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ReStructuredText
294 lines
12 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=========================
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Resilient Next-hop Groups
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=========================
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Resilient groups are a type of next-hop group that is aimed at minimizing
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disruption in flow routing across changes to the group composition and
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weights of constituent next hops.
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The idea behind resilient hashing groups is best explained in contrast to
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the legacy multipath next-hop group, which uses the hash-threshold
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algorithm, described in RFC 2992.
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To select a next hop, hash-threshold algorithm first assigns a range of
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hashes to each next hop in the group, and then selects the next hop by
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comparing the SKB hash with the individual ranges. When a next hop is
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removed from the group, the ranges are recomputed, which leads to
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reassignment of parts of hash space from one next hop to another. RFC 2992
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illustrates it thus::
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+-------+-------+-------+-------+-------+
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| 1 | 2 | 3 | 4 | 5 |
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+-------+-+-----+---+---+-----+-+-------+
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| 1 | 2 | 4 | 5 |
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+---------+---------+---------+---------+
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Before and after deletion of next hop 3
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under the hash-threshold algorithm.
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Note how next hop 2 gave up part of the hash space in favor of next hop 1,
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and 4 in favor of 5. While there will usually be some overlap between the
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previous and the new distribution, some traffic flows change the next hop
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that they resolve to.
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If a multipath group is used for load-balancing between multiple servers,
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this hash space reassignment causes an issue that packets from a single
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flow suddenly end up arriving at a server that does not expect them. This
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can result in TCP connections being reset.
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If a multipath group is used for load-balancing among available paths to
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the same server, the issue is that different latencies and reordering along
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the way causes the packets to arrive in the wrong order, resulting in
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degraded application performance.
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To mitigate the above-mentioned flow redirection, resilient next-hop groups
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insert another layer of indirection between the hash space and its
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constituent next hops: a hash table. The selection algorithm uses SKB hash
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to choose a hash table bucket, then reads the next hop that this bucket
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contains, and forwards traffic there.
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This indirection brings an important feature. In the hash-threshold
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algorithm, the range of hashes associated with a next hop must be
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continuous. With a hash table, mapping between the hash table buckets and
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the individual next hops is arbitrary. Therefore when a next hop is deleted
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the buckets that held it are simply reassigned to other next hops::
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|1|1|1|1|2|2|2|2|3|3|3|3|4|4|4|4|5|5|5|5|
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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v v v v
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|1|1|1|1|2|2|2|2|1|2|4|5|4|4|4|4|5|5|5|5|
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Before and after deletion of next hop 3
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under the resilient hashing algorithm.
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When weights of next hops in a group are altered, it may be possible to
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choose a subset of buckets that are currently not used for forwarding
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traffic, and use those to satisfy the new next-hop distribution demands,
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keeping the "busy" buckets intact. This way, established flows are ideally
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kept being forwarded to the same endpoints through the same paths as before
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the next-hop group change.
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Algorithm
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---------
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In a nutshell, the algorithm works as follows. Each next hop deserves a
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certain number of buckets, according to its weight and the number of
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buckets in the hash table. In accordance with the source code, we will call
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this number a "wants count" of a next hop. In case of an event that might
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cause bucket allocation change, the wants counts for individual next hops
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are updated.
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Next hops that have fewer buckets than their wants count, are called
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"underweight". Those that have more are "overweight". If there are no
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overweight (and therefore no underweight) next hops in the group, it is
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said to be "balanced".
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Each bucket maintains a last-used timer. Every time a packet is forwarded
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through a bucket, this timer is updated to current jiffies value. One
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attribute of a resilient group is then the "idle timer", which is the
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amount of time that a bucket must not be hit by traffic in order for it to
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be considered "idle". Buckets that are not idle are busy.
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After assigning wants counts to next hops, an "upkeep" algorithm runs. For
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buckets:
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1) that have no assigned next hop, or
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2) whose next hop has been removed, or
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3) that are idle and their next hop is overweight,
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upkeep changes the next hop that the bucket references to one of the
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underweight next hops. If, after considering all buckets in this manner,
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there are still underweight next hops, another upkeep run is scheduled to a
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future time.
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There may not be enough "idle" buckets to satisfy the updated wants counts
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of all next hops. Another attribute of a resilient group is the "unbalanced
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timer". This timer can be set to 0, in which case the table will stay out
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of balance until idle buckets do appear, possibly never. If set to a
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non-zero value, the value represents the period of time that the table is
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permitted to stay out of balance.
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With this in mind, we update the above list of conditions with one more
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item. Thus buckets:
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4) whose next hop is overweight, and the amount of time that the table has
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been out of balance exceeds the unbalanced timer, if that is non-zero,
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\... are migrated as well.
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Offloading & Driver Feedback
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----------------------------
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When offloading resilient groups, the algorithm that distributes buckets
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among next hops is still the one in SW. Drivers are notified of updates to
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next hop groups in the following three ways:
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- Full group notification with the type
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``NH_NOTIFIER_INFO_TYPE_RES_TABLE``. This is used just after the group is
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created and buckets populated for the first time.
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- Single-bucket notifications of the type
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``NH_NOTIFIER_INFO_TYPE_RES_BUCKET``, which is used for notifications of
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individual migrations within an already-established group.
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- Pre-replace notification, ``NEXTHOP_EVENT_RES_TABLE_PRE_REPLACE``. This
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is sent before the group is replaced, and is a way for the driver to veto
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the group before committing anything to the HW.
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Some single-bucket notifications are forced, as indicated by the "force"
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flag in the notification. Those are used for the cases where e.g. the next
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hop associated with the bucket was removed, and the bucket really must be
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migrated.
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Non-forced notifications can be overridden by the driver by returning an
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error code. The use case for this is that the driver notifies the HW that a
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bucket should be migrated, but the HW discovers that the bucket has in fact
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been hit by traffic.
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A second way for the HW to report that a bucket is busy is through the
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``nexthop_res_grp_activity_update()`` API. The buckets identified this way
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as busy are treated as if traffic hit them.
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Offloaded buckets should be flagged as either "offload" or "trap". This is
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done through the ``nexthop_bucket_set_hw_flags()`` API.
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Netlink UAPI
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------------
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Resilient Group Replacement
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Resilient groups are configured using the ``RTM_NEWNEXTHOP`` message in the
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same manner as other multipath groups. The following changes apply to the
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attributes passed in the netlink message:
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=================== =========================================================
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``NHA_GROUP_TYPE`` Should be ``NEXTHOP_GRP_TYPE_RES`` for resilient group.
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``NHA_RES_GROUP`` A nest that contains attributes specific to resilient
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groups.
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=================== =========================================================
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``NHA_RES_GROUP`` payload:
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=================================== =========================================
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``NHA_RES_GROUP_BUCKETS`` Number of buckets in the hash table.
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``NHA_RES_GROUP_IDLE_TIMER`` Idle timer in units of clock_t.
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``NHA_RES_GROUP_UNBALANCED_TIMER`` Unbalanced timer in units of clock_t.
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=================================== =========================================
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Next Hop Get
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^^^^^^^^^^^^
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Requests to get resilient next-hop groups use the ``RTM_GETNEXTHOP``
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message in exactly the same way as other next hop get requests. The
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response attributes match the replacement attributes cited above, except
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``NHA_RES_GROUP`` payload will include the following attribute:
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=================================== =========================================
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``NHA_RES_GROUP_UNBALANCED_TIME`` How long has the resilient group been out
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of balance, in units of clock_t.
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=================================== =========================================
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Bucket Get
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^^^^^^^^^^
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The message ``RTM_GETNEXTHOPBUCKET`` without the ``NLM_F_DUMP`` flag is
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used to request a single bucket. The attributes recognized at get requests
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are:
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=================== =========================================================
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``NHA_ID`` ID of the next-hop group that the bucket belongs to.
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``NHA_RES_BUCKET`` A nest that contains attributes specific to bucket.
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=================== =========================================================
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``NHA_RES_BUCKET`` payload:
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======================== ====================================================
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``NHA_RES_BUCKET_INDEX`` Index of bucket in the resilient table.
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======================== ====================================================
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Bucket Dumps
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^^^^^^^^^^^^
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The message ``RTM_GETNEXTHOPBUCKET`` with the ``NLM_F_DUMP`` flag is used
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to request a dump of matching buckets. The attributes recognized at dump
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requests are:
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=================== =========================================================
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``NHA_ID`` If specified, limits the dump to just the next-hop group
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with this ID.
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``NHA_OIF`` If specified, limits the dump to buckets that contain
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next hops that use the device with this ifindex.
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``NHA_MASTER`` If specified, limits the dump to buckets that contain
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next hops that use a device in the VRF with this ifindex.
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``NHA_RES_BUCKET`` A nest that contains attributes specific to bucket.
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=================== =========================================================
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``NHA_RES_BUCKET`` payload:
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======================== ====================================================
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``NHA_RES_BUCKET_NH_ID`` If specified, limits the dump to just the buckets
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that contain the next hop with this ID.
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======================== ====================================================
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Usage
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-----
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To illustrate the usage, consider the following commands::
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# ip nexthop add id 1 via 192.0.2.2 dev eth0
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# ip nexthop add id 2 via 192.0.2.3 dev eth0
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# ip nexthop add id 10 group 1/2 type resilient \
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buckets 8 idle_timer 60 unbalanced_timer 300
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The last command creates a resilient next-hop group. It will have 8 buckets
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(which is unusually low number, and used here for demonstration purposes
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only), each bucket will be considered idle when no traffic hits it for at
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least 60 seconds, and if the table remains out of balance for 300 seconds,
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it will be forcefully brought into balance.
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Changing next-hop weights leads to change in bucket allocation::
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# ip nexthop replace id 10 group 1,3/2 type resilient
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This can be confirmed by looking at individual buckets::
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# ip nexthop bucket show id 10
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id 10 index 0 idle_time 5.59 nhid 1
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id 10 index 1 idle_time 5.59 nhid 1
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id 10 index 2 idle_time 8.74 nhid 2
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id 10 index 3 idle_time 8.74 nhid 2
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id 10 index 4 idle_time 8.74 nhid 1
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id 10 index 5 idle_time 8.74 nhid 1
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id 10 index 6 idle_time 8.74 nhid 1
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id 10 index 7 idle_time 8.74 nhid 1
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Note the two buckets that have a shorter idle time. Those are the ones that
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were migrated after the next-hop replace command to satisfy the new demand
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that next hop 1 be given 6 buckets instead of 4.
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Netdevsim
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---------
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The netdevsim driver implements a mock offload of resilient groups, and
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exposes debugfs interface that allows marking individual buckets as busy.
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For example, the following will mark bucket 23 in next-hop group 10 as
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active::
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# echo 10 23 > /sys/kernel/debug/netdevsim/netdevsim10/fib/nexthop_bucket_activity
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In addition, another debugfs interface can be used to configure that the
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next attempt to migrate a bucket should fail::
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# echo 1 > /sys/kernel/debug/netdevsim/netdevsim10/fib/fail_nexthop_bucket_replace
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Besides serving as an example, the interfaces that netdevsim exposes are
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useful in automated testing, and
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``tools/testing/selftests/drivers/net/netdevsim/nexthop.sh`` makes use of
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them to test the algorithm.
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