1.. SPDX-License-Identifier: GPL-2.0 2 3========================= 4Resilient Next-hop Groups 5========================= 6 7Resilient groups are a type of next-hop group that is aimed at minimizing 8disruption in flow routing across changes to the group composition and 9weights of constituent next hops. 10 11The idea behind resilient hashing groups is best explained in contrast to 12the legacy multipath next-hop group, which uses the hash-threshold 13algorithm, described in RFC 2992. 14 15To select a next hop, hash-threshold algorithm first assigns a range of 16hashes to each next hop in the group, and then selects the next hop by 17comparing the SKB hash with the individual ranges. When a next hop is 18removed from the group, the ranges are recomputed, which leads to 19reassignment of parts of hash space from one next hop to another. RFC 2992 20illustrates it thus:: 21 22 +-------+-------+-------+-------+-------+ 23 | 1 | 2 | 3 | 4 | 5 | 24 +-------+-+-----+---+---+-----+-+-------+ 25 | 1 | 2 | 4 | 5 | 26 +---------+---------+---------+---------+ 27 28 Before and after deletion of next hop 3 29 under the hash-threshold algorithm. 30 31Note how next hop 2 gave up part of the hash space in favor of next hop 1, 32and 4 in favor of 5. While there will usually be some overlap between the 33previous and the new distribution, some traffic flows change the next hop 34that they resolve to. 35 36If a multipath group is used for load-balancing between multiple servers, 37this hash space reassignment causes an issue that packets from a single 38flow suddenly end up arriving at a server that does not expect them. This 39can result in TCP connections being reset. 40 41If a multipath group is used for load-balancing among available paths to 42the same server, the issue is that different latencies and reordering along 43the way causes the packets to arrive in the wrong order, resulting in 44degraded application performance. 45 46To mitigate the above-mentioned flow redirection, resilient next-hop groups 47insert another layer of indirection between the hash space and its 48constituent next hops: a hash table. The selection algorithm uses SKB hash 49to choose a hash table bucket, then reads the next hop that this bucket 50contains, and forwards traffic there. 51 52This indirection brings an important feature. In the hash-threshold 53algorithm, the range of hashes associated with a next hop must be 54continuous. With a hash table, mapping between the hash table buckets and 55the individual next hops is arbitrary. Therefore when a next hop is deleted 56the buckets that held it are simply reassigned to other next hops:: 57 58 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 59 |1|1|1|1|2|2|2|2|3|3|3|3|4|4|4|4|5|5|5|5| 60 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 61 v v v v 62 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 63 |1|1|1|1|2|2|2|2|1|2|4|5|4|4|4|4|5|5|5|5| 64 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 65 66 Before and after deletion of next hop 3 67 under the resilient hashing algorithm. 68 69When weights of next hops in a group are altered, it may be possible to 70choose a subset of buckets that are currently not used for forwarding 71traffic, and use those to satisfy the new next-hop distribution demands, 72keeping the "busy" buckets intact. This way, established flows are ideally 73kept being forwarded to the same endpoints through the same paths as before 74the next-hop group change. 75 76Algorithm 77--------- 78 79In a nutshell, the algorithm works as follows. Each next hop deserves a 80certain number of buckets, according to its weight and the number of 81buckets in the hash table. In accordance with the source code, we will call 82this number a "wants count" of a next hop. In case of an event that might 83cause bucket allocation change, the wants counts for individual next hops 84are updated. 85 86Next hops that have fewer buckets than their wants count, are called 87"underweight". Those that have more are "overweight". If there are no 88overweight (and therefore no underweight) next hops in the group, it is 89said to be "balanced". 90 91Each bucket maintains a last-used timer. Every time a packet is forwarded 92through a bucket, this timer is updated to current jiffies value. One 93attribute of a resilient group is then the "idle timer", which is the 94amount of time that a bucket must not be hit by traffic in order for it to 95be considered "idle". Buckets that are not idle are busy. 96 97After assigning wants counts to next hops, an "upkeep" algorithm runs. For 98buckets: 99 1001) that have no assigned next hop, or 1012) whose next hop has been removed, or 1023) that are idle and their next hop is overweight, 103 104upkeep changes the next hop that the bucket references to one of the 105underweight next hops. If, after considering all buckets in this manner, 106there are still underweight next hops, another upkeep run is scheduled to a 107future time. 108 109There may not be enough "idle" buckets to satisfy the updated wants counts 110of all next hops. Another attribute of a resilient group is the "unbalanced 111timer". This timer can be set to 0, in which case the table will stay out 112of balance until idle buckets do appear, possibly never. If set to a 113non-zero value, the value represents the period of time that the table is 114permitted to stay out of balance. 115 116With this in mind, we update the above list of conditions with one more 117item. Thus buckets: 118 1194) whose next hop is overweight, and the amount of time that the table has 120 been out of balance exceeds the unbalanced timer, if that is non-zero, 121 122\... are migrated as well. 123 124Offloading & Driver Feedback 125---------------------------- 126 127When offloading resilient groups, the algorithm that distributes buckets 128among next hops is still the one in SW. Drivers are notified of updates to 129next hop groups in the following three ways: 130 131- Full group notification with the type 132 ``NH_NOTIFIER_INFO_TYPE_RES_TABLE``. This is used just after the group is 133 created and buckets populated for the first time. 134 135- Single-bucket notifications of the type 136 ``NH_NOTIFIER_INFO_TYPE_RES_BUCKET``, which is used for notifications of 137 individual migrations within an already-established group. 138 139- Pre-replace notification, ``NEXTHOP_EVENT_RES_TABLE_PRE_REPLACE``. This 140 is sent before the group is replaced, and is a way for the driver to veto 141 the group before committing anything to the HW. 142 143Some single-bucket notifications are forced, as indicated by the "force" 144flag in the notification. Those are used for the cases where e.g. the next 145hop associated with the bucket was removed, and the bucket really must be 146migrated. 147 148Non-forced notifications can be overridden by the driver by returning an 149error code. The use case for this is that the driver notifies the HW that a 150bucket should be migrated, but the HW discovers that the bucket has in fact 151been hit by traffic. 152 153A second way for the HW to report that a bucket is busy is through the 154``nexthop_res_grp_activity_update()`` API. The buckets identified this way 155as busy are treated as if traffic hit them. 156 157Offloaded buckets should be flagged as either "offload" or "trap". This is 158done through the ``nexthop_bucket_set_hw_flags()`` API. 159 160Netlink UAPI 161------------ 162 163Resilient Group Replacement 164^^^^^^^^^^^^^^^^^^^^^^^^^^^ 165 166Resilient groups are configured using the ``RTM_NEWNEXTHOP`` message in the 167same manner as other multipath groups. The following changes apply to the 168attributes passed in the netlink message: 169 170 =================== ========================================================= 171 ``NHA_GROUP_TYPE`` Should be ``NEXTHOP_GRP_TYPE_RES`` for resilient group. 172 ``NHA_RES_GROUP`` A nest that contains attributes specific to resilient 173 groups. 174 =================== ========================================================= 175 176``NHA_RES_GROUP`` payload: 177 178 =================================== ========================================= 179 ``NHA_RES_GROUP_BUCKETS`` Number of buckets in the hash table. 180 ``NHA_RES_GROUP_IDLE_TIMER`` Idle timer in units of clock_t. 181 ``NHA_RES_GROUP_UNBALANCED_TIMER`` Unbalanced timer in units of clock_t. 182 =================================== ========================================= 183 184Next Hop Get 185^^^^^^^^^^^^ 186 187Requests to get resilient next-hop groups use the ``RTM_GETNEXTHOP`` 188message in exactly the same way as other next hop get requests. The 189response attributes match the replacement attributes cited above, except 190``NHA_RES_GROUP`` payload will include the following attribute: 191 192 =================================== ========================================= 193 ``NHA_RES_GROUP_UNBALANCED_TIME`` How long has the resilient group been out 194 of balance, in units of clock_t. 195 =================================== ========================================= 196 197Bucket Get 198^^^^^^^^^^ 199 200The message ``RTM_GETNEXTHOPBUCKET`` without the ``NLM_F_DUMP`` flag is 201used to request a single bucket. The attributes recognized at get requests 202are: 203 204 =================== ========================================================= 205 ``NHA_ID`` ID of the next-hop group that the bucket belongs to. 206 ``NHA_RES_BUCKET`` A nest that contains attributes specific to bucket. 207 =================== ========================================================= 208 209``NHA_RES_BUCKET`` payload: 210 211 ======================== ==================================================== 212 ``NHA_RES_BUCKET_INDEX`` Index of bucket in the resilient table. 213 ======================== ==================================================== 214 215Bucket Dumps 216^^^^^^^^^^^^ 217 218The message ``RTM_GETNEXTHOPBUCKET`` with the ``NLM_F_DUMP`` flag is used 219to request a dump of matching buckets. The attributes recognized at dump 220requests are: 221 222 =================== ========================================================= 223 ``NHA_ID`` If specified, limits the dump to just the next-hop group 224 with this ID. 225 ``NHA_OIF`` If specified, limits the dump to buckets that contain 226 next hops that use the device with this ifindex. 227 ``NHA_MASTER`` If specified, limits the dump to buckets that contain 228 next hops that use a device in the VRF with this ifindex. 229 ``NHA_RES_BUCKET`` A nest that contains attributes specific to bucket. 230 =================== ========================================================= 231 232``NHA_RES_BUCKET`` payload: 233 234 ======================== ==================================================== 235 ``NHA_RES_BUCKET_NH_ID`` If specified, limits the dump to just the buckets 236 that contain the next hop with this ID. 237 ======================== ==================================================== 238 239Usage 240----- 241 242To illustrate the usage, consider the following commands:: 243 244 # ip nexthop add id 1 via 192.0.2.2 dev eth0 245 # ip nexthop add id 2 via 192.0.2.3 dev eth0 246 # ip nexthop add id 10 group 1/2 type resilient \ 247 buckets 8 idle_timer 60 unbalanced_timer 300 248 249The last command creates a resilient next-hop group. It will have 8 buckets 250(which is unusually low number, and used here for demonstration purposes 251only), each bucket will be considered idle when no traffic hits it for at 252least 60 seconds, and if the table remains out of balance for 300 seconds, 253it will be forcefully brought into balance. 254 255Changing next-hop weights leads to change in bucket allocation:: 256 257 # ip nexthop replace id 10 group 1,3/2 type resilient 258 259This can be confirmed by looking at individual buckets:: 260 261 # ip nexthop bucket show id 10 262 id 10 index 0 idle_time 5.59 nhid 1 263 id 10 index 1 idle_time 5.59 nhid 1 264 id 10 index 2 idle_time 8.74 nhid 2 265 id 10 index 3 idle_time 8.74 nhid 2 266 id 10 index 4 idle_time 8.74 nhid 1 267 id 10 index 5 idle_time 8.74 nhid 1 268 id 10 index 6 idle_time 8.74 nhid 1 269 id 10 index 7 idle_time 8.74 nhid 1 270 271Note the two buckets that have a shorter idle time. Those are the ones that 272were migrated after the next-hop replace command to satisfy the new demand 273that next hop 1 be given 6 buckets instead of 4. 274 275Netdevsim 276--------- 277 278The netdevsim driver implements a mock offload of resilient groups, and 279exposes debugfs interface that allows marking individual buckets as busy. 280For example, the following will mark bucket 23 in next-hop group 10 as 281active:: 282 283 # echo 10 23 > /sys/kernel/debug/netdevsim/netdevsim10/fib/nexthop_bucket_activity 284 285In addition, another debugfs interface can be used to configure that the 286next attempt to migrate a bucket should fail:: 287 288 # echo 1 > /sys/kernel/debug/netdevsim/netdevsim10/fib/fail_nexthop_bucket_replace 289 290Besides serving as an example, the interfaces that netdevsim exposes are 291useful in automated testing, and 292``tools/testing/selftests/drivers/net/netdevsim/nexthop.sh`` makes use of 293them to test the algorithm. 294