17-03-2010, 11:59 AM
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Introduction to Computer Networks
Internet Structure
Autonomous Systems (AS)
Internet is not a single network!
The Internet is a collection of networks, each controlled by different administrations
An autonomous system (AS) is a network under a single administrative control
AS Numbers (ASNs)
Implications
ASs want to choose own local routing algorithm
AS takes care of getting packets to/from their own hosts
Intradomain routing: RIP, OSPF, etc
ASs want to choose own non-local routing policy
Interdomain routing must accommodate this
BGP is the current interdomain routing protocol
BGP: Border Gateway Protocol
Example
Intra-Domain
Inter-Domain
Inter-Domain Routing
Global connectivity is at stake
Inevitably leads to one single protocol that everyone must speak
Unlike many choices in intra-domain routing
What are the requirements?
Scalability
Flexibility in choosing routes
If you were to choose, page link state based or distance vector based?
BGP is sort of a hybrid: Path vector protocol
Border Gateway Protocol Part I: E-BGP
Purpose of E-BGP
Part II: I-BGP, Carrying Info within an AS
I-BGP
Join I-BGP with IGP to Create Forwarding Table
Multiple Routing Processes on a Single Router
Routing between ISPs
Routing protocol (BGP) contains reachability information (no metrics)
Not about optimizing anything
All about policy (business and politics)
Why?
Metrics optimize for a particular criteria
AT&Tâ„¢s idea of a good route is not the same as UUnetâ„¢s
Scale
What a BGP speaker announces or not announces to a peer determines what routes may get used by whom
Nontransit vs. Transit ASes
Selective Transit
Customers and Providers
Customers Donâ„¢t Always Need BGP
Customer-Provider Hierarchy
The Peering Relationship
Peering Provides Shortcuts
BGP: Path Vector Protocol
Distance vector algorithm with extra information
For each route, store the complete path (ASs)
No extra computation, just extra storage
Advantages:
can make policy choices based on set of ASs in path
can easily avoid loops
BGP Operations (Simplified)
Four Types of BGP Messages
Open : Establish a peering session.
Keep Alive : Handshake at regular intervals.
Notification : Shuts down a peering session.
Update : Announcing new routes or withdrawing previously announced routes.
Attributes are Used to Select Best Routes
Example: Multiple AS Paths
Shorter Doesnâ„¢t Always Mean Shorter
Implementing Customer/Provider and Peer/Peer relationships
What you announce determines what route can be used by whom
Enforce transit relationships
Outbound route filtering
Enforce order of route preference
provider < peer < customer
Import Routes
Export Routes
How Can Routes be Colored? BGP Communities!
Example AS Graph
BGP Issues
BGP designed for policy not performance
Susceptible to router misconfiguration
Blackholes: announce a route you cannot reach
Incompatible policies
Solutions to limit the set of allowable policies
More Issues
Scaling the I-BGP mesh
Confederations
Route Reflectors
BGP Table Growth
140K prefixes and growing
Address aggregation (CIDR)
Address allocation
AS number allocation and use
Dynamics of BGP
Inherent vs. accidental oscillation
Rate limiting and route flap dampening
Lots and lots of redundant info
Slow convergence time
please read http://studentbank.in/report-computer-ne...ull-report for computer networks full seminar report
