CIRCE Technology Center The high amount of small packets present in nowaday's networks results in a low efficiency, as the size of both the headers and the payload in these packets can be comparably large, often falling within the same order of magnitude. In some situations, multiplexing (i.e. aggregating) a number of small packets into a bigger one is desirable to improve the efficiency. For example, a number of small packets can be sent together between a pair of machines if they share a common network path. This may happen between machines in different locations or even inside a datacenter with a number of servers hosting virtual machines. The traffic profile can be shifted from small to larger packets, thus reducing the network overhead and the number of packets per second to be managed by intermediate devices. This document describes Simplemux, a protocol able to encapsulate a number of packets belonging to different protocols into a single packet. Small headers (separators) are added at the beginning of each multiplexed packet, including some flags, the packet length and a "Protocol" field. The presence of this "Protocol" field allows it to aggregate packets belonging to any protocol (the "multiplexed packets"), in a single packet belonging to other (or the same) protocol (the "tunneling protocol"). To reduce the overhead, the size of the multiplexing headers is kept very low (it may be a single byte when multiplexing packets of small size).