Internet Engineering Task Force E. Vyncke Internet-Draft P. Thubert Intended status: Informational E. Levy-Abegnoli Expires: August 10, 2014 Cisco February 6, 2014 Why Network-Layer Multicast is Not Always Efficient At Datalink Layer draft-vyncke-6man-mcast-not-efficient-00 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on August 10, 2014. Copyright Notice Copyright (c) 2014 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 (http://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 Vyncke, et al. Expires August 10, 2014 [Page 1] Internet-Draft L3 Multicast is not always efficient February 2014 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Issue on Wired Ethernet Network . . . . . . . . . . . . . . . 2 3. Issues on IEEE 802.11 Wireless Network . . . . . . . . . . . 4 3.1. Multicast over Wireless . . . . . . . . . . . . . . . . . 4 3.2. Low Power and Sleep Mode . . . . . . . . . . . . . . . . 6 3.3. Battery Impact . . . . . . . . . . . . . . . . . . . . . 7 3.4. Even Unicast NDP is not Optimum . . . . . . . . . . . . . 7 4. Measuring the Amount of IPv6 Multicast . . . . . . . . . . . 7 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. Informative References . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 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. 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 Vyncke, et al. Expires August 10, 2014 [Page 2] Internet-Draft L3 Multicast is not always efficient February 2014 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. 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 Vyncke, et al. Expires August 10, 2014 [Page 3] Internet-Draft L3 Multicast is not always efficient February 2014 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 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 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). Vyncke, et al. Expires August 10, 2014 [Page 4] Internet-Draft L3 Multicast is not always efficient February 2014 +--------------+---------------+---------------+--------------------+ | 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 (STAs). 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 AP on the same datalink domain, then, all AP also broadcast this multicast frame. 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 not negligible packet loss into account. This could have a negative impact for Duplicate Address Detection (DAD) if the multicast NS and the multicast NA with override are lost. 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. Vyncke, et al. Expires August 10, 2014 [Page 5] Internet-Draft L3 Multicast is not always efficient February 2014 3.2. Low Power and Sleep Mode In order to save their battery, Low Power hosts go into sleep mode until awaken by a user interaction or by an incoming frame destined for the host. When a host wakes up, 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. +---------+---------+------------+----------+---------+-------------+ | WiFi | Wake-up | Mcast | Mcast | Lowest | Mcast | | Clients | Cycle | packet/sec | bit/sec | WiFi | Utilization | | | | | | Rate | | +---------+---------+------------+----------+---------+-------------+ | 6 000 | 600 | 60 | 57.6 | 1 Mbps | 6 % | | | | | kbps | | | +---------+---------+------------+----------+---------+-------------+ Table 2: Multicast WiFi Usage by Sleeping Devices Vyncke, et al. Expires August 10, 2014 [Page 6] Internet-Draft L3 Multicast is not always efficient February 2014 3.3. Battery Impact As the IP multicast frame is actually broadcasted over the wireless network, this also means that all wireless receivers must process this frame even if it will be dropped quickly by the host kernel because the host is not part of the IP multicast group. This goes beyond a waste of CPU and affect mobile device batteries... because in order to save the battery, it is common for mobile device to go into sleep mode when inactive to be awaken by the radio receiver (which is always on) in order to process wireless frame received either to the unicast MAC address or to any multicast MAC address or to the broadcast MAC address. Even if the device can go quickly back to sleep mode after discarding the IP packet, it drains the battery. 3.4. Even Unicast NDP is not Optimum OLE SUGGESTION: Wrong thing to add as NDP cache are refreshed only when needed, i.e., this would anyway wake up the mobile. Also, it is not really related to the title of the document... 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. 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); Vyncke, et al. Expires August 10, 2014 [Page 7] Internet-Draft L3 Multicast is not always efficient February 2014 * 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, Steve Simlo, Ole Troan, Stig Venaas and Andrew Yourtchenko 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. [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", . Vyncke, et al. Expires August 10, 2014 [Page 8] Internet-Draft L3 Multicast is not always efficient February 2014 Authors' Addresses Eric Vyncke Cisco De Kleetlaan, 6A Diegem 1831 BE Phone: +32 2 778 4677 Email: evyncke@cisco.com Pascal Thubert Cisco Email: pthubert@cisco.com Eric Levy-Abegnoli Cisco Email: elevyabe@cisco.com Vyncke, et al. Expires August 10, 2014 [Page 9]