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Background

The IEEE 802.1D bridge protocol specifies how the packets are switched between LAN

segments. As mentioned in this specification, for the packets addressed with multicast destination,

switch always forward copies of them to each of the non ingress ports in forwarding state, just like

it done for broadcast packets. That means whether the nodes connecting to the switch ports are

interesting in the multicast packets, they always receive all of these packets. As we know, the

original purpose for multicast is send the information to those really interest for it, to broadcast the

multicast frames is against to this purpose. Further more, the multicast currently is used to carry

the videos, voices, etc. These applications will occupy more and more bandwidth, if the multicast

frames are forwarded as broadcast same as before, the network nodes will be drawn in the

multicast waves soon.

To avoid such kind of situation, many mechanisms are introduced into the L2 switch devices,

such as IGMP snooping, IGMP proxy, controllable multicasting, and so on. In our product, some

of them will be implemented to protect the end users from the annoying from the multicast storm.

And one of them will be described in this document, that is IGMP snooping.

The block surrounded with the dash line shows the position of the IGMP module in the whole

system. The module is divided into two major parts: one is the IGMP core module, the other is

IGMP control logic. The IGMP core module is mainly responding for the IGMP protocol frames

sniffer, switch forward table control and membership management. On the other hand, the IGMP

control logic is in charge of the procession to the customer control request to the IGMP module.

IGMP control logic receives the customer control request, and translates them to the IGMP

core through system call ioctl, and show the result to the customer. For IGMP core module, on one

hand, it will sniffer the IGMP protocol frames passing through the switch, and maintain the state

of the membership, then manage the switch forward table according to the state, on the other hand,

it will accept the control requests from the control logic, and change the current behavior

according to these requests.

Module Design

As mentioned in the previous section, there are six elements need to be implemented for

IGMP module. In all this elements, the IGMP core module is the most important one. It handles

the whole module state transition, controls the switch behavior, and responses to the customer

control requests. We can regard it as the brain of the IGMP module, so that the design of it will

affect and decide the other elements’ design. For the IGMP core is so important that we will

describe the mechanism of it, after then we define the other elements one by one.

IGMP Data Flow Control Mechanism

As shown in the figure, the data flow of the IGMP module is divided into two parts, one is

control flow, the other is multicast data flow. The control flow includes all kinds of the IGMP

protocol messages, such as ‘query’, ‘report’, ‘leave’ and so on. All of the control messages should

be sent to IGMP control block at first, then forwarded to the ports decided by IGMP control block.

IGMP control block is also in charge of the data flow control according to the IGMP control

messages.

When the IGMP snooping function enabled, it will block all of the multicast streams ingress

from the router ports and uplink ports (This is a general sense, but for IGMPv3 compatible, there

may be some special procession, this will be discussed later), while a join report to a group ingress

from a host port is snooped, the corresponding multicast stream will be sent to that port with a

specified time window (so called aging time). If the time window has elapsed and there is no more

report snooped, the stream will be stop to send to the port.

Beside the aging time, a query to a group will start an additional time window according to

the maximum response time embedded in the query. In this time window, if the aging time

window has elapsed, the event will be ignored. If the query time window has elapsed and no more

report is received, the multicast stream will be stopped to send to that port.

IGMP Core Element

This element implements a serious of logic to satisfy the recommends described by the

RFC4541, as listed below:

Type No Rules

1

A snooping switch should forward IGMP Membership Reports only

to those ports where multicast routers are attached

2

IGMP networks may also include devices that implement

"proxy-reporting", in which reports received from downstream hosts

are summarized and used to build internal membership states

3

The switch that supports IGMP snooping must flood all

unrecognized IGMP messages to all other ports and must not

attempt to make use of any information beyond the end of the

network layer header

4

An IGMP snooping switch should be aware of link layer topology

changes caused by Spanning Tree operation

Control

(IGMP forwarding)

5

An IGMP snooping switch must not make use of information in

IGMP packets where the IP or IGMP headers have checksum or

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xidianjunnan

2012-03-20

全是英文而且有些图片和文字都是乱的（重叠或是显示不全）不怎么好
lxahnu

2012-02-20

里面的图片里面的文字很多都没有了，全部是英语，感觉不是很好
gzlzxmh

2012-10-11

英文版本有乱码，不过还是谢谢了，可以参考一下！

dayushu

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