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