MAGMA Working Group B. Haberman
Internet Draft Caspian Networks
draft-ietf-magma-mrdisc-01.txt J. Martin
July 2004 Netzwert AG
Expires January 2005
Multicast Router Discovery
Status of this Memo
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Abstract
The concept of Internet Group Membership Protocol (IGMP) and
Multicast Listener Discovery (MLD) snooping requires the ability to
identify the location of multicast routers. Since snooping is not
standardized, there are many mechanisms in use to identify the
multicast routers. However, this can lead to interoperability
issues between multicast routers and snooping switches from
different vendors.
This document introduces a general mechanism that allows for the
discovery of multicast routers. This new mechanism, Multicast
Router Discovery (MRD), introduces a standardized means of
identifying multicast routers without a dependency on particular
multicast routing protocols.
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Internet Draft Multicast Router Discovery February 2004
1. Introduction
Multicast Router Discovery messages are useful for determining which
nodes attached to a switch have multicast routing enabled. This
capability is useful in a layer-2 bridging domain with snooping
switches. By listening to MRD messages, layer-2 switches can
determine where to send multicast source data and group membership
messages [RFC1112][RFC2236]. Multicast source data and group
membership Reports must be received by all multicast routers on a
segment. Using the group membership protocol Query messages to
discover multicast routers is insufficient due to query suppression.
Although MRD messages could be sent as ICMP messages, the group
management protocols were chosen since this functionality is
multicast specific. The addition of this functionality to the group
membership protocol also allows operators to have congruency between
multicast router discovery problems and data forwarding issues.
The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. Due to the lack of italics, emphasis is indicated herein
by bracketing a word or phrase in "*" characters. Furthermore,
square brackets are used to denote the value of the enclosed
variable, as opposed to the variable itself, written without
brackets.
2. Protocol Overview
Multicast Router Discovery consists of three messages for
discovering multicast routers. The Multicast Router Advertisement
is sent by routers to advertise that IP multicast forwarding is
enabled. Devices may send Multicast Router Solicitation messages in
order to solicit Advertisement messages from multicast routers. The
Multicast Router Termination messages are sent when a router stops
IP multicast routing functions on an interface.
Multicast routers send Advertisements periodically on all interfaces
on which multicast forwarding is enabled. Advertisement messages
are also sent in response to Solicitations. In addition to
advertising the location of multicast routers, Advertisements also
convey useful information concerning group management protocol
variables. This information can be used for consistency checking on
the subnet.
A device sends Solicitation messages whenever it wishes to discover
multicast routers on a directly attached link.
A router sends Termination messages when it terminates multicast
routing functionality on an interface.
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Internet Draft Multicast Router Discovery February 2004
All MRD messages are sent with an IPv4 TTL or IPv6 Hop Limit of 1
and contain the Router Alert Option [RFC2113][RFC2711].
Advertisement and Termination messages are sent to the All-Snoopers
multicast address.
Solicitation messages are sent to the All-Routers multicast address.
Any data beyond the fixed message format MUST be ignored.
3. Multicast Router Advertisement
Multicast Router Advertisements are sent periodically on all router
interfaces on which multicast forwarding is enabled. They are also
sent in response to Multicast Router Solicitation messages.
Advertisements are sent
1. Upon the expiration of a periodic (modulo randomization) timer
2. As a part of a router's start up procedure
3. During the restart of a multicast forwarding interface
4. On receipt of a Solicitation message
All Advertisements are sent as IGMP (for IPv4) or MLD (for IPv6)
messages to the All-Snoopers multicast address. These messages
SHOULD be rate-limited.
3.1 Advertisement Configuration Variables
An MRD implementation MUST support the following variables being
configured by system management. Default values are specified to
make it unnecessary to configure any of these variables in many
cases.
3.1.1 MaxAdvertisementInterval
This variable is the maximum time (in seconds) allowed between the
transmissions of Advertisements on an interface. This value MUST be
no less than 4 seconds and no greater than 180 seconds.
Default: 20 seconds
3.1.2 MinAdvertisementInterval
This is the minimum time (in seconds) allowed between the
transmissions of Advertisements on an interface. This value MUST be
no less than 3 seconds and no greater than MaxAdvertisementInterval.
Default: 0.75 * MaxAdvertisementInterval
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Internet Draft Multicast Router Discovery February 2004
3.1.3 MaxInitialAdvertisementInterval
The first Advertisement transmitted on an interface is sent after
waiting a random interval (in seconds) less than this variable.
This prevents a flood of Advertisements when multiple routers start
up at the same time.
Default: 2 seconds
3.1.4 MaxInitialAdvertisements
This variable is the maximum number of Advertisements that will be
transmitted by the advertising interface when MRD starts up.
Default: 3
3.1.5 NeighborDeadInterval
This variable is the maximum time (in seconds) allowed to elapse
before a neighbor can be declared unreachable. In order for all
devices to have a consistent state, it is necessary for the
MaxAdvertisementInterval to be configured consistently in all
devices on the subnet.
Default: 3 * MaxAdvertisementInterval
3.2 Advertisement Packet Format
The Advertisement message has the following format:
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 | Ad. Interval | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Query Interval | Robustness Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2.1 Type Field
The Type field identifies the message as an Advertisement. It is
set to X1 (to be assigned by IANA) for IPv4 and X2 (to be assigned
by IANA) for IPv6.
3.2.2 Advertisement Interval Field
This field specifies the periodic time interval at which
Advertisement messages are transmitted in units of seconds. This
value is set to the configured MaxAdvertisementInterval variable.
3.2.3 Checksum Field
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Internet Draft Multicast Router Discovery February 2004
The checksum field is set as follows:
1. For IPv4 it is the 16-bit one's complement of the one's
complement sum of the IGMP message, starting with the Type
field. For computing the checksum, the checksum field is set
to 0.
2. For IPv6 it is ICMPv6 checksum as specified in [RFC2463].
3.2.4 Query Interval Field
The Query Interval field is set to the Query Interval value in use
by IGMP or MLD on the interface. If IGMP or MLD is not enabled on
the advertising interface, this field MUST be set to 0. Note that
this is the Querier's Query Interval (QQI), not the Querier's Query
Interval Code (QQIC) as specified in the IGMP/MLD specifications.
3.2.5 Robustness Variable Field
This field is set to the Robustness Variable in use by IGMPv2
[RFC2236], IGMPv3 [RFC3376], or MLD [RFC2710][MLDV2] on the
advertising interface. If IGMPv1 is in use or no group management
protocol is enabled on the interface, this field MUST be set to 0.
3.3 IP Header Fields
3.3.1 Source Address
The IP source address is set to an IP address configured on the
advertising interface. For IPv6, a link-local address MUST be used.
3.3.2 Destination Address
The IP destination address is set to the All-Snoopers multicast
address.
3.3.3 Time-to-Live / Hop Limit
The IPv4 TTL and IPv6 Hop Limit are set to 1.
3.3.4 IPv4 Protocol
The IPv4 Protocol field is set to IGMP (2).
3.4 Sending Multicast Router Advertisements
Advertisement messages are sent when the following events occur:
1. The expiration of the periodic advertisement interval timer.
Note that it this timer is not strictly periodic since it is
a random number between MaxAdvertisementInterval and
MinAdvertisementInterval.
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Internet Draft Multicast Router Discovery February 2004
2. After a random delay less than
MaxInitialAdvertisementInterval when an interface is first
enabled, is (re-)initialized, or MRD is enabled. A router
may send up to a maximum of MaxInitialAdvertisements
Advertisements, waiting for a random delay less than
MaxInitialAdvertisementInterval between each successive
message. Multiple Advertisements are sent for robustness in
the face of packet loss on the network.
This is to prevent an implosion of Advertisements. An example of
this occurring would be when many routers are powered on at the same
time. When a Solicitation is received, an Advertisement is sent in
response with a random delay less than MAX_RESPONSE_DELAY. If a
Solicitation is received while an Advertisement is pending, that
Solicitation MUST be ignored.
Changes in the Query Interval or Robustness Variable MUST NOT
trigger a new advertisement, however the new values MUST be used all
future Advertisement messages.
When an Advertisement is sent, the periodic advertisement interval
timer MUST be reset.
3.5 Receiving Multicast Router Advertisements
Upon receiving an Advertisement message, devices validate the
message with the following criteria:
1. The checksum is correct
2. The IP destination address is equal to the All-Snoopers
multicast address
3. For IPv6, the IP source address is a link-local address
An Advertisement not meeting the validity requirements MUST be
silently discarded and may be logged in a rate-limited manner.
If an Advertisement is not received for a particular neighbor within
a NeighborDeadInterval time interval, then the neighbor is
considered unreachable.
4. Multicast Router Solicitation
Multicast Router Solicitation messages are used to solicit
Advertisements from multicast routers on a segment. These messages
are used when a device wishes to discover multicast routers. Upon
receiving a solicitation on an interface with IP multicast
forwarding and MRD enabled, a router will respond with an
Advertisement.
Solicitations may be sent when:
1. An interface is (re-)initialized
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Internet Draft Multicast Router Discovery February 2004
2. MRD is enabled
Solicitations are sent to the All-Routers multicast address and
SHOULD be rate-limited.
4.1 Solicitation Packet Format
The Solicitation message has the following format:
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 | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.1 Type Field
The Type field identifies the message as a Solicitation. It is set
to Y1 (to be assigned by IANA) for IPv4 and Y2 (to be assigned by
IANA) for IPv6.
4.1.2 Reserved Field
The Reserved field is set to 0 on transmission and ignored on
reception.
4.1.3 Checksum Field
The checksum field is set as follows:
. For IPv4 it is the 16-bit one's complement of the one's
complement sum of the IGMP message, starting with the Type
field. For computing the checksum, the checksum field is set
to 0.
. For IPv6 it is ICMPv6 checksum as specified in [RFC2463].
4.2 IP Header Fields
4.2.1 Source Address
The IP source address is set to an IP address configured on the
soliciting interface. For IPv6, a link-local address MUST be used.
4.2.2 Destination Address
The IP destination address is set to the All-Routers multicast
address.
4.2.3 Time-to-Live / Hop Limit
The IPv4 TTL and IPv6 Hop Limit are set to 1.
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Internet Draft Multicast Router Discovery February 2004
4.2.4 IPv4 Protocol
The IPv4 Protocol field is set to IGMP (2).
4.3 Sending Multicast Router Solicitations
Solicitation messages are sent when the following events occur:
. After waiting for a random delay less than MAX
SOLICITATION_DELAY when an interface first becomes
operational, is (re-)initialized, or MRD is enabled. A
device may send up to a maximum of MAX_SOLICITATIONS,
waiting for a random delay less than MAX SOLICITATION_DELAY
between each solicitation.
. Optionally, for an implementation specific event.
Solicitations MUST be rate-limited; the implementation MUST send no
more than MAX_SOLICITATIONS in MAX SOLICITATION_DELAY seconds.
4.4 Receiving Multicast Router Solicitations
A Solicitation message MUST be validated before a response is sent.
A router MUST verify that:
. The checksum is correct
. The IP destination address is the All-Routers multicast
address
. For IPv6, the IP source address MUST be a link-local address
Solicitations not meeting the validity requirements SHOULD be
silently discarded and may be logged in a rate-limited manner.
5. Multicast Router Termination
The Multicast Router Termination message is used to expedite the
notification of a change in the status of a router's multicast
forwarding functions. Multicast routers send Terminations when
multicast forwarding is disabled on the advertising interface.
5.1 Termination Packet Format
The Termination message has the following format:
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 | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.1.1 Type Field
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Internet Draft Multicast Router Discovery February 2004
The Type field identifies the message as a Termination. It is set
to Z1 (to be assigned by IANA) for IPv4 and Z2 (to be assigned by
IANA) for IPv6.
5.1.2 Reserved Field
The Reserved field is set to 0 on transmission and ignored on
reception.
5.1.3 Checksum Field
The checksum field is set as follows:
. For IPv4 it is the 16-bit one's complement of the one's
complement sum of the IGMP message, starting with the Type
field. For computing the checksum, the checksum field is
set to 0.
. For IPv6 it is ICMPv6 checksum as specified in [RFC2463].
5.2 IP Header Fields
5.2.1 Source Address
The IP source address is set to an IP address configured on the
advertising interface. For IPv6, a link-local address MUST be used.
5.2.2 Destination Address
The IP destination address is set to the All-Snoopers multicast
address.
5.2.3 Time-to-Live / Hop Limit
The IPv4 TTL and IPv6 Hop Limit are set to 1.
5.2.4 IPv4 Protocol
The IPv4 Protocol field is set to IGMP (2).
5.3 Sending Multicast Router Terminations
Termination messages are sent by multicast routers when:
. Multicast forwarding is disabled on an interface
. An interface is administratively disabled
. The router is gracefully shutdown
. MRD is disabled
5.4 Receiving Multicast Router Terminations
Upon receiving a Termination message, devices validate the message.
The validation criteria is:
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Internet Draft Multicast Router Discovery February 2004
. Checksum MUST be correct
. IP destination address MUST equal the All-Snoopers multicast
address
. For IPv6, the IP source address MUST be a link-local address
Termination messages not meeting the validity requirements MUST be
silently discarded and may be logged in a rate-limited manner.
If the message passes these validation steps, a Solicitation is
sent. If an Advertisement is not received within
NeighborDeadInterval, the sending router is removed from the list of
active multicast routers.
6. Protocol Constants
The following list identifies constants used in the MRD protocol.
These constants are used in the calculation of parameters.
. MAX_RESPONSE_DELAY 2 seconds
. MAX_SOLICITATION_DELAY 1 second
. MAX_SOLICITATIONS 3 transmissions
7. Security Considerations
The Multicast Router Advertisement message may allow rogue machines
to masquerade as multicast routers. This could allow those machines
to eavesdrop on multicast data transmissions. Additionally, it could
constitute a denial of service attack to other hosts in the same
snooping domain or sharing the same device port in the presence of
high rate multicast flows.
This issue stems from the fact that there is currently no mechanism
for hosts to authenticate and authorize messages being sent from
local routers. This problem is shared by all IGMP and ICMPv6
messages, as well as other protocols such as IPv6 Neighbor
Discovery.
While solving this problem is beyond the scope of this document, it
is worth noting that work in the Secure Neighbor Discovery Working
Group may be applicable to Multicast Router Discovery. Should this
work prove successful, appropriate mechanisms will be incorporated
into a later extension to MRD.
8. IANA Considerations
This document introduces three new IGMP messages. Each of these
messages requires a new IGMP Type value. This document requests
IANA to assign three new IGMP Type values to the Multicast Router
Discovery Protocol (for IPv4 Advertisements, Solicitations, and
Terminations).
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Internet Draft Multicast Router Discovery February 2004
This document also introduces three new MLD messages. Each of these
messages requires a new ICMPv6 Type value. This document requests
IANA to assign three new ICMPv6 Type values from the Informational
range to the Multicast Router Discovery Protocol (for IPv6
Advertisements, Solicitations, and Terminations).
This document also requires the assignment of an All-Snoopers
multicast address for IPv4. This multicast address should be in the
224.0.0/24 range since it is used for link-local, control message.
A corresponding IPv6 multicast address is also requested. Following
the guidelines in [RFC3307], the IPv6 multicast address should be
link-local in scope and have a group-ID value equal to the lowest-
order 8 bits of the requested IPv4 multicast address.
9. Acknowledgements
ICMP Router Discovery [RFC1256] was used as a general model for
Multicast Router Discovery.
Morten Christensen, Pekka Savola, Hugh Holbrook, and Isidor Kouvelas
provided helpful feedback on various versions of this document.
10. References
10.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1112] Deering, S., "Host Extensions for IP Multicasting",
RFC1112, August 1989.
[RFC2236] Fenner, W., "Internet Group Management Protocol,
Version 2", RFC2236, November 1997.
[RFC2113] Katz, D., "IP Router Alert Option", RFC2113,
February 1997.
[RFC2711] Partridge, C. and Jackson, A., "IPv6 Router Alert
Option", RFC2711, October 1999.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., et al., "Internet
Group Management Protocol, Version 3", RFC3376, October
2002.
[RFC2710] Deering, S., Fenner, W., and Haberman, B., "Multicast
Listener Discovery (MLD) for IPv6", RFC2710, October
1999.
[RFC3810] Vida, R. and Costa, L., "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC3810, December 2003.
[RFC2463] Conta, A. and Deering, S., "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC2463, December 1998.
[RFC3307] Haberman, B., "Allocation Guidelines for IPv6 Multicast
Addresses", RFC3307, August 2002.
10.2 Informative References
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Internet Draft Multicast Router Discovery February 2004
[RFC1256] Deering, S., "ICMP Router Discovery Messages", September
1991.
[BCP78] Bradner, S., "IETF Rights in Contributions"
RFC3677, BCP78, February 2004.
[BCP79] Bradner, S., "Intellectual Property Rights in IETF
Technology", RFC3668, BCP79, February 2004.
11. Authors
Brad Cain and Shantam Biswas were initial authors on this document.
12. Editors' Addresses
Brian Haberman Jim Martin
Caspian Networks Netzwert AG
753 Bridgewater Drive An den Treptowers 1
Sykesville, MD 21784 D-12435 Berlin
USA Germany
brian@innovationslab.net jim@netzwert.ag
+1-443-280-0932 +49.30/5 900 800-180
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14. Disclaimer of Validity
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Internet Draft Multicast Router Discovery February 2004
This document and the information contained herein are provided on
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Copyright (C) The Internet Society (2004). This document is subject
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Funding for the RFC Editor function is currently provided by the
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Haberman, Martin 13
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