Download Chapter 14-16a

Chapter 14-16a
Internet Routing
Review
1
Introduction

Motivation: Router performance is critical
to overall performance and QoS in the
Internet and private internetworks.
Consider:
– selecting best routes (optimize delay, load)
– maintaining routing tables (router overhead)
– router-generated management traffic

Topics:
– Overview of Graph Theory
– Interior Routing Protocols
– Exterior Routing Protocols
Chapter 14-16: Internet Routing Review
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Graph Theory and Networks
Network
Links
Network
Nodes
Size of G: |E| = 10
Order of G: |V| = 6
Chapter 14-16: Internet Routing Review
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Graph Theory and Networks
Weighted Digraph to Represent Link Costs
Chapter 14-16: Internet Routing Review
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Spanning Trees
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Dijkstra’s Link-State Algorithm
Based on breadth-first search
Running time order of |V|2
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Bellman-Ford Distance Vector
Algorithm
Running time order of |V| x |E|
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Ex: Dijkstra vs. Bellman-Ford
Least-cost routing algorithms
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Interior/Exterior Routing
Protocols
C.b
a
Host
h1
C
A.a
b
Exterior
routing
between
networks
A and B
A.c
d
A
a
b
c
Interior routing
within network
(AS) A
Chapter 14-16: Internet Routing Review
B.a
a
c
B
Host
h2
b
Interior routing
within network
(AS) B
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Chapter 15
Interior Routing
Protocols
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Internet Routing Example
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Routing Table
Fixed vs. Adaptive Routing?
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Routing Information Protocol
(RIP)

Distance Vector Algorithm

Small networks

Adaptive

Uses UDP transport (port 520)
– typically uses simple link delay count
metric
– fast and simple
– max hop count = 15 (RIPv1)
– updates sent approximately every 30
seconds
– links are invalidated if no update in 180
seconds
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RIP Messages

Command
– 1 for request
– 2 for reply

Version
– 1: RIPv1, RFC 1058
– 2: RIP-2, RFC 1723

Address Family ID
– always 2 for IP addresses

IP address
– non-zero network portion
– zero host portion

Cost Metric
– typically 1 (i.e. hop count)
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Open Shortest Path First
Protocol (OSPF)

Link-State Algorithm

Preferred for TCP/IP-based internets

Adaptive

Sent as payload in IP datagram
– Dijkstra’s Algorithm
– uses flooding to advertise, request info
– scale, TOS, hierarchy of areas, etc.
– multiple metrics… monetary cost,
reliability, throughput, delay, arbitrary
– routers flood with new data when a
significant change occurs
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Open Shortest Path First
Protocol (Scale, Hierarchy)
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OSPF Messages







OSPF Header
Chapter 14-16: Internet Routing Review
Version
– 2: current version
Type
– 1: “Hello”, for discovery
– 2: database structure
– 3: request link-state
– 4: update link-state
– 5: acknowledge update
Length of message
Router ID
– unique in a single area
Area ID
– unique to an AS
Internet checksum
Authentication Info
– Type of authentication
– 64 bit authentication value
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Chapter 16a
Exterior Routing
Protocols
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Exterior Routing
BGP
OSPF
RIP
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Border Gateway Protocol
(BGP-4)


Path Vector Algorithm
– cost metrics not used or advertised
– each router specifies a complete path to all
other nodes (e.g., from node A to node Z =
ADFHKXZ)
Facilitates routing based on policy instead
of cost metrics
– use preferred routes, providers (based on policy)
– avoid competitors, poor performers, etc.
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Adaptive

Uses TCP transport (port 179)
– peers exchange updates on demand
– nodes may choose not to participate
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Border Gateway Protocol
Example
B
W
X
A
C
Y



legend:
provider
network
customer
network
(Stub AS)
W, X, Y, A, B and C are networks
Paths are specified as, for example:
 Path (w, y) = wACy
 Path (x, y) = xCy
BGP routers advertise (update) only those routes
that are specified by administrative policy
 would A advertise …ABx… or …ACx… ?
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BGP Messages
Common Attributes
 Marker
– synchronization and
authentication
mechanism


Chapter 14-16: Internet Routing Review
Length of message
Type
– open: establish a
neighbor relationship
– update: send/withdraw
route information
– keepalive: acknowledge
or confirm relationship
– notification: error
detected
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