1 ACL plugin constant-time lookup design {#acl_hash_lookup}
2 ======================================
4 The initial implementation of ACL plugin performs a trivial for() cycle,
5 going through the assigned ACLs on a per-packet basis. This is not very
6 efficient, even if for very short ACLs due to its simplicity it can beat
9 However, to cover the case of longer ACLs with acceptable performance,
10 we need to have a better way of matching. This write-up proposes
11 a mechanism to make a lookup from O(M) where M is number of entries
12 to O(N) where N is number of different mask combinations.
17 The ACL plugin will maintain a global list of "mask types", i.e. the specific
18 configurations of "do not care" bits within the ACEs.
19 Upon the creation of a new ACL, a pass will be made through all the
20 ACEs, to assign and possibly allocate the "mask type number".
22 Each ACL has a structure *hash_acl_info_t* representing the "hash-based"
23 parts of information related to that ACL, primarily the array of
24 *hash_ace_info_t* structures - each of the members of that array
25 corresponding to one of the rules (ACEs) in the original ACL,
26 for this they have a pair of *(acl_index, ace_index)* to keep track,
27 predominantly for debugging.
29 Why do we need a whole separate structure, and are not adding new fields
30 to the existing rule structure? First, encapsulation, to minimize
31 the pollution of the main ACL code with the hash-based lookup artifacts.
32 Second, one rule may correspond to more than one "hash-based" ACE.
33 In fact, most of the rules do correspond to two of those. Why ?
35 Consider that the current ACL lookup logic is that if a packet
36 is not the initial fragment, and there is an L4 entry acting on the packet,
37 the comparison will be made only on the L4 protocol field value rather
38 than on the protocol and port values. This behavior is governed by
39 *l4_match_nonfirst_fragment* flag in the *acl_main*, and is needed to
40 maintain the compatibility with the existing software switch implementation.
42 While for the sequential check in *single_acl_match_5tuple()*
43 it is very easy to implement by just breaking out at the right moment,
44 in case of hash-based matching this cost us two checks:
45 one on full 5-tuple and the flag *pkt.is_nonfirst_fragment* being zero,
46 the second on 3-tuple and the flag *pkt.is_nonfirst_fragment* being one,
47 with the second check triggered by the *acl_main.l4_match_nonfirst_fragment*
48 setting being the default 1. This dictates the necessity of having a "match"
49 field in a given *hash_ace_info_t* element, which would reflect the value
50 we are supposed to match after applying the mask.
52 There can be other circumstances when it might be beneficial to expand
53 the given rule in the original ACL into multiple - for example, as an
54 optimization within the port range handling for small port ranges
55 (this is not done as of the time of writing).
57 Assigning ACLs to an interface
58 ------------------------------
60 Once the ACL list is assigned to an interface, or, rather, a new ACL
61 is added to the list of the existing ACLs applied to the interface,
62 we need to update the bihash accelerating the lookup.
64 All the entries for the lookups are stored within a single *48_8* bihash,
65 which captures the 5-tuple from the packet as well as the miscellaneous
66 per-packet information flags, e.g. *l4_valid*, *is_non_first_fragment*,
67 and so on. To facilitate the use of the single bihash by all the interfaces,
68 the *is_ip6*, *is_input*, *sw_if_index* are part of the key,
69 as well as *mask_type_index* - the latter being necessary because
70 there can be entries with the same value but different masks, e.g.:
71 `permit ::/0, permit::/128`.
73 At the moment of an ACL being applied to an interface, we need to
74 walk the list of *hash_ace_info_t* entries corresponding to that ACL,
75 and update the bihash with the keys corresponding to the match
76 values in these entries.
78 The value of the hash match contains the index into a per-*sw_if_index* vector
79 of *applied_ace_hash_entry_t* elements, as well as a couple of flags:
80 *shadowed* (optimization: if this flag on a matched entry is zero, means
81 we can stop the lookup early and declare a match - see below),
82 and *need_portrange_check* - meaning that what matched was a superset
83 of the actual match, and we need to perform an extra check.
85 Also, upon insertion, we must keep in mind there can be
86 multiple *applied_ace_hash_entry_t* for the same key and must keep
87 a list of those. This is necessary to incrementally apply/unapply
88 the ACLs as part of the ACL vector: say, two ACLs have
89 "permit 2001:db8::1/128 any" - we should be able to retain the entry
90 for the second ACL even if we have deleted the first one.
91 Also, in case there are two entries with the same key but
92 different port ranges, say 0..42 and 142..65535 - we need
93 to be able to sequentially match on those if we decide not
94 to expand them into individual port-specific entries.
99 The simple single-packet lookup is defined in
100 *multi_acl_match_get_applied_ace_index*, which returns the index
101 of the applied hash ACE if there was a match, or ~0 if there wasn't.
103 The future optimized per-packet lookup may be batched in three phases:
105 1. Prepare the keys in the per-worker vector by doing logical AND of
106 original 5-tuple record with the elements of the mask vector.
107 2. Lookup the keys in the bihash in a batch manner, collecting the
108 result with lowest u64 (acl index within vector, ACE index) from
109 the hash lookup value, and performing the list walk if necessary
111 3. Take the action from the ACL record as defined by (ACL#, ACE#) from the
112 resulting lookup winner, or, if no match found, then perform default deny.
114 Shadowed/independent/redundant ACEs
115 ------------------------------------
117 During the phase of combining multiple ACLs into one rulebase, when they
118 are applied to interface, we also can perform several optimizations.
120 If a given ACE is a strict subset of another ACE located up in the linear
121 search order, we can ignore this ACE completely - because by definition
122 it will never match. We will call such an ACE *redundant*. Here is an example:
125 permit 2001:db8:1::/48 2001:db8:2::/48 (B)
126 deny 2001:d8b:1:1::/64 2001:db8:2:1::/64 (A)
129 A bit more formally, we can define this relationship of an ACE A to ACE B as:
132 redundant(aceA, aceB) := (contains(protoB, protoA) && contains(srcB, srcA)
133 && contains(dstB, dstA) && is_after(A, B))
136 Here as "contains" we define an operation operating on the sets defined by
137 the protocol, (srcIP, srcPortDefinition) and (dstIP, dstPortDefinition)
138 respectively, and returning true if all the elements represented by
139 the second argument are represented by the first argument. The "is_after"
140 is true if A is located below B in the ruleset.
142 If a given ACE does not intersect at all with any other ACE
143 in front of it, we can mark it as such.
145 Then during the sequence of the lookups the successful hit on this ACE means
146 we do not need to look up other mask combinations - thus potentially
147 significantly speeding up the match process. Here is an example,
148 assuming we have the following ACL:
151 permit 2001:db8:1::/48 2001:db8:2::/48 (B)
152 deny 2001:db8:3::/48 2001:db8:2:1::/64 (A)
155 In this case if we match the second entry, we do not need to check whether
156 we have matched the first one - the source addresses are completely
157 different. We call such an ACE *independent* from another.
159 We can define this as
162 independent(aceA, aceB) := (!intersect(protoA, protoB) ||
163 !intersect(srcA, srcB) ||
164 !intersect(dstA, dstB))
167 where intersect is defined as operation returning true if there are
168 elements belonging to the sets of both arguments.
170 If the entry A is neither redundant nor independent from B, and is below
171 B in the ruleset, we call such an entry *shadowed* by B, here is an example:
174 deny tcp 2001:db8:1::/48 2001:db8:2::/48 (B)
175 permit 2001:d8b:1:1::/64 2001:db8:2:1::/64 (A)
178 This means the earlier rule "carves out" a subset of A, thus leaving
179 a "shadow". (Evidently, the action needs to be different for the shadow
180 to have an effect, but for for the terminology sake we do not care).
182 The more formal definition:
185 shadowed(aceA, aceB) := !redundant(aceA, aceB) &&
186 !independent(aceA, aceB) &&
190 Using this terminology, any ruleset can be represented as
191 a DAG (Directed Acyclic Graph), with the bottom being the implicit
192 "deny any", pointing to the set of rules shadowing it or the ones
195 These rules may in turn be shadowing each other. There is no cycles in
196 this graph because of the natural order of the rules - the rule located
197 closer to the end of the ruleset can never shadow or make redundant a rule
200 The optimization that enables can allow for is to skip matching certain
201 masks on a per-lookup basis - if a given rule has matched,
202 the only adjustments that can happen is the match with one of
205 Also, another avenue for the optimization can be starting the lookup process
206 with the mask type that maximizes the chances of the independent ACE match,
207 thus resulting in an ACE lookup being a single hash table hit.
213 All the new routines are located in a separate file,
214 so we can cleanly experiment with a different approach if this
215 does not fit all of the use cases.
217 The constant-time lookup within the data path has the API with
218 the same signature as:
222 multi_acl_match_5tuple (u32 sw_if_index, fa_5tuple_t * pkt_5tuple, int is_l2,
223 int is_ip6, int is_input, u32 * acl_match_p,
224 u32 * rule_match_p, u32 * trace_bitmap)
227 There should be a new upper-level function with the same signature, which
228 will make a decision whether to use a linear lookup, or to use the
229 constant-time lookup implemented by this work, or to add some other
230 optimizations (e.g. by keeping the cache of the last N lookups).
232 The calls to the routine doing preparatory work should happen
233 in `acl_add_list()` after creating the linear-lookup structures,
234 and the routine doing the preparatory work populating the hashtable
235 should be called from `acl_interface_add_del_inout_acl()` or its callees.
237 The initial implementation will be geared towards looking up a single
238 match at a time, with the subsequent optimizations possible to make
239 the lookup for more than one packet.