1 CMPT 471 Networking II RIP. © Janice Regan, 2006 2 Dynamic Routing In very simple small and stable networks static routing may be adequate. As networks

1

CMPT 471Networking II

RIP

© Janice Regan, 2006 2

Dynamic Routing In very simple small and stable networks static

routing may be adequate. As networks grow, become more complex, include

multiple or redundant paths between hosts and routers, or change frequently, static routing becomes difficult to manage Redundant routes are only useful if they can be

used immediately on failure of the original route In a large network the reporting of failure and

response by a network manager cannot be accomplished on a short enough time scale for the redundant routes to be useful


Dynamic Routing A routing algorithm to update the routing tables to

respond to changes and failures in the network becomes a necessary component of the system

Addition of a routing algorithm does not change the forwarding algorithm used to deliver packets, segments …

Addition of a routing algorithm allows dynamic changes to the entries in the routing table, so those entries can be updated to respond to Changes in network topology Changes in load through the network


Autonomous Systems (AS) Group of connected networks and routers, or

nodes, controlled by a single administrative authority. May contain one or more networks (need not be

subnets of one network, or networks of the same hardware type)

There is at least one route between any pair of nodes in the AS

Administrative authority can specify if packets originating in their AS are permitted to pass through any other AS (Company A may not wish it’s data to pass through Company B’s routers)


Autonomous Systems (AS) Routers within an AS use a common routing

protocol (interior routing protocol or IRP) for all members of the group Defines mechanisms for discovering,

validating, and maintaining routes within the autonomous system

Some routers in the autonomous system (those connected to the larger Internet) also support a routing protocol for propagating subset of routing information to other autonomous systems on the internet (Exterior routing protocol or ERP)


Conceptual view of AS

Figure 16.3 Comer (2000)


Sample routing protocols IRP (Internal routing protocol, within AS)

GGP: gateway to gateway protocol (no longer used) RIP: routing information protocol (implemented using

routed, for simple networks only) OSPF: Open Shortest Path First

ERP (External routing protocol, between AS’s) BGP: ERP usually used in the internet.


Routing algorithm A routing algorithm or routing protocol process

determines How much information is sent to other routers Which information is sent to other routers How often information is sent to other routers Which other routers to exchange information with best route from present router to every other router

in the AS When and on what basis to change routes due to

changes in the AS


Static Routing All routing information specified by

administrator and installed at system boot time. Details are determined by the operating system being used For Ubuntu LINUX,

Default static routing information can be found in /etc/network/network/interfaces

To add additional static routes through the interface

up route add [-net|-host] <host/net>/<mask> gw <host/IP> dev <Interface>


Distance-Vector Routing Each node exchanges information only with

neighbor nodes Neighbors are both directly connected to same AS

Each node maintains vector of link costs for each directly attached node and distance and next-hop values for each destination node in the system

A node must transmit large amounts of information Distance vector to all neighbors, Containing estimated

path cost to all nodes in a configuration and possibly next hop labels


Distance-Vector Routing Changes take long time to propagate

(count to infinity) Used by first generation routing algorithm

for ARPANET and by Routing Information Protocol (RIP, routed) RIP is an internal gateway protocol (IGP) used between routers within an AS

There is no built in mechanism to prevent routing loops


Once every T seconds each node i, sends each of its neighbors (one hop distant) a list, of estimated delays from itself to every other node j.

Similarly each node will receive routing information from each of its neighbors every T sec. The routing data received are used to update

the routing table of the node

Distance Vector Routing


Similarly each station will receive routing information from each of its neighbors every T sec. Note: the existing routing table for station i at

time t is not used directly to compute the routing table for station i at time t+T, the ‘distance’ to each neighbor station k, lki, and the delay vectors from each neighbor station j, dij are used

Distance Vector Routing

][min kiijAi

kj ldd


Updating the routing table Use the distributed version of the Bellman-Ford

algorithm.

Di delay vector from node i

Si successor node vector

sij next node in minimum delay route

from i to j

N # of nodes in network

lki current estimate of delay between neighbors

k, I

The set of neighbor nodes

Ni

i

i

i

Ni

i

i

i

s

s

s

S

d

d

d

D

2

1

2

1

][min kiijAi

kj ldd


Distance Vector Routing : from node J

II

J LK

E F HG

B DC

2

4

3

2

5

1

6

6

2

4

4

8

5

44

24

A

4

3

Original network and original delaysF I J K

A A 2 E 3 F 6 F 7

B B 4 F 7 F 8 F 9

C G 8 F 11 F 12 H 12

D G 6 F 9 F 10 H 8

E A 3 E 2 I 6 F 8

F 0 0 F 3 F 4 F 5

G G 2 F 5 F 6 F 7

H K 5 J 11 K 7 H 4

I I 3 0 0 I 4 J 7

J J 4 J 4 0 0 J 3

K K 5 J 7 K 3 0 0

L K 7 J 9 K 5 L 2

lJF

4Delay lJi

lJI

4lJK

3

Janice Regan © 2005-101216

Calculate new routes at node J

TkNkkK

kiijAi

kj

dddD

ldd

],,,[

][min

21

A A 2 E 5 F 5

B A 7 E 10 F 10

C G 9 F 13 H 9

D G 7 F 11 H 6

E A 5 E 2 F 8

F 0 0 F 4 F 3

G G 3 F 7 F 6

H K 3 F 10 H 4

I I 4 0 0 F 7

J J 2 J 3 J 4

K K 3 J 7 0 0

L K 6 F 10 L 3

lJF4

Measured Delay lJi

lJI4

lJK3

Delay vectors received

SF DF SI DISK DK

6 9 8 F 611 14 13 F 1113 17 12 K 1211 15 9 K 99 6 11 I 64 8 6 F 47 11 9 F 77 14 7 K 78 4 10 I 46 7 7 0 07 11 3 K 3

10 14 6 K 6

DF+IJF DF+4

DI+IJI

DI+4

DK+IJK

DK+3

New table for node J

iskj

SJ DJ

Later time, some routes weights have changed


Actual Practice The routing table is updated each time a packet

of routing information arrives from a neighbor The updating process consists of

Comparing the sum of the delay to the particular node from which the update came and the cost of propagation to that node to the present cost. If the cost decreases replace the path for that

node If there is presently no such route add the route Always replace the cost to the directly connected

nodes


Problems: distance vector routing Changes take long time to propagate

(count to infinity) slow convergence There is no built in mechanism to prevent

routing loops Oscillation and instability is possible


Count to Infinity problem In practice when conditions in a

network change, the change will take time to propagate across the network.

Good news propagates quickly across a network

Bad news propagates slowly across the network


Count to Infinity problem: Consider a linear network with 6 stations

A is down initially and all other stations know this A comes up, at the time of the first exchange of routing

information after A comes up B learns that A is alive At the time of the second exchange of routing

information after A comes up C learns that A is alive This pattern continues till the time of the sixth data

exchange after A comes up, when F learns that A is alive. At this point all stations in the network have learned the good news

A B C D E F


Count to Infinity problem:

Initially B can reach A and C can reach A through B When the link from B to A fails then B updates its routing table

to indicate it cannot reach A directly. When a update arrives at B from C, B notices that C has a

route to A with a cost of 2, and adopts that route. The new route has a cost of (2+1)=3

When an update from B reaches C, C notices the cost of the route to A through B has increased to 3 so it updates it own routes cost to A to (3+1)=4

The process continues as the cost to route A counts to infinity. In the more general network the costs increment as shown on

the next slides

A B C


Count to Infinity problem: Linear network with 6 stations (all single hop

costs 1)

The number of iterations necessary to indicate that the link to 1 is down (cost infinite) is in fact infinite. It is clear that the delay increases as the number of iterations increase, but it is still necessary to ‘count to infinity’ to reach the correct link costs for link A-B down.

A B C D E F


Count to Infinity problem: Linear network with 6 stations (all single hop costs 1)

A is up initially and all other stations know this A goes down, at the time of the first exchange of routing

information after A goes down B hears nothing from A. Since there is no direct path to A, B chooses an indirect path to A through C (which actually goes through B itself, but B doesn’t know this)

The first eight exchanges are illustrated on the next slide.

A B C D E F


Count to Infinity problem: Linear network with 6 stations, single hop costs 1)

1 2 3 4 5

3 2 3 4 5

3 4 3 4 5

5 4 5 4 5

5 6 5 6 5

7 6 7 6 7

7 8 7 8 7

9 8 9 8 9

9 10 9 10 9

A B C D E F

INITIALLY

AFTER 1 EXCHANGE

AFTER 2 EXCHANGES

AFTER 3 EXCHANGES

AFTER 4 EXCHANGES

AFTER 5 EXCHANGES

AFTER 6 EXCHANGES

AFTER 7 EXCHANGES

AFTER 8 EXCHANGES


Routing Information Protocol RIP, The simplest dynamic distance vector

routing protocol still in use, was built and adopted before a formal standard was available (RFC 1058 RIPv1, 2453 RIPv2 )

Implemented in LINUX as the routed process. Adequate only for small and stable ASs based on the Bellman-Ford (or distance vector)

algorithm helps control count to infinity problem by

specifying a maximum hop count of 15


Routing Information Protocol Uses a simple metric, hop count

Not designed to deal with more complicated dynamic metrics such as delay, reliability or load (these can cause route oscillations)

Helps control oscillation between equal cost routes by retaining original route unless a route with a lower cost is found.

Helps prevent slow convergence (after changes in the topology of the network) by sending update messages immediately after updates have been completed (triggered updates)


Routing Information Protocol Helps prevent routing loops by

omitting routes learned from a particular neighbor in the updates sent to that neighbor or other neighbors reached through the same network interface (split-horizon)

advertising routes learned from a particular neighbor with cost 16 (∞) in the updates sent to that neighbor or other neighbors on the same broadcast network (split-horizon with poison reverse),

Nodes are either active (routers) or passive (silent hosts) Active nodes advertise their routing information Passive nodes listen for advertisements and update

their routing tables accordingly


Count to Infinity: split infinity

A learns the route to C from B C learns the route to A from B Initially B can reach A and C can reach A through B When the link from B to A fails then B updates its routing table

to indicate it cannot reach A directly. Without split infinity: When a update arrives at B from C, B

notices that C has a route to A with a cost of 2, and adopts that route. The new route has a cost of (2+1)=3

Split infinity: When an update arrives at B from C, it does not include an entry for A because C learned the route to A from B. B does not update its entry for A, it has no alternative for the broken direct route to A. Eventually, C’s route to A times out .

A B C


Count to ∞ : split infinity poison reverse

A learns the route to C from B C learns the route to A from B Initially B can reach A and C can reach A through B When the link from B to A fails then B updates its routing table

to indicate it cannot reach A directly. Without split infinity: When a update arrives at B from C, B

notices that C has a route to A with a cost of 2, and adopts that route. The new route has a cost of (2+1)=3

Split infinity with poison reverse: B sends an update with a cost of infinity back to C. If C really has a route that is not through B the route sent by B will never be used, otherwise C will immediately update its path cost to A to infinity (Protects reverse routes)

A B C


Nodes in RIP Nodes can be hosts or routers Each node keeps a routing table for every

possible destination network (or active host) in the AS. Each entry in the table includes network or host address, gateway router address,

metric and interface a timer measuring the amount of time since the entry

was last updated. Active nodes send routing information to all

neighbors


Neighbors in RIP All hosts or routers 1 hop from a node are the

neighbors of that node This means that all hosts on a network

segment, including the router or routers for that segment (with a single broadcast address) are neighbors

Directly connected routers are also neighbors Update messages containing routing

information are sent only to neighbors


Initialization: RIP Each node needs to know its own address Each node must be able to identify the links (interfaces)

to which it is attached. Each node starts with a routing table that contains a

single entry, its own localhost Each router broadcasts (advertises) that single entry

through its interfaces to all attached networks. The information reaches all neighbor nodes (directly connected routers)

All nodes directly connected to a network segment receive the information from all routers on that network segment, and add that information to their routing tables.


Updating and RIP Then tables are updated or maintained

Each router will periodically broadcast its routing table

Each router will process the received updates, adding new entries, updating entries for which a lower cost path has been located and updating entries for directly connected nodes whose cost changed

If the routing information changes during the update process the router will immediately broadcast the modified tables to its neighbors (called a triggered broadcast)


Updating and RIP When no further changes occur the

routing table has converged to the topology of the network

When a change is made to the network topology (removing, adding, or modifying a node) The neighbor nodes will detect the change

and update their routing tables the routing algorithm will converges to a new

topology consistent with the changes


Timers in RIP RIP uses three timers

The first timer keeps track of the route advertisement time. Each time it expires (usually every 30 seconds) a message containing the routing information is sent

The second and third types of timers are associated with each entry in the routing table. Each route is given an expiration timer and a garbage collection timer


Timers in RIP RIP uses three timers

The second and third types of timers are an expiration timer and a garbage collection timer If no update information is received before the

expiration timer (usually 180 seconds) expires the route is considered unreachable and its cost is set to ∞ (16)

When the expiration timer expires a garbage collection timer is set (usually 120 seconds). When the garbage collection timer expires the routing table entry is deleted


RIP1 message format

Comer 2000: fig 16.5


RIP2 message format

Comer 2000: fig 16.6


RIP messages RIP messages are sent encapsulated in UDP

packets (port 520). The length of the message is transmitted only by UDP

RIPv1 cannot be used for variable length subnets or for CIDR addresses (must use RIPv2). If subnets are used all receivers must be able to interpret them.

Family gives the family of protocols used (TCP/IP 2)

0.0.0.0 is address for default route


RIP messages (RIPv1 and RIPv2) Two types of messages request (1) and

response (2) A router may request routing information A router may reply to a request by sending a

response Solicited responses will contain only the

requested routes, not all available routes A router may sent unsolicited responses

(periodic broadcasts of routing table information for all routes)


Improvements in RIP2 Next hop field helps eliminate “double

hops” a-b-a The netmask means that RIP2

Can handle subnet routing (not just within subnetted network, extension to RIP1 could do that)

Can deal with CIDR addresses The route tag identifier

Used to flag external (to AS) routes


Improvements in RIP2 Includes an authentication procedure

The first entry in a packet can be replaced by an authentication segment (command will become a 32 bit field containing the command and authentication information, packet length, key … see RFC 2453, 2082 for details )

RIP2 uses random variations on time delays Triggered updates are delayed by 1-5 s Periodic advertisements are randomized by

adding between -5 and 5 seconds


Why use RIP2 “With the advent of OSPF and IS-IS,

there are those who believe that RIP is obsolete. While it is true that the newer IGP routing protocols are far superior to RIP, RIP does have some advantages. Primarily, in a small network, RIP has very little overhead in terms of bandwidth used and configuration and management time. RIP is also very easy to implement, especially in relation to the newer IGPs. “

Quote from RFC 2453 (RIP2)

Documents

1 CMPT 471 Networking II RIP. © Janice Regan, 2006 2 Dynamic Routing In very simple small and stable networks static routing may be adequate. As networks