Engineering Workshops 1 Multicast on the LAN. Engineering Workshops 2 Multicast Addressing at Layer 2 An IPv4 multicast address is 32 bits, of which the

Engineering Workshops

1

Multicast on the LAN


2

Multicast Addressing at Layer 2

• An IPv4 multicast address is 32 bits, of which the first 4 bits are always the same, leaving 28 bits.

• A MAC multicast address is 48 bits, of which the first 24 bits are always the same. One of the remaining bits is reserved, leaving 23 bits.

• So, one multicast MAC address maps to 32 multicast IP addresses.


3

000000010000000001011110

0 8 31

0

32-bit IP address

48-bit Ethernet address

00 24

0 = Internet multicast1 = Reserved for other use

IANA owns 01-00-5E vendor address block; half of it is assigned for IP multicast.

IEEE Ethernet multicast bit

1110 ignored, leaving 28 bits

47

Ethernet Multicast Addressing

23 bits

01-00-5E-

Class D address

00-00-00 thru 7F-FF-FF


4

IGMP• Internet Group Management Protocol - how hosts tell routers

about group membership• Routers also solicit group membership from directly connected

hosts• RFC 1112 specifies version 1 of IGMP

– Supported on Windows 95• RFC 2236 specifies version 2 of IGMP

– Supported on latest service pack for Windows, newer Windows releases, and most UNIX systems

• RFC 3376 specifies version 3 of IGMP– Provides source include-list capabilities (SSM!)– Included in Linux kernel 2.6 and later– See http://videolab.uoregon.edu/projects.html


5

IGMPv2• Router:

– sends Membership Query messages to All Hosts (224.0.0.1)• query-interval = 125 secs default

– router with lowest IP address is Querier (rest non-queriers)– If lower-IP address query heard, back off to non-querier

state• Other Querier Present Interval default: (robust-count x

query-interval) + (0.5 x query-response-interval) = 255 secs

– listens for reports (whether querier or not) and adds group to membership list for that interface

• query-response-interval = 10 secs default– timeout (Group member interval) default:

• (robust-count x query-interval) + (1 x query-response-interval) = 260 sec

– robust-count - provides fine-tuning to allow for expected packet loss on a subnet. Default = 2 (tunable from 2-10)


6

IGMPv2• Host:

– sends Membership Report messages to groups it is a member of• waits 0-10 sec (default) • Hosts listen to other host reports• Only 1 host responds

– sends unsolicited Membership Reports (i.e., Join Messages) to group address (e.g. 224.10.8.5)

– sends Leave messages to All Routers (224.0.0.2)

– reports group membership ONLY – no sources. Only the existence of local group members is reported, not the actual members themselves


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IGMP Protocol Flow - Join a Group

• Router triggers group membership request to PIM.• Hosts can send unsolicited join membership messages –

called reports in the RFC (usually more than 1)• Or hosts can join by responding to periodic query from router

I want 230.0.0.1

230.0.0.1

230.0.0.1Forwards stream

Router adds group

I want to JOIN!

230.0.0.1


8

IGMP Protocol Flow - Querier

• Hosts respond to query to indicate (new or continued) interest in group(s)– only one host should respond per group

• Hosts fall into idle-member state when same-group report heard.

• After 260 sec with no response, router times out group.

224.0.0.1

Still interested

?(general query)

224.0.0.1125 sec

I want 230.0.0.1

230.0.0.1

230.0.0.1

0-10 sec

230.0.0.1 group

Yes, me!


9

IGMP Protocol Flow - Leave a Group

• Hosts that support IGMPv2 send leave messages to all-routers group indicating group they’re leaving.– Router follows up with 2 group-specific queries messages

• IGMPv1 hosts leave by not responding to queries (260 sec timeout)

I don’t want 230.0.0.1 anymore

224.0.0.2<230.0.0.1>

230.0.0.1 group

I wantto leave!

Anyone still want this group?

224.0.0.1<230.0.0.1>

224.0.0.1<230.0.0.1>

1 sec (re-transmit timer)


10

Soft State• Say I set up an active Multicast group, say by

issuing a membership report. What happens if my computer goes down and never directly leaves the group ?

• This is fixed with “Soft State”– Everything has a timer, and if not periodically

reinitiated the timer will expire and the state will be removed.

– So there is no danger of some rogue group lasting forever.


11

Source = 1.1.1.1Group = 224.1.1.1

H1 - Member of 224.1.1.1

R1

R3

R2

Source = 2.2.2.2Group = 224.1.1.1

IGMPv3: MODE_IS_INCLUDEJoin 1.1.1.1, 224.1.1.1

IGMPv3

• H1 wants to receive from S = 1.1.1.1 but not from S = 2.2.2.2

• With IGMPv3, specific sources can be pruned back - S = 2.2.2.2 in this case

Specified in RFC 3376Enables hosts to listen only to a specified subset of the sources sending to the group

Video Server

Video Server


12

IGMPv3 Enhancements

• Group-Source Report message is defined. Enables hosts to specify which senders it can receive or not receive data from.

• Group-Source Leave message is defined. Enables host to specify the specific IP addresses of a (source,group) that it wishes to leave.


13

Switches and Snooping

• IGMP host reports (Joins) tell the router to start sending multicast traffic to the LAN, since one or more hosts on the LAN are members of the group.

• In a conventional shared broadcast LAN using switches that have no multicast smarts, the traffic is sent to all hosts.

• With multiple high bandwidth multicast sources (e.g. video at 5 Mbps), this does not scale beyond approximately one source.

• There are a few techniques used to deal with this...


14

IGMP Snooping

• Implemented by several vendors. Support for IGMPv2 is common; support for IGMPv3 is rare, but becoming more common.

• What happens at the MAC layer:– IGMP snoopers add a bridge table entry for each

multicast group destination address (GDA) to each switch port that has the interested member's unicast source address (USA) already on it. (Remember that there are likely to be dumb hubs downstream of switches, so more than one USA can be on a single port.)

– When an IGMP Leave is received, the GDA entries are pruned.


15

Why IGMP snooping isharder than it looks

• The IGMP membership reports have to be captured from each host and suppressed to other hosts to prevent the others from going into idle-member state; every interested host has to be spoofed into thinking it is the only member of the group, so that it actively sends membership reports. The IGMP snooper then forwards one of these membership reports up to the router (or makes up a fake membership report for itself).


16

Why IGMP snooping is harder than it looks, continued

• Since multiple USAs can be on a port (via dumb hub), the switch has to actually do the IGMP membership query/timeout before pruning a port.

• Since membership reports are sent to the same GDA as the (possibly high-bandwidth) multicast traffic, there is a potential for heavy loading of the switch CPU, unless you use more expensive ASICs that can separate the IGMP protocol messages from general traffic and route only the IGMP messages to the CPU.

• The switch has to know which is the multicast router port. It does this by snooping for IGMP queries.


17

CGMP• The proprietary Cisco Group Management Protocol puts the

bulk of the Layer 3 logic in Layer 3 devices rather than cramming it into Layer 2 devices like IGMP snooping does.

• The router sends CGMP Joins and Leaves to the switch, specifying the USA and GDA.

• On receipt of an IGMP Membership Report, the router sends the switch a CGMP Join.

• On receipt of an IGMP Leave, the router sends the switch a CGMP Leave.

• IGMP membership reports still have to be suppressed so that hosts don't go into idle-member state.

• CGMP does not work correctly with IGMPv3.– Leaves are not implemented, resulting in channel surfers causing a

multicast flooding DoS on their subnet.– See

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcpt3/1cfmulti.htm#1046127.


18

PIM Snooping and RGMP• For Layer 2 networks with routers but no hosts

(transit LANs). • PIM, not IGMP, is spoken among routers, so

IGMP snooping does not work in this case.• PIM snooping and the Cisco-proprietary Router

Group Management Protocol (RGMP) are used by the Layer 2 switch to send only the multicast flows that the router needs to the router's port. These work analogously to IGMP snooping (smarts in the switch) and CGMP (smarts in the router).

• PIM snooping is still mostly experimental. Some Foundry Networks switches support it.


19

Problems with Multicast on the LAN

• In general, multicast on the LAN is not as well understood as multicast on the WAN.

• Switch behaviors are not standardized. • Problems with switches:

– when snooping is enabled, they may drop packets that shouldn’t be dropped.

– even without snooping, sometimes they step outside their bailiwick, trying to do non-Layer-2 tasks.


20

Case StudyA few months ago I converted all our interfaces over to IGMP Version 3. Then I started getting complaints from our lab/classroom support group that Norton Ghost was failing for them. It would hang after about 3 minutes. So far the fix, without understanding why it works, has been to revert the interfaces to IGMP version 2. The switches downstream from these interfaces are running CGMP and CGMP LEAVE (which is actually a form of IGMP snooping/spoofing for IGMP Leaves sent to 224.0.0.2). I suspect that the fact that these switches are actually looking at IGMP packets may have something to do with the problem that reverting to v2 fixed...

— Alan Crosswell


21

Case Study

This author traveled to Los Alamos, New Mexico to help debug a multicast problem that had everyone stumped. Everyone was assuming the only known router on the subnet was also acting as the multicast gateway. Unfortunately, this wasn’t the case. A nominally Layer 2 switch on the subnet was accidentally configured with PIM active, and won the PIM Designated Router election. Of course, this Layer 2 switch had no upstream to anywhere.

— Bill Nickless


22

One Approach to Multicast on the LAN

• Avoid snooping, as it causes more problems than it solves.

• Keep subnets small. A smaller subnet is less likely to have people joining several different multicast groups, traffic for each of which is sent to the entire subnet.

• If at all possible, use routers, not switches or bridges.

• If you have to use switches, try to at least buy them all from the same vendor, so you won’t have inconsistent behavior as well as unexpected behavior.


23

Another Approach toMulticast on the LAN

• The previous approach reflects gigaPoP/WAN bias.

• On a campus, it just isn't possible to use routers everywhere.

• Switches and snooping may be evils, but they are necessary evils. Learn to cope with them. http://www.cisco.com/warp/public/473/22.htmlis a good place to start.


24

Lab 1: Multicast on the LAN


25

SSM


26

PIM-SM• SM stands for “Sparse Mode.”

– RFC 2362 and draft-ietf-pim-sm-v2-new-06.txt

– There is also a Dense Mode, but we don’t recommend using it.

– Cisco has a proprietary “Sparse-Dense” mode which is used for RP discovery.

• PIM-SM allows for both RPTs and SPTs.

• There are two ways to use PIM-SM…


27

ASM and SSM• ASM: Any-Source Multicast. Traditional multicast – data

and joins are forwarded to an RP. – All routers in a PIM domain must have RP mapping.– When load exceeds threshold, forwarding switches

to an SPT. The default threshold is one packet; in this case, the sole purpose of the RPT is to learn which sources are active. (With IGMPv2, the receiver can only specify the group, not specific sources.)

– State increases (not everywhere) as number of sources and number of groups increase.

– SPT state is refreshed when data is forwarded and with Join/Prune control messages.

• SSM: Source-Specific Multicast. PIM-SM without RPs – instead, the source is learned out-of-band, and the SPT is built directly to it.


28

SSM• Source-Specific Multicast (SSM) is a subset of ASM,

so – SSM concepts apply directly to ASM, but– SSM is a lot simpler than ASM.

For these reasons, we cover SSM first in this workshop.

• 232 / 8 is assigned to SSM as an address space. Other address ranges can also be set up for SSM — this is primarily a function of the receiving network.

• Source activity and IP addresses are assumed known.

• IGMPv3 allows for “Include” lists of (S,G) pairs.


29

SSM• SSM - draft-ietf-ssm-arch-01.txt

– 232/8 – IANA assigned– No RPTs– Guarantees ONE source on any delivery tree

• Content security – no unwanted sources– Reduced protocol dependence – more later...– Solves address allocation issues for inter-domain one-to-many

• tree address is 64 bits – S,G– Host must learn source address out-of-band (e.g, from a web

page)– Host-to-router join request specifies source as well as group

• requires IGMPv3 for include-source list– SSM behavior in 232/8 by default

• Configurable to expand range


30

SSM in Action• Each (S,G) pair listed in the IGMPv3

include list generates a (S,G) Join directly towards the source.

• That’s it. It’s very simple. All you need to implement is :– Edge routers need IGMPv3– Interior routers need filters to prevent

RP (*,G) Joins & other RP state for the SSM address block


31

SSM Group Addresses• 232 / 8 is assigned to SSM as an address

space. – You don’t have to ask, you can just pick one

and use it.• How can this be ?

– Note that all joins are unique as long as the combination of S and G are unique. Not only can one source support multiple groups, but if there are two sources using the same group address, everything works just fine.


32

SSM

Receiver

RP

Receiver announces desireto join group G AND source Swith an IGMPv3 include-list.IGMPv3 host report

(S, G) JoinLast-hop router joins the SPT.

Source

Shortest Path Tree

Traffic Flow (S,G) state is built between the source and the receiver.


33

SSM

Receiver

RP

Data flows down the shortest path tree to the receiver.

Source

Shortest Path Tree

Traffic Flow


34

Lab 2: SSM

Documents

Engineering Workshops 1 Multicast on the LAN. Engineering Workshops 2 Multicast Addressing at Layer 2 An IPv4 multicast address is 32 bits, of which the