Thus spake "Vadim Antonov" <avg@kotovnik.com>
In fact, a method to encrypt small parcels of data efficiently is well-known for decades. It is called "stream cypher" (surprise). Besides LFSR-based and other stream cyphers, any block cypher can be used in this mode. Its application to RTP is trivial and straight-forward. Just leave sequence number in clear text, so that position in the stream is recoverable in case of packet loss.
Most stream modes are chained in some way to intentionally disrupt decryption if part of the ciphertext is missing; that is why IPsec resets the stream for each packet (currently). When NIST was standardizing AES, they added CTR mode specifically to address IPsec implementations. I think there's already been a draft out of the IRTF on how to modify IPsec for this, but it's not something I've followed closely.
It also allows precomputation of the key stream, adding nearly zero latency/jitter to the actual packet processing.
You fail to note that this requires precomputing and storing a keystream for every SA on the encrypting device, which often number in the thousands. This isn't feasible in a software implementation, and it's unnecessary in hardware. S Stephen Sprunk "God does not play dice." --Albert Einstein CCIE #3723 "God is an inveterate gambler, and He throws the K5SSS dice at every possible opportunity." --Stephen Hawking