Download 16. Exterior Routing Protocols

Chapter 16
Exterior Routing Protocols
And Multicasting
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Chapter 16 Exterior Routing Protocols and Multicasting
Problems with Distance-Vector
and 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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Chapter 16 Exterior Routing Protocols and Multicasting
Path Vector Routing
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Dispense with routing metrics
Provide information about:
– Which networks can be reached by given router
– Which ASs must be crossed to get there
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No distance or cost element
Routing information includes all Ass visited to
reach destination
– Allows policy routing
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Chapter 16 Exterior Routing Protocols and Multicasting
Boarder Gateway Protocol
(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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Chapter 16 Exterior Routing Protocols and Multicasting
BGP v4 Messages

Open
– Start neighbor relationship with another router
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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– List multiple routes to be withdrawn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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Chapter 16 Exterior Routing Protocols and Multicasting
Neighbor Acquisition
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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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Chapter 16 Exterior Routing Protocols and Multicasting
Neighbor Reachability
Periodic issue of keepalive messages
 Between all routers that are neighbors

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Chapter 16 Exterior Routing Protocols and Multicasting
Network 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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Chapter 16 Exterior Routing Protocols and Multicasting
BGP Message
Formats

Marker:
– Reserved for
authentication

Length:
– In octets

Type:
– Open, Update,
Keepalive,
Notification
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Chapter 16 Exterior Routing Protocols and Multicasting
Neighbor Acquisition Detail

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Router opens TCP connection with neighbor
Sends open message
– Identifies sender’s AS and gives IP address
– Includes Hold Time
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As proposed by sender
If recipient prepared to open neighbor
relationship
– Calculate hold time

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min [own hold time, received hold time]
Max time between keepalive/update messages
– Reply with keepalive
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Chapter 16 Exterior Routing Protocols and Multicasting
Keepalive Detail
Header only
 Often enough to prevent hold time
expiring

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Chapter 16 Exterior Routing Protocols and Multicasting
Update 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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Chapter 16 Exterior Routing Protocols and Multicasting
Path 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
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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
Chapter 16 Exterior Routing Protocols and Multicasting
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Withdrawal of Route(s)

Route identified by IP address of
destination subnetwork(s)
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Chapter 16 Exterior Routing Protocols and Multicasting
Notification Message

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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

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Hold time expired
Finite state machine error
Cease
– Close connection in absence of any other error
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Chapter 16 Exterior Routing Protocols and Multicasting
Diagram for BGP Routing
Information Exchange
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Chapter 16 Exterior Routing Protocols and Multicasting
BGP Routing Information
Exchange

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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

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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
Chapter 16 Exterior Routing Protocols and Multicasting
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Inter-Domain Routing Protocol
(IDRP)
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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

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Variable length AS identifiers
Handles multiple internet protocols and address schemes
Aggregates path information using routing domain
confederations
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Chapter 16 Exterior Routing Protocols and Multicasting
Routing Domain Confederations
Set of connected AS
 Appear to outside world as single AS

– Recursive

Effective scaling
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Chapter 16 Exterior Routing Protocols and Multicasting
Multicasting

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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Chapter 16 Exterior Routing Protocols and Multicasting
Multicasting 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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Chapter 16 Exterior Routing Protocols and Multicasting
Example
Configuration
for Multicast
Internet
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Chapter 16 Exterior Routing Protocols and Multicasting
Broadcast
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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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Chapter 16 Exterior Routing Protocols and Multicasting
Multiple 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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Chapter 16 Exterior Routing Protocols and Multicasting
True Multicast
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Least cost path from source to each network
containing member of group is determined
– Gives spanning tree configuration
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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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Chapter 16 Exterior Routing Protocols and Multicasting
Multicast Transmission
Example
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Chapter 16 Exterior Routing Protocols and Multicasting
Requirements for Multicasting
(1)
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Router must forward two or more copies of incoming
packet
Addressing
– IPv4 uses class D
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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
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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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Chapter 16 Exterior Routing Protocols and Multicasting
Requirements for Multicasting
(2)
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Multicast addresses may be permanent or dynamic
Individual hosts may join or leave dynamically
– Need mechanism to inform routers
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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
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Routes determined based on source and destination
addresses
– Avoids unnecessary duplication of packets
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Chapter 16 Exterior Routing Protocols and Multicasting
Internet Group Management
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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Chapter 16 Exterior Routing Protocols and Multicasting
IGMP Operation
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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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Chapter 16 Exterior Routing Protocols and Multicasting
Group Membership with IPv6
Function incorporated in ICMPv6
 Includes all ICMPv4 plus IGMP
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– Includes group membership query and report
– Addition of new group membership
termination message
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Chapter 16 Exterior Routing Protocols and Multicasting
Multicast Extension to OSPF
(MOSPF)
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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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Chapter 16 Exterior Routing Protocols and Multicasting
Forwarding 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 sourcedestination pair and forward to other
routers if required
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Chapter 16 Exterior Routing Protocols and Multicasting
Equal Cost Multipath
Ambiguities

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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Chapter 16 Exterior Routing Protocols and Multicasting
Interarea 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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Chapter 16 Exterior Routing Protocols and Multicasting
Inter-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
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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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Chapter 16 Exterior Routing Protocols and Multicasting
MOSPF Routing Illustration
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Chapter 16 Exterior Routing Protocols and Multicasting
Multicast Routing Protocol
Characteristics

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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Chapter 16 Exterior Routing Protocols and Multicasting
Protocol Independent Multicast
(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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Chapter 16 Exterior Routing Protocols and Multicasting
PIM Strategy
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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
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For intra-AS
Alternative to MOSPF
– Sparse mode

Inter-AS multicast routing
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Chapter 16 Exterior Routing Protocols and Multicasting
Spares 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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Chapter 16 Exterior Routing Protocols and Multicasting
Group Destination Router
Group Source Router
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Group Destination Router
– Has local group members
– Router becomes destination router for given
group when at least one host joins group

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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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Chapter 16 Exterior Routing Protocols and Multicasting
PIM Approach
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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
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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
Chapter 16 Exterior Routing Protocols and Multicasting
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Example of PIM Operation
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Chapter 16 Exterior Routing Protocols and Multicasting