Information Network 1 Routing (1) - IPLab | IPLab · 2017-03-14 · A 3 B 2 Router metric A 1 C 2...
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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�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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.
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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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S� D�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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.�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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?
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
Distance Vector Routing�
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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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved. 17
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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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
��
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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.
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved. 23
Link-State Routing�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved. 24
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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�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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�©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�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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Shortest path tree; rooted at RT6�
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Questions?
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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From directed graph to shortest-path trees: Dijkstra Algorithm ! ����������
! � �+��! ���+�� ��������#��%���-���–�& '��! ��+�& '��
! ��� �������������������� ���! ���+������-���–���&��'�! ��+���/�&�'�
! � ����������! ���+������-���–���&������������'�
! ����������� �������! ������
! ����������"!�� �� �� !��!������"!���������������� !���!$����(�)�����(�)�*������� !��� !���!$����( )�!��(�)�������"!���$�!����� !��� !���!��������
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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)
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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)�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved. 37
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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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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Questions?
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
Summary�
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved. 40
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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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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
©2015 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.
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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)�
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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
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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
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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