Multicast - Part 1

Multicast transmission is communication a method that allows send traffic from the source to the group of receivers simultaneously. Multicast communication relays on Internet Group Management Protocol (IGMP) and Protocol Independent Multicast (PIM).

IP Multicast concept:

-receivers use Internet Group Management Protocol (IGMP) to join multicast groups – Layer 2

-protocol Independent Multicast (PIM) is used to route multicast traffic(Layer 3)

-only members of the group receive data

-once send information to the group address all members receive it

-non members can send information to a group

-source address can never be a Class D multicast group address.

IPv4 address space reserved for Multicast is 224.0.0.0 – 239.255.255.255 (Class D) which includes:

 

1. 224.0.0.0 – 224.0.0.255 – Reserved Link-Local Address ie:

- 224.0.0.5 – used by OSPF all routers

-224.0.0.6 – used by OSPF DRs

-224.0.0.9 – used by RIP

-224.0.0.10 – used by EIGRP

-224.0.0.13 – All PIM routers

2. 224.0.1.0 – 238.255.255.255 – Globally Scoped Addresses 

   a) 224.0.1.0 - 224.0.1.255 - Internetwork Control Block i.e.:

   -224.0.1.1 – NTP Network Rime Protocol

   -224.0.1.32 – Mtrace routers

   -224.0.1.39 – cisco-rp-announce

   -224.0.1.40 - cisco-rp-discovery 

   b) 232.0.0.0 – 232.255.255.255 – Source Specific Multicast (SSM) addresses, default range used by SSM which is a PIM extension (RFC 4607) 

   c)233.0.0.0 – 233.255.255.255 – GLOP Block (The assignment is made, for a domain with a 16bit  Autonomous System Number (ASN))

   d) 239.0.0.0-239.255.255.255-Administratively Scoped IP addresses

More details about IPv4 addresses (IANA):
https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-2

Internet Group Management Protocol (IGMP) is the layer 2 protocol used by receivers to join multicast groups (i.e. between the receiver and its gateway).

 

We have tree versions on IGMP, here is the brief:

-         in IGMPv1 – JOIN message is send to the router, there is a 60s timer on the router that will request an update from the receiver if it still needs to belong to the group

-         in IGMPv2 – Receiver sends JOIN group message as well as the LEAVE group message

-         in IGMPv3 – Receiver sends JOIN group message, LEAVE group message and define SOURCE IP for multicast traffic (if there is more than one source),  two different modes are available: include mode and exclude mode.

IGMP snooping is used by i.e. switch to learn about multicast traffic so it can use the information to be forwarded to receivers, benefits:

-Optimized bandwidth utilization

-Security – prevents denial of service attacks from unknown sources

IGMP filtering can be used to control:

-           specific multicast groups membership

-          number of multicast groups per port/interface

-          version of IGMP allowed

What is Protocol Independent Multicast (PIM)?

-         multicast routing protocol necessary to route multicast traffic used by routers to locate and request multicast streams utilize Distribution Trees that define the path that multicast traffic follows through the network. There are two types of distribution trees: source trees (Shortest Path Trees - SPT) and shared trees (RPT)

PIM control messages:

-          use IP protocol 103

-          unicast or multicast

-          TTL value of 1

-          All PIM router address 224.0.0.13

-          by default “Hello” messages are being send every 30 seconds

Source three – multicast distribution tree often referred as SPT, source is the root of the tree and the branches are all the way through the network to receivers, known by the notation (S, G) where “S” is the source and “G” is the multicast group

Shared trees – multicast distribution tree where the root is the designated router as the rendezvous point (RP) not the source (S) of the multicast traffic. Shared trees are also known as RP trees (RPTs). Traffic is being forwarded to the group address regardless of the source, notation (*,G). Advantage is that RPT require fewer multicast entries (*,G) and (S,G). Downside is that receivers receive traffic from all multicast sources which will cause lots of unwanted traffic, waste of bandwidth and also it opens potential security risk (unwanted packets send to receivers).

PIM operating modes:

-          PIM dense Mode

-          PIM Sparse Mode

-          PIM Sparse Dense Mode

-          PIM Bidirectional Mode

-          PIM Source Specific Multicast Mode