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Diffstat (limited to 'Documentation/networking/fib_trie.txt')
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diff --git a/Documentation/networking/fib_trie.txt b/Documentation/networking/fib_trie.txt deleted file mode 100644 index fe719388518b..000000000000 --- a/Documentation/networking/fib_trie.txt +++ /dev/null @@ -1,145 +0,0 @@ - LC-trie implementation notes. - -Node types ----------- -leaf - An end node with data. This has a copy of the relevant key, along - with 'hlist' with routing table entries sorted by prefix length. - See struct leaf and struct leaf_info. - -trie node or tnode - An internal node, holding an array of child (leaf or tnode) pointers, - indexed through a subset of the key. See Level Compression. - -A few concepts explained ------------------------- -Bits (tnode) - The number of bits in the key segment used for indexing into the - child array - the "child index". See Level Compression. - -Pos (tnode) - The position (in the key) of the key segment used for indexing into - the child array. See Path Compression. - -Path Compression / skipped bits - Any given tnode is linked to from the child array of its parent, using - a segment of the key specified by the parent's "pos" and "bits" - In certain cases, this tnode's own "pos" will not be immediately - adjacent to the parent (pos+bits), but there will be some bits - in the key skipped over because they represent a single path with no - deviations. These "skipped bits" constitute Path Compression. - Note that the search algorithm will simply skip over these bits when - searching, making it necessary to save the keys in the leaves to - verify that they actually do match the key we are searching for. - -Level Compression / child arrays - the trie is kept level balanced moving, under certain conditions, the - children of a full child (see "full_children") up one level, so that - instead of a pure binary tree, each internal node ("tnode") may - contain an arbitrarily large array of links to several children. - Conversely, a tnode with a mostly empty child array (see empty_children) - may be "halved", having some of its children moved downwards one level, - in order to avoid ever-increasing child arrays. - -empty_children - the number of positions in the child array of a given tnode that are - NULL. - -full_children - the number of children of a given tnode that aren't path compressed. - (in other words, they aren't NULL or leaves and their "pos" is equal - to this tnode's "pos"+"bits"). - - (The word "full" here is used more in the sense of "complete" than - as the opposite of "empty", which might be a tad confusing.) - -Comments ---------- - -We have tried to keep the structure of the code as close to fib_hash as -possible to allow verification and help up reviewing. - -fib_find_node() - A good start for understanding this code. This function implements a - straightforward trie lookup. - -fib_insert_node() - Inserts a new leaf node in the trie. This is bit more complicated than - fib_find_node(). Inserting a new node means we might have to run the - level compression algorithm on part of the trie. - -trie_leaf_remove() - Looks up a key, deletes it and runs the level compression algorithm. - -trie_rebalance() - The key function for the dynamic trie after any change in the trie - it is run to optimize and reorganize. It will walk the trie upwards - towards the root from a given tnode, doing a resize() at each step - to implement level compression. - -resize() - Analyzes a tnode and optimizes the child array size by either inflating - or shrinking it repeatedly until it fulfills the criteria for optimal - level compression. This part follows the original paper pretty closely - and there may be some room for experimentation here. - -inflate() - Doubles the size of the child array within a tnode. Used by resize(). - -halve() - Halves the size of the child array within a tnode - the inverse of - inflate(). Used by resize(); - -fn_trie_insert(), fn_trie_delete(), fn_trie_select_default() - The route manipulation functions. Should conform pretty closely to the - corresponding functions in fib_hash. - -fn_trie_flush() - This walks the full trie (using nextleaf()) and searches for empty - leaves which have to be removed. - -fn_trie_dump() - Dumps the routing table ordered by prefix length. This is somewhat - slower than the corresponding fib_hash function, as we have to walk the - entire trie for each prefix length. In comparison, fib_hash is organized - as one "zone"/hash per prefix length. - -Locking -------- - -fib_lock is used for an RW-lock in the same way that this is done in fib_hash. -However, the functions are somewhat separated for other possible locking -scenarios. It might conceivably be possible to run trie_rebalance via RCU -to avoid read_lock in the fn_trie_lookup() function. - -Main lookup mechanism ---------------------- -fn_trie_lookup() is the main lookup function. - -The lookup is in its simplest form just like fib_find_node(). We descend the -trie, key segment by key segment, until we find a leaf. check_leaf() does -the fib_semantic_match in the leaf's sorted prefix hlist. - -If we find a match, we are done. - -If we don't find a match, we enter prefix matching mode. The prefix length, -starting out at the same as the key length, is reduced one step at a time, -and we backtrack upwards through the trie trying to find a longest matching -prefix. The goal is always to reach a leaf and get a positive result from the -fib_semantic_match mechanism. - -Inside each tnode, the search for longest matching prefix consists of searching -through the child array, chopping off (zeroing) the least significant "1" of -the child index until we find a match or the child index consists of nothing but -zeros. - -At this point we backtrack (t->stats.backtrack++) up the trie, continuing to -chop off part of the key in order to find the longest matching prefix. - -At this point we will repeatedly descend subtries to look for a match, and there -are some optimizations available that can provide us with "shortcuts" to avoid -descending into dead ends. Look for "HL_OPTIMIZE" sections in the code. - -To alleviate any doubts about the correctness of the route selection process, -a new netlink operation has been added. Look for NETLINK_FIB_LOOKUP, which -gives userland access to fib_lookup(). |