Anti-replay checking with multiple sequence number spaces

Processes and systems to create a plurality of sequence number spaces in a security association at a transmission device. Each sequence number space corresponds to a respective class of traffic. Each sequence number space is identified by a unique selector value. For each sequence number space, a sequence number counter is created for counting a sequence of outbound packets of a class of traffic corresponding to the sequence number space. For an outbound packet of a particular class of traffic, a selector value of a sequence number space of the particular class of traffic is written into a first portion of a sequence number field in the outbound packet. Low-order bits of the current value of a sequence number counter, associated with the sequence number space of the particular class of traffic, is written into a second portion of the sequence number field. The sequence number counter is then incremented.