l6 Intra-Domain Routing

L6 Intra-Domain - Lecture 6 Intra-Domain Routing Overview Internet structure ASes Forwarding vs Routing Distance vector and link state Example

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Lecture 6: Intra-Domain Routing
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Overview Internet structure, ASes Forwarding vs. Routing Distance vector and link state Example distance vector: RIP Example link state: OSPF
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The Internet, 1990 NSFNET backbone Stanford BARRNET regional Berkeley PARC NCAR UA UNM Westnet regional UNL KU ISU MidNet regional Hierarchical structure w. single backbone
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Address allocation, 1990 Network number Host number Class B address Subnet mask (255.255.255.0) Subnetted address 111111111111111111111111 00000000 Network number Host ID Subnet ID Hierarchical IP addresses - Class A (8-bit prefix), B (16-bit), C (24-bit) Subnetting adds another level within organizations - Subnet masks define variable partition of host part - Subnets visible only within site
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Example Subnet mask: 255.255.255.128 Subnet number: 128.96.34.0 128.96.34.15 128.96.34.1 H1 R1 128.96.34.130 Subnet mask: 255.255.255.128 Subnet number: 128.96.34.128 128.96.34.129 128.96.34.139 R2 H2 128.96.33.1 128.96.33.14 Subnet mask: 255.255.255.0 Subnet number: 128.96.33.0 H3
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The Internet, today Backbone service provider Peering point Peering point Large corporation Large corporation Small corporation “Consumer” ISP “Consumer” ISP “Consumer” ISP Multiple “backbones”
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Address allocation, today Class system makes inefficient use of addresses - class C with 2 hosts (2/255 = 0.78% efficient) - class B with 256 hosts (256/65535 = 0.39% efficient) - Causes shortage of IP addresses (esp. class B) - Makes address authorities reluctant to give out class Bs Still Too Many Networks - routing tables do not scale - route propagation protocols do not scale
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Supernetting Assign block of contiguous network numbers to nearby networks Called CIDR: Classless Inter-Domain Routing Represent blocks with a single pair ( first network address , count ) Restrict block sizes to powers of 2 - Represent length of network in bits w. slash - E.g.: 128.96.34.0/25 means netmask has 25 1 bits, followed by 7 0 bits, or 0xffffff80 = 255.255.255.128 - E.g.: 128.96.33.0/24 means netmask 255.255.255.0 All routers must understand CIDR addressing
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IP Connectivity For each destination address, must either: 1. Have prefix mapped to next hop in forwarding table, or 2. know “smarter router”—default for unknown prefixes Route using longest prefix match, default is prefix 0.0.0.0/0 Core routers know everything—no default Manage using notion of Autonomous System (AS) Two-level route propagation hierarchy - interior gateway protocol (each AS selects its own) - exterior gateway protocol (Internet-wide standard)
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Autonomous systems Correspond to an administrative domain - Internet is not a single network - ASes reflect organization of the Internet - E.g., Stanford, large company, etc. Goals: - ASes want to choose their own local routing algorithm - ASes want to set policies about non-local routing Each AS assigned unique 16-bit number
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Types of traffic & AS Local traffic – packets with src or dst in local AS Transit traffic – passes through an AS Stub AS - Connects to only a single other AS Multihomed AS - Connects to multiple ASes - Carries no transit traffic Transit AS - Connects to multiple ASes and carries transit traffic
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Intra-domain routing
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This note was uploaded on 04/02/2012 for the course CS 144 at Stanford.

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L6 Intra-Domain - Lecture 6 Intra-Domain Routing Overview Internet structure ASes Forwarding vs Routing Distance vector and link state Example

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