IGMPv3/MLDv2 TLVs: Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery Version 2 (MLDv2) Message Extension
A generic list of type-length-value extensions for IGMPv3 and MLDv2 multicast membership messages.
Sivakumar, M., Venaas, S., Zhang, Z., and H. Asaeda, "Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery Version 2 (MLDv2) Message Extension", RFC 9279, DOI 10.17487/RFC9279, August 2022, <https://www.rfc-editor.org/info/rfc9279>.
@misc{rfc9279,
series = {Request for Comments},
number = 9279,
howpublished = {RFC 9279},
publisher = {RFC Editor},
doi = {10.17487/RFC9279},
url = {https://www.rfc-editor.org/info/rfc9279},
author = {M. Sivakumar and S. Venaas and Z. Zhang and H. Asaeda},
title = {{Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery Version 2 (MLDv2) Message Extension}},
pagetotal = 12,
year = 2022,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) M. Sivakumar
Request for Comments: 9279 Juniper Networks
Category: Standards Track S. Venaas
ISSN: 2070-1721 Cisco Systems, Inc.
Z. Zhang
ZTE Corporation
H. Asaeda
NICT
August 2022
Internet Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Version 2 (MLDv2) Message Extension
Abstract
This document specifies a generic mechanism to extend IGMPv3 and
Multicast Listener Discovery Version 2 (MLDv2) by using a list of
TLVs (Type, Length, and Value).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9279.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Conventions Used in This Document
3. Extension Format
3.1. Multicast Listener Query Extension
3.2. Version 2 Multicast Listener Report Extension
3.3. IGMP Membership Query Extension
3.4. IGMP Version 3 Membership Report Extension
4. No-op TLV
5. Processing the Extension
6. Applicability and Backwards Compatibility
7. Security Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
This document defines a generic method to extend IGMPv3 [RFC3376] and
MLDv2 [RFC3810] messages to accommodate information other than what
is contained in the current message formats. This is done by
allowing a list of TLVs to be used in the Additional Data section of
IGMPv3 and MLDv2 messages. This document defines a registry for such
TLVs. Other documents will define their specific types, and their
values and 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
[RFC5790].
When this extension mechanism is used, it replaces the Additional
Data section defined in IGMPv3/MLDv2 with TLVs.
Additional Data is defined for Query messages in IGMPv3
(Section 4.1.10 of [RFC3376]) and MLDv2 (Section 5.1.12 of
[RFC3810]), and for Report messages in IGMPv3 (Section 4.2.11 of
[RFC3376]) and MLDv2 (Section 5.2.11 of [RFC3810]).
2. Conventions Used in This Document
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Extension Format
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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension Type 1 | Extension Length 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension Value 1 |
. . .
. . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension Type 2 | Extension Length 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension Value 2 |
. . .
. . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension Type n | Extension Length n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension Value n |
. . .
. . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Extension Format
Extension Type: 2 octets. This identifies a particular Extension
Type as defined in the "IGMP/MLD Extension Types" 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 specification
defining the Extension Type describes the length and contents of
this field.
IGMPv3 and MLDv2 messages are defined so they can fit within the
network MTU in order to avoid fragmentation. An IGMPv3/MLDv2 Report
message contains a number of records. The records are called Group
Records for IGMPv3 and Address Records for MLDv2. When this
extension mechanism is used, the number of records in each Report
message SHOULD be kept small enough so that the entire message,
including any extension TLVs, can fit within the network MTU.
3.1. Multicast Listener Query Extension
The MLDv2 Query message format [RFC3810] 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 130 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Response Code | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* *
| |
* Multicast Address *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| Resv|S| QRV | QQIC | Number of Sources (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* *
| |
* Source Address [1] *
| |
* *
| |
+- -+
| |
* *
| |
* Source Address [2] *
| |
* *
| |
+- . -+
. . .
. . .
+- -+
| |
* *
| |
* Source Address [N] *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: MLD Query Extension
3.2. Version 2 Multicast Listener Report Extension
The MLDv2 Report message format [RFC3810] 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 143 | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| Reserved |Nr of Mcast Address Records (M)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Multicast Address Record [1] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Multicast Address Record [2] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
. . .
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Multicast Address Record [M] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: MLD Report Extension
3.3. IGMP Membership Query Extension
The IGMPv3 Query message format [RFC3376] 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x11 | Max Resp Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| Resv|S| QRV | QQIC | Number of Sources (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address [1] |
+- -+
| Source Address [2] |
+- . -+
. . .
. . .
+- -+
| Source Address [N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: IGMP Query Extension
3.4. IGMP Version 3 Membership Report Extension
The IGMPv3 Report message format [RFC3376] 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x22 | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| Reserved | Number of Group Records (M) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Group Record [1] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Group Record [2] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
. . .
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Group Record [M] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: IGMP Report Extension
4. No-op TLV
The No-op TLV is a No-Operation TLV that MUST be ignored during
processing. This TLV may be used to verify that the extension
mechanism has been implemented correctly. Note that there is no
alignment requirement, so there is no need to use this Extension Type
to provide alignment.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| No-op Type = 0 | No-op Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
. . .
. . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: No-op TLV Format
No-op Type: 2 octets. The type of the No-op TLV extension is 0.
Extension Length: 2 octets. This specifies the length in octets of
the following Value field. The length may be zero if no value is
needed.
Value: This field contains the value. As this Extension Type is
always ignored, the value can be arbitrary data. The number of
octets used MUST match the specified length.
5. Processing the Extension
The procedure specified in this document only applies 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, only the content of
the Extension Length fields is examined.
Future documents defining a new Extension Type MUST specify any
additional processing and validation. These rules, if any, will be
examined only after the general validation succeeds.
TLVs with unsupported Extension Types MUST be ignored.
6. Applicability and Backwards Compatibility
IGMP and MLD implementations, particularly implementations on hosts,
rarely change. The adoption process of this extension mechanism is
expected to be slow. As new extension TLVs are defined, it may take
a long time for them to be supported. Due to this, defining new
extension TLVs 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]. As mentioned in
the previous section, unsupported extension TLVs are ignored.
It is possible that a new extension TLV will only apply to queries or
only to reports, or that there may be other specific conditions for
when it is to be used. A document defining a new Extension Type MUST
specify the conditions under which the new Extension Type should be
used, including 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 Extension Types, the effect of partial support for
the new TLV, by either the hosts or routers, on the same link should
be carefully considered. Further, whether there are any dependencies
or restrictions on combinations between the new Extension Types and
any preexisting Extension Types must be considered.
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.
7. Security Considerations
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 not to
exceed the packet boundary when parsing the TLVs, because 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. This would
magnify a denial-of-service attack.
8. IANA Considerations
IANA has created a new registry called "IGMP/MLD Extension Types" in
the "Internet Group Management Protocol (IGMP) Type Numbers" section
and lists this document as the reference. The registration procedure
is "IETF Review" [RFC8126]. The registry is common for IGMP and MLD.
Two Extension Types (65534 and 65535) are provided for "Experimental
Use" [RFC8126]. Any experiments should be confined to closed
environments where it is unlikely that they may conflict with other
experiments; see [RFC3692].
IANA has initially populated the registry as shown in Table 1
+================+==========+==================+===========+
| Extension Type | Length | Name | Reference |
+================+==========+==================+===========+
| 0 | variable | No-op | RFC 9279 |
+----------------+----------+------------------+-----------+
| 1-65533 | | Unassigned | |
+----------------+----------+------------------+-----------+
| 65534-65535 | variable | Reserved for | |
| | | Experimental Use | |
+----------------+----------+------------------+-----------+
Table 1: IGMP/MLD Extension Types
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>.
[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692,
DOI 10.17487/RFC3692, January 2004,
<https://www.rfc-editor.org/info/rfc3692>.
[RFC5790] Liu, H., Cao, W., and H. Asaeda, "Lightweight Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Version 2 (MLDv2) Protocols", RFC 5790,
DOI 10.17487/RFC5790, February 2010,
<https://www.rfc-editor.org/info/rfc5790>.
Acknowledgements
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.
Authors' Addresses
Mahesh Sivakumar
Juniper Networks
64 Butler St
Milpitas, CA 95035
United States of America
Email: sivakumar.mahesh@gmail.com
Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose, CA 95134
United States of America
Email: stig@cisco.com
Zheng(Sandy) Zhang
ZTE Corporation
No. 50 Software Ave, Yuhuatai District
Nanjing
210000
China
Email: zhang.zheng@zte.com.cn
Hitoshi Asaeda
National Institute of Information and Communications Technology
4-2-1 Nukui-Kitamachi, Koganei, Tokyo
184-8795
Japan
Email: asaeda@nict.go.jp
AI ELI5
On this page
§1Why Extend IGMPv3 and MLDv2
This RFC defines a generic way to extend IGMPv3 and MLDv2 messages so they can carry information beyond what the current formats hold. It does this by allowing a list of TLVs in the Additional Data section of those messages. It also defines a registry for the TLV types. Other documents will define specific types and what they mean.
The extension is used only when at least one TLV is to be added to a message. It also applies to the lightweight versions of IGMPv3 and MLDv2 defined in RFC 5790. When the mechanism is used, the TLVs replace the Additional Data section defined in IGMPv3 and MLDv2.
Imagine a form with no blank lines left for notes. This RFC adds a spot at the bottom where anyone can staple on extra labeled notes.
§3The Extension Format
A previously reserved bit in each IGMPv3 and MLDv2 header, the E-bit, signals that the extension is present. It MUST be 1 when the extension is there and 0 otherwise. When it is used, the Additional Data is a variable number of TLVs, and it MUST contain at least one.
Each TLV has an Extension Type of 2 octets, an Extension Length of 2 octets, and an Extension Value. The length counts the octets of the value that follows, and may be zero. The next TLV follows immediately, with no alignment or padding.
IGMPv3 and MLDv2 messages are designed to fit within the network MTU to avoid fragmentation. With this extension, the number of records in each Report SHOULD be kept small enough that the whole message, including the TLVs, fits within the MTU.
A little flag at the top says whether extra notes are stuck on. Each note has a label, a size and then its contents, and notes sit one right after another.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
EXT-TYPE | Extension Type | 2 bytes | Identifies the type in the IGMP/MLD Extension Types registry. | |
EXT-LENGTH | Extension Length | 2 bytes | Length in octets of the value that follows; may be zero. |
§3.1Where the E-bit Sits in Each Message
The RFC shows the extension in four message types. In the MLDv2 Query (Type 130) and the IGMPv3 Membership Query (Type 0x11), the E-bit is the first bit of the byte that also holds a reserved field, the S flag and the QRV field, followed by QQIC and the Number of Sources. In the MLDv2 Report (Type 143) and the IGMPv3 Membership Report (Type 0x22), the E-bit is the first bit of the field that precedes the record count.
In every case the Extension follows the last source address (in queries) or the last record (in reports).
In each kind of message the flag lives in the same spot near the start, and the extra notes always go at the very end.
§4The No-op TLV
The only type defined here is the No-op TLV, with type 0. It MUST be ignored during processing. It may be used to check that the extension mechanism has been implemented correctly. Because there is no alignment requirement, it is not needed for padding.
Its value can be arbitrary data, but the number of octets used MUST match the specified length.
A No-op note is a note that says nothing on purpose. It is useful for checking that notes are being read and skipped correctly.
§5Processing and Validation
The procedure applies only when the E-bit is set. If validation of the TLVs fails, the entire Additional Data field MUST be ignored, as IGMPv3 and MLDv2 already specify. Validation fails unless all of these hold:
- At least one TLV is present.
- No data follows the last TLV. A parser walks the TLVs until fewer than four octets are left in the IP payload; if any octets remain, validation fails.
- The total length of the Extension does not exceed the rest of the IP payload. Only the Extension Length fields are examined for this check.
TLVs with unsupported Extension Types MUST be ignored. Future documents that define a new type MUST specify any extra processing and validation, which is applied only after the general validation succeeds.
If the stack of notes is broken, for example a note claims to be longer than the page, the receiver throws the whole stack away. A note it does not understand is simply skipped.
§6Applicability and Backwards Compatibility
IGMP and MLD implementations, especially on hosts, rarely change, so adoption is expected to be slow and new TLVs may take a long time to be supported. For that reason, defining new extension types should not be taken lightly. Implementations that do not support the mechanism ignore it, as IGMPv3 and MLDv2 specify.
A document defining a new type MUST say when it is used, including which message types, and what to do if it shows up in the wrong kind of message. Authors should also consider partial support by hosts or routers on the same link, and any dependencies between the new type and existing ones. The mechanism does not apply when hosts or routers send older-version messages.
Old devices will just not read the new notes. So anyone inventing a new kind of note has to make sure things still work when some devices ignore it.
§7Security and IANA Considerations
The security considerations of IGMPv3 and MLDv2 also apply. Because TLVs are variable in number, implementations must not read past the packet boundary, since an attacker could specify a TLV whose length exceeds it. An implementation could also add a large number of minimal TLVs to a message to raise the cost of processing it, which would magnify a denial-of-service attack.
IANA created the "IGMP/MLD Extension Types" registry, common to IGMP and MLD, with the "IETF Review" procedure. Type 0 is the No-op, types 1 to 65533 are unassigned, and types 65534 and 65535 are for experimental use, which should be confined to closed environments.
A liar could write a note that claims to be huge, or pile on thousands of tiny notes to keep a receiver busy, so receivers must be careful when reading.
| Extension Type | Name |
|---|---|
| 0 THIS RFC | No-op |
| 1-65533 | Unassigned |
| 65534-65535 | Reserved for Experimental Use |