Fast decoding of tagged message formats

Many important protocols, such as Q.2931 or any protocol based on the ASN.1 basic encoding rules, are transmitted using tagged message formats, in which a message can be considered as a sequence of interleaved tag and data fields, where tag fields define the meaning of subsequent fields. These messages are computationally expensive to decode, partly because decoding each data field requires resting one or more tag fields. Evidence suggests that in some applications, although the potential space of message encodings may be very large, only a small number of message layouts are seen frequently, and thus some of the work required in decoding can be amortized over many messages. This paper analyzes the use of run-time code generation to generate optimized decoding instruction sequences for received messages matching previously observed layouts, and describes a prototype system that applies the techniques to decoding the Q.2931 and ASN.1 BER protocols. In the average case, substantial performance gains are seen