Spanning Tree and Multicast

The Story So Far

• Switched ethernet is good– Besides switching needed to join even multiple

classical ethernet networks• Routing is expensive, learning switches seem

cheaper• Flooding in a network with a cycle of switches

is bad (why?)

Flooding With Cycles

1

2 4

3

B

A

C

D

A wants to send a packet to D- A sends packet to 1- 1 Floods to 2 and 4- 2 Floods to B and 3- 4 Floods to D and 3 <- D receives packet- 3 Floods packet from 2 to C and 4- 3 Floods packet from 4 to C and 2- 4 Floods packet from 3 to D and 1- 2 Floods packet from 3 to B and 1- 1 Floods packet from 2 to A and 4- 1 Floods packet from 4 to B and 2- ….- When does this craziness stop?

Fixing with Cycles

• DEC (which is long dead) faced the same problem in the mid-1980s (connecting Catenets)

• Choices offered– Rely on network administrators to build loop-free

topologies• Turns out to be hard

– Build a protocol• Given a loopy graph, want no loops– Trees are nice, they don’t have loops– Need a tree that connects all the nodes somehow– Spanning Trees <- Trees spanning over all nodes

The High Level Protocol

• Pick a root– For the purposes of

this class, root node is the one with the lowest MAC address.• In reality also adds a

user specified “priority” into the mix

1

2 4

3

B

A

C

D

The High Level Protocol

• For each node, determine shortest path to root– Break ties by choosing

the lower of two MAC addresses

– In the example 3 picks the path through 2 rather than 4

1

2 4

3

B

A

C

D

The High Level Protocol

• Disable all links not used by the previously picked paths. 1

2 4

3

B

A

C

D

The High Level Protocol

• Recomputation for resilience:– If root goes down,

select a new root, rerun algorithm

– If another node goes down adjust links to recreate the tree.

1

2 4

3

B

A

C

D

The Protocol in Real Life

• Uses messages of the form– (proposed root, distance to root, node sending message)– From example (1, 0, 1) <- “Node 1 proposes that 1

be the root, also it is distance 0 away from 1”• Messages allow for election of root and

determining distances• Messages (when sent described next) are

always flooded out all ports of a switch– This is not a problem even in the presence of

loops. Why?

The Protocol in Real Life• Initially all switches send a message proposing themselves

as the root – Messages like (1, 0, 1), (2, 0, 2) etc

• Switches update their view of the root– On receiving a message (Y, d, Z) from Z, if id(Y) < id(root), root = Y

• Compute distance from the root• If root or shortest distance has changed, flood an update

message notifying neighbors of new root and distance• Periodically everyone reannounces their distance and

perceived root– Includes the root which sends (root, 0, root)– Used to detect failures and recompute tree when needed.

Multicast

• Promise of the 90s– All TV and live events broadcast over the internet• More viewers, more revenue, all over the world.

– Too expensive to run one stream per user.

The ProblemYour favorite media conglomerate (The Producer)

The Network

You A B ZZZZEveryone else in the world

…

• The individual connections from the “producer” to the network all carry the exact same data.• Similarly each individual stream uses network bandwidth, so there might be 15 copies of the

same data needlessly using up bandwidth.• IDEA: Why not just have the network deal with this, give it one copy of the data and have it

determine where the packets go.• Does this violate End-to-End?

• Multicast!!!!

Multicast

• Fundamentally things to do– Join a multicast group (set of end hosts listening to packets)– Send to group

• Different implementation at different layers– Link layer

• Easiest to implement, used in LANs– IP

• Harder to implement, but allows for greater efficiency (as implemented)

– Application level• We ignore this.

Link Layer Multicast

• Each multicast groups is denoted by an address G• Join a group by telling NIC about G– NIC then listens for packets sent to address G

• Send by broadcasting packet with a destination address of G.

• Very efficient in terms of state (end-host stores everything)

• Inefficient in terms of bandwidth

IP Multicast• We focus on intra-domain• Portion of IP address

space is reserved for Multicast

• Receivers join group using IGMP (anyone can join)

• Anyone can send (don’t need to be a part of the group)

Receiver Sender

Receiver

IP Multicast

• Take graph on right.• Want Sender to be able

to multicast to receivers• Minimize number of

packets sent to get one packet from sender to all receivers.

• A few ways to do this

Receiver Sender

Receiver

IP Multicast

• Must build a tree from source to all destinations

• We know how to flood along a tree

• Can build a tree based on a specific source– Distance Vector Multicast

Routing Protocol• Build one tree for all

possible sources– Core Based Trees

Receiver Sender

Receiver

DVMRP• An extension to distance

vector routing.• Consider distances to

source (use source as root of spanning tree).

• Three steps, each getting us closer to the ideal.– Reverse Path Flooding– Reverse Path Broadcasting– Truncated Reverse Path

Broadcasting Receiver Sender

Receiver

DVMRP• RPF

– If incoming link is shortest path to source• Send on all links except incoming

– Otherwise drop• Packets sent along the black

links in the direction marked will be flooded.

• Sometimes two of the same packet are sent. – For instance Node X receives

the same packet along both L0 and L1, forwards both of them along L2.

X

Receiver Sender

Receiver

L0

L1

L2

DVMRP• RPB

– Pick a single parent for each link.

– Send packets from parent along a link.

• In example to the right Y is picked as the parent for L2.

• Packet from Z is not forwarded by X, while packet from Y is, therefore only one packet goes through L2.

• Still suboptimal, X does not really need to receive the packet.

Y

X Z

Receiver Sender

Receiver

L0

L1

L2

DVMRP• Pruning

– Do not send packet to destinations not in the multicast group.

• Start by sending to everyone as in RPB

• Nodes send an explicit non-membership request if no one below them on the tree wants the data.

• In this example X might send a NMR.

• Do not send data to a pruned node.

• NMR eventually expires, at which point data is again transmitted.

Y

X Z

Receiver Sender

Receiver

L0

L1

L2

Core Based Trees

• Build a common tree• For each group pick a

“rendezvous point” (the Core)

• Build a spanning tree rooted at the rendezvous point

• Flood using spanning tree algorithm

Receiver Sender

Receiver