diff options
| -rw-r--r-- | filter/trie.c | 113 |
1 files changed, 69 insertions, 44 deletions
diff --git a/filter/trie.c b/filter/trie.c index cf805afc..dbed5ace 100644 --- a/filter/trie.c +++ b/filter/trie.c @@ -1,7 +1,8 @@ /* * Filters: Trie for prefix sets * - * Copyright 2009 Ondrej Zajicek <santiago@crfreenet.org> + * (c) 2009--2020 Ondrej Zajicek <santiago@crfreenet.org> + * (c) 2009--2020 CZ.NIC z.s.p.o. * * Can be freely distributed and used under the terms of the GNU GPL. */ @@ -9,53 +10,68 @@ /** * DOC: Trie for prefix sets * - * We use a (compressed) trie to represent prefix sets. Every node - * in the trie represents one prefix (&addr/&plen) and &plen also - * indicates the index of the bit in the address that is used to - * branch at the node. If we need to represent just a set of - * prefixes, it would be simple, but we have to represent a - * set of prefix patterns. Each prefix pattern consists of - * &ppaddr/&pplen and two integers: &low and &high, and a prefix - * &paddr/&plen matches that pattern if the first MIN(&plen, &pplen) - * bits of &paddr and &ppaddr are the same and &low <= &plen <= &high. + * We use a (compressed) trie to represent prefix sets. Every node in the trie + * represents one prefix (&addr/&plen) and &plen also indicates the index of + * bits in the address that are used to branch at the node. Note that such + * prefix is not necessary a member of the prefix set, it is just a canonical + * prefix associated with a node. Prefix lengths of nodes are aligned to + * multiples of &TRIE_STEP (4) and there is 16-way branching in each + * node. Therefore, we say that a node is associated with a range of prefix + * lengths (&plen .. &plen + TRIE_STEP - 1). * - * We use a bitmask (&accept) to represent accepted prefix lengths - * at a node. As there are 33 prefix lengths (0..32 for IPv4), but - * there is just one prefix of zero length in the whole trie so we - * have &zero flag in &f_trie (indicating whether the trie accepts - * prefix 0.0.0.0/0) as a special case, and &accept bitmask + * The prefix set is not just a set of prefixes, it is defined by a set of + * prefix patterns. Each prefix pattern consists of &ppaddr/&pplen and two + * integers: &low and &high. The tested prefix &paddr/&plen matches that pattern + * if the first MIN(&plen, &pplen) bits of &paddr and &ppaddr are the same and + * &low <= &plen <= &high. + * + * There are two ways to represent accepted prefixes for a node. First, there is + * a bitmask &local, which represents independently all 15 prefixes that extend + * the canonical prefix of the node and are within a range of prefix lengths + * associated with the node. E.g., for node 10.0.0.0/8 they are 10.0.0.0/8, + * 10.0.0.0/9, 10.128.0.0/9, .. 10.224.0.0/11. This order (first by length, then + * lexicographically) is used for indexing the bitmask &local, starting at + * position 1. I.e., index is 2^(plen - base) + offset within the same length, + * see function trie_local_mask6() for details. + * + * Second, we use a bitmask &accept to represent accepted prefix lengths at a + * node. The bit is set means that all prefixes of given length that are either + * subprefixes or superprefixes of the canonical prefix are accepted. As there + * are 33 prefix lengths (0..32 for IPv4), but there is just one prefix of zero + * length in the whole trie so we have &zero flag in &f_trie (indicating whether + * the trie accepts prefix 0.0.0.0/0) as a special case, and &accept bitmask * represents accepted prefix lengths from 1 to 32. * - * There are two cases in prefix matching - a match when the length - * of the prefix is smaller that the length of the prefix pattern, - * (&plen < &pplen) and otherwise. The second case is simple - we - * just walk through the trie and look at every visited node - * whether that prefix accepts our prefix length (&plen). The - * first case is tricky - we don't want to examine every descendant - * of a final node, so (when we create the trie) we have to propagate - * that information from nodes to their ascendants. + * One complication is handling of prefix patterns with unaligned prefix length. + * When such pattern is to be added, we add a primary node above (with rounded + * down prefix length &nlen) and a set of secondary nodes below (with rounded up + * prefix lengths &slen). Accepted prefix lengths of the original prefix pattern + * are then represented in different places based on their lengths. For prefixes + * shorter than &nlen, it is &accept bitmask of the primary node, for prefixes + * between &nlen and &slen - 1 it is &local bitmask of the primary node, and for + * prefixes longer of equal &slen it is &accept bitmasks of secondary nodes. * - * Suppose that we have two masks (M1 and M2) for a node. Mask M1 - * represents accepted prefix lengths by just the node and mask M2 - * represents accepted prefix lengths by the node or any of its - * descendants. Therefore M2 is a bitwise or of M1 and children's - * M2 and this is a maintained invariant during trie building. - * Basically, when we want to match a prefix, we walk through the trie, - * check mask M1 for our prefix length and when we came to - * final node, we check mask M2. + * There are two cases in prefix matching - a match when the length of the + * prefix is smaller that the length of the prefix pattern, (&plen < &pplen) and + * otherwise. The second case is simple - we just walk through the trie and look + * at every visited node whether that prefix accepts our prefix length (&plen). + * The first case is tricky - we do not want to examine every descendant of a + * final node, so (when we create the trie) we have to propagate that + * information from nodes to their ascendants. * - * There are two differences in the real implementation. First, - * we use a compressed trie so there is a case that we skip our - * final node (if it is not in the trie) and we came to node that - * is either extension of our prefix, or completely out of path - * In the first case, we also have to check M2. + * There are two kinds of propagations - propagation from child's &accept + * bitmask to parent's &accept bitmask, and propagation from child's &accept + * bitmask to parent's &local bitmask. The first kind is simple - as all + * superprefixes of a parent are also all superprefixes of appropriate length of + * a child, then we can just add (by bitwise or) a child &accept mask masked by + * parent prefix length mask to the parent &accept mask. This handles prefixes + * shorter than node &plen. * - * Second, we really need not to maintain two separate bitmasks. - * Checks for mask M1 are always larger than &applen and we need - * just the first &pplen bits of mask M2 (if trie compression - * hadn't been used it would suffice to know just $applen-th bit), - * so we have to store them together in &accept mask - the first - * &pplen bits of mask M2 and then mask M1. + * The second kind of propagation is necessary to handle superprefixes of a + * child that are represented by parent &local mask - that are in the range of + * prefix lengths associated with the parent. For each accepted (by child + * &accept mask) prefix length from that range, we need to set appropriate bit + * in &local mask. See function trie_amask_to_local() for details. * * There are four cases when we walk through a trie: * @@ -65,8 +81,7 @@ * - we are beyond the end of path (node length > &plen) * - we are still on path and keep walking (node length < &plen) * - * The walking code in trie_match_prefix() is structured according to - * these cases. + * The walking code in trie_match_net() is structured according to these cases. */ #include "nest/bird.h" @@ -166,6 +181,10 @@ attach_node(struct f_trie_node *parent, struct f_trie_node *child, int v4) } +/* + * Compute appropriate mask representing prefix px/plen in local bitmask of node + * with prefix length nlen. Assuming that nlen <= plen < (nlen + TRIE_STEP). + */ static inline uint trie_local_mask4(ip4_addr px, uint plen, uint nlen) { @@ -182,6 +201,12 @@ trie_local_mask6(ip6_addr px, uint plen, uint nlen) return 1u << pos; } +/* + * Compute an appropriate local mask (for a node with prefix length nlen) + * representing prefixes of px that are accepted by amask and fall within the + * range associated with that node. Used for propagation of child accept mask + * to parent local mask. + */ static inline uint trie_amask_to_local(ip_addr px, ip_addr amask, uint nlen) { |