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Network Protocols

Network Protocols. Internet Protocol (IP). Will layer 2 networking suffice?. Layer 3 usually provides. Internetworking for data link technologies Globally unique addresses Scalable routing A common communications format Packet fragmentation capability A hardware independent interface

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Network Protocols

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  1. Network Protocols • Internet Protocol (IP)

  2. Will layer 2 networking suffice?

  3. Layer 3 usually provides... • Internetworking for data link technologies • Globally unique addresses • Scalable routing • A common communications format • Packet fragmentation capability • A hardware independent interface • Packet independence

  4. An IP router (or gateway)... • Is usually a special purpose, dedicated device • Connects heterogenous networks • Directs packets toward their ultimate destination • Often uses dynamic routing algorithms/protocols • which make automatic forwarding decisions • which make decisions based on various metrics • Official pronounciation is rooter • layer 3 switch = router = layer 3 switch

  5. IP routing • Scope • autonomous system (AS), interior, exterior • Dynamic routing • protocol for route exchange and computation • Static routing • manually configured routes • Destination address driven

  6. Internet Protocol (IP) • Documented in IETF RFC 791 • Connectionless • Unreliable • Simple (relatively) • The thin waist in the TCP/IP suite hourglass model

  7. IP address • Virtual, not specific to a hardware device • 32-bit fixed address length (IPv4) • Unique address for each interface (typically) • Global registrar assigns network bits (prefix) • Local administrator assigns host bits (suffix) • Usually written in dotted decimal (dotted quad) • e.g. 140.192.5.1

  8. IP address types • Unicast (one-to-one) • source addresses should always be unicast • Multicast (one-to-many) • receivers join/listen to group destination address • Broadcast (one-to-all) • special case of multicast, usually unnecessary • Anycast (one-to-one-of-many) • usually one-to-nearest, often used for reliability

  9. IP address notation

  10. Special IP addresses

  11. Classful IP addressing

  12. Classful address sizes

  13. Example IP network

  14. Example IP router addressing

  15. Classful addressing limitations • Internet growth • Route table size • Address depletion • Misappropriation of addresses • Lack of support for varying sized networks • Class B is often too big, Class C often too small

  16. IP addressing solutions • Subnetting • Supernetting • Classless interdomain routing (CIDR) • Variable length subnet masks (VLSM)

  17. Subnetting

  18. Subnet masks • The bit length of the prefix (network bits) • Prefix (network bits) no longer classful (fixed size) • Use of the slash '/' notation to represent addresses • 140.192.5.1 with mask of 255.255.255.128 is: • 140.192.5.1/25 • As viewed in binary for clarity, a /25 mask is: • 11111111.11111111.11111111.10000000

  19. Subnet masks example • Given 140.192.50.8/20 what is the... • subnet mask in dotted decimal notation? • directed broadcast address in dotted quad? • total number of hosts that can be addressed?

  20. Supernetting • Combine smaller address blocks into an aggregate • If class B is too big and class C is too small... • Combine 199.63.0.0/24 to 199.63.15.0/24 • To form 199.63.0.0/20

  21. Supernetting example • Given an ISP that has 128.15.0.0/16: • what block might be assigned to a customer needing to address 300 hosts? • how does the ISP manage their IP address allocation if there are many customers with varying address requirements?

  22. CIDR • Routers using aggregated prefixes (CIDR blocks) • primarily through the use of supernetting • So instead of adding multiple class C blocks... • ...advertise some larger aggregate, e.g. /20 • The Internet CIDR report: • http://www.cidr-report.org

  23. CIDR example • Given an ISP that announces: • 64.5.0.0/20 • 64.5.16.0/20 • 192.0.2.0/25 • 192.0.2.192/26 • 192.0.2.128/26 • What is the least number of CIDR announcements that can be made for this ISP? • Why might address blocks not be aggregated?

  24. VLSM • Many subnet sizes in an autonomous system (AS) • Allows for efficient use of address space • Can be used to build an internal hierarchy • External view of the AS does not change • An AS may be allocated 140.192.0.0/16, but... • internally may use: • 140.192.0.0/17 • 140.192.128.0/24 • 140.192.129.0/25 and so on...

  25. VLSM example • Given an assignment of 140.192.0.0/16, create an addressing strategy to support: • 6 satellite offices and 1 large headquarter site • 6000 total hosts on all combined networks • headquarters needs about 50% of all addresses • satellite offices need 200 to 700 addresses • overall growth per year is 500 hosts

  26. Obtaining IP addresses • IANA has global authority for assignment • Regional registries delegate to ISPs and large nets • ISPs assign addresses to end users • RFC 1918 defines private address blocks • NOT globally unique • NOT for hosts attached directly to public Internet • 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 • You will understand RFC 1918 consequences

  27. IP datagram layout

  28. Inside an IP datagram • Version field • usually set to binary 0100 (is what decimal?) • Header length • length of IP header in 32-bit words • typically set to 5 (as in 5 octets) • Type of Service (redefined in newer RFCs) • an indication of quality/class of service • rarely used with success outside a single AS

  29. Inside an IP datagram [cont.] • Total length • total IP datagram length in octets • maximum value is 65535, but rarely > 1500 • Identification • to identify fragments of a single IP datagram • experimentally used in tracing DDoS sources • Flags • bit 0 reserved, others for fragmentation (or not)

  30. Inside an IP datagram [cont.] • Fragment offset • helps piece together IP fragments • Time to live (TTL) • limts the time/hops of datagram in the net • counts down to zero, at zero it is discarded • Protocol type • indicates next layer protocol in payload

  31. Inside an IP datagram [cont.] • Header checksum • used to verify header validity at each hop • Source/Destination address • 32-bit IP address • Options (optional) • rarely used, padded to 32-bit boundary if needed • Payload • variable length

  32. Basic tools • ping • traceroute • arp • route • netstat • packet capture • tcpdump, ethereal • routing table monitors, looking glass servers

  33. Final thoughts • IP is unreliable, connectionless • IP addressing is a pain, wait til you see IPv6 • IP addresses today are both a who and a where • IP addresses make for poor trust relationships • IP fragmentation is generally best avoided • Private IPs and NATs, yuck!

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