1 / 24

Internetworking: IP Packet Switching

Internetworking: IP Packet Switching. Reading: 4.1.1 - 4.1.7 (except Implementation; pp. 245-250 ) . Terminology. “ i nternetwork”, i nternet: an arbitrary collection of physical networks interconnected to provide some sort of host- to-host packet delivery service

imelda
Télécharger la présentation

Internetworking: IP Packet Switching

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Internetworking:IP Packet Switching Reading: 4.1.1 - 4.1.7 (except Implementation; pp. 245-250 )

  2. Terminology • “internetwork”, internet: an arbitrary collection of physical networks interconnected to provide some sort of host- to-host packet delivery service • interconnect physical networks (e.g. Ethernets, FDDIs, ATMs, PPP,..) • form a logical network (an internet) • Internet: widely used, global internetwork to which a largepercentage of networks are now connected • Learn the principles of internetworking • illustrate ideas with real-world examples from Internet

  3. Network of “Single Technology Networks” • Internet Protocol, IP: a tool used to build scalable, heterogeneous internetwork • net 1, net 2 : Ethernets • net 3: FDDI • net 4: point-to-point link • R1, R2, R3: routers for interconnection

  4. Service Model • Connectionless (datagram-based) • Best-effort delivery (unreliable service to transport {or other higher} layer protocols) • packets are lost • packets are delivered out of order • duplicate copies of a packet are delivered • packets can be delayed for a long time

  5. Packet Format: 20 to 24 byte Header I • Version • HLen: length of header in 32-bit words • TOS, Type of Service: allow packets to be treated differently based on application needs • Length: bytes of datagram (including header, max 65,535) • Indent, Offset , Flag: information used for fragmentation

  6. Packet Format: 20 to 24 byte Header II • TTL, time to live: discard looping packets; 64 is the current default • Protocol: higher-level protocol (TCP = 6, UDP =17, …) • Checksum: calculated for IP header considered as a sequence of 16-bit words • SourceAddr, DestinationAddr: IP defines its own global address space, independent of physical networks • Options, Pad: rarely use

  7. Fragmentation and Reassembly • Each physical network has some maximum transmission unit (MTU). • Examples • Ethernet packets up to 1500 bytes • FDDI packets up to 4500 bytes Fragmentation and Reassembly Design Decisions: • try to avoid fragmentation at source host • source host chooses size of IP datagram equals to MTU of physical network to which it is directly attached • if transport protocol gives IP a packet larger than local MTU, then source host fragments it • fragment when necessary • whenever the path to destination includes a network with MTU < Datagram

  8. Design Decisions (cont.) • re-fragmentation is possible • a router wants to forward a datagram over a network with MTU < received datagram • fragments are self-contained datagrams • delay reassembly until destination host • fragments carry same identifier in Ident • Ident chosen by sending host • unique among datagrams that might arrive at destination from source over some reasonable time period • do not recover from lost fragments • If all fragments do not arrive at receiving host, it discards fragments that arrived

  9. Example 1500 MTU 4500 MTU 532 MTU • IP on H1 wants to send to H8 a 1420-byte packet (20-byte IP header plus 1400 bytes of data) • MTU of net2 = 1500 byte • MTU of net3 = 4500 byte • MTU of net4 = 532 byte • MTU of net4 = 1500 byte R2 must fragment the packet before routing it to RT3

  10. Example (continued) Start of header • First fragment • M bit 1 in the Flags field (more fragments to follow) • Offset 0 (fragment contains first part of original datagram) Ident = x 0 Offset = 0 Rest of header (a) 1400 data bytes 1420-byte datagram (20-byte IP header plus 1400 bytes of data) • Second fragment • starts with the 513th byte • Offset field in header set to 64, which is 512/8 • fragmentation is done on 8-byte boundaries • Offset field counts 8-byte chunks Start of header Ident = x 1 Offset = 0 Rest of header 512 data bytes (b) (a) Start of header Ident = x 1 Offset = 64 Rest of header 4500 MTU 1500 MTU 512 data bytes (b) 532 MTU Start of header Ident = x 0 Offset = 128 Rest of header • Third fragment • contains last 376 bytes • offset (2 x 512)/ 8 = 128 • M bit is 0 376 data bytes

  11. Global Addresses • Although globally unique, Ethernet addresses have no structure to provide clues to routing protocols • IP addresses: 32 bits • globally unique (4,294,967,296 possible addresses) • hierarchical: network + host • Dot Notation • 10.3.2.4 • 128.96.33.81 • 192.12.69.77

  12. Yes. Yes. Yes. Yes. Yes. Yes. Yes.\ Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. IP addresses (cont. I) • The original idea was that the Internet would consist of • a small number of wide area networks (these would be class A networks), 126 • 126 class A networks (the values 0 and 127 are reserved) • each class A network accommodate up to 224 - 2 (about 16 million) hosts (again, there are two reserved values)

  13. Yes. Yes. Yes. Yes. Yes. Yes. Yes.\ Yes. Yes. Yes. Yes. Yes. Yes. Yes. IP addresses (cont. II) • The original idea was that the Internet would consist of • a small number of wide area networks (these would be class A networks), 126 • a modest number of site- (campus-) sized networks (these would be class B networks) • 65,534 hosts /network

  14. IP addresses (cont. III) • The original idea was that the Internet would consist of • a small number of wide area networks (these would be class A networks), 126 • a modest number of site- (campus-) sized networks (these would be class B networks) • 65,534 hosts /network • a large number of LANs (these would be class C networks) • 254 hosts/LAN • 255 reserved for broadcast • 0 not a valid host number

  15. Datagram Forwarding Strategy • every datagram contains destination’s address • if router is connected to destination network, then forward to host • if not directly connected, then forward to some router • forwarding table maps network number into next hop (router) • each host has a default router • each router maintains a forwarding table

  16. Datagram Forwarding (cont.) • Example: R2 Table Network Number Next Hop 1 R3 2 R1 3 interface 1 4 interface 0

  17. Address Translation a packet reaches a new physical network • map IP address into a physical address • Either • determine physical address of destination host • OR • determine physical address of next hop router • encapsulate IP datagram inside a frame that contains link-level address

  18. Address Resolution Protocol (ARP) • ARP enables each host on a physical network to dynamically build up a table of mappings between IP addresses and link-level addresses Invoking ARP: • an “originator” host wants to send a datagram to a host (or router) on the same physical network • if no mapping is found in its ARP table, it invokes the Address Resolution Protocol

  19. Executing ARP • “originator” host broadcast an ARP query containing “target IP address” and {IP address; link-layer address} of originator host • if a host on network already has an entry for originator host, it “refreshes” this entry (resets length of time until it discards entry) • target host adds information about originator to its table & sends back a response message that contains its link-layer address • originator adds information contained in the response to its ARP table • all other hosts do not add an entry for the originator host

  20. ARP: Notes • mappings may change over time • entries are timed out periodically and removed • discarded if not refreshed (in about 10 minutes)

  21. 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

  22. 0 8 16 31 Hardware type = 1 ProtocolType = 0x0800 HLen = 48 PLen = 32 Operation SourceHardwareAddr (bytes 0 ― 3) ― 5) ― 1) SourceHardwareAddr (bytes 4 SourceProtocolAddr (bytes 0 SourceProtocolAddr (bytes 2 TargetHardwareAddr (bytes 0 ― 3) ― 1) ― 5) TargetHardwareAddr (bytes 2 TargetProtocolAddr (bytes 0 ― 3) Mapping IP into Ethernet Addresses ARP Packet Format

  23. Internet Control Message Protocol (ICMP) • a companion protocol to IP • defines a collection of error messages that are sent back to source host whenever a router or host is unable to process an IP datagram successfully

  24. Error Messages Defined by ICMP • Destination unreachable • TTL exceeded (so datagrams don’t cycle forever) • Checksum failed • Reassembly failed • Cannot fragment • Echo (ping) • Redirect (one of the control messages a router can send back to a source host) • there is a better route to the destination

More Related