I assume you're coming from a hardware perspective - HDLC/PPP parsing in hardware might make sense in niche cases; though most protocols benefit much more from

I assume you're coming from a hardware perspective - HDLC/PPP parsing in hardware might make sense in niche cases; though most protocols benefit much more from the flexibility and upgrade possibilities of a software implementation. In this case, it is a messaging protocol. The incoming message essentially must be copied somewhere, therefore space must be allocated to store it, and therefore the length must be known. CRLF require either two passes (one to get string length, another to copy data) or continuously expanding storage, both of which are significantly more expensive than just parsing a short number in the beginning of the string.