MLD. MLDv1 (defined in RFC 2710), which is derived from IGMPv2. MLDv2 (defined in RFC 3810), which is derived from IGMPv3.

Transcription

1 Introduction to Multicast listener discovery protocol () is used by an IPv6 router to discover the presence of multicast listeners on directly-attached subnets. Multicast listeners are nodes wishing to receive multicast packets. Through, the router can learn whether there are any IPv6 multicast listeners on directly-connected subnets, put corresponding records in the database, and maintain timers related to IPv6 multicast addresses. Routers running use an IPv6 unicast link-local address as the source address to send messages. messages are Internet control message protocol for IPv6 (ICMPv6) messages. All messages are confined to the local subnet, with a hop count of 1. Versions So far, two versions are available: v1 (defined in RFC 2710), which is derived from IGMPv2. v2 (defined in RFC 3810), which is derived from IGMPv3. How v1 Works v1 implements IPv6 multicast listener management based on the query/response mechanism. v1 uses two types of query messages: I. querier election Of multiple IPv6 multicast routers on the same subnet, all the routers can hear listener report messages (often referred to as reports) from hosts, but only one router is needed for sending query messages (often referred to as queries). So, a querier election mechanism is required to determine which router will act as the querier on the subnet. 1) Initially, every router assumes itself as the querier and sends general query messages (often referred to as general queries) to all hosts and routers on the local subnet (the destination address is FF02::1). 2) Upon hearing a general query, every router compares the source IPv6 address of the query message with its own interface address. After comparison, the router with the lowest IPv6 address wins the querier election and all other routers become non-queriers. 1

2 3) All the non-queriers start a timer, known as other querier present timer. If a router receives an query from the querier before the timer expires, it resets this timer; otherwise, it assumes the querier to have timed out and initiates a new querier election process. II. Joining an IPv6 multicast group Querier Router A Router B Ethernet Host A (G2) Host B (G1) Host C (G1) Query Report Figure 1 Joining IPv6 multicast groups Assume that Host B and Host C are expected to receive IPv6 multicast data addressed to IPv6 multicast group G1, while Host A is expected to receive IPv6 multicast data addressed to G2, as shown in Figure 1. The basic process that the hosts join the IPv6 multicast groups is as follows: 1) The querier (Router B in the figure) periodically multicasts queries (with the destination address of FF02::1) to all hosts and routers on the local subnet. 2) Upon receiving a query message, Host B or Host C (the delay timer of whichever expires first) sends an report to the IPv6 multicast group address of G1, to announce its interest in G1. Assume it is Host B that sends the report message. 3) Host C, which is on the same subnet, hears the report from Host B for joining G1. Upon hearing the report, Host C will suppress itself from sending a report message for the same IPv6 multicast group, because the routers (Router A and Router B) already know that at least one host on the local subnet is interested in G1. This mechanism, known as report suppression, helps reduce traffic over the local subnet. 4) At the same time, because Host A is interested in G2, it sends a report to the IPv6 multicast group address of G2. 2

3 5) Through the above-mentioned query/report process, the routers learn that members of G1 and G2 are attached to the local subnet, and generate (*, G1) and (*, G2) multicast forwarding entries, which will be the basis for subsequent IPv6 multicast forwarding, where * represents any IPv6 multicast source. 6) When the IPv6 multicast data addressed to G1 or G2 reaches an router, because the (*, G1) and (*, G2) multicast forwarding entries exist on the router, the router forwards the IPv6 multicast data to the local subnet, and then the receivers on the subnet receive the data. III. Leaving an IPv6 multicast group When a host leaves a multicast group: 1) This host sends a done message to all IPv6 multicast routers (the destination address is FF02::2) on the local subnet. 2) Upon receiving the leave message, the querier sends a configurable number of multicast-address-specific queries to the group being left. The destination address field and group address field of the message are both filled with the address of the IPv6 multicast group being queried. 3) One of the remaining members, if any on the subnet, of the group being queried should send a report within the time of the maximum response delay set in the query messages. 4) If the querier receives a report for the group within the maximum response delay time, it will maintain the memberships of the IPv6 multicast group; otherwise, the querier will assume that no hosts on the subnet are still interested in IPv6 multicast traffic addressed to that group and will stop maintaining the memberships of the group. How v2 Works Compared with v1, v2 provides the following new features: I. IPv6 multicast group filtering In addition to multicast-address-specific queries, v2 has introduced IPv6 multicast source filtering modes (Include and Exclude), so that a host not only can join a designated IPv6 multicast group but also can explicitly specify to receive or reject IPv6 multicast data from a designated IPv6 multicast source. When a host joins an IPv6 multicast group G: If it needs to receive IPv6 multicast data from specific IPv6 multicast sources like S1, S2,, it sends a report with the Filter-Mode denoted as Include Sources (S1, S2, ). If it needs to reject IPv6 multicast data from specific IPv6 multicast sources like S1, S2,, it sends a report with the Filter-Mode denoted as Exclude Sources (S1, S2, ). 3

4 As shown in I. Figure 2, the network comprises two IPv6 multicast sources, Source 1 (S1) and Source 2 (S2), both of which can send IPv6 multicast data to IPv6 multicast group G. Host B is interested only in the IPv6 multicast data that Source 1 sends to G but not in the data from Source 2. Source 1 Host A Receiver Host B Source 2 Host C Packets (S1,G) Packets (S2,G) Figure 2 Flow paths of source-and-group-specific multicast traffic In the case of v1, Host B cannot select IPv6 multicast sources when it joins IPv6 multicast group G. Therefore, IPv6 multicast streams from both Source 1 and Source 2 will flow to Host B whether it needs them or not. When v2 is running on the hosts and routers, Host B can explicitly express its interest in the IPv6 multicast data Source 1 sends to G (denoted as (S1, G)), rather than the IPv6 multicast data Source 2 sends to G (denoted as (S2, G)). Thus, only IPv6 multicast data from Source 1 will be delivered to Host B. II. state A multicast router running v2 maintains the multicast address state per multicast address per attached subnet. The multicast address state consists of the following: Filter mode: The router keeps tracing the Include or Exclude state. List of sources: The router keeps tracing the newly added or deleted IPv6 multicast source. Timers: Filter timer (the time the router waits before switching to the Include mode after an IPv6 multicast address times out), source timer (for source recording), and so on. III. Receiver host state listening By listening to the state of receiver hosts, a multicast router running v2 records and maintains information of hosts joining the source group on the attached subnet. 4

5 Message Types The following descriptions are based on v2 messages. I. query message An querier learns the multicast listening state of neighbor interfaces by sending query messages. Figure 3 shows the format of an query message. The dark blue area in the figure shows the format of an v1 message Type = 130 Code Checksum Maximum Response Delay Reserved Multicast Address (128 bits) Reserved S QRV QQIC Number of Sources (n) Source Address [1] (128 bits)... Source Address [n] (128 bits) Figure 3 Format of v2 query message Table 1 describes the fields in Figure 3. Table 1 Description on fields in an v2 query message Field Description Type = 130 Code Checksum Message type. For a query message, this field is set to 130. Initialized to zero Standard IPv6 checksum 5

6 Field Maximum Response Delay Reserved Multicast Address S QRV QQIC Number of Sources Source Address( i ) Description Maximum response delay allowed before a host sends a report message Reserved field and initialized to zero This field is set to 0 in a general query message. It is set to a specific IPv6 multicast address in a multicast-address-specific query message or multicast-address-and-source-specifi c query message. Flag indicating whether a router updates the timer for suppression after receiving a query message. Querier s Robustness Variable Querier s Query Interval Code This field is set to 0 in a general query message or a multicast-address-specific query message. This field represents the number of source addresses in a multicast-address-and-source-specifi c query message IPv6 multicast source address in a multicast-address-specific query message (i = 1, 2,.., n, where n represents the number of multicast source addresses.) II. report message A host sends an report message to report the current multicast listening state Figure 4 shows the format of an report message. 6

7 Type = 143 Code Checksum Reserved Number of Multicast Address Records (m) Multicast Address Record [1]... Multicast Address Record [m] Figure 4 Format of v2 report message Table 2 describes the fields in Figure 4. Table 2 Description on fields in an v2 report message Field Description Type = 143 Reserved Checksum Number of Multicast Address Records Multicast Address Record(i) Message type. For a report message, this field is set to 143. The Reserved fields are set to 0 on transmission and ignored on reception. Standard IPv6 checksum This field indicates how many IPv6 multicast address records are present in this report message. This field represents information of each IPv6 multicast address the host listens to on the interface from which the report message is sent, including record type, IPv6 multicast address, and IPv6 multicast source address on the sender (i= 1, 2,... m, where m represents the number of IPv6 multicast address records). 7

8 Protocols and Standards is documented in: RFC 2710: Multicast Listener Discovery () for IPv6 RFC 3810: Multicast Listener Discovery Version 2 (v2) for IPv6 8