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

Chapter 16 Exterior Routing Protocols and Multicasting1

Chapter Chapter 1616Exterior Routing ProtocolsAnd Multicasting


Problems with Distance-Vector Problems with Distance-Vector and Link-State Routingand Link-State RoutingNeither 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


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


Boarder Gateway Protocol Boarder Gateway Protocol (BGP)(BGP)Allows routers (gateways) in different ASs

to exchange routing informationMessages sent over TCP

– See next slideThree functional procedures

– Neighbor acquisition– Neighbor reachability– Network reachability


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Update– Transmit information about single route– List multiple routes to be withdrawn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


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


Neighbor ReachabilityNeighbor Reachability

Periodic issue of keepalive messagesBetween all routers that are neighbors


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


BGP MessageBGP MessageFormatsFormatsMarker:

– Reserved for authentication

Length:– In octets

Type:– Open, Update,

Keepalive, Notification


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


Keepalive DetailKeepalive Detail

Header onlyOften enough to prevent hold time

expiring


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


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


Withdrawal of Route(s)Withdrawal of Route(s)

Route identified by IP address of destination subnetwork(s)


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


Diagram for BGP Routing Diagram for BGP Routing Information ExchangeInformation Exchange


BGP Routing Information BGP 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


Inter-Domain Routing Protocol Inter-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


Routing Domain ConfederationsRouting Domain Confederations

Set of connected ASAppear to outside world as single AS

– RecursiveEffective scaling


MulticastingMulticasting

Sending message to multicast address– Multicast address refers to a group of hosts

MultimediaTeleconferencingDatabasesDistributed computationReal-time workgroup


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 onceMuch harder on internet


Example Example Configuration Configuration for Multicast for Multicast InternetInternet


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


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


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


Multicast Transmission Multicast Transmission ExampleExample


Requirements for Multicasting Requirements 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


Requirements for Multicasting Requirements 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


Internet Group Management Internet Group Management Protocol (IGMP)Protocol (IGMP)

Type:Membership query (general or group specific), membership report, leave group, max. response time

Checksum: uses IPv4 algorithmGroup address: zero for request, valid IP

multicast for report or leave


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


Group Membership with IPv6Group Membership with IPv6

Function incorporated in ICMPv6Includes all ICMPv4 plus IGMP

– Includes group membership query and report– Addition of new group membership

termination message


Multicast Extension to OSPF Multicast 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


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


Equal Cost Multipath Equal Cost Multipath AmbiguitiesAmbiguitiesDijkstra’ 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


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


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


MOSPF Routing IllustrationMOSPF Routing Illustration


Multicast Routing Protocol Multicast Routing Protocol CharacteristicsCharacteristicsExtension to existing protocol

– MOSPF v OSPFDesigned to be efficient for high

concentration of group membersAppropriate with single ASNot for large internet


Protocol Independent Multicast Protocol Independent Multicast (PIM)(PIM)Independent of unicast routing protocolsExtract required routing information from

any unicast routing protocolWork across multiple AS with different

unicast routing protocols


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


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


Group Destination Router Group Destination Router Group Source RouterGroup Source RouterGroup Destination Router

– Has local group members– Router becomes destination router for given

group when at least one host joins groupUsing IGMP or similar

Group source router– Attaches to network with at least one host

transmitting on multicast address via that router


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


Example of PIM OperationExample of PIM Operation

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