Unicast and Multicast

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Chapter 21
Unicast and Multicast Routing Routing Protocols
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21.1 Unicast Routing
Metric
Interior and Exterior Routing
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Figure 21.1 Unicasting

• A metric is the cost assigned for passage of a
  packet through a network.
• A router consults its routing table to determine
  the best path for a packet.

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Note
In unicast routing, the router forwards the
received packet through only one of its ports.
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21.2 Unicast Routing Protocols
RIP (Routing Information Protocol)
OSPF (Open Shortest Path First)
BGP (Border Gateway Protocol)
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Figure 21.2 Popular routing protocols

• RIP and OSPF are popular interior routing
  protocols used to update routing tables in an
  autonomous system (AS).
• BGP is an interautonomous system routing protocol
  used to update routing tables.

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Figure 21.3 Autonomous systems

• An autonomous system (AS) is a group of networks
  and routers under the authority of a single
  administration.

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Table 21.1 A distance vector routing table
Destination Hop Count Next Router Other information
163.5.0.0 7 172.6.23.4
197.5.13.0 5 176.3.6.17
189.45.0.0 4 200.5.1.6
115.0.0.0 6 131.4.7.19

• A RIP routing table entry consists of a
  destination network address, the hop count to
  that destination, and the IP address of the next
  router.

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RIP Updating Algorithm
Receive a response RIP message 1. Add one hop to
the hop count for each advertised destination. 2.
Repeat the following steps for each advertised
destination 1. If (destination not in the
routing table) 1. Add the advertised
information to the table. 2. Else 1.
If (next-hop field is the same) 1.
Replace entry in the table with the advertised
one. 2. Else 1. If (advertised
hop count smaller than one in the table)
1. Replace entry in the routing table. 3.
Return.

• RIP is based on distance vector routing, in which
  each router shares, at regular intervals, its
  knowledge about the entire AS with its neighbor.

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Figure 21.4 Example of updating a routing table

• Each routing table is updated upon receipt of RIP
  messages using the RIP updating algorithm

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Figure 21.5 Initial routing tables in a small
autonomous system

• The table contains only the directly attached
  networks
• The hop count is initialized to 1

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Figure 21.6 Final routing tables for Figure
21.5

• Previous AS with final routing tables

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Figure 21.7 Areas in an autonomous system

• OSPF is based on link state routing, in which
  each router sends the state of its neighborhood
  to every other router in the area. A packet is
  sent only if there is a change in the
  neighborhood.
• OSPF divides an AS into areas, defined as
  collections of networks, hosts, and routers

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Figure 21.8 Types of links

• OSPF defines four types of links (networks)
  point-to-point, transient, stub, and virtual.

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Figure 21.9 Point-to-point link

• A point-to-point link connects two routers
  without any other host or router in between.

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Figure 21.10 Transient link

• A transient link is a network with several
  routers attached to it

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Figure 21.11 Stub link

• A stub link is a network that is connected to
  only one router

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Figure 21.12 Example of an internet

• DS Digital Signal service
• T-1 line DS-1, 1.544 Mbps, 24 voice channels
• T-3 line DS-3, 44.736 Mbps, 672 voice channels

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Figure 21.13 Graphical representation of an
internet

• Square nodes are routers
• Ovals are networks (represented by designated
  routers)

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Figure 21.14 Types of LSAs

• An LSA (link state advertisement) is a mutifield
  entry in a link state update packet.
• Five types of LSAs disperse information in OSPF
  router link, network link, summary link to
  network, summary link to AS boundary router, and
  external link.

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Figure 21.15 Router link

• A router link advertisement defines the links of
  a true router.
• A true router uses this advertisement to announce
  information about all its links and what is at
  the other side of the link (neighbors)

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Figure 21.16 Network link

• A router compiles all the information from the
  LSAs it receives into a link state database. This
  database is common to all routers in an area.
• A network link advertisement defines the links of
  a network.
• A designated router, on behalf of the transient
  network, distributes this type of LSA packet the
  packet announces the existence of all router
  connected to the network

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Figure 21.17 Summary link to network

• R1 floods area 1 with information about how to
  reach a network located in area 0
• R2 floods area 2 with information about how to
  reach the same network in area 0

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Figure 21.18 Summary link to AS boundary router

• The area border routers flood their areas with
  information on routes to autonomous boundary
  routers to allow a router inside an area to send
  a packet to outside the AS

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Figure 21.19 External link

• The external link advertisement provides
  information that allows a router inside an AS to
  know which networks are available outside the AS

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Note
In OSPF, all routers have the same link state
database.
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Dijkstra Algorithm
1. Start with the local node (router) the root
of the tree. 2. Assign a cost of 0 to this node
and make it the first permanent node. 3. Examine
each neighbor node of the node that was the last
permanent node. 4. Assign a cumulative cost to
each node and make it tentative. 5. Among the
list of tentative nodes 1. Find the node with
the smallest cumulative cost and make it
permanent. 2. If a node can be reached from
more than one direction 1. Select the
direction with the shortest cumulative cost.6.
Repeat steps 3 to 5 until every node becomes
permanent.

• The Dijkstra algorithm calculates the shortest
  path between to points on a network, using a
  graph made of nodes and edges
• The algorithm divides the nodes into two sets
  tentative and permanent

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Figure 21.20 Shortest-path calculation

• Some steps of Dijkstra algorithm applied to node
  A of figure 21.13

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Table 21.2 Link state routing table for router A
Network Cost Next Router Other Information
N1 5 C
N2   7 D
N3 10 B
N4 11 D
N5 15 C

• To find the cost of reaching networks outside of
  the are, the routers use the summary link to
  network, the summary link to boundary router, and
  the external link advertisements

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Table 21.3 Path vector routing table
Network Next Router Path
N01 R01 AS14, AS23, AS67
N02 R05 AS22, AS67, AS05, AS89
N03 R06 AS67, AS89, AS09, AS34
N04 R12 AS62, AS02, AS09

• BGP is based of a routing method called path
  vector routing

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Figure 21.21 Path vector messages

• In BGP, the ASs through which a packet must pass
  are explicitly listed.

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Figure 21.22 Types of BGP messages

• There are four types of BGP messages open,
  update, keep-alive, and notification.

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21.3 Multicast Routing
IGMP (Internet Group Management Protocol)
Multicast Trees
MBONE (multicast backbone)
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Figure 21.23 Multicasting

• Multicasting applications include distributed
  databases, information dissemination,
  teleconferencing, and distance learning.
• For efficient multicasting we use a shortest-path
  spanning tree to represent the communication path.

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Note
In multicast routing, the router may forward the
received packet through several of its ports.
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Note
IGMP is a group management protocol. It helps a
multicast router create and update a list of
loyal members related to each router interface.
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Figure 21.24 IGMP message types

• A host or router can have membership in a group.
• The three IGMP message types are the query
  message, the membership report, and the leave
  report.

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Figure 21.25 IGMP message format

• Type Type of message
• Maximum response time Amount of time in which a
  query must be answered
• Checksum Calculated over the 8-byte message
• Group address Groupid (multicast address of the
  group in the special query, the membership
  report, and the leave report messages.

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Table 21.4 IGMP type field
Type Value
General or special query 0x11 or 00010001
Membership report 0x16 or 00010110
Leave report 0x17 or 00010111

• The value of the field is shown in both
  hexadecimal and binary notation

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Figure 21.26 IGMP operation

• IGMP operates locally

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Figure 21.27 Membership report

• A host maintains a list of processes that have
  membership in a group.
• A router maintains a list of groupids that shows
  group membership for each interface.
• A membership report is sent out of all interfaces
  except the one from which the new interest comes

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Note
In IGMP, a membership report is sent twice, one
after the other.
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Figure 21.28 Leave report
No Response

• When a host sees that no process is interested in
  a specific group it sends a leave report

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Note
The general query message does not define a
particular group.
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Figure 21.29 General query message
No Response

• The query message must be sent by only one router
  (normally called the query router)

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Example 1
Imagine there are three hosts in a network, as
shown in Figure 21.30 (below). A query message
was received at time 0 the random delay time (in
tenths of seconds) for each group is shown next
to the group address. Show the sequence of report
messages.

• The events occur in the sequence Time 12, Time
  30, Time 50 and Time 70

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Solution

• The events occur in this sequence
• Time 12. The timer for 228.42.0.0 in host A
  expires and a membership report is sent, which is
  received by the router and every host including
  host B which cancels its timer for 228.42.0.0.
• Time 30. The timer for 225.14.0.0 in host A
  expires and a membership report is sent, which is
  received by the router and every host including
  host C which cancels its timer for 225.14.0.0.
• Time 50. The timer for 251.71.0.0 in host B
  expires and a membership report is sent, which is
  received by the router and every host.
• Time 70. The timer for 230.43.0.0 in host C
  expires and a membership report is sent, which is
  received by the router and every host including
  host A which cancels its timer for 230.43.0.0.

• If each host had sent a report for every group in
  the list, there would have been seven reports
  with this strategy only four reports are sent

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Note
In a source-based tree approach, the combination
of source and group determines the tree.
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Note
In the group-shared tree approach, the group
determines the tree.
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Figure 21.31 Logical tunneling

• The multicast routers are seen as a group of
  routers on the top of the unicast routers.
• The multicast routers may not be connected
  physically, but they are connected logically

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Figure 21.32 MBONE

• For multicasting between two noncontiguous
  multicast routers, we make a multicast backbone
  (MBONE) to enable tunneling.

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21.4 Multicast Routing Protocols
DVMRP (Distance Vector Multicast Routing Protocol)
MOSPF (Multicast Open Shortest Path First)
CBT (Core-Based Tree)
PIM (Protocol Independent Multicast)
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Figure 21.33 Multicast routing protocols

• DVRMP is a multicast routing protocol that uses
  the distance routing protocol to create a
  source-based tree.
• MOSPF is a multicast protocol that uses multicast
  link state routing to create a source-based
  least-cost tree.
• The Core-Based Tree (CBT) protocol is a multicast
  routing protocol that uses a core as the root of
  the tree.
• PIM-DM is a source-based routing protocol that
  uses RPF and pruning and grafting strategies to
  handle multicasting
• PIM-SM is a group-shared routing protocol that is
  similar to CBT and uses a rendezvous point as the
  source of the tree

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Figure 21.34 Reverse path forwarding

• To find if the packet has traveled the shortest
  path, RPF uses the unicast routing table of RIP

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Note
In reverse path forwarding, the router forwards
only the packets that have traveled the shortest
path from the source to the router all other
copies are discarded. RPF prevents the formation
of loops.
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Figure 21.35 RPF versus RPB

• For each source, the router sends the packet only
  out of those ports for which it is designated
  parent.

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Note
RPB creates a shortest-path broadcast tree from
the source to each destination. It guarantees
that each destination receives one and only one
copy of the packet.
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Figure 21.36 RPF, RPB, and RPM

• Pruning is a procedure that stops the sending of
  messages from an interface
• Grafting is a procedure that resumes the sending
  of multicast messages from and interface
• Pruning and drafting are done by IGMP

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Note
RPM adds pruning and grafting to RPB to create a
multicast shortest-path tree that supports
dynamic membership changes.
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Figure 21.37 Unicast tree and multicast tree

• The least-cost trees generated by Dijkstras
  algorithm in MOSPF are different fro each router,
  unlike the database and the topology , which are
  the same for each router

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Figure 21.38 Shared-group tree with rendezvous
router

• When the rendezvous router has received all join
  messages from every member of the group, the tree
  is formed

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Figure 21.39 Sending a multicast packet to the
rendezvous router

• The rendezvous router distributes the packet to
  all members of the group

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Note
In CBT, the source sends the multicast packet to
the core router. The core router decapsulates the
packet and forwards it to all interested hosts.
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Note
PIM-DM uses RPF and pruning and grafting
strategies to handle multicasting. However, it is
independent of the underlying unicast protocol.
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Note
PIM-SM is similar to CBT but uses a simpler
procedure.