15-441 Computer NetworkingOutlineIPv4 Header – RFC791 (1981)IP Header FieldsSlide 5IP Type of ServiceICMP: Internet Control Message ProtocolFragmentationIP Fragmentation & ReassemblyReassemblyFragmentation Related FieldsIP Fragmentation and ReassemblyFragmentation is HarmfulPath MTU DiscoverySlide 15IP Address Utilization (‘98)Network Address Translation (NAT)NAT IllustrationProblems with NATSlide 20Slide 21IPv6IPv6 ChangesIPv6 HeaderSlide 25Slide 26Transition From IPv4 To IPv6Dual Stack ApproachTunnelingSlide 30Router Architecture OverviewWhat Does a Router Look Like?Router ProcessingNetwork ProcessorThree Types of Switching FabricsSwitching Via MemorySwitching Via BusSwitching Via An Interconnection NetworkSwitch Design IssuesInput Port FunctionsOutput PortsSwitch BufferingInput Port QueuingOutput Port QueuingVirtual Output QueuingSlide 46How To Do Variable Prefix MatchSpeeding up Prefix Match - AlternativesSpeeding up Prefix MatchPrefix TreeSlide 51Slide 52Slide 5315-441 Computer NetworkingLecture 8 – IP Packets, RoutersLecture 8: 9-20-01 2Outline•IP Packet Format•NAT•IPv6•Router Internals•Route LookupLecture 8: 9-20-01 3IPv4 Header – RFC791 (1981)verlength32 bitsdata (variable length,typically a TCP or UDP segment)16-bit identifierHeader checksumtime tolive32 bit source IP addressheaderlengthtype ofserviceflagsfragment offsetProtocol32 bit destination IP addressOptions (if any)0 4 16 24 328 19Padding (if any)Lecture 8: 9-20-01 4IP Header Fields•Version 4 for IPv4•Header length (in 32 bit words)•Minimum value is 5 (header without any options)•Length of entire IP packet in octets (including header)•Identifier, flags, fragment offset used primarily for fragmentation•Time to live•Must be decremented at each router•Packets with TTL=0 are thrown away•Ensure packets exit the networkLecture 8: 9-20-01 5IP Header Fields•Protocol•Demultiplexing to higher layer protocols•TCP = 6, ICMP = 1, UDP = 17…•Header checksum•Ensures some degree of header integrity•Relatively weak – 16 bit•Source/Dest address•Options•E.g. Source routing, record route, etc.•Performance issues•Poorly supportedLecture 8: 9-20-01 6IP Type of Service•Typically ignored•Values•3 bits of precedence•1 bit of delay requirements•1 bit of throughput requirements•1 bit of reliability requirements•Replaced by DiffServLecture 8: 9-20-01 7ICMP: Internet Control Message Protocol•Used by hosts, routers, gateways to communication network-level information•Error reporting: unreachable host, network, port, protocol•Echo request/reply (used by ping)•Network-layer “above” IP:•ICMP msgs carried in IP datagrams•ICMP message: type, code plus first 8 bytes of IP datagram causing errorType Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP headerLecture 8: 9-20-01 8Fragmentation•IP packets can be up to 64KB•Different link-layers have different MTUs •Split IP packet into multiple fragments•IP header on each fragment•Intermediate router may fragment as neededLecture 8: 9-20-01 9IP Fragmentation & Reassembly•Network links have MTU (max.transfer size) - largest possible link-level frame.•different link types, different MTUs •Large IP datagram divided (“fragmented”) within net•one datagram becomes several datagrams•IP header bits used to identify, order related fragmentsfragmentation: in: one large datagramout: 3 smaller datagramsreassemblyLecture 8: 9-20-01 10Reassembly•Where to do reassembly?•End nodes•Avoids unnecessary work where large packets are fragmented multiple times•Dangerous to do at intermediate nodes•How much buffer space required at routers?•What if routes in network change?•Multiple paths through network•All fragments only required to go through destinationLecture 8: 9-20-01 11Fragmentation Related Fields•Length•Length of IP fragment•Identification •To match up with other fragments•Flags•Don’t fragment flag•More fragments flag•Fragment offset•Where this fragment lies in entire IP datagram•Measured in 8 octet units (13 bit field)Lecture 8: 9-20-01 12IP Fragmentation and ReassemblyID=xoffset=0fragflag=0length=4000ID=xoffset=0fragflag=1length=1500ID=xoffset=1480fragflag=1length=1500ID=xoffset=2960fragflag=0length=1040One large datagram becomesseveral smaller datagramsLecture 8: 9-20-01 13Fragmentation is Harmful•Uses resources poorly•Forwarding costs per packet•Best if we can send large chunks of data•Worst case: packet just bigger than MTU•Poor end-to-end performance•Loss of a fragment •Reassembly is hard•Buffering constraintsLecture 8: 9-20-01 14Path MTU Discovery•Hosts dynamically discover minimum MTU of path •Algorithm:•Initialize MTU to MTU for first hop•Send datagrams with Don’t Fragment bit set•If ICMP “pkt too big” msg, decrease MTU•What happens if path changes?•Periodically (>5mins, or >1min after previous increase), increase MTU•Some routers will return proper MTU•MTU values cached in routing tableLecture 8: 9-20-01 15Outline•IP Packet Format•NAT•IPv6•Router Internals•Route LookupLecture 8: 9-20-01 16IP Address Utilization (‘98)•Address space depletion•In danger of running out of classes A and B•32-bit address space completely allocated by 2008•Two solutions•NAT•IPv6Lecture 8: 9-20-01 17Network Address Translation (NAT)•Possible solution to address space exhaustion•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)•Uses special unallocated addresses (RFC 1597) locally•10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16Lecture 8: 9-20-01 18NAT 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•How
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