Internet DRAFT - draft-vyncke-6man-mcast-not-efficient
draft-vyncke-6man-mcast-not-efficient
Internet Engineering Task Force E. Vyncke, Ed.
Internet-Draft P. Thubert
Intended status: Informational E. Levy-Abegnoli
Expires: August 18, 2014 A. Yourtchenko
Cisco
February 14, 2014
Why Network-Layer Multicast is Not Always Efficient At Datalink Layer
draft-vyncke-6man-mcast-not-efficient-01
Abstract
Several IETF protocols (IPv6 Neighbor Discovery for example) rely on
IP multicast in the hope to be efficient with respect to available
bandwidth and to avoid generating interrupts in the network nodes.
On some datalink-layer network, for example IEEE 802.11 WiFi, this is
not the case because of some limitations in the services offered by
the datalink-layer network. This document lists and explains all the
potential issues when using network-layer multicast over some
datalink-layer networks.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 18, 2014.
Copyright Notice
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document authors. All rights reserved.
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Issue on Wired Ethernet Network . . . . . . . . . . . . . . . 3
3. Issues on IEEE 802.11 Wireless Network . . . . . . . . . . . 4
3.1. Multicast over Wireless . . . . . . . . . . . . . . . . . 4
3.2. Host Sleep Mode . . . . . . . . . . . . . . . . . . . . . 6
3.3. Low Power WiFi Clients . . . . . . . . . . . . . . . . . 7
3.4. Vendor and Configuration Optimizations . . . . . . . . . 8
3.5. Even Unicast NDP is not Optimum . . . . . . . . . . . . . 8
4. Measuring the Amount of IPv6 Multicast . . . . . . . . . . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Informative References . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Several IETF protocols rely on the use of link-local scoped IP
multicast in the hope of reducing traffic over the underlying
datalink network and generating less operating systems interrupts for
the receiving nodes. For example, IPv6 Neighbor Discovery [RFC4861]
uses link-local multicast to:
o advertise the presence of a router by sending router advertisement
to IPv6 address link-local multicast address (LLMA), ff02::1,
whose members are only the IPv6 nodes but per [RFC4291] section 3
those messages must be forwarded on all ports. This IPv6 LLMA is
mapped to the Ethernet Multicast Address (EMA) 33:33:00:00:00:01;
o solicit the data-link layer address of an adjacent on-link node by
sending a neighbor solicitation to the solicited-node multicast
address corresponding to the target address such as
ff02:0:0:0:0:1:ffXX:XXXX (where the last 24 bits are the last 24
bits of the target address) as described in [RFC4291]. This IPv6
LLMA is mapped to the EMA 33:33:ff:XX:XX:XX.
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2. Issue on Wired Ethernet Network
Most switch vendors implement MLD snooping [RFC4541] in order to
forward multicast frames only to switch ports where there is a member
of the IPv6 multicast group. This optimization works by installing
hardware forwarding states in the switch. As there is a finite
amount of memory in the switches, especially when the memory is used
by the data plane forwarding, there is also a limit to the number of
MLD optimization states i.e. a limit to the number of IPv6 multicast
groups that can be optimized by the switch; frames destined to groups
without such a state are flooded on all ports in the same datalink
domain, and generally the use of MLD snooping is reserved to groups
with a scope wider than link local.
With IPv6, all nodes have usually at least two IPv6 addresses: a
link-local and a global address. If both addresses are based on
EUI-64, then they share the same 24 least-significant bits, hence
there is only one solicited-node multicast address per node. Else,
there is a high probability that the 24 least-significant bits are
different, hence requiring the membership to two solicited-node
multicast addresses. If a switch uses MLD snooping to install
hardware-optimized multicast forwarding states for LLMA, then the
switch installs two hardware-optimized states per node as EUI-64
addresses are no more commonly used. If privacy extension addresses
[RFC4941] are used, then every node can have multiple IPv6 global
addresses, most of which are not based on EUI-64, a large switch
fabric will have to support multiple times more states for multicast
EMA than it does for unicast addresses, resulting in an excessive
amount of resources in each individual switch to be built at an
affordable price.
Therefore, due to cost reason, the multicast optimization by MLD
snooping of solicited-node LLMA is disabled on most Ethernet
switches. This means wasting:
o the switch bandwidth as it works as a full-duplex hub;
o the nodes CPU as all nodes will have to receive the multicast
frame (if their network adapter is not optimized to support MAC
multicast) and quickly drop it.
A special mention must be paid when a layer-2 domain includes legacy
devices working on at 10 Mbps half-duplex; for example, in hospitals
having old equipments dated back of 1990. For this case, it takes
only 100 300-byte frames per second to already utilize the media to
2.4 % not to mention that the NIC and the processor have to process
those frames and that the processor is probably also dated from
1990...
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It is unclear what the impact is on virtual machines with different
MAC addresses and different IPv6 address connected with a virtual
layer-2 switch hosted on a single physical server... The MLD snooping
done by the virtual switch will consume CPU by the hypervisor, hence,
also reducing the amount of CPU available for the virtual machines.
Leveraging MLD snooping to save layer-2 switches from flooding link-
local multicast messages carries additional challenges. Unsolicited
MLD reports are usually sent once (when link comes up) and not
acknowledged. There exist a retransmission mechanism, but it is not
generally deployed, and it does not guarantee that subsequent
retransmission won't also get lost. The switch could easily end up
with incomplete forwarding states for a given group, with some of the
listeners ports, but not all (much worse than no state at all). As
the switch does not know one of its forwarding entry is incomplete,
it can't fall back to broadcasting. As ordinary MLD routers, the
switch could query reports on a periodic basis. However, it is not
practical for layer-2 access switches to send periodic general MLD
queries to maintain forwarding states accuracy for at least 2
reasons:
o The queries must be sourced with a link-local IPv6 address, one
per link, and, for many practical reasons, layer-2 switches don't
have such address on each link (vlan) they operate on.
o Since address resolution uses a multicast group, and may happen
quite frequently on the link, in order to avoid black holing
resolution, the interval for a switch to issue MLD general query
would have to be very small (a few seconds). These MLD queries
are themselves sent to a multicast group that all nodes would need
to get. That would completely defeat the purpose of reducing
multicast traffic towards end nodes.
3. Issues on IEEE 802.11 Wireless Network
3.1. Multicast over Wireless
Wireless networks are a shared half-duplex media: when one station
transmits, then all others must be silent. A multicast or broadcast
transmission from an AP is physically transmitted to all WiFi cliens
(STAs) and no other node can use the wireless medium at that time.
This is the first issue with the use of wireless for multicast: the
medium access behaves as a Ethernet hub.
Depending on distance and radio propagation, different wireless
clients may use different transmission encodings and data rates. A
lower data rate effectively locks the medium for a longer time per
bit. In order to reach all nodes, and considering that multicast and
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broadcast frames are not protected by ARQ (retries), the AP is
constrained to transmit all multicast or broadcast frames at the
lowest rate possible, which in practice is often translated to rates
as low as 1 Mbps or 6 Mbps, even when the unicast rate can reach a
hundred of Mbps and above. It results that sending a single
multicast frame can consume as much bandwidth as dozens of unicast
frames. Table Table 1 provides some example values of the bandwidth
used by multicast frames transmitted from the AP (i.e. not counting
the original multicast frame transmitted by the WiFi client to the AP
when he source is effectively wireless).
+--------------+---------------+---------------+--------------------+
| Lowest WiFi | Highest WiFi | Mcast frame | WiFi Utilization |
| rate | rate | %-age | by Mcast |
+--------------+---------------+---------------+--------------------+
| 1 Mbps | 11 Mbps | 1 % | 9 % |
| 6 Mbps | 54 Mbps | 1 % | 9 % |
| 6 Mbps | 54 Mbps | 5 % | 45 % |
| 6 Mbps | 54 Mbps | 10 % | 90 % |
+--------------+---------------+---------------+--------------------+
Table 1: Multicast WiFi Usage
If multiple APs cover the same wireless LAN, then the multicast
frames must be transmitted by all APs to all their WiFi clients.
Communication of a multicast frame by a WiFi client requires three
steps:
1. The WiFi client sends a datalink unicast frame to the AP at its
maximum possible rate.
2. The WiFi AP forwards this frame on its wired interface and
broadcasts it (as explained above) to all its WiFi clients. If
there are multiple APs on the same datalink domain, then, all APs
also broadcast this multicast frame to their WiFi clients.
3. A WiFi NIC that implements the STA in the client filters the
frames that are effectively expected by this device based on
destination address.
Another side effect of multicast frames is that there cannot be an
acknowledgement mechanism (ARQ) similar to that used for unicast
frame, therefore frames can be missed and NDP does not take this non
negligible packet loss into account. This could have a negative
impact for Duplicate Address Detection (DAD) if the multicast NS or
the multicast NA with override are lost. Assuming a error rate of 8%
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of corrupted frame, this means a 8% chance of loosing a complete
frame, this means a 16% chance of not detecting a duplicate address.
For a well-distributed multicast group where relatively few devices
actually participate to any given group, there should be no
transmission at all if none of the clients expects the multicast
destination address, and there should be very few unicast but fast
transmissions to the limited set of interest STAs when there is
effectively a match in the set of associated devices. But there is
no mechanism in place to ensure that functionality.
3.2. Host Sleep Mode
When a sleeping host wakes up by a user interaction, it cannot
determine whether it has moved to another network (SSID are not
unique), hence, it has to send a multicast Router Solicitation (which
triggers a Router Advertisement message from all adjacent routers)
and the mobile host has to do Duplicate Address Detection for its
link-local and global addresses, thus means transmitting at least two
multicast Neighbour Solicitation messages which will be repeated by
the AP to all other WiFi clients.
This process creates a lot of multicast packets:
o one multicast Router Solicitation from the WiFi client, which is
received by the AP and if the AP is not optimized, then the Router
Solitication is broadcasted again over the wireless link;
o one multicast Neighbor Solitication for the host LLA from the WiFi
client, which is received by the AP and if the AP is not
optimized, the message is transmitted back over the wireless link;
o per global address (usually 1 or 2 depending on whether privacy
extension is active), same behavior as above.
In conclusion and in the good case of not having privacy extension,
this means 6 WiFi broadcast packets plus the unicast replies on each
wake-up of the device. Assuming a packet size of 80 bytes, this
translates into about 120 bytes to take into account the WiFi frame
format which is larger than the usual Ethernet frame, the table
Table 2 gives some result of the WiFi utilization just for the
multicast part of the wake-up of sleeping devices... This does not
take into account the rest of the multicast utilization used by RS,
RA, NS, NA, MLD, ... and the associated unicast traffic.
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+---------+---------+------------+----------+---------+-------------+
| WiFi | Wake-up | Mcast | Mcast | Lowest | Mcast |
| Clients | Cycle | packet/sec | bit/sec | WiFi | Utilization |
| | | | | Rate | |
+---------+---------+------------+----------+---------+-------------+
| 100 | 600 sec | 1 | 960 bps | 1 Mbps | 0.1 % |
| 1 000 | 600 sec | 1 | 9600 bps | 1 Mbps | 1.0 % |
| 5 000 | 600 sec | 50 | 48 kbps | 1 Mbps | 4.8 % |
| 5 000 | 300 sec | 100 | 96 kbps | 1 Mbps | 9.6 % |
+---------+---------+------------+----------+---------+-------------+
Table 2: Multicast WiFi Usage by Sleeping Devices
3.3. Low Power WiFi Clients
In order to save their batteries, Low Power (LP) hosts go into radio
sleep mode until there is a local need to send a wireless frame.
Before going into radio sleep mode, the LP hosts signal to the AP
that they are going into sleep; this allows the AP to store unicast
and multicast frames destined for those sleeping LP clients. LP
clients wake up periodically to listen to the WiFi beacon frames
transmitted periodically (default every 100 ms) because this beacon
frame contains a bit mask (Traffic Indication Map - TIM) indicating
for which STA there is waiting unicast traffic and whether there is
multicast traffic waiting. If there is multicast traffic waiting,
that ALL LP hosts must stay awake to receive all multicast frames
sent immediately after by the AP and process them. If there is a bit
indicating that unicast traffic is waiting for a specific LP host,
then only this LP host will stay awake to poll the AP later to
collect its traffic. The TIM maximum length is 2008 bits and the
complete beacon frame is less than 300 bytes long.
The table Table 2 indicates the ration of active/sleeping time for LP
hosts when multicast is present. In the absence of multicast
traffic, the radio is active only 2.4 % of the time while if there
are 50 multicast frames of 300 bytes per second, the radio is active
14.4 % of the time, nearly 6 times more often... with a battery life
probably reduced by 6...
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+-------------+------------+---------------+------------+-----------+
| Beacon | Mcast | Mcast frame | Lowest | Awake |
| frames/sec | frames/sec | size (bytes) | WiFi Rate | time/sec |
+-------------+------------+---------------+------------+-----------+
| 10 | 0 | 300 bytes | 1 Mbps | 2.4 % |
| 10 | 5 | 300 bytes | 1 Mbps | 3.6 % |
| 10 | 10 | 300 bytes | 1 Mbps | 4.8 % |
| 10 | 50 | 300 bytes | 1 Mbps | 14.4 % |
+-------------+------------+---------------+------------+-----------+
Table 3: Multicast WiFi Impact on Low Power Hosts
3.4. Vendor and Configuration Optimizations
Vendors have noticed the problem and have come with several
optimizations such as
o LP hosts not waking up the main processor when they are not member
of the multicast group;
o APs no transmitting back over radio received Router Sollication
multicast messages;
o ...
AP can also work in 'AP isolation mode' where there is no direct
traffic between WiFi clients, this mode has a positive side-effect
when a WiFi client transmits a multicast frame as this frame is
transmitted at the highest possible rate over the WiFi medium and the
AP will not re-transmit if back to all other WiFi clients at the
lowest rate.
3.5. Even Unicast NDP is not Optimum
While this is not directly related to the subject of this document,
it is worth mentioning anyway as this is important for devices
running on battery.
NDP cache needs to be maintained by refreshing the neighbor cache for
entries which are in the STALE state. This requires yet another
Neighbor Solicitation / Neighbor Advertisement round. Even if the
destination IP and MAC addresses are unicast, this traffic is
generated and again wakes up mobile devices.
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4. Measuring the Amount of IPv6 Multicast
There are basically three ways to measure the amount of IPv6
multicast traffic:
o sniffing the traffic and generating statistics, somehow an
overkill:
o exporting IPfix data and doing aggregation on the ff02::/16 link-
local multicast prefix
o using SNMP to query on the AP the IP-MIB [RFC4293] with commands
such as:
* snmpwalk -c private -v 1 udp6:[2001:db8::1] -Ci -m IP-MIB
ifDesc: to get the interface names and index;
* snmpwalk -c private -v 1 udp6:[2001:db8::1] -Ci -m IP-MIB
ipIfStatsOutTransmits.ipv6: to get the global transmit counters
(i.e. unicast and multicast as there is no broadcast in IPv6);
* snmpwalk -c private -v 1 udp6:[2001:db8::1] -Ci -m IP-MIB
ipIfStatsOutMcastPkts.ipv6: to get the multicast packet
counter.
5. Acknowledgements
The authors would like to thank Norman Finn, Michel Fontaine, Steve
Simlo, Ole Troan, and Stig Venaas for their suggestions and comments.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
The only security considerations about this document is that by
forcing a lot of traffic to be multicast, then, a denial of service
(DoS) attack could be mounted on available bandwidth and battery of
some network nodes.
8. Informative References
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4293] Routhier, S., "Management Information Base for the
Internet Protocol (IP)", RFC 4293, April 2006.
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[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping
Switches", RFC 4541, May 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[packet_loss]
Department of Computer Sciences, University of Wisconsin
Madison, USA, "Diagnosing Wireless Packet Losses in
802.11: Separating Collision from Weak Signal",
<http://pages.cs.wisc.edu/~suman/pubs/diagnose.pdf>.
Authors' Addresses
Eric Vyncke (editor)
Cisco
De Kleetlaan, 6A
Diegem 1831
BE
Phone: +32 2 778 4677
Email: evyncke@cisco.com
Pascal Thubert
Cisco
Batiment D, 45 Allee des Ormes
MOUGINS, PROVENCE-ALPES-COTE D'AZUR 06250
France
Email: pthubert@cisco.com
Eric Levy-Abegnoli
Cisco
Batiment D, 45 Allee des Ormes
MOUGINS, PROVENCE-ALPES-COTE D'AZUR 06250
France
Email: elevyabe@cisco.com
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Andrew Yourtchenko
Cisco
De Kleetlaan, 6A
Diegem 1831
BE
Phone: +32 2 704 5494
Email: ayourtch@cisco.com
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