Link State Routing Algorithm Shortest path calculations

Link State Routing Algorithm • Use a routing protocol to collect the whole network topology • Obtain destination reachability information as well as link weights/states •Compute shortest paths using Dijkstra’s algorithm from a node to all other nodes •Construct routing tables that show the destination addresses and the next hop addresses •Note that while Dijkstra’s algorithm gives you end-to-end routes, the routing tables may only store the next hop address. Malathi Veeraraghavan (originals by Jörg Liebeherr)

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Shortest path calculations Calculate shortest paths for node s to all other nodes Dijkstra’s Algorithm: s source node. Dn cost of the least-cost path from node s to node n M = {s}; for each n ∉ M Dn = dsn; while (M ≠ all nodes) do Find w ∉ M for which Dw = min{Dj ; j ∉ M}; Add w to M; for each n ∉ M Dn = minw [ Dn, Dw + dwn ]; Update route; enddo Malathi Veeraraghavan (originals by Jörg Liebeherr)

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Example • Assume node 1 has obtained the entire network topology using some link state routing protocol •Construct the routing table at node 1 using Dijkstra’s algorithm to determine shortest paths from node 1 to all other nodes in the network

5 2

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6 2

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Malathi Veeraraghavan (originals by Jörg Liebeherr)

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Example (at node 1) Iteration

M

D1

D2

D3

D4

D5

D6

Init


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Solution • Use Dijkstra’s algorithm described in slide 2 • Start with M consisting of only node 1.

0 1 2 3 4

M {1} {1,4} {1,4,2,5} {1,4,2,5,3} {1,4,2,5,3,6}

D1 0 0 0 0 0

D2 2 2 2 2 2

D3 5 4 3 3 3

D4 1 1 1 1 1


D5 inf 2 2 2 2

D6 inf inf 4 4 4

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Resulting Routing Tree 2

2

3 1

1 1 4

1

6 2

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•The tree is translated into a routing table at node 1: Destination 2 3 4 5 6 Malathi Veeraraghavan (originals by Jörg Liebeherr)

Next Hop 2 4 4 4 4 6

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Link State Discussion • Each node requires complete topology information. • Link state Information must be flooded to all nodes. Guaranteed to converge. • Each node must maintain a global database. • Convergence of the algorithm is guaranteed.


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IP Routing • IP Forwarding is performed by IP (in OS kernel) • IP Routing is performed by a user-level process • In Unix, by the daemon processes routed and gated


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IP Routing routing daemon

route command

netstat command

UDP

TCP YES For me ?

ICMP

MP IC

routing table

t ec dir e R

IP Output: Calculate Next Hop Router

: a NO g en n i ard rw fi fo Source g Routin

d ble

Process IP Options

IP Input Queue Network Interfaces


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Routing Table Lookup • For each IP packet, there is one routing table lookup. 1. Find matching host address 2. Find matching network address 3. Find default entry

• Routing table printout with netstat -rn • Example: Destination 140.252.23.32 127.0.0.1 default


Gateway 140.252.23.1 127.0.0.1 140.252.13.33

Flags Refcnt Use Interface UGH UH UG

3 1 0

25000 0 0

emd0 lo0 emd0

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Explanation of the printout •

Flags: – U: route is up – G: route is to a gateway (router); if flag is not set, destination is directly connected – H: route is to a host, I.e., destination address is the complete host address; if flag is not set, route is to a network and destination address is netID or subnetID – D: route created by redirect – M: route modified by redirect

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Refcnt: reference count: number of active uses of each route; for each TCP connection passing through route, this is increased by 1

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Use: number of packets sent on route

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Interface: local interface


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Statically Setting IP Routing Tables • There are several ways for setting IP routing tables without a routing protocol (=Static Routing) 1. Automatic creation of entry during initialization of a local interface (with ifconfig) 2. During bootstrap with route command 3. Via ICMP redirect messages 4. Via ICMP router advertisement/router discovery messages


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Route Command • Route commands are put in a system file that is read during system bootstrap • System file is:

/etc/rc2.d/S69inet in Solaris, /etc/netstart in FreeBSD.

• Example: “default:” destination “sun:” gateway or router 1: metric route add default sun 1 route add slip bsdi 1


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ICMP Redirect •

Based on routing data in host, it does an arp for router 1 and sends packet to router 1

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When router 1 detects that an IP datagram should have gone to a different router, the router: • forwards the IP datagram to the correct router • sends an ICMP redirect message to the host

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Host uses ICMP message to update its routing table

(1) IP datagram (3) ICMP Redirect (2) IP datagram Router 1 Malathi Veeraraghavan (originals by Jörg Liebeherr)

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ICMP Router Solicitation ICMP Router Advertisement • Specified in RFC 1256 (1991) • After bootstrapping a host broadcasts an ICMP router solicitation message. • In response, routers send an ICMP router advertisement message • Also, routers periodically broadcast ICMP router advertisement


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Dynamic IP Routing Protocols • In Unix systems, the dynamic setting of routing tables is done by the routed or gated daemons •

The routing daemons execute the following intradomain and interdomain routing protocols interdomain

intradomain

Daemon

Hello

rrouted Gated (Version 3)


RIP

OSPF

EGP

BGP

V2

Yes

V2, V3

V1 Yes

V1 V2

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RIP - Routing Information Protocol • A simple intradomain protocol • Straightforward implementation of Distance Vector Routing • Each router advertises its minimum distances to destinations every 30 seconds (or whenever its routing table changes) • RIP always uses the hop-count as link metric. Maximum hop count is 15, with “16” equal to “∞”. • Routes timeout after 3 minutes if they are not updated. Route metric is set to ∞ (16) and marked for deletion


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RIP Packet Format IP header 1: request 2: reply 3, 4: unused 5: poll 6: poll entry addr. family: 2 for IP

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RIP Message

UDP header

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15 16

Command Version (1-6) (1) address family

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Set to 00...0 Set to 00.00

32-bit address 20 bytes long

IP address for which a route is requested

Unused (Set to 00...0) Unused (Set to 00...0) metric (1-16) Up to 24 more routes (each 20 bytes)


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Routing with RIP • This is the operation of RIP in routed. Dedicated port for RIP is UDP port 520. •

Initialization: Broadcast a request packet (command = 1, metric=16; address family=0, metric=16) on the interfaces requesting current routing tables from routers.

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Request received: Routers that receive above request send their entire routing table.

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Response received: Update the routing table (see distance vector algorithm).

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Regular routing updates: Every 30 seconds, send all or part of the routing tables to every neighbor.

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Triggered Updates: Whenever the metric for a route changes, send data that has changed.


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RIPv2 IP header

RIPv2 Message

UDP header

Process ID of routing daemon

Command Version (1-6) (=2) address family

routing domain route tag

32-bit address Subnet Mask (32 bits)

Support of EGP and BGP

Subnet Mask of IP address (RIP version 1 is not aware of subnet masks)

Next-Hop IP address (32 bits) Metric (1-16)

Identifies next hop; value of 0 means packets

Identifies next hop: value of 0 means packets should be should be sent to node sending this RIP message sent to node sending this RIP message

Up to 24 more routes (each 20 bytes)

•The next-hop IP address is where packets to that dest. IP address should be sent; a value of 0 means packets should be sent to the router sending this RIP message Malathi Veeraraghavan (originals by Jörg Liebeherr)

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OSPF • OSPF = Open Shortest Path First • RFC 1247 from 1991 • Alternative solution to RIP as interior gateway protocol • OSPF is a link state protocol, i.e., each node has complete topology information • OSPF messages are sent directly in IP (and not as payload of UDP packets) • Hellos and Link State Advertisements (LSAs) – To get the topology of the network • Shortest-path algorithm, e.g., Dijkstra’s to precompute routing tables. Malathi Veeraraghavan (originals by Jörg Liebeherr)

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Features of OSPF • Provides authentication of routing messages • Enables load balancing by allowing traffic to be split evenly across routes with equal cost • Supports subnetting • Supports multicasting


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BGP • BGP = Border Gateway Protocol • Currently in version 4 • Note: In the context of BGP, a gateway is nothing else but an IP router that connects autonomous systems. • Interdomain routing protocol for routing between autonomous systems • Uses TCP to send routing messages • BGP is a distance vector protocol, but unlike in RIP, routing messages in BGP contain complete routes. • Network administrators can specify routing policies Malathi Veeraraghavan (originals by Jörg Liebeherr)

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BGP • BGP’s goal is to find any path (not an optimal one). Since the internals of the AS are never revealed, finding an optimal path is not feasible. • For each autonomous system (AS), BGP distinguishes: • local traffic = traffic with source or destination in AS • transit traffic = traffic that passes through the AS • Stub AS = has connection to only one AS, only carry local traffic • Multihomed AS = has connection to >1 AS, but does not carry transit traffic • Transit AS = has connection to >1 AS and carries transit traffic Malathi Veeraraghavan (originals by Jörg Liebeherr)

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BGP

AS 1

AS 2 Router

Router

Router

AS 3

Router

Router

Router Router

AS 4


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Exercise 135.60.81 .1

R1 161.6.5

.1

11.4.6

.2

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R2 180.11.22

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.1 190.4.6

•Assume route commands in the bootstrap sequence in both routers allows them to immediately recognize their directly connected subnets •Show the routing tables at nodes R1 and R2 initially before they exchange RIP messages and after. •See Page 131 of your textbook for guidance. Malathi Veeraraghavan (originals by Jörg Liebeherr)

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Exercise solution Routing table at router R1 soon after reboot Destination Gateway Flags RIP Metric 135.60.81 135.60.81.1 U 1 161.6.5 180.11.22

161.6.5.1 U 180.11.22.1 U

1 1

•Table created by route commands executed during boot-up procedure - directly-connected interfaces have a route command metric of 0 (pg. 116) but a RIP metric of 1 (pg. 131) •See page 116- it states that the metric is 0 if the G flag is not set, which means a direct route. The gateway column has the IP address of the outgoing interface. Malathi Veeraraghavan (originals by Jörg Liebeherr)

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Exercise solution contd. Routing table at router R2 soon after reboot Destination Gateway Flags RIP Metric 190.4.6 190.4.6.1 U 1 11.4.6 180.11.22

11.4.6.4 U 180.11.22.2 U

1 1

•Same as on previous slide


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Exercise solution contd. • RIP messages sent. As per fig. 10.4 on page 131 of your text book each router advertises routes to each subnet on all other subnets with a hop count of 1. • Thus, each router listens to the broadcasts about the other router’s subnets. • Updated routing tables are shown on the next two slides.


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Exercise solution contd. Routing table at router R1 after receiving RIP messages from R2 Destination Gateway Flags RIP Metric 135.60.81 135.60.81.1 U 1 161.6.5 180.11.22 190.4.6 11.4.6

161.6.5.1 180.11.22.1 180.11.22.2 180.11.22.2

U U UG UG

1 1 2 2

•See page 131 of your textbook for RIP metrics for subnets on adjacent routers Malathi Veeraraghavan (originals by Jörg Liebeherr)

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Exercise solution contd. Routing table at router R2 after receiving RIP messages from R1 Destination Gateway Flags RIP Metric 190.4.6 190.4.6.1 U 1 11.4.6

11.4.6.4

U

1

180.11.22

180.11.22.2 U

1

135.60.81

180.11.22.1 UG

2

161.6.5

180.11.22.1 UG

2


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