Distance Vector Routing

1
Distance Vector Routing

• Using the Hop Count as a Metric

2
Distance Vector Forwarding

• Also called Bellman-Ford forwarding
• Used for ARPANET routing several years ago
• This category of routers use some concept of
  route distance to determine the best path
• In addition to destination network addresses and
  ports, distance vector routing tables contain the
  distance (in hops) to each network
• Routing tables in distance vector (DV) routing
  typically contain a number of routes to a given
  network address
• If these tables are sorted based on distance
  vectors (i.e. hop count), the first entry can be
  used (shortest distance)
• This distance can propagate from router to router
  (each incrementing or decrementing as necessary)
  when new networks join

3
Distance Vector Forwarding
N5
N4
R3
N1
3
R1
1
2
R2
N2
N3
4
Distance Vector Tables

• DV routing tables are created dynamically
• DV routers initialize their routing tables with
  entries for networks in which they are directly
  connected (i.e. zero distance links)
• Distance is 0, port is the port which connects
  the router directly to that network
• Periodically, DV routers announce their routing
  table entries
• If a neighbouring router R2 is advertising a
  route for network N with distance D, we can route
  to N with distance (D1)
• Our routing table will get a new entry for each
  of these remote entries
• Distance is D1, port is the port which connects
  the router to R2

5
Distance Vector Tables
N1
N4
1
1
3
4
N2
R1
R2
2
T1
T1
T1
T1
2
3
N5
N3
6
Distance Vector Tables
N1
N4
1
1
3
4
N2
R1
R2
2
T2
T2
T2
T2
2
3
N5
N3
7
Distance Vector Tables
N1
N4
1
1
3
4
N2
R1
R2
2
2
3
N5
N3
8
Distance Vector Tables

• Routers share their routing table information by
  sending datagrams
• These datagrams contain a number of pairs of
  destinations (V) and distances (D)

9
DV Routing Conceptual

• Distance Vector routing requires that each router
  publicly announce the distance to all its
  reachable destinations
• Consider this analogy
• Think of a router as a road intersection in the
  middle of nowhere
• At this location, several signs are posted
  indicating the distance (in kilometres),
  direction (with an arrow on the sign), and the
  name of the destination
• These 3 values correspond to hop count, port, and
  network address, respectively

10
DV Routers Conceptual View
Welcome to Bobton Population 1
Whoohoo! A friend!
11
DV Routing Conceptual

• Continuing this analogy, routers establish
  information the same way you would create these
  signposts
• Travel down each of your paths, and look for
  either
• Other signposts Create a signpost with the same
  elements as the other signpost
• Measure the distance to the signpost
• Add that distance to each distance
• Towns Measure the distance, and post it verbatim
  onto your signpost

12
DV Routing Conceptual

• It is possible that two or more directions could
  lead to the same destination
• In this case, the shortest path is used
• Considering our analogy, two streets could
  potentially lead to the same town
• However, one direction may be a less efficient
  route than the other
• Clearly, we wish to direct traffic towards the
  more efficient route

13
DV Routing Conceptual

• In this way, routers (and sign builders) only
  need to travel down one link in each direction to
  gather routing information
• Until they either find another router (signpost),
  or a destination network (town)

14
RIP

• Routing Information Protocol

15
RIP

• Distance vector forwarding
• Employed (but not exclusively) in the following
  networks
• IP
• IPX

16
How RIP Works

• Participants on an RIP network are either passive
  or active
• Active nodes
• Are routers
• They collect routing information from neighbours
  to build their own routing tables
• They advertise their own routes (using distance
  vectors) to other nodes
• Passive nodes
• Usually hosts or bridges
• They collect routing information from neighbours
  to build their own routing tables
• They do not, however, advertise their own routes,
  because no datagrams are forwarded through
  passive nodes
• In other words, passive nodes are the end links
  in the chain only

17
How RIP Works

• Passive and active nodes create their routing
  tables by listening to RIP response messages
  (explained later)
• These response messages contain lists of distance
  vectors
• When a shorter route (lower distance) is found,
  it overwrites the previous route in the routing
  table
• Equal distances are not overwritten
• Saves processing time

18
RIP Packet Format Version 1

• Version has the value of 1 (for RIPv1)
• Reserved is not presently utilized

octets
command
1
version
1
reserved
2
address family ID
2
address
14
metric
4
May be repeated
19
RIP Packets Command

• Denotes whether the packet is a request message
  (1) or a response message (2)
• These are the only 2 kinds of packets in RIP
• Requests might be transmitted by routers that
  have timed out or outdated information
• Requests essentially ask for information (i.e.
  DVs) from specific (or all) nodes
• Responses might be transmitted in response to a
  request
• Thus the response messages usually contain a list
  of distance vectors

20
RIP Responses

• Are sent
• In response to a request
• Usually issued by nodes with outdated information
  or new nodes
• Periodically
• The RIP specification defines that distance
  vectors should be publicized every 30 seconds
• In response to a change
• If a neighbour (or link to a neighbour) changes
  (e.g. fails), a response message is automatically
  transmitted to neighbour nodes

21
RIP Packets DV Lists

• Can be up to 25 in each packet
• Contain 3 parts
• Address family ID Identifier describing that
  kind of address is specified
• Address The address of the node to which this
  DV refers
• Metric The distance in some metric (e.g. hop
  count)
• The large space allocated for addresses (14
  octets) can be used in different ways in networks
  that have smaller addresses
• E.g. In IP networks, 2 octets is left unused
  before the 4 octet address, and 8 octets is left
  unused after the address (see page 301 for a more
  complete picture)

22
DV Routing Problems Example

• Consider the network shown below

B
A
C
1
1
1
2
23
DV Routing Problems Example

• What happens if we have link failure between B
  C?
• B will realize there is a problem (ignored
  messages, etc.), and tries to determine another
  route to C

B
A
C
1
1
1
2
X
24
DV Routing Problems Example

• B will ask its neighbours (i.e. A), if they can
  reach C
• How will A respond?
• Look at As routing table to find the answer

B
A
C
1
1
1
2
X
25
DV Routing Problems Example

• A thinks that it can reach C in 2 hops
• This path is through B, but that is not known to
  A
• B will get this information and (falsely) think
  it can access C through A
• B believes that the path is 3 hops

B
A
C
1
1
1
2
X
26
DV Routing Problems Example

• Now suppose B has a message for C
• It will send the message to A
• Just as its routing table suggests
• A will send the message back to B
• This will put the packet into a continuous loop

B
A
C
1
1
1
2
X
27
DV Routing Problems Example

• This loop could eventually be resolved by network
  management protocols, but
• It takes a while for A and B to realize there is
  a routing loop
• A will eventually invalidate its route, and use
  Bs distance vector (314) for its new route (it
  now seems possible to get to C via B)
• A will soon after send out its distance vector
• This will cause B to invalidate its route, and
  use As distance vector (415), etc. (to
  infinity)
• Once A and B determine that no route is possible
  the process may continue backward (if A had other
  neighbours)

B
A
C
1
1
1
2
X
28
Count to Infinity

• The example shows a route towards a host
  (destination node) D

Network
R1
R2
R3
D
29
Count to Infinity

• What happens if there is link failure between R1
  and D?
• R1 will detect the change and look for
  alternative routes

Network
R1
R2
R3
D
X
30
Count to Infinity

• R2 will advertise that it has a route to D (with
  distance 2)
• R1 is unaware that this route passes through
  itself to D (and as such is invalid)
• Because only the distance is advertised (not the
  link state), there is no way for R1 to know that
  R2 does not have a viable route

Network
R1
R2
R3
?
D
31
Count to Infinity

• R1 will replace its routing table entry for D
  with one pointing to R2 for packets addressed to
  D
• The distance will be 3 (21)
• The new distance vector is sent to R2

D
3
Network

?
R1
R2
R3
D
32
Count to Infinity

• R2 will replace its routing table entry for D
  with one pointing to R1 for packets addressed to
  D
• The distance will be 4 (31)
• The new distance vector is sent to R1

D
4
Network

?
R1
R2
R3
D
33
Count to Infinity

• R1 will replace its routing table entry for D
  with one pointing to R2 for packets addressed to
  D
• The distance will be 5 (41)
• The new distance vector is sent to R2
• etc (to infinity)

D
5
Network

?
R1
R2
R3
D
34
Count to Infinity

• Once infinity is reached, R2 concludes
  (correctly) that R1 cannot deliver packets to D
• R2 will use the distance vector from R3 to find
  an alternative path to D
• If none exists (as in the example below), this
  process will propagate backwards to the very
  source of the message

Network
?
R1
R2
R3
D
35
Slow Convergence

• Slow convergence describes when this situation
  occurs
• The routers in the network spend a large amount
  of time in an inconsistent state
• Some routers know of the failure, while others do
  not
• During this time, messages addressed to D are
  being lost and/or looping between routers
• The routers of the network (as a whole) slowly
  converge on the correct configuration to route
  (or determine routing impossible) to D

36
Split Horizon Updates

• One preventative measure for counting to
  infinity
• Distance vectors are not transmitted to the same
  port where the original distance vector came
  from, on which this distance vector is based
• For example
• R1 may advertise a distance vector to D (D,1)
• R2 will receive this distance vector and create
  its own (D,2)
• R2 will not propagate this distance vector to R1
  at any time
• It will, however, propagate it to through all
  other links

37
Split Horizon Updates

• Problems
• Information about where distance vectors came
  from must be stored in the routing table
• It is possible to create the count to infinity
  problem using three or more routers in a cycle
• Thus, split horizon is not a complete solution to
  the count to infinity problem

38
Hold Down

• Routers wait before distributing their
  information about a disconnected network for a
  period of time
• For RIP, the distance vectors expire in 180
  seconds
• Before this time, routers will continue to
  forward this data incorrectly
• After this time, all routers will remove all
  paths containing the disconnected link from their
  routing tables
• Distance vectors are considered out-of-date if
  the vector is not retransmitted
• After this time, the new routing information
  (minus the failed link) is distributed and new
  routes are created
• If a route to the new destination is not found at
  this time, the routers will not have entries
  (thus it is unreachable)

39
Hold Down

• Problem
• The routers are expected to wait a long time
  (180s)
• During this time, routers are in an inconsistent
  state
• An ideal solution would result in routers
  remaining consistent throughout the process

40
Poison Reverse

• Routers detecting a disconnected host or network
  keep its entry, but increase its cost to infinity
• The new distance vector(s) are immediately
  transmitted
• This prompts all other routers to find an
  alternative route if possible
• If no alternative routes are found, the routers
  will determine the host to be unreachable

41
RIP Count to Infinity Solutions

• Infinity in RIP is 16
• This way, the number of messages during count to
  infinity is limited
• However, it also means that the width of the
  network (host to host) must be less than 16
• If costs gt 1 are to be used, the width must be
  even less
• This imposes a serious restriction on the network
  size
• Most RIP implementations use split horizon
• However, some RIP implementations use poison
  reverse