Exterior Routing Protocols And Multi casting Chapter 16

Chapter 16 Exterior Routing Protocols and Multicasting
1
ChapterChapter 1616
Exterior Routing Protocols
And Multicasting

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Problems with Distance-VectorProblems with Distance-Vector
and Link-State Routingand Link-State Routing
Neither distance-vector (RIP) nor link state
(OSPF) protocols effective for exterior
routing
Distance vector and link state protocols
assume all routers share common metric
Priorities and restrictions may differ
between ASs
Flooding of link state information may
become unmanageable

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Path Vector RoutingPath Vector Routing
 Dispense with routing metrics
 Provide information about:
– Which networks can be reached by given router
– Which ASs must be crossed to get there
 No distance or cost element
 Routing information includes all Ass visited to
reach destination
– Allows policy routing

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Boarder Gateway ProtocolBoarder Gateway Protocol
(BGP)(BGP)
Allows routers (gateways) in different ASs
to exchange routing information
Messages sent over TCP
– See next slide
Three functional procedures
– Neighbor acquisition
– Neighbor reachability
– Network reachability

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BGP v4 MessagesBGP v4 Messages
 Open
– Start neighbor relationship with another router
 Update
– Transmit information about single route
– List multiple routes to be withdrawn
 Keepalive
– Acknowledge open message
– Periodically confirm neighbor relationship
 Notification
– Send when error condition detected

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Neighbor AcquisitionNeighbor Acquisition
 Neighbors attach to same subnetwork
 If in different ASs routers may wish to exchange
information
 Neighbor acquisitionis when two neighboring
routers agree to exchange routing information
regularly
– Needed because one router may not wish to take part
 One router sends request, the other acknowledges
– Knowledge of existence of other routers and need to
exchange information established at configuration
time or by active intervention

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Neighbor ReachabilityNeighbor Reachability
Periodic issue of keepalive messages
Between all routers that are neighbors

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Network ReachabilityNetwork Reachability
Each router keeps database of subnetworks
it can reach and preferred route
When change made, router issues update
message
All BGP routers build up and maintain
routing information

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BGP MessageBGP Message
FormatsFormats
Marker:
– Reserved for
authentication
Length:
– In octets
Type:
– Open, Update,
Keepalive,
Notification

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Neighbor Acquisition DetailNeighbor Acquisition Detail
 Router opens TCP connection with neighbor
 Sends open message
– Identifies sender’s AS and gives IP address
– Includes Hold Time
 As proposed by sender
 If recipient prepared to open neighbor
relationship
– Calculate hold time
 min [own hold time, received hold time]
 Max time between keepalive/update messages
– Reply with keepalive

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Keepalive DetailKeepalive Detail
Header only
Often enough to prevent hold time
expiring

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Update DetailUpdate Detail
 Information about single route through internet
– Information to be added to database of any recipient
router
– Network layer reachability information (NLRI)
 List of network portions of IP addresses of subnets reached
by this route
– Total path attributes length field
– Path attributes field (next slide)
 List of previously advertised routes being
withdrawn
 May contain both

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Path Attributes FieldPath Attributes Field
 Origin
– Interior (e.g. OSPF) or exterior (BGP) protocol
 AS_Path
– ASs traversed for this route
 Next_Hop
– IP address of boarder router for next hop
 Multi_Exit_disc
– Information about routers internal to AS
 Local_Pref
– Tell other routers within AS degree of preference
 Atomic_Aggregate, Aggregator
– Uses subnet addresses in tree view of network to reduce
information needed in NLRI

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Withdrawal of Route(s)Withdrawal of Route(s)
Route identified by IP address of
destination subnetwork(s)

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Notification MessageNotification Message
 Error notification
 Message header error
– Includes authentication and syntax errors
 Open message error
– Syntax errors and option not recognised
– Proposed hold time unacceptable
 Update message error
– Syntax and validity errors
 Hold time expired
 Finite state machine error
 Cease
– Close connection in absence of any other error

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Diagram for BGP RoutingDiagram for BGP Routing
Information ExchangeInformation Exchange

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BGP Routing InformationBGP Routing Information
ExchangeExchange
 R1 constructs routing table for AS1 using OSPF
 R1 issues update message to R5 (in AS2)
– AS_Path: identity of AS1
– Next_Hop: IP address of R1
– NLRI: List of all subnets in AS1
 Suppose R5 has neighbor relationship with R9 in AS3
 R9 forwards information from R1 to R9 in update
message
– AS_Path: list of ids {AS2,AS1}
– Next_Hop: IP address of R5
– NLRI: All subnets in AS1
 R9 decides if this is prefered route and forwards to
neighbors

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Inter-Domain Routing ProtocolInter-Domain Routing Protocol
(IDRP)(IDRP)
 Exterior routing protocol for IPv6
 ISO-OSI standard
 Path-vector routing
 Superset of BGP
 Operates over any internet protocol (not just TCP)
– Own handshaking for guaranteed delivery
 Variable length AS identifiers
 Handles multiple internet protocols and address schemes
 Aggregates path information using routing domain
confederations

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Routing Domain ConfederationsRouting Domain Confederations
Set of connected AS
Appear to outside world as single AS
– Recursive
Effective scaling

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MulticastingMulticasting
Sending message to multicast address
– Multicast address refers to a group of hosts
Multimedia
Teleconferencing
Databases
Distributed computation
Real-time workgroup

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Multicasting within LANMulticasting within LAN
MAC level multicast addresses
– IEEE 802 uses highest order bit 1
All stations that recognise the multicast
address accept the packet
Works because of broadcast nature of
LAN
Packet only sent once
Much harder on internet

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ExampleExample
ConfigurationConfiguration
for Multicastfor Multicast
InternetInternet

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BroadcastBroadcast
 Assume location of recipients not know
 Send packet to every network
 Packet addressed to N3 traverses N1, link L3, N3
 Router B translates IP multicast address to MAC
multicast address
 Repeat for each network
 Generates lots of packets
– In example, 13

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Multiple UnicastMultiple Unicast
Location of each member of multicast
group known to source
Table maps multicast address to list of
networks
Only need to send to networks containing
members of multicast group
Reduced traffic (a bit)
– In example, 11

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True MulticastTrue Multicast
 Least cost path from source to each network
containing member of group is determined
– Gives spanning tree configuration
 For networks containing group members only
 Source transmits packet along spanning tree
 Packet replicated by routers at branch points of
spanning tree
 Reduced traffic
– In example, 8

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Multicast TransmissionMulticast Transmission
ExampleExample

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Requirements for MulticastingRequirements for Multicasting
(1)(1)
 Router must forward two or more copies of incoming
packet
 Addressing
– IPv4 uses class D
 Start 1110 plus 28 bit group id
– IPv6 uses 8 bit prefix of all 1s, 4 bit flags field, 4 bit scope field
112 bit group id
 Node must translate between multicast address and list of
networks containing members of group
 Router must translate between IP multicast address and
subnet multicast address to deliver to destination network

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Requirements for MulticastingRequirements for Multicasting
(2)(2)
 Multicast addresses may be permanent or dynamic
 Individual hosts may join or leave dynamically
– Need mechanism to inform routers
 Routers exchange information on which subnets contain
members of groups
 Routers exchange information to calculate shortest path
to each network
– Need routing protocol and algorithm
 Routes determined based on source and destination
addresses
– Avoids unnecessary duplication of packets

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Internet Group ManagementInternet Group Management
Protocol (IGMP)Protocol (IGMP)
Type:Membership query (general or group
specific), membership report, leave group,
max. response time
Checksum: uses IPv4 algorithm
Group address: zero for request, valid IP
multicast for report or leave

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IGMP OperationIGMP Operation
 Host uses IGMP to make itself know as member of group
to other hosts and routers
 To join, send IGMP membership report message
– Send to multicast destination of group being joined
 Routers periodically issue IGMP query
– To all-hosts multicast address
– Hosts respond with report message for each group to which it
belongs
 Only one host in group needs to respond to keep group alive
 Host keeps timer and reponds if no other reply heard in time
 Host sends leave group message
– Group specific query from router determins if any members
remain

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Group Membership with IPv6Group Membership with IPv6
Function incorporated in ICMPv6
Includes all ICMPv4 plus IGMP
– Includes group membership query and report
– Addition of new group membership
termination message

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Multicast Extension to OSPFMulticast Extension to OSPF
(MOSPF)(MOSPF)
 Enables routing of IP multicast datagrams within
single AS
 Each router uses MOSPF to maintain local group
membership information
 Each router periodically floods this to all routers
in area
 Routers build shortest path spanning tree from a
source network to all networks containing
members of group (Dijkstra)
– Takes time, so on demand only

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Forwarding Multicast PacketsForwarding Multicast Packets
If multicast address not recognised, discard
If router attaches to a network containing a
member of group, transmit copy to that
network
Consult spanning tree for this source-
destination pair and forward to other
routers if required

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Equal Cost MultipathEqual Cost Multipath
AmbiguitiesAmbiguities
Dijkstra’ algorithm will include one of
multiple equal cost paths
– Which depends on order of processing nodes
For multicast, all routers must have same
spanning tree for given source node
MOSPF has tiebreaker rule

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Interarea MulticastingInterarea Multicasting
Multicast groups amy contain members
from more than one area
Routers only know about multicast groups
with members in its area
Subset of area’s border routers forward
group membership information and
multicast datagrams between areas
– Interarea multicast forwarders

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Inter-AS MulticastingInter-AS Multicasting
 Certain boundary routers act as inter-AS
multicast forwarders
– Run and inter-AS multicast routing protocol as well as
MOSPF and OSPF
– MOSPF makes sure they receive all multicast
datagrams from within AS
– Each such router forwards if required
– Use reverse path routing to determine source
 Assume datagram from X enters AS at point advertising
shortest route back to X
 Use this to determine path of datagram through MOSPF AS

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MOSPF Routing IllustrationMOSPF Routing Illustration

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Multicast Routing ProtocolMulticast Routing Protocol
CharacteristicsCharacteristics
Extension to existing protocol
– MOSPF v OSPF
Designed to be efficient for high
concentration of group members
Appropriate with single AS
Not for large internet

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Protocol Independent MulticastProtocol Independent Multicast
(PIM)(PIM)
Independent of unicast routing protocols
Extract required routing information from
any unicast routing protocol
Work across multiple AS with different
unicast routing protocols

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PIM StrategyPIM Strategy
 Flooding is inefficient over large sparse internet
 Little opportunity for shared spanning trees
 Focus on providing multiple shortest path unicast
routes
 Two operation modes
– Dense mode
 For intra-AS
 Alternative to MOSPF
– Sparse mode
 Inter-AS multicast routing

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Spares Mode PIMSpares Mode PIM
A spare group:
– Number of networks/domains with group
members present significantly small than
number of networks/domains in internet
– Internet spanned by group not sufficiently
resource rich to ignore overhead of current
multicast schemes

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Group Destination RouterGroup Destination Router
Group Source RouterGroup Source Router
Group Destination Router
– Has local group members
– Router becomes destination router for given
group when at least one host joins group
 Using IGMP or similar
Group source router
– Attaches to network with at least one host
transmitting on multicast address via that
router

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PIM ApproachPIM Approach
 For a group, one router designated rendezvous point (RP)
 Group destination router sends join message towards RP
requesting its members be added to group
– Use unicast shortest path route to send
– Reverse path becomes part of distribution tree for this RP to
listeners in this group
 Node sending to group sends towards RP using shortest
path unicast route
 Destination router may replace group-shared tree with
shortest path tree to any source
– By sending a join back to source router along unicast shortest
path
 Selection of RP dynamic
– Not critical

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Example of PIM OperationExample of PIM Operation

Exterior Routing Protocols And Multi casting Chapter 16

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