Each transmitted packet has niceness level, as Unix has
command for controlling processes priority. Higher nicer level means
that packet is "nicer" and allows other to bypass him – that means
lower transmission precedence.
Send big files with higher nice level! That will guarantee you that higher priority packets, like mail messages, will pass first, even when lower priority packet was already been partially downloaded.
There are default niceness levels built-in for nncp-exec, nncp-file and nncp-freq commands. But pay attention that it can give information about underlying payload to the adversary!
There are 1-255 niceness levels. They could be specified either as integer, or using aliases with delta modifiers:
High priority. Command execution/mail use that priority by default.
Normal priority. File requests use that priority by default.
Bundles shipped on a "least effort" basis. File transmission use that priority by default.
1: F-31 65: P-31 129: N-31 193: B-31 2: F-30 66: P-30 130: N-30 194: B-30 ... ... ... ... 32: F 96: P 160: N 224: B 33: F+1 97: P+1 161: N+1 225: B+1 34: F+2 98: P+2 162: N+2 226: B+2 ... ... ... ... 64: F+32 128: P+32 192: N+32 255: B+31 | MAX
Precedence could be specified both with single-letter aliases and with whole strings. They are case insensitive. MAX is an alias for 255 niceness level.