Network Protocols Internet Protocol IP TDC375 Autumn 03 04 John Kristoff DePaul University 1 Will layer 2 networking suffice TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 IP routing Scope Dynamic routing protocol for route exchange and computation Static routing autonomous system AS interior exterior manually configured routes Destination address driven TDC375 Autumn 03 04 John Kristoff DePaul University 1 Internet Protocol IP Documented in IETF RFC 791 Connectionless Unreliable Simple relatively The thin waist in the TCP IP suite hourglass model TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 IP address types Unicast one to one Multicast one to many receivers join listen to group destination address Broadcast one to all source addresses should always be unicast special case of multicast usually unnecessary Anycast one to one of many usually one to nearest often used for reliability TDC375 Autumn 03 04 John Kristoff DePaul University 1 IP address notation TDC375 Autumn 03 04 John Kristoff DePaul University 1 Special IP addresses TDC375 Autumn 03 04 John Kristoff DePaul University 1 Classful IP addressing TDC375 Autumn 03 04 John Kristoff DePaul University 1 Classful address sizes TDC375 Autumn 03 04 John Kristoff DePaul University 1 Example IP network TDC375 Autumn 03 04 John Kristoff DePaul University 1 Example IP router addressing TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 IP addressing solutions Subnetting Supernetting Classless interdomain routing CIDR Variable length subnet masks VLSM TDC375 Autumn 03 04 John Kristoff DePaul University 1 Subnetting TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 IP datagram layout TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 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 TDC375 Autumn 03 04 John Kristoff DePaul University 1 Inside an IP datagram cont