Apple iPhone Tech Talk London 2009: Networking: From Sockets to GameKit

Paul Danbold danbold@apple.com — Evangelist

This was another really fast presentation with loads of detail. I look forward to grabbing the presentation from the iPhone dev site.

• don’t want to use CFSocket or BSD Socket
  • don’t let you bring up wireless, or involve security
• service discovery:
  • NSNetServices, CFNetServices, dns-sd, BSD Sockets
  • dns-sd is Bonjour or Zeroconf

URL Loading system

• NSURLRequest, NSURLConnection & about 10 other classes (also mutable ones)
• there’s sample code for validating URLs
• recommend to use asynchronous API style:
  • use delegate mechanism — calls connection:didReceiveResponse:
• also connection:didReceiveAuthenticationChallenge:
• finally connectionDidFinishLoading:
• default behaviours should work fine

Local area networking — service discovery

• kick off NSNetServiceBrowser in asynchronous mode, w/delegation
• browser searchForServicesOfType:inDomain:
  • e.g. type _http._tcp in domain local. — don’t go beyond any routers
  • can look for your own types too such as _foo._tcp
• then get myServiceBrowser:didFindService:moreComing:
• call NSNetworkService getInputStream:outputStream to resolve
  • this takes time (many seconds)
  • wait for the user to choose before resolving
  • and let the user decide when to cancel
  • resolveWithTimeout:0.0
• must release input & output stream objects returned by NSNetService
  • bug in Apple’s code — doesn’t obey normal Cocoa behaviour

OutputStream & InputStream

• NSStreamEventHasSpaceAvailable is probably the interesting one
• though you want to handle the errors too…

advertising & publishing

• advertise service with NSNetService initWithDomain:type:name:port
  • domain can be empty — means local
  • name can be blank too — will use device’s iTunes name
• then call publish on service

stop when you’re done

• browsing is fairly lightweight on the network, but you should stop anyway
• the same goes for publishing, when you’ve got all your connected clients

register types & ports

• to register a service type:
  • http://www.dns-sd.org/ServiceTypes.html
  • send them an email, no money changes hands
  • and http://www.iana.org/assignments/port-numbers for one further

game kit — peer to peer

• 3.1 adds WiFi support as well as Bluetooth
• GKSession initWithSessionID:displayName:sessionMode
  • session ID is your Bonjour service type — how you advertise yourself
  • again, can leave displayName as nil for iTunes name
  • sessionMode can be peer to peer (1 to 1) or client/server (multiple clients)
• can’t physically go over 10-20 clients with Bluetooth
  • 3.1 and later is a lot better, but still no more than 3-4 clients
  • Bluetooth also has low bandwidth
• set up delegate and set available to YES
• check other peers with peersWithConnectionState:
  • can be available, already connected, in the process of getting connected
• connectToPeer:withTimeout:
  • generally set timeout to zero to let user cancel if they want
• actual connection made with session:didReceiveConnectionRequestFromPeer: and acceptConnectionFromPeer:error:
• must monitor the network using session:peer:didChangeState:
  • other players can drop out, so you’d have to tear down your session
• send data with sendDataToAllPeers:withDataMode:error: and sendDataToPeers:...
  • can choose unreliable (UDP) or reliable (TCP) data modes
  • as always TCP imposes a performance cost so use sparingly
• receive data with setDataReceiveHandler:withContext: and receiveData:fromPeer:inSession:context:
• always check for errors — buffers fill up on low bandwidth networks
  • check it by using a slow network
• when you’re finished, tear down with disconnectFromAllPeers:
• there’s a GKPeerPickerController for setting up Bluetooth peers
  • doesn’t work for client/server or WiFi
• the only way to find out if Bluetooth is turned on is to use the GKPeerPickerController (it uses private APIs…)
• all data sent & received is NSData-wrapped — design your packaging for network efficiency first and coding efficiency second

Voice chat

• GKVoiceChatClient and ~Service
• sets up socket interface
• handles microphone, echo suppression, etc

network challenges

scoped routing and reachability

• iphone 3.0 and later provides scoped routing
  • keeps both cellular and wifi up at the same time — so cellular connections keep going when you enter a wifi hotspot
  • but this puts a toll on the battery, so you should behave and check for networking changes
• use SystemConfiguration APIs to monitor network state
  • Am I on the network and what kind of network is it?
  • cannot tell you that packets will arrive…
  • if network changes, finish what you’re doing, close connection and start a new one
• use SCNetworkReachabilityRef and check for flags
  • but again use callback APIs since the calls take about 30 seconds
• no point using this to pre-flight as things may change
  • just use to monitor
• can set UIRequiresPersistentWiFi key
  • keeps WiFi live while your app is live
  • shows up UI elements automatically (like offering to turn off Airplane mode)
  • but if the device sleeps, you still lose your WiFi connection

battery use

• listening is cheap, transmitting is expensive
• 3GS sending leaves the antenna in high powered state for 5 seconds
  • so don’t send every 5 seconds — it will leave it in high power continuously
• compress data, send large chunks
• tear down connections when reachability changes

other tips

• try to use the cache as much as possible
• try to resume downloads rather than restart
• use pipelining if poss.
  • ask for lots of things at once and be careful about return order
  • some web proxies and apache configs just break pipelining
• also use multiple connections
• but different network conditions and different servers require different tuning
• test in areas where you know the network is flaky!
• use a packet analyzer — see QA1176
• try to isolate the user from network problems
  • often there’s a transient glitch — try again before alerting the user
• see also Technote TN2152
• look for developer forum postings by eskimo1 :-)

there’s some new sample code available:

• SimpleNetworkStreams
• SimpleURLConnection
• SimpleFTPSample
• Reachability — there’s a new version! The old version had bad code :-)
• BonjourWeb
• WiTap — peer to peer, but doesn’t use GameKit
• GKTank
• GKRocket — work in progress, with voice chat too