Multicast has brought a lot of efficiencies to IP networks. But multicast wasn’t designed for wireless, and especially isn’t well suited for high-bandwidth multicast applications like video. I’ll cover the challenges of multicast over wireless and design considerations.
But first, an overview of multicast:
To level set, I’ll briefly cover IP multicast. For the purposes of this article, I’ll focus specifically on Layer 2, If you’re already familiar with multicast over ethernet, feel free to skip this section.
What is multicast?
In short, multicast is a means of sending the same data to multiple recipients at the same time without the source having to generate copies for each recipient. Whereas broadcast traffic is sent to every device whether they want it or not, multicast allows recipients to subscribe to the traffic they want. As a result, efficiency is improved (traffic is only sent once) and overhead is reduced (unintended recipients don’t receive the traffic).
How does it work?
With multicast, the sender doesn’t know who the recipients are, or even how many there are. In order for a given set of traffic to reach its intended recipients, we send traffic to multicast groups. IANA has reserved 18.104.22.168 – 22.214.171.124 for multicast groups, with 126.96.36.199/8 commonly used within private organizations. Traffic is sent with the unicast source IP of the sender, and a destination IP of the chosen multicast group.
On the receiving side, recipients subscribe to a multicast group using Internet Group Management Protocol (IGMP). A station that wishes to join a multicast group sends an IGMP Membership Report / Join message for that given group. Most enterprise switches, WLCs, or APs use IGMP snooping to inspect IGMP packets and populate their multicast table, which matches ports/devices to multicast groups. Then, when a multicast packet is received, the network device can forward that packet to the intended receivers. Network devices that don’t support IGMP snooping will forward the packet the same as it would a broadcast, to every port except the port the packet came in on. Here’s an example of an IGMP Join request:
The problems with multicast over WiFi vs wired
In a switched wired network, all traffic is sent at the same data rate (generally 1Gbps today) and with each port being its own collision domain, collisions are rare. In addition, wired traffic uses a bounded medium, so interference and frame corruption is also rare. Because of this, there is no network impact to sending large amounts of wired traffic as multicast. WiFi does not share either of these characteristics, which makes multicast more complicated. Below are some of the issues with multicast to multicast over WiFi:
- Multicast traffic is sent at a mandatory data rate. As mentioned, WiFi clients share a collision domain. Because multicast is a single transmission that must be received by all transmitted receivers, access points are forced to send that frame at the lowest-common-denominator settings, to give the receivers the best chance of hearing the transmission uncorrupted. While this is fine for small broadcast traffic like beacons, it’s unsustainable for high-bandwidth applications.
- Low data-rate traffic consumes more air time. Because multicast traffic is sent at a low data rate, it takes longer for each of those transmissions to complete. A 1MB file sent at a data rate of 1 Mbps will take significantly longer than the same file at a data rate of 54Mbps. This means that all other stations must spend more time waiting for their turn to transmit.
- Battery-powered clients have reduced battery life. Multicast and broadcast traffic are sent at the DTIM interval, which all stations keep track of. When a multicast frame is sent, all stations must wake up to listen to the frame, and discard it if they don’t need it. This results in battery-powered devices staying awake for a lot longer than needed. If the DTIM interval is too high, the increased latency can impact real-time applications like video. But the lower the DTIM interval, the more often stations need to wake up.
- Multicast senders will not resend corrupt frames. Frame corruption and retransmissions are a standard part of any WiFi transaction. Every unicast frame, even if unacknowledged at upper OSI layers such as when using UDP, are acknowledged at Layer 2, and retransmitted by the sending station if necessary. This may not seem like a big deal at first, as unacknowledged traffic on a wired network works fine most of the time. But in an area of interference or poor RSSI level, it’s not unusual to see 10% of wireless frames retransmitted. 10% loss would be considered extremely high on a wired network, and most applications are unable to handle this level of loss.
So how do we fix it?
There’s no silver bullet to “fixing” multicast over wireless, but there are a few ways to design around the shortcomings.
- Increasing the minimum data rate. An increase to the minimum data rate means that broadcast and multicast frames must be sent at the higher rate. Unicast traffic is acknowledged at Layer 2, reducing loss experienced by the upper layers. As mentioned earlier, higher data rates reduce the time spent transmitting, and increase throughput for the multicast traffic. It also reduces the amount of time a battery powered device must spend listening to the frames. However, other design and configuration considerations must be made to ensure the wireless network can support this, as changing the minimum data rate can impact roaming, as well as connectivity for low-powered devices.
- Multicast-to-Unicast Conversion (M-to-U). Many vendors of wireless APs support multicast-to-unicast conversion, which sends a unicast copy the frame to each intended receiver, using IGMP snooping to determine those stations. This means that the frame can be sent at the receiving station’s best data rate, which should almost always be above the minimum. Several unicast transmissions at 54Mbps would still use less channel time than the same multicast transmission at 1Mbps. In addition, stations which aren’t the intended receivers don’t need to wake up to listen to the frame, reducing their battery consumption.
Let’s take a look at the same multicast frame sent with and without Multicast-to-Unicast Conversion. Using iperf2 (since iperf3 doesn’t support multicast), we’ll generate multicast traffic at a rate of 20Mbps from a wired client and send it to a wireless client, using multicast address 188.8.131.52.
Parameters for this test:
Receiver: MacBook Pro (2015 edition). 3 spatial stream 802.11ac airport card.
Access Point: Cisco Meraki MR42E (802.11ac Wave 2, 3×3:3) with omni-directional dipole antennas.
Wired Multicast Source (10.1.1.216):
Wireless Multicast Recipient (M-to-U enabled):
Wireless Multicast Recipient (M-to-U disabled):
The first thing to notice is the loss rate. With M-to-U enabled, my 20Mbps stream was successfully being transmitted with almost no loss. With M-to-U disabled, throughput was reduce by roughly 95%, with an average of 1Mbps throughput. There are two reasons for this: first, the mandatory data rate used for the multicast transmission was 6Mbps, of which ~40% is attributed to protocol overhead. In addition, with a unicast transmission the AP can buffer frames to a receiver, whereas a multicast transmission is best effort: it has no layer 2 acknowledgement or communication from the receivers. This can be improved with application-level handling, such as the application deciding to transmit at a lower quality, but there are no guarantees that the application is set up to handle that. iperf has no such throttling/accommodation.
To dive in further, let’s take a look at the differences in the frames transmitted:
Frame Capture (M-to-U enabled):
Frame Capture (M-to-U disabled):
We can verify that the second frame is using multicast by the MAC address in the Destination Address field, since all multicast MACs begin with 01-00-5E. Notice also that the source address of the unicast frame is set to the MAC address of the access point as the AP had to generate that frame, whereas the multicast frame’s source is that of the sending station since there was no frame modification needed.
Next, we’ll look at the data rate. Multicast is always sent at the basic rate, which was 6 Mbps for this BSSID, and a transmission time of 2072μs. Compared to M-to-U with a data rate of 540Mbps and a transmission time of 46μs. That means that the multicast transmission held the channel 45 times longer than the unicast, and still only sent half as much data.
Also, since multicast must use the lowest-common-denominator parameters, it cannot take advantage of efficiency improvements such as A-MPDU and multiple spatial streams offered by this AP.
So wouldn’t M-to-U be the silver bullet solution?
As is often the case, the answer is “it depends”. In a lab where my 3 spatial stream MacBook Pro can connect at MCS 8, it may appear so. But, if the majority of clients are connected at a low data rate, and the content only consumes a small amount of bandwidth, the overhead caused by retransmitting small frames for a large number of receivers could add delay and consume more aggregate airtime than simply transmitting once at a low data rate.