Position of sequence element depends on offset from message beginning

[treiher]

The three byte offset to next extension indicates the position of the next extension as offset from the beginning of the SLP message.

The offset value is 0 if there are no extensions following the current extension.

If the offset is 0, the length of the current Extension Data is determined by subtracting total length of the SLP message as given in the SLP message header minus the offset of the current extension.

https://tools.ietf.org/html/rfc2608

The mechanism is similar to the references/pointers used in DNS (#52).