[XFRM]: Add documentation for async events.

Documentation to describe asynchronous xfrm events to help people
writting HA code in user space.

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/Documentation/networking/xfrm_sync.txt b/Documentation/networking/xfrm_sync.txt
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+
+The sync patches work is based on initial patches from
+Krisztian <hidden@balabit.hu> and others and additional patches
+from Jamal <hadi@cyberus.ca>.
+
+The end goal for syncing is to be able to insert attributes + generate
+events so that the an SA can be safely moved from one machine to another
+for HA purposes.
+The idea is to synchronize the SA so that the takeover machine can do
+the processing of the SA as accurate as possible if it has access to it.
+
+We already have the ability to generate SA add/del/upd events.
+These patches add ability to sync and have accurate lifetime byte (to
+ensure proper decay of SAs) and replay counters to avoid replay attacks
+with as minimal loss at failover time.
+This way a backup stays as closely uptodate as an active member.
+
+Because the above items change for every packet the SA receives,
+it is possible for a lot of the events to be generated.
+For this reason, we also add a nagle-like algorithm to restrict
+the events. i.e we are going to set thresholds to say "let me
+know if the replay sequence threshold is reached or 10 secs have passed"
+These thresholds are set system-wide via sysctls or can be updated
+per SA.
+
+The identified items that need to be synchronized are:
+- the lifetime byte counter
+note that: lifetime time limit is not important if you assume the failover
+machine is known ahead of time since the decay of the time countdown
+is not driven by packet arrival.
+- the replay sequence for both inbound and outbound
+
+1) Message Structure
+----------------------
+
+nlmsghdr:aevent_id:optional-TLVs.
+
+The netlink message types are:
+
+XFRM_MSG_NEWAE and XFRM_MSG_GETAE.
+
+A XFRM_MSG_GETAE does not have TLVs.
+A XFRM_MSG_NEWAE will have at least two TLVs (as is
+discussed further below).
+
+aevent_id structure looks like:
+
+   struct xfrm_aevent_id {
+             struct xfrm_usersa_id           sa_id;
+             __u32                           flags;
+   };
+
+xfrm_usersa_id in this message layout identifies the SA.
+
+flags are used to indicate different things. The possible
+flags are:
+        XFRM_AE_RTHR=1, /* replay threshold*/
+        XFRM_AE_RVAL=2, /* replay value */
+        XFRM_AE_LVAL=4, /* lifetime value */
+        XFRM_AE_ETHR=8, /* expiry timer threshold */
+        XFRM_AE_CR=16, /* Event cause is replay update */
+        XFRM_AE_CE=32, /* Event cause is timer expiry */
+        XFRM_AE_CU=64, /* Event cause is policy update */
+
+How these flags are used is dependent on the direction of the
+message (kernel<->user) as well the cause (config, query or event).
+This is described below in the different messages.
+
+The pid will be set appropriately in netlink to recognize direction
+(0 to the kernel and pid = processid that created the event
+when going from kernel to user space)
+
+A program needs to subscribe to multicast group XFRMNLGRP_AEVENTS
+to get notified of these events.
+
+2) TLVS reflect the different parameters:
+-----------------------------------------
+
+a) byte value (XFRMA_LTIME_VAL)
+This TLV carries the running/current counter for byte lifetime since
+last event.
+
+b)replay value (XFRMA_REPLAY_VAL)
+This TLV carries the running/current counter for replay sequence since
+last event.
+
+c)replay threshold (XFRMA_REPLAY_THRESH)
+This TLV carries the threshold being used by the kernel to trigger events
+when the replay sequence is exceeded.
+
+d) expiry timer (XFRMA_ETIMER_THRESH)
+This is a timer value in milliseconds which is used as the nagle
+value to rate limit the events.
+
+3) Default configurations for the parameters:
+----------------------------------------------
+
+By default these events should be turned off unless there is
+at least one listener registered to listen to the multicast
+group XFRMNLGRP_AEVENTS.
+
+Programs installing SAs will need to specify the two thresholds, however,
+in order to not change existing applications such as racoon
+we also provide default threshold values for these different parameters
+in case they are not specified.
+
+the two sysctls/proc entries are:
+a) /proc/sys/net/core/sysctl_xfrm_aevent_etime
+used to provide default values for the XFRMA_ETIMER_THRESH in incremental
+units of time of 100ms. The default is 10 (1 second)
+
+b) /proc/sys/net/core/sysctl_xfrm_aevent_rseqth
+used to provide default values for XFRMA_REPLAY_THRESH parameter
+in incremental packet count. The default is two packets.
+
+4) Message types
+----------------
+
+a) XFRM_MSG_GETAE issued by user-->kernel.
+XFRM_MSG_GETAE does not carry any TLVs.
+The response is a XFRM_MSG_NEWAE which is formatted based on what
+XFRM_MSG_GETAE queried for.
+The response will always have XFRMA_LTIME_VAL and XFRMA_REPLAY_VAL TLVs.
+*if XFRM_AE_RTHR flag is set, then XFRMA_REPLAY_THRESH is also retrieved
+*if XFRM_AE_ETHR flag is set, then XFRMA_ETIMER_THRESH is also retrieved
+
+b) XFRM_MSG_NEWAE is issued by either user space to configure
+or kernel to announce events or respond to a XFRM_MSG_GETAE.
+
+i) user --> kernel to configure a specific SA.
+any of the values or threshold parameters can be updated by passing the
+appropriate TLV.
+A response is issued back to the sender in user space to indicate success
+or failure.
+In the case of success, additionally an event with
+XFRM_MSG_NEWAE is also issued to any listeners as described in iii).
+
+ii) kernel->user direction as a response to XFRM_MSG_GETAE
+The response will always have XFRMA_LTIME_VAL and XFRMA_REPLAY_VAL TLVs.
+The threshold TLVs will be included if explicitly requested in
+the XFRM_MSG_GETAE message.
+
+iii) kernel->user to report as event if someone sets any values or
+thresholds for an SA using XFRM_MSG_NEWAE (as described in #i above).
+In such a case XFRM_AE_CU flag is set to inform the user that
+the change happened as a result of an update.
+The message will always have XFRMA_LTIME_VAL and XFRMA_REPLAY_VAL TLVs.
+
+iv) kernel->user to report event when replay threshold or a timeout
+is exceeded.
+In such a case either XFRM_AE_CR (replay exceeded) or XFRM_AE_CE (timeout
+happened) is set to inform the user what happened.
+Note the two flags are mutually exclusive.
+The message will always have XFRMA_LTIME_VAL and XFRMA_REPLAY_VAL TLVs.
+
+Exceptions to threshold settings
+--------------------------------
+
+If you have an SA that is getting hit by traffic in bursts such that
+there is a period where the timer threshold expires with no packets
+seen, then an odd behavior is seen as follows:
+The first packet arrival after a timer expiry will trigger a timeout
+aevent; i.e we dont wait for a timeout period or a packet threshold
+to be reached. This is done for simplicity and efficiency reasons.
+
+-JHS