┌──────────────────────────────────────────────┐
│ headroom │   packet data   │   tailroom      │
└──────────────────────────────────────────────┘
                    tail →

tailroom is the contiguous region after packet data in a packet buffer.

it is used to append headers or trailers without reallocating memory.

RX:
[ alloc ]
┌──────────────────────────────┐
│ head | data | tailroom       │
└──────────────────────────────┘

TX:
push()
┌──────────────────────────────┐
│ head | expanded data | tail  │
└──────────────────────────────┘

used in linux skb, xdp, af_xdp, dpdk.

if tailroom is wrong you corrupt memory or fall off fast path.

// idea
data + tailroom == in-place packet growth

move pointers, not packets.