Improving the Routing and Addressing of IP

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Improving the Routing and Addressing of IP

• Ford, Rekhter, and Braun
• IEEE Network Magazine
• May 1993
• George Lee

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Abstract

• The current routing and addressing structure dose
  not adequately scale to meet the needs generated
  by the accelerated growth of the Internet.
• This article introduces hierarchy into the
  interdomain routing system.
• Routing on IP address prefixes
• Summarization to aggregate multiple routing
  entries
• Renumbering when site moving
• To change host addresses within a site to their
  new network service provider
• Advantages
• reduction in routing overhead

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IP Address Structure

• 32-bit long address represented as dotted quads
• Each octet is represented as a decimal number,
  separated by dots
• Consisting of a network address field and a host
  identifier field
• Threeclassesofunicast addresses

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• Subnetting with a subnet mask
• represented in dotted quad form
• 1s network address
• 0s host identifier
• Subnet information is not globally distributed.

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Routing

• IP routing domain
• A collection of hosts and routers under the
  control of a single administrative entity using a
  common routing system
• Intradomain Routing
• Interior Routing Protocols (IGPs)
• OSPF (Open Shortest Path First)
• IGRP (Interior Gateway Routing Protocol)
• RIP (Routing Information Protocol)
• IS-IS (Intermediate-System, Intermediate-System)
• Interdomain Routing
• Exterior Routing Protocols (EGPs)
• EGP (Exterior Routing Protocol)
• BGP (Border Gateway Protocol)

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• The relationship between Intradomain and
  Interdomain routing can be illustrated by the
  NSFNET
• Hierarchical Architecture
• Sites ? IGP ? EGP ?
• Regional Networks ? IGP ? ? EGP ?
• NSFNET Backbone ? ? EGP
• Using IGP
• Within Sites
• Within Regional Networks
• Using EGP
• Between Sites and Regional Networks
• Between Sites and NSFNET Backbone
• Between Regional Networks and NSFNET Backbone

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• Hierarchical structure of NSFNET
• Simplifying the routing
• Sites only need to maintain routing information
  for their networks and a default route to the
  regional network
• Regional networks only need to maintain routes to
  their member sites and a default route to the
  NSFNET backbone
• NSFNET backbone only need to maintain routes to
  their member sites and regional networks
• Five top-level routing domains
• NSFNET Backbone
• Commercial Internet eXchange (CIX)
• NASA Science Internet (NSI)
• SprintLink
• European IP Backbone (EBONE)

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Current Status of Routing

• The current routing of flat interdomain routing
  by network number dose not scale adequately in
  light of the Internets continued growth.
• Memory and computational overhead for routing
  information
• Bandwidth for routing information distribution
• Stability of distributed routing computations
• Class B networks only advertise a single routing
  entry (214). But each Class C network will
  require a separate route entry (222) resulting
  in even faster growth of Internet Interdomain
  routing system.

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• Hierarchical routing has desirable scalable
  properties, but requires the use of hierarchical
  abstractions of network addresses.
• Router software will need to be changed, but
  these changes can be made transparent to host
  software.

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Routing on IP Address Prefixes

• Classless Interdomain Rouging (CIDR)
• IP Address Prefixes (up to 32 bits)
• Tuple representation ltIP network address, bit
  maskgt
• bit mask
• specifying contiguous leading address bits that
  are significant to Internet routing (similar to
  subnet mask)
• represented in dotted quad form
• Example lt193.128.0.0, 255.128.0.0gt
• 9-bit IP prefixed(a leading one bit and followed
  by eight zeros)
• 100000000 - 11000001.10000000.00000000.00000000
• Significant network field 11000001.1

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• Hierarchical Abstraction can be provided by the
  Summarization
• A pair of prefixes of length N can be summarized
  to a single prefix of length N-1 if the prefixes
  have the first N-1 bits in common.
• Example 1010 and 1011 ? 101
• Summarization can be repeatedly applied to
  aggregate multiple routing entries into a single
  entry? Minimizing the number of routes
  distributed ? Scaling better

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• OSPF, RIP-2, and BGP-4 can carry 32-bit prefix
  bit masks.
• CIDR was Initially proposed for Class C address,
  but it can also be used for Class A and B.
• The current CIDR specifies three-level
  architecture, however, additional levels can be
  added if needed.
• At the site level
• At the network service providers
• At continental boundaries
• Service providers may operate on several regions,
  but they should obtain addresses out of the the
  continental regions they serve to maximize the
  summarization capability for each region.

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The Best Match is the Longest

• The destination address is matched with the
  candidate prefixes in the routing table.
• The list of prefixes is sorted by length and
  searched in the descending order.
• Example (See the picture on the next page)
• Provider A lt198.1.0.0, 255.255.0.0gtSite
  S lt198.1.8.0, 255.255.248.0gt
• ?Summarization ? A advertises lt198.1.0.0,
  255.255.0.0gt
• Provider B lt199.3.0.0, 255.255.0.0gtSite
  T lt199.3.128.0, 255.255.128.0gt
• ?Summarization ? B advertises lt199.3.0.0,
  255.255.0.0gt
• Suppose that Site S switches to provider B
• ? B advertises lt199.3.0.0, 255.255.0.0gt

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• ?Summarization? Provider A advertises
  lt198.1.0.0, 255.255.0.0gt? Provider B
  advertises lt199.3.0.0, 255.255.0.0gt
• Routing tables are sorted by prefix length
• When Site S switches to provider B,B adds one
  entry for S
• If B needs to forward packet to Site S, it will
  use the longest matching prefix S instead of
  prefix A.

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Names, Not Addresses, Are Permanent

• Renumbering
• When a site switches to a new service provider,
  but keeps its old address, additional routing
  prefix are advertised.
• But, if hosts addresses within that site are
  changed to related prefix of the new provider,
  additional routing overhead is reduced.
• However, there is not an automatic, easy-to-use
  method for changing host addresses within a site.

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• 1) Ss prefix is part of As prefix
• 2) S switches to provider Band Ss prefix is
  till part of As prefix
• 3) S renumbers into Bs prefix, Interdomain
  routing tables shrink

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Utilization of the IP Address Space

• Summarization
• A site with contiguous Class C addresses can
  summarize to a single routing advertisement
• Variable length subnetting
• A site can split a Class B network into smaller
  area using prefixes of different length, thus
  avoiding the waste on IP addresses

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Conclusion

• To use IP prefixes instead of IP network
  addresses
• Scale the routing system
• Reduce the rate of IP address consumption