Lesson 06 · A length-prefixed label

A length-prefixed label

Domain names on the wire are not dotted strings - they are sequences of length-prefixed labels. Today you encode a single label, the building block of every name, by putting its byte count in front of its characters.

The goal

Encode one label as its length byte followed by its ASCII characters.

Start here - the target

TO DO

Scenario: A label is its length followed by its bytes

Giventhe label text "example"

Whenit is encoded as a label

Thenthe result is the 8 bytes 07 65 78 61 6d 70 6c 65 (a length byte 7, then the ASCII of example)

Andthe label "com" encodes to 03 63 6f 6d

Background

A DNS name like www.example.com is never sent as text with dots. Instead it is a run of labels, and each label is a single length byte (how many characters follow) then those characters as raw ASCII. The dots are implied by where one label ends and the next begins. So example becomes the byte 7 followed by the seven bytes of example.

This length-prefix framing is what lets a parser walk a name without needing a delimiter: read one byte, then read that many characters, repeat. A single label’s length must fit in six bits (at most 63), because the top two bits of a length byte are reserved for something you will meet in chapter three. Today just nail the one-label case; stringing labels together into a full name is next.

Make it work

func encodeLabel(s string) []byte {
  out := []byte{byte(len(s))} // the length prefix
  return append(out, s...)    // then the raw characters
}
// "example" -> 07 'e' 'x' 'a' 'm' 'p' 'l' 'e'

CheckpointDONE

You can encode a single length-prefixed label. Commit and stop here.