NewAllocator initiates a []byte allocator for frames less than 65536 bytes, the waste(memory fragmentation) of space allocation is guaranteed to be no more than 50%.

MaxSegmentSize is the largest possible UDP datagram size.

io.Copy default buffer size is 32 KiB but the maximum packet size of vmess/shadowsocks is about 16 KiB so define a buffer of 20 KiB to reduce the memory of each TCP relay.

Allocator for incoming frames, optimized to prevent overwriting after zeroing.