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CMU 15441 Computer Networking - Lecture

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2/23/2008115-441 Computer NetworkingLecture 14 – IP Wrap Up1Peter SteenkisteDepartments of Computer Science andElectrical and Computer Engineering15-441 Networking, Spring 2008http://www.cs.cmu.edu/~dga/15-441/S08Outlinez The recurring IP address space problemz IPv6.z NAT.2zTunneling / Overlaysz Network Management» Autoconfiguration» SNMP (notes only)IP Address spacez Address space crunch 1: Classful routing» 128 “class A” blocks of 2^24 addresses (too big)» 16k “class B” blocks of 2^16 addresses (still too big)» 2M “class C” blocks (often too small)3» Result: Exceptionally wasteful allocation– MIT still has 18.0.0.0/8 -- 16M addresses for 30k people» Solution: CIDR (“cider”). Classful Inter-Domain Routing.– Removed classness.– Now can route on arbitrary power of two boundary– “slash” notation: /8 = 255.0.0.0, /16 = 255.255.0.0, etc.IP address space 2z How many IP addresses? 4Bz How many…» People? 6.5B» Cell phones? 2.2B (roughly)» Embedded computers? (???, but huge)4p(,g)» What happens if you network all of the devices in your house?» Big reality in 2005: We’re about 50% used.– CIDR– Tighter allocation policies; voluntary IP reclamation– NAT (later today)z Will it happen? Maybe, maybe not. But I wouldn’t bet against it.z But even if not, put on architecture hat and think about the problems…Efficiency vs. Conveniencez Exact allocation vs. Routing Table Size» If I have 20 computers, how do you alloc?– /27 = 32 (30 usable): Waste 12– /28 = 16 (14) + /29 (8, 6 usable): Waste 4– Can’t get any better…5–We’ve traded a bit of address efficiency for two BGP routing table entries.zExact allocation vs. Future Growth» Suppose you buy more computers– Could add a new netblock (more table entries)– Could move to a bigger one (re-addressing)z Update computersz Update routersz Update DNSz Update address allocation registriesz Maybe have to fix some hard-coded addresses, if you were bad» Partial solution: DHCP (we’ll talk about later today)IP v6z “Next generation” IP.z Most urgent issue: increasing address space.» 128 bit addresses zSimplified header for fasterV/PrV/Pr Flow labelFlow labelLengthLengthNextNext Hop LHop L6zSimplified header for faster processing:» No checksum (why not?)» No fragmentation (?)z Support for guaranteed services: priority and flow idz Options handled as “next header”» reduces overhead of handling optionsSource IP addressSource IP addressDestination IP addressDestination IP address2/23/20082IPv6 Addressingz Do we need more addresses? Probably, long term» Big panic in 90s: “We’re running out of addresses!”» Big worry: Devices. Small devices. Cell phones, toasters, everything.z 128 bit addresses provide space for structure (good!)» Hierarchical addressing is much easier7g» Assign an entire 48-bit sized chunk per LAN -- use Ethernet addresses» Different chunks for geographical addressing, the IPv4 address space,» Perhaps help clean up the routing tables - just use one huge chunk per ISP and one huge chunk per customer.Registry010 Provider HostSubNetSubscriberIPv6 Cleanup – Consider Router Architecturesz Common case: Switched in silicon (“fast path”)z Weird cases: Handed to CPU (“slow path”, or “process switched”)z Typical division:» Fast path: Almost everything» Slow path:Ftti8–Fragmentation– TTL expiration (traceroute)– IP option handlingz Slow path is evil in today’s environment» “Christmas Tree” attack sets weird IP options, bits, and overloads router.» Developers can’t (really) use things on the slow path for data flow– If it became popular, they’d be in the soup!z Other speed issue: Touching data is expensive. Designers like to minimize accesses to packet during forwardingIPv6 Header Cleanup: Checksumz No checksumz Why checksum just the IP header?» Efficiency: If packet corrupted at hop 1, don’t waste b/w transmitting on hops 2..N.» Useful when corruption frequent, b/w expensiveTd C ti b/h9»Today: Corruption rare, b/w cheapIPv6 Header Cleanup: Option Handlingz IPv4 options: Variable length header field. 32 different options.» Rarely used» No development / many hosts/routers do not support– Worse than useless: Packets w/options often even get dropped!10get dropped!» Processed in “slow path”.zIPv6 options: “Next header” pointer» Combines “protocol” and “options” handling– Next header: “TCP”, “UDP”, etc.» Extensions header: Chained together» Makes it easy to implement host-based options» One value “hop-by-hop” examined by intermediate routers– Things like “source route” implemented only at intermediate hopsIPv6 Fragmentation Cleanupz IPv4:z IPv6:» Discard packets, send ICMP “Packet Too Big”– Similar to IPv4 “Don’t Fragment” bit handlingLarge MTUSmall MTURouter must fragment11» Sender must support Path MTU discovery– Receive “Packet too Big” messages and send smaller packets» Increased minimum packet size– Link must support 1280 bytes;– 1500 bytes if link supports variable sizesz Reduced packet processing and network complexity.z Increased MTU a boon to application writersz Hosts can still fragment - using fragmentation header. Routers don’t deal with it any more.Migration from IPv4 to IPv6z Interoperability with IP v4 is necessary for gradual deployment.z Two complementary mechanisms:» dual stack operation: IP v6 nodes support both 12address types» tunneling: tunnel IP v6 packets through IP v4 cloudsz Alternative is to create IPv6 islands, e.g. corporate networks, ...» Use of form of NAT to connect to the outside world» NAT must not only translate addresses but also translate between IPv4 and IPv6 protocols2/23/20083IPv6 Discussionz IPv4 Infrastructure got better» Address efficiency» Co-opted IPv6 ideas: IPSec, diffserv, autoconfiguration via DHCP, etc.Mi hll13zMassive challenge» Huge installed base of IPv4-speaking devices» Chicken & Egg problem– Who’s the first person to go IPv6-only?z Steady progress in deployment.» Most hosts & big routers support.» Long-term: The little devices will probably force IPv6– Used now on many mobile phones in JapanNATs and Tunnelsz NATs originally invented as a way to help migrate to a hybrid IPv4 IPv6 world» Took on a life of their own»May have substantially delayed IPv6 deployment by14»May have substantially delayed IPv6 deployment by reducing address pressure!»


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