TCPIP培训.ppt

* Query-Response Process The router multicasts periodic IGMPv1 Membership Queries to the “All-Hosts” () group address.Only one member per group responds with a report to a query. This is to save bandwidth on the subnet network and processing by the hosts. This is process is called “Response Suppression”. (See section below.) Response Suppression Mechanism The “Report Suppression” mechanism is accomplished as follows: When a host receives the Query, it starts a count-down timer for each multicast group of which it is a member. The count-down timers are each initialized to a random count within a given time range. (In IGMPv1 this was a fixed range of 10 seconds. Therefore the count-down timers were randomly set to some value between 0 and 10 seconds.) When a count-down timer reaches zero, the host sends a Membership Report for the group associated with the count-down timer to notify the router that the group is still active. However, if a host receives a Membership Report before its associated count-down timer reaches zero, it cancels the count-down timer associated with the multicast group, thereby suppressing its own report.In the example shown in the slide, H2’s time expired first so it responded with its Membership Report. H1 and H3 cancelled their timers associated with the group; thereby suppressing their reports. * * * Purpose: This figure explains what is contained in a TCP segment. Emphasize: Source Port and Destination Port are the connections to the upper-layer protocol. Sequence and Acknowledgment numbers are the position in the user’s byte stream of this segment. Sequence numbers are used for establishing reliability. HLEN is the header length. It tells us where the data begins. Six bits are reserved for future use. Code Bits distinguish session management messages from data. Window is a term we will come back to in a few slides. For now, consider it the size of the receivers buffers. Checksum is a cyclic redundancy check (CRC). It ver