MAGMA Working Group M. Christensen
Internet Draft morten@jagd-christensen.com
June 2002 F. Solensky
Expiration Date: December 2002 Premonitia
IGMP and MLD snooping switches
<draft-ietf-magma-snoop-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [RFC2026].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other docu¡
ments at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This memo describes the requirements for IGMP and MLD snooping
switches. The requirements for IGMPv2 snooping switches are based
on best current practices. IGMPv3 and MLDv2 snooping are also cov¡
ered in this draft although we are not aware of any such implemen¡
tations at the time of writing. Areas which are of relevance to
IGMP and MLD snooping switches, such as link layer topology changes
and Ethernet specific encapsulation issues are also considered.
Interoperability issues that arise betweed different versions of
IGMP are not discussed in this document. Interested readers are
directed to [IGMPv3] for a thorough description of problem area.
This document is intended as an accompanying document to the IGMPv3
and MLDv2 specifications.
Christensen, Solensky [Page 1]
RFC DRAFT June 2002
11.. IInnttrroodduuccttiioonn
When a packet with a broadcast or multicast destination address is
received, the switch will forward a copy into each of the remaining
network segments in accordance with [BRIDGE]. Eventually, the
packet is made accessible to all nodes connected to the network.
This approach works well for broadcast packets that are intended to
be seen or processed by all connected nodes. In the case of multi¡
cast packets, however, this approach could lead to less efficient
use of network bandwidth, particularly when the packet is intended
for only a small number of nodes. Packets will be flooded into
network segments where no node has any interest in receiving the
packet. While nodes will rarely incur any processing overhead to
filter packets addressed to unrequested group addresses, they are
unable to transmit new packets onto the shared media for the period
of time that the multicast packet is flooded.
The problem of wasting bandwidth is even worse when the LAN segment
is not shared, for example in Full Duplex links. Full Duplex is
standard today for most switches operating at 1Gbps or above. In
this case the bandwidth that is wasted is proportional to the num¡
ber of attached nodes.
In recent years, a number of commercial vendors have introduced
products described as "IGMP snooping switches" to the market.
These devices do not adhere to the conceptual model that provides
the strict separation of functionality between different communica¡
tions layers in the ISO model and instead utilizes information in
the upper level protocol headers as factors to be considered in the
processing at the lower levels. This is analogous to the manner in
which a router can act as a firewall by looking into the transport
protocol's header before allowing a packet to be forwarded to its
destination address.
In the case of multicast traffic, an IGMP snooping switch provides
the benefit of conserving bandwidth on those segments of the net¡
work where no node has expressed interest in receiving packets
addressed to the group address. This is in contrast to normal
switch behaviour where multicast traffic is typically forwarded on
all interfaces.
Many switch datasheets state support for IGMP snooping, but no
requirements for this exist today. It is the authors hope that the
information presented in this draft will supply this information.
The requirements presented here is based on the following informa¡
tion sources: The IGMP specifications [RFC112][RFC2236][IGMPv3],
Christensen, Solensky [Page 2]
RFC DRAFT June 2002
vendor supplied technical documents [CISCO], bug reports [MSOFT],
discussions with people invovled in design of IGMP snooping
switches, MAGMA mailinglist discussions, and on replies by switch
vendors to an implementation questionnaire.
The discussions in this document are based on IGMP which applies to
IPv4 only. For IPv6 we must use MLD instead. Because MLD is based
on IGMP we do not repeat the whole discussion and requirements for
MLD snooping switches. Instead we point out the few cases where
there is a difference compared to IGMP.
22.. IIGGMMPP SSnnooooppiinngg RReeqquuiirreemmeennttss
The following sections list the requirements for an IGMP snooping
switch. The requirement is stated and is supplemented by a discus¡
sion. All implementation discussions are examples only and there
may well be other ways to achieve the same functionality.
22..11.. FFoorrwwaarrddiinngg rruulleess
The IGMP snooping functionality is here separated in a control sec¡
tion (IGMP forwarding) and data section (Data forwarding).
22..11..11.. IIGGMMPP FFoorrwwaarrddiinngg RRuulleess
1) A snooping switch SHOULD forward IGMP Membership Reports only
to those ports where multicast routers are attached. Alterna¡
tively stated: A snooping switch SHOULD NOT forward IGMP Mem¡
bership Reports to ports on which only hosts are attached. An
administrative control MAY be provided to override this
restriction, allowing the report messages to be flooded to
other ports.
This is the main IGMP snooping functionality. Sending member¡
ship reports (as described in IGMP versions 1 and 2) to other
hosts can result in unintentionally preventing a host from
joining a specific multicast group. This is not a problem in
an IGMPv3 only network because there is no cancellation of IGMP
Membership reports.
The administrative control allows IGMP Membership Report mes¡
sages to be processed by network monitoring equipment such as
packet analysers or port replicators.
Christensen, Solensky [Page 3]
RFC DRAFT June 2002
The switch supporting IGMP snooping MUST maintain a list of
multicast routers and the ports on which they are attached.
This list can be constructed in any combination of the follow¡
ing ways:
a) This list SHOULD be built using IGMP Multicast Router Dis¡
covery [MRDISC] by the snooping switch sending Multicast
Router Solicitation messages on its own. It MAY also snoop
Multicast Router Advertisement messages sent by and to
other nodes.
b) The arrival port for IGMP Queries (sent by multicast
routers) where the source where the address is not 0.0.0.0.
c) A list of ports configured by management as described in
the previous step.
2) IGMP snooping switches MAY implement "proxy-reporting" in which
reports received from downstream hosts are summarized and used
to build internal membership states as described in [PROXY].
An IGMP proxy-reporting switch would then report its own state
in response to upstream queriers. If the switch does not
alreay have an IP address it SHOULD use the address 0.0.0.0 as
source in these reports.
An IGMP proxy-reporting switch may act as Querier for the down¡
stream hosts while proxy reporting to the 'real' upstream
queriers.
It should be noted that there may be multiple IGMP proxy-
reporting switches in the network all using the 0.0.0.0 source
IP address. In this case the switches can be uniquely identi¡
fied through their link layer source MAC address.
IGMP membership reports MUST NOT be rejected because of a
source IP address of 0.0.0.0; however, these messages MUST NOT
be included the election process so that a snooping switch does
not elected over an active router.
3) The switch that supports IGMP snooping MUST flood all unrecog¡
nized IGMP messages to all other ports and MUST NOT attempt to
make use of any information beyond the end of the network layer
header. In particular, messages where any reserved fields in
the IGMP header are non-zero MUST NOT be subject to "normal"
snooping since this could indicate an incompatible change to
the IGMP message format.
Christensen, Solensky [Page 4]
RFC DRAFT June 2002
4) An IGMP snooping switch SHOULD be aware of link layer topology
changes. Following a topology change the switch SHOULD initi¡
ate the transmission of a General Query on all ports in order
to reduce network convergence time.
5) An IGMP snooping switch MUST NOT make use of information in
IGMP packets where the IP or IGMP headers have checksum or
intregity errors. The switch SHOULD NOT flood such packets but
if it does, it SHOULD take some note of the event (i.e.: incre¡
ment a counter). These errors and their processing are further
discussed in [IGMPv3], [MLD] and [MLDv2].
22..11..22.. DDaattaa FFoorrwwaarrddiinngg RRuulleess
1) Packets with a destination IP (DIP) address in the 224.0.0.X
range which are not IGMP MUST be forwarded on all ports.
This requirement is based on fact that many hosts exist today,
which does not Join IP multicast addresses in this range before
sending or listening to IP multicast. Furthermore since the
224.0.0.X address range is defined as link local (not to be
routed) it seems unnecessary to keep state for each address in
this range.
2) Packets with a destination IP address outside 224.0.0.X which
are not IGMP SHOULD be forwarded according to group based port
membership tables and MUST also be forwarded on router ports.
This is the core IGMP snooping requirement for the data path.
Discussion: An implementation could maintain separate member¡
ship and multicast router tables in software and then "merge"
these tables into a current forwarding cache.
3) If a switch receives a non-IGMP multicast packet without having
first processed Membership Reports for the group address, it
MAY forward the packet on all ports, but it MUST forward the
packet on router ports. A switch MAY forward an unregistered
packet only on router ports, but the switch MUST have a config¡
uration option that suppresses this restrictive operation and
forces flooding of unregistered packets on all ports.
4) IGMP snooping switches MAY maintain forwarding tables based on
either MAC addresses or IP addresses. If a switch supports
both types of forwarding tables then the default behavior
SHOULD be to use IP addresses.
Christensen, Solensky [Page 5]
RFC DRAFT June 2002
Discussion: Forwarding based on MAC addresses is subject to the
problem associated with the 32-fold IP address to 1 MAC address
mapping.
5) Switches which rely on information in the IP header SHOULD ver¡
ify that the IP header checksum is correct. If the checksum
fails, the information in the packet MUST NOT be incorporated
into the forwarding table. Further, the packet SHOULD be dis¡
carded.
22..22.. IIGGMMPP ssnnooooppiinngg rreellaatteedd pprroobblleemmss
A special problem arise in the network consisting of IGMPv3 routers
as well as IGMPv2 and IGMPv3 hosts interconnected by a IGMPv2
snooping switch. IGMPv3 has a mechanism to fall back to IGMPv2
when receiving IGMPv2 membership reports. This means that the net¡
work will converged on IGMPv2 eventually. However, the convergence
time will lead to supression of v3 Hosts for several minutes.
Therefore it is recommended that in such a network, the multicast
router is configured to use IGMPv2.
33.. IIPPvv66 CCoonnssiiddeerraattiioonnss
In order to avoid confusion, the previous discussions have been
based on IGMPv3 functionality which only applies to IPv4 multicast.
In the case of IPv6 most of the above discussions are still valid
with a few exceptions which we will describe here.
In IPv6 the protocol for multicast group maintenance is called Mul¡
ticast Listener Discovery (MLDv2). IPv6 is not widely deployed
today and neither is IPv6 multicast. However, it is anticipated
that at some time IPv6 switches capable of MLD snooping will
appear.
The three main differences between IGMPv3 and MLDv2 are:
- MLDv2 uses ICMPv6 message types instead of IGMP message types.
- The ethernet encapsulation is a mapping of 32 bits of the 128
bit DIP addresses into 48 bit DMAC addresses [IPENCAPS].
- Multicast router discovery is done using Neighbor Discovery Pro¡
tocol (NDP) for IPv6. NDP uses ICMPv6 message types.
Christensen, Solensky [Page 6]
RFC DRAFT June 2002
The IPv6 packet header does not include a checksum field. Never¡
theless, the switch SHOULD detect other packet integrity issues.
When the snooping switch detects such an error, it MUST NOT include
information from the corresponding packet in the IGMP forwarding
table. The forwarding code SHOULD drop the packet and take further
reasonable actions as advocated above.
The fact that MLDv2 is using ICMPv6 adds new requirements to a
snooping switch because ICMPv6 has multiple uses aside from MLD.
This means that it is no longer sufficient to detect that the next-
header field of the IP header is ICMPv6 in order to redirect pack¡
ets to the CPU. If this was the case the CPU queue assigned for
MLD would potentially be filled with non-MLD related packets. Fur¡
thermore ICMPv6 packets destined for other hosts would not reach
their destination. A solution is either to require that the snoop¡
ing switch looks further into the packets or to be able to detect a
multicast DMAC address in conjunction with ICMPv6. The first solu¡
tion is desirable only if it is configurable which message types
should trigger a CPU redirect and which should not. The reason is
that a hardcoding of message types is unflexible for the introduc¡
tion of new message types. The second solution introduces the risk
of new protocols, which are not related to MLD but uses ICMPv6 and
multicast DMAC addresses wrongly being identified as MLD. It is
suggested that solution one is the preferred if the switch is capa¡
ble of triggering CPU redirects on individual ICMPv6 message types.
If this is not the case then use solution two.
The mapping from IP multicast addresses to multicast DMAC addresses
introduces a potentially enormous overlap. The structure of an IPv6
multicast address is shown in figure 5. Theoretically 2**80, two
to the power of 80 (128 - 8 - 4 - 4 - 32) unique DIP addresses
could map to one DMAC address. This should be compared to 2**5 in
the case of IPv4.
Initial allocation of IPv6 multicast addresses, however, uses only
the lower 32 bits of group ID. This eliminates the address ambigu¡
ity for the time being but it should be noted that the allocation
policy may change in the future.
| 8 | 4 | 4 | 112 bits |
+--------+----+----+---------------------------------------+
|11111111|flgs|scop| group ID |
+--------+----+----+---------------------------------------+
Figure 5
In the case of IPv6 forwarding can be made on the basis of DMAC
Christensen, Solensky [Page 7]
RFC DRAFT June 2002
addresses in the forseable future.
Finally, we mention the reserved address range FF02::/96. This
range is similar to 224.0.0.X for IPv4 and is reserved to routing
protocols and resource discovery [RFC2375]. In the case of IPv6 it
is suggested that packets in this range are forwarded on all ports
if they are not MLD packets.
44.. SSeeccuurriittyy CCoonnssiiddeerraattiioonnss
Security considerations for IGMPv3 are accounted for in [IGMPv3].
The introduction of IGMP snooping switches adds the following con¡
siderations with regard to IP multicast.
The exclude source failure which could cause traffic from sources
that are 'black listed' to reach hosts that have requested other¡
wise. This can also occur in certain network topologies without
IGMP snooping.
It is possible to generate packets which make the switch wrongly
believe that there is a multicast router on the segment on which
the source is attached. This will potentially lead to excessive
flooding on that segment. The authentication methods discussed in
[IGMPv3] will also provide protection in this case.
IGMP snooping switches which rely on the IP header of a packet for
their operation and which do not validate the header checksum
potentially will forward packets on the wrong ports. Even though
the IP headers are protected by the ethernet checksum this is a
potential vulnerability.
Generally though, it is worth to stress that IP multicast must so
far be considered insecure until the work of for example the sug¡
gested Multicast Security (MSEC) working group or similar is com¡
pleted or at least has matured.
55.. IIGGMMPP QQuueessttiioonnnnaaiirree
As part of this work the following questions were asked both on the
MAGMA discussion list and sent to known switch vendors implementing
IGMP snooping. The individual contributions have been anonymized
upon request and do not necessarily apply to all of the vendors'
products.
Christensen, Solensky [Page 8]
RFC DRAFT June 2002
The questions were:
Q1 Does your switches perform IGMP Join aggregation? In other
words, are IGMP joins intercepted, absorbed by the hard¡
ware/software so that only one Join is forwarded to the
querier?
Q2 Is multicast forwarding based on MAC addresses? Would data¡
grams addressed to multicast IP addresses 224.1.2.3 and
239.129.2.3 be forwarded on the same ports-groups?
Q3 Is it possible to forward multicast datagrams based on IP
addresses (not routed). In other words, could 224.1.2.3 and
239.129.2.3 be forwarded on different port-groups with unal¡
tered TTL?
Q4 Are multicast datagrams within the range 224.0.0.1 to
224.0.0.255 forwarded on all ports whether or not IGMP Joins
have been sent?
Q5 Are multicast frames within the MAC address range
01:00:5E:00:00:01 to 01:00:5E:00:00:FF forwarded on all ports
whether or not IGMP joins have been sent?
Q6 Does your switch support forwarding to ports on which IP multi¡
cast routers are attached in addition to the ports where IGMP
Joins have been received?
Q7 Is your IGMP snooping functionality fully implemented in hard¡
ware?
Q8 Is your IGMP snooping functionality partly software imple¡
mented?
Q9 Can topology changes (for example spanning tree configuration
changes) be detected by the IGMP snooping functionality so that
for example new queries can be sent or tables can be updated to
ensure robustness?
Christensen, Solensky [Page 9]
RFC DRAFT June 2002
The answers were:
---------------------------+-----------------------+
| Switch Vendor |
---------------------------+---+---+---+---+---+---+
| 1 | 2 | 3 | 4 | 5 | 6 |
---------------------------+---+---+---+---+---+---+
Q1 Join aggregation | x | x | x | | x | x |
Q2 Layer-2 forwarding | x | x | x | x |(1)| |
Q3 Layer-3 forwarding |(1)| |(1)| |(1)| x |
Q4 224.0.0.X aware |(1)| x |(1)|(2)| x | x |
Q5 01:00:5e:00:00:XX aware | x | x | x |(2)| x | x |
Q6 Mcast router list | x | x | x | x | x | x |
Q7 Hardware implemented | | | | | | |
Q8 Software assisted | x | x | x | x | x | x |
Q9 Topology change aware | x | x | x | x | |(2)|
---------------------------+---+---+---+---+---+---+
x Means that the answer was Yes.
(1) In some products (typically high-end) Yes, in others No.
(2) Currently no, but will be real soon.
66.. RReeffeerreenncceess
[BRIDGE] IEEE 802.1D, "Media Access Control (MAC) Bridges"
[CISCO] Cisco Tech Notes, "Multicast In a Campus Network: CGMP
and IGMP snooping", http://www.cisco.com/warp/pub¡
lic/473/22.html
[IANA] Internet Assigned Numbers Authority, "Internet Multicast
Addresses", http://www.isi.edu/in-notes/iana/assign¡
ments/multicast-addresses
[IGMPv3] Cain, B., "Internet Group Management Protocol, Version
3", draft-ietf-idmr-igmp-v3-11.txt, May 2002.
[IPENCAPS] Crawford, M., "Transmission of IPv6 Packets over Ether¡
net Networks", RFC2464, December 1998.
[MLD] Deering, S., Fenner, B., and Haberman, B. "Multicast
Listener Discovery (MLD) for IPv6", RFC2710, October
1999.
[MLDv2] Vida, R., "Multicast Listener Discovery Version 2
(MLDv2) for IPv6", draft-vida-mld-v2-03.txt, June 2002.
Christensen, Solensky [Page 10]
RFC DRAFT June 2002
[MRDISC] Biswas, S. "IGMP Multicast Router Discovery", draft-
ietf-idmr-igmp-mrdisc-08.txt, January 2002.
[MSOFT] Microsoft support article Q223136, "Some LAN Switches
with IGMP Snooping Stop Forwarding Multicast Packets on
RRAS Startup", http://support.microsoft.com/sup¡
port/kb/articles/Q223/1/36.ASP
[PROXY] Fenner, B. et al, "IGMP-based Multicast Forwarding (IGMP
Proxying)", draft-ietf-magma-proxy-02(?).txt.
[RFC1112] Deering, S., "Host Extensions for IP Multicasting", RFC
1112, August 1989.
[RFC2026] Bradner, S. "The Internet Standards Process -- Revision
3", RFC2026, October 1996.
[RFC2236] Fenner, W., "Internet Group Management Protocol, Version
2", RFC2236, November 1997.
[RFC2375] Hinden, R. "IPv6 Multicast Address Assignments",
RFC2375, July 1998.
77.. AAcckknnoowwlleeddggeemmeennttss
We would like to thank Martin Bak, Les Bell, Yiqun Cai, Paul Cong¡
don, Toerless Eckert, Bill Fenner, Brian Haberman, Edward Hilquist,
Hugh Holbrook, Kevin Humphries, Karen Kimball and Jaff Thomas for
comments and suggestions on this document.
Furthermore, the following companies are acknowledged for their
contributions: 3Com, Alcatel, Cisco Systems, Enterasys Networks,
Hewlett-Packard, Vitesse Semiconductor Corporation. The ordering
of these names do not necessarily correspond to the column numbers
in the response table.
88.. RReevviissiioonn HHiissttoorryy
This section, while incomplete, is provided as a convenience to the
working group members. It will be removed when the document is
released in its final form.
draft-ietf-magma-snoop-01.txt: January 2002
Extensive restructuring of the original text.
Christensen, Solensky [Page 11]
RFC DRAFT June 2002
draft-ietf-magma-snoop-02.txt: June 2002
Status section removes document history; moved into this sec¡
tion instead.
Introduction restores text from the -00 revision that
describes snooping and its goals
IGMP flooding rules eased, allowing management option to
broaden beyond "routers only".
Removed a SHOULD/MAY inconsistancy between IPv4 Forwarding and
IPv6 processing of checksums.
IGMP Forwarding Rules: clarify text describing processing of
non-zero reserved fields.
Data Forwarding Rules, item 3 is changed from "MUST forward to
all ports" to "MAY"; item 4 default changes from "MUST" to
"SHOULD use network addresses".
Added two sets of additional responses to the questionnaire
and text indicating that responses don't cover all products.
Removed (commented out) description of IPR issues: IESG is
aware of them.
The next revision:
In the interest of getting this version of the draft released
before the deadline (less than seven hours from the moment
this paragraph is being typed), we briefly summarize some of
the comments on the previous version that need to be included
in the next one. We believe that other comments have been
addressed in this draft; please let the authors know if this
they have either not been included or need to be corrected.
IGMP Forwarding rules:
Add a reference to and become consistant with the next
revision of the IGMP proxy draft,
In item 'b': include a description on how the switch
determines that a Query came from the router and not
another switch. Is there some way to make this distinc¡
tion beyond the source address?
Proxy reporting: further analysis of the impact on the
Christensen, Solensky [Page 12]
RFC DRAFT June 2002
election process when using 0.0.0.0 as the source address
in membership report messages. Also consider the case
where the switch assumes the role of Querier when no
routers are detected and forfeits the role as soon as one
is announced.
Include some discussion about how entries are to be aged
from the list, perhaps similar to spanning tree algorithm
for unicast MAC address processing.
Data Forwarding rules:
Link-local range to mention the problem is due to routing
protocols not sending Report Messages for their respec¡
tive multicast addresses.
Expand discussion of non-IGMP packet forwarding for data
that matches an IGMPv3 record. Do snooping switches add
intelligence to recognize SSM versus ASM groups?
IPv6 Considerations:
Is having MLD a subset of ICMPv6 an issue? Should MLDv2
be a separate protocol?
Add reference to ICMPv6 specification for message pro¡
cessing rules.
99.. AAuutthhoorr''ss AAddddrreesssseess
Morten Jagd Christensen
email: morten@jagd-christensen.com
Frank Solensky
Premonitia, Inc.
1 Nanog Park
Acton, MA 01720
email: fsolensky@premonitia.com
Christensen, Solensky [Page 13]
RFC DRAFT June 2002
TTaabbllee ooff CCoonntteennttss
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. IGMP Snooping Requirements . . . . . . . . . . . . . . . . . 3
2.1. Forwarding rules . . . . . . . . . . . . . . . . . . . . . 3
2.1.1. IGMP Forwarding Rules . . . . . . . . . . . . . . . . . 3
2.1.2. Data Forwarding Rules . . . . . . . . . . . . . . . . . 5
2.2. IGMP snooping related problems . . . . . . . . . . . . . . 6
3. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . 8
5. IGMP Questionnaire . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
8. Revision History . . . . . . . . . . . . . . . . . . . . . . 11
9. Author's Addresses . . . . . . . . . . . . . . . . . . . . . 13
Christensen, Solensky [Page i]
