Multicast

In my previous post, I mentioned my frustration that certain Android phones (including my HTC EVO) cannot receive multicast datagrams. I'd like to get feedback from my friends and colleagues about multicast support on their phones, so I wrote a simple app for testing multicast.

The Multicast Test Tool continually monitors the network for Multicast DNS (mDNS) packets while the app is running in the foreground, and presents the contents of these packets to the user. The app also allows the user to perform simple mDNS queries on the local network. If you run the app and touch the "Query" button, it will query for the default _services._dns-sd._udp.local name, which will solicit mDNS responses from devices on your network that advertise services via mDNS.

Mac and Linux machines will respond to this discovery query. Windows machines will also respond if you are running iTunes and have checked "Share my library on my local network" in the preferences. If you think your network may not have any of these devices which support mDNS service discovery, you can run the attached perl script to transmit a gratuitous mDNS packet.

You can also query specific hosts using the format <hostname>.local. My home network is always buzzing with mDNS traffic, so on my multicast-capable virtual machine I see lots of activity without performing any queries.

If you see any activity at all, it means that your phone supports multicast. If you see no activity, then your phone likely does not support multicast.

Downloads

• multicast_test.apk - This is the Multicast Test Tool app, which you can download and install on your Android phone.
• send_mdns.pl - This perl script transmits a single mDNS packet, which should be detected by the Multicast Test Tool. This is only needed if you don't already have devices on your network (Macs, Linux boxes, etc.) which implement the mDNS service discovery protocol.
• multicast_test_tool.tar.gz - The source code for the Multicast Test Tool. This is only needed if you are curious about the inner workings. (Added 2011-02-02.)

It looks like some (most?) HTC phones running Android, such as my HTC EVO, are not capable of receiving multicast or broadcast datagram packets over the Wi-Fi network. This means that apps which rely on such communication will fail, often with no indication of the problem. From the app's perspective, no obvious error is happening — it can only assume that no other devices on the network are transmitting such datagrams. Multicast communication is becoming increasingly common as a technique for devices to discover each other on a network, and the absence of this capability represents serious breakage that leaves apps crippled. Examples of resources an app might use multicast to discover are:

• iTunes music libraries
• entertainment center components such as Roku and AppleTV
• network shares
• other devices running the same app, to set up multiplayer games

Without a functioning multicast capability, these apps either don't work, or require the user to manually configure the IP addresses of the other devices.

There is speculation that HTC's restriction is an attempt to save power and prolong battery life — it takes energy to process a network's many multicast and broadcast packets, many of which are useless to the device and its applications. However, Android already has a facility for allowing prudent use of multicast when needed. Applications acquire a WifiManager.MulticastLock for the duration of their multicast needs, which causes the Wi-Fi chip to stop filtering multicast packets until the app releases the lock. Thus, multicast processing in the software TCP/IP stack only happens when it's really needed.

This is a bit frustrating, as I'm currently tinkering with some multicast code. Some people have been able to fix the problem by rooting their phones and replacing the /system/bin/wpa_supplicant binary with a stock version. This doesn't seem to help on my EVO. To add to the frustration, it can be difficult to test multicast in the Android emulator. I finally had some success in testing multicast code by running android-x86 in a VirtualBox virtual machine, with the network configured for bridging.

References:

• HTC Desire Android 2.1-update1 does not receive udp broadcasts (Android issue #8407)
• Multicast on Android 2.2 (Stack Overflow)