xctrace --template Leaks identified this as a leak:
NSString *uuid = [NSString stringWithUTF8String:connectionID];
NSData *contentData = [NSData dataWithBytes:data length:length];
id<ConnexctionProtocol> proxy = [connection asyncConnectionProxy];
[proxy handleData:uuid data:contentData];
return;
(Which is to say: a few thousand objects show up in the Leaks pane, the stack for them goes up to the NSData creation, and Leaks apparently thinks it's never released.)
That doesn't look like it should be a leak, with ARC? Which probably means I'm doing something wrong?