Interdomain Routing - Networking - Lecture Slides, Slides of Computer Networks

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Interdomain Routing

Internet Routing

• So far, only considered routing within a domain

• Many issues can be ignored in this setting because there is central administrative control over routers - Issues such as autonomy , privacy , policy
• But the Internet is more than a single domain

Autonomous Systems (AS)

• AS is a network under a single administrative control
  • currently over 30,000 ASes
  • Think AT&T, France Telecom, UCB, IBM, etc.
• ASes are sometimes called “domains”.
  • Hence, “interdomain routing”
• Each AS is assigned a unique identifier
  • 16 bit AS Number (ASN)

Routing between ASes

Two key challenges

• Scaling
• Administrative structure
  • Issues of autonomy, policy, privacy

Recall Also… to MIT to UW UCB to NYU Destination Next Hop UCB 4 UW 5 MIT 2 NYU 3 Forwarding Table 111010010 MIT switch# switch# switch# switch#

Scaling

• Every router must be able to forward packets to any destination - Given address, it needs to know “next hop” (table)
• Naive: Have an entry for each address
  • There would be over 10^ 8 entries!
  • And routing updates per destination!
• Any ideas on how to improve scalability?

Two Key Challenges

• Scaling
• Administrative structure
  • Issues of autonomy, policy, privacy

Administrative structure shapes

Interdomain routing

• ASes want freedom to pick routes based on policy
  • “My traffic can’t be carried over my competitor’s network”
  • “I don’t want to carry A’s traffic through my network”
  • Not expressible as Internet-wide “shortest path”!
• ASes want autonomy
  • Want to choose their own internal routing protocol
  • Want to choose their own policy
• ASes want privacy
  • choice of network topology, routing policies, etc.

Today

• Addressing
• BGP
  • today: context and key ideas
  • next lecture: details and issues

Addressing Goal: Scalable Routing

• State: Small forwarding tables at routers
  • Much less than the number of hosts
• Churn: Limited rate of change in routing tables
  • Traffic, inconsistencies, complexity

Ability to aggregate addresses is crucial for both

(one entry to summarize many addresses)

Hence, IP Addressing: Hierarchical

• Hierarchical address structure
• Hierarchical address allocation
• Hierarchical addresses and topology

IP Addresses (IPv4)

• Unique 32 - bit number associated with a host
• Represented with the dotted-quad notation, e.g., 12.34.158.5 :

Hierarchy in IP Addressing

• 32 bits are partitioned into a prefix and suffix components
• Prefix is the network component; suffix is host component
• Interdomain routing operates on the network prefix
• Notation and terminology: 12.34.158.0/23 represents a “slash 23” network with a 23 bit prefix and 2 9 host addresses

Network (23 bits) Host (9 bits) 12 34 158 5

History of Internet Addressing

• Always dotted-quad notation
• Always network/host address split
• But nature of that split has changed over time