| Internet Area | C. Perkins |
| Internet-Draft | Futurewei |
| Intended status: Informational | D. Stanley |
| Expires: January 20, 2018 | HPE |
| W. Kumari | |
| JC. Zuniga | |
| SIGFOX | |
| July 19, 2017 |
Multicast Considerations over IEEE 802 Wireless Media
draft-perkins-intarea-multicast-ieee802-03
Performance issues have been observed when multicast packet transmissions of IETF protocols are used over IEEE 802 wireless media. Even though enhamcements for multicast transmissions have been designed at both IETF and IEEE 802, there seems to exist a disconnect between specifications, implementations and configuration choices. This draft describes the different issues that have been observed, the multicast enhancement features that have been specified at IETF and IEEE 802 for wireless media, as well as the operational chioces that can be taken to improve the performace of the network. Finally, it provides some recommendations about the usage and combination of these features and operational choices.
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Many IETF protocols depend on multicast/broadcast for delivery of control messages to multiple receivers. Multicast is used for various purposes such as neighborhood discovery, network flooding, address resolution, as well minimizing media occupancy for the transmission of data that is intended for multiple receivers.
IETF protocols typically rely on network protocol layering in order to reduce or eliminate any dependence of higher level protocols on the specific nature of the MAC layer protocols or the physical media. In the case of multicast transmissions, higher level protocols have traditionally been designed as if transmitting a packet to an IP address had the same cost in interference and network media access, regardless of whether the destination IP address is a unicast address or a multicast or broadcast address. This model was reasonable for networks where the physical medium was wired, like Ethernet. Unfortunately, for many wireless media, the costs to access the medium can be quite different. Some enhancements have been designed in IETF protocols that are assumed to work primarily over wireless media. However, these enhancements are usually implemented in limited deployments and not widely spread on most wireless networks.
IEEE 802 wireless protocols have been designed with certain features to support multicast traffic. For instance, lower modulations are used to transmit multicast frames, so that these can be received by all stations in the cell, regardless of the distance or path attenuation from the base station or access point. However, these lower modulation transmissions occupy the medium longer; they hamper efficient transmission of traffic using higher order modulations to nearby stations. For these and other reasons, IEEE 802 working groups such as 802.11 have designed features to improve the performance of multicast transmissions at Layer 2 [REF 11-15-1261-03]. In addition to protocol design features, certain operational and configuration enhancements can ameliorate the network performance issues created by multicast traffic.
This Internet Draft details various problems caused by multicast transmission over wireless networks. It also explains some enhancements that have been designed at IETF and IEEE 802, as well as the operational choices that can be taken, to ameliorate the effects of multicast traffic. Recommendations about how to use and combine these enhancements are also provided.
This document uses the following definitions:
In this section we list some of the issues related to the use of multicast transmissions over IEEE 802 wireless technologies.
Multicast traffic is typically much less reliable than unicast traffic. Since multicast makes point-to-multipoint communications, multiple acknowledgements would be needed to guarantee the reception on all recipients.
Because more robust MCSs have longer range but also lower data rate, multicast / broadcast traffic is generally transmitted at the lowest common denominator rate, also known as the basic rate. On IEEE 802.11 networks (aka WiFi), this rate might be as low as 6 Mbps, when some unicast links in the same cell can be operating at rates up to 600 Mbps. Transmissions at a lower rate require longer occupancy of the wireless medium and thus take away from the airtime of other communications and degrade the overall capacity.
Wired multicast also affects wireless LANs when the AP extends the wired segment; in that case, multicast / broadcast frames on the wired LAN side are copied to WLAN. Since broadcast messages are transmitted at the most robust MCS, many large frames are sent at a slow rate over the air.
Multicast can work poorly with the power-save mechanisms defined in IEEE 802.11.
This section identifies some representative IETF protocols, and describes possible negative effects due to performance degradation when using multicast transmissions for control messages. Common uses of multicast include:
The following list contains a few representative IPv4 protocols using multicast.
After initial configuration, ARP and DHCP occur much less commonly.
IPv6 makes much more extensive use of multicast, including the following:
Address Resolution
Service Discovery
Route Discovery
Decentralized Address Assignment
Geographic routing
Multicast Listener Discovery(MLD) [RFC4541] is often used to identify members of a multicast group that are connected to the ports of a switch. Forwarding multicast frames into a WiFi-enabled area can use such switch support for hardware forwarding state information. However, since IPv6 makes heavy use of multicast, each STA with an IPv6 address will require state on the switch for several and possibly many multicast solicited-node addresses. Multicast addresses that do not have forwarding state installed (perhaps due to hardware memory limitations on the switch) cause frames to be flooded on all ports of the switch.
On the Internet there is a "background radiation" of scanning traffic (people scanning for vulnerable machines) and backscatter (responses from spoofed traffic, etc). This means that routers very often receive packets destined for machines whose IP addresses may or may not be in use. In the cases where the IP is assigned to a host, the router broadcasts an ARP request, gets back an ARP reply, and caches it; then traffic can be delivered to the host. When the IP address is not in use, the router broadcasts one (or more) ARP requests, and never gets a reply. This means that it does not populate the ARP cache, and the next time there is traffic for that IP address the router will rebroadcast the ARP requests.
The rate of these ARP requests is proportional to the size of the subnets, the rate of scanning and backscatter, and how long the router keeps state on non-responding ARPs. As it turns out, this rate is inversely proportional to how occupied the subnet is (valid ARPs end up in a cache, stopping the broadcasting; unused IPs never respond, and so cause more broadcasts). Depending on the address space in use, the time of day, how occupied the subnet is, and other unknown factors, on the order of 2000 broadcasts per second have been observed at the IETF NOCs.
On a wired network, there is not a huge difference amongst unicast, multicast and broadcast traffic; but this is not true in the wireless realm. Wireless equipment often is unable to send this amount of broadcast and multicast traffic. Consequently, on the wireless networks, we observe a significant amount of dropped broadcast and multicast packets. This, in turn, means that when a host connects it is often not able to complete DHCP, and IPv6 RAs get dropped, leading to users being unable to use the network.
This section lists some optimizations that have been specified in IEEE 802 and IETF that are aimed at reducing or eliminating the issues discussed in Section 3.
The AP knows the MAC address and IP address for all associated STAs. In this way, the AP acts as the central "manager" for all the 802.11 STAs in its BSS. Proxy ARP is easy to implement at the AP, and offers the following advantages:
Here is the specification language as described in clause 10.23.13 of [dot11-proxyarp]:
As used in this section, a Low-Power Wireless Personal Area Network (6LoWPAN) denotes a low power lossy network (LLN) that supports 6LoWPAN Header Compression (HC). A 6TiSCH network is an example of a 6LowPAN. In order to control the use of IPv6 multicast over 6LoWPANs, the 6LoWPAN Neighbor Discovery (6LoWPAN ND) standard defines an address registration mechanism that relies on a central registry to assess address uniqueness, as a substitute to the inefficient Duplicate Address Detection (DAD) mechanism found in the mainstream IPv6 Neighbor Discovery Protocol (NDP) [RFC4861][RFC4862].
The 6lo Working Group is now completing an update to RFC6775. The update enables the registration to a Backbone Router, which proxies for the registered addresses with the mainstream IPv6 NDP running on a high speed aggragating backbone. The update also enables a proxy registration on behalf of the registered node, e.g. by a 6LoWPAN router to which the mobile node is attached.
The general idea behind the backbone router concept is that in a variety of Wireless Local Area Networks (WLANs) and Wireless Personal Area Networks (WPANs), the broadcast/multicast domain should be controlled, and connectivity to a particular link that provides the subnet should be left to Layer-3. The model for the Backbone Router operation is represented in Figure 1.
|
+-----+
| | Gateway (default) router
| |
+-----+
|
| Backbone Link
+--------------------+------------------+
| | |
+-----+ +-----+ +-----+
| | Backbone | | Backbone | | Backbone
| | router | | router | | router
+-----+ +-----+ +-----+
o o o o o o
o o o o o o o o o o o o o o
o o o o o o o o o o o o o o o
o o o o o o o o o o
o o o o o o o
LLN LLN LLN
Figure 1: Backbone Link and Backbone Routers
LLN nodes can move freely from an LLN anchored at one IPv6 Backbone Router to an LLN anchored at another Backbone Router on the same backbone, keeping any of the IPv6 addresses they have configured. The Backbone Routers maintain a Binding Table of their Registered Nodes, which serves as a distributed database of all the LLN Nodes. An extension to the Neighbor Discovery Protocol is introduced to exchange that information across the Backbone Link in the reactive fashion of mainstream IPv6 Neighbor Discovery.
RFC6775 and follow-on work are designed to address the needs of LLNs, but the techniques are likely to be valuable on any type of link where sleeping devices are attached, or where the use of broadcast and multicast operations should be limited.
The AP acts on behalf of STAs in various ways. In order to improve the power-saving feature for STAs in its BSS, the AP buffers frames for delivery to the STA at the time when the STA is scheduled for reception.
IPv6 uses Neighbor Discovery Protocol (NDP) instead of ARP. Every IPv6 node subscribes to a special multicast address for this purpose.
Here is the specification language from clause 10.23.13 of [dot11-proxyarp]:
NDP may be used to request additional information
NDP messages are sent as group addressed (broadcast) frames in 802.11. Using the proxy operation helps to keep NDP messages off the wireless medium.
It is often possible to transmit multicast control and data messages by using unicast transmissions to each station individually.
There are situations where more is needed than simply converting multicast to unicast. For these purposes, DMS enables a client to request that the AP transmit multicast group addressed frames destined to the requesting clients as individually addressed frames [i.e., convert multicast to unicast]. Here are some characteristics of DMS:
DMS is not currently implemented in products.
GCR (defined in [dot11aa]) provides greater reliability by using either unsolicited retries or a block acknowledgement mechanism. GCR increases probability of broadcast frame reception success, but still does not guarantee success.
For the block acknowledgement mechanism, the AP transmits each group addressed frame as conventional group addressed transmission. Retransmissions are group addressed, but hidden from non-11aa clients. A directed block acknowledgement scheme is used to harvest reception status from receivers; retransmissions are based upon these responses.
GCR is suitable for all group sizes including medium to large groups. As the number of devices in the group increases, GCR can send block acknowledgement requests to only a small subset of the group. GCR does require changes to both AP and STA implementation.
GCR may introduce unacceptable latency. After sending a group of data frames to the group, the AP has do the following:
This latency may not be acceptable for some traffic.
There are ongoing extensions in 802.11 to improve GCR performance.
This section lists some operational optimizations that can be implemented when deploying wireless IEEE 802 networks to mitigate the issues discussed in Section 3.
Many of the causes of performance degradation described in earlier sections are also observable for wireless media other than 802.11.
For instance, problems with power save, excess media occupancy, and poor reliability will also affect 802.15.3 and 802.15.4. However, 802.15 media specifications do not include mechanisms similar to those developed for 802.11. In fact, the design philosophy for 802.15 is oriented towards minimality, with the result that many such functions would more likely be relegated to operation within higher layer protocols. This leads to a patchwork of non-interoperable and vendor-specific solutions. See [uli] for some additional discussion, and a proposal for a task group to resolve similar issues, in which the multicast problems might be considered for mitigation.
This section provides some recommendations about the usage and combinations of the multicast enhancements described in Section 4 and Section 5.
(FFS)
This document does not introduce any security mechanisms, and does not have any impact on existing security mechanisms.
This document does not specify any IANA actions.
This document has benefitted from discussions with the following people, in alphabetical order: Pascal Thubert