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gRPC iOS Network Transition Behaviors

Network connectivity on an iOS device may transition between cellular, WIFI, or no network connectivity. This document describes how these network changes should be handled by gRPC and current known issues.

Expected Network Transition Behaviors

The expected gRPC iOS channel and network transition behaviors are:

  • Channel connection to a particular host is established at the time of starting the first call to the channel and remains connected for future calls to the same host.
  • If the underlying connection to the remote host is broken, the channel is disconnected and enters TRANSIENT_FAILURE state.
  • A channel is broken if the channel connection is no longer viable. This happens when
    • The network interface is no longer available, e.g. WiFi or cellular interface is turned off or goes offline, airplane mode turned on, etc;
    • The underlying TCP connection is no longer valid, e.g. WiFi connects to another hotspot, cellular data switched from LTE to 4G, etc;
    • A network interface more preferable by the OS is valid, e.g. WiFi gets connected when the channel is already connected via cellular.
  • A channel in TRANSIENT_FAILURE state attempts reconnection on start of the next call to the same host, but only after a certain backoff period (see corresponding doc). During the backoff period, any call to the same host will wait until the first of the following events occur:
    • Connection succeeded; calls will be made using this channel;
    • Conncetion failed; calls will be failed and return UNAVAILABLE status code;
    • The call's deadline is reached; the call will fail and return DEADLINE_EXCEEDED status code. The length of backoff period of a channel is reset whenever a connection attempt is successful.

Implementations

gRPC iOS with TCP Sockets

gRPC's default implementation is to use TCP sockets for networking. It turns out that although Apple supports this type of usage, it is not recommended by Apple and has some issues described below.

Issues with TCP Sockets

The TCP sockets on iOS is flawed in that it does not reflect the viability of the channel connection. Particularly, we observed the following issues when using TCP sockets:

  • When a TCP socket connection is established on cellular data and WiFi becomes available, the TCP socket neither return an error event nor continue sending/receiving data on it, but still accepts write on it.
  • A TCP socket does not report certain events that happen in the background. When a TCP connection breaks in the background for the reason like WiFi connects to another hotspot, the socket neither return an error nor continue sending/receiving data on it, but still accepts write on it. In both situations, the user will see the call hang for an extended period of time before the TCP socket times out.

gRPC iOS library's resolution to TCP socket issues

We introduced ConnectivityMonitor in gRPC iOS library v0.14.0 to alleviate these issues in TCP sockets, which changes the network transition behaviors a bit.

We classify network connectivity state of the device into three categories based on flags obtained from SCNetworkReachability API:

Reachable ConnectionRequired IsWWAN Category
0 X X None
X 1 X None
1 0 0 WiFi
1 0 1 Cellular

Whenever there is a transition of network between two of these categories, all previously existing channels are assumed to be broken and are actively destroyed. If there is an unfinished call, the call should return with status code UNAVAILABLE.

ConnectivityMonitor is able to detect the scenario of the first issue above and actively destroy the channels. However, the second issue is not resolvable. To solve that issue the best solution is to switch to CFStream implementation which eliminates all of them.

gRPC iOS with CFStream

gRPC iOS with CFStream implementation (introduced in v1.13.0) uses Apple's networking API to make connections. It resolves the issues with TCP sockets mentioned above. Users are recommended to use this implementation rather than TCP socket implementation. The detailed behavior of streams in CFStream is not documented by Apple, but our experiments show that it accords to the expected behaviors. With CFStream implementation, an event is always received when the underlying connection is no longer viable. For more detailed information and usages of CFStream implementation, refer to the user guide.