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Internet Protocol: IP Routing

Internet Protocol: IP Routing. Linda Wu (CMPT 471 • 2003-3). Content. Definitions Direct / indirect delivery Table-driven IP routing IP routing algorithms Routing with IP addresses Incoming datagram handling Routing in Linux References: chapter 8 & 10. Definitions.

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Internet Protocol: IP Routing

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  1. Internet Protocol: IP Routing Linda Wu (CMPT 471 • 2003-3)

  2. Content • Definitions • Direct / indirect delivery • Table-driven IP routing • IP routing algorithms • Routing with IP addresses • Incoming datagram handling • Routing in Linux References: chapter 8 & 10 CMPT 471  2003-3

  3. Definitions In packet-switched systems: • Routing • The process of choosing a path over which to send packet • IP routing • The process of choosing a path within a single network or across multiple networks over which a datagram should be sent • 2 forms of routing • Direct delivery • Indirect delivery CMPT 471  2003-3

  4. Definitions (cont.) • Router • A computer which interconnects multiple physical networks and makes routing decisions • Host • A computer that usually connects directly to one physical network • Multi-homed host • A host that connects directly to multiple networks * TCP/IP standards draw a sharp distinction between the functions of a router and of a host CMPT 471  2003-3

  5. Direct / Indirect Delivery • Direct delivery • The ultimate destination connects to the same physical network as the deliverer • No router involved • Direct delivery occurs when: • Source and destination are in the same physical network, or, • The delivery is between the last router and the destination CMPT 471  2003-3

  6. Direct delivery Host Host Direct delivery Net 1 To the rest of internet Router Direct / Indirect Delivery (cont.) CMPT 471  2003-3

  7. Direct / Indirect Delivery (cont.) • The source determines whether the delivery is direct by: • Extracting the destination network address from the destination IP address • Comparing the destination network address with its own network address • If a match is found  direct delivery • How is datagram delivered? • Map the destination IP address into a physical address (mapping table, ARP) • Encapsulate datagram in a frame • Use physical hardware to deliver it CMPT 471  2003-3

  8. Direct / Indirect Delivery (cont.) • Indirect delivery • The destination is not on the same physical network as the source • The datagram goes from router to router till it reaches the one connected to the same physical network as the destination • Note: a delivery always involves one direct delivery but zero or more indirect delivery, with the direct delivery as the last one CMPT 471  2003-3

  9. Host Net 1 Indirect delivery Router Net 2 Router Indirect delivery Net 3 Direct delivery Host Direct / Indirect Delivery (cont.) (source) (destination) CMPT 471  2003-3

  10. Direct / Indirect Delivery (cont.) • How is datagram delivered? • The source maps the nearest router’s IP address into a physical address, encapsulates the datagram in a frame, and sends the frame to the router • The router extracts the datagram and selects the next router on the path towards the destination • Datagram is again placed in a frame and sent over next physical network to a second router • So on till it can be delivered directly CMPT 471  2003-3

  11. Table-Driven IP Routing • Routing table • Stores information about possible destinations and how to reach them • Both hosts and routers have routing tables • Reduce the size of routing table • Contains next hop addresses instead of the routes to the ultimate destinations • Contains destination network address instead of every possible IP address (i.e., network-specific routing instead of host-specific routing) CMPT 471  2003-3

  12. B A Net 1 Net 2 Net 3 R1 R2 Table-Driven IP Routing (cont.) (a) Routing tables based on route (b) Routing tables based on next hop CMPT 471  2003-3

  13. S A B C D Net 1 Net 2 R1 Table-Driven IP Routing (cont.) CMPT 471  2003-3

  14. Table-Driven IP Routing (cont.) • Consequences of routing based on next hop and destination network address • All traffic destined for a given network takes the same path without regard to the delay or throughput of the physical network • Only the final router along the path can determine whether the destination host exists or is operational • Datagrams from A to B may follow an entirely different path than that from B to A CMPT 471  2003-3

  15. Table-Driven IP Routing (cont.) • Next-hop routing • The routing table holds only the address of next hop instead of the complete route • A machine M’s routing table contains pairs (N, R) • N: the destination network address • R: the IP address of the next router along the path to N; router R is called next hop * R must lie on the network to which M connects directly * The routing table on M only specifies one step along the path from M to a destination network; M does not know the complete path to a destination CMPT 471  2003-3

  16. 20.0.0.5 30.0.0.6 40.0.0.7 Net 1 10.0.0.0 Net 2 20.0.0.0 Net 3 30.0.0.0 Net 4 40.0.0.0 Q R S 30.0.0.7 10.0.0.5 20.0.0.6 Table-Driven IP Routing (cont.) • The size of routing table depends on the number of networks in the internet • The size of routing table is independent of the number of individual hosts connect to the networks CMPT 471  2003-3

  17. A Net 1 R1 R3 Net 2 Net 3 R2 B Table-Driven IP Routing (cont.) • Host-specific routing • The destination host address is given in the routing table • Efficiency is sacrificed for other advantages: • The local network administrator is given more control over routing CMPT 471  2003-3

  18. A Net 1 R1 Net 2 R2 (default router) To the rest of internet Table-Driven IP Routing (cont.) • Default routing • Consolidates multiple entries into a default case • First look in the routing table for the destination network; if no route appears in the table, send the datagram to the default router • Especially useful when a site has a small set of local addresses and only one connection to the rest of the internet CMPT 471  2003-3

  19. Table-Driven IP Routing (cont.) • Static v.s. dynamic routing table • Static routing table • The administrator enters the route for each destination into the table • The table cannot update automatically when there is a change in internet; it must be manually altered by the administrator • Can be used in a small internet that does not change very often • Dynamic routing table • The routing table is updated periodically using dynamic routing protocol: RIP, OSPF, BGP • Used in large internet CMPT 471  2003-3

  20. IP Routing Algorithms • Standard routing algorithm for classful addressing • routing table entries (network address, next hop) • Hierarchical strategy for routing lookup • Direct delivery • Host-specific routing • Network-specific routing • Default routing CMPT 471  2003-3

  21. IP Routing Algorithms (cont.) Input: datagram DG, routing table T D  extract destination IP address from DG N  compute destination network address if N matches any directly connected network address map D to a physical address PD encapsulate DG in a frame send the frame to PD over that network else if T contains a host-specific route for D send DG to next hop specified in T else if T contains a route for network N send DG to next hop specified in T else if T contains a default route send DG to the default router else declare a routing error CMPT 471  2003-3

  22. IP Routing Algorithms (cont.) • Subnet routing algorithm • Routing table entries (subnet mask, network address, next hop) • Subsumes all special cases of the standard algorithm • Host-specific routing: mask = all 1s, network address = host IP address • Routing to a classful network: using default mask of that class • Default routing: mask = all 0s, network address = all 0s CMPT 471  2003-3

  23. IP Routing Algorithms (cont.) Input: datagram DG, routing table T D  extract destination IP address from DG if prefix of D matches any directly connected network address map D to a physical address PD encapsulate DG in a frame send the frame to PD over that network else for each entry in T do N = D & subnet mask if (N == network address field of the entry) route DG to the specified next hop endfor if no match found, declare a routing error CMPT 471  2003-3

  24. Routing with IP Addresses • IP routing does not alter datagram except for, • Decrementing time to live (TTL) • Recomputing checksum • Source and destination IP addresses always specify the original source and ultimate destination • Next-hop address • Selected by IP routing algorithm • Used by network interface software to get physical address • Discarded after physical address is found CMPT 471  2003-3

  25. Routing with IP Addresses (cont.) • Why use IP address in routing table? • Provide an clean interface between IP routing software and high-level software that manipulates routes • The objective of IP protocol is to hide the details of underlying networks CMPT 471  2003-3

  26. Income Datagram Handling When a datagram arrives at a machine: • The machine compares the destination address with the IP address for each of its network connection • If the destination address matches the machine’s IP address • IP software accepts the datagram • Passes it to the appropriate higher-level protocol for further processing • If not match • Host: discard the datagram • Router: forward the datagram using routing algorithm CMPT 471  2003-3

  27. Routing in Linux • Route command • Show / manipulate the IP routing table • Run “route” on July (172.16.1.7 / 172.18.1.7) CMPT 471  2003-3

  28. Routing in Linux (cont.) • Flags • U (up): the router is up and running • G (gateway): the destination is in another network, use gateway for delivery • H (host-specific): destination is a host • D (added by redirection): the entry is added to the routing table by a redirect message • M (modified by redirection): the entry is modified by a redirect message • Metric: distance (# of hops) to the destination address • Ref: # of users that are using this route • Use: # of packets transmitted through this router for the corresponding destination • Iface: the name of the interface CMPT 471  2003-3

  29. Routing in Linux (cont.) • Manipulate routing table route add/del –net route add/del –host Examples: route add -net 192.56.76.0 255.255.0.0 gw 192.56.76.9 eth0 route del -net 192.56.76.0 255.255.0.0 gw 192.56.76.9 eth0 route add -host 192.168.0.253 gw 192.168.1.3 • More details: man route CMPT 471  2003-3

  30. Routing in Linux (cont.) • Traceroute command • Track packet’s routing path to a host • Run “traceroute spring.net17” on July Path: july  cisco.net18  january  spring.net17 traceroute to spring.net17 (172.17.1.250), 30 hops max, 38 byte packets 1 cisco.net18 (172.18.1.254) 3.964 ms 1.787 ms 1.925 ms 2 january (172.16.1.253) 6.898 ms 0.935 ms 0.827 ms 3 spring.net17 (172.17.1.250) 1.712 ms 0.566 ms 0.599 ms CMPT 471  2003-3

  31. Routing in Linux (cont.) • How traceroute works? • Launch UDP probe packet with ttl =1, and listen for reply; increase ttl by 1, or stop probing, after receiving the reply • 2 types of reply • ICMP time exceeded message (from router) • ICMP port unreachable message (from the destination) • -p: to set the base UDP port number used in probes • Default base port: 33434; traceroute hopes that nothing is listening on UDP port (base ~ base + nhops – 1) on destination • The port number is incremented by 1 for each subsequent packet CMPT 471  2003-3

  32. Routing in Linux (cont.) • -q: number of probes • Default: 3 (3 probes are sent at each ttl setting) • -m: maximal ttl used in outgoing probe packets • Default: 30 • -w: time to wait for a response to a probe • Default: 3s (if there is no response within 3s, “*” is printed for that probe) • Output • Each line is composed of ttl, address of router, round trip time of each probe • More details: man traceroute CMPT 471  2003-3

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