bind/doc/design/compression

			Name Compression

Overview.

	BIND 4.x and BIND 8.x only had one methods of compression to deal
	with 14 bit compression.  BIND 9 has 3 methods of compression
	to deal with 14 bit, 16 bit and local compression.

	In addition to this the allowed compression methods vary across
	types and across client revisions thanks to EDNS.

	To be able to compress a domain name you need to some or all of
	the following pieces of information.

	1. where the message starts.
	2. where the current rdata starts in the message (local compression).
	3. what the current owner name is (local compression).
	4. existing global 14 bit compression targets.
	5. existing global 16 bit compression targets.
	6. existing local compression targets.
	7. the current domain name.
	8. what are allowable compression methods, these are not constant
	   across a message.

	BIND 4.x and BIND 8.x used a table of existing 14 bit compression
	targets. 

	The implicit assumption is that we will use compression whenever
	possible and when ever there are multiple alternatives available
	we will choose the one that minimises the size of the message.

	We will need functions that determine the allowable compression
	methods, find the "best" match among the available compression
	targets, add new compression targets.

	We need to be able to back out any changes made to the compression
	targets if we are unable to add a complete RR (RRset?). This is
	only a problem for the global compression targets.

Implementation:

	We will maintain two RBT, one for local compression targets and
	one for global compression targets.  The data for these RBT will
	be the offset values.  The local compression RBT only needs to
	be maintained when local compression is possible. The global
	compression RBT is maintained regardless.  Unless there is a
	perfect match (or the name is ".") we will add the name to the
	compression RBTs provide the offset would not be too large for
	the valid compression methods of the RBT.  All nodes of the RBT
	will have an offset excluding the root node.

	The local compression RBT will be initalised with the owner name
	and the start of the rdata will be recorded.

	We will use deepest partial match to find the potential
	compression targets.

	We only need to maintain one global RBT as 16 bit compression
	pointers are either valid or invalid for the whole message.

Functions:
	unsigned int
	dns_compress_allowed(dns_rdatatype_t type, int edns,
				  isc_boolean_t isowner);

	Returns allowed compression methods based on type, edns, and whether
	we are about to compress a owner name.

	void
	dns_compress_init(dns_compress_t *cctx, isc_boolean_t global16,
			  isc_mem_t *mctx);

	Initalises cctx to empty and sets whether 16 bit global
	compression targets are to be added to the global RBT.

	void
	dns_compress_localinit(dns_compress_t *cctx, dns_name_t owner,
			       isc_buffer_t *target);

	Initalise a RBT for local compression, freeing and existing RBT.
	Record current offset.

	dns_compress_invalidate(dns_compress_t *cctx);

	Free any RBT's and make empty.

	void
	dns_compress_setmethods(dns_compress_t *cctx, unsigned int allowed);

	unsigned int
	dns_compress_getmethods(dns_compress_t *cctx);

	dns_result_t
	dns_name_towire(dns_name_t *name, dns_compress_t *cctx,
			isc_buffer_t *target);

	'name' contains the current name to be added to the message 'target'.
	'target' is assumed to only contain the message.
	'cctx' contains the compression context and has to hold all the
	information required that cannot be obtained from 'name' or 'target'.

	struct dns_compress {
		unsigned int allowed;	/* Allowed methods. */
		unsigned int rdata;	/* Start of local rdata */
		isc_boolean_t global16;	/* 16 bit offsets allowed */
		dns_rbt_t *local;	/* Local RBT */
		dns_rbt_t *global;	/* Global RBT */
		isc_mem_t *mctx;	/* Required by RBT */
	};