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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 Network Layer
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.
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
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 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: 140.192.23.23 MAC 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.