Information Network 1 Routing (1) - IPLab | IPLab · 2017-03-14 · A 3 B 2 Router metric A 1 C 2 Router metric B 1 C 3 18 Problems of distance vector routing ! Poor scalability

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Information Network 1 Routing (1)�

Youki Kadobayashi NAIST

©2015 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?

S� D�


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?

S� D�

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 ©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.

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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S� D�


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) ©2015 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


13

Questions?


Distance Vector Routing�

©2015 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

!   �� !   ��

��+�.��0� ��

! � ��+��

!   ��!   ��

��+��-��&��'�

!   �� !   ��

��

!   �� !��!��"!�� !��!$��"!��(�)��( )��%��!�� !��!�#��

��"��!��"!�� "!��(�)�� !��!$��"!��(�)��

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


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�

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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 ,�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 ,�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 ,�inf

D

��


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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


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


27

Graph representation of routers and links�

Source: OSPF Version 2, RFC 1583�©2015 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 !   ��

!   � �+��! ��+�� #��%��-��–�& '��!   ��+�& '��

!   �� !   ��+��-��–��&��'�!   ��+��/�&�'�

!   � ��! ��+��-��–��&��'�

!   �� !   ��

!   ��"!�� !��!��"!�� !��!$��(�)��(�)�*�� !�� !��!$��( )�!��(�)��"!��$�!�� !�� !��!��


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�

!   2select designated router and backup designated router3�

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


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Information exchange among routers�

!   Neighbor1→1Adjacent !   Routers don’t exchange routing

information unless they are adjacent

!   Formation process:�!   Hello ,�Neighbor�!   Synchronize each LSDB�

!   Synchronized → Adjacent�


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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Questions?


Summary�


41

Gateway Model Revisited

FIB

Input interfaces Output interfaces

RIP

Multiple RIBs

OSPF

Routing software Topology info, Link status info

Topology info, Link status info


42

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


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Routing(2) – Inter-domain Routing

Information Network I Youki Kadobayashi

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Outline

! Continued from previous lecture on: ! Distance vector routing

! Link state routing

!   IGP and EGP •  Interior gateway protocol,

Exterior gateway protocol

!  Path vector routing !  BGP: Border Gateway Protocol !  Scaling tricks

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Hierarchical Routing: a natural result of growth

!  Routing domain !  Defines the boundary between domains !  Fault isolation, route aggregation

!  Distinction between intra-domain routing protocol and inter-domain routing protocol

!   IGP (Interior Gateway Protocol) !  EGP (Exterior Gateway Protocol)

IGP IGP IGP EGP

Hierarchical Routing: in reality it is two-tier routing

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IGP

EGP

IGP

It’s like: train network plus airline network�

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IGP and EGP has different goal

!   IGP !   Focus on propagating the state of each link/

router as fast as possible !   RIP-2, RIPng: distance vector routing !   OSPF, IS-IS: link state routing

!   EGP !   Focus on routing stability of the entire Internet !   BGP4, BGP4+: path vector routing

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Path Vector Routing: a small trick for stable routing

S T1

T3

T2

T5

D

T4

Representing route from S to D •  (D, { T1, T2 }, ...) •  (D, { T3, T4, T5 }, ...)

!   Derived from the Bellman-Ford algorithm !  Exchanged information:

!   Attaches distance as well as path information to the route information → Embodies “routing without loops” This protocol prioritizes route that has the shortest path vector.

Distance Vector� (prefix, metric)�Path Vector� (prefix, path, attributes)�

Path Vector Routing: Background

S T1

T2 D

T3

!  Multiple alternative routes !   Dense connections between ISPs !   Which route should we prioritize?

•  constrained by cost, contract, load, etc. !   Routing policy

!   Encodes the intention of the intermediate ISPs !   Route selection policies enable each domain to

select a particular route among multiple routes 1111Policy can’t be expressed by scalar cost.

!  Cost of loops !   Convergence time from transient state � RTT

49

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Loop Avoidance in Path Vector Routing

A C B

Path vectors from “A” to C (C, { B }) (C, { D, B })

• When B-C link fails, B is deleted from path vector.

• Rejects path vector that include itself • Loop avoidance

D

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BGP: Border Gateway Protocol !  Algorithm

•  Path vector !  Transport

•  TCP •  TCP provides retransmission and acknowledgement

!  Adjacency: manually configured (i.e., no auto discovery)

!  Routing information: (prefix, path, attributes)

!  Topology: full mesh (internal), arbitrary (external)

RFC 4271, 4760�

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Adjacency Relationship of BGP

!  Adjacency relationship is defined by ISP operator !  Adjacency relationship must be explicitly

configured !  Why?

!   c.f. OSPF : if parameters match, adjacency relationship is enabled ... ...

R4

R1 R2

R3

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Representing Path Vector in BGP

!  AS (Autonomous System) is represented as an AS number

•  AS: routing domain that is operated by single policy

!  BGP concisely encodes (prefix, AS-path, attributes)

•  (AS-path, attributes, { prefix1, prefix2, ... } ) !   Reduction of traffic

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Example of Path Vector in BGP Try it yourself w/

looking glass

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Q&A

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Reducing routing information

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Route Reduction: Default route

Stub AS

Transit AS

R1

R2

“prefix ::0/0 next-hop R1”

!   0.0.0.0/0 (IPv4), ::0/0 (IPv6) !  Longest prefix match

→ matches in the end of route search !  Results in hiding of routes and reduction of the number of routes

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When and where routes can’t be reduced?

!  Default-free !   No default route

!  Tier-1 ISPs, North American backbone

(source: old UUNet network maps, www.uu.net) ‏

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Aggregation

!   163.221.10.0/24 and 163.221.11.0/24 are aggregated into 163.221.10.0/23

163.221.0/18 163.221.128/18

163.221.64/18

163.221.192/18

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Route Aggregation at Domain Edge

!  Routes are aggregated at the edge of the routing domain.

163.221.10.0 163.221.52.0 163.221.56.0

OSPF area 1

R1

OSPF backbone

“163.221.0/18 via R1” (Summary LSA)

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Hierarchical Aggregation of Routes

OSPF areas

“163.221.0/18 via R1” (Summary LSA)

OSPF backbone

OSPF backbone

BR

“163.221.0/16 via BR” (BGP advertisement)

163.221.10.0 163.221.52.0 163.221.56.0

R1

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Challenges in Route Aggregation

!  Route aggregation depends on address assignment •  Planned address assignment is important for route aggregation

!  Can we predict the extent of future growth of NAIST?

•  Can we predict future growth of an ISP?

!  → Prefix renumbering •  Renumber to aggregatable addresses •  Provide operational means to unanticipated changes •  Technology under development

!  Manual operation is necessary for route aggregation

•  Internet full-route : 250,000 •  BGP table growth trends - Telstra

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Q&A

64

Summary

!  Hierarchical routing concepts !   IGP, EGP

!  Path Vector Routing !   Loop-free, policy-aware

!  BGP !   State transition, route information and topology !   Limitation of policy-based routing

!  Scaling trick: Route aggregation !   Aggregation concepts, challenges

Documents

Information Network 1 Routing (1) - IPLab | IPLab · 2017-03-14 · A 3 B 2 Router metric A 1 C 2 Router metric B 1 C 3 18 Problems of distance vector routing ! Poor scalability