Internet Group Management Protocol version 3 (IGMPv3) and Multicast Listener Discovery version 2 (MLDv2) Message Extension

This document defines a generic method to extend IGMPv3 and MLDv2 messages to accommodate information other than what is contained in the current message formats. This is done by allowing a list of TLVs (Type, Length and Value) to be used in the Additional Data part of IGMPv3 and MLDv2 messages. This document defines a registry for such TLVs, while other documents will define the specific types and their values, and their semantics. The extension would only be used when at least one TLV is to be added to the message. This extension also applies to the lightweight versions of IGMPv3 and MLDv2 as defined in . When this extension mechanism is used, it replaces the Additional Data section defined in IGMPv3/MLDv2 for TLVs. Additional Data is defined for Query messages in IGMPv3 Section 4.1.10 and MLDv2 Section 5.1.12, and for Report messages in IGMPv3 Section 4.2.11 and MLDv2 Section 5.2.11.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

For each of the IGMPv3 and MLDv2 headers, a previously reserved bit is used to indicate the presence of this extension. When this extension is used, the Additional Data of IGMPv3 and MLDv2 messages is formatted as follows. Note that this format contains a variable number of TLVs. It MUST contain at least one TLV.

Extension Type: 2 octets. This identifies a particular Extension Type as defined in the IGMP/MLD Extension Type Registry. If this is not the first TLV, it will follow immediately after the end of the previous one. There is no alignment or padding. Extension Length: 2 octets. This specifies the length in octets of the following Extension Value field. The length may be zero if no value is needed. Extension Value: This field contains the value. The length and the contents of this field is according to the specification of the Extension Type. IGMPv3 and MLDv2 messages are defined so that they can fit within the network MTU, in order to avoid fragmentation. When this extension mechanism is used, the number of Group Records in each Report message SHOULD be kept small enough that the entire message, including any extension TLVs can fit within the network MTU.

The MLDv2 Query Message format with extension is shown below. The E-bit MUST be set to 1 to indicate that the extension is present. Otherwise, it MUST be 0.

The MLDv2 Report Message format with extension is shown below. The E-bit MUST be set to 1 to indicate that the extension is present. Otherwise, it MUST be 0.

The IGMPv3 Query Message format with the extension is shown below. The E-bit MUST be set to 1 to indicate that the extension is present. Otherwise, it MUST be 0.

The IGMPv3 Report Message format with the extension is shown below. The E-bit MUST be set to 1 to indicate that the extension is present. Otherwise, it MUST be 0.

The procedure specified in this document applies only when the E-bit is set. If the validation of the TLVs fails, the entire Additional Data field MUST be ignored as specified in IGMPv3 [RFC3376] and MLDv2 [RFC3810]. The following checks must pass for the validation of the TLVs not to fail: At least one TLV MUST be present. There MUST NOT be any data in the IP payload after the last TLV. To check this, the parser needs to walk through each of the TLVs until there are less than four octets left in the IP payload. If there are any octets left, validation fails. The total length of the Extension MUST NOT exceed the remainder of the IP payload length. For this validation, one only examines the content of the Extension Length fields. Future documents defining a new type MUST specify any additional processing and validation. These rules, if any, will be examined only after the general validation (above) succeeds. Unsupported types MUST be ignored.

IGMP and MLD implementations, particularly implementations on hosts, rarely change, and the adoption process of this extension mechanism is expected to be slow. Also as new extensions are defined, it may take a long time before they are supported. Due to this, defining extensions should not be taken lightly, and it is crucial to consider backwards compatibility. Implementations that do not support this extension mechanism will ignore it, as specified in [RFC3376] and [RFC3810]. It is possible that a new extension type only applies to queries, or only to reports, or there may be other specific conditions for when it is to be used. A document defining a new type MUST specify under what conditions the new type should be used, including for which message types. It MUST also be specified what the behavior should be if a message is not used in the defined manner, e.g., if it is present in a query message, when it was only expected to be used in reports. When defining new types, care should be taken to consider the effect of partial support for the new TLV, by either the hosts or routers, on the same link. Further, it must be considered whether there are any dependencies or restrictions on combinations between the new types and any pre-existing types. This document defines an extension mechanism only for IGMPv3 and MLDv2. Hence this mechanism does not apply if hosts or routers send older version messages.

The Security Considerations of [RFC3376] and [RFC3810] also apply here. This document extends the IGMP and MLD message formats, allowing for a variable number of TLVs. Implementations must take care when parsing the TLVs to not exceed the packet boundary, an attacker could intentionally specify a TLV with a length exceeding the boundary. An implementation could add a large number of minimal TLVs in a message to increase the cost of processing the message to magnify a Denial of Service attack.

IANA is asked to create a new registry called "IGMP/MLD Extension Types" in the "Internet Group Management Protocol (IGMP) Type Numbers" section, with registration procedure "IETF Review" , and with this document as a reference. The registry is common for IGMP and MLD. The initial content of the registry should be as below (empty).

The authors thank Ron Bonica, Ian Duncan, Wesley Eddy, Leonard Giuliano, Jake Holland, Tommy Pauly, Pete Resnick, Alvaro Retana and Zhaohui Zhang for reviewing the document and providing valuable feedback.