Network Layer Fundamentals

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Network Layer Fundamentals

• 3rd Tutorial Session for CEG3180B
• February 1st, 2005

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The ISO OSI Model

• A conceptual, layered model for designing
  networked systems (i.e., both the hardware and
  software components that relate to networking a
  certain system)
• 7 Layers (from top to bottom) Application,
  Presentation, Session, Transport, Network, Data
  Link, Physical
• The higher the layer, the more abstract its
  functions are with respect to the actual physical
  transmission

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The ISO OSI Model

• A conceptual, layered model for designing
  networked systems (i.e., both the hardware and
  software components that relate to networking a
  certain system)
• 7 Layers (from top to bottom) Application,
  Presentation, Session, Transport, Network, Data
  Link, Physical
• The higher the layer, the more abstract its
  functions are with respect to the actual physical
  transmission

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The ISO OSI Model (contd)

• Allows for transparent peer-to-peer communication
  between same layers of two networked systems
• Top four layers network layers the other
  three layers host layers
• Beginning with the upmost half of the Data Link
  Layer (the LLC Sub-Layer), operations are
  media-independent

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The Network Layer

• Two fundamental functions
• Logical network topology and Addressing
• Path determination (i.e., Datagram routing)
• The rest of this discussion focuses on the IP
  (Internet Protocol), version 4 (IPv4) of the
  TCP/IP Protocol Stack

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Original IPv4 Addressing

• 32-bit addresses (010010111)
• Most of the times written in the dotted-decimal
  format 4 numbers between 0 and 255, separated by
  dots
• E.g., 137.122.14.100
• Theoretically to yield 232 4.3 billion addresses

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Original IPv4 Addressing (contd)

• Address space divided into classes of addresses
  based on the size of the networks it was supposed
  to be allocated to
• Class A large size networks
• Class B medium size networks
• Class C small size networks
• Class D special (multicast)
• Class E special (reserved)

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Original IPv4 Addressing (contd)

• Address space divided into classes of addresses
  based on the size of the networks it was supposed
  to be allocated to
• Class A large size networks
• Class B medium size networks
• Class C small size networks
• Class D special (multicast)
• Class E special (reserved)

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Path Determination

• I.e., datagram (packet) routing
• The hop-by-hop routing paradigm packet passes
  from router to router, each step bringing it
  closer to the destination
• If a packet travels too many hops, it is
  discarded (in order to prevent routing loops)

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Path Determination

• I.e., datagram (packet) routing
• The hop-by-hop routing paradigm packet passes
  from router to router, each step bringing it
  closer to the destination
• If a packet travels too many hops, it is
  discarded (in order to prevent routing loops)

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Path Determination (contd)

• Routers maintain routing tables containing, for
  each known destination network address
• The output interface for that destination
• The next hop address for that destination
• Routing tables updated statically (by hand) or
  dynamically (by using dynamic routing protocols)

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Static vs. Dynamic Routing

• Static is
• Simpler to configure, yet more difficult to
  maintain
• Very low CPU time-consuming and memory-consuming
• Not at all suited for large networks and only
  marginally suited for redundant topologies
• Dynamic is
• More difficult to configure, but need not be
  manually maintained up to date
• Usually more CPU time-consuming and
  memory-consuming
• Virtually a must for redundant topologies and
  larger networks

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Simple Routing Algorithm

• Examine destination address to determine if class
  A, B or C
• Extract the network part from the address
• Search for the destination network in the routing
  table
• If found, and next hop is reachable route out
  the specified interface to the next hop
• Otherwise, discard the packet and send ICMP
  Destination Host/Network Unreachable message to
  the sender

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Original IPv4 Addressing Issues

• Inefficient address space allocation - a large
  part of the address space is being wasted
• Inefficient routing large routing tables,
  routing processes very CPU intensive

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Solutions Devised

• Subnetting
• Default routing Classless Inter-Domain Routing
  (CIDR), also known as Supernetting

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Subnetting

• Borrowing bits from the host portion for the
  network portion of the address
• Network addresses expressed as pairs of address
  and subnet mask
• The concept of classes becomes obsolete, yet
  designs have sometimes to accommodate older
  equipment with no knowledge of subnetting

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Subnetting

• Borrowing bits from the host portion for the
  network portion of the address
• Network addresses expressed as pairs of address
  and subnet mask
• The concept of classes becomes obsolete, yet
  designs have sometimes to accommodate older
  equipment with no knowledge of subnetting

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Subnet Masks

• 32-bit strings with a contiguous left side of 1s
  and a contiguous right side of 0s
• The number of 1s (the length of the subnet
  mask) how many bits of the address corresponds
  to the network part

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Subnet Masks (contd)

• Written either in dotted-decimal format, or as
  /number_of_1s (/length)
• Original classes of addresses
• A 255.0.0.0 (/8)
• B 255.255.0.0 (/16)
• C 255.255.255.0 (/24)

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Default Routing

• Specifies a way to handle packets for which no
  specific entry exists in the routing table
• Fall-back the packed is routed via a default
  gateway that is supposed to know better what to
  do with it
• Especially useful for stub networks
• Helps keeping routing tables small
• Default route entry 0.0.0.0/0

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Classless Inter-Domain Routing

• Grouping a number of contiguous network addresses
  into a larger routing table entry
• E.g., 192.168.8.0/24 through 192.168.15.0/24 can
  be written as 192.168.8.0/21
• Helps keeping routing tables small

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Modified Routing Algorithm

• For each routing table entry perform AND between
  destination address and entry subnet mask if
  result equals the entry network address and entry
  more specific (i.e., longer subnet mask) than the
  previous one, keep it and discard the other
• If matched, and next hop is reachable route out
  the specified interface to the next hop
• Otherwise, discard the packet and send ICMP
  Destination Host/Network Unreachable message to
  the sender

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Subnetting Examples

• Given the following two address/mask pairs, how
  can we tell whether they are on the same subnet
  or not?
• 192.168.0.5/28 and 192.168.0.18/28
• AND 192.168.0.5 and 255.255.255.240 (/28)
  192.168.0.0
• AND 192.168.0.18 and 255.255.255.240
  192.168.0.16
• NO (192.168.0.0 ! 192.168.0.16)

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Subnetting Examples (contd)

• Given the following two address/mask pairs, how
  can we tell whether they are on the same subnet
  or not?
• 192.168.0.66/26 and 192.168.0.90/26
• AND 192.168.0.66 and 255.255.255.192 (/26)
  192.168.0.64
• AND 192.168.0.90 and 255.255.255.192
  192.168.0.64
• YES (192.168.0.64 192.168.0.64)

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Subnetting Examples (contd)

• Given the following address/mask pair, can you
  determine the subnet address and the address
  range for that subnet?
• 192.168.32.115/29
• AND 192.168.32.115 and 255.255.255.248 (/29)
  192.168.32.112 (subnet address)
• OR 192.168.32.112 and NOT 255.255.255.248
  192.168.32.119 (broadcast address)
• Address range 192.168.32.112-119 (6 usable
  addresses, 113-118)

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Routing Table Example

• Given the following routing table
• 192.168.1.0 255.255.255.0 Serial0
• 192.168.1.0 255.255.255.240 Serial1
• 0.0.0.0 0.0.0.0 Serial2
• Address 192.168.1.20 will route by entry 1
• Address 192.168.1.5 will route by entry 2
• Address 192.168.3.35 will route by entry 3 (via
  the default gateway)

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Dynamic Routing Protocols

• Can be classified from multiple points of view
• By the algorithm they use for building routing
  tables
• Distance Vector use distance metrics
• Link State use cost metrics and SPF algorithms
• Hybrid
• By the way they use and advertise subnet
  information
• Classless they accept and advertise subnets
• Classful they ignore and dont advertise subnets
• By their intended use
• Exterior Gateway Protocols (EGP) inter-AS
• Interior Gateway Protocols (IGP) intra-AS

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Examples of Routing Protocols

• RIPv1 IGP, distance vector, classful
• RIPv2 IGP, distance vector, classless
• IGRP (Cisco) IGP, distance vector, classful
• EIGRP (Cisco) IGP, advanced distance vector
  (sometimes called hybrid), classless
• OSPF, IS-IS IGP, link state, classless
• BGP-4 EGP, hybrid, classless

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Distance Vector vs. Link State

• Distance Vector are
• Simpler
• Less CPU time-consuming and often less
  memory-consuming
• Slower-converging
• More bandwidth-consuming
• Less scalable
• Link State are
• More complicated
• CPU and memory intensive
• Faster-converging
• Less bandwidth-consuming
• Very scalable

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So, Which One To Choose?

• Distance vector in small and simple networks, or
  in networks with slower-CPU and small-sized
  memory routers
• Link state in large networks, and in networks
  requiring Shortest Path Tree calculation for the
  purpose of Traffic Engineering (i.e., MPLS-TE)

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Our Labs

• Will consist of configuring Cisco routers for
  Static Routing (Lab 2), for OSPF routing within a
  single area (Lab 3), and for OSPF routing within
  a multi-area topology (Lab 4)
• Technical documentation to be consulted listed in
  the References section of this presentation

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References

• J. Postel, STD0005/RFC0791 Internet Protocol
• J. Postel, STD0005/RFC0792 Internet Control
  Message Protocol
• J. C. Mogul, J. Postel, STD0005/RFC0950 Internet
  Standard Subnetting Procedure
• Y. Rekhter, T. Li, RFC1518 An Architecture for
  IP Address Allocation with CIDR
• IANA, RFC3330 Special-Use IPv4 Addresses
• Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de
  Groot, E. Lear, RFC1918 Address Allocation for
  Private Internets
• Cisco IOS IP Command Reference, Volume 1 of 4
  Addressing and Services, Release 12.3
• http//www.cisco.com/univercd/cc/td/doc/product/s
  oftware/ios123/123cgcr/ipras_r/ip1bookg.pdf
• Cisco IOS IP Command Reference, Volume 2 of 4
  Routing Protocols, Release 12.3
• http//www.cisco.com/univercd/cc/td/doc/product/s
  oftware/ios123/123cgcr/iprrp_r/ip2bookg.pdf