Each remote notification includes a payload. The payload contains information about how the system should alert the user as well as any custom data you provide. The maximum size allowed for a notification payload is 2 kilobytes; Apple Push Notification service refuses any notification that exceeds this limit.

https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ApplePushService.html

看这个官方文档，其中有新的协议描述。