Network Protocols: Design and Analysis-Multicast Routing

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Network Protocols:

Design and Analysis

Polly Huang EE NTU

http://cc.ee.ntu.edu.tw/~phuang phuang@cc.ee.ntu.edu.tw

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Multicast Routing

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Polly Huang, NTU EE 3

Key ideas

• lays foundation for IP multicast

– defines IP service model

• ex. best effort, packet based, anon group

• compare to ISIS with explicit group membership, guaranteed ordering (partial or total ordering)

• several algorithms

– extended/bridged LANs – distance-vector extensions – link-state extensions

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Why Multicast

• save bandwidth

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Characterizing Groups

• pervasive or dense

– most LANs have a receiver

• sparse

– few LANs have receivers

• local

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Service Model

• same delivery characteristics as unicast

– best effort packet delivery

– open-loop (no built-in congestion/flow control)

• scoping as control mechanism

• groups identified by a single IP address

• group membership is open

– anyone can join or leave

– do security at higher levels

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Routing Algorithms

• single spanning tree

– for bridged LANs

• distance-vector based

• link-state based

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Distance-vector Mcast Rtg

• Basic idea: flood and prune

• flood: send info about new sources everywhere

• prune: routers will tell us if they don’t have

receivers

• routing info is soft state; periodically re-flood (and

prune) to refresh this info

– if no refresh, then the info goes away => easy fault recovery

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Example Topology

g g

s

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Phase 1:

Flood

using

Truncated Broadcast

g g

s

g

truncated broadcast:

this router knows it has no gropus on its LAN, so it doesn’t b roadcast

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Phase 2:

Prune

g g s prune (s,g) prune (s,g) g

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Polly Huang, NTU EE 12 graft (s,g) graft (s,g)

Phase 3:

Graft

g g s g g report (g)

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Phase 4: Steady State

g g

s

g g

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Sending Data in DVMRP

• Data packets are sent on all branches of the tree

– send on all interfaces except the one they came in on

• RPF (Reverse Path Forwarding) Check:

– drop packets that arrive on incorrect interfaces (i.e., not from the unicast direction to the sending host)

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DVMPR Pros and Cons

• Pros:

– simple

– works well with many receivers. why? overhead is per-sender, receivers are passive

• Cons:

– works poorly with many groups (why? every sender in every group floods the nets)

– works poorly with sparse groups (why? flood data

everywhere and then prune back, expensive if only

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Link-state Multicast Routing

• Basic idea: treat group members (receivers) as ne

w links

– flood info about them to everyone in LSA msg (just lik e LSA rtg)

• Compute next-hop for mcast routes on-demand (la

zily)

– unlike for LSA unicast where all are computed as soon as LSA arrives

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Polly Huang, NTU EE 17 S1 R1 R2 X Y Z

Link state: Each router floods link state advertisement Multicast: add membership information to “link state”

Each router computes multicast tree for each active source, builds forwarding entry with outgoing interface list.

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Polly Huang, NTU EE 18 S1 R1 R2 X Y

Z has network map, including membership at X and Y Z computes shortest path tree from S1 to X and Y

(when it gets a data packet on G), puts in rtg table W, Q, R, each do same thing as data arrives at them

Z W

Q

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Polly Huang, NTU EE 19 R1 R2 X Y Z W Q R S1

Link state advertisement with new topology may require re-computation of tree and forwarding entry

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Polly Huang, NTU EE 20 R1 R2 X Y Z W Q R S1

T

R3

Link state advertisement (T) with new membership (R3) may require incremental computation and addition of interface

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MOSPF Pros and Cons

• Pros:

– simple add on to OSFP

– works well with many senders. why? no per-sender state

• Cons:

– works poorly with many receivers (why? per-receiver costs)

– works poorly with sparse groups (why? lots of info goes places tha t don’t want it)

– works poorly with large domains (why? link-state scales wrt numb er of links—many links causes frequent changes)

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PIM

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Key ideas

• want a mcast routing protocol that works w

ell with sparse users

• use a single shared tree; fix one host as rend

ezvous point

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Why not just DVMRP or

MOSPF?

• With sparse groups, both are expensive

– DVMRP problem with many senders – MOSPF problem with many receivers – neither works well with sparse groups

• Solution: PIM-SM

– use rendezvous point as a place to meet – but dowside:

• single point of failure

• don’t necessarily get shortest path

• also concerned about “concentration” of all data going through rendezvous point

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New Design Questions

• Where to place RP?

• How to make the RP robust?

– don’t want a single point of failure

• How to build the tree given an RP?

• How to send data with a shared tree?

• What is the overhead of going through RP (a

shared tree)?

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Where to place RP?

• RP is a node to which people send join mes

sages

• place it in the core

– at the edge is more expensive since tfc must go

through it

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Robustness

• single RP is single point of failure, so must

have backup plan

• approach:

– start with a set of cores

– hash the group name to form an ordered list

• basic idea: order RPs, hash(G) selects one, use it • if it fails, hash(G) to find the next one

• if everyone uses the same hash function, people find the same RPs

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Building the Shared Tree

• Simply send a message towards the RP

– use the unicast routing table to get there

• Add links to the tree as you go

• Stop if you get to a rtr that’s already in the t

ree

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PIM Example: build Shared tree

Shared tree after R1,R2,R3 join Join message toward RP RP R1 R2 _R3 R4 (*, G) (*, G) (*, G) (*, G) (*, G) (*, G) (*, G) (*, G)

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PIM: Sending Data

• If you are on the tree, you just send it as wit

h other mcast protocols

– it follows the tree

• If you are not on the tree (say, you’re a send

er but not a group member), the pkt is tunne

led to the RP that sends it

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PIM Example: sending data on

the tree

RP R1 R2 _R3 R4 (*, G) (*, G) (*, G) (*, G) (*, G) (*, G) (*, G) (*, G) R4 sends data

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Sending data if not on the tree

RP

R1

R2 _R3

R4

S1 unicast encapsulated data packet to RP in Register

RP decapsulates, forwards down shared tree

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What is the cost of the shared

tree?

• Some data goes further than it should

– but latency is bounded to 2x SPT

• All data goes on one tree, rather than on

many trees

– but no guarantee you get multiple paths with

source-specific trees

• But to optimize things, PIM-SM supports

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Build source-specific tree

RP R1 R2 _R3 R4 Join messages toward S1 RP distribution tree

Build source-specific tree for high data rate source

S1 (S1, G) (*,G) (S1, G) (S1, G) (S1, G), (*,G) (S1, G) (*,G) (*, G)

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Forward packets on

“longest-match” entry

RP R1 R2 _R3 R4 R5 S1 Source (S1)-specific distribution tree Shared tree Source-specific entry is

“longer match” for source S1 than is Shared tree entry

that can be used by any source

(S1, G) (*,G) (S1, G) (S1, G) (S1, G), (*,G) (S1, G) (*,G) (*, G) (*,G) (*,G)

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SPT and Shared Trees

• Many more details to be careful about

– need to handle switchover from shared-tree to SPT gracefully

– need to support pruning for both SPT and shared-tree

• and have to worry about LANs with multiple

routers, multiple senders, etc.

• Uses similar protocols (soft-state, refresh, etc.),

but lots of details

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PIM-SM observations

• does a good job at intra-domain mcast routing that

scales to

– many senders – many receivers – many groups – large bandwidth

• preserves original (simple) service model

• but quite complex

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Multi-AS Mcast Routing

• Fine, PIM-SM (or DVMRP or MOSPF) work insi

de an AS, what about between ASes?

– lots of policy questions

– and have to show ISPs why they should deploy (how th ey can make money :-)

– and convince them the world won’t end

• multicast, that’s for high-bandwidth video, right? • multicast can flood all my links with data, right? • what apps, again?

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MSDP

• Support for inter-domain PIM-SM

• Temporary solution

• Basic approach:

– send all sources to all ASes (like original

flood-and-prune)

– AS border routers are PIM-SM RPs for their do

main

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But does this seem complicated?

• some people thought so

• and commercial deployment has been slow

• if we change the service model, maybe we c

an greatly simplify things

– and make it easier for ISPs to understand how t

o change/manage mcast

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Express

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Key ideas

• use channels: a single sender, many subscri

bes

– makes mcast tree easier to config

– easier to tell who can send

• add mechanism to let you count subscribers

• easier to think about billing

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Multicast Problems

• need billing mechanism

– need to know number of subscribers

• need access control

– need to limit who can send and subscribe

– ISPs concerned about mcast

• IPv4 mcast addresses too limited

• current protocols too complex

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Express vs. Multicast Problems

• need billing mechanism

– record sources

– count receivers

• need access control

– only subscriber can send

• IPv4 mcast addresses too limited

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Express Approach

• all addresses are source specific (S,E)

– 224 channels per source, (232 sources)

• access control

– only source can send

– channels optionally protected by “key” (really just a sec ret)

• sub-cast support (encapsulate pkt to any router on

the tree [if you know who they are])

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Express Components

• ECMP: Express Count Mgt Protocol

– like IGMP, but also adds count support

– counts used to determine receivers or for other

things like voting

• not clear how general

• session relays

– service at source that can relay data on to tree

(similar to PIM tunneling)

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Observations

• Simpler? yes

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Another Alternative:

Application-level Multicast

• if the ISPs won’t give us multicast, we’ll ta

ke it :-)

• just do it all at the app

• results in some duplicated data on links

• and app doesn’t have direct access to unicas

t routing

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Application-level Multicast

Example

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App-level Multicast

• Simplest approach:

– send data to central site that forwards

• Better approaches:

– try to balance load on any one link

– try to topologically cluster relays

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