Solicited-node multicast address
Encyclopedia
A Solicited-Node multicast address is an IPv6
multicast address valid within the local-link (e.g. an Ethernet
segment or a Frame Relay
cloud). Every IPv6 host will have at least one such address per interface. Solicited-Node multicast addresses are used in Neighbor Discovery Protocol
for obtaining the layer 2 link-layer
addresses of other nodes.
A Solicited-Node multicast
address is created by taking the last 24 bits of a unicast
or anycast
address and appending them to the prefix ff02:0:0:0:0:1:ff00::/104.
A host is required to join a Solicited-Node multicast group for each of its configured unicast or anycast addresses.
We can see this clearly if we look at an example using the equivalent uncompressed IPv6 address
.
The result of this process is the IPv6 link-local solicited node multicast address that the Neighbor Solicitation packet is sent to.
(ARP) in IPv4
. ARP uses broadcasts to send an ARP Request to the broadcast address 255.255.255.255, which is received by all stations on the local link, although only one station—the one being queried—would need to respond. The other stations still have to process and discard the request. This interruption can cause problems on networks if the amount of broadcast traffic becomes excessive. Devices, such as embedded print servers, might not be able to cope with the amount of traffic they are processing, and fail to operate in a timely manner.
Because a Solicited-node Multicast Addresses is a function of the last 24-bits of an IPv6 unicast (or anycast) address, the number of hosts that are subscribed to each Solicited-node Multicast Address is very small. This number would typically be one, but there could be a few because the mapping function is not a 1:1 mapping. This means that a host should not need to be interrupted as often to service Neighbor Solicitation requests, compared to ARP in IPv4.
However, to prevent any intervening Ethernet switches from flooding the multicast frames out of all switch-ports, which turns the traffic profile in something more like broadcast, intermediate switches should implement MLD Snooping, which would allow them to send traffic that is addressed to a Solicited-node Multicast Address (or any other multicast address) to be sent out only on the ports that lead to stations that have subscribed to receive that traffic.
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
multicast address valid within the local-link (e.g. an Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
segment or a Frame Relay
Frame relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
cloud). Every IPv6 host will have at least one such address per interface. Solicited-Node multicast addresses are used in Neighbor Discovery Protocol
Neighbor Discovery Protocol
The Neighbor Discovery Protocol is a protocol in the Internet Protocol Suite used with Internet Protocol Version 6 . It operates in the Link Layer of the Internet model and is responsible for address autoconfiguration of nodes, discovery of other nodes on the link, determining the Link Layer...
for obtaining the layer 2 link-layer
Link Layer
In computer networking, the link layer is the lowest layer in the Internet Protocol Suite , the networking architecture of the Internet . It is the group of methods or protocols that only operate on a host's link...
addresses of other nodes.
| bits | 8 | 4 | 4 | 79 | 9 | 24 |
|---|---|---|---|---|---|---|
| field | prefix | flgs | sc | zeroes | ones | unicast address |
A Solicited-Node multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
address is created by taking the last 24 bits of a unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
or anycast
Anycast
Anycast is a network addressing and routing methodology in which datagrams from a single sender are routed to the topologically nearest node in a group of potential receivers all identified by the same destination address.-Addressing methodologies:...
address and appending them to the prefix ff02:0:0:0:0:1:ff00::/104.
A host is required to join a Solicited-Node multicast group for each of its configured unicast or anycast addresses.
- Example 1: fc00::1/64 would become ff02::1:ff00:1.
- Example 2: fe80::2aa:ff:fe28:9c5a would become ff02::1:ff28:9c5a.
Worked Example
Assume a host needs to make a local delivery to another host on the local network, and the target host has an IPv6 address of fe80::2aa:ff:fe28:9c5a (as in Example 2 above). In order to make a Layer-2 (eg. Ethernet) delivery, it needs to know the target host's hardware address (eg. "Ethernet MAC address"). But in order to do this, it must first determine which hardware address to send it to. To do this, an IPv6 host will construct the Solicited-node Multicast Address related to the target address.We can see this clearly if we look at an example using the equivalent uncompressed IPv6 address
IPv6 address
An Internet Protocol Version 6 address is a numerical label that is used to identify a network interface of a computer or other network node participating in an IPv6-enabled computer network....
.
fe80::2aa:ff:fe28:9c5a Target address (compressed notation)
fe80:0000:0000:0000:02aa:00ff:fe28:9c5a Target address (uncompressed notation)
-- ---- the last 24-bits
ff02::1:ff00:0000/104 Solicited-node Multicast Address prefix
ff02:0000:0000:0000:0000:0001:ff00:0000/104 (uncompressed)
---- ---- ---- ---- ---- ---- -- The first 104 bits
ff02:0000:0000:0000:0000:0001:ff28:9c5a Result
ff02::1:ff28:9c5a Result (compressed notation)
The result of this process is the IPv6 link-local solicited node multicast address that the Neighbor Solicitation packet is sent to.
Efficiency Compared to IPv4 and ARP
Solicited-node Multicast Addresses are used with IPv6 Neighbor Discovery to provide the same function as the Address Resolution ProtocolAddress Resolution Protocol
Address Resolution Protocol is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. ARP was defined by RFC 826 in 1982. It is Internet Standard STD 37...
(ARP) in IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
. ARP uses broadcasts to send an ARP Request to the broadcast address 255.255.255.255, which is received by all stations on the local link, although only one station—the one being queried—would need to respond. The other stations still have to process and discard the request. This interruption can cause problems on networks if the amount of broadcast traffic becomes excessive. Devices, such as embedded print servers, might not be able to cope with the amount of traffic they are processing, and fail to operate in a timely manner.
Because a Solicited-node Multicast Addresses is a function of the last 24-bits of an IPv6 unicast (or anycast) address, the number of hosts that are subscribed to each Solicited-node Multicast Address is very small. This number would typically be one, but there could be a few because the mapping function is not a 1:1 mapping. This means that a host should not need to be interrupted as often to service Neighbor Solicitation requests, compared to ARP in IPv4.
However, to prevent any intervening Ethernet switches from flooding the multicast frames out of all switch-ports, which turns the traffic profile in something more like broadcast, intermediate switches should implement MLD Snooping, which would allow them to send traffic that is addressed to a Solicited-node Multicast Address (or any other multicast address) to be sent out only on the ports that lead to stations that have subscribed to receive that traffic.