Tracking P4 Program Execution in the Data Plane

While programmable switches provide operators with much-needed control over the network, they also increase the potential sources of packet-processing errors. Bugs can happen anywhere: in the P4 program, the controller installing rules into tables, or the compiler that maps the P4 program into the resource-constrained switch pipelines. Most of these bugs manifest themselves after certain sequences of packets with certain combinations of rules in the tables. Tracking each packet's execution path through the P4 program, i.e., the sequence of tables hit and the actions applied, directly in the data plane is useful in localizing such bugs as they occur in real time. The fact that programmable data planes require P4 programs to be loop-free and can perform simple integer arithmetic operations makes them amenable to Ball-Larus encoding, a well-known technique in profiling execution paths in software programs that can efficiently encode all N paths in a single [log(N)]-bit variable. However, for real-world P4 programs, the path variable can get quite large, making it inefficient for integer arithmetic at line rate. Moreover, the encoding could require a subset of tables, that would otherwise have no data dependency, to update the same variable. By carefully breaking up the P4 program into disjoint partitions and tracking each partition's execution path separately, we show how to minimally augment P4 programs to track the execution path of each packet.