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b2f57102 RO |
1 | LC-trie implementation notes. |
2 | ||
3 | Node types | |
4 | ---------- | |
5 | leaf | |
6 | An end node with data. This has a copy of the relevant key, along | |
7 | with 'hlist' with routing table entries sorted by prefix length. | |
8 | See struct leaf and struct leaf_info. | |
9 | ||
10 | trie node or tnode | |
11 | An internal node, holding an array of child (leaf or tnode) pointers, | |
12 | indexed through a subset of the key. See Level Compression. | |
13 | ||
14 | A few concepts explained | |
15 | ------------------------ | |
16 | Bits (tnode) | |
17 | The number of bits in the key segment used for indexing into the | |
18 | child array - the "child index". See Level Compression. | |
19 | ||
20 | Pos (tnode) | |
21 | The position (in the key) of the key segment used for indexing into | |
22 | the child array. See Path Compression. | |
23 | ||
24 | Path Compression / skipped bits | |
25 | Any given tnode is linked to from the child array of its parent, using | |
26 | a segment of the key specified by the parent's "pos" and "bits" | |
27 | In certain cases, this tnode's own "pos" will not be immediately | |
28 | adjacent to the parent (pos+bits), but there will be some bits | |
29 | in the key skipped over because they represent a single path with no | |
30 | deviations. These "skipped bits" constitute Path Compression. | |
31 | Note that the search algorithm will simply skip over these bits when | |
32 | searching, making it necessary to save the keys in the leaves to | |
33 | verify that they actually do match the key we are searching for. | |
34 | ||
35 | Level Compression / child arrays | |
36 | the trie is kept level balanced moving, under certain conditions, the | |
37 | children of a full child (see "full_children") up one level, so that | |
38 | instead of a pure binary tree, each internal node ("tnode") may | |
39 | contain an arbitrarily large array of links to several children. | |
40 | Conversely, a tnode with a mostly empty child array (see empty_children) | |
41 | may be "halved", having some of its children moved downwards one level, | |
42 | in order to avoid ever-increasing child arrays. | |
43 | ||
44 | empty_children | |
45 | the number of positions in the child array of a given tnode that are | |
46 | NULL. | |
47 | ||
48 | full_children | |
49 | the number of children of a given tnode that aren't path compressed. | |
50 | (in other words, they aren't NULL or leaves and their "pos" is equal | |
51 | to this tnode's "pos"+"bits"). | |
52 | ||
53 | (The word "full" here is used more in the sense of "complete" than | |
54 | as the opposite of "empty", which might be a tad confusing.) | |
55 | ||
56 | Comments | |
57 | --------- | |
58 | ||
59 | We have tried to keep the structure of the code as close to fib_hash as | |
60 | possible to allow verification and help up reviewing. | |
61 | ||
62 | fib_find_node() | |
63 | A good start for understanding this code. This function implements a | |
64 | straightforward trie lookup. | |
65 | ||
66 | fib_insert_node() | |
67 | Inserts a new leaf node in the trie. This is bit more complicated than | |
68 | fib_find_node(). Inserting a new node means we might have to run the | |
69 | level compression algorithm on part of the trie. | |
70 | ||
71 | trie_leaf_remove() | |
72 | Looks up a key, deletes it and runs the level compression algorithm. | |
73 | ||
74 | trie_rebalance() | |
75 | The key function for the dynamic trie after any change in the trie | |
76 | it is run to optimize and reorganize. Tt will walk the trie upwards | |
77 | towards the root from a given tnode, doing a resize() at each step | |
78 | to implement level compression. | |
79 | ||
80 | resize() | |
81 | Analyzes a tnode and optimizes the child array size by either inflating | |
a2ffd275 | 82 | or shrinking it repeatedly until it fulfills the criteria for optimal |
b2f57102 RO |
83 | level compression. This part follows the original paper pretty closely |
84 | and there may be some room for experimentation here. | |
85 | ||
86 | inflate() | |
87 | Doubles the size of the child array within a tnode. Used by resize(). | |
88 | ||
89 | halve() | |
90 | Halves the size of the child array within a tnode - the inverse of | |
91 | inflate(). Used by resize(); | |
92 | ||
93 | fn_trie_insert(), fn_trie_delete(), fn_trie_select_default() | |
94 | The route manipulation functions. Should conform pretty closely to the | |
95 | corresponding functions in fib_hash. | |
96 | ||
97 | fn_trie_flush() | |
98 | This walks the full trie (using nextleaf()) and searches for empty | |
99 | leaves which have to be removed. | |
100 | ||
101 | fn_trie_dump() | |
102 | Dumps the routing table ordered by prefix length. This is somewhat | |
103 | slower than the corresponding fib_hash function, as we have to walk the | |
104 | entire trie for each prefix length. In comparison, fib_hash is organized | |
105 | as one "zone"/hash per prefix length. | |
106 | ||
107 | Locking | |
108 | ------- | |
109 | ||
110 | fib_lock is used for an RW-lock in the same way that this is done in fib_hash. | |
111 | However, the functions are somewhat separated for other possible locking | |
112 | scenarios. It might conceivably be possible to run trie_rebalance via RCU | |
113 | to avoid read_lock in the fn_trie_lookup() function. | |
114 | ||
115 | Main lookup mechanism | |
116 | --------------------- | |
117 | fn_trie_lookup() is the main lookup function. | |
118 | ||
119 | The lookup is in its simplest form just like fib_find_node(). We descend the | |
120 | trie, key segment by key segment, until we find a leaf. check_leaf() does | |
121 | the fib_semantic_match in the leaf's sorted prefix hlist. | |
122 | ||
123 | If we find a match, we are done. | |
124 | ||
125 | If we don't find a match, we enter prefix matching mode. The prefix length, | |
126 | starting out at the same as the key length, is reduced one step at a time, | |
127 | and we backtrack upwards through the trie trying to find a longest matching | |
128 | prefix. The goal is always to reach a leaf and get a positive result from the | |
129 | fib_semantic_match mechanism. | |
130 | ||
131 | Inside each tnode, the search for longest matching prefix consists of searching | |
132 | through the child array, chopping off (zeroing) the least significant "1" of | |
133 | the child index until we find a match or the child index consists of nothing but | |
134 | zeros. | |
135 | ||
136 | At this point we backtrack (t->stats.backtrack++) up the trie, continuing to | |
137 | chop off part of the key in order to find the longest matching prefix. | |
138 | ||
139 | At this point we will repeatedly descend subtries to look for a match, and there | |
140 | are some optimizations available that can provide us with "shortcuts" to avoid | |
141 | descending into dead ends. Look for "HL_OPTIMIZE" sections in the code. | |
142 | ||
143 | To alleviate any doubts about the correctness of the route selection process, | |
144 | a new netlink operation has been added. Look for NETLINK_FIB_LOOKUP, which | |
145 | gives userland access to fib_lookup(). |