Routing (1) - NAIST · Loops can be easily formed, however. ! ... Discovering neighbor routers with OSPF Hello ! Discover neighbor router on the same link • send Hello packet to

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Information Network 1 Routing (1)

Youki Kadobayashi NAIST

©2013 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.

Image: Part of the entire Internet topology based on CAIDA dataset, using NAIST Internet viewer

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The Routing Problem

!   How do I get from source to destination?

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163.221.10.10 203.178.136.61

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The Routing Problem: add some realistic constraints…

!   How do I get from source to destination?

!   Which path is best? In terms of: !   Number of hops !   Delay, bandwidth !   Policy constraints, cost…

!   Who will make decision? !   Router? !   Source?

!   How can we detect failures? !   How much the overhead will be?

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Rogue ISP

500ms 500ms

unreliable


Routing problem can be solved in many ways

!   Represent network in: !   a graph, or !   a matrix

!   Collect information: !   across the network, or !   toward some routers, or !   only locally among neighbors

!   Compute route at: !   every router, or !   some routers

… solutions are instantiated in routing systems.

4 ©2013 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.

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Characterization of routing systems

!  Static routing !   Compute route a priori

!  Dynamic routing !   Reflect dynamic state of network

!  Source-based routing !   Source node computes path to destination

!  Hop-by-hop routing !   Every node computes next hop

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Focus of this lecture: dynamic, hop-by-hop routing

!  Static routing !   Compute route a priori

!  Dynamic routing !   Reflect dynamic state of network

!  Source-based routing !   Source node computes path to destination

!  Hop-by-hop routing !   Every node computes next hop


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Routing system has many functions

!   Provision end-to-end reachability !   Automatically compute best path !   Distribute traffic among multiple links !   Avoid failing links

!   Isolate faults

!   Reflect administrative policies

… in one system. Isn’t it awesome.


Routing system can be characterized by…

!   Representation of network !   Network topology !   Attributes associated with each link

!   Exchange of information !   Communication overhead !   Propagation speed

!   Computation algorithm !   Computation overhead !   Convergence speed

!   Routing system: protocol + information + algorithm


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Routing system: its structure

!   Routing protocol !   discovers neighbor router;

!   exchanges topology information; !   exchanges link information

!   Routing algorithm !   computes route (result: RIB - Routing Information Base)

!   Integrate information from multiple routing protocols !   Multiple routing protocols → Multiple RIBs !   Consolidate Multiple RIBs into single FIB

(FIB: Forwarding Information Base) ©2013 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.

Typically, routing protocol discovers network topology

!  Topology: geometric configurations that are unaltered by elastic deformations

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1

2

3

1

3

2

1 2 3

1 2 3

∞ 1 3 1 ∞ 2 3 2 ∞

Graph representation Matrix representation


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Gateway Model: a conceptual model of routing system

FIB (forwarding information base)

Input interfaces Output interfaces

Multiple RIBs (routing information base)

Routing software Topology info, Link status info

Topology info, Link status info


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Types of Routing Algorithm

1.  Distance vector 2.  Link state 3.  Path vector

!   Key difference of these algorithms: !   Topology representation !   Propagation range / frequency / timing of link state !   Algorithm for computing shortest path

!   Design trade-offs: !   Scalability !   Convergence time !   Algorithm simplicity


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Questions?


Distance Vector Routing

©2013 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved. 14

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Distance vector routing w/ Bellman-Ford algorithm

!   1. assign distance vector to myself 0; for others, assign ∞

!   2. send my distance vectors to all neighbor routers !   3. router calculates minimum distance vectors by 1) distance vectors

advertised by neighbor routers and 2) distance from myself to individual neighbor router

!   Initial condition !   bi

(0) ← ∞ (i ≠ D)

! bD(0) ← 0

!   Repetition !   bi

(m) ← minj ∈ Vi{bj(m-1) + dij}

!   Terminal condition !   bi

(m) = bi(m-1)

!   D: destination router, bi(m): distance between router “i” and “D” by m-times iterative

calculation, Vi: neighbor routers of router “i”, dij: distance of link between router “i” and j

DN p.397


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RIP: Routing Information Protocol

!  Distance vector routing !  RIP-2 (IPv4), RIPng (IPv6) !  Used in relatively small network

•  Due to ease of implementation and operation

!  Textbook classics

RFC 2453, RFC 2080


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Informal description of RIP

!   Each router sends a list of distance-vectors (route, cost) to each neighbor periodically

!   Every router selects the route with smallest metric !   Metric: integer of 1..16, where 16 implies infinity


A B C cost: 2 cost: 1

Router metric

A 3

B 2

Router metric

A 1

C 2

Router metric

B 1

C 3

Hands on: see how RIP works

!   (description of GNS3)

!   (basic instruction to keep RIP up and running)


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Hands on task: try to expand your RIP network

!  Add 3rd router to your RIP network !  Example topology:


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1 3 2

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Problems of distance vector routing

!   Poor scalability. For the given number of routers N, !   Time complexity: O(N3) !   Traffic: O(N2)

!   Slow convergence speed, as it sends distance vector periodically

!   Slow convergence induces inconsistent and transitional state

!   => Counting to infinity problem


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Counting to infinity problem

A C B

Suppose link B-C went down. B thinks: (C, inf) A says: (C, 2) B thinks: (C, 3) via A B says: (C, 3) A thinks: (C, 4) via B …

1 1 → inf


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Split Horizon

A C B

Suppose link B-C went down. B thinks: (C, inf) A says: (C, 2) to everyone except B …

1 1 → inf

!  Workaround for counting to infinity

!  Router doesn’t send learned information to source router


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Limitation of split horizon

A C B

Suppose link B-C went down. B thinks: (C, inf) A says: (C, 2) to everyone except B D thinks: (C, 3) via A D says: (C, 3) B thinks: (C, 4) via D …

1 1 → inf

D

1 1


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Questions?

!   (Dumb questions welcomed while you’re a student)

!  Extra hands-on if you are further motivated: !   Introduce link failure and observe counting-to-

infinity problem !   Add 4th router as in the last slide and observe that

split-horizon does not save us


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Lessons from RIP

!   Bellman-Ford: very simple !   But with many traps and pitfalls…

!   Guiding principle for using RIP: avoid loops!

!   Loops can be easily formed, however. !   Backup links !   Guiding principle can be easily forgotten

•  OMG!

!   A better alternative: link state routing.


Link-State Routing


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Link-state routing

!  Collect router and link information → Directed graph: router as a node, link as an arc

!  … then create link state database (LSDB) •  A network map that collects link state information

!  Based on LSDB, calculate the shortest path with the Dijkstra’s shortest path algorithm

!   Time complexity: O(N2) •  Can be further optimized by improved data structure


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A high level view of link-state routing

Router

Link

Router

Link

LSDB RIB Dijkstra

directed graph shortest path tree

fragments of directed graph


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Graph representation of routers and links

Source: OSPF Version 2, RFC 1583 ©2013 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.

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A more complex network


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and its directed graph representation


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Shortest path tree; rooted at RT6


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Questions?


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From directed graph to shortest-path trees: Dijkstra Algorithm !   Initially,

!   ds ← 0 ! dj ← csj (for every j ∈ V – {s}) !   P ← {s}

!   Find the next closest node: !   di ← minj ∈ V – P {dj} !   P ← P ∪ {i}

!   Update labels: ! dk ← mink ∈ V – P {dk, di + cik}

!   Terminal condition: !   P = V

!   V: all routers, s: starting router, cik: link cost between “i” and “k”,dj: least cost between “s” to “j”, P: router with least cost determined


Interconnections p.223

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Pros and Cons of Link-State Routing

!   Traffic increases in proportion to the number of links and routers

!   Storage complexity increases in proportion to the number of links and routers

!   Time complexity: lower than distance vector routing

!   Convergence time: must be short, for transient loop issue !   Counting to infinity doesn’t happen

(read: fewer traps and pitfalls) !   Flexible configuration of link cost is possible

(no nonsense like 16 = infinity)


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OSPF: Open Shortest Path First

!   The link state routing protocol for the Internet !   OSPFv2 (IPv4), OSPFv3 (IPv6)

!   Functions !   Recognize neighbor router !   Exchange link state information and create LSDB

!   Calculate shortest path tree (spanning tree) !   + Designated Router, Backup Designated Router !   + Hierarchical structure by area

!   + Collaboration with EGP

RFC 2328, 5340


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Discovering neighbor routers with OSPF Hello

!   Discover neighbor router on the same link •  send Hello packet to 224.0.0.5, ff02::5 (AllSPFRouters)

!   List of neighbor routers in Hello packet •  check bidirectional communication

!   （select designated router and backup designated router）

!   Send Hello packet periodically to detect link down !   keeps pinging, as in ICMP


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Information exchange among routers

!   Neighbor　→　Adjacent !   Routers don’t exchange routing

information unless they are adjacent

!   Formation process: !   Hello → Neighbor !   Synchronize each LSDB

!   Synchronized → Adjacent


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Hands on: say hello in OSPF!

!   (basic instruction to bring OSPF up and running)

!   (establish OSPF adjacency)

!   (add 1 or 2 routers)


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Topology Representation in OSPF

!   LSA (Link State Advertisement) •  Type 1: Router LSA •  Type 2: Network LSA •  Type 3: Summary LSA (network) •  Type 4: Summary LSA (AS boundary) •  Type 5: AS External LSA

!   LSA common header •  Validity period and sequence number in LSA header •  Helps routers to tell if given LSA is fresh


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Hands on:

!   (some more extra work that lays foundation for Assignment)


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Questions?


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Summary


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Gateway Model Revisited

FIB

Input interfaces Output interfaces

RIP

Multiple RIBs

OSPF

Routing software Topology info, Link status info

Topology info, Link status info


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Summary

!   Routing system: design space, characterization

!   Distance vector routing !   Bellman-Ford algorithm !   RIP protocol, information model !   Traps and pitfalls

!   Link state routing !   Graph and its spanning trees ! Dijkstra algorithm

!   OSPF protocol, algorithm, information model


Assignment


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Assignment:


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Routing (1) - NAIST · Loops can be easily formed, however. ! ... Discovering neighbor routers with OSPF Hello ! Discover neighbor router on the same link • send Hello packet to