15-744: Computer NetworkingRoutingOutlineIP Address Problem (1991)IP Address Utilization (‘98)Solution 1 – CIDRClassless Inter-Domain RoutingSolution 2 - NATNAT IllustrationSolution 3 - IPv6Summary: Internet ArchitectureSummary: Minimalist ApproachSummary: IP DesignSlide 15Techniques for Forwarding PacketsSource RoutingSlide 18Virtual Circuits/Tag SwitchingVirtual Circuits ExamplesVirtual CircuitsIP Datagrams on Virtual CircuitsSlide 23Global Addresses (IP)Global Address ExampleRouter Table SizeGlobal AddressesSummarySlide 29Original IP Route LookupOriginal IP Route Lookup – ExampleCIDR RevisitedCIDR ExampleCIDR IllustrationCIDR ShortcomingsRouting to the NetworkSlide 40Compressed Trie Using Sample DatabaseSpeeding up Prefix Match (P+98)Prefix TreeSlide 46Slide 47Slide 48Speeding up Prefix Match - AlternativesBinary Search on RangesSlide 51Next Lecture: Routers & Routing15-744: Computer NetworkingL-3 Addressing/ForwardingL-3; 9-17-02© Srinivasan Seshan, 2002 2Routing•How do routers process IP packets•Forwarding lookup algorithms•Assigned reading•[D+97] Small Forwarding Tables for Fast Routing Lookups•[BV01] Scalable Packet ClassificationL-3; 9-17-02© Srinivasan Seshan, 2002 3Outline•IP address allocation•Alternative methods for packet forwarding•IP route lookup•Variable prefix match algorithmsL-3; 9-17-02© Srinivasan Seshan, 2002 4IP Address Problem (1991)•Address space depletion•In danger of running out of classes A and B•Why?•Class C too small for most domains•Very few class A – IANA (Internet Assigned Numbers Authority) very careful about giving•Class B – greatest problem•Sparsely populated – but people refuse to give it backL-3; 9-17-02© Srinivasan Seshan, 2002 5IP Address Utilization (‘98)http://www.caida.org/outreach/resources/learn/ipv4space/L-3; 9-17-02© Srinivasan Seshan, 2002 6Solution 1 – CIDR•Assign multiple class C addresses•Assign consecutive blocks•RFC1338 – Classless Inter-Domain Routing (CIDR)L-3; 9-17-02© Srinivasan Seshan, 2002 7Classless Inter-Domain Routing•Do not use classes to determine network ID•Assign any range of addresses to network•Use common part of address as network number•e.g., addresses 192.4.16 - 196.4.31 have the first 20 bits in common. Thus, we use this as the network number•netmask is /20, /xx is valid for almost any xx•Enables more efficient usage of address space (and router tables)L-3; 9-17-02© Srinivasan Seshan, 2002 8Solution 2 - NAT•Network Address Translation (NAT)•Alternate solution to address space•Kludge (but useful)•Sits between your network and the Internet•Translates local network layer addresses to global IP addresses•Has a pool of global IP addresses (less than number of hosts on your network)L-3; 9-17-02© Srinivasan Seshan, 2002 9NAT IllustrationGlobal InternetPrivateNetworkPool of global IP addresses•Operation: Source (S) wants to talk to Destination (D):•Create Sg-Sp mapping•Replace Sp with Sg for outgoing packets•Replace Sg with Sp for incoming packets•D & S can be just IP addresses or IP addresses + port #’sPGDgSpDataNATDestination SourceDgSgDataL-3; 9-17-02© Srinivasan Seshan, 2002 10Solution 3 - IPv6•Scale – addresses are 128bit•Header size?•Simplification•Removes infrequently used parts of header•40byte fixed size vs. 20+ byte variable•IPv6 removes checksum•Relies on upper layer protocols to provide integrity•IPv6 eliminates fragmentation•Requires path MTU discovery•Requires 1280 byte MTU•Flows•Help soft state systems•Maps well onto TCP connection or stream of UDP packets on host-port pairL-3; 9-17-02© Srinivasan Seshan, 2002 12Summary: Internet Architecture•Packet-switched datagram network•IP is the “compatibility layer” •Hourglass architecture•All hosts and routers run IP•Stateless architecture•no per flow state inside networkIPTCP UDPATMSatelliteEthernetL-3; 9-17-02© Srinivasan Seshan, 2002 13Summary: Minimalist Approach•Dumb network•IP provide minimal functionalities to support connectivity•Addressing, forwarding, routing•Smart end system•Transport layer or application performs more sophisticated functionalities•Flow control, error control, congestion control•Advantages•Accommodate heterogeneous technologies (Ethernet, modem, satellite, wireless)•Support diverse applications (telnet, ftp, Web, X windows)•Decentralized network administrationL-3; 9-17-02© Srinivasan Seshan, 2002 14Summary: IP Design•Relatively simple design•Some parts not so useful (TOS, options)•Beginning to show age•Unclear what the solution will be probably IPv6L-3; 9-17-02© Srinivasan Seshan, 2002 15Outline•IP address allocation•Alternative methods for packet forwarding•IP route lookup•Variable prefix match algorithmsL-3; 9-17-02© Srinivasan Seshan, 2002 16Techniques for Forwarding Packets•Source routing•Packet carries path•Table of virtual circuits•Connection routed through network to setup state•Packets forwarded using connection state •Table of global addresses (IP)•Routers keep next hop for destination•Packets carry destination addressL-3; 9-17-02© Srinivasan Seshan, 2002 17Source Routing•List entire path in packet•Driving directions (north 3 hops, east, etc..)•Router processing•Examine first step in directions•Strip first step from packet•Forward to step just stripped offReceiverPacketR2, R3, RSender234123412341R2R3R1R3, RRL-3; 9-17-02© Srinivasan Seshan, 2002 18Source Routing•Advantages•Switches can be very simple and fast•Disadvantages•Variable (unbounded) header size•Sources must know or discover topology (e.g., failures)•Typical use•Ad-hoc networks (DSR)•Machine room networks (Myrinet)L-3; 9-17-02© Srinivasan Seshan, 2002 19Virtual Circuits/Tag Switching•Connection setup phase•Use other means to route setup request •Each router allocates flow ID on local link•Creates mapping of inbound flow ID/port to outbound flow ID/port•Each packet carries connection ID•Sent from source with 1st hop connection ID•Router processing•Lookup flow ID – simple table lookup•Replace flow ID with outgoing flow ID•Forward to output portL-3; 9-17-02© Srinivasan Seshan, 2002 20Virtual Circuits ExamplesReceiverPacket1,5 3,7Sender23411,7 4,2234123412,2 3,6R2R3R15 726L-3; 9-17-02© Srinivasan Seshan, 2002 21Virtual Circuits•Advantages•More efficient lookup (simple table lookup)•More flexible
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