cost paths (as in ECMP)
• Maintenance schedules
• Compute a sequence of weight changes
• Avoid link congestion in each step
• Convergence models
• What actually happens during convergence?
• Simulation of forwarding-plane behavior
• Effective pre-computation on routers
• Routers precompute reactions to certain failures
• E.g., all single-link failures or single-router
  failures

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For Next Time, on Tuesday PlanetLab

• Guest lecture
• Professor Larry Peterson
• Three papers (two short, one regular)
• A Blueprint for Introducing Disruptive
  Technology into the Internet
• Overcoming the Internet Impasse through
  Virtualization
• Operating System Support for Planetary-Scale
  Network Services
• No written reviews
• But, be ready to ask hard questions
• PlanetLab is very useful for course projects

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Next Thursday Interdomain Convergence

• Two papers (intradomain and interdomain)
• Experience in Black-box OSPF Measurement
• Route Flap Damping Exacerbates Internet Routing
  Convergence
• Written reviews
• Summary
• Reasons to accept
• Reasons to reject
• Avenues for future work
• Optional
• NANOG video about the second paper
• Really great essay on You and Your Research