Internet DRAFT - draft-ietf-pim-explicit-tracking
draft-ietf-pim-explicit-tracking
PIM Working Group H. Asaeda
Internet-Draft NICT
Intended status: Experimental November 1, 2015
Expires: May 4, 2016
IGMP/MLD-Based Explicit Membership Tracking Function for Multicast
Routers
draft-ietf-pim-explicit-tracking-13
Abstract
This document describes the IGMP/MLD-based explicit membership
tracking function for multicast routers and IGMP/MLD proxy devices
supporting IGMPv3/MLDv2. The explicit membership tracking function
contributes to saving network resources and shortening leave latency.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 4, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Motivation and Experimentation . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Membership State Information . . . . . . . . . . . . . . . . 4
4. Specific Query Suppression . . . . . . . . . . . . . . . . . 5
5. Shortening Leave Latency . . . . . . . . . . . . . . . . . . 6
6. Risk of Wrong Membership State . . . . . . . . . . . . . . . 7
7. All-Zero and Unspecified Source Addresses . . . . . . . . . . 8
8. Compatibility with Older Version Protocols . . . . . . . . . 9
9. Interoperability . . . . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 9
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
13.1. Normative References . . . . . . . . . . . . . . . . . . 10
13.2. Informative References . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The Internet Group Management Protocol (IGMP) version 3 [2] for IPv4
and the Multicast Listener Discovery Protocol (MLD) version 2 [3] for
IPv6 are the standard protocols used by member hosts and multicast
routers. Lightweight IGMPv3 and Lightweight MLDv2 (or LW-IGMPv3 and
LW-MLDv2) [4] are subsets of the standard IGMPv3 and MLDv2.
When a host starts/finishes listening to particular multicast
channels, it sends IGMP/MLD State-Change Report messages specifying
the corresponding channel information as the join/leave request to
its upstream router (i.e., an adjacent multicast router or IGMP/MLD
proxy device [8]). The "unsolicited" report messages are sent only
when the host joins/leaves the channels. Since IGMP/MLD are non-
reliable protocols, unsolicited report messages may be lost or may
not reach upstream routers. To alleviate this problem, unsolicited
report messages are retransmitted a number of times according to the
value of the [Robustness Variable] defined in [2][3].
In addition, a querier router periodically sends IGMP/MLD General
Query messages every General Query timer interval (i.e. [Query
Interval] value defined in [2][3]). Upon receiving the query
messages, the member hosts reply with "solicited" report messages.
Routers then keep their membership state information up to date.
However, this approach still does not guarantee that the membership
state is always perfectly synchronized. To minimize the possibility
of having outdated membership information, routers may shorten the
periodic General Query timer interval. Unfortunately, this increases
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the number of transmitted solicited report messages and induces
network congestion. And the greater the amount of network
congestion, the greater the potential for IGMP/MLD report messages
being lost and the membership state information being outdated in the
router.
IGMPv3 [2], MLDv2 [3], and these lightweight protocols [4] can
provide the ability to keep track of the downstream (adjacent)
multicast membership state in multicast routers, yet the
specifications are not clearly given. This document describes the
"IGMP/MLD-based explicit member tracking function" for multicast
routers and a way for routers to implement the function. By enabling
this explicit tracking function, routers can keep track of the
downstream multicast membership state. The explicit tracking
function contributes to saving network resources and shortening leave
latency.
The explicit tracking function does not change message formats used
by IGMPv3 [2], MLDv2 [3], and their lightweight version protocols
[4]; nor does it change a multicast data sender's and receiver's
behavior.
1.1. Motivation and Experimentation
This document specifies an experimental extension to IGMPv3/MLDv2.
It makes some fundamental changes to how IGMPv3/MLDv2 works in that
membership state does not require periodic updates, and it partly
turns IGMPv3/MLDv2 into a hard-state protocol. Also, a mechanism
called "specific query suppression" with a robust link state is a new
concept. It does not make the router send any specific query
message(s) and immediately leave the group or sources when the sole
member has left according to its membership state information.
The explicit tracking function does not change the reliability of the
message transmission. The list of tracked member hosts may be
outdated in the router because of host departure from the network
without sending State-Change Report messages or loss of such messages
due to network congestion. This document describes the risk of
having wrong membership state and guides for setting up appropriate
values or mechanisms used with the explicit tracking function in
routers.
The explicit tracking function potentially requires a large amount of
memory so that routers keep all membership states. Particularly when
a router needs to maintain a large number of member hosts, this
resource requirement might be sensitive. As the security
consideration, this document describes that operators may decide to
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disable this function when their routers have insufficient memory
resources, despite the benefits.
It is likely that experiences from early implementations and
deployments will lead to at least minor changes in the protocol. Any
experiments using this protocol extension are encouraged. Reports
from such experiments planned with pre-specified objectives and
scenarios are particularly encouraged. Results from such
experiments, documenting the following, are of particular importance:
o Operation in networks that contain both routers implementing this
extension, and routers implementing only [2][3][4], in particular
are there any unexpected interactions that can break the network?
o Operation in networks that contain both routers implementing this
extension, and routers implementing older versions of IGMP or MLD,
IGMPv1 [5], IGMPv2 [6], or MLDv1 [7].
o Operation in networks with dynamic topology changes.
o Operation in networks with invalid or outdated membership states.
o Network performance, including leave latency and packet delivery
results.
o Resource requirements observed from running the protocol,
including processing, storage, and bandwidth.
o Any other implementation issues.
Once there is some deployment experience, making this a Standards
Track protocol should be considered.
2. Terminology
The 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 RFC 2119 [1].
3. Membership State Information
A router enabling the explicit tracking function maintains the
"membership state information". When a multicast router receives a
Current-State or State-Change Report message, it creates or modifies
this membership state information to maintain the membership state up
to date.
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The membership state information consists of the following
information:
(S, G, number of receivers, (receiver records))
where "S" denotes source address, "G" denotes group or multicast
address, and each receiver record is of the form:
(IGMP/MLD membership/listener report sender's address)
In the state information, each S and G indicates a single IPv4/IPv6
address. S is set to "All sources" for Any-Source Multicast (ASM)
communication (i.e., (*,G) join reception). In order to simplify the
implementation, Lightweight-IGMPv3/MLDv2 [4] do not keep the state of
(S,G) joined with EXCLUDE filter mode; if a router receives an (S,G)
join/leave request with EXCLUDE filter mode from the downstream
hosts, the router translates the request to a (*,G) join state/leave
request and records the state and the receivers' addresses in the
maintained membership state information.
The membership state information must be identified properly even
though a receiver (i.e., IGMP/MLD Report sender) sends the identical
report messages multiple times. And the maintained membership state
information will be flushed when the router reboots or restarts the
multicast routing processes.
4. Specific Query Suppression
In accordance with [2] and [3], when a router receives the State-
Change Report and needs to confirm whether any hosts are still
interested in a channel or not, the router sends the corresponding
Group-Specific or Group-and-Source Specific Query messages as defined
in Section 6.4.2 of [2] and Section 7.4.2 of [3]. The queries sent
by actions defined in these sections need to be transmitted [Last
Member Query Count] (LMQC) or [Last Listener Query Count] (LLQC)
times, once every [Last Member Query Interval] (LMQI) or [Last
Listener Query Interval] (LLQI), in order to confirm the sole member.
(The default values for LMQI/LLQI defined in [2][3] are 1 second.
The default values for LMQC/LLQC are the [Robustness Variable] value
whose default value is 2.) All member hosts joining the identical
channel then reply with their own states after acquiring these query
messages. However, transmitting a large number of IGMP/MLD Report
messages consumes network resources, and this may pose a particular
problem especially when many hosts joining the identical channel send
these reports simultaneously.
The explicit tracking function provides a mechanism called "specific
query suppression". With the specific query suppression, regardless
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of the LMQC/LLQC values, if the router receives one or more replies
from the downstream member(s), it SHOULD stop (i.e., cancel)
retransmitting the specific query message(s) for the specified source
and/or group. It reduces the number of Group-Specific or Group-and-
Source Specific Query messages transmitted from a router, and in turn
reduces the number of Current-State Report messages transmitted from
member hosts. This contributes to saving network resources.
The specific query suppression MAY define an option called "robust
link state". If an operator is confident that the link is stable and
robust enough and thus the tracked membership state information is
perfectly synchronized with the current (actual) member hosts, s/he
can enable the specific query suppression with a robust link state.
A router enabling the specific query suppression with a robust link
state does not send any specific query message(s) and immediately
leave the group or sources when the sole member has left according to
its membership state information. The specific query suppression
with a robust link state hence does not rely on LMQC/LLQC and LMQI/
LLQI values. This contributes to shortening leave latency described
in Section 5. However, this behavior requires that the router
perfectly tracks all member hosts. (See a risk of wrong membership
expectation described in Section 6.)
Note that the default behavior of the router that supports the
explicit tracking function SHOULD disable this specific query
suppression, in order to avoid the risk caused by the wrong
membership expectation or by the case in which multiple multicast
routers exist on a LAN and the querier router is not the forwarder
router. The former case is described in Section 6. For the latter
case, when the querier suppresses the specific query message
transmission, and expects that the State-Change Report sender is not
the sole member of the channel, it does not send the specific query.
Then the routers (including the forwarder) on the same LAN do not
receive a Current-State Report message from the corresponding member
hosts. The forwarder in this case may prune the routing path,
although there are other member hosts subscribing to the channel on
the LAN.
5. Shortening Leave Latency
A router enabling the explicit tracking function can shorten leave
latencies by tuning the following values; [Last Member Query Count]
(LMQC), [Last Listener Query Count] (LLQC), [Last Member Query
Interval] (LMQI), [Last Listener Query Interval] (LLQI), and
[Robustness Variable] values.
The [Last Member Query Interval] (LMQI) and [Last Listener Query
Interval] (LLQI) values defined in the standard specifications [2][3]
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specify the maximum time allowed for a member host to send a
responding Report. The [Last Member Query Count] (LMQC) and [Last
Listener Query Count] (LLQC) are the number of Group-Specific Queries
or Group-and-Source Specific Queries sent before the router assumes
there are no local members. The [Last Member Query Time] (LMQT) and
[Last Listener Query Time] (LLQT) values are the total time the
router should wait for a report after the Querier has sent the first
query.
The default values for LMQI/LLQI defined in [2][3] are 1 second, yet,
for a router enabling the explicit tracking function, the LMQI/LLQI
may be set to 1 second or shorter. As well, the default values for
LMQC/LLQC are the [Robustness Variable] value whose default value is
2, yet the LMQC/LLQC may be set to 1 for the router. Smaller LMQC/
LLQC values give shorter LMQT/LLQT, which shorten the leave
latencies.
Furthermore, if operators are confident that their link is fairly
robust (e.g., the [Robustness Variable] value is appropriately
configured so that the chances of unsolicited messages being lost are
sufficiently low), and if the querier router always acts as the
forwarder router for all multicast channels in the LAN, they will set
smaller LMQC/LLQC and shorter LMQI/LLQI (and hence shorter LMQT/LLQT)
with the specific query suppression, or enable the specific query
suppression with a robust link state (Section 4) for their routers.
Note that setting smaller LMQC/LLQC and shorter LMQI/LLQI values or
adopting the specific query suppression with a robust link state
poses the risk of wrong membership state described in Section 6.
Operators setting these values or enabling that mechanism must
recognize this tradeoff.
6. Risk of Wrong Membership State
There are possibilities that a router's membership expectation is
inconsistent due to an outdated membership state. For example, (1) a
router expects that more than one corresponding member host exists on
its LAN, but in fact no member host exists for that multicast
channel, or (2) a router expects that no corresponding member host
exists on its LAN, but in fact one or more than one member host
exists for that multicast channel.
The first case may occur in an environment where the sole member host
departs the network without sending a State-Change Report message.
The router later detects that there is no member host for the
corresponding channels when it does not receive a Current-State
Report within the timeout of the response for the periodic General
Query (and then the group or source timers are expired). However,
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this situation prolongs leave latency and wastes network resources
since the router forwards unneeded traffic for a while.
The second case occurs when a router sends a specific query but does
not receive a Current-State Report from a downstream host within an
LMQT or LLQT period. It recognizes that no member host exists on the
LAN and might prune the routing path. The router reestablishes the
routing path when it receives the solicited report message for the
channels. However, the downstream hosts may loose the data packets
until the routing path is reestablished and the data forwarding is
restarted.
If operators do not believe that their link is fairly robust or that
they can configure the [Robustness Variable] value appropriately,
they may configure the LMQC/LLQC value to 2 (the default value of the
[Robustness Variable] value) or bigger value for their routers. In
this case, the routers would enable the explicit tracking function
but may want to disable the specific query suppression specified in
Section 4. Such configurations will not contribute to saving network
resources, but reduce the risk of the incorrect membership
expectation.
7. All-Zero and Unspecified Source Addresses
The IGMPv3 specification [2] mentions that an IGMPv3 report is
usually sent with a valid IP source address, yet it permits a host to
use the 0.0.0.0 source address (since the host has not yet acquired
an IP address), and routers must accept a report with this source
address.
When a router enabling the explicit tracking function receives IGMP
report messages with an all-zero source address, it deals with the
IGMP report messages correctly as defined in [2] and continuously
keeps track of the membership state. However, the router SHOULD NOT
maintain the host specifying all-zero source address in its
membership state information. The router will maintain its
membership state information by checking Current-State reports as
ordinary routers do.
On the other hand, the MLDv2 specification [3] mentions that routers
silently discard a message that is sent with an invalid link-local
address or sent with the unspecified address (::), without taking any
action, because of security considerations. According to this
specification, whether the explicit tracking function is used or not,
a router does not deal with a member hosts sending an MLD report
message with the unspecified source address.
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8. Compatibility with Older Version Protocols
The explicit tracking function does not work with older versions of
IGMP or MLD, IGMPv1 [5], IGMPv2 [6], or MLDv1 [7], because a member
host using these protocols enables "membership report suppression" by
which the host will cancel sending pending membership reports if a
similar report is observed from another member on the network.
To preserve compatibility with older versions of IGMP/MLD, routers
supporting IGMPv3/MLDv2 enable the host compatibility mode defined in
[2][3]. The host compatibility mode of an interface changes the
operational protocol version on the LAN whenever an older version
query (than the current compatibility mode) is heard or when certain
timer conditions occur. The routers can hence support downstream
hosts that are not upgraded to the latest versions and run membership
report suppression.
Therefore, if a multicast router supporting IGMPv3/MLDv2 and enabling
the explicit tracking function changes its compatibility mode to the
older versions, the router SHOULD disable the explicit tracking
function while it acts as the older version router.
9. Interoperability
There might be various ways to implement the explicit tracking
function. Some existing implementations may not implement the
mechanisms such as specific query suppression described in this
document. Yet, the explicit tracking function does not change on-
wire behavior, and the function or mechanisms described in this
document do not break the interoperability between the existing
implementations and the implementation based on this specification.
On the other hand, for the future implementation for the explicit
tracking function, since this document specifies the minimum but
effective sets of the explicit tracking function, it is RECOMMENDED
to refer and follow this specification as the standard implementation
for that function.
10. IANA Considerations
This document has no actions for IANA.
11. Security Considerations
The explicit tracking function potentially requires a large amount of
memory so that routers keep all membership states. It gives some
impact in the cases where (1) a router attaches to a link or an IGMP/
MLD proxy device [8] that has a large number of member hosts, and a
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router has insufficient memory resources to maintain a large number
of member hosts, or (2) a malicious host sends a large number of
invalid IGMP/MLD State-Change Report messages without any intent to
join the specified channels.
For the first case, operators may disable the explicit tracking
function, despite the benefits mentioned above. For the second case,
some serious threats may be induced. For instance;
1. Transmitting a large number of invalid IGMP/MLD report messages
consumes network resources.
2. Keeping a large number of invalid membership states on a router
consumes the router's memory resources.
3. Dealing with a large number of invalid membership states on a
router consumes the router's CPU resources.
In order to mitigate such threats, a router enabling the explicit
tracking function may limit a total amount of membership information
the router can store, or may rate-limit State-Change Report messages
per host. When the router enables rate-limiting per host, the router
MAY ignore the received State-Change Report messages to minimize the
processing overhead or prevent DoS attacks. The rate limit is left
to the router's implementation.
12. Acknowledgements
Luis M. Contreras, Toerless Eckert, Adrian Farrel, Sergio
Figueiredo, Bharat Joshi, Nicolai Leymann, Magnus Nystrom, Alvaro
Retana, Stig Venaas, and others provided many constructive and
insightful comments.
13. References
13.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to indicate
requirement levels", RFC 2119, March 1997.
[2] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[3] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
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[4] 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,
February 2010.
13.2. Informative References
[5] Deering, S., "Host Extensions for IP Multicasting",
RFC 1112, August 1989.
[6] Fenner, W., "Internet Group Management Protocol, Version
2", RFC 2236, November 1997.
[7] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October
1999.
[8] Fenner, B., He, H., Haberman, B., and H. Sandick,
"Internet Group Management Protocol (IGMP) / Multicast
Listener Discovery (MLD)-Based Multicast Forwarding
("IGMP/MLD Proxying")", RFC 4605, August 2006.
Author's Address
Hitoshi Asaeda
National Institute of Information and Communications Technology
4-2-1 Nukui-Kitamachi
Koganei, Tokyo 184-8795
Japan
Email: asaeda@nict.go.jp
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