l6 Intra-Domain Routing - Lecture 6 Intra-Domain Routing...

Lecture 6: Intra-Domain Routing

Overview
• Internet structure, ASes
• Forwarding vs. Routing
• Distance vector and link state
• Example distance vector: RIP
• Example link state: OSPF

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

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

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

The Internet, today
Backbone service provider
Peering
point
Peering
point
Large corporation
Large corporation
Small
corporation
“Consumer” ISP
“Consumer” ISP
“Consumer” ISP
• Multiple “backbones”

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

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

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)

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

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

Intra-domain routing
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