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ECE 4450:427/527 - Computer Networks Spring 2014

ECE 4450:427/527 - Computer Networks Spring 2014. Dr. Nghi Tran Department of Electrical & Computer Engineering. Lecture 6.2: IP. Internetworking: Discussions. For Internetworking , we shall look at few sub-problems: Interconnect links of the same type : Switches

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ECE 4450:427/527 - Computer Networks Spring 2014

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  1. ECE 4450:427/527 - Computer NetworksSpring 2014 Dr. Nghi Tran Department of Electrical & Computer Engineering Lecture 6.2: IP ECE 4450:427/527

  2. Internetworking: Discussions • For Internetworking, we shall look at few sub-problems: • Interconnect links of the same type: Switches • We consider an important of class switch: Bridges to interconnect Ethernet segments. • We also look a way to interconnect disparate networks and links: Gateways, or now mostly known as routers. We shall focus on the IP • Once we are able to interconnect a whole lot of links and networks with switches and routers, we will look at a way to find a suitable path, or route through a new working: • Paths that are efficient, loop free, etc.: Routing ECE 4450:427/527

  3. Internetworking • What is internetwork • An arbitrary collection of networks interconnected to provide some sort of host-host to packet delivery service A simple internetwork where H represents hosts and R represents routers ECE 4450:427/527

  4. IP: Internet Protocol • What is IP • IP stands for Internet Protocol • Key tool used today to build scalable, heterogeneous internetworks • It runs on all the nodes in a collection of networks and defines the infrastructure that allows these nodes and networks to function as a single logical internetwork A simple internetwork showing the protocol layers A simple internetwork showing the protocol layers ECE 4450:427/527

  5. IP: “Best Effort” Service • IP is a datagram connectionless protocol • Does not provide any type of guarantee about packet delivery • Out of order packet deliveries, duplicate packets, no error correction • If packets are lost, IP does not try to recover or retransmit (though lower or higher layer functionalities may do so) • Global addressing scheme ECE 4450:427/527

  6. IPv4: Packet Format 4-bit Header Length 8-bit Type of Service (TOS) 4-bit Version 16-bit Total Length (Bytes) 3-bit Flags 16-bit Identification 13-bit Fragment Offset 8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum 32-bit Source IP Address 32-bit Destination IP Address Options (if any) Payload ECE 4450:427/527

  7. IP Packet Format • Version number (4 bits) • Indicates the version of the IP protocol • Necessary to know what other fields to expect • Typically “4” (for IPv4), and sometimes “6” (for IPv6) • Header length (4 bits) • Number of 32-bit words in the header • Typically “5” (for a 20-byte IPv4 header) • Can be more when “IP options” are used • Type-of-Service (8 bits) • Allow packets to be treated differently based on needs • E.g., low delay for audio, high bandwidth for bulk transfer ECE 4450:427/527

  8. IP Packet Format • Total length (16 bits) • Number of bytes in the packet • Maximum size is 63,535 bytes (216 -1) • … though underlying links may impose harder limits • Fragmentation information (32 bits) • Packet identifier, flags, and fragment offset • Supports dividing a large IP packet into fragments • … in case a link cannot handle a large IP packet • Time-To-Live (8 bits) • Used to identify packets stuck in forwarding loops • … and eventually discard them from the network ECE 4450:427/527

  9. Time-to-Live (TTL) • Potential robustness problem • Forwarding loops can cause packets to cycle forever • Confusing if the packet arrives much later • Time-to-live field in packet header • TTL field decremented by each router on the path • Packet is discarded when TTL field reaches 0… • …and “time exceeded” message is sent to the source ECE 4450:427/527

  10. Protocol Field • Protocol (8 bits) • Identifies the higher-level protocol • E.g., “6” for the Transmission Control Protocol (TCP) • E.g., “17” for the User Datagram Protocol (UDP) • Important for demultiplexing at receiving host • Indicates what kind of header to expect next protocol=6 protocol=17 IP header IP header TCP header UDP header ECE 4450:427/527

  11. IP Addresses Field • Two IP addresses • Source IP address (32 bits) • Destination IP address (32 bits) • Destination address • Unique identifier for the receiving host • Allows each node/router to make forwarding decisions • Source address • Unique identifier for the sending host • Recipient can decide whether to accept packet • Enables recipient to send a reply back to source ECE 4450:427/527

  12. IP Fragmentation and Reassembly • Each network has some MTU (Maximum Transmission Unit): largest IP datagram it can carry in a frame • Ethernet (1500 bytes), FDDI (4500 bytes) • Strategy • Fragmentation occurs in a router when it receives a datagram that it wants to forward over a network which has (MTU < received datagram) • Reassembly is done at the receiving host • All the fragments carry the same identifier in the Ident field • Fragments are self-contained datagrams • IP does not recover from missing fragments ECE 4450:427/527

  13. IP Fragmentation and Reassembly IP datagrams traversing the sequence of physical networks ECE 4450:427/527

  14. IP Fragmentation and Reassembly Header fields used in IP fragmentation. (a) Unfragmented packet; (b) fragmented packets. ECE 4450:427/527

  15. IP Datagram Forwarding • Strategy • every datagram contains destination's address • if directly connected to destination network, then forward to host • if not directly connected to destination network, then forward to some router • forwarding table maps network number into next hop • each host has a default router • each router maintains a forwarding table ECE 4450:427/527

  16. Forwarding Table ECE 4450:427/527

  17. IPv4 Addressing 00001100 00100010 10011110 00000101 A unique 32-bit number Identifies an interface (on a host, on a router, …) Represented in dotted-quad notation 12 34 158 5 ECE 4450:427/527

  18. IPv4 Addressing • A host usually has a single link into network: When IP in host wants to send datagram, it does so over the link -> Boundary between host and link: interface. • IP address: technically associated with an interface, rather with the host • How about a router? • Receive datagram on a link and forward to on some other link. • How many interfaces? How many IP addresses? ECE 4450:427/527

  19. IPv4: Early Addressing • Properties • globally unique • hierarchical: network + host • 4 Billion IP address, half are A type, ¼ is B type, and 1/8 is C type • Format • Dot notation • 10.3.2.4 • 128.96.33.81 • 192.12.69.77 ECE 4450:427/527

  20. IP Addressing: CIDR CIDR:Classless InterDomain Routing • subnet portion of address of arbitrary length • address format: a.b.c.d/x, where x is # bits in subnet portion of address host part subnet part 11001000 0001011100010000 00000000 200.23.16.0/23 ECE 4450:427/527

  21. Subnets 223.1.1.1 • IP address: • subnet part (high order bits) • host part (low order bits) • What’s a subnet ? • device interfaces with same subnet part of IP address • can physically reach each other without intervening router 223.1.2.1 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.1.3 223.1.3.27 subnet 223.1.3.2 223.1.3.1 network consisting of 3 subnets ECE 4450:427/527

  22. Subnet and Subnet Mask 223.1.1.0/24 223.1.2.0/24 223.1.3.0/24 Recipe • to determine the subnets, detach each interface from its host or router, creating islands of isolated networks • each isolated network is called a subnet. Subnet mask: /24 Or 255.255.255.000 ECE 4450:427/527

  23. Subnets 223.1.1.2 How many? 223.1.1.1 223.1.1.4 223.1.1.3 223.1.7.0 223.1.9.2 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.3.27 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 ECE 4450:427/527

  24. ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B’s IP address? • Each IP node (host, router) on LAN has ARP table in ARP module • ARP table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> • TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) 137.196.7.78 1A-2F-BB-76-09-AD 137.196.7.23 137.196.7.14 LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 137.196.7.88 ECE 4450:427/527

  25. ARP: Same LAN • A wants to send datagram to B, and B’s MAC address not in A’s ARP table. • A broadcastsARP query packet, containing B's IP address • dest MAC address = FF-FF-FF-FF-FF-FF • all machines on LAN receive ARP query • B receives ARP packet, replies to A with its (B's) MAC address • frame sent to A’s MAC address (unicast) • A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) • ARP is “plug-and-play”: • nodes create their ARP tables without intervention from net administrator ECE 4450:427/527

  26. ARP: Packet Format • HardwareType: type of physical network (e.g., Ethernet) • ProtocolType: type of higher layer protocol (e.g., IP) • HLEN & PLEN: length of physical and protocol addresses • Operation: request or response • Source/Target Physical/Protocol addresses ECE 4450:427/527

  27. Addressing: Routing to another LAN 111.111.111.110 E6-E9-00-17-BB-4B B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.221 88-B2-2F-54-1A-0F walkthrough: send datagram from A to B via R. • focus on addressing - at both IP (datagram) and MAC layer (frame) • assume A knows B’s IP address • How can A know whether B is in the same subnet/network? • assume A knows B’s MAC address (how?) • assume A knows IP address of first hop router, R (how?) • assume A knows MAC address of first hop router interface (how?) – How many MAC addresses? A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D ECE 4450:427/527

  28. Addressing: Routing to another LAN MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy 111.111.111.110 E6-E9-00-17-BB-4B B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.221 88-B2-2F-54-1A-0F • A creates IP datagram with IP source A, destination B • A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D ECE 4450:427/527

  29. Addressing: Routing to another LAN MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B IP Eth Phy IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy 111.111.111.110 E6-E9-00-17-BB-4B B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.221 88-B2-2F-54-1A-0F • frame sent from A to R • frame received at R, datagram removed, passed up to IP A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D ECE 4450:427/527

  30. Addressing: Routing to another LAN IP Eth Phy MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP Eth Phy IP src: 111.111.111.111 IP dest: 222.222.222.222 111.111.111.110 E6-E9-00-17-BB-4B B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.221 88-B2-2F-54-1A-0F • R forwards datagram with IP source A, destination B • R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D ECE 4450:427/527

  31. Addressing: Routing to another LAN IP Eth Phy MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP Eth Phy IP src: 111.111.111.111 IP dest: 222.222.222.222 111.111.111.110 E6-E9-00-17-BB-4B B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.221 88-B2-2F-54-1A-0F • R forwards datagram with IP source A, destination B • R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D ECE 4450:427/527

  32. Addressing: Routing to another LAN IP Eth Phy MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 111.111.111.110 E6-E9-00-17-BB-4B B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.221 88-B2-2F-54-1A-0F • R forwards datagram with IP source A, destination B • R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D ECE 4450:427/527

  33. IP Addresses: How to get one? • MAC address: configured in adapter, globally unique • IP address: • Not only be unique on a given internetwork, but needs to reflect the structure of the internetwork • Not possible to be configured once into host; Hosts might change to another network: IP needs to be reconfigurable • Usually, automatic configuration methods are required: Dynamic Host Configuration Protocol (DHCP): Textbook, 3.2.7 ECE 4450:427/527

  34. A day in the life: Connecting to Internet browser DNS server Comcast network 68.80.0.0/13 school network 68.80.2.0/24 web page web server Google’s network 64.233.160.0/19 64.233.169.105 ECE 4450:427/527

  35. A day in the life: Connecting to Internet DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP • connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP • DHCP request encapsulatedin UDP, encapsulated in IP, encapsulated in 802.1 Ethernet router (runs DHCP) • Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server • Ethernet demuxed to IP demuxed, UDP demuxed to DHCP ECE 4450:427/527

  36. A day in the life: Connecting to Internet DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP • DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server • encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client router (runs DHCP) • DHCP client receives DHCP ACK reply Client now has IP address, knows name & addr of DNS server, IP address of its first-hop router ECE 4450:427/527

  37. A day in the life: ARP (before DNS, HTTP) ARP ARP Eth Phy ARP query ARP reply DNS UDP IP Eth Phy DNS DNS DNS • before sending HTTPrequest, need IP address of www.google.com:DNS • DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. In order to send frame to router, need MAC address of router interface: ARP • ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface • client now knows MAC address of first hop router, so can now send frame containing DNS query ECE 4450:427/527

  38. NAT rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3 Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers ECE 4450:427/527

  39. NAT Implementation: NAT router must: • outgoing datagrams:replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #) . . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr. • remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair • incoming datagrams:replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table ECE 4450:427/527

  40. NAT 3 2 1 4 S: 10.0.0.1, 3345 D: 128.119.40.186, 80 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table S: 128.119.40.186, 80 D: 10.0.0.1, 3345 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 NAT translation table WAN side addr LAN side addr 138.76.29.7, 5001 10.0.0.1, 3345 …… …… 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 3: Reply arrives dest. address: 138.76.29.7, 5001 ECE 4450:427/527

  41. NAT: Network Address Translation • NAT is controversial: • routers should only process up to layer 3 • violates end-to-end argument: Hosts should be talking directly with each other, without interfereing nodes modifying IP addresses and port numbers • Address shortage should instead be solved by IPv6 But like it or not, NAT becomes an important component of the Internet ECE 4450:427/527

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