1. Lecture 15 and 16 Ch 12: Multicast Routing Section 12
Lecture 15 and 16 Ch 12: Multicast Routing Section 12.3 IGMP Chapter 9: ICMPv4
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2. Lecture 15 Ch 12: Multicast Routing
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3. Put Your Unicast Routing HAT on
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4. Recall Unicasting Vs Multicasting Vs Broadcasting
Unicasting Case:
In unicast routing, the router forwards the received packet through only one of its interfaces.
Both the source and destination addresses are unicast addresses
One-to-One Relationship
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5. Multicasting Case
Packet starts from source, S1, and goes to all destinations belonging to group, G1
In multicast routing, the router may forward the received packet through several of its interfaces.
The source address is unicast and the destination address is a group address (Class D)
Group address define a set of Rx’s
One-to-Many Relationship
Actually, the packet is DUPLICATED at each router – only one copy travels in between any two routers
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6. Broadcasting Case One-to-all case would cause traffic problems
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7. Multicasting versus multiple unicasting
Recall Multicasting
One packet start from source and is duplicated at each router
Only one copy of the packet travels in between any two routers
Packet has a single group address
Multiple Unicast Case
More than one copy of the packet starts from the source
Each copy of the packet has a different destination address
In this case, there could be multiple copies traveling between any two routers
Multicast is more efficient than multiple unicast because in the unicast cast, some links will use more bandwidth in handling more packet copies. Also for the unicast case, there is more delay at the source due to packet duplication
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8. MULTICAST ROUTING Now we understand what multicasting is
Now let’s understand how packets are routed in the multicast case
Some Objectives of multicast routing (very complex)
Each Rx of the group must get only one copy of the packet
Rx’s not belonging to the group DO NOT get a copy of the packet
The packet can not visit the same router more than once (no loops)
The route from Tx to various Rx’s must be optimal (shortest path)
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9. MULTICAST TREES
Recall in the RIP/OSPF unicast World, we converted networks to graphs in finding the optimal routes
For graphs, nodes can have both successors and predecessors
In the multicast World, networks are converted to trees
Tree has a hierarchical structure
Each node on a tree has (1) a single parent and (2) zero to multiple off-springs (children)
The root (source or Tx) of the tree is the initial node (has no parent)
A leaf (group members) of a tree has no child
Called Spanning Tree provided all nodes are connected
NOTE: show students difference between graph & tree
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10. Two Types of Trees are used for multicasting by protocols:
Source-Based Trees – a single tree is created for each Source-to-Group combination. For example, given M sources and N groups, there would be a maximum of MxN trees
Group-Shared Trees – each group has it’s own tree. Given N groups, there would be a maximum of N trees.
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11. Source-Based Tree
Given a source needs to send a packet to group-1, a certain tree is used
Given the same source needs to send a packet to group-5, a different tree is used
Challenge: (1) determining all source-to-group combinations, (2) each tree needs to be optimal
Two approaches used to create optimal source-based trees:
(1st) An extension to the unicast distance vector routing we covered in regards to RIP – used by DVMRP (Distance Vector Multicasting Routing Protocol)
(2nd) An extension to the unicast link state routing we covered in regards to OSPF – used by MOSPF (Multicast Open Shortest Path First)
Another protocol called PIM-DM (Protocol Independent Multicast – Dense Mode) uses either RIP or OSPF depending on need
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12. Group-Shared Tree
Given a source (source x) needs to send a packet to group-1, a certain tree is used
Given a different source (source y) needs to send a packet to the same group, group-1, the same tree is used
If either source-x or source-y need to send to a different group, the tree would change
Note: the tree changes when the group changes – the tree remains the same for a group regardless if the source changes – the group determines the tree
Two approaches used to create optimal group-shared trees:
(1st) Steiner Tree – the optimal tree has the minimum cost routes like Dijkstra’ algorithm however, instead of it being based around a source node, it is not based around any particular source (very complex and has to re-run every time the topology changes) – not really used by the Internet
(2nd) Rendezvous-Point Tree – a tree is created for each group and a single router is selected as the core or rendezvous point or root of the tree. The CBT (Core-Based Tree Protocol) and PIM-SM (Protocol Independent Multicast – Sparse Mode) use the rendezvous-point tree approach.
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13. Multicast routing protocols
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14. DVMRP
Distance Vector Multicast Routing Protocol –similar to the distance vector routing protocol we covered for the unicast case – next hop scenario.
For DVMRP, the optimal tree is not pre-defined – only the next hop
How do we build a tree using the DVMRP approach ?
Use a modified “flooding” approach
Recall what flooding is: a router sends a copy of a packet out of all of it’s interfaces – all interfaces except the interface the packet came in on
Flooding will cause looping problems (ie. the same packet copy that left the router will re-visit the router)
The flooding is modified to stop the looping problem
How is the flooding modified ??????
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15. DVMRP - How is the flooding modified ??????
Instead of forwarding copies of the packet through all interfaces (except the receiving interface), ONLY FORWARD THE PACKET IF IT CAME IN ON THE SHORTEST PATH
If it comes in on the non-shortest path – drop it
This approach of only forwarding the packet if it comes in on the shortest path is called Reverse Path Forwarding (RPF) – RPF prevents looping
How does the router determines if the packet came in on the shortest path ???
Recall that the unicast routing tables contain the next hop based around the shortest path – the table has destinations, interfaces and next hops en route to destinations
If the router used the packet’s source address (instead of destination address), the router could determine the NEXT HOP and desired INTERFACE to exit en route to the packet’s source address
Punch Line: if the INTERFACE the packet arrived at, is the same INTERFACE the packet needs to take in achieving the shortest path en route to the source address – then the PACKET ARRIVED USING THE SHORTEST PATH – make sense ??
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16. DVMRP Continuing EXAMPLE
A multicast router receives a packet with source address and destination address from interface 2. Should the router discard or forward the packet based on the following unicast table ?
Destination
Interface 1 2 3
SOLUTION: In interpreting the source address of using the default mask, the router would send the packet to network via interface 3 (not interface 2). Recall the packet came in on interface 2 – therefore, the router would DROP the packet (and not forward it)
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17. DVMRP Continuing
What RPF guarantees is: each network will receive a copy of the multicast packet WITHOUT the loop problem
What RPF doesn’t guarantee is: each network will receive ONLY ONE COPY
With the Reverse Path Forwarding approach, networks could received multiple copies (see example below)
In fixing this problem, a policy called Reverse Path Broadcasting (RPB) can be implemented.
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18. Reverse Path Broadcasting (RPB)
To eliminate networks (nodes) receiving more than one copy, ONLY THE PARENT HAS THE RIGHT TO FORWARD (this is the RPB policy)
Recall: with a tree, each node has only ONE PARENT
Therefore, if the parent is the only node that can forward, no node should receive duplicates
Policy: the router sends the packet only out of those interfaces for which it is the designated parent.
See the example 
The next question: “How is the parent determined ????”
The router with the shortest path to the source is designated as the PARENT
Recall: because routers share info with their neighbors, they can easily determine which router has the shortest path to the source
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19. RPB creates a shortest path broadcast tree (not multicast 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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20. Reverse Path Multicasting (RPM)
Recall RPB broadcast a packet versus multicast
How is multicasting achieved ? (1) the first packet is broadcasted no matter what, (2) the remaining packets are multicasted based on pruning and grafting
Another name for pruning and grafting is Reverse Path Multicasting (RPM)
Based on pruning
Using the IGMP (Internet Group Management Protocol), each PARENT ROUTER holds a membership and knows which groups it is not responsible for.
The PARENT ROUTER sends a “prune message” to it’s upstream router letting the upstream router know NOT to send any packets belonging to certain no-interest groups through that corresponding interface.
That upstream router will do the same to it’s upstream router
This creates a “pruning” effect in that only the packets belonging to a group are forwarded through a particular interface
Based on grafting
Suppose a “leaf” router (a router with NO children) had previously sent a prune message and suddenly realize it NOW INTERESTED in receiving the multicast packet
The leaf router will issue a grafting message upstream and as a result, multicasting will resume
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21. RPM adds pruning and grafting to RPB to create a multicast shortest path tree that supports dynamic membership changes.
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22. MOSPF MOSPF stands for Multicast Open Shortest Path First
Extension of the OSPF protocol
Instead of the tree being generated gradually – it’s generated all at once – by using the link state database (recall)
With the link state database, the router can see the entire topology
Each router could then use Dijikstra’s algorithm and obtain a least cost tree for each router (or node)
For multicasting routing, we need a tree for each source/group pair
For the source/group trees, the only hosts with the particular group address are included
We do the previous by associating the unicast address with the group address – with this approach, we do the calculation the same way using the unicast address however, the associated group address dictates if the host is added to the tree or not
MOSPF is a data-driven protocol – the first time a MOSPF router sees a datagram with a given source and group address, the router calculates Dijkstra
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23. Core-Based Tree (CBT) Protocol
Is a group-shared protocol
Autonomous systems are divided into regions and a core router or rendezvous point is used for each region
In forming a tree:
1st: the core router or rendezvous router is selected (very complex - will not cover this process – not covered in your book as well)
2nd: all other routers are informed of the unicast address of rendezvous router
3rd: all routers wanting to join group sends a “join message” to the rendezvous router
4th: the intermediate routers between the rendezvous router and Tx router record the address of the source and the interface in which the packet came into the router on
5th: after the rendezvous has received all joined messages – the tree is formed
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24. CBT - Sending a multicast packet
Now that the tree is formed, how are multicast packets sent ?
Any particular source can send a multicast packet to the group by:
1st: source (inside or outside of the shared tree) sends packet to rendezvous router (via the rendezvous router’s unicast address)
2nd: rendezvous router then sends the packet to the group members
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25. DVMRP & MOSPF Versus CBT
For DVMRP and MOSPF, the tree is created from the root
For CBT, the tree is created starting from the leaves
For DVMRP, the tree is first made via broadcast and then pruned into a multicast tree
For CBT, initially there is no tree and then a tree is created gradually via grafting (ie. announcing to the core you want to be apart of the group)
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26. Protocol Independent Multicast – Dense Mode
(PIM-DM)
PIM-DM is used in a dense multicast environment, such as a LAN environment.
PIM-DM is justifiably used when each router is involved in multicasting – therefore broadcasting is justified
PIM-DM uses reverse path forwarding, pruning and grafting techniques for multicasting
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27. Protocol Independent Multicast – Sparse Mode
(PIM-SM)
PIM-SM is used in a sparse multicast environment, such as a WAN environment.
PIM-SM is used when there is a slight possibility each router is involved in multicasting – therefore NOT justifying broadcasting
PIM-SM operates more like CBT
PIM-SM allows the ability to switch between source-based tree and group- shared tree strategies
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28. Multicast Backbone (MBONE)
There are many more unicast oriented routers in the Internet than multicast routers (ie. routers able to multicast)
In creating more links between multicast routers, the concept of “tunneling” is used
Tunneling - via unicast routers, multicast routers are logically connected – in essence we create a multicast backbone in logically linking the multicast routers
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29. MBONE – How are tunnels created ?
How to create a tunnel
1st: encapsulate multicast packet inside a unicast packet (in the data field)
2nd: the unicast intermediate routers route the packet to the next multicast router
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30. Internet Control Message Protocol
Chapter 9
Internet Control Message Protocol
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31. Recall - (1) Explain Creating a Table
Recall – (2) Explain How the Router Uses the Table
Mask Destination Next Hop I. m0 m1 m1 m0 U UG
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32. ICMP
IP, as an unreliable protocol, is not concerned with error checking and error control. ICMP was designed, in part, to compensate for this shortcoming. ICMP does not correct errors, it simply reports them.
ICMP messages are divided into error-reporting messages and query messages. The error-reporting messages report problems that a router or a host (destination) may encounter. The query messages get specific information from a router or another host.
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33. ICMP encapsulation
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34. ICMP messages
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35. 9.2 MESSAGE FORMAT
An ICMP message has an 8-byte header and a variable-size data section. Although the general format of the header is different for each message type, the first 4 bytes are common to all.
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36. ICMP always reports error messages to the original source.
Error-reporting messages
ICMP always reports error messages to the original source.
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37. Note: The following are important points about ICMP error messages:
No ICMP error message will be generated in response to a datagram carrying an ICMP error message.
No ICMP error message will be generated for a fragmented datagram that is not the first fragment.
No ICMP error message will be generated for a datagram having a multicast address.
No ICMP error message will be generated for a datagram having a special address such as or
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38. Other destination-unreachable messages can be created only by routers.
Destination-unreachable format
Destination-unreachable messages with codes 2 or 3 can be created only by the destination host.
Other destination-unreachable messages can be created only by routers.
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39. NOTE: IP doesn’t have Flow Control.
Source-quench format
NOTE: IP doesn’t have Flow Control.
A source-quench message informs the source that a datagram has been discarded due to congestion in a router or the destination host.
The source must slow down the sending of datagrams until the congestion is relieved.
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40. Time-exceeded message format
Whenever a router decrements a datagram with a time- to-live value to zero, it discards the datagram and sends a time-exceeded message to the original source.
When the final destination does not receive all of the fragments in a set time, it discards the received fragments and sends a time-exceeded message to the original source.
In a time-exceeded message, code 0 is used only by routers to show that the value of the time-to-live field is zero. Code 1 is used only by the destination host to show that not all of the fragments have arrived within a set time.
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41. Parameter-problem message format
A parameter-problem message can be created by a router or the destination host.
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42. Redirection concept
A host usually starts with a small routing table that is gradually augmented and updated. One of the tools to accomplish this is the redirection message.
A redirection message is sent from a router to a host on the same local network.
Router forwards packet to correct router and sends “redirection” message to host so host can correct table
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43. 9.4 QUERY
ICMP can also diagnose some network problems through the query messages, a group of four different pairs of messages. In this type of ICMP message, a node sends a message that is answered in a specific format by the destination node.
The topics discussed in this section include:
Echo Request and Reply
Timestamp Request and Reply
Address-Mask Request and Reply
Router Solicitation and Advertisement
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44. Echo-request and echo-reply messages
An echo-request message can be sent by a host or router. An echo- reply message is sent by the host or router which receives an echo- request message.
Echo-request and echo-reply messages can be used by network managers to check the operation of the IP protocol.
Echo-request and echo-reply messages can test the reachability of a host. This is usually done by invoking the ping command.
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45. Timestamp-request and Timestamp-reply message format
Timestamp-request and timestamp-reply messages can be used to calculate the round-trip time between a source and a destination machine even if their clocks are not synchronized.
The timestamp-request and timestamp-reply messages can be used to synchronize two clocks in two machines if the exact one-way time duration is known.
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46. Mask-request and mask-reply message format
Mask-request and Mask-reply messages can be used to get a mask for a particular IP address
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47. Router-solicitation/advertisement message format
Router-Solicitation Message – router uses this message in determining if adjacent routers are alive or not
Router-Advertisement Message – router uses this message in gathering info on the other routers connected to the same network
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48. ICMP CHECKSUM
In ICMP the checksum is calculated over the entire message (header and data).
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49. 9.6 DEBUGGING TOOLS
We introduce two tools that use ICMP for debugging: ping and traceroute.
The topics discussed in this section include:
Ping
Traceroute
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50. The ping program operation
We use the ping program to test the server fhda.edu. The result is shown below:
$ ping fhda.edu PING fhda.edu ( ) 56 (84) bytes of data. 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=0 ttl=62 time=1.91 ms 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=1 ttl=62 time=2.04 ms 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=2 ttl=62 time=1.90 ms 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=3 ttl=62 time=1.97 ms 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=4 ttl=62 time=1.93 ms
64 bytes from tiptoe.fhda.edu ( ): icmp_seq=5 ttl=62 time=2.00 ms 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=6 ttl=62 time=1.94 ms 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=7 ttl=62 time=1.94 ms 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=8 ttl=62 time=1.97 ms 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=9 ttl=62 time=1.89 ms 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=10 ttl=62 time=1.98 ms --- fhda.edu ping statistics packets transmitted, 11 received, 0% packet loss, time 10103ms
rtt min/avg/max = 1.899/1.955/2.041 ms
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51. Trace Route
We use the traceroute program to find the route from the computer voyager.deanza.edu to the server fhda.edu. The following shows the result:
$ traceroute fhda.edu traceroute to fhda.edu ( ), 30 hops max, 38 byte packets 1 Dcore.fhda.edu ( ) ms ms ms 2 Dbackup.fhda.edu ( ) ms ms ms 3 tiptoe.fhda.edu ( ) ms ms ms
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52. Section 12.3: Internet Group Management Protocol
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53. Unicast – one-to-one relationship
Position of IGMP in the network layer
Unicast – one-to-one relationship
Multicast – one-to-many relationship – IGMP helps facilitate that one-to-many relationship
Like ICMP, IGMP is a companion to IP
IGMP is NOT a multicast routing protocol – but rather a protocol that manages the group membership
IGMP gives the multicast routers info about the membership status of hosts (routers) connected to the network. .
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54. 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.
(Visualize a set of “multicast” routers amongst a set of “unicast” routers – and IGMP’s job is to facilitate this communication and info amongst the “multicast” routers”)
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55. IGMP MESSAGES
IGMP has three types of messages: the query, the membership report, and the leave report. There are two types of query messages, general and special.
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56. IGMP message format
Amount of time a query must be answered in – 10ths of a second units
Checksum over the entire 8-byte message
Shows the type of message
0 for general query: contains group id for special query, membership report and leave report messages
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57. IGMP OPERATION
A multicast router connected to a network has a list of multicast addresses of the groups with at least one loyal member in that network. For each group, there is one router that has the duty of distributing the multicast packets destined for that group.
The topics discussed in this section include:
Joining a Group
Leaving a Group
Monitoring Membership
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58. IGMP operation
A multicast router connected to a network has a list of multicast addresses of the groups with at least one loyal member in that network. For each group, there is one router that has the duty of distributing the multicast packets destined for that group.
A host can have a membership in a group – this means one of that host’s processes receives a multicast packet
Routers R1, R2 and R list of groupids are mutually exclusive
A muticast router can have a membership in a group – this means one of that router’s interfaces receives a multicast packet
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59. Membership report – Joining A Group
A host or router can join a group
A host maintains a list of processes that have group membership
If a process wants to join a group, the host adds process and the desired group to its list
If it is the first time entry, the host sends a “membership report” message to the distributing router (in order to receive multicast packets fro that desired group)
A router can join a group
A router maintains a list of interfaces that have group membership
If an interface wants to join a group, the router adds the interface and the desired group to its list
If it is the first time entry, the router sends a “membership report” message. The message is sent out of all interfaces other than one from which the new interest comes
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60. In IGMP, a membership report is sent twice, one after the other.
Note:
In IGMP, a membership report is sent twice, one after the other.
(if the first is lost or damaged, the second one should make it.)
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61. Leave report
When a host (or router) sees that no process is interested in a specific group, it sends a leave report
After receiving a leave report, the router doesn’t automatic remove the groupid – there could be other interested hosts or interfaces – therefore the router sends a special query message – if no feedback is received in a specified amount of time – it then purges the groupid from the list
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62. General query message
What about the case when there is only 1 host interested in a particular groupid and that host goes down ? Does the router maintain that groupid or what ?
The router periodically sends “general query” messages – the general query message queries for membership continuation for all groups (not just one) – if no response is received for a particular groupid (it is removed) – if more than one host/router are interested in the same group – only one host/router responds – cuts down on traffic
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63. Delayed Response
If more than one host/router are interested in the same group – only one host/router responds – cuts down on traffic – how is this implemented ? Delayed Response
Each router needing to send a response has randomly generated wait times before sending a report FOR EACH group – because the reports are broadcasted – the router will know if some other router has already sent a report regarding the groupid (therefore relinquishing it from having to send a report
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64. Example 1
Imagine there are three hosts in a network as shown 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.
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65. Example 1 (Continued) Solution The events occur in this sequence:
a. Time 12: The timer for 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
b. Time 30: The timer for 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
c. Time 50: The timer for in host B expires and a membership report is sent, which is received by the router and every host.
d. Time 70: The timer for 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 timerfore
Note that if each host had sent a report for every group in its list, there would have been seven reports; with this strategy only four reports are sent.
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66. Encapsulation of IGMP packet
The IGMP message is encapsulated in an IP datagram, which is itself encapsulated in a frame.
Because the IGMP occurs within the physical LAN, the TTL of the IP is set to 1 – guarantees the message doesn’t leave the LAN
Regarding the data link layer:
Because the IP packet has a MULTICAST address, ARP can’t be used in finding the physical address and forwarding – therefore, the data link layer (or underlying technology) must support multicast addressing
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67. Mapping class D to Ethernet physical address
Ethernet supports physical multicast addressing
If the first 25 bits indicate this pattern, then the remaining 23 bits can take on a group
The router extracts the least significant 23 bits of the class D – however, the class D is 28 bits – therefore, 25 (32) multicast addresses are mapped to a single multicast address at the IP level
Therefore, the host must check the IP and discard any packets that do not belong to it.
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68. Note:
An Ethernet multicast physical address is in the range 01:00:5E:00:00:00 to 01:00:5E:7F:FF:FF.
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69. Solution We can do this in two steps:
Example 2
Change the multicast IP address to an Ethernet multicast physical
Solution We can do this in two steps:
a. We write the rightmost 23 bits of the IP address in hexadecimal. This can be done by changing the rightmost 3 bytes to hexadecimal and then subtracting 8 from the leftmost digit if it is greater than or equal to 8. In our example, the result is 2B:0E:07.
b. We add the result of part a to the starting Ethernet multicast address, which is (01:00:5E:00:00:00). The result is
01:00:5E:2B:0E:07
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70. Example 3
Change the multicast IP address to an Ethernet multicast address.
Solution
a. The right-most three bytes in hexadecimal are D4:18:09. We need to subtract 8 from the leftmost digit, resulting in 54:18:09..
b. We add the result of part a to the Ethernet multicast starting address. The result is
01:00:5E:54:18:09
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71. Tunneling
Most WANs do not support physical multicast addressing – therefore tunneling is used – the multicast packet is encapsulated in the unicast packet and sent through the network
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72. Example 4
We use netstat with three options, -n, -r, and -a. The -n option gives the numeric versions of IP addresses, the -r option gives the routing table, and the -a option gives all addresses (unicast and multicast). Note that we show only the fields relative to our discussion.
$ netstat -nra Kernel IP routing table Destination Gateway Mask Flags Iface U eth U eth U lo U eth UG eth0
Any packet with a multicast address from to is masked and delivered to the Ethernet interface.
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