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152 lines
5.2 KiB
Plaintext
152 lines
5.2 KiB
Plaintext
Copyright (C) 1999, 2000 Internet Software Consortium.
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See COPYRIGHT in the source root or http://www.isc.org/copyright for terms.
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Name Compression
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$Id: compression,v 1.6 2000/08/01 01:18:07 tale Exp $
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Overview.
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BIND 4.x and BIND 8.x only had one methods of compression to deal
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with 14 bit compression. BIND 9 has 3 methods of compression
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to deal with 14 bit, 16 bit and local compression (14 and 16 bit).
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In addition to this the allowed compression methods vary across
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types and across client revisions thanks to EDNS.
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To be able to compress a domain name you need to some or all of
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the following pieces of information.
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1. where the message starts.
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2. where the current rdata starts in the message (local compression).
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3. what the current owner name is (local compression).
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4. existing global 14 bit compression targets.
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5. existing global 16 bit compression targets.
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6. existing local compression targets.
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7. the current domain name.
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8. what are allowable compression methods, these are not constant
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across a message.
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BIND 4.x and BIND 8.x used a table of existing 14 bit compression
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targets.
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The implicit assumption is that we will use compression whenever
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possible and when ever there are multiple alternatives available
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we will choose the one that minimises the size of the message.
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We will need functions that determine the allowable compression
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methods, find the "best" match among the available compression
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targets, add new compression targets.
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We need to be able to back out any changes made to the compression
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targets if we are unable to add a complete RR (RRset?). This is
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only a problem for the global compression targets.
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Implementation:
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We will maintain two RBT, one for local compression targets and
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one for global compression targets. The data for these RBT will
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be the offset values. The local compression RBT only needs to
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be maintained when local compression is possible. The global
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compression RBT is maintained regardless. Unless there is a
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perfect match (or the name is ".") we will add the name to the
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compression RBTs provide the offset would not be too large for
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the valid compression methods of the RBT. All nodes of the RBT
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will have an offset excluding the root node.
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The local compression RBT will be initalised with the owner name
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and the start of the rdata will be recorded.
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We will use deepest partial match to find the potential
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compression targets.
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We only need to maintain one global RBT as 16 bit compression
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pointers are either valid or invalid for the whole message.
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dns_rdata_towire() will set the allowed methods based on the
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edns version.
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Functions:
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dns_result_t
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dns_compress_init(dns_compress_t *cctx, int edns, isc_mem_t *mctx);
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Initalises cctx to empty and sets whether 16 bit global
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compression targets are to be added to the global RBT based on the
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edns value.
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dns_result_t
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dns_compress_localinit(dns_compress_t *cctx, dns_name_t *owner,
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isc_buffer_t *target);
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Initalise a RBT for local compression, freeing and existing RBT.
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Record current offset.
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dns_compress_invalidate(dns_compress_t *cctx);
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Free any RBT's and make empty.
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dns_compress_localinvalidate(dns_compress_t *cctx);
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Free the local RBT.
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void
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dns_compress_setmethods(dns_compress_t *cctx, unsigned int allowed);
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unsigned int
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dns_compress_getmethods(dns_compress_t *cctx);
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int
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dns_compress_getedns(dns_compress_t *cctx);
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dns_result_t
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dns_name_towire(dns_name_t *name, dns_compress_t *cctx,
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isc_buffer_t *target);
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'name' contains the current name to be added to the message 'target'.
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'target' is assumed to only contain the message.
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'cctx' contains the compression context and has to hold all the
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information required that cannot be obtained from 'name' or 'target'.
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struct dns_compress {
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unsigned int allowed; /* Allowed methods. */
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unsigned int rdata; /* Start of local rdata */
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isc_boolean_t global16; /* 16 bit offsets allowed */
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dns_rbt_t *local; /* Local RBT */
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dns_rbt_t *global; /* Global RBT */
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isc_mem_t *mctx; /* Required by RBT */
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};
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sets allowed based on the value of edns.
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isc_boolean_t
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dns_compress_findglobal(dns_compress_t *cctx, dns_name_t *name,
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dns_name_t *prefix, dns_name_t *suffix,
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isc_uint16_t *offset, isc_buffer_t *workspace);
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isc_boolean_t
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dns_compress_findlocal(dns_compress_t *cctx, dns_name_t *name,
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dns_name_t *prefix, dns_name_t *suffix,
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isc_uint16_t *offset, isc_buffer_t *workspace);
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Find the best best match in the global / local RBT. Returns prefix,
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suffix and offset of the bestmatch. Findglobal(), findlocal()
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requires as workspace as it may be neccessary to spit a bit stream
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label. The result prefix will be such that it can be added to the
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wire format followed by a compression pointer pointing to offset.
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Suffix is returned so that it is possible to add the compression
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pointers via dns_compress_add().
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void
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dns_compress_add(dns_compress_t *cctx, dns_name_t *prefix,
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dns_name_t *suffix, isc_uint16_t offset);
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Add compression pointers pointing to lebels (if any) in prefix.
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The offset to the first label is passed in offset.
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Dependancy:
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Requires RBT deepest match.
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Requires the ability to walk the RBT and remove any node which
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meets the removal condition.
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