Intradomain Routing

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Intradomain Routing

• Jennifer Rexford
• Advanced Computer Networks
• http//www.cs.princeton.edu/courses/archive/fall06
  /cos561/
• Tuesdays/Thursdays 130pm-250pm

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What is Routing?

• A famous quotation from RFC 791
• A name indicates what we seek.An address
  indicates where it is.A route indicates how we
  get there. -- Jon Postel

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Forwarding vs. Routing

• Forwarding data plane
• Directing a data packet to an outgoing link
• Individual router using a forwarding table
• Routing control plane
• Computing the paths the packets will follow
• Routers talking amongst themselves
• Individual router creating a forwarding table

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Internet Structure

• Federated network of Autonomous Systems
• Routers and links controlled by a single entity
• Routing between ASes, and within an AS

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Two-Tiered Internet Routing System

• Interdomain routing between ASes
• Routing policies based on business relationships
• No common metrics, and limited cooperation
• BGP policy-based, path-vector routing protocol
• Intradomain routing within an AS
• Shortest-path routing based on link metrics
• Routers all managed by a single institution
• OSPF and IS-IS link-state routing protocol
• RIP and EIGRP distance-vector routing protocol

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Shortest-Path Routing

• Path-selection model
• Destination-based
• Minimum hop count or sum of link weights
• Dynamic vs. static link weights

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Distance Vector Routing Bellman-Ford

• Define distances at each node x
• dx(y) cost of least-cost path from x to y
• Update distances based on neighbors
• dx(y) min c(x,v) dv(y) over all neighbors v

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du(z) minc(u,v) dv(z),
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E.g., RIP and EIGRP
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Link-State Routing Dijsktras Algorithm

• Each router keeps track of its incident links
• Link cost, and whether the link is up or down
• Each router broadcasts the link state
• To give every router a complete view of the graph
• Each router runs Dijkstras algorithm
• To compute shortest paths and forwarding table

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E.g., OSPF and IS-IS
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Routing Protocols (COS 461 15 and 16)
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History Packet-Based Load-Sensitive Routing

• Packet-based routing
• Forward packets based on forwarding table
• Load-sensitive
• Compute table entries based on load or delay
• Questions
• What link metrics to use?
• How frequently to update the metrics?
• How to propagate the metrics?
• How to compute the paths based on metrics?

Still a popular area of research
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Original ARPANET Algorithm (1969)

• Delay-based routing algorithm
• Shortest-path routing based on link metrics
• Instantaneous queue length plus a constant
• Distributed shortest-path algorithm (Bellman-Ford)

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congested link
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Performance of Original ARPANET Algorithm

• Light load
• Delay dominated by the constant part
  (transmission delay and propagation delay)
• Medium load
• Queuing delay is no longer negligible
• Moderate traffic shifts to avoid congestion
• Heavy load
• Very high metrics on congested links
• Busy links look bad to all of the routers
• All routers avoid the busy links
• Routers may send packets on longer paths

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Improvements in the Second ARPANET Algorithm
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Problem of Long Alternate Paths

• Picking alternate paths
• Long path chosen by one router consumes resource
  that other packets could have used
• Leads other routers to pick other alternate paths
• Solution limit path length
• Bound the value of the link metric
• This link is busy enough to go two extra hops
• Extreme case
• Limit path selection to the shortest paths
• Pick the least-loaded shortest path in the network

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Problem of Out-of-Date Information

• Routers make decisions with old information
• Propagation delay in flooding link metrics
• Thresholds applied to limit number of updates
• Old information leads to bad decisions
• All routers avoid the congested links
• leading to congestion on other links
• and the whole things repeats

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Intradomain Routing Today

• Link-state routing with static link weights
• Static weights avoid stability problems
• Link state faster reaction to topology changes
• Most common protocols in backbones
• OSPF Open Shortest Path First
• IS-IS Intermediate SystemIntermediate System
• Some use of distance vector in enterprises
• RIP Routing Information Protocol
• EIGRP Enhanced Interior Gateway Routing Protocol
• Growing use of Multi-Protocol Label Switching

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What do Operators Worry About?

• Topology design
• Small propagation delay and low congestion
• Ability to tolerate node and link failures
• Convergence delay
• Limiting the disruptions during topology changes
• E.g., by trying to achieve faster convergence
• Traffic engineering
• Limiting propagation delay and congestion
• E.g., by carefully tuning the static link
  weights
• Scalable routing designs
• Avoiding excessive protocol overhead
• E.g., by introducing hierarchy in routing

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Topology Design Intra-AS Topology

• Node router
• Edge link

Hub-and-spoke
Backbone
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Topology Design Abilene Internet2 Backbone
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Topology Design Points-of-Presence (PoPs)

• Inter-PoP links
• Long distances
• High bandwidth
• Intra-PoP links
• Short cables between racks or floors
• Aggregated bandwidth
• Links to other networks
• Wide range of media and bandwidth

Inter-PoP
Intra-PoP
Other networks
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Convergence Detecting Topology Changes

• Beaconing
• Periodic hello messages in both directions
• Detect a failure after a few missed hellos
• Performance trade-offs
• Detection speed
• Overhead on link bandwidth and CPU
• Likelihood of false detection

hello
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Convergence Transient Disruptions

• Inconsistent link-state database
• Some routers know about failure before others
• The shortest paths are no longer consistent
• Can cause transient forwarding loops

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Convergence Delay for Converging

• Sources of convergence delay
• Detection latency
• Flooding of link-state information
• Shortest-path computation
• Creating the forwarding table
• Performance during convergence period
• Lost packets due to blackholes and TTL expiry
• Looping packets consuming resources
• Out-of-order packets reaching the destination
• Very bad for VoIP, online gaming, and video

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Convergence Reducing Convergence Delay

• Faster detection
• Smaller hello timers
• Link-layer technologies that can detect failures
• Faster flooding
• Flooding immediately
• Sending link-state packets with high-priority
• Faster computation
• Faster processors on the routers
• Incremental Dijkstra algorithm
• Faster forwarding-table update
• Data structures supporting incremental updates

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Traffic Engineering Tuning Link Weights

• Problem congestion along the blue path
• Second or third link on the path is overloaded
• Solution move some traffic to bottom path
• E.g., by decreasing the weight of the second link

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Traffic Engineering Problem Formulation

• Topology
• Connectivity capacity of routers links
• Traffic matrix
• Offered load between points in the network
• Link weights
• Configurable parameters for the protocol
• Performance objective
• Balanced load, low latency, service agreements
• Question Given topology and traffic matrix,
  which link weights to use?

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Traffic Engineering Key Ingredients of Approach

• Instrumentation
• Topology monitoring of the routing protocols
• Traffic matrix fine-grained traffic measurement
• Network-wide models
• Representations of topology and traffic
• What-if models of shortest-path routing
• Network optimization
• Efficient algorithms to find good configurations
• Operational experience to identify key
  constraints

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Scalability Overhead of Link-State Protocols

• Protocol overhead depends on the topology
• Bandwidth flooding of link state advertisements
• Memory storing the link-state database
• Processing computing the shortest paths

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Scalability Improving the Scaling Properties

• Dijkstras shortest-path algorithm
• Simplest version O(N2), where N is of nodes
• Better algorithms O(Llog(N)), where L is
  links
• Incremental algorithms great for small changes
• Timers to pace operations
• Minimum time between LSAs for the same link
• Minimum time between path computations
• More resources on the routers
• Routers with more CPU and memory

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Scalability Introducing Hierarchy Through Areas

• Divide network into regions
• Backbone (area 0) and non-backbone areas
• Each area has its own link-state database
• Advertise only path distances at area boundaries

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Scalability Dividing into Multiple ASes

• Divide the network into regions
• Separate instance of link-state routing per
  region
• Interdomain routing between regions (i.e., BGP)
• Loss of visibility into differences within region

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Limitations of Conventional Intradomain Routing

• Overhead of hop-by-hop forwarding
• Large routing tables and expensive look-ups
• Paths depend only on the destination
• Rather than differentiating by source or class
• Only the shortest path(s) are used
• Even if a longer path has enough resources
• Transient disruptions during convergence
• Cannot easily prepare in advance for changes
• Limited control over paths after failure
• Depends on the link weights and remaining graph

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Multi-Protocol Label Switching (MPLS)

• Multi-Protocol
• Encapsulate a data packet
• Could be IP, or some other protocol (e.g., IPX)
• Put an MPLS header in front of the packet
• Actually, can even build a stack of labels
• Label Switching
• MPLS header includes a label
• Label switching between MPLS-capable routers

MPLS header
IP packet
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Multi-Protocol Label Switching (MPLS)

• Key ideas of MPLS
• Label-switched path spans group of routers
• Explicit path set-up, including backup paths
• Flexible mapping of data traffic to paths
• Motivating applications
• Small routing tables and fast look-ups
• Virtual Private Networks
• Traffic engineering
• Path protection and fast reroute

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MPLS Forwarding Based on Labels

• Hybrid of packet and circuit switching
• Logical circuit between a source and destination
• Packets with different labels multiplex on a link
• Basic idea of label-based forwarding
• Packet fixed length label in the header
• Switch mapping label to an outgoing link

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MPLS Swapping the Label at Each Hop

• Problem using label along the whole path
• Each path consumes a unique label
• Starts to use up all of label space in the
  network
• Label swapping
• Map the label to a new value at each hop
• Table has old label, next link, and new label
• Allows reuse of the labels at different links

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MPLS Pushing, Swapping, and Popping

• Pushing add the initial in label
• Swapping map in label to out label
• Popping remove the out label

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MPLS Forwarding Equivalence Class (FEC)

• Rule for grouping packets
• Packets that should be treated the same way
• Identified just once, at the edge of the network
• Example FECs
• Destination prefix
• Longest-prefix match in forwarding table at entry
  point
• Useful for conventional destination-based
  forwarding
• Src/dest address, src/dest port, and protocol
• Five-tuple match at entry point
• Useful for fine-grain control over the traffic

A label is just a locally-significant identifier
for a FEC
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Status of MPLS

• Deployed in practice
• Small control and data plane overhead in core
• Virtual Private Networks
• Traffic engineering and fast reroute
• Challenges
• Protocol complexity
• Configuration complexity
• Difficulty of collecting measurement data
• Continuing evolution
• Standards
• Operational practices and tools

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Conclusion

• Two-tiered Internet routing system
• Interdomain between Autonomous Systems
• Intradomain within an Autonomous System
• Intradomain routing
• Shortest path routing based on link metrics
• Stability problems with dynamic link metrics
• Link-state vs. distance-vector protocols
• MultiProtocol Label Switching (MPLS)
• Forwarding packets based on a label
• Explicit path set-up