1.. SPDX-License-Identifier: BSD-3-Clause 2 Copyright(c) 2010-2014 Intel Corporation. 3 4Longest Prefix Match 6 (LPM6) Library 5===================================== 6 7The LPM6 (LPM for IPv6) library component implements the Longest Prefix Match (LPM) table search method for 128-bit keys 8that is typically used to find the best match route in IPv6 forwarding applications. 9 10LPM6 API Overview 11----------------- 12 13The main configuration parameters for the LPM6 library are: 14 15* Maximum number of rules: This defines the size of the table that holds the rules, 16 and therefore the maximum number of rules that can be added. 17 18* Number of tbl8s: A tbl8 is a node of the trie that the LPM6 algorithm is based on. 19 20This parameter is related to the number of rules you can have, 21but there is no way to accurately predict the number needed to hold a specific number of rules, 22since it strongly depends on the depth and IP address of every rule. 23One tbl8 consumes 1 kb of memory. As a recommendation, 65536 tbl8s should be sufficient to store 24several thousand IPv6 rules, but the number can vary depending on the case. 25 26An LPM prefix is represented by a pair of parameters (128-bit key, depth), with depth in the range of 1 to 128. 27An LPM rule is represented by an LPM prefix and some user data associated with the prefix. 28The prefix serves as the unique identifier for the LPM rule. 29In this implementation, the user data is 21-bits long and is called "next hop", 30which corresponds to its main use of storing the ID of the next hop in a routing table entry. 31 32The main methods exported for the LPM component are: 33 34* Add LPM rule: The LPM rule is provided as input. 35 If there is no rule with the same prefix present in the table, then the new rule is added to the LPM table. 36 If a rule with the same prefix is already present in the table, the next hop of the rule is updated. 37 An error is returned when there is no available space left. 38 39* Delete LPM rule: The prefix of the LPM rule is provided as input. 40 If a rule with the specified prefix is present in the LPM table, then it is removed. 41 42* Lookup LPM key: The 128-bit key is provided as input. 43 The algorithm selects the rule that represents the best match for the given key and returns the next hop of that rule. 44 In the case that there are multiple rules present in the LPM table that have the same 128-bit value, 45 the algorithm picks the rule with the highest depth as the best match rule, 46 which means the rule has the highest number of most significant bits matching between the input key and the rule key. 47 48Implementation Details 49~~~~~~~~~~~~~~~~~~~~~~ 50 51This is a modification of the algorithm used for IPv4 (see :ref:`lpm4_details`). 52In this case, instead of using two levels, one with a tbl24 and a second with a tbl8, 14 levels are used. 53 54The implementation can be seen as a multi-bit trie where the *stride* 55or number of bits inspected on each level varies from level to level. 56Specifically, 24 bits are inspected on the root node, and the remaining 104 bits are inspected in groups of 8 bits. 57This effectively means that the trie has 14 levels at the most, depending on the rules that are added to the table. 58 59The algorithm allows the lookup operation to be performed with a number of memory accesses 60that directly depends on the length of the rule and 61whether there are other rules with bigger depths and the same key in the data structure. 62It can vary from 1 to 14 memory accesses, with 5 being the average value for the lengths 63that are most commonly used in IPv6. 64 65The main data structure is built using the following elements: 66 67* A table with 2^24 entries 68 69* A number of tables, configurable by the user through the API, with 2^8 entries 70 71The first table, called tbl24, is indexed using the first 24 bits of the IP address be looked up, 72while the rest of the tables, called tbl8s, 73are indexed using the rest of the bytes of the IP address, in chunks of 8 bits. 74This means that depending on the outcome of trying to match the IP address of an incoming packet to the rule stored in the tbl24 75or the subsequent tbl8s we might need to continue the lookup process in deeper levels of the tree. 76 77Similar to the limitation presented in the algorithm for IPv4, 78to store every possible IPv6 rule, we would need a table with 2^128 entries. 79This is not feasible due to resource restrictions. 80 81By splitting the process in different tables/levels and limiting the number of tbl8s, 82we can greatly reduce memory consumption while maintaining a very good lookup speed (one memory access per level). 83 84 85.. figure:: img/tbl24_tbl8_tbl8.* 86 87 Table split into different levels 88 89 90An entry in a table contains the following fields: 91 92* next hop / index to the tbl8 93 94* depth of the rule (length) 95 96* valid flag 97 98* valid group flag 99 100* external entry flag 101 102The first field can either contain a number indicating the tbl8 in which the lookup process should continue 103or the next hop itself if the longest prefix match has already been found. 104The depth or length of the rule is the number of bits of the rule that is stored in a specific entry. 105The flags are used to determine whether the entry/table is valid or not 106and whether the search process have finished or not respectively. 107 108Both types of tables share the same structure. 109 110The other main data structure is a table containing the main information about the rules (IP, next hop and depth). 111This is a higher level table, used for different things: 112 113* Check whether a rule already exists or not, prior to addition or deletion, 114 without having to actually perform a lookup. 115 116When deleting, to check whether there is a rule containing the one that is to be deleted. 117This is important, since the main data structure will have to be updated accordingly. 118 119Addition 120~~~~~~~~ 121 122When adding a rule, there are different possibilities. 123If the rule's depth is exactly 24 bits, then: 124 125* Use the rule (IP address) as an index to the tbl24. 126 127* If the entry is invalid (i.e. it doesn't already contain a rule) then set its next hop to its value, 128 the valid flag to 1 (meaning this entry is in use), 129 and the external entry flag to 0 (meaning the lookup process ends at this point, 130 since this is the longest prefix that matches). 131 132If the rule's depth is bigger than 24 bits but a multiple of 8, then: 133 134* Use the first 24 bits of the rule as an index to the tbl24. 135 136* If the entry is invalid (i.e. it doesn't already contain a rule) then look for a free tbl8, 137 set the index to the tbl8 to this value, 138 the valid flag to 1 (meaning this entry is in use), 139 and the external entry flag to 1 140 (meaning the lookup process must continue since the rule hasn't been explored completely). 141 142* Use the following 8 bits of the rule as an index to the next tbl8. 143 144* Repeat the process until the tbl8 at the right level (depending on the depth) has been reached 145 and fill it with the next hop, setting the next entry flag to 0. 146 147If the rule's depth is any other value, prefix expansion must be performed. 148This means the rule is copied to all the entries (as long as they are not in use) which would also cause a match. 149 150As a simple example, let's assume the depth is 20 bits. 151This means that there are 2^(24-20) = 16 different combinations of the first 24 bits of an IP address that would cause a match. 152Hence, in this case, we copy the exact same entry to every position indexed by one of these combinations. 153 154By doing this we ensure that during the lookup process, if a rule matching the IP address exists, 155it is found in, at the most, 14 memory accesses, 156depending on how many times we need to move to the next table. 157Prefix expansion is one of the keys of this algorithm, since it improves the speed dramatically by adding redundancy. 158 159Prefix expansion can be performed at any level. 160So, for example, is the depth is 34 bits, it will be performed in the third level (second tbl8-based level). 161 162Lookup 163~~~~~~ 164 165The lookup process is much simpler and quicker. In this case: 166 167* Use the first 24 bits of the IP address as an index to the tbl24. 168 If the entry is not in use, then it means we don't have a rule matching this IP. 169 If it is valid and the external entry flag is set to 0, then the next hop is returned. 170 171* If it is valid and the external entry flag is set to 1, then we use the tbl8 index to find out the tbl8 to be checked, 172 and the next 8 bits of the IP address as an index to this table. 173 Similarly, if the entry is not in use, then we don't have a rule matching this IP address. 174 If it is valid then check the external entry flag for a new tbl8 to be inspected. 175 176* Repeat the process until either we find an invalid entry (lookup miss) or a valid entry with the external entry flag set to 0. 177 Return the next hop in the latter case. 178 179Limitations in the Number of Rules 180~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 181 182There are different things that limit the number of rules that can be added. 183The first one is the maximum number of rules, which is a parameter passed through the API. 184Once this number is reached, it is not possible to add any more rules to the routing table unless one or more are removed. 185 186The second limitation is in the number of tbl8s available. 187If we exhaust tbl8s, we won't be able to add any more rules. 188How to know how many of them are necessary for a specific routing table is hard to determine in advance. 189 190In this algorithm, the maximum number of tbl8s a single rule can consume is 13, 191which is the number of levels minus one, since the first three bytes are resolved in the tbl24. However: 192 193* Typically, on IPv6, routes are not longer than 48 bits, which means rules usually take up to 3 tbl8s. 194 195As explained in the LPM for IPv4 algorithm, it is possible and very likely that several rules will share one or more tbl8s, 196depending on what their first bytes are. 197If they share the same first 24 bits, for instance, the tbl8 at the second level will be shared. 198This might happen again in deeper levels, so, effectively, 199two 48 bit-long rules may use the same three tbl8s if the only difference is in their last byte. 200 201The number of tbl8s is a parameter exposed to the user through the API in this version of the algorithm, 202due to its impact in memory consumption and the number or rules that can be added to the LPM table. 203One tbl8 consumes 1 kilobyte of memory. 204 205Use Case: IPv6 Forwarding 206------------------------- 207 208The LPM algorithm is used to implement the Classless Inter-Domain Routing (CIDR) strategy used by routers implementing IP forwarding. 209