A Method for Generating Link-Scoped IPv6 Multicast Addresses
updates
- rfc-3306 — Unicast-Prefix-based IPv6 Multicast Addresses
Extracted elements (15)
IID-based link-local multicast addresses are preferred over unicast-prefix-based IPv6 multicast addresses (RFC 3306) because they allow each node to generate multicast addresses automatically without allocation servers, making them better suited for serverless environments such as ad-hoc networks and mobile environments.
In mobile environments, the lifetime of IID-based link-scoped multicast addresses is linked to the period during which the IID is known to be unique, creating a potential conflict when a new node joins a merged network using the same IID as an existing powered node; this document acknowledges but does not resolve that issue.
This document updates RFC 3306 by restricting the usage of the scop, plen, and network prefix fields; the plen value of 255 and scop <= 2 signal this new link-local format, distinguishing it from standard unicast-prefix-based multicast addresses.
The creation of the IID-based multicast address MUST only occur after Duplicate Address Detection (DAD) has completed as part of the auto-configuration process.
The flgs field in the link-scoped multicast address MUST be '0011'.
The IID embedded in the link-scoped multicast address MUST only come from the IID of the link-local unicast address on the interface after DAD has completed.
The reserved field in the link-scoped multicast address format MUST be zero.
The scop field in the link-scoped multicast address MUST be less than or equal to 2, indicating link-local scope.
This technique MUST only be used for link-scoped multicast addresses. For multicast addresses greater than link-local scope, other methods such as unicast-prefix-based IPv6 multicast addresses must be used.
The 32-bit Group ID field is generated to indicate a multicast application and guarantees uniqueness only within the host; it may be set based on the guidelines in RFC 3307.
The 64-bit IID field replaces the 64-bit network prefix field of RFC 3306 and is used to distinguish each node from others on the link.
The plen field takes the special value '1111 1111' (decimal 255) to distinguish this link-scoped format from RFC 3306 unicast-prefix-based addresses, where plen indicates the prefix length.
RFC 3041 does not protect the privacy of link-local unicast addresses, and therefore it does not appear necessary to protect the privacy of IID-based link-local multicast addresses derived from them.
The uniqueness of multicast addresses using this method is guaranteed by the DAD process, so a secure DAD process is needed for stability. RFC 3041 (Privacy Extensions for Stateless Address Autoconfiguration) is proposed to secure the DAD process, though it cannot be used to make a link-local unicast address directly.
Link-scoped multicast addresses are 128 bits structured as: 8-bit prefix (11111111), 4-bit flgs (0011), 4-bit scop (<=2), 8-bit reserved (zero), 8-bit plen (255/0xFF), 64-bit IID, and 32-bit group ID.