A Policy-aware Switching Layer for Data Centers

This paper proposes a switching architecture to ensure that some traffic is always routed through certain middleboxes. Given some collection of middleboxes of various types, the administrator specifies that the traffic matching some (source interface, source address/port, destination address/port) pattern must be forwarded through a certain type of middlebox (or routed to their final destination). Chains of middleboxes are specified by using a prior middlebox as the source interface type.
Switches along the paths of packets handle traffic that needs to be routed through a middlebox by encapsulating the ethernet frame in another ethernet frame destined for the middlebox. The router adjacent to the middlebox chosen removes this encapsulation, allowing the middlebox to act as if it were on the actual path.

The primary challenges of implementing this scheme comes with handling changes. To ensure compliance during a policy change without being able to mark packets passing through middleboxes, the authors propose dividing a change from policy P1 to P2 into multiple steps. First, each type of middlebox (relevant to the policy change) is split into two types (say, old and new) of middleboxes. Then (after the new-type middleboxes have processed all old traffic), the new policy is installed replacing the old type with the new type everywhere. Finally, after the new policy has been installed on all switches (and the old type middleboxes have processed all pending traffic), all middleboxes of the old type are switched to middleboxes of the new type.

The primary disadvantage of this routing scheme is latency. Compared to the traditional approach of placing middleboxes directly in the natural path of the packets, this scheme imposes an extra approx. 0.3 ms (based on the authors' testbed) of latency. The authors argue that this difference is insignificant given the general low-latency of the environment, but, for data-center-internal traffic, this may double the latency, which is very likely to substantially affect performance.