1 0940_03F8_c1 NW97_US_106 Multicast Issues for Gigapop Operators David Meyer Gigapop Operators II...

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Multicast Issues for Gigapop Operators

David MeyerGigapop Operators II Workshop 26 June 1998

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• Introduction

• First Some Basic TechnologyBasic Host Model

Basic Router Model

Data Distribution Concepts

• What Are the Deployment ObstaclesWhat Are the Non-technical Issues

What Are the Technical Scaling Issues

Agenda

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Agenda (Cont.)

• Potential Solutions (Cisco Specific)

Multi-level RP, Anycast Clusters, MSDP

Using Directory Services

• Industry Solutions

BGMP and MASC

• Possible Deployment Scenarios

• References

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Introduction—Level Set

• This presentation focuses on large-scale multicast routing for Gigapops and their customers

Note that the problem is essentially the same as the inter-domain multicast routing problem

• The problems/solutions presented are related to inter-enterprise or Gigapop deployment of IP multicast

• The current set of deployed technology is sufficient for enterprise environments

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Introduction—Why Would You Want to Deploy IP Multicast?

• You don’t want the same data traversing your links many times— bandwidth saver

• You want to join and leave groups dynamically without notifying all data sources—pay-per-view

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Introduction—Why Would You Want to Deploy IP Multicast?

• You want to discover a resource but don’t know who is providing it or if you did, don’t want to configure it— expanding ring search

• Reduce startup latency for subscribers

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Introduction—Why Would a Gigapop Want to Deploy IP Multicast?

• All of the previous, plus revenue potential for deploying IP multicast

• Initial applicationsRadio station transmissions

Real-time stock quote service

• Future applicationsDistance learning

Entertainment

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Basic Host Model

• Strive to make the host model simpleWhen sourcing data, just send the data

Map network layer address to link layer address

Routers will figure out where receivers are and are not

When receiving data, need to perform two actions

Tell routers what group you’re interested in (via IGMP)

Tell your LAN controller to receive for link-layer mapped address

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Basic Host Model

• Hosts can be receivers and not send to the group

• Hosts can send but not be receivers of the group

• Or they can be both

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Basic Host Model

• There are some protocol and architectural issues

Multiple IP group addresses map into a single link-layer address

You need IP-level filtering

Hosts join groups, which means they receive traffic from all sources sending to the group

Wouldn’t it be better if hosts could say what sources they were willing to receive from

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Basic Host Model

• There are some protocol and architectural issues (continued)

You can access control sources but you can’t access control receivers in a scalable way

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Basic Router Model

• Since hosts can send any time to any group, routers must be prepared to receive on all link-layer group addresses

And know when to forward or drop packets

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Basic Router Model

• What does a router keep track of?

interfaces leading to receivers

sources when utilizing source distribution trees

prune state depending on the multicast routing protocol (e.g. Dense Mode)

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• Routers maintain state to deliver data down a distribution tree

• Source trees

Router keeps (S,G) state so packets can flow from the source to all receivers

Trades off low delay from source against router state

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• Shared trees

Router keeps (*,G) state so packets flow from the root of the tree to all receivers

Trades off higher delay from source against less router state

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• How is the tree built?

On demand, in response to data arrival

Dense-mode protocols (PIM-DM and DVMRP)

MOSPF

Explicit control

Sparse-mode protocols (PIM-SM and CBT)

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• Building distribution trees requires knowledge of where members are

flood data to find out where members are not (Dense-mode protocols)

flood group membership information (MOSPF), and build tree as data arrives

send explicit joins and keep join state (Sparse-mode protocols)

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• Construction of source trees requires knowledge of source locations

In dense-mode protocols you learn them when data arrives (at each depth of the tree)

Same with MOSPF

In sparse-mode protocols you learn them when data arrives on the shared tree (in leaf routers only)

Ignore since routing based on direction from RP

Pay attention if moving to source tree

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• To build a shared tree you need to know where the core (RP) is

Can be learned dynamically in the routing protocol (Auto-RP, PIMv2)

Can be configured in the routers

Could use a directory service

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• Source trees make sense forBroadcast radio transmissions

Expanding ring search

Generic few-sources-to-many-receiver applications

High-rate, low-delay application requirements

Per source policy from a service provider’s point of view

Per source access control

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• Shared trees make sense for

Many low-rate sources

Applications that don’t require low delay

Consistent policy and access control across most participants in a group

When most of the source trees overlap topologically with the shared tree

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Deployment Obstacles—Non-Technical Issues

• How to bill for the serviceIs the service what runs on top of multicast?

Or is it the transport itself?

Do you bill based on sender or receiver, or both?

• How to control accessShould sources be rate-controlled (unlike unicast routing)

Should receivers be able to receive at a specific rate only?

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Deployment Obstacles—Non-Technical Issues

• How to make your peers fan-out instead of you (reduce the replication factor in you own network)

Closest exit versus latest entrance—all a wash

• How to avoid multicast from opening a lot of security holes

Network-wide denial of service attacks

Eaves-dropping simpler since receivers are unknown

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Deployment Obstacles—Technical Issues

• Source tree state will become a problem as IP multicast gains popularity

When policy and access control per source will be the rule rather than the exception

• Group state will become a problem as IP multicast gains popularity

10,000 three member groups across the Internet

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Deployment Obstacles— Technical Issues

• Hopefully we can upper bound the state in routers based on their switching capacity

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• Gigapop customers are telling us they don’t want to depend on another customer’s (or gigapop) RP

Do we connect shared trees together?

Do we have a single shared tree across domains?

Do we use source trees only for inter-domain groups?

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• Customers are telling us that the unicast and multicast topologies won’t be congruent across domains

Due to physical/topological constraints

Due to policy constraints

• We need a inter-domain routing protocol that distinguishes unicast versus multicast policy

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How to Control Multicast Routing Table State in the Network?

• Fundamental problem of learning group membership

Flood and PruneDVMRPPIM-DM

Broadcast MembershipMOSPFDWRs

Rendezvous MechanismPIM-SMBGMP

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Rendezvous Mechanism

• Why not use sparse-mode PIM?

Where to put the root of the shared tree (the RP)

Third-party RP problem

• If you did use sparse-mode PIM

Group-to-RP mappings would have to be distributed throughout the Internet

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Rendezvous Mechanism

• Lets try using sparse-mode PIM for inter-domain multicast

• Look at four possibilities

Multi-level RP

Anycast clusters

MSDP

Use directory services

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Connect Shared Trees Together—Multi-Level RP

• Idea is to have a hierarchy of shared trees

Level-0 RPs are inside of domains

They propagate joins from routers to a Level-1 RP that may be in another domain

All level-0 shared trees get connected together via a Level-1 RP

If multiple Level-1 RPs, iterate up to Level-2 RPs

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Connect Shared Trees Together—Multi-Level RP

• ProblemsRequires PIM protocol changes

If you don’t locate the Level-0 RP at the border, intermediate PIM routers think there may be two RPs for the group

Still has the third-party problem, there is ultimately one node at the root of the hierarchy

Data has to flow all the way to the highest-level RP

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Connect Shared Trees Together—Anycast Clusters

• Share the burden of being an RP among service providers

Each RP in each domain is a border router

• Build RP clusters at interconnect points (or dense-mode clouds)

• Group allocation is per cluster and not per-user or per-domain

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Connect Shared Trees Together—Anycast Clusters

• Closest border router in cluster is used as the RP

• Routers in a domain will use the domain’s RP

Provided you have an RP for that group range at an interconnect point

If not, you use the closest RP at the interconnect point (could be RP in another domain)

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Connect Domains Together—MSDP

• If you can’t connect shared trees together easily, then don’t

• Multicast Source Discovery Protocol

Different paradigm

Rather than getting trees connected, get sources known to all trees

Sounds non-scalable, but the trick is in the implementation

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• An RP in a domain has a MSDP peering session with an RP in another domain

Runs over TCP

Source-Active (SA) messages are sent to describe active sending sources in a domain

Logical topology is built for the sole purpose to distribute SA messages

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• How it worksSource goes active in PIM-SM domain

It’s packets get PIM registered to domain’s RP

RP sends SA message to it’s MSDP peers

Other MSDP peers forward to their peers away from the originating RP

If a peer in another domain has receivers for the group the source is sending to, it joins the source (Flood-and-Join model)

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Connect Domains Together—MSDP• There is no shared tree across domains

Therefore each domain can depend solely on it’s own RP (no third-party problem)

• SA state is not stored at each MSDP peer

• You could encapsulate data in SA messages for low-rate bursty sources

• You could have SA caching peers to speed up join latency

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Use Directory Services

• You can use directory services to:

Enable a single shared tree across domains

Enable use of source tree only and avoid using a single shared tree across domains

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• How it works with a single shared tree across domains

Put RP in client’s domain

Optimal placement of the RP if the domain had a multicast source or receiver active

Policy for RP is consistent with policy for domain’s unicast prefixes

Use directory to find RP address for a given group

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• For example

Receiver host sends IGMP report for 224.1.1.1

First-hop router performs DNS name resolution on

1.1.1.224.pim.mcast.net

An A record is returned with the IP address of RP

First-hop router sends PIM join message towards RP

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• All routers get consistent RP addresses via DNS

• When dynamic DNS is widely deployed it will be easier to change A records

• In the mean time, use loopback addresses on routers and move them around in your domain

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• When domain group allocation exists, a domain can be authoritative for a DNS zone

1.224.pim.mcast.net

128/17.1.224.pim.mcast.net

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• Another approach—avoid using shared trees all together

Build PIM-SM source trees across domains

• Put multiple A records in DNS to describe sources for the group

1.0.2.224.sources.pim.mcast.net IN CNAME dmm-home IN CNAME dino-homedmm-home IN A 171.69.58.81dino-home IN A 171.69.127.178

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Standards Solutions

• Ultimate scalability of both routing and group allocation can be achieved using BGMP/MASC

• Use BGP4+ (MBGP) to deal with non-congruency issues

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Border Gateway Multicast Protocol (BGMP)

• Use a PIM-like protocol that runs between domains (BGP equivalent for multicast)

• The protocol builds a shared tree of domains for a group

So we can use a rendezvous mechanism at the domain level

Shared tree is bi-directional

Root of shared tree of domains is at root domain

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Border Gateway Multicast Protocol (BGMP)

• Runs in routers that border a multicast routing domain

• Runs over TCP like BGP

• Joins and prunes travel at domain hops

• Can build unidirectional source trees

• MIGP tells the borders about group membership

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Multicast Address Set Claim (MASC)

• How does one determine the root domain for a given group?

• Group prefixes are temporarily leased to domains

• They are allocated out of a service provider’s allocation which in turn get from upstream provider

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• Claims for group allocation resolve collisions

• Group allocations are advertised across domains

• Lots of machinery for aggregating group allocations

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• Tradeoff between aggregation and anticipated demand for group addresses

• Group prefix allocations are not assigned to domains—they are leased

Application must be written to know that group addresses may go away

• Work in progress

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Using BGP4+ (MBGP) for Non-Congruency Issues

• Multiprotocol extensions to BGP4—RFC 2283

• MBGP allows you to build a unicast RIB and multicast RIB independently with one protocol

• Can use the existing or new BGP peering topology

• MBGP carries unicast prefixes of multicast capable sources

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Possible Deployment Scenarios

• Environment:Multiple customers multicast peer at a Gigapop

• Deployment proposalEach customer puts their own administered RP attached to the Gigapop

That RP as well as all border routers run MBGP

The interconnect runs dense-mode PIM

Each customer runs PIM-SM/DM internally

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• What about multiple interconnect points between Gigapop customers?

If multiple Gigapop customers connect at different interconnect points, they can multicast peer for any groups as long as their respective RPs are collocated on the same Gigapop (and the interconnect is Dense mode)

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• What if all RPs are not at the same interconnect point?

Use MSDP so the sources known to the interconnect with RPs can tell the RPs at other interconnects where to join

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• Use a group range that depends on DNS for rendezvousing or building trees

Customers decide which domains will have RPs

Customers decide which groups will use source trees and don’t have to administer RPs

Customers administer DNS databases

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