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 AssessmentSANS 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)|
Loading up the packet capture in Wireshark, I applied a display filter to include traffic only from or to my address:
ip.addr eq 172.16.0.114Using 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;
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):
|184.108.40.206||No DNS Name||Apple, Inc system in the 17/8 netblock|
|220.127.116.11||init.ess.apple.com.edgesuite.net||An Akamai server, a239.da1.akamai.net|
|18.104.22.168||crl.verisign.net||Verisign's CRL server|
|22.214.171.124||No DNS Name||Apple, Inc system in the 17/8 netblock|
|126.96.36.199||No DNS Name||Apple, Inc system in the 17/8 netblock|
|188.8.131.52||No DNS Name||Apple, Inc system in the 17/8 netblock|
|184.108.40.206||cdns1.cox.net||My ISP's DNS server|
|220.127.116.11||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:
|1||172.16.0.114 -> 18.104.22.168||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 -> 22.214.171.124||Several UDP connections from the iPhone 4g to Apple's server over UDP/59007.|
|3||172.16.0.114 -> 126.96.36.199||More UDP traffic to a host with the next 4th octet over UDP/59007|
|4||172.16.0.114 -> 188.8.131.52||HTTP traffic to the Akamai server over XML, retrieving certificate information from Apple's servers.|
|5||172.16.0.114 -> 184.108.40.206||HTTPS traffic to an Apple server.|
|6||172.16.0.114 -> 220.127.116.11||UDP STUN traffic to an Apple server for NAT traversal.|
|7||18.104.22.168 -> 172.16.0.114||UDP SIP traffic from Apple revealing phone numbers, among other details.|
|8||22.214.171.124 -> 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.|
SummaryBased 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.