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Transcript
Computer Networks Network Layer 1 Where are we? 2 Will Layer 2 Networking Suffice? 3 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 4 Connecting Heterogeneous Networks Computer System used Special purpose Dedicated Works with LAN or WAN technologies Known as router gateway 5 Illustration of a Router Cloud denotes an arbitrary network One interface per network 6 Important Idea A router can interconnect networks that use different technologies, including different media and media access techniques, physical addressing schemes or frame formats. 7 The Internet Concept 8 Key Functions of the Network Layer Global Addressing Fragmentation Routing We’ll be primarily concerned with addressing and routing 9 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! 10 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 11 IP Address Virtual only understood by software Used for all communication across an internetwork 32-bit integer Unique value for each host/interface 12 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. 13 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 14 Class of IP Addresses (Historical) Initial bits determined the class The class determines the boundary between prefix and suffix 15 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 16 Examples of Dotted Decimal Notation Four decimal values per 32-bit address Each decimal number represents eight bits is between 0 and 255 inclusive 17 Class Hierarchy and Network Size (Historical) Maximum size determined by class of address Class A large Class B medium Class C small 18 Addressing Example 19 Illustration of Router Addresses Address prefix identifies the network Need one address per router connection 20 Special Addresses Network Address not used in packets Loopback addresses never leave the local computer 21 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) 22 IP Addressing: Problems with Classes Internet growth Routing table size Exhaustion of addresses Administration overhead Misappropriation of addresses 23 IP Addressing: Solutions Subnetting Supernetting Classless InterDomain Routing (CIDR) Variable Length Subnet Mask (VLSM) 24 Subnetting Split the suffix into a local network portion and a smaller host id portion 25 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 26 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? 27 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 28 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 29 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. 30 IP Packet (datagram) Format 31 IP Datagrams Can be delayed Duplicated Delivered out of order Lost Can change routes from packet to packet Are connectionless 32 Address Resolution Protocol (ARP) Resolves IP address to Layer 2 (MAC) address Node sends MAC broadcast looking for another node IP src: 140.192.23.1MAC 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. 33 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 34 IP Routing Performed by routers Table-driven Forwarding on a hop-by-hop basis Destination address used for route determination 35 Example IP Routing Table Table (b) is for center router in (a) 36 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. 37 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 38 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. 39 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 40 Routing Protocol Requirements Efficient routing table size Efficient routing control messages Robustness and reliability prevent loops avoid black holes reconvergence time is short 41 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 42 Compute Shortest/Best Path Possible metric geographic distance economic cost capacity 43 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 44 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 45 Count to Infinity Problem What happens when link 1<->5 goes down? Does 5 think it can get to 1 through 2? 46 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. 47 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. 48 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 49 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. 50 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. 51 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 52 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 53 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 54 Network Layer: Final Notes Basic IP requirements on an end host IP address, mask, gateway, DNS BOOTP ICMP BGP IPX AppleTalk CLNP 55