IP - IGMP OV (Study Note) for IPv4

1.1. IGMP Overview Summary

IGMP Summary

What is IGMP

§ Internet Group Management Protocol (IGMP) is a protocol that manages host membership in IP multicast groups. IGMP（Internet Group Management Protocol）協定很簡單，它由主機成員關係協定發展而來。

§ IGMP: Internet Group Management Protocol - Used in IP multicast

§ IGMP is an Internet Layer protocol

§ IGMP is an asymmetric protocol.

§ IGMP carries group membership information

§ IGMP: “signaling” protocol to establish, maintain, remove groups on a subnet.

§ IGMP is required for all machines that receive IP multicast

§ IGMP operates at the IP Layer

§ Technically embeds its information in IP packets

§ IP Protocol Number = 2 to identify IGMP messages

Host and Group Membership in IGMP

§ Host and router exchange of multicast group info. IGMP在主機和它直接連接的路由器間工作。

§ Hosts use IGMP to dynamically register themselves in a multicast group on a particular LAN. 主機使用IGMP消息通告本地的組播路由器它想接收組播流量的組播組位址

§ Host registration is accomplished using IGMP.

§ How hosts tell routers about group membership. Hosts identify group memberships by sending IGMP messages to their local multicast router.

§ IGMP is used by host receivers to join or leave a multicast host group.

§ IGMP is used by IPv4-based receivers.

§ An IP multicast group, also known as a host group, is a set of hosts that listen for IP traffic destined for a specific multicast IP address.

§ A host group can be of any size.

§ Host group membership is dynamic, hosts can join and leave the group at any time.

§ Members of a host group can span IP routers across multiple networks. This situation requires IP multicast support on the IP routers and the ability for hosts to register their group membership with local routers.

§ A host can send traffic to an IP multicast address without belonging to the corresponding host group.

§ Each host keeps track of which mcast groups are subscribed to

• Socket API informs IGMP process of all joins

§ Hosts send memberships reports

• use response suppression

Multicast Router (Mrouter) in IGMP

§ Objective: keep router up-to-date with group membership of entire LAN

• Routers need not know who all the members are, only that members exist

§ Used by mrouters to learn about Multicast Group Memberships on their directly attached subnets

• the existence of at least one member/group

§ Routers and multilayer switches, configured for IGMP, listen to IGMP messages and periodically send out queries to discover which groups are active or inactive on a particular subnet or VLAN.

§ Routers solicit group membership from directly connected hosts

§ IGMP messages not forwarded by routers

§ Router queries periodically (every 60s.)

• mcast query to all-hosts group: 224.0.0.1
• maintains list of “active” groups
• entries time-out if hosts do not respond

§ 已使用的有兩個版本：IGMPv1和IGMPv2。

§ The latest version at press time was Version 3.

• IGMPv3 supports receivers that explicitly signal sources from which they wish to receive traffic.

The following list indicates the current versions of IGMP:

§ IGMP version 1 (IGMPv1) RFC 1112

§ IGMP version 2 (IGMPv2) RFC 2236

§ IGMP version 3 (IGMPv3) RFC 3376

§ IGMP version 3 lite (IGMPv3 lite)

IGMP v1 – version v1

§ No way to expressly leave a multicast group.

§ It’s up to the router to timeout the group membership

§ 組播路由器通過IGMP協議為其每個埠都維護一張組播組成員表，並定期探詢表中的組播組成員，以確定該組播組是否存活（即該網段是否仍有屬於某個組播組的成員），實現所連網路組成員關係的收集與維護。

§ 當路由器接收到某個組G的資料分組後，只向那些有G的成員的埠上轉發資料分組。

§ 而資料分組在路由器之間如何轉發則由路由協定決定，IGMP協定並不負責。

IGMP v2 – version v2

§ Includes “leave processing” mechanism

§ IGMPv2向前相容IGMPv1協定，IGMPv1的設備可以接收處理IGMPv2的消息分組。

§ IGMPv2中允許路由器對指定的組播組位址做"成員詢問"，非該組的主機不必回應。如果某主機想退出，它可以主動向路由器發送"退出組播組"消息，而不必像IGMPv1中那樣只能被動退出。

• explicit leave message - reduces leave latency
• group-specific query – reduces bandwidth

IGMP v3 – version v3

§ Supports "source filtering," which enables a multicast receiver host to signal to a router which groups it wants to receive multicast traffic from, and from which source(s) this traffic is expected.

Please refer further for the following topics: http://docs.google.com/fileview?id=0B71FtRlv0N1DNWJhNTI1NDktNDlmYy00OWUzLTljMGEtZmM4ZDI1NTY3Yzdh&hl=en

1.2. Overview - Multicast

Multicast

Broadcast

Multicast Overview

Possible levels of participation of a host in multicasting:

Multicast Routing

Unicast vs. Multicast routing

Multicast Forwarding Algorithms

IP Multicast Addresses

1.3. IGMP Protocol Overview

1.4. IGMP Protocol Packet Overview

1.5. IGMPv1 Message and Packet Format

1.6. IGMPv2 Message and Packet Format

1.7. IGMPv3 Message and Packet Format

1.8. IGMP Operation

To join a group, hosts sends report (Membership Report) message

Routers periodically issue request (Membership Query) message

Host Operations

Multicast Router Operations

IGMP Operation

1.9. Functions in IGMP Operation

Switches Using IGMP Snooping

Multicast Forwarding and Routing

Flooding:

DoS: IGMP Blasting

Spanning tree

Group Table for IGMP