Debugging Multicast

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Debugging Multicast

• John Barlow

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Status

• Multicast enabled on all GrangeNet routers.
• Multicast enabled in all AARNet RNOs except for
  NT RNO.
• Multicast running natively across SCCN and
  throughout GrangeNet and AARNet
• Curtin, VU, JCU, UoW, CSIRO, UQ, CQU connected
  (whole-of-campus ?)

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Overview

• Terminology
• Intro. to multicast protocols
• Good design
• Caveats
• The document you should read/use
• Brief walkthrough

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Terminology

• SA
• RP
• RPF
• mroute
• MSDP
• MBGP
• IGMP

• PIM-sparse
• PIM-sparse-dense
• PIM-dense

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SA (Source Active)

• Information about a singular source of multicast
  packets
• Source address (eg 192.94.63.41)
• Multicast group (eg 233.2.178.9)
• Rendezvous Point (remote or local )
• AS, age, peer address
• Your router should have a cache of all sources
  for all multicast groups

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SA

• edge1.actsh ip msdp sa-cache
• MSDP Source-Active Cache - 2867 entries
• (62.40.99.241, 224.0.1.32), RP 62.40.102.39,
  MBGP/AS 20965, 061311/000523, Peer
  202.0.98.13
• (62.40.103.30, 224.0.1.32), RP 62.40.102.9,
  MBGP/AS 20965, 061247/000523, Peer
  202.0.98.13

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RP (Rendezvous Point)

• A repository for multicast source information.
• Local source information
• Remote source information
• Acts as central bootstrap point for subscribing
  to a multicast source.

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RP

• edge1.actsh ip pim rp mapping
• PIM Group-to-RP Mappings
• Group(s) 224.0.0.0/4, Static
• RP 202.0.98.49 (loopback0.edge1.act.grangenet
  .net)
• edge1.act

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Reverse Path Forwarding (RPF)

• Used to discard/ignore multicast packets that
  might be looping (ie multicast packet claiming
  to come from a source must flow in the interface
  that the unicast routing table says you should
  take to get _to_ the source).
• Also used to discard MSDP SA packets

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RPF

• edge1.actsh ip rpf 192.94.63.10
• RPF information for clix.aarnet.edu.au
  (192.94.63.10)
• RPF interface Vlan23
• RPF neighbor vlan23.gig0-0.aarnet1.yarralumla.a
  arnet.edu.au (202.0.98.82)
• RPF route/mask 192.94.63.0/24
• RPF type mbgp
• RPF recursion count 0
• Doing longest-match lookups across tables
• Multicast Multipath enabled

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Multicast ROUTE (mroute)

• When some multicast is flowing, there will be an
  mroute entry that tells the router which
  interface the SA is coming in on, and which
  interface(s) it is going out of.

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mroute

• edge1.actsh ip mroute active
• Active IP Multicast Sources - sending gt 4 kbps
• Group 233.29.147.222, (?)
• Source 192.94.63.35 (arrowroot.aarnet.edu.au)
• Rate 9 pps/7 kbps(1sec), 6 kbps(last 30
  secs), 1 kbps(life avg)
• Source 192.231.212.2 (lattice.broadway.aarnet.
  net.au)
• Rate 9 pps/6 kbps(1sec), 6 kbps(last 30
  secs), 1 kbps(life avg)

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Multicast Source Discovery Protocol (MSDP)

• Is a unicast (TCP) protocol to pass SA
  information from one RP to another RP
• You should configure MSDP sa-cache to save SA
  entries (saves time compared to the alternative
  of querying peers)

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MSDP

• edge1.actsh ip msdp summary
• MSDP Peer Status Summary
• Peer Address AS State Uptime/ Reset SA
  Peer Name
• Downtime Count
  Count
• 202.0.98.1 18062 Up 3d01h 2 15
  loopback0.edge1.qld.grangenet.net
• 202.0.98.68 18062 Up 1w0d 4 0
  loopback0.edge1.vic.grangenet.net
• 202.0.98.102 64607 Up 2w6d 5846 0
  g102.anu.edu.au
• 202.0.98.118 65522 Down 4w6d 0 0
  port2.extreme.apac.edu.au

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Multiprotocol Border Gateway Protocol (MBGP)

• Used to feed reverse path routing information
  (can be used to enforce policy to route multicast
  separately from unicast routes)
• Also used for IPv6 unicast, IPv6 multicast (MBGP
  is an expansion on standard BGP to handle lots of
  things)

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MBGP

• edge1.actsh ip mbgp summary
• BGP
• Neighbor V AS MsgRcvd MsgSent TblVer
  InQ OutQ Up/Down State/PfxRcd
• 202.0.98.1 4 18062 50194 49994 465329
  0 0 3d01h 21
• 202.0.98.12 4 18062 50110 50213 465329
  0 0 3w2d 11
• 202.0.98.13 4 18062 336849 50255 465329
  0 0 3w2d 4348

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Internet Group Membership Protocol (IGMP)

• Protocol that a host uses to chat with a router
  to subscribe (and possibly unsubscribe) from a
  multicast source
• Several versions exist (usually want the latest,
  version 3, if you have any option)
• Switches can snoop IGMP so they can olptimise
  which ports do _not_ get some multicast source

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PIM sparse / dense

• PIM-sparse, PIM-dense, PIM-sparse-dense
• Sparse mode uses the RP and is network efficient
• Dense mode does not use the RP, flood-and-prune
  mechanism to inform every router, required for
  some protocols (eg Novell).

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Intro. to Multicast

• Protocol Independent Multicast (PIM)
• Peering between PIM clouds
• MBGP
• MSDP

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Protocol Independent Multicast

• Operates with some arbitrary administrative
  domain (eg your campus).
• Provides RPF (Reverse Path Forwarding)
  verification by using the unicast routing table
  (which can be populated by any protocol OSPF,
  ISIS, static, etc) hence the name.

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Intro. to PIM

• Sources tell the nearest router about themselves,
  and the nearest router passes that information to
  the RP.
• Consumers ask the nearest router to join them
  to multicast source(s), and the router passes
  this request to the RP.

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Intro. to PIM

• Need to specify an RP (Rendezvous Point) which
  doesnt have to be inside your PIM cloud, but it
  is nice to have it inside (self-contained, more
  robust).
• Client subscribes to multicast via RP, but this
  quickly changes to a native subscription.

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Intro. to PIM

• After the RP has set up the multicast join, the
  router nearest the consumer has enough
  information to join independently of the RP, so
  it does this, and the RP stops forwarding the
  multicast.
• The RP passes the multicast stream in an
  encapsulated unicast packet

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Intro. to PIM Peering

• Became politically impractical to run one PIM
  cloud for the entire Internet
• MSDP
• My RP and your RP swap SA information
• MBGP
• I can enforce policy on multicast routing
• Remote site might need your MBGP info.

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Good Design

• No tunnels native multicast everywhere (easier
  debugging)
• Advertise your prefix(es) via MBGP(or have the
  RNO do so on your behalf)
• Keep unicast and multicast on the same link
  (debugging and problems are simpler) avoid
  special policy for multicast where possible

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Good Design

• Single RP to start with (easier debugging)
• Use a dedicated loopback interface with a /32
  address
• Later upgrade to anycast RP for robustness

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Good Design

• Monitor packets blocked by firewalls and access
  lists
• PIM-sparse mode unless you need dense (ie
  Novell), then PIM-sparse-dense

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Caveats

• IGMP snooping
• If you have enough switch CPU, use it
• Ghost
• Uses multicast block it at your border
• You may not have multicast officially enabled,
  but might need IGMP snooping to help when running
  ghost
• Access lists firewalls

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Caveats

• Reverse path forwarding (reverse path
  verification) and a lack of error messages
• Wireless
• 10Mbps half duplex
• hubs

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The Doco. you should use

• The following is an Internet2 debugging guide
• http//www.aarnet.edu.au/engineering/trouble/multi
  cast/troubleshoot-multicast.pdf

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Brief Walkthrough

• Router-by-router process between source and
  destination.
• Need to have a source trying to transmit and a
  consumer trying to receive (even if this isnt
  working !)
• A good way to achieve this is with the multicast
  beacon service
• If you are an access-grid site, subscribe to the
  access-grid beacon
• Otherwise (maybe as well) subscribe to the AARNet
  beacon server

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References

• General multicast noteshttp//www.aarnet.edu.au/
  network/design/multicast/
• IPTV software downloadhttp//videolab.uoregon.ed
  u/download.html
• Troubleshooting guideshttp//www.aarnet.edu.au/e
  ngineering/trouble/multicast/in particular,
  check the NANOG 2003 tutorial link !
• http//beaconserver.accessgrid.org9999/