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Addressing

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Addressing

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  1. Addressing Jennifer Rexford Advanced Computer Networks http://www.cs.princeton.edu/courses/archive/fall06/cos561/ Tuesdays/Thursdays 1:30pm-2:50pm

  2. Goals of the Course • Study networked systems • Large in size and scope • Heterogeneous components • Decentralized control • Explore design trade-offs • Scalability, performance, reliability, flexibility, … • And design principles • Layering, end-to-end argument, late binding, hierarchy, randomization, indirection, caching, … • Today’s class: addressing

  3. What is Addressing? • Providing suitable identifiers to nodes • So you can direct data to a node • So you know which node sent the data • … and how to send data back to that node • Addressing in the U.S. mail • Zip code: 08540 • Street: Olden Street • Building on street: 35 • Room in building: 306 • Name of occupant: Jennifer Rexford ???

  4. Phone Numbers • Hierarchical • Country code (1) • Area code (609) • Local exchange (258) • Subscriber number (5182) • Some exceptions • 800: indirection service (free for the caller) • 900: indirection service (billed to the caller) • Cell phone numbers, where the node is mobile • ... blurring distinction between name and address

  5. Overview of Today’s Lecture • Two widely-used addressing schemes • Medium Access Control (MAC) addresses • Internet Protocol (IP) addresses • Key concepts in addressing • Number of unique addresses • Allocating addresses to nodes • Flat vs. hierarchical structure • Persistent vs. temporary identifiers • Handling diminishing address space • Spoofing of source addresses • Discussion of the Cerf/Kahn 1974 paper

  6. Some Questions • Could every host on the Internet have an arbitrary, unique numerical address? • Would it scale? • If hierarchy is necessary, how to do it? • Tying the addressing to the topology & routing? • What about mobile hosts? Temporary addresses? • Who should allocate the addresses? • Network provider? Device manufacturer? • Does the sender of the traffic need to authenticate itself? The destination? • What about spoofing and impersonation?

  7. Comparing MAC and IP Addresses E.g., social security number vs. postal address

  8. MAC Addresses

  9. MAC Addresses • Flat name space of 48 bits • Typically written in six octets in hex • E.g., 00-15-C5-49-04-A9 for my Ethernet • Organizationally unique identifier • Assigned by IEEE Registration Authority • Determines the first 24 bits of the address • E.g., 00-15-C5 corresponds to “Dell Inc” • Remainder of the MAC address • Allocated by the manufacturer • E.g., 49-04-A9 for my Ethernet card

  10. Scalability Challenges • MAC addresses are flat • Multiple hosts on the same network • No relationship between MAC addresses • Data plane • Forwarding based on MAC address • Table size? Look-up overhead? • Control plane • Determining where the host is located • Keeping the information up-to-date

  11. Forwarding Frames to Destination Adapter • Shared media • Forward all frames on the shared media • Adapter grabs frames with matching dest address • Multi-hop switched networks • Flood every frame over every link? • Learn where the MAC address is located? ... host host host host host host host

  12. When to Learn? • When the adapter connects to the network? • Requires adaptor to register its presence • Overhead even when not sending/receiving • Leading to control messages and large tables • When the adapter sends a frame? • Source MAC address is in the frame • Allows switch to learn about the adapter • When the adapter needs to receive a frame? • Destination MAC address is in the frame • Switch needs to figure out how to get there

  13. Motivation For Self Learning • Switches forward frames selectively • Forward frames only on segments that need them • Switch table • Maps dest MAC address to outgoing interface • Goal: construct the switch table automatically B A C switch D

  14. Self Learning: Building the Table • When a frame arrives • Inspect the source MAC address • Associate the address with the incoming interface • Store the mapping in the switch table • Use a TTL field to eventually forget the mapping B Switch learns how to reach A. A C D

  15. Self Learning: Handling Misses • When frame arrives with unfamiliar dest • Forward the frame out all of the interfaces • … except for the one where the frame arrived • Hopefully, this case won’t happen very often Switch floods frame that is destined to C. B A C D

  16. When switch receives a frame: index switch table using MAC dest address if entry found for destinationthen { if dest on segment from which frame arrivedthen drop the frame else forward the frame on interface indicated } else flood Switch Filtering/Forwarding forward on all but the interface on which the frame arrived

  17. MAC Addresses • Disadvantages • Large forwarding tables in the data plane • Flooding overhead to learn location information • Lack of privacy • Advantages • Persistent identifier (well, except for spoofing) • Mobile hosts are easy to handle • Forwarding-table look-up is a simple match

  18. COS 461: Internet Control Protocols (#8) • Dynamic Host Configuration Protocol (DHCP) • End host learns how to send packets • Learn IP address, DNS servers, and gateway • Address Resolution Protocol (ARP) • Others learn how to send packets to the end host • Learn mapping between IP and MAC addresses ??? 1.2.3.7 1.2.3.156 ... ... host DNS host DNS host host 5.6.7.0/24 1.2.3.0/24 1.2.3.19 router router router

  19. COS 461: Hubs and Switches (#11) • Different devices switch different things • Physical layer: electrical signals (repeaters, hubs) • Link layer: frames (bridges, switches) • Network layer: packets (routers) • Key ideas in switches • Self learning of the switch table • Cut-through switching • Spanning trees • Virtual LANs (VLANs) Application gateway Transport gateway Router Bridge, switch Repeater, hub Frameheader Packetheader TCPheader User data

  20. IP Addresses

  21. IP Addressing: Scalability Through Hierarchy • Hierarchy through IP prefixes • Routing between networks • Allocation of address blocks • Non-uniform hierarchy • More efficient address allocation • More complex packet forwarding • Dealing with limited address space • Larger address space (IPv6 with 128 bits) • Sharing a small set of addresses (NAT) • Dynamic assignment of addresses (DHCP)

  22. Grouping Related Hosts • The Internet is an “inter-network” • Used to connect networks together, not hosts • Needs a way to address a group of hosts ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN LAN = Local Area Network WAN = Wide Area Network

  23. forwarding table Scalability Challenge • Suppose hosts had arbitrary IP addresses • Then every router would need a lot of information • …to know how to direct packets toward the host 1.2.3.4 5.6.7.8 2.4.6.8 1.2.3.5 5.6.7.9 2.4.6.9 ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN 1.2.3.4 1.2.3.5

  24. 00001100 00100010 10011110 00000101 Hierarchy Through Prefixes • Divided into network and host portions • 12.34.158.0/24 is 24-bit prefix (28 addresses) 12 34 158 5 Network (24 bits) Host (8 bits)

  25. 11111111 00001100 00100010 11111111 10011110 11111111 00000101 00000000 Example IP Address and Subnet Mask Address 12 34 158 5 255 255 255 0 Mask

  26. Scalability Improved • Number related hosts from a common subnet • 1.2.3.0/24 on the left LAN • 5.6.7.0/24 on the right LAN 1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212 ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN 1.2.3.0/24 5.6.7.0/24 forwarding table

  27. Easy to Add New Hosts • No need to update the routers • E.g., adding a new host 5.6.7.213 on the right • Doesn’t require adding a new forwarding entry 1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212 ... ... host host host host host host LAN 2 LAN 1 router router router host WAN WAN 5.6.7.213 1.2.3.0/24 5.6.7.0/24 forwarding table

  28. Classful Addressing (and Dotted Quad Notation) • In the olden days… • Class A: 0* • Very large /8 blocks (e.g., MIT has 18.0.0.0/8) • Class B: 10* • Large /16 blocks (e.g,. Princeton has 128.112.0.0/16) • Class C: 110* • Small /24 blocks (e.g., AT&T Labs has 192.20.225.0/24) • Class D: 1110* • Multicast groups • Class E: 11110* • Reserved for future use (sounds a bit scary…) • And then, address space became scarce…

  29. 00001100 00000100 00000000 00000000 Address 11111111 11111110 00000000 00000000 Mask Network Prefix for hosts Classless Inter-Domain Routing (CIDR) Use two 32-bit numbers to represent a network. Network number = IP address + Mask IP Address : 12.4.0.0 IP Mask: 255.254.0.0 Usually written as 12.4.0.0/15

  30. 12.0.0.0/16 : : : 12.1.0.0/16 12.3.0.0/24 12.2.0.0/16 12.3.1.0/24 : : 12.3.0.0/16 : : : 12.0.0.0/8 12.3.254.0/24 12.253.0.0/19 12.253.32.0/19 12.253.64.0/19 12.253.0.0/16 12.253.96.0/19 12.254.0.0/16 12.253.128.0/19 12.253.160.0/19 12.253.192.0/19 CIDR = Hierarchy in Address Allocation • Prefixes are key to Internet scalability • Routing protocols and packet forwarding based on prefixes • Today, routing tables contain ~150,000-200,000 prefixes

  31. Obtaining a Block of Addresses • Separation of control • Prefix: assigned to an institution • Addresses: assigned to nodes by the institution • Who assigns prefixes? • Internet Corp. for Assigned Names and Numbers • Allocates large blocks to Regional Internet Registries • Regional Internet Registries (RIRs) • E.g., ARIN (American Registry for Internet Numbers) • Allocated to ISPs and large institutions in a region • Internet Service Providers (ISPs) • Allocate address blocks to their customers • Who may, in turn, allocate to their customers…

  32. whois –h whois.arin.net 128.112.136.35 OrgName: Princeton University OrgID: PRNU Address: Office of Information Technology Address: 87 Prospect Avenue City: Princeton StateProv: NJ PostalCode: 08544-2007 Country: US NetRange: 128.112.0.0 - 128.112.255.255 CIDR: 128.112.0.0/16 NetName: PRINCETON NetHandle: NET-128-112-0-0-1 Parent: NET-128-0-0-0-0 NetType: Direct Allocation RegDate: 1986-02-24

  33. Longest Prefix Match Forwarding • Forwarding tables in IP routers • Maps each IP prefix to next-hop link(s) • Destination-based forwarding • Packet has a destination address • Router identifies longest-matching prefix • Pushing complexity into forwarding decisions forwarding table 4.0.0.0/8 4.83.128.0/17 12.0.0.0/8 12.34.158.0/24 126.255.103.0/24 destination 12.34.158.5 outgoing link Serial0/0.1

  34. Are 32-bit Addresses Enough? • Not all that many unique addresses • 232 = 4,294,967,296 (just over four billion) • Plus, some are reserved for special purposes • And, addresses are allocated in larger blocks • And, many devices need IP addresses • Computers, PDAs, routers, tanks, toasters, … • Long-term solution: a larger address space • IPv6 has 128-bit addresses (2128 = 3.403 × 1038)

  35. 138.76.29.7 10.0.0.1 outside NAT inside 10.0.0.2 Short-Term Solutions: Limping Along • Network Address Translation (COS 461 lecture #9) • Allowing multiple hosts to share an IP address • IP addresses not unique and not end-to-end

  36. DHCP discover (broadcast) DHCP server 233.1.2.5 DHCP offer arrivingclient DHCP request (broadcast) DHCP ACK Short-Term Solutions: Limping Along • Dynamic Host Configuration Protocol (lecture #8) • Share a pool of addresses among many hosts • Dynamically assign an IP address upon request

  37. Growth in the Number of IP Prefixes Internet bust Internet boom recovery? CIDR pre-CIDR

  38. “A Protocol for Packet Network Intercommunication”(IEEE Trans. on Communications, May 1974) Vint Cerf and Bob Kahn Written when Vint Cerf was an assistant professor at Stanford, and Bob Kahn was working at ARPA.

  39. Life in the Early 1970s • Multiple unconnected networks • ARPAnet • Data-over-cable • Packet satellite (Aloha) • Packet radio satellite net ARPAnet

  40. Differences Across Packet-Switched Networks • Addressing • Maximum packet size • Timing for handling success/failure of delivery • Handling of lost or corrupted data • Routing, fault detection, status information, … satellite net ARPAnet

  41. Where to Handle Heterogeneity? • Application process? • End host? • Packet switches? • Someplace else? • Compatible process and host conventions • Obviate the need to support all combinations • Retain the unique features of each network • Avoid changing the local network components • Introduce the notion of a gateway

  42. Gateway “Embed internetwork packets in local packet format or extract them” Route (at internetwork level) to next gateway Gateways Between Different Kinds of Networks Internetwork layer • Internetwork appears as a single, uniform entity • Despite the heterogeneity of the local networks • Network of networks gateway satellite net ARPAnet

  43. source address dest. address seq. # byte count flag field Internetwork Packet Format internetwork header • Internetwork header in standard format • Interpreted by the gateways and end hosts • Source and destination addresses • Uniformly and uniquely identify every host • Ensure proper sequencing of the data • Include a sequence number and byte count • Enable detection of corrupted text • Checksum for an end-to-end check on the text local header checksum text

  44. Process-Level Communication • Enable pairs of processes to communicate • Full duplex • Unbounded but finite-length messages • E.g., keystrokes or a file • Key ideas • Port numbers to (de)multiplex packets • Breaking messages into segments • Sequence numbers and reassembly • Retransmission and duplicate detection • Window-based flow control

  45. Differences in Max Packet Size • Select smallest packet size as the new max? • Coordinate to determine max size on a path? • Enable gateway to fragment a large packet? • Reassembly by the next gateway? The receiver? • Design trade-offs • Coordination overhead for identifying the max • Overhead of sending many small packets • Overhead of buffering packets for reassembly

  46. Discussion • What did they get right? • Which ideas were key to the Internet’s success? • Which decisions still seem right today? • What did they miss? • Which ideas had to be added later? • Which decisions seem wrong in hindsight? • What would you do in a clean-slate design? • If your goal wasn’t to support communication between disparate packet-switched networks • Would you do anything differently?