1/30/2019 Outline • IP design goals 15-441/641: Computer Networks • Traditional IP addressing Intradomain Routing • Addressing approaches • Class-based addressing 15-441 Spring 2019 Profs Peter Steenkiste & Justine Sherry • Subnetting • CIDR • Packet forwarding Fall 2019 https://computer-networks.github.io/sp19/ 2 So far you know how to build a Local Logical Structure of an Internet Area Network host router router host router How do we get them to router talk to each other? router router • Interconnection of separately managed networks using routers • Individual networks can use different (layer 1-2) technologies • Packet travels from source to destination by hopping through networks • “Network” layer responsibility • How do routers connect heterogeneous network technologies? 4 3 1
1/30/2019 The Packet as an Envelope Solution: Internet Protocol (IP) • Inter-network connectivity provided by But need a local Local Address Header (Ethernet) the Internet protocol addressing header to To: Destination Host From: Sender Host • Hosts use Internet Protocol to send travel between routers Network email WWW phone... packets destined across networks. Applications SMTP HTTP RTP... IP Header IP address identifies TCP UDP… To: 123.45.67.89 (Destination Host) • IP creates abstraction layer that hides final destination IP From: 169.229.49.157 (Sender Host) underlying technology from network ethernet PPP… HTTP Packet Payload: application software CSMA async sonet... Network GET nyan.cat…. copper fiber radio... Host wants to send… Allows range of current & future technologies Technology • WiFi, traditional and switched Ethernet, • personal area networks, … 5 6 Traveling through the Internet The Packet as an Envelope Set of envelopes • Source adds all headers (HTTP, transport -> PHY) • Each router: Datalink Header Local Address Header (Token Ring) But need a local Datalink headers may • Removes datalink layer To: Next hop router addressing header to To: Destination Host differ across networks From: This router From: Local Router travel between routers • Uses IP header to make forwarding decision IP Header IP Header • Adds data link layer header for next network IP address identifies But IP header To: 123.45.67.89 To: 123.45.67.89 (Destination Host) • Destination removes all headers (PHY -> HTTP) final destination remains unchanged! From: 169.229.49.157 From: 169.229.49.157 (Sender Host) host HTTP Packet Payload: HTTP Packet Payload: router GET nyan.cat…. GET nyan.cat…. Host wants to send… router host router router router router 7 2
1/30/2019 Protocol Demultiplexing What are the Goals? • LANs: “Connect hosts” switching: • “Wire” abstraction: behaves like Ethernet – helps manageability FTP HTTP SMTP DNS • Only has to scale up a “LAN size” TCP UDP TCP HTTP: • Availability Datalink IP UDP GET .. • Internet: “Connect networks” routing: IP Type Protocol Port • Scalability Field Field Number Ether LTE … WiFi • Manageability of individual networks – contributes to scalability • Availability • Affects addressing, protocols, routing ● What layers do not need a protocol field? 9 10 Outline Addressing and Forwarding • Flat address space with smart routers • IP design goals • Packets carry destination • Traditional IP addressing • Routers know location of every host • Addressing approaches • Flat address space with dumb routers • Class-based addressing • Packet carries a path • Subnetting • Heirarchical Routing Space • CIDR • What we actually do in IP • (Table of virtual circuits ids) • Packet forwarding • More on this later, but not today 11 12 3
1/30/2019 Flat Address Forwarding Flat Address Forwarding Why is this not a good solution for the Internet? MAC MAC Bridge Bridge Address Address 1 Port Age 1 Port Age 1 1 36 36 A21032C9A591 A21032C9A591 2 2 3 3 Each router tracking 2 2 01 01 99A323C90842 99A323C90842 2 15 2 15 8711C98900AA 8711C98900AA 2^32 addresses = 2 2 16 16 301B2369011C 301B2369011C scalability nightmare • Bridge/switch has a table that shows for each • Bridge/switch has a table that shows for each 3 3 11 11 695519001190 695519001190 MAC Address which port to use for forwarding MAC Address which port to use for forwarding • For every packet, the bridge “looks up” the entry for the packets • For every packet, the bridge “looks up” the entry for the packets destination MAC address and forwards the packet on that port. destination MAC address and forwards the packet on that port. Other packets are broadcast – why? Other packets are broadcast – why? • • • Timer is used to flush old entries • Timer is used to flush old entries 14 16 Hierarchical Addressing Source Routing • Flat addresses – one address for every host • List entire path in packet • Peter Steenkiste: 123-45-6789 • Driving directions (north 3 hops, east, etc..) • Does not scale – router table size explodes • Router processing • 630M (1/09) entries, doubling every 2.5 years • Why does it work for Ethernet? • Strip first step from packet • Hierarchical – add structure • Examine next step in directions and forward • Pennsylvania / Pittsburgh / Oakland / CMU / Gates / 9 th fl / Steenkiste • Defined for IPv4 but rarely used • Common “trick” to simplify forwarding, reduce forwarding table • End points need to know a lot about network • What type of Hierarchy do we need for the Internet? • How many levels? • Economic and security concerns • Same hierarchy depth for everyone? • Variable header size • Who controls the hierarchy? 15 16 4
1/30/2019 IP Addresses (IPv4) Hierarchy in IP Addressing • 32 bits are partitioned into a prefix and suffix components • Unique 32-bit number associated with a host • Prefix is the network component: CMU 00001100 00100010 10011110 00000101 • Suffix is host component: Prof. Sherry’s laptop at CMU 12 34 158 5 • Represented with the “dotted quad” notation • e.g., 12.34.158.5 00001100 00100010 10011110 00000101 12 34 158 5 Network (23 bits) Host (9 bits) • Interdomain routing operates on the network prefix 00001100 00100010 10011110 00000101 • Destination network operates on the host component 19 21 History of Internet Addressing Original Internet Addresses • First eight bits: network component • Always dotted-quad notation • Last 24 bits: host component • Always network/host address split Assumed 256 networks were more than enough! • But nature of that split has changed over time 22 23 5
1/30/2019 IP Address Structure, ca 1981 IP Route Lookup, ca 1981 Routers know how to get to network ID, but not individual hosts. Address specifies prefix for forwarding table • Extract address type and network ID • Network ID Host ID Forwarding table structure reflects address structure • 8 16 24 32 Logically, a separate forwarding table for each address class Class A • 0 Network ID Host ID For unicast address (classes A-C) entries contain • The prefix for a destination network (length 8/16/24) Class B • 10 Information on how to forward the packet, e.g., exit port, .. • Class C 110 www.cmu.edu address 128.2.11.43 • Class B address – class + network is 128.2 • Class D Multicast Addresses 1110 Lookup 128.2 in forwarding table for class B • Tables are still large! • Class E 1111 Reserved for experiments 2 Million class C networks • 24 23 Subnetting Outline • Add another layer to hierarchy • IP design goals • Variable length subnet masks • Could subnet a network internally into several chunks • Traditional IP addressing • Subnetting is done internally in the organization • Addressing approaches • It is not visible outside – important for management • Class-based addressing • Subnetting Network Host • CIDR Network Subnet Host • Packet forwarding Subnet 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 Mask 24 25 6
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