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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;
- xfrm_address_t saddr;
- __u32 flags;
- __u32 reqid;
- };
-
-The unique SA is identified by the combination of xfrm_usersa_id,
-reqid and saddr.
-
-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