Special Look: Face Time (part 1: Introduction)

Facetime Introduction

With the iPhone 4g, video chat through Facetime is a reality in a mobile device. As a frequent traveler, I use Skype on my laptop or netbook to stay in touch with family and friends, but it usually requires some planning and coordination. With Facetime, we can initiate a voice call over the cellular network, then switch to video on demand, when WiFi service is also available (which hopefully not be a requirement in the future).

As a packet junkie, I find the concept of Facetime very interesting. The intended usage for Facetime, as described by SteveEsteban, is for a user to place a call over the cellular network with the freedom to switch to video, then back and forth as desired. Focusing on the network protocol components, there are several interesting challenges:

• Device capabilities negotiation and call setup over WiFi;
• Video content streaming between devices;
  
• Authorization to accept the video stream by recipient;
• NAT traversal for users behind a WiFi NAT interface;
• Binding between GSM and WiFi traffic to mitigate spoofing attacks.

Knowing this, a lot of interesting questions come to mind. How is the management and streaming traffic protected? How is the call authorized by the end-user? What can we deduce by sniffing the WiFi-side of a Facetime transaction?

In this multi-part series, we'll look at how the Facetime protocol works, answering these and other questions while looking at tools and techniques for network protocol analysis. It's my hope that you'll learn about the Facetime protocol by reading this series, and furthermore, be able to apply these techniques to other protocols as well.

High-Level Assessment

To assess the protocol, I've taken several packet captures from my unencrypted wireless network, c alling 888-Facetime (Apple's service for customers to try out Facetime) and a colleague at the SANS Institute. Most of the analysis will be on the call to 888-Facetime, though I'll introduce other packet captures as needed.

The Facetime call with 888-Facetime was initiated by Apple's representative, which I'll herein refer to as an "inbound" session, due to the differences in Facetime calls in the role of initiator or responder. The details of my iPhone 4g are as follows:

iOS Version:	4.0 (8A293)
IP Address:	172.16.0.114
MAC Address:	5c:59:48:02:8a:65

My AP was running in 802.11b mode (for simplifying the packet capture process), also acting as a NAT at 172.16.0.1.

Loading up the packet capture in Wireshark, I applied a display filter to include traffic only from or to my address:

ip.addr eq 172.16.0.114

Using Wireshark's Protoco l Hierarchy summary (Statistics | Protocol Hierarchy), we can get a quick look at all the protocols in this 28,034 packet capture file, as shown.

Besides the low-layer protocols, we can see different activity here:

• UDP DNS traffic (to be expected);
• Session Traversal Utilities for NAT (STUN);
  
• Session Initiation Protocol (SIP);
  
• Lots of unrecognized UDP data packets;
• HTTP traffic transmitting XML data;
  
• HTTPS traffic;
  
• Unrecognized TCP traffic;
  
• ICMP.
  

Wireshark doesn't give us the option to sort this traffic view by time, but we can switch to the Conversations view (Statistics | Conversations) to view time-relative data by protocol, as shown (TCP first, then UDP):

We can see a few nodes are involved here:

Address	Name	Note
17.149.36.103	No DNS Name	Apple, Inc system in the 17/8 netblock
72.215.224.43	init.ess.apple.com.edgesuite.net	An Akamai server, a239.da1.akamai.net
199.7.52.190	crl.verisign.net	Verisign's CRL server
17.155.4.14	No DNS Name	Apple, Inc system in the 17/8 netblock
17.155.5.251	No DNS Name	Apple, Inc system in the 17/8 netblock
17.155.5.252	No DNS Name	Apple, Inc system in the 17/8 netblock
68.105.28.11	cdns1.cox.net	My ISP's DNS server
17.109.28.227	No DNS Name	Apple, Inc system in the 17/8 netblock

Using the timing and address information, we can construct a timeline of what happens in this session:

Step	Nodes	Description
1	172.16.0.114 -> 17.149.36.103	The iPhone 4g initiates a TCP session to the remote host over TCP/5223. Wireshark does not have a dissector for this protocol, though it believes the port number is associated with the HP Virtual Group protocol.
2	172.16.0.114 -> 17.155.5.251	Several UDP connections from the iPhone 4g to Apple's server over UDP/59007.
3	172.16.0.114 -> 17.155.5.252	More UDP traffic to a host with the next 4th octet over UDP/59007
4	172.16.0.114 -> 72.215.224.43	HTTP traffic to the Akamai server over XML, retrieving certificate information from Apple's servers.
5	172.16.0.114 -> 17.155.4.14	HTTPS traffic to an Apple server.
6	172.16.0.114 -> 17.109.28.227	UDP STUN traffic to an Apple server for NAT traversal.
7	17.109.28.227 -> 172.16.0.114	UDP SIP traffic from Apple revealing phone numbers, among other details.
8	17.155.5.14 -> 172.16.0.114	UDP traffic over port 16402; making up the majority of the packet capture data, this is likely the video stream information which continues until a SIP BYE message is observed.

Summary

Based on this analysis we can determine several critical pieces of how Facetime works:

• Unknown TCP protocol starts the conversation, likely initiated following an event that starts on the GSM network;
  
• Unknown UDP traffic between two hosts with similar IP addresses;
  
• Certificate validation through an Akamai server, followed by an HTTPS request to an Apple server;
  
• STUN traffic for NAT traversal;
  
• SIP traffic for call setup and negotiation;
  
• UDP stream data for video/audio.

In the next part of this series,we'll spend some more time look at the SIP and video/audio streaming traffic and look at some tools we can use to extract that data. Stay tuned!

-Josh