Olivier Bonaventure

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Is your network ready for iOS7 and Multipath TCP ?

During the last days, millions of users have installed iOS7 on their iphones and ipad. Estimates published by The Guardian reveal that more than one third of the users have already upgraded their devices to support the new release. As I still don’t use a smartphone, I usually don’t check these new software releases. From a networking viewpoint, this iOS update is different because it is the first step towards a wide deployment of Multipath TCP [RFC 6824]. Until now, Multipath TCP has mainly been used by researchers. With iOS7, the situation changes since millions of devices are capable of using Multipath TCP.

From a networking viewpoint, the deployment of Multipath TCP is an important change that will affect many network operators. In the 20th century, networks were only composed of routers and switches. These devices are completely transparent to TCP and never change any field of the TCP header or payload. Today’s networks, mainly enterprise and cellular networks are much more complex. They include various types of middleboxes that process the IP header but also analyze the TCP headers and payload and sometimes modify them for various. Michio Honda an his colleagues presented at IMC2011 a paper that reveals the impact of these middleboxes on TCP and its extensibility. In a nutshell, this paper revealed the following behaviors :

  • some middleboxes drop TCP options that they do not understand
  • some middleboxes replace TCP options by dummy options
  • some middleboxes change fields of the TCP header (source and destination ports for NAT, but also sequence/acknowledgement numbers, window fields, ...)
  • some middleboxes inspect the payload of TCP segments, reject out-of-sequence segments and sometimes modify the TCP payload (e.g. ALG for ftp on NAT)

These results had a huge influence on the design of Multipath TCP that includes various mechanisms that enable it to work around most of these middleboxes and fallback to regular TCP in case of problems (e.g. payload modifications) to preserve connectivity.

Of course, Multipath TCP will achieve the best performance when running in a network which is fully transparent and does not include middleboxes that interfere with it. Network operators might have difficulties to check the possible interference between their devices and TCP extensions like Multipath TCP. While implementing Multipath TCP in the Linux kernel, we spent a lot of time understanding the interference caused by our standard firewall that randomizes TCP sequence numbers.

To support network operators who want to check the transparency of their network, we have recently released a new open-source software called tracebox. tracebox is described in a forthcoming paper that will be presented at IMC2013.

In a nutshell, tracebox can be considered as an extension to traceroute. Like traceroute, it allows to discover devices in a network. However, while traceroute only detects IP routers, tracebox is able to detect any type of middlebox that modify some fields of the network or transport header. tracebox can be used as a command-line tool but also includes a scripting language that allows operators to develop more complex tests.

For example, tracebox can be used to verify that a path is transparent for Multipath TCP as shown below

# tracebox -n -p IP/TCP/MSS/MPCAPABLE/WSCALE bahn.de
tracebox to (bahn.de): 64 hops max
1: IP::CheckSum
2: IP::TTL IP::CheckSum
3: IP::TTL IP::CheckSum
4: IP::TTL IP::CheckSum
5: IP::TTL IP::CheckSum
6: IP::TTL IP::CheckSum
7: IP::TTL IP::CheckSum
8: IP::TTL IP::CheckSum
9: IP::TTL IP::CheckSum
10: TCP::CheckSum IP::TTL IP::CheckSum TCPOptionMaxSegSize::MaxSegSize -TCPOptionMPTCPCapable -TCPOptionWindowScale

At each hop, tracebox verifies which fields of the IP/TCP headers have been modified. In the trace above, tracebox sends a SYN TCP segment on port 80 that contains MSS, MP_CAPABLE and WSCALE option. The last hop corresponds to a middlebox that changes the MSS option and removes the MP_CAPABLE and WSCALE option. Thanks to the flexibility of tracebox, it is possible to use it to detect almost any type of middlebox interference.

You can use it on Linux and MacOS to verify whether the network that you use is fully transparent to TCP. If not, tracebox will point you to the offending middlebox.