IPv6 Routing.pdf

 Routing is the process of selecting paths in a network along which to
send network traffic. It is to find path between source and destination.
All devices or nodes in the way to destination use the destination IP
address to send the packet in the right direction to reach its
destination. The path a packet takes is determined after a router
consults its routing table. After router determines the best path the
packet is encapsulated into a frame and frame is then placed on
network medium.
 Routing protocol is a software process running on the router. It will
exchange routing information with other routers，studying route
information of network not directly connected and adjusting the route
information when topology changes

 Routing protocols are based either on a distance vector, link state,
or path vector technology.
Distance Vector
 Distance vector routing protocols propagate routing information in
the form of an address prefix and its ―distance‖ (hop count).
Routers use distance vector-based routing protocols to periodically
advertise the routes in their routing tables. Routing information
exchanged between typical distance vector-based routers is
unsynchronized and unacknowledged.
 The advantages of distance vector based routing protocols include
simplicity and ease of configuration. The disadvantages of distance
vector-based routing protocols include relatively high network
traffic, a long convergence time, and inability to scale to a large or
very large network.

Link State
 Routers using link state–based routing protocols exchange link
state advertisements (LSAs) throughout the network to update
routing tables. LSAs consist of a router’s attached network
prefixes and their assigned costs. Routers advertise LSAs upon
startup and when changes in the network topology are detected.
Link state updates are sent using unicast or multicast traffic rather
than broadcasting.
 Link state routers build a database of link state advertisements
and use the database to calculate the optimal routes to add to the
routing table. Routing information exchanged between link state–
based routers is synchronized and acknowledged.
 The advantages of link state–based routing protocols are low
network overhead, low convergence time, and the ability to scale
to large and very large networks. The disadvantages of link state–
based routing protocols are that they can be more complex and
difficult to configure.

 Unicast routing is a process that enable sender to send an unicast IP
packets to the destination node.
 1 router or more intermediate routers may be used, depending to the
destination of the node. (Figure 1)
 Unicast routing protocol is a set of rules of forwarding unicast traffic
from a source to a destination on an internetwork.
`
Source (S)
`
Destination (D)
S D S D
S
D
S D S D
Fig. 1. Unicast Routing
The router is using only 1
port to forwards the
received unicast packet

 Unicast Routing Protocol consists of:
◦ RIP (Routing Information Protocol)
◦ OSPF (
◦ BGP
 They each serve a different purpose.
Routing
Interior Exterior
RIP OSPF BGP
Fig. 2. Types of Unicast Routing Protocol

 Based on Distance Vector algorithm known as bellman ford algorithm.
RIPng for IPv6 has a maximum distance of 15, where 15 is the accumulated
cost (hop count). Locations that are a distance of 16 or further are
considered unreachable.
 RIPng for IPv6 is a simple routing protocol with a periodic route-
advertising mechanism designed for use in small to midsize IPv6 networks.
RIPng for IPv6 does not scale well to a large or very large IPv6 network.
 RIPng uses a simple mechanism to determine the metric (cost) of a route. It
basically counts the number of routers (hops) to the destination. Each router
counts as one hop. Routes with a distance greater than or equal to 16 are
considered to be unreachable. The router periodically distributes information
about its routes to its directly connected neighbors using RIPng response
messages. Upon receiving RIPng response messages from its neighbor, the
router adds the distance between the neighbor and itself (usually one, as in
one hop) to the metric of each route received.
 After initialization, the RIPng for IPv6 router periodically announces (every
30 seconds, by default) the appropriate routes in its routing table for each
interface

 Router keeps the following entries in the routing table
◦ IPv6 Route
 Address prefix and prefix length of the destination address
◦ Next Hop Address
 The IPv6 Address (link-local) of the first router along the path
◦ Next Hop Interface
 The physical interface used to reach the next hop
◦ Metric
 Number indicating the total distance to the destination. RIPng
advertizes directly connected routes with the configured outgoing
metric of 1.
◦ Timer: Amount of time since the information about the route was last
updated
◦ Route change flag: Set to control triggered routing updates
◦ Route Source: Entity to provide route information
◦ eg: Ripng, OSPF etc..

RIPng is a UDP-based protocol. Each router that uses RIPng has a routing process
that sends and receives datagrams on UDP port number 521, the RIPng port. All
communications intended for another router's RIPng process are sent to the RIPng
port
20 Bytes/RTE
 Command 1: ask
system to send all
or part of its
routing table
 Command 2: sends
an update message
containing all or
parts of the
senders routing
table.

 RIPng header is followed by one or more routing
table entries (format of Routing table entry)
….
16 B IPv6 Prefix
2 B Route Tag
1 B Prefix Length
1 B Metric(1-16)

 IPv6 Prefix: 128 bits address of the network whose information is being carried.
 Route Tag
It may be used to carry additional information about a route learned from another
routing protocol eg: BGP
◦ Prefix Length: The no of bits of the adress that represents the network portion.
The number of RTEs within single updates depends on the MTU of the medium
between two neighboring routers
No of RTEs=[INT(MTU-IPv6 Hdr len-UDP Hdr len -RIPng Hdr len) / RTE-Size]
 Timers
◦ RIPng uses different timers to control updates of the routing information
◦ Update timer
 By default, every 30 seconds, RIPng process wakesup on each interface to
send an unsolicited routing response to the neighboring routers

 Timeout Timer
◦ Each time a route entry is updated and the timeout
timer is reset to zero
◦ If the route entry reaches 180 secs (default), without
another update, it is considered to have expired,
metric set to 16 and garbage collection process
starts
 Garbage collection timer (hold down timer)
◦ Set to 120 secs (default) that have timeout or been
received with a metric of 16 after expiration, the
route entry finally be removed from the routing table.

Each router maintains a database describing the link states within the
autonomous system. This database is being built by exchanging Link
State Advertisements (LSAs) between neighboring routers. Depending
on its contents, an LSA is flooded to all routers in the autonomous
system (AS flooding scope), all routers within the same area (area
flooding scope), or simply to its neighbors. The flooding always occurs
along a path of neighboring routers, so a stable neighbor relationship is
extremely important for OSPF to work properly. The neighbor
relationship is called adjacency. Each router originates router LSAs
advertising the local state of its interfaces to all routers within the same
area. Additional LSAs are originated to identify links with multiple
routers (multi-access networks), IPv6 routes from other areas, or IPv6
routes external to the OSPF autonomous system. Each router puts the
received LSA into its LSA database, called the Link-State Database
(LSDB).

Using the LSDB as the input, each router runs the same algorithm to
build a tree of least-cost paths (shortest-path-first tree [SPF tree]) to each
route. The LSDB is like having a map of the network used to plot the
shortest paths to each destination. The cost is described by a single
dimensionless metric, which is configurable on each interface of the
router.
The metric assigned to the interface is usually inversely proportional to
its line speed, i.e., higher bandwidth means lower cost. A common
formula was to divide 10^8 by the line speed in bits per second. This
formula is outdated as interface speed today is in the range of 10^9 (e.g.,
Gigabit Ethernet) or even 10^10. Most vendors today apply a nonlinear
formula. You can always choose and implement your own cost metrics
according to corporate standards. OSPF can put multiple equal-cost paths
to the same route into the routing table. The algorithm for distributing
traffic among these paths is at the discretion of the routing process itself,
normally based on the source and destination IPv6 addresses.

 An implementation of a Link-State Routing Protocol
 Open Standard
 Interior Gateway Protocol(inside an AS) developed by
IETF
 Cost/Metric: Link-Bandwidth
 Manages Hello and Link State advertisements
 Implements Shortest-Path-First Algorithm to select
best path
 Supports
◦ Authentication and encryption
◦ VLSM
◦ Multiple areas
 Area functionality allows OSPF to only function over a
subset of a network
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 Can be used in Scalable network.
 Use a hierarchical design principles( Multiple areas connect
to a distribution area or backbone area or Area 0)
 It supports area allocation where OSPF can divide the
network into 2 level areas, they are backbone and non-
backbone, in this way, each area will maintain the
independent LSDB and run SPF algorithm respectively, it is
easier and takes less time to calculate the routes, so, area
allocation can reduce the protocol impact on CPU memory.
Only LSDB of routers in the same area can be synchronized.
The changes of network topology structure are first updated
within the area..
 After areas are allocated, route aggregation is performed on
the boundary router.
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• Each router connects to the backbone called area 0, or the backbone area.
• Routers that connect other areas to the backbone within an AS are called Area Border
Routers (ABRs).
• One interface must be in area 0.
• OSPF runs inside an autonomous system, but can also connect multiple autonomous
systems together.
• The router that connects these ASes together is called an Autonomous System
Boundary Router (ASBR).

 Link State routing
◦ Each node within the autonomous system has the
information about the entire topology.
◦ Each node in the domain build up the routing table using
Dijkstra’s algorithm.
 Link State Database (LSDB) contains link state advertisement is send to
every router in the same domain.
◦ Each router will be updated with the latest copy of LSDB
 Based on the LSDB, router creates a Shortest Path First (SPF) tree
◦ Using Dijkstra’s a Algorithm
 A routing table can be derived from the SPF tree which contains the best
route to each router.

 OSPF for IPv6, also known as OSPFv3, is a link state routing
protocol defined in RFC 5340. It is designed to be run as a routing
protocol for a single autonomous system. OSPF for IPv6 is an
adaptation of the OSPF routing protocol version 2 for IPv4. The
OSPF cost of each router link is a unitless number that the network
administrator assigns, and it can include delay, bandwidth, and other
cost factors. The accumulated cost between network segments in an
OSPF network must be less than 65,535.
 It was designed to overcome some of the limitations introduced by
RIP, such as the small diameter, long convergence time, and a metric
that does not reflect the characteristics of the network. In addition,
OSPF handles a much larger routing table to accommodate large
number of routes.

 Protocol Processing Per-Link, Not Per-Subnet
Multiple IPv6 subnets can be assigned to a single link, and two nodes can talk directly over a single
link, even if they do not share a common IPv6 subnet (IPv6 prefix). For this reason, OSPF for IPv6
runs per-link instead of the IPv4 behavior of per-IP-subnet. The terms "network" and "subnet" used in
the IPv4 OSPF specification ([OSPFV2]) should generally be replaced by link.
 Removal of addressing semantics
IPv6 addresses are no longer present in OSPF packet headers. They are only allowed as payload
information
 Addition of Flooding Scope
Flooding scope for LSAs has been generalized and is now explicitly coded in the
LSA’s LS type field. There are now three separate flooding scopes for LSAs:
o Link-local scope. LSA is only flooded on the local link and no further. Used for the
new link-LSA.
o Area scope. LSA is only flooded throughout a single OSPF area. Used for router-
LSAs, network-LSAs, inter-area-prefix-LSAs, interarea-router-LSAs, and intra-area-
prefix-LSAs.
o AS scope. LSA is flooded throughout the routing domain. Used for AS-external-
LSAs. A router that originates AS scoped LSAs is considered an AS Boundary Router
(ASBR) and will set its E-bit in router-LSAs for regular areas.

 Explicit Support for Multiple Instances per Link
Ability to run multiple OSPF protocol instances on a single link. Support for multiple
protocol instances on a link is accomplished via an "Instance ID" contained in the
OSPF packet header and OSPF interface data structures.
 Use of Link-Local Addresses
IPv6 link-local addresses are for use on a single link, for purposes of neighbor
discovery, auto-configuration, etc. IPv6 routers do not forward IPv6 datagrams having
link-local source addresses [IP6ADDR]. Link-local unicast addresses are assigned
from the IPv6 address range FE80/10
 Authentication Changes
In OSPF for IPv6, authentication has been removed from the OSPF protocol. The
"AuType" and "Authentication" fields have been removed from the OSPF packet
header, and all authentication-related fields have been removed from the OSPF area
and interface data structures. When running over IPv6, OSPF relies on the IP
Authentication Header (see [IPAUTH]) and the IP Encapsulating Security Payload (see
[IPESP]) as described in [OSPFV3-AUTH] to ensure integrity and
authentication/confidentiality of routing exchanges.

 Packet format changes
OSPF for IPv6 runs directly over IPv6. Aside from this, all addressing
semantics have been removed from the OSPF packet headers, making it
essentially"network-protocol-independent".(Details from book)
 Identifying Neighbors by Router ID
In OSPF for IPv6, neighboring routers on a given link are always
identified by their OSPF Router ID. This contrasts with the IPv4
behavior where neighbors on point-to-point networks and virtual links
are identified by their Router IDs while neighbors on broadcast and
point-to-multipoint links are identified by their IPv4 interface addresses
 Handling unknown LSA types
Instead of simply discarding them, OSPF for IPv6 introduces a more
flexible way of handling unknown LSA types. A new LSA handling bit
has been added to the LS Type field to allow flooding of unknown LSA
types

IPv6 Header
NH: 89
OSPF Header OSPF Message
Version 1B
16 Byte
Instance ID 1B
Packet
Length 2B
40Byte
RouterID 4B
AreaID 4B
Checksum 2B
Unused 1B
Packet
Type 1B

 Version: 3
Type: Type of OSPF packet
1- Hello Message
2- Database Description message
3- Link State Request
4- Link State Update
5- Link State Acknowledgement
Packet Length: Length of OSPF packet in bytes. It includes the
standard 16 bytes as well.
Router ID: The 32-bit Router ID of the packet source
Area ID: A 32-bit Area ID indicating the area that this packet belongs
to. Every packet belongs to a single area.
Checksum: Standard 16-bit checksum
InstanceID: Enables multiple instances of OSPF to be run over a single
link. It has local significance only. Received packets whose Instance ID
is not equal to the receiving interface's Instance ID, are discarded.

 How do OSPF maintain Adjacencies
 How they synchronize their link state
database and update using link state update
packet.
(You should submit it writing in a paper next
week later by Tuesday. )

 Border Gateway Protocol (BGP) is an inter-autonomous
routing protocol used on the edge of autonomous
systems (AS).It is the protocol which is used to make
core routing decisions on the Internet.
 This is considered to use a path-vector routing
algorithm( it tracks the path in terms of which AS it
passes through, and does NOT track the 'route'
through individual routers within an AS.)
 BGP is a kind of enhanced distance vector routing
protocol
 Transmission protocol: TCP，port number： 179
 Support CIDR（classless inter domain routing）
 Route updates only send added route
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 BGP is used among ASs to ensure that there
is no loop in the network
BGP
AS300
AS100
B
C C
D
A
130.1.0.0/16
AS 400
AS200
130.1.0.0/16
AS:100
130.1.0.0/16
AS:200 100
130.1.0.0/16
AS:200 100
130.1.0.0/16
AS:400 200 100

BGP has four kinds of messages:
 OPEN – used to establish BGP connection
 KEEPALIVE – used to keep BGP connection
 UPDATE – used to update or withdraw BGP route
 NOTIFICATION – BGP error notification
(Full routing updates are sent at the start of the session,
trigger updates are sent subsequently. This creates and
maintains connections between peers, using TCP port
179. The connection is maintained by periodic keep alive.
The failure to see a keep alive, an update, or a
notification is the means by which destination networks
and paths to those destinations are tracked. Any change
in the network results in a triggered update. )
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There are two types of BGP neighbor relationships:
• iBGP Peers – BGP neighbors within the same
autonomous system.
• eBGP Peers – BGP neighbors connecting separate
autonomous systems
 Once BGP peers form a neighbor relationship, they
share their full routing table. Afterwards, only
changes to the routing table are forwarded to
peers.

Accept those routes without AS loop and with valid next-hop
address. Then make decision for route as follows:
 Local preference—routers will prefer the path with the largest
value.
 Local router—if local preferences are the same, the preferred
route is one that was originated by the BGP process on this
local router (it might have redistributed into BGP from an IGP
also running on the router)
 AS path—if the route wasn’t originated here, then choose the
path with the shortest AS path value
 Origin code- if the path lengths are the same, prefer a route
with lowest origin type, where IGP is less than EGP
 MED—if there’s still no difference, the path with the lowest
Multi Exit Discriminator is preferred. Since MED is an optional
configuration, this may not apply, and you’ll need to check
whether a missing MED value is counted as worst or best case
in your implementation of the protocol, as they differ. MED
Informs external neighbors about the preferred path into an
AS that has multiple entry points.
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 EBGP vs. IBGP—a route learned via EBGP is
preferred
 Smallest internal path cost to the next hop
 Choose the route with the lowest neighbor Router-ID
 Choose the route with the lowest neighbor interface
address
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 BGP Multiprotocol Extension for IPv6
 The only three pieces of information carried by BGP-4 that are IPv4
specific are
(a) the NEXT_HOP attribute (expressed as an IPv4 address),
(b) AGGREGATOR (contains an IPv4 address), and
(c) NLRI (expressed as IPv4 address prefixes).
 To enable BGP-4 to support routing for multiple Network Layer protocols,
the only two things that have to be added to BGP-4 are
(a) the ability to associate a particular Network Layer protocol with the next
hop information
(b) the ability to associate a particular Network Layer protocol with NLRI.
(Network Layer Reachability Information)

This is an optional non-transitive attribute that can be used for the following
purposes:
 (a) to advertise a feasible route to a peer
 (b) to permit a router to advertise the Network Layer address of the router
that should be used as the next hop to the destinations listed in the Network
Layer Reachability Information field of the MP_NLRI attribute
+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Length of Next Hop Network Address (1 octet) |
+---------------------------------------------------------+
| Network Address of Next Hop (variable) |
+---------------------------------------------------------+
| Reserved (1 octet) |
+---------------------------------------------------------+
| Network Layer Reachability Information (variable) |

 Address Family Identifier (AFI):
This field in combination with the Subsequent Address Family Identifier
field identifies the set of Network Layer protocols to which the address
carried in the Next Hop field must belong, the way in which the address
of the next hop is encoded, and the semantics of the Network Layer
Reachability Information that follows.( If the Next Hop is allowed to be
from more than one Network Layer protocol, the encoding of the Next
Hop MUST provide a way to determine its Network Layer protocol)
 Subsequent Address Family Identifier (SAFI):
This field in combination with the Address Family Identifier field
identifies the set of Network Layer protocols to which the address
carried in the Next Hop must belong, the way in which the address of
the next hop is encoded, and the semantics of the Network Layer
Reachability Information that follows
 Length of Next Hop Network Address:
A 1-octet field whose value expresses the length of the "Network
Address of Next Hop" field, measured in octets.

 Network Address of Next Hop:
A variable-length field that contains the Network Address of the next
router on the path to the destination system. The Network Layer
protocol associated with the Network Address of the Next Hop is
identified by a combination of <AFI, SAFI> carried in the attribute.
 Reserved:
A 1 octet field that MUST be set to 0, and SHOULD be ignored upon
receipt.
 Network Layer Reachability Information (NLRI):
A variable length field that lists NLRI for the feasible routes that are
being advertised in this attribute. The semantics of NLRI is identified
by a combination of <AFI, SAFI> carried in the attribute.

 This is an optional attribute that can be used for the purpose of
withdrawing multiple unfeasible routes from service.
 The attribute is encoded as shown below:
+-------------------------------------------------+
| Address Family Identifier (2 octets) |
+------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+-------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------+
Withdrawn Routes Network Layer Reachability Information:
A variable-length field that lists NLRI for the routes that are being
withdrawn from service. The semantics of NLRI is identified by a
combination of <AFI, SAFI> carried in the attribute

The Network Layer Reachability information is encoded as one or more
2-tuples of the form <length, prefix>, whose fields are described below:+------
---------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length, in bits, of the address prefix. A length of
zero indicates a prefix that matches all (as specified by the address family)
addresses (with prefix, itself,of zero octets)
b) Prefix:
The Prefix field contains an address prefix followed by enough trailing bits to
make the end of the field fall on an octet boundary.

 Error Handling
If a BGP speaker receives from a neighbor an UPDATE message that contains
the MP_REACH_NLRI or MP_UNREACH_NLRI attribute, and if the
speaker determines that the attribute is incorrect, the speaker MUST delete all
the BGP routes received from that neighbor whose AFI/SAFI is the same as
the one carried in the incorrect MP_REACH_NLRI or MP_UNREACH_NLRI
attribute. For the duration of the BGP session over which the UPDATE
message was received, the speaker then SHOULD ignore all the subsequent
routes with that AFI/SAFI received over that session.
 Use of BGP Capability Advertisement
 A BGP speaker that uses Multiprotocol Extensions should use the Capability
Advertisement procedures [BGP-CAP] to determine whether the speaker
could use Multiprotocol Extensions with a particular peer. The Capability
Code field is set to 1 (which indicates Multiprotocol Extensions
capabilities). The Capability Length field is set to 4. The Capability Value
field is defined as:
0 7 15 23 31
+-------+-------+-------+-------+
| AFI | Res. | SAFI |
+-------+-------+-------+-------+

+------------------------------+
| Capability Code (1 octet) |
+------------------------------+
| Capability Length (1 octet) |
+------------------------------+
| Capability Value (variable) |
+------------------------------+
The use and meaning of these fields are as follows:
 Capability Code:
Capability Code is a one octet field that unambiguously identifies individual
capabilities.
 Capability Length:
Capability Length is a one octet field that contains the lengthof the Capability Value
field in octets.
 Capability Value:
Capability Value is a variable length field that is interpreted according to the value of
the Capability Code field.

 Multicast Routing are used to distribute data to multiple recipients. Using
multicast, a source can send a single copy of data to a single multicast
address, which is then distributed to an entire group of recipients. In
multicasting, the router may forward the received packets through several
of its interface. In this case, router may copy the data when it is necessary,
and forward it to the receivers.
 [A multicast is similar to a broadcast in the sense that its target is a number
of machines on a network, but not all. Where a broadcast is directed to all
hosts on the network, a multicast is directed to a group of hosts. The hosts
can choose whether they wish to participate in the multicast group (often
done with the Internet Group Management Protocol), whereas in a
broadcast, all hosts are part of the broadcast group whether they like it or
not!]
 A multicast group identifies a set of recipients that are interested in a
particular data stream, and is represented by an IP address from a well-
defined range. Data sent to this IP address is forwarded to all members of
the multicast group.

 Routers between the source and recipients duplicate data packets and
forward multiple copies wherever the path to recipients diverges.
Group membership information is used to calculate the best routers at
which to duplicate the packets in the data stream to optimize the use
of the network
 A multicast packet is not directed to one host but a number of hosts,
so the destination MAC address will not match the unique MAC
address of any computer, but the computers which are part of the
multicast group will recognize the destination MAC address and
accept it for processing.
 (Note:
Each host on an Ethernet network has a unique MAC address. So the
important point to understand in Multicasting is: How do you talk to a
group of hosts (our multicast group), where each host has a different
MAC address, and at the same time ensure that the other hosts, which
are not part of the multicast group, don't process the information)

 IPv4 Multicast Address :224.0.0.0/4
 Data transmissions on Ethernet
◦ Sent directly to specific layer 2 MAC addresses
◦ ARP, one to one mapping between layer 2 and layer 3
 To accommodate multicast transmissions, a set of Ethernet MAC
addresses has been reserved specifically for this purpose
◦ Class D IP is mapping to this MAC address
(01:00:5e:00:00:00 - 01:00:5e:7f:ff:ff )

We have an IP Address of 224.0.0.5, this is then converted into binary so we can
clearly see the mapping of the 23 bits to the MAC address of the computer. The MAC
Address part which is in yellow has been defined by the IEEE group. So
the yellow and pink line make the one MAC Address as shown in binary mode, then
we convert it from binary to hex and that's about it

 IPv6 multicast addresses have the Format Prefix (FP) of 1111 1111. An
IPv6 address is simple to classify as multicast because it always begins
with FF. eg:
FF01::1 (node-local scope all-nodes address)
FF02::1 (link-local scope all-nodes address)
 Same as IPv4, a set of Ethernet MAC address has been reserved
specifically for this IPv6 multicasting
◦ Take the low order 32 bits of IPv6 multicast addresses uses it to create
a MAC address by mapping it into MAC 33:33:00:00:00:00 (RFC
2464)
◦ Thus, an IPv6 packet addressed to FF02::1:FF68:12CB would be sent
to the Ethernet address 33-33-FF-68-12-CB
◦ Other Example:
 FF02:ABCD:EF12::1:3 will have a MAC address 33:33:00:01:00:03
 FF32::8000:9 will have a MAC address 33:33:80:00:00:09

◦ DVMRP (Distance Vector Multicast Routing Protocol)
◦ PIM-DM (Protocol Independent Multicast - Dense Mode)
◦ CBT (Core Based Tree)
◦ PIM-SM (Protocol Independent Multicast – Sparse Mode)
Routing
Dense Mode
DVMRP PIM-DM
Sparse Mode
PIM-SM CBT

Reverse Path Forwarding
 Goal: avoid flooding duplicates
 In multicast routing, the decision to forward traffic is based upon source
address and not on destination address as in unicast routing. When a
multicast packet enters a router's interface, it will look up the list of
networks that are reachable via that interface i.e., it checks the reverse path
of the packet. If the router finds a matching routing entry for the source IP
address of the multicast packet, the RPF check passes and the packet is
forwarded to all other interfaces that are participating in multicast for that
multicast group. If the RPF check fails, the packet will be dropped. As a
result, the forwarding of the packet is decided based upon the reverse path
of the packet rather than the forward path.
 It is an optimized form of flooding, where the router accepts a packet from
source S through interface I only if I is the interface the router would use in
order to reach S. It determines whether the interface is correct by consulting
its unicast routing tables. This technique dramatically decreases the
overhead associated with standard flooding.

 Because a router accepts a packet from only one neighbor, it floods the packet only
once, which means (assuming point-to-point links) each packet is transmitted over
each link once in each direction.
 RPF routers only forward packets that come into the interface that also holds the
routing entry for the source of the packet, thus breaking any loop.
 This is critically important in redundant multicast topologies. Because the same
multicast packet could reach the same router via multiple interfaces, RPF checking
is integral in the decision to forward packets or not.
 Assumptions:
1. A wants to broadcast
2. all nodes know predecessor node on shortest path back to A
 Reverse path forwarding(working): if node receives a broadcast packet
 And if packet arrived on predecessor on shortest path to A, then flood to all
neighbors otherwise ignore broadcast packet – either already arrived, or will arrive
from predecessor
 flood if packet arrives from source on link that router would use to send packets to
source otherwise discard
 rule avoids flooding loops uses shortest path tree from destinations to source
(reverse tree)

The two basic types of multicast distribution trees:
1. Source trees 2. Shared trees
Source-based tree
Separate shortest path tree for each source
Flood and prune (DVMRP, PIM-DM)
» Send multicast traffic everywhere
» Prune edges that are not actively subscribed to group
Link-state (MOSPF)
» Routers flood groups they would like to receive
» Compute shortest-path trees on demand
Shared tree (PIM-SM)
Single distributed tree shared among all sources
Specify rendezvous point (RP) for group
Senders send packets to RP, receivers join at RP
RP multicasts to receivers; Fix-up tree for optimization
Note : A Rendezvous Point (RP) is a router in a multicast network domain that acts as a shared
root for a multicast shared tree. Any number of routers can be configured to work as RPs and
they can be configured to cover different group ranges

 Dense Vector Multicasting Routing Protocol
 The basic assumption behind dense mode is that the multicast packet
stream has receivers at most locations. Sparse mode assumes
relatively fewer receivers. Dense mode is ideal for groups where
many of the nodes will subscribe to receive the multicast packets, so
that most of the routers must receive and forward these packets.
 It provides an efficient mechanism for connectionless datagram
delivery to a group of hosts across an internetwork. It is a distributed
protocol that dynamically generates IP multicast delivery trees using
a technique called Reverse Path Multicasting. DVMRP uses a
distance vector distributed routing algorithm in order to build per-
source-group multicast delivery trees.
 Dense Mode uses a fairly simple approach to handle IP multicast
routing. The source initially broadcasts to every router directly
connected to it. These neighboring routers further forward the data to
their neighbors.

 When a router does not wish to receive this group's data (i.e if no
other neighboring PIM routers are present and no host is interested), it
sends a Prune message to indicate its lack of interest or stop the
communication. Upon receiving a Prune message, the router will
modify its state so that it will not forward those packets out
that interface If every interface on a router is pruned, the router will
also be pruned.
 The routers will use reverse-path forwarding to ensure that there are
no loops for packet forwarding among routers that wish to receive
multicast packets.
 Multicast Forwarding in DVMRP
1. check incoming interface: discard if not on shortest path to source
2. forward to all outgoing interfaces
3. don’t forward if interface has been pruned
4. prunes time out every minute

 Protocol Independent Multicasting- Sparse Mode
 PIM-SM is called "protocol independent" because it can use the route
information that any routing protocol enters into the multicast Routing
Information Base (RIB).
 Examples of these routing protocols include unicast protocols such as the
Routing Information Protocol (RIP) and Open Shortest Path First (OSPF),
but multicast protocols that populate the routing tables—such as the
Distance Vector Multicast Routing Protocol (DVMRP)—can also be used.
 Sparse mode means that the protocol is designed for situations where
multicast groups are thinly populated across a large region. Sparse-mode
protocols can operate in LAN environments, but they are most efficient over
WANs. A sparse group can be defined as "one in which
 a) the number of networks or domains with group members present is
significantly smaller than the number of networks/domains in the Internet,
b) group members span an area that is too large/wide to rely on a hop-count
limit or some other form of limiting the scope of multicast packet
propagation, and c) the internetwork is not sufficiently resource rich to
ignore the overhead of current [dense mode] schemes.

PIM-SM was designed to support the following goals:
 Maintain the traditional IP multicast service model of receiver-initiated multicast
group membership. In this model, sources simply put packets on the first-hop
Ethernet, without any signaling. Receivers signal to routers in order to join the
multicast group that will receive the data.
 Leave the host model unchanged. PIM-SM is a router-to-router protocol, which
means that the hosts don't have to be upgraded, but that PIM-SM-enabled routers
must be deployed in the network.
 Support both shared and source distribution trees. For shared trees, PIM-SM uses a
central router, called the Rendezvous Point (RP), as the root of the shared tree. All
source hosts send their multicast traffic to the RP, which in turn forwards the packets
through a common tree to all the members of the group. Source trees directly
connect sources to receivers. There is a separate tree for every source. Source trees
are considered shortest-path trees from the perspective of the unicast routing tables.
PIM-SM can use either type of tree or both simultaneously.
 Maintain independence from any specific unicast routing protocol (see above).
 Use soft-state mechanisms to adapt to changing network conditions and multicast
group dynamics. Soft-state means that, unless it is refreshed, the router's state
configuration is short-term and expires after a certain amount of time.
(Source: https://msdn.microsoft.com/en-us/library/bb742462.aspx)

Concluding:
 protocol independent
 PIM SM implements forwarding trees for each multicast group
o creating routing tree for a group with Rendezvous Point (RP) as a root for
the tree
o Rendezvous Point Tree (RPT)
 PIM SM implements explicit join model to maintain a routing tree
Receivers send Join towards the RP
Sender send Register towards the RP
Supports both Source based and Shared distribution tree
For further information :
1.http://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/Whit
e_papers/mcst_ovr.html#wp1009081
2. https://msdn.microsoft.com/en-us/library/bb742462.aspx

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