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2. Terminology

This section explains some terms used in this document.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Subheader

  An IPv6 base header, an IPv6 extension header, an IPv4 header, a
  UDP header, or a TCP header.

Header

  A chain of subheaders.

Compress

  The act of reducing the size of a header by removing header fields
  or reducing the size of header fields. This is done in a way such
  that a decompressor can reconstruct the header if its context
  state is identical to the context state used when compressing the
  header.

Decompress

  The act of reconstructing a compressed header.

Context identifier (CID)

  A small unique number identifying the context that should be used
  to decompress a compressed header.  Carried in full headers and
  compressed headers.

Context

  The state which the compressor uses to compress a header and the
  decompressor uses to decompress a header.  The context is the
  uncompressed version of the last header sent (compressor) or
  received (decompressor) over the link, except for fields in the
  header that are included "as-is" in compressed headers or can be
  inferred from, e.g., the size of the link-level frame.

  The context for a packet stream is associated with a context
  identifier.  The context for non-TCP packet streams is also
  associated with a generation.

Generation

  For non-TCP packet streams, each new version of the context for a
  given CID is associated with a generation: a small number that is
  incremented whenever the context associated with that CID changes.
  Carried by full and compressed non-TCP headers.

Packet stream

  A sequence of packets whose headers are similar and share context.
  For example, headers in a TCP packet stream have the same source
  and final destination address, and the same port numbers in the
  TCP header.  Similarly, headers in a UDP packet stream have the
  same source and destination address, and the same port numbers in
  the UDP header.

Full header (header refresh)

  An uncompressed header that updates or refreshes the context for a
  packet stream. It carries a CID that will be used to identify the
  context.

  Full headers for non-TCP packet streams also carry the generation
  of the context they update or refresh.

Regular header

  A normal, uncompressed, header.  Does not carry CID or generation
  association.

Incorrect decompression

  When a compressed and then decompressed header is different from
  the uncompressed header. Usually due to mismatching context
  between the compressor and decompressor or bit errors during
  transmission of the compressed header.

Differential coding

  A compression technique where the compressed value of a header
  field is the difference between the current value of the field and
  the value of the same field in the previous header belonging to
  the same packet stream. A decompressor can thus obtain the value
  of the field by adding the value in the compressed header to its
  context.  This technique is used for TCP streams but not for non-
  TCP streams.