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Routing Essentials

Routing Essentials. E. Damiani. Common Routing Configuration. Routing and routing protocols Three common routing configurations Minimal routing Static routing Dynamic routing The minimal Routing table:. netstat -rn Routing tables Destination Gateway Refcnt Interface

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Routing Essentials

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  1. Routing Essentials E. Damiani University of Milan

  2. Common Routing Configuration • Routing and routing protocols • Three common routing configurations • Minimal routing • Static routing • Dynamic routing • The minimal Routing table: • netstat -rn • Routing tables • Destination Gateway Refcnt Interface • 127.0.0.1 127.0.0.1 1 lo0 • 172.16.12.0 172.16.12.2 26 1e0 University of Milan

  3. Building a static routing table • To reach remote hosts, routes through external gateways must be added to the routing table. • For example, on Solaris • #route add 207.25.98.0 172.16.12.1 1 • #route add 192.0.2.32/27 somegateway • route [-fnvq] add | delete [-net | -host] destination gateway [args] or • route [-fnvq] change | get [-net | -host] destination gateway [args] • destination is the destination host or network • gateway is the next-hop intermediary through which packets should be routed. University of Milan

  4. Example • Add a default route on peanut through gateway almond • # route -n add default 172.16.12.1 1 • Try to ping host on other local host connected to pecan, • What will happen? University of Milan

  5. Example 2 % ping 172.16.1.2 PING 172.16.1.2: 56 data bytes ICMP Host redirect from gateway almond.nuts.com (172.16.12.1) to pecan.nuts.com (172.16.12.3) for filbert.nuts.com (172.16.1.2) 64 bytes from filbert.nuts.com (172.16.1.2): icmp_seq=1. time=30. ms ^C ----172.16.1.2 PING Statistics---- 1 packets transmitted, 1 packets received, 0% packet loss round-trip (ms) min/avg/max = 30/30/30 University of Milan

  6. Static routing • ICMP redirect works well for some old models with limited routing functions. • To avoid redirect specific routes can installed for each subnet using individual route statement. # route -n add 172.16.1.0 172.16.12.3 1 add net 172.16.1.0: gateway 172.16.12.3 # route -n add 172.16.6.0 172.16.12.3 1 add net 172.16.6.0: gateway 172.16.12.3 # route -n add 172.16.3.0 172.16.12.3 1 add net 172.16.3.0: gateway 172.16.12.3 # route -n add 172.16.9.0 172.16.12.3 1 add net 172.16.9.0: gateway 172.16.12.3 University of Milan

  7. Installing static routes at startup • Add the desired route statements to a startup file. • On Solaris, /etc/init.d/inetinit • route -n add default 172.16.12.1 1 > /dev/console • route -n add 172.16.1.0 172.16.12.3 1 > /dev/console • route -n add 172.16.6.0 172.16.12.3 1 > /dev/console • For Linux, /etc/rc.d/rc.local • Disable the routing protocol if it was setup running. University of Milan

  8. Interior Routing Protocol • Interior Protocols: • The Routing Information Protocol (RIP) • Hello • Intermediate System to Intermediate System (IS-IS) • Open Shortest Path First (OSPF) • RIP • Delivered with many Unix system, Daemon routed • On Solaris, if more than two interfaces or /etc/gateways exists, routed will be started. University of Milan

  9. Base of RIP: Distance Vector Routing • Limited state information. Just the next hop and cost. A D A D F C B H G E University of Milan

  10. I A D F C B H G E Distance Vector Routing – Dynamic Programming • Suppose a new node comes on line. I University of Milan

  11. I A D F C B H G E Distance Vector Routing – Dynamic Programming • Suppose a new node comes on line. • Suppose I first talks to A. A I University of Milan

  12. I A D F C B H G E Distance Vector Routing – Dynamic Programming • Suppose a new node comes on line. • Suppose I first talks to A. • Next I talks to D. I D University of Milan

  13. Distance Vector - Algorithm • Start with all destinations with infinite distance, except for the actual node, which is distance 0. • Every 30 seconds (RIP), or when a change occurs in the table, send table to neighbors. • If the distance to a prefix advertised by a neighbor is less plus the distance to the neighbor is less than known distance, reduce distance to prefix and route packets with that destination prefix to that neighbor. University of Milan

  14. A A B B C C D D E E Count to Infinity Problem University of Milan

  15. Approaches to Mitigate Count to Infinity • Why is count to infinity a problem? • It generates tons of routing updates – too much traffic • The network should report that a route is unreachable. • One possible answer: Put upper bound an upper bound the the diameter of the network. • But what is the network grows? (as it did). • Techniques • Split horizon. A router does not report a distance to the neighbor it learned the distance from. • Split horizon with poison reverse. If A advertises the best cost to E to B, then B advertises a cost of infinity to E back to A. • This only works for loops that involve two nodes. With larger loops, the mitigation is more difficult and these remedies reduce the rate of convergence. • The way to fix it is to use link state routing. University of Milan

  16. RIP • For non-gateway hosts, prevent advertising routes: -q option • /etc/gateways • Additional routing information • Define an active default route • net 0.0.0.0 gateway 172.16.12.1 metric 1 active • Active route • Can be updated by RIP • Active gateway is expected to supply routing information • Active gateway will be removed if it does not provide routing updates for a while • Passive route • Stay as long as system is up • Permanent static route • Prevent routing protocol from dynamically updating the route to reflect the changing network conditions. University of Milan

  17. RIP • Shortcomings: • Limited network diameter • 15 hops • Slow convergence • Classful routing • Improvement: • Split horizon • A router does not advertise routes on the link from which those routes were obtained. • Poison reverse • Router should advertise an infinite distance for routes on this link • Triggered update • In stead of waiting, a triggered update is send immediately. • RIP2 adds network mask and a next-hop address in original RIP packets. University of Milan

  18. Open Shortest Path First • Based on link-state: Each routers shares info about its neighbors with the entire network. • OSPF defines a hierarchy of routing area within an autonomous systems • Areas • Backbone • Stub area • OSPF link-state database can be big • Dividing the autonomous system into areas improved efficiency • Use designated router • OSPF is used on dedicated routers, e.g. Cisco. • Use Zebra on Linux University of Milan

  19. Link State Routing (IS-IS and OSPF) • Each router learns the entire network. (Compare to distance vector) • If the entire network is known, the shortest cost routing can be computed. • Each router advertises to its neighbors who it is connected to. • Each router floods any advertisement it receives. University of Milan

  20. Link State Routing PHASES: • HELLO – Determining who the router is connected to. • Reliable Flooding of LSA (link state advertisement) and keeping/getting up-to-date information. • Calculate shortest path. University of Milan

  21. HELLO • Periodically, a node sends a HELLO LSP (Link State Packet) to its neighbors. • The neighbor responds with a HELLO reply. • This way the router can determine which router it is connected to. • The default period is 10 – 30sec. University of Milan

  22. Reliable Flooding • Each LSP contains • The ID of the node that created the LSP. • The list of the neighbors directly connected to that node with the cost of each link. • A sequence number • A time to live • Each LSP reception is ACKed. University of Milan

  23. Sequence Number • The node that generated the LSA increments the sequence number for each LSA it sends out. • The sequence number is 32 bits long, so wrap around is not possible. • When router receives an LSA, it checks if the seq num of the received LSA is larger than the LSA in memory. • If the LSA has a larger seq. num, it is stored, the old one discarded and the new LSA is flooded to all neighbors, except the one that sent it. • If the LSA has a smaller or equal seq. no, it is discarded. University of Milan

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  25. Time to Live (TTL) • Each second a LSA sits in memory, its TTL is decremented. • Each time the LSA is transmitted, its TTL is decremented. • When the TTL reaches zero, the LSA is discarded and the router floods the LSA with TTL=0 to tell other routers to delete this information. University of Milan

  26. OSPF Header Type: 1=hello 2=database description 3=link status request 4=link state update 5=link state ack version type length Source address Address of the sender Area ID Checksum Authentication type Authentication Which area the packet originated University of Milan

  27. OSPF LSA Like TTL LS Age Options Type=1 Link State ID The same Advertising Router Seq. No Checksum Length Number of Links Link ID Link Data Link Type Num TOS Metric Optional TOS More Links University of Milan

  28. Managing Rebooting • When a node reboots, it doesn’t know its seq. no. • It floods its link state with seq. no. 0. • It sends a Link State Request to its neighbors • The neighbors respond with the most up to date LSA they have. These LSA may contain the LSA of the before it crashed. In this case, the node updates its seq. no. • Remember that every time a router gets a new info, it floods the information. University of Milan

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  31. Security Issues • An attacker could send fake LSAs. • Must filter out LSA from any other source but the neighbor and use authentication. • If a router is compromised, it could advertise a low cost to all nodes. Then all nodes will send packets to this node and the network would stop working. University of Milan

  32. metrics • ARPANET – the link cost included the size of the queue (a measure of congestion). • This does not work.. • Static cost – just use delay (Juniper seems to do this) • Dynamic cost – must make sure that link costs do not vary too much. • The cost can is restricted to only change slowly over time. • The cost between links cannot vary much. There cannot be more than a factor of 7 difference between the most and least expensive link. • The cost of a link can only vary by a factor of 3. • Cost only depends on utilization at moderate or high loads. • Only send updates when the cost crosses a threshold. • Is this stable? University of Milan

  33. TOS – Type of Service • A data packet can identify the type of service it wants. • The router can provide different routing according to the TOS. • To support this, OSPF allows the link cost to depend on the TOS. • This is not widely deployed. University of Milan

  34. Exterior routing protocols • Exchange routing information between autonomous systems • Reachability information • Exterior Gateway Protocol (EGP) • Acquiring a neighbor: Establish dialog between two EGP gateways using hello and I-heard-you. • Requesting routing information called poll • Sending a packet of reachability called update • EGP Does not attempt to choose the best route • In the old time core gateways were expected to have the information necessary to choose the best external routes. University of Milan

  35. Exterior Routing Protocols • Border Gateway Protocol (BGP) • Support policy-based routing • No-technical reasons to make routing decisions. • Political, organizational, or security • Routing policies are not part of the BGP protocol. Policies are provided externally as configuration information. • Routing Arbiters (RAs) at the NAP can be queries for routing policy information. • Bilateral agreements on private policy between ISPs • How BGP uses policy? • Control the routes it announces to others • Control the routes it accept form others University of Milan

  36. BGP • BGP is implemented on TCP, port 179 • BGP is a path-vector protocol. • Entire end-to-end path of a route in the form of a sequence of autonomous system numbers. • Eliminate the possibility of routing loops and count-to-infinity problems. • Most systems never run exterior protocols • Only on gateways that connect AS to another AS. University of Milan

  37. Choosing a routing protocol • Local area networks, RIP is the choice • Larger networks, OSPF is the choice • Exterior routing protocol, BGP or what the other part is running. • Equipment affects the choices: • Routers support a wide range • Most Unix systems are delivered with only RIP. • Daemon gated give the options of using Unix System as a router. University of Milan

  38. Gateway Routing Daemon • Combines several different routing protocols in a single software package. • System can run more than one routing protocol. • Routers learned from interior protocol can be announced via exterior routing protocol. • Routing policy can be implemented • All protocols are configured from a single file ( /etc/gated.conf) • Constantly upgraded University of Milan

  39. Route Type Default Preference direct route 0 OSPF 10 Internally generated default 20 ICMP redirect 30 static route 60 Hello protocol 90 RIP 100 OSPF ASE routes 150 BGP 170 EGP 200 Gated’s preference value • Routing implementation has two sides: • External: exchange routing info • Internal: update routing table • Gated can pick the best route from multiple protocols. • Metric may be different: hop account, delay, … • Gated’s own value: preference University of Milan

  40. Configuring gated • Available at http://www.gated.org • /etc/gated.conf • Multiple protocols are configured in the same file. • Sections: • Option statement • Interface statement • Definition statement • Unicast statement • Multicast protocol statement • Static statements • Control statement • Aggregate statement University of Milan

  41. A host configuration # enable rip, don't broadcast updates, # listen for RIP-2 updates on the multicast address, # check that the updates are authentic. # rip yes { nobroadcast ; interface 172.16.9.23 version 2 multicast authentication simple "REALstuff" ; } ; University of Milan

  42. Interior Gateway configurations # Don't time-out subnet 9 interfaces { interface 172.16.9.1 passive ; } ; # Define the OSPF router id routerid 172.16.1.9 ; # Enable RIP-2; announce OSPF routes to # subnet 9 with a cost of 5. rip yes { broadcast ; defaultmetric 5 ; interface 172.16.9.1 version 2 multicast authentication simple "REALstuff" ; } ; University of Milan

  43. Interior Gateway configurations (cont) # Enable OSPF; subnet 1 is the backbone area; # use password authentication. ospf yes { backbone { authtype simple ; interface 172.16.1.9 { priority 5 ; authkey "It'sREAL" ; } ; } ; } ; University of Milan

  44. Exterior Gateway Configuration # Defines the OSPF router id routerid 172.16.1.1; # Disable RIP rip no; # Enable BGP bgp yes { preference 50 ; group type external peeras 164 { peer 10.6.0.103 ; peer 10.20.0.72 ; }; }; # Enable OSPF; subnet 1 is the backbone area; # use password authentication. ospf yes { backbone { authtype simple ; interface 172.16.1.1 { priority 10 ; authkey "It'sREAL" ; } ; } ; }; University of Milan

  45. Exterior Gateway Configuration (cont) # Announce routes learned from OSPF and route # to directly connected network via BGP to AS 164 export proto bgp as 164 { proto direct ; proto ospf ; }; # Announce routes learned via BGP from # AS number 164 to our OSPF area. export proto ospfase type 2 { proto bgp as 164 { all ; }; }; University of Milan

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