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Computer Networks
Network Layer
Autumn 2000
John Kristoff
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Where are we?
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Will Layer 2 Networking
Suffice?
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Motivation
Ñ Connect various link technologies to form a larger
internetwork
Ñ
Ñ
Ñ
Ñ
Ñ
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
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Connecting Heterogeneous
Networks
Ñ Computer System used
ÑSpecial purpose
ÑDedicated
ÑWorks with LAN or WAN technologies
ÑKnown as
Ñrouter
Ñgateway
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Illustration of a Router
Ñ Cloud denotes an arbitrary network
Ñ One interface per network
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Important Idea
A router can interconnect networks that use
different technologies, including different
media and media access techniques,
physical addressing schemes or frame
formats.
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The Internet Concept
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Key Functions of the
Network Layer
Ñ Global Addressing
Ñ Fragmentation
Ñ Routing
We’ll be primarily concerned with addressing and routing
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Example Network Layer:
Internet Protocol (IP)
Ñ Standardized by IETF as RFC 791
Ñ Most popular Layer 3 protocol
Ñ Core protocol used on the public Internet
Ñ Connectionless protocol
Ñ datagrams contain identity of the destination
Ñeach datagram sent/handled independently
Ñ Of utmost importance for this class!
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IP Addressing
Ñ Provides an abstraction
Ñ Independent of hardware (MAC)
addressing
Ñ Used by
Ñhigher layer protocols
ÑApplications
Good IP addressing tutorial: http://www.3com.com/nsc/501302.html
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IP Address
Ñ Virtual
Ñonly understood by software
Ñ Used for all communication across an
internetwork
Ñ 32-bit integer
Ñ Unique value for each host/interface
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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.
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IP Address Details
Ñ Divided into two parts
Ñprefix identifies the network
Ñsuffix identifies the host/interface
Ñ Global authority assigns unique prefix for
the network
Ñ Local administrator assigns unique suffix
for the host/interface
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Class of IP Addresses
(Historical)
Ñ Initial bits determined the class
Ñ The class determines the boundary between prefix and suffix
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Dotted Decimal Notation
Ñ Shorthand for IP addresses
Ñ Allows humans to avoid binary
Ñ Represents each octet in decimal
separated by dots
Ñ NOT the same as names like
www.depaul.edu
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Examples of Dotted
Decimal Notation
Ñ Four decimal values per 32-bit address
Ñ Each decimal number
Ñ represents eight bits
Ñ is between 0 and 255 inclusive
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Class Hierarchy and
Network Size (Historical)
Ñ
Maximum size determined by class of address
Ñ Class A large
Ñ Class B medium
Ñ Class C small
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Addressing Example
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Illustration of Router
Addresses
Ñ Address prefix identifies the network
Ñ Need one address per router connection
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Special Addresses
Ñ Network Address not used in packets
Ñ Loopback addresses never leave the local computer
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Getting IP Addresses
Ñ IANA has global authority for allocation
ÑRegional registries: ARIN, RIPE, APNIC
Ñ RFC 1918 defines private address space
ÑNOT globally unique
Ñ10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
Ñ End users obtain address space from their
Internet Service Provider (ISP)
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IP Addressing: Problems
with Classes
Ñ Internet growth
Ñ Routing table size
Ñ Exhaustion of addresses
Ñ Administration overhead
Ñ Misappropriation of addresses
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IP Addressing: Solutions
Ñ Subnetting
Ñ Supernetting
Ñ Classless InterDomain Routing (CIDR)
Ñ Variable Length Subnet Mask (VLSM)
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Subnetting
Ñ Split the suffix into a local network portion and a smaller
host id portion
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Subnet Masks
Ñ Cannot determine prefix on first few bits
Ñ Need a 'bit mask' that specifies prefix/suffix
Ñ Dotted decimal notation used, but...
Ñ I told you binary was important!
Ñ Examples:
Ñ Network: 140.192.9.0 Subnet mask: 255.255.255.0
Ñ Network: 140.192.9.0 Subnet mask: 255.255.255.128
Ñ Network and mask: 140.192.9.0/24
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More Subnet Examples
Ñ 63.85.18.5/22 - What is the network?
Ñ 32.152.6.1/26 - How many hosts possible?
Ñ 219.52.33.8/20 - What is the directed
broadcast address?
Ñ How might you allocate a 10.5.0.0/16 block
for an organization with 4 offices of 500
users each?
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Supernetting
Ñ Combine multiple smaller address classes
into a larger block
Ñ Class B was too big
Ñ Class C was too small
Ñ Combine contiguous Class C addresses
Ñ e.g. 199.242.64.0 to 199.242.67.255
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Classless InterDomain
Routing (CIDR)
Ñ Employ supernetting style information in
IP routers
Ñ Advertise smaller CIDR blocks
Ñ Decreases the routing table size
Advertise 199.242.64.0/22 instead of 199.242.64.0,199.242.65.0,
199.242.66.0 and 199.242.67.0
The CIDR Report: http://www.employees.org/~tbates/cidr-report.html
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Variable Length Subnet
Masks (VLSM)
Ñ Ability to use multiple subnet sizes in a single
autonomous system
Ñ Allows more efficient use of addresses
Ñ Routers must support subnets masks
Ñ e.g. RIPv1 did not support this!
Ñ For example:
Ñ 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.
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IP Packet (datagram)
Format
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IP Datagrams
Ñ Can be delayed
Ñ Duplicated
Ñ Delivered out of order
Ñ Lost
Ñ Can change routes from packet to packet
Ñ Are connectionless
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Address Resolution
Protocol (ARP)
Ñ Resolves IP address to Layer 2 (MAC)
address
Ñ Node sends MAC broadcast looking for another node
Ñ IP src: 140.192.23.1 MAC src: 0x00:80:05:1A:F0
Ñ IP dst: 140.192.23.23 MAC dst: 0xFF:FF:FF:FF:FF
Ñ Node with that IP dst address replies with its MAC
Ñ 140.192.23.23 replies with 0x00:60:0A:34:AA:3C
Ñ ARP Table: contains records of learned
relationships.
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Configuration Protocol
(DHCP)
Ñ Standardized in RFC 1531
Ñ Allows hosts to obtain IP address
information upon startup from a server
Ñ Eliminates cumbersome manual
configuration
Ñ Grants IP addresses based on a
predefined "lease" period
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IP Routing
Ñ Performed by routers
Ñ Table-driven
Ñ Forwarding on a hop-by-hop basis
Ñ Destination address used for route
determination
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Example IP Routing Table
Ñ Table (b) is for center router in (a)
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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.
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Routing/Forwarding
Overview
Ñ Given a datagram
Ñ Extract destination address field, D
Ñ Look up D in the routing table
Ñ Find next hop address, N
Ñ Send datagram to N
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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.
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Routing/Forwarding
Overview
Ñ Strip off layer 2 information
Ñ Extract destination IP address field
Ñ Look up IP address in the routing table
Ñ Find next hop address to forward to
Ñ Send datagram to the next hop
Ñ Add on necessary layer 2 information
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Routing Protocol
Requirements
Ñ Efficient routing table size
Ñ Efficient routing control messages
Ñ Robustness and reliability
Ñprevent loops
Ñavoid black holes
Ñ reconvergence time is short
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Source of Route Table
Information
Ñ Manual
ÑTable created by hand
ÑUseful in small networks
ÑUseful if routes never change
Ñ Automatic
Ñsoftware creates/updates tables
ÑNeeded in large networks
ÑChanges routes when failures occur
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Compute Shortest/Best
Path
Ñ Possible metric
Ñ geographic distance
Ñ economic cost
Ñ capacity
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Algorithms for Computing
Shortest Path
Ñ Distance Vector
ÑExchange routing tables with neighboring
routers
Ñe.g., RIP, RIPv2
Ñ Link State
ÑRouters exchange link status information
Ñe.g., OSPF, IS-IS
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Distance Vector
Ñ Routers periodically advertise and learn
about IP networks
Ñ Cost of the route is based on hops to the
network (number of routers to pass)
Ñ Recalculation occurs when links fail
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Count to Infinity Problem
Ñ What happens when link 1<->5 goes down?
Ñ Does 5 think it can get to 1 through 2?
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Solving the Count to
Infinity Problem
Ñ Hold down
Ñ Wait for a period of time before switching paths. Advertise
route cost as infinity. Based on timers.
Ñ Report the entire path
Ñ Guarantees no loops, but expensive.
Ñ Split horizon
Ñ Do not advertise routes to neighbors if the route was received
from that neighbor. Not foolproof.
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Other Distance Vector
Improvements
Ñ Triggered updates
Ñ Advertise changes as soon as you learn of them. May help
convergence time. May create routing instability for flapping
routes.
Ñ Poison reverse
Ñ Used with split horizon. Report infinity rather than nothing at
all.
Ñ Diffusing Update ALgorithm (DUAL)
Ñ Somewhat like hold down, but routers are alerted of broken
paths. Complex. Not popular.
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Example Distance Vector
Protocol: RIP
Ñ Standardized in RFC 1058 and 2453
Ñ An interior gateway protocol (IGP)
Ñ Simple
Ñ RIPv2 includes subnet mask in updates
Ñ Hop count based (> 15 = unreachable)
Ñ Widely used in small to medium sized
organizations
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Link State
Ñ Routers distribute link cost and topology
information to all other routers in its area.
Ñ All routers have complete information
about the network.
Ñ Each router computes its own optimal
path to destinations.
Ñ Ensures loop free environments.
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Link State Procedure
Ñ
Ñ
Ñ
Ñ
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.
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Dijkstra Algorithm:
Databases
Ñ
Link State Database
Ñ contains latest LSPs from all other routers
Ñ
PATH (permanent) Database
Ñ Consists of ID, path cost and forwarding direction
Ñ
TENT (tentative/temporary) Database
Ñ Consists of ID, path cost and forwarding direction
Ñ
Forwarding Database
Ñ Contains ID and forwarding direction
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Dijkstra Algorithm:
Procedure
Ñ
Ñ
Ñ
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
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Example Link State
Protocol: Open Shortest
Path First (OSPF)
Ñ Standardized in RFC 2328
Ñ An interior gateway protocol (IGP)
Ñ Used in medium to large organizations
Ñ Supports internal/external routes
Ñ Supports TOS based routing, but never
implemented in practice
Ñ Two-tier hierarchy for scalability
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Network Layer: Final
Notes
Ñ Basic IP requirements on an end host
Ñ IP address, mask, gateway, DNS
Ñ BOOTP
Ñ ICMP
Ñ BGP
Ñ IPX
Ñ AppleTalk
Ñ CLNP
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