Autumn 2000John Kristoff1 Computer Networks Network Layer

Autumn 2000 John Kristoff 1

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 Facilitate communicate between autonomous

domains Able to move packets between any host on the

internetwork


Connecting Heterogeneous Networks

Computer System used Special purpose Dedicated Works with LAN or WAN technologies Known as

router gateway


Illustration of a Router

Cloud denotes an arbitrary network One interface per network


Important Idea

A router can interconnect networks that use different technologies,

including different media and media access techniques, physical

addressing schemes or frame formats.


The Internet Concept


Key Functions of the Network Layer

Global Addressing Fragmentation Routing

Well be primarily concerned with addressing and routing


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

Provides an abstraction Independent of hardware (MAC)

addressing Used by

higher layer protocols Applications

Good IP addressing tutorial: http://www.3com.com/nsc/501302.html


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

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

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)

Initial bits determined the class The class determines the boundary between prefix and suffix


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

www.depaul.edu


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)

Maximum size determined by class of address Class A large Class B medium Class C small


Addressing Example


Illustration of Router Addresses

Address prefix identifies the network Need one address per router connection


Special Addresses

Network Address not used in packets Loopback addresses never leave the local computer


Getting IP Addresses

IANA has global authority for allocation Regional registries: ARIN, RIPE, APNIC

RFC 1918 defines private address space NOT globally unique 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16

End users obtain address space from their Internet Service Provider (ISP)


IP Addressing: Problems with Classes

Internet growth Routing table size Exhaustion of addresses Administration overhead Misappropriation of addresses


IP Addressing: Solutions

Subnetting Supernetting Classless InterDomain Routing

(CIDR) Variable Length Subnet Mask (VLSM)


Subnetting

Split the suffix into a local network portion and a smaller host id portion


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: 140.192.9.0 Subnet mask: 255.255.255.0 Network: 140.192.9.0 Subnet mask: 255.255.255.128 Network and mask: 140.192.9.0/24


More Subnet Examples

63.85.18.5/22 - What is the network? 32.152.6.1/26 - How many hosts

possible? 219.52.33.8/20 - What is the directed

broadcast address? How might you allocate a 10.5.0.0/16

block for an organization with 4 offices of 500 users each?


Supernetting

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. 199.242.64.0 to

199.242.67.255


Classless InterDomain Routing (CIDR)

Employ supernetting style information in IP routers

Advertise smaller CIDR blocks 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


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 Packet (datagram) Format


IP Datagrams

Can be delayed Duplicated Delivered out of order Lost Can change routes from packet to

packet Are connectionless


Address Resolution Protocol (ARP)

Resolves IP address to Layer 2 (MAC) address Node sends MAC broadcast looking for another

node IP src: 140.192.23.1 MAC src: 0x00:80:05:1A:F0 IP dst: 140.192.23.23MAC dst: 0xFF:FF:FF:FF:FF

Node with that IP dst address replies with its MAC 140.192.23.23 replies with 0x00:60:0A:34:AA:3C

ARP Table: contains records of learned relationships.


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

Performed by routers Table-driven Forwarding on a hop-by-hop basis Destination address used for route

determination


Example IP Routing Table

Table (b) is for center router in (a)


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

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

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 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

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

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

Possible metric geographic distance economic cost capacity


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

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

What happens when link 1<->5 goes down? Does 5 think it can get to 1 through 2?


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

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

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

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

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

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

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) 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

Basic IP requirements on an end host IP address, mask, gateway, DNS

BOOTP ICMP BGP IPX AppleTalk CLNP

Documents

Autumn 2000John Kristoff1 Computer Networks Network Layer