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Computer Networks. Network Layer. Where are we?. Will Layer 2 Networking Suffice?. Motivation. Connect various link technologies to form a larger internetwork Universal addressing scheme required General purpose use Hides underlying technologies from end user

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Computer Networks

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computer networks
Computer Networks

Network Layer

  • Connect various link technologies to form a larger internetwork
  • Universal addressing scheme required
  • General purpose use
  • Hides underlying technologies from end user
  • Facilitate communicate between autonomous domains
  • Able to move packets between any host on the internetwork
connecting heterogeneous networks
Connecting Heterogeneous Networks
  • Computer System used
    • Special purpose
    • Dedicated
    • Works with LAN or WAN technologies
    • Known as
      • router
      • gateway
illustration of a router
Illustration of a Router
  • Cloud denotes an arbitrary network
  • One interface per network
important idea
Important Idea

A router can interconnect networks that use different technologies, including different media and media access techniques, physical addressing schemes or frame formats.

key functions of the network layer
Key Functions of the Network Layer
  • Global Addressing
  • Fragmentation
  • Routing

We’ll be primarily concerned with addressing and routing

example network layer internet protocol ip
Example Network Layer: Internet Protocol (IP)
  • Standardized by IETF as RFC 791
  • Most popular Layer 3 protocol
  • Core protocol used on the public Internet
  • Connectionless protocol
    • datagrams contain identity of the destination
    • each datagram sent/handled independently
  • Of utmost importance for this class!
ip addressing
IP Addressing
  • Provides an abstraction
  • Independent of hardware (MAC) addressing
  • Used by
    • higher layer protocols
    • Applications

Good IP addressing tutorial:

ip address
IP Address
  • Virtual
    • only understood by software
  • Used for all communication across an internetwork
  • 32-bit integer
  • Unique value for each host/interface
ip address assignment
IP Address Assignment

An IP address does not identify a specific computer. Instead, each IP address identifies a connection between a computer and a network. A computer with multiple network connections (e.g., a router) must be assigned one IP address for each connection.

ip address details
IP Address Details
  • Divided into two parts
    • prefix identifies the network
    • suffix identifies the host/interface
  • Global authority assigns unique prefix for the network
  • Local administrator assigns unique suffix for the host/interface
class of ip addresses historical
Class of IP Addresses (Historical)
  • Initial bits determined the class
  • The class determines the boundary between prefix and suffix
dotted decimal notation
Dotted Decimal Notation
  • Shorthand for IP addresses
  • Allows humans to avoid binary
  • Represents each octet in decimal separated by dots
  • NOT the same as names like
examples of dotted decimal notation
Examples of Dotted Decimal Notation
  • Four decimal values per 32-bit address
  • Each decimal number
    • represents eight bits
    • is between 0 and 255 inclusive
class hierarchy and network size historical
Class Hierarchy and Network Size (Historical)
  • Maximum size determined by class of address
    • Class A large
    • Class B medium
    • Class C small
illustration of router addresses
Illustration of Router Addresses
  • Address prefix identifies the network
  • Need one address per router connection
special addresses
Special Addresses
  • Network Address not used in packets
  • Loopback addresses never leave the local computer
getting ip addresses
Getting IP Addresses
  • IANA has global authority for allocation
    • Regional registries: ARIN, RIPE, APNIC
  • RFC 1918 defines private address space
    • NOT globally unique
  • End users obtain address space from their Internet Service Provider (ISP)
ip addressing problems with classes
IP Addressing: Problems with Classes
  • Internet growth
  • Routing table size
  • Exhaustion of addresses
  • Administration overhead
  • Misappropriation of addresses
ip addressing solutions
IP Addressing: Solutions
  • Subnetting
  • Supernetting
  • Classless InterDomain Routing (CIDR)
  • Variable Length Subnet Mask (VLSM)
  • Split the suffix into a local network portion and a smaller host id portion
subnet masks
Subnet Masks
  • Cannot determine prefix on first few bits
  • Need a 'bit mask' that specifies prefix/suffix
  • Dotted decimal notation used, but...
  • I told you binary was important!
  • Examples:
    • Network: Subnet mask:
    • Network: Subnet mask:
    • Network and mask:
more subnet examples
More Subnet Examples
  • - What is the network?
  • - How many hosts possible?
  • - What is the directed broadcast address?
  • How might you allocate a block for an organization with 4 offices of 500 users each?
  • Combine multiple smaller address classes into a larger block
  • Class B was too big
  • Class C was too small
  • Combine contiguous Class C addresses
    • e.g. to
classless interdomain routing cidr
Classless InterDomain Routing (CIDR)
  • Employ supernetting style information in IP routers
  • Advertise smaller CIDR blocks
  • Decreases the routing table size

Advertise instead of,, and

The CIDR Report:

variable length subnet masks vlsm
Variable Length Subnet Masks (VLSM)
  • Ability to use multiple subnet sizes in a single autonomous system
  • Allows more efficient use of addresses
  • Routers must support subnets masks
    • e.g. RIPv1 did not support this!
  • For example:
    • May use /24 in most places, but may have a small office with only 10 users! May want to use a /28 for that network.
ip datagrams
IP Datagrams
  • Can be delayed
  • Duplicated
  • Delivered out of order
  • Lost
  • Can change routes from packet to packet
  • Are connectionless
address resolution protocol arp
Address Resolution Protocol (ARP)
  • Resolves IP address to Layer 2 (MAC) address
    • Node sends MAC broadcast looking for another node
      • IP src: MAC src: 0x00:80:05:1A:F0
      • IP dst: MAC dst: 0xFF:FF:FF:FF:FF
    • Node with that IP dst address replies with its MAC
      • replies with 0x00:60:0A:34:AA:3C
  • ARP Table: contains records of learned relationships.
dynamic host configuration protocol dhcp
Dynamic Host Configuration Protocol (DHCP)
  • Standardized in RFC 1531
  • Allows hosts to obtain IP address information upon startup from a server
  • Eliminates cumbersome manual configuration
  • Grants IP addresses based on a predefined "lease" period
ip routing
IP Routing
  • Performed by routers
  • Table-driven
  • Forwarding on a hop-by-hop basis
  • Destination address used for route determination
example ip routing table
Example IP Routing Table
  • Table (b) is for center router in (a)
routing table size
Routing Table Size

Since each destination in a routing table corresponds to a network, the number of entries in a routing table is proportional to the number of networks in the internetwork. Caveat: you can use a "default" route to forward to when route is unknown or when no route specific information is available.

routing forwarding overview
Routing/Forwarding Overview
  • Given a datagram
  • Extract destination address field, D
  • Look up D in the routing table
  • Find next hop address, N
  • Send datagram to N
key concept
Key Concept

The destination address in a datagram header always refers to the ultimate destination. When a router forwards the datagram to another router, the address of the next hop does not appear in the datagram header.

routing forwarding overview40
Routing/Forwarding Overview
  • Strip off layer 2 information
  • Extract destination IP address field
  • Look up IP address in the routing table
  • Find next hop address to forward to
  • Send datagram to the next hop
  • Add on necessary layer 2 information
routing protocol requirements
Routing Protocol Requirements
  • Efficient routing table size
  • Efficient routing control messages
  • Robustness and reliability
    • prevent loops
    • avoid black holes
    • reconvergence time is short
source of route table information
Source of Route Table Information
  • Manual
    • Table created by hand
    • Useful in small networks
    • Useful if routes never change
  • Automatic
    • software creates/updates tables
    • Needed in large networks
    • Changes routes when failures occur
compute shortest best path
Compute Shortest/Best Path
  • Possible metric
    • geographic distance
    • economic cost
    • capacity
algorithms for computing shortest path
Algorithms for Computing Shortest Path
  • Distance Vector
    • Exchange routing tables with neighboring routers
    • e.g., RIP, RIPv2
  • Link State
    • Routers exchange link status information
    • e.g., OSPF, IS-IS
distance vector
Distance Vector
  • Routers periodically advertise and learn about IP networks
  • Cost of the route is based on hops to the network (number of routers to pass)
  • Recalculation occurs when links fail
count to infinity problem
Count to Infinity Problem
  • What happens when link 1<->5 goes down?
  • Does 5 think it can get to 1 through 2?
solving the count to infinity problem
Solving the Count to Infinity Problem
  • Hold down
    • Wait for a period of time before switching paths. Advertise route cost as infinity. Based on timers.
  • Report the entire path
    • Guarantees no loops, but expensive.
  • Split horizon
    • Do not advertise routes to neighbors if the route was received from that neighbor. Not foolproof.
other distance vector improvements
Other Distance Vector Improvements
  • Triggered updates
    • Advertise changes as soon as you learn of them. May help convergence time. May create routing instability for flapping routes.
  • Poison reverse
    • Used with split horizon. Report infinity rather than nothing at all.
  • Diffusing Update ALgorithm (DUAL)
    • Somewhat like hold down, but routers are alerted of broken paths. Complex. Not popular.
example distance vector protocol rip
Example Distance Vector Protocol: RIP
  • Standardized in RFC 1058 and 2453
  • An interior gateway protocol (IGP)
  • Simple
  • RIPv2 includes subnet mask in updates
  • Hop count based (> 15 = unreachable)
  • Widely used in small to medium sized organizations
link state
Link State
  • Routers distribute link cost and topology information to all other routers in its area.
  • All routers have complete information about the network.
  • Each router computes its own optimal path to destinations.
  • Ensures loop free environments.
link state procedure
Link State Procedure
  • Each router is responsible for meeting its neighbors and learning their names.
  • Each router advertises LSPs which contain costs to its neighbors.
  • Most current LSPs are stored by all routers.
  • Each router now computes routes.
dijkstra algorithm databases
Dijkstra Algorithm: Databases
  • Link State Database
    • contains latest LSPs from all other routers
  • PATH (permanent) Database
    • Consists of ID, path cost and forwarding direction
  • TENT (tentative/temporary) Database
    • Consists of ID, path cost and forwarding direction
  • Forwarding Database
    • Contains ID and forwarding direction
dijkstra algorithm procedure
Dijkstra Algorithm: Procedure
  • 1. Each router starts with itself as root of the tree by putting its ID and 0 cost and 0 forwarding direction in PATH
  • 2. For each node placed in PATH, examine its LSP and place those neighbors in TENT if not already in PATH or TENT
  • 3. If TENT is empty, terminate, otherwise find the ID in TENT with the lowest cost and move it to PATH
example link state protocol open shortest path first ospf
Example Link State Protocol: Open Shortest Path First (OSPF)
  • Standardized in RFC 2328
  • An interior gateway protocol (IGP)
  • Used in medium to large organizations
  • Supports internal/external routes
  • Supports TOS based routing, but never implemented in practice
  • Two-tier hierarchy for scalability
network layer final notes
Network Layer: Final Notes
  • Basic IP requirements on an end host
    • IP address, mask, gateway, DNS
  • ICMP
  • BGP
  • IPX
  • AppleTalk
  • CLNP