12-exterior routing p#88016

8/7/2019 12-Exterior Routing P#88016

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Computer Networks with

Internet TechnologyWilliam Stallings

Chapter 12

Exterior Routing Protocols

And Multicasting


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Boarder Gateway Protocol

(BGP)

Allows routers (gateways) in different ASs toexchange routing information

Messages sent over TCP

See next slide Three functional procedures

Neighbor acquisition

Neighbor reachability

Network reachability


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BGP Messages

Open

Start neighbor relationship with another router

Update

Transmit information about single routeList multiple routes to be withdrawn

Keepalive

Acknowledge open message

Periodically confirm neighbor relationship

Notification

Send when error condition detected


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Neighbor Acquisition

Neighbors attach to same subnetwork

If in different ASs routers may wish to exchangeinformation

Neighbor acquisitionis when two neighboring routersagree 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 activeintervention


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Neighbor Reachability

Periodic issue of keepalive messages

Between all routers that are neighbors


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Network Reachability

Each router keeps database of subnetworks itcan reach and preferred route

When change made, router issues update

message All BGP routers build up and maintain routing

information


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Figure 12.1

BGP Message

Formats


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Neighbor Acquisition Detail

Router opens TCP connection with neighbor

Sends open messageIdentifies senders AS and gives IP address

Includes Hold Time As proposed by sender

If recipient prepared to open neighborrelationshipCalculate hold time

min [own hold time, received hold time] Max time between keepalive/update messages

Reply with keepalive


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Keepalive Detail

Header only

Often enough to prevent hold time expiring


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Update Detail

Information about single route through internetInformation to be added to database of any recipient

router

Network layer reachability information (NLRI)

List of network portions of IP addresses of subnets reachedby this route

Total path attributes length field

Path attributes field (next slide)

List of previously advertised routes beingwithdrawn

May contain both


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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

Local_Pref

Tell other routers within AS degree of preference Atomic_Aggregate, Aggregator

Uses subnet addresses in tree view of network to reduce informationneeded in NLRI


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Withdrawal of Route(s)

Route identified by IP address of destinationsubnetwork(s)


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Notification 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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BGP Routing Information

Exchange 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 Protocol

(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 addressschemes

Aggregates path information using routing domainconfederations


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Routing Domain Confederations

Set of connected AS

Appear to outside world as single AS

Recursive

Effective scaling


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Multicasting

Addresses that refer to group of hosts on one ormore networks

Uses

Multimedia broadcastTeleconferencing

Database

Distributed computing

Real time workgroups


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Figure 12.2

Example

Configuration


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Broadcast and Multiple Unicast

Broadcast a copy of packet to each network

Requires 13 copies of packet

Multiple Unicast

Send packet only to networks that have hosts ingroup

11 packets


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True Multicast

Determine least cost path to each network thathas host in group

Gives spanning tree configuration containingnetworks with group members

Transmit single packet along spanning tree

Routers replicate packets at branch points ofspanning tree

8 packets required


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Figure 12.3 Multicast

Transmission Example


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Requirements for

Multicasting (1)

Router may have to forward more than one copy ofpacket

Convention needed to identify multicast addresses IPv4 - Class D - start 1110

IPv6 - 8 bit prefix, all 1, 4 bit flags field, 4 bit scope field, 112bit group identifier

Nodes must translate between IP multicast addressesand list of networks containing group members

Router must translate between IP multicast address andnetwork multicast address


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Requirements for

Multicasting (2)

Mechanism required for hosts to join and leavemulticast group

Routers must exchange info

Which networks include members of given groupSufficient info to work out shortest path to each

network

Routing algorithm to work out shortest path

Routers must determine routing paths based onsource and destination addresses


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Figure 12.4 Spanning Tree from

Router C to Multicast Group


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Internet Group Management

Protocol (IGMP)

RFC 3376

Host and router exchange of multicast groupinfo

Use broadcast LAN to transfer info amongmultiple hosts and routers


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Principle Operations

Hosts send messages to routers to subscribe toand unsubscribe from multicast group

Group defined by multicast address

Routers check which multicast groups of interestto which hosts

IGMP currently version 3

IGMPv1

Hosts could join group

Routers used timer to unsubscribe members


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Operation of IGMPv1 & v2

Receivers have to subscribe to groups

Sources do not have to subscribe to groups

Any host can send traffic to any multicast group

Problems:Spamming of multicast groups

Even if application level filters drop unwantedpackets, they consume valuable resources

Establishment of distribution trees is problematic

Location of sources is not known

Finding globally unique multicast addresses difficult


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IGMP v3

Allows hosts to specify list from which they wantto receive traffic

Traffic from other hosts blocked at routers

Allows hosts to block packets from sources thatsend unwanted traffic


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Figure 12.5a IGMP Message

Formats Membership Query


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Membership Query

Sent by multicast router

General query

Which groups have members on attached network

Group-specific queryDoes group have members on an attached network

Group-and-source specific query

Do attached device want packets sent to specified

multicast addressFrom any of specified list of sources


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Membership Query Fields (1)

Type

Max Response Time Max time before sending report in units of 1/10 second

Checksum

Same algorithm as IPv4

Group Address

Zero for general query message

Multicast group address for group-specific or group-and-source

S Flag 1 indicates that receiving routers should suppress normal timer

updates done on hearing query


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Membership Query Fields (2)

QRV (querier's robustness variable) RV value used by sender of query

Routers adopt value from most recently received query

Unless RV was zero, when default or statically configured valueused

RV dictates number of retransmissions to assure report notmissed

QQIC (querier's querier interval code) QI value used by querier

Timer for sending multiple queries

Routers not current querier adopt most recently received QI Unless QI was zero, when default QI value used

Number of Sources

Source addresses

One 32 bit unicast address for each source


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Figure 12.5b IGMP Message

Formats Membership Report


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Membership Reports

Type

Checksum

Number of Group Records

Group RecordsOne 32-bit unicast address per source


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Figure 12.5c IGMP Message

Formats Group Record


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Group Record

Record TypeSee later

Aux Data LengthIn 32-bit words

Number of Sources

Multicast Address

Source Addresses

One 32-bit unicast address per source Auxiliary Data

Currently, no auxiliary data values defined


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IGMP Operation - Joining

Host using IGMP wants to make itself known as groupmember to other hosts and routers on LAN

IGMPv3 can signal group membership with filteringcapabilities with respect to sources EXCLUDE mode all group members except those listed

INCLUDE mode Only from group members listed

To join group, host sends IGMP membership reportmessageAddress field multicast address of group

Sent in IP datagram with Group Address field of IGMP messageand Destination Address encapsulating IP header same

Current members of group will receive learn of new member

Routers listen to all IP multicast addresses to hear all reports


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IGMP Operation

Keeping Lists Valid

Routers periodically issue IGMP general query message

In datagram with all-hosts multicast address

Hosts that wish to remain in groups must read datagrams withthis all-hosts address

Hosts respond with report message for each group to which itclaims membership

Router does not need to know every host in a group Needs to know at least one group member still active

Each host in group sets timer with random delay

Host that hears another claim membership cancels own report If timer expires, host sends report

Only one member of each group reports to router


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IGMP Operation - Leaving

Host leaves group, by sending leave group message toall-routers static multicast address

Send membership report message with EXCLUDE optionand null list of source addresses

Router determine if there are any remaining groupmembers using group-specific query message


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Group Membership with IPv6

IGMP defined for IPv4

Uses 32-bit addresses

IPv6 internets need functionality

IGMP functions incorporated into InternetControl Message Protocol version 6 (ICMPv6)

ICMPv6 includes all of functionality of ICMPv4 andIGMP

ICMPv6 includes group-membership query andgroup-membership report message

Used in the same fashion as in IGMP


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Multicast Extension to OSPF

(MOSPF)

Enables routing of IP multicast datagrams withinsingle 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 containingmembers of group (Dijkstra)

Takes time, so on demand only


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Forwarding Multicast Packets

If multicast address not recognised, discard

If router attaches to a network containing amember of group, transmit copy to that network

Consult spanning tree for this source-destinationpair and forward to other routers if required


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Equal Cost Multipath

Ambiguities

Dijkstra algorithm will include one of multipleequal cost paths

Which depends on order of processing nodes

For multicast, all routers must have samespanning tree for given source node

MOSPF has tiebreaker rule


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Interarea Multicasting

Multicast groups amy contain members frommore than one area

Routers only know about multicast groups with

members in its area Subset of areas border routers forward group

membership information and multicastdatagrams between areas

Interarea multicast forwarders


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Inter-AS Multicasting

Certain boundary routers act as inter-ASmulticast forwarders

Run and inter-AS multicast routing protocol as well asMOSPF and OSPF

MOSPF makes sure they receive all multicastdatagrams from within AS

Each such router forwards if required

Use reverse path routing to determine source

Assume datagram from X enters AS at point advertisingshortest route back to X

Use this to determine path of datagram through MOSPF AS


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Figure 12.6

Illustrations of MOSPF Routing


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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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Protocol Independent Multicast

(PIM)

Independent of unicast routing protocols

Extract required routing information from anyunicast routing protocol

Work across multiple AS with different unicastrouting protocols


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PIM 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 MOSPFSparse mode

Inter-AS multicast routing


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Spares Mode PIM

A spare group:Number of networks/domains with group members

present significantly small than number ofnetworks/domains in internet

Internet spanned by group not sufficiently resourcerich to ignore overhead of current multicast schemes


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Group Destination Router Group

Source Router

Group Destination RouterHas local group members

Router becomes destination router for given groupwhen at least one host joins group

Using IGMP or similar

Group source router

Attaches to network with at least one hosttransmitting on multicast address via that router


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PIM Approach

For a group, one router designated rendezvous point (RP)

Group destination router sends join message towards RP requestingits 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 shortestpath 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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Figure 12.7

Example of PIM Operation


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Required Reading

Stallings chapter 12

12-exterior routing p#88016

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