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Chapter 4: Advanced Internetworking. Networking CS 3470, Section 1. C.b. B.a. A.a. c. A.c. b. a. c. b. Intra-AS and Inter-AS Routing. Gateways: perform inter-AS routing amongst themselves perform intra-AS routers with other routers in their AS. b. a. a. C. B. d. A. C.b.

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Chapter 4 advanced internetworking

Chapter 4: Advanced Internetworking


CS 3470, Section 1

Intra as and inter as routing










Intra-AS and Inter-AS Routing

  • Gateways:

    • perform inter-AS routing amongst themselves

    • perform intra-AS routers with other routers in their AS








Intra as and inter as routing1










Intra-AS and Inter-AS Routing

  • Gateways:

    • perform inter-AS routing amongst themselves

    • perform intra-AS routers with other routers in their AS








network layer

inter-AS, intra-AS routing in

gateway A.c

link layer

physical layer

Intra as routing algorithms
Intra-AS Routing Algorithms

  • We have already talked about two intra-AS routing algorithms:

    • Link state routing

    • Distance vector routing

Link state vs distance vector
Link State vs Distance Vector

  • Tells everyone about neighbors

  • Controlled flooding to exchange link state

  • Dijkstra’s algorithm

  • Each router computes its own table

  • May have oscillations

  • Tells neighbors about everyone

  • Exchanges distance vectors with neighbors

  • Bellman-Ford algorithm

  • Each router’s table is used by others

  • May have routing loops


  • RIP == Routing Information Protocol

  • RIP is a distance vector implementation

  • Instead of advertising costs to the next router, RIP advertises the cost to the next network.




Large routing
Large Routing

  • OSPF

  • BGP


  • One of the most widely-used link-state routing protocols is Open Shortest Path First

    • Open, nonproprietary standard created by the Internet Engineering Task Force

    • Shortest Path First is an alternative name for link-state routing

  • Hierarchical – can divide the system into “areas.”

Ospf roles
OSPF Roles

  • Internal router :: a level 1 router.

  • Backbone router :: a level 2 router.

  • Area border router (ABR) :: a backbone router that attaches to more than one area.

  • AS border router :: (an interdomain router), namely, a router that attaches to routers from other ASs across AS boundaries.

Ospf advertisement
OSPF advertisement


LSA type


link cost

Ospf lsa types
OSPF LSA types

  • Router link advertisement [Hello message]

  • Network link advertisement

  • Network summary link advertisement

  • AS border router’s summary link advertisement

  • AS external link advertisement

Border gateway protocol and autonomous systems
Border Gateway Protocol and Autonomous Systems

  • Border Gateway Protocol (BGP)

    • Assumes that the Internet is an arbitrarily interconnected set of ASs.

    • Today’s Internet consists of an interconnection of multiple backbone networks (they are usually called service provider networks, and they are operated by private companies rather than the government)

    • Sites are connected to each other in arbitrary ways

Border gateway protocol and autonomous systems1
Border Gateway Protocol and Autonomous Systems

  • Assumes the Internet is an arbitrarily interconnected set of AS's.

  • Define local traffic as traffic that originates at or terminates on nodes within an AS, and transit traffic as traffic that passes through an AS.

Border gateway protocol and autonomous systems2
Border Gateway Protocol and Autonomous Systems

  • We can classify AS's into three types:

    • Stub AS: an AS that has only a single connection to one other AS; such an AS will only carry local traffic

Border gateway protocol and autonomous systems3
Border Gateway Protocol and Autonomous Systems

  • We can classify AS's into three types:

    • Multihomed AS: an AS that has connections to more than one other AS, but refuses to carry transit traffic

Border gateway protocol and autonomous systems4
Border Gateway Protocol and Autonomous Systems

  • We can classify AS's into three types:

    • Transit AS: an AS that has connections to more than one other AS, and is designed to carry both transit and local traffic (backbone provider)


  • The goal of Inter-domain routing is to find any path to the intended destination that is loop free

    • We are concerned with reachability than optimality

    • Finding path anywhere close to optimal is considered to be a great achievement


  • Scalability: An Internet backbone router must be able to forward any packet destined anywhere in the Internet

    • Having a routing table that will provide a match for any valid IP address

  • Autonomous nature of the domains

    • It is impossible to calculate meaningful path costs for a path that crosses multiple ASs

    • A cost of 1000 across one provider might imply a great path but it might mean an unacceptable bad one from another provid

  • Issues of trust

    • Provider A might be unwilling to believe certain advertisements from provider B


Each AS has:

  • One BGP speaker that advertises:

    • local networks

    • other reachable networks (transit AS only)

    • gives path information

  • In addition to the BGP speakers, the AS has one or more border “gateways” which need not be the same as the speakers

  • The border gateways are the routers through which packets enter and leave the AS


  • BGP does not belong to either of the two main classes of routing protocols (distance vectors and link-state protocols)

  • BGP advertises complete paths as an enumerated lists of ASs to reach a particular network


  • Moving on to IPv6!

  • For more information, refer to Section 4.1.3 in your textbooks

Why not ipv4
Why not IPv4?

  • IPv4 addresses have become relatively scarce

  • NATs help by promoting reuse of address space, but

    • They do not support standards-based network layer security or the correct mapping of all higher layer protocols

    • Can create problems when connecting two organizations that use the private address space.

Why not ipv41
Why not IPv4?

  • Additionally, the rising prominence of Internet-connected devices and appliances ensures that the public IPv4 address space will eventually be depleted.

  • It would be nice to not have to rely on protocols like DHCP to configure addresses

Why not ipv42
Why not IPv4?

  • Private communication over a public medium like the Internet requires encryption services that protect the data being sent from being viewed or modified in transit.

Why ipv6
Why IPv6?

  • IPv6 is required to include IPsec.

  • IPsec allows authentication, encryption, and compression of IP traffic.

  • The benefit of this is that all applications on a machine can benefit from encryption and authentication, and that policies can be set on a per-host (or even per-network) basis, not per application/service.

Why ipv61
Why IPv6?

  • IPv6 uses a 128-bit address instead of the 32-bit address of IPv4.

  • This doesn't give 4 times the addresses of IPv4 but rather the number of IPv4 addresses squared twice.

  • A couple of articles out there have stated that this works out to billions of billions of addresses for every square meter on the planet.

Ipv6 addressing
IPv6 Addressing

  • An IPv6 address is written as hexadecimal values (0-F) in groups of four separated by colons, like:A223:BB34:0000:0000:0000:0099:DA78:5679

  • Strings of zeros can be dropped and leading zeros in a number group can be dropped, so the example above would shorten to


Ipv6 addressing1
IPv6 Addressing

  • IPv4 isn't left out completely

  • IPv4 addresses can be expressed in IPv6 form as follows:0000:0000:0000:0000:0000:0000:–which can be shortened to ::

  • This makes transitioning a bit easier.

Ipv6 headers
IPv6 Headers

  • 4-bit version field that defines the protocol in use (default is 6),

  • 8-bit traffic class field to allow for Quality of Service (QOS)-type services

  • 20-bit flow label field that provides path management services

Ipv6 headers1
IPv6 Headers

  • 8-bit next header field that indicates to routers that there is another header following the main header,

  • a 16-bit payload length field that gives the length of the payload in octets

  • hop limit field that indicates the number of hops a packet can take before being discarded

Ipv6 headers2
IPv6 Headers

  • The header ends with the 128-bit source and destination fields.

  • The total length of the header is 40 bytes compared to the 20-byte header in Ipv4.

  • Use of the next header field allows headers to be chained after the main header. These auxiliary headers can add routing, encryption and authentication features.


  • IPv6 adds significant extra features that were not possible with IPv4.

    • Automatic configuration of hosts (similar and DHCP)

    • Extensive multicasting capabilities

    • Built-in security using authentication headers and encryption

    • Built-in support for QOS and path control

Ipv4 and ipv6 addressing
IPV4 and IPV6 Addressing

  • Ipv4:

    • 32 bits

    • ~ 4,200,000,000 addresses

  • IPV6

    • 128 bits

    • 340,282,366,920,938,463,463,374,607,431,768,211,456 nodes

    • Addresses have “scope”

      • Link Local

      • Unique Local

      • Global

    • Addresses have lifetime

      • Valid and preferred lifetime facets

    • Unicast, Multicast, and Anycast...but no broadcast

Ipv6 addressing2
IPv6 Addressing

  • Same “longest-prefix match” routing as IPv4 CIDR

    • e.g: 2001:db8:12::/40

  • Two Different Classes

    • Link-State (i.e., OSPF, ISIS, etc.)

    • Distance-Vector (i.e., RIP, IGRP, etc.)

  • Autonomous System / Routing Domain

    • Interior Gateway Protocols (IGPs)i.e., OSPFv3, ISIS for IPv6, RIPng, EIGRP for IPv6

    • Exterior Gateway Protocols (EGPs) Multi-Protocol Extensions for BGP4

Ipv6 addressing3
IPv6 Addressing

  • The idea behind having fixed-width, 64-bit wide host identifiers is that they aren't assigned manually as in IPv4.

  • Instead, v6 host addresses are recommended to be built from so-called EUI64 addresses.

  • EUI64 addresses are 64-bits wide, and derived from MAC addresses of the underlying network interface.

  • For example, with Ethernet, the 6-byte (48-bit) MAC address is usually filled with the hex bits "fffe" in the middle

What s your address mac
What's your address, MAC?

  • For example, with Ethernet, the 6-byte (48-bit) MAC address is usually filled with the hex bits "fffe" in the middle -- the MAC address 01:23:45:67:89:abresults in the EUI64 address 01:23:45:ff:fe:67:89:abwhich again gives the host bits for the IPv6 address. ::0123:45ff:fe67:89ab