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Documentation / networking / xfrm_sync.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	
2	The sync patches work is based on initial patches from
3	Krisztian <hidden@balabit.hu> and others and additional patches
4	from Jamal <hadi@cyberus.ca>.
5	
6	The end goal for syncing is to be able to insert attributes + generate
7	events so that the an SA can be safely moved from one machine to another
8	for HA purposes.
9	The idea is to synchronize the SA so that the takeover machine can do
10	the processing of the SA as accurate as possible if it has access to it.
11	
12	We already have the ability to generate SA add/del/upd events.
13	These patches add ability to sync and have accurate lifetime byte (to
14	ensure proper decay of SAs) and replay counters to avoid replay attacks
15	with as minimal loss at failover time.
16	This way a backup stays as closely uptodate as an active member.
17	
18	Because the above items change for every packet the SA receives,
19	it is possible for a lot of the events to be generated.
20	For this reason, we also add a nagle-like algorithm to restrict
21	the events. i.e we are going to set thresholds to say "let me
22	know if the replay sequence threshold is reached or 10 secs have passed"
23	These thresholds are set system-wide via sysctls or can be updated
24	per SA.
25	
26	The identified items that need to be synchronized are:
27	- the lifetime byte counter
28	note that: lifetime time limit is not important if you assume the failover
29	machine is known ahead of time since the decay of the time countdown
30	is not driven by packet arrival.
31	- the replay sequence for both inbound and outbound
32	
33	1) Message Structure
34	----------------------
35	
36	nlmsghdr:aevent_id:optional-TLVs.
37	
38	The netlink message types are:
39	
40	XFRM_MSG_NEWAE and XFRM_MSG_GETAE.
41	
42	A XFRM_MSG_GETAE does not have TLVs.
43	A XFRM_MSG_NEWAE will have at least two TLVs (as is
44	discussed further below).
45	
46	aevent_id structure looks like:
47	
48	   struct xfrm_aevent_id {
49	             struct xfrm_usersa_id           sa_id;
50	             xfrm_address_t                  saddr;
51	             __u32                           flags;
52	             __u32                           reqid;
53	   };
54	
55	The unique SA is identified by the combination of xfrm_usersa_id,
56	reqid and saddr.
57	
58	flags are used to indicate different things. The possible
59	flags are:
60	        XFRM_AE_RTHR=1, /* replay threshold*/
61	        XFRM_AE_RVAL=2, /* replay value */
62	        XFRM_AE_LVAL=4, /* lifetime value */
63	        XFRM_AE_ETHR=8, /* expiry timer threshold */
64	        XFRM_AE_CR=16, /* Event cause is replay update */
65	        XFRM_AE_CE=32, /* Event cause is timer expiry */
66	        XFRM_AE_CU=64, /* Event cause is policy update */
67	
68	How these flags are used is dependent on the direction of the
69	message (kernel<->user) as well the cause (config, query or event).
70	This is described below in the different messages.
71	
72	The pid will be set appropriately in netlink to recognize direction
73	(0 to the kernel and pid = processid that created the event
74	when going from kernel to user space)
75	
76	A program needs to subscribe to multicast group XFRMNLGRP_AEVENTS
77	to get notified of these events.
78	
79	2) TLVS reflect the different parameters:
80	-----------------------------------------
81	
82	a) byte value (XFRMA_LTIME_VAL)
83	This TLV carries the running/current counter for byte lifetime since
84	last event.
85	
86	b)replay value (XFRMA_REPLAY_VAL)
87	This TLV carries the running/current counter for replay sequence since
88	last event.
89	
90	c)replay threshold (XFRMA_REPLAY_THRESH)
91	This TLV carries the threshold being used by the kernel to trigger events
92	when the replay sequence is exceeded.
93	
94	d) expiry timer (XFRMA_ETIMER_THRESH)
95	This is a timer value in milliseconds which is used as the nagle
96	value to rate limit the events.
97	
98	3) Default configurations for the parameters:
99	----------------------------------------------
100	
101	By default these events should be turned off unless there is
102	at least one listener registered to listen to the multicast
103	group XFRMNLGRP_AEVENTS.
104	
105	Programs installing SAs will need to specify the two thresholds, however,
106	in order to not change existing applications such as racoon
107	we also provide default threshold values for these different parameters
108	in case they are not specified.
109	
110	the two sysctls/proc entries are:
111	a) /proc/sys/net/core/sysctl_xfrm_aevent_etime
112	used to provide default values for the XFRMA_ETIMER_THRESH in incremental
113	units of time of 100ms. The default is 10 (1 second)
114	
115	b) /proc/sys/net/core/sysctl_xfrm_aevent_rseqth
116	used to provide default values for XFRMA_REPLAY_THRESH parameter
117	in incremental packet count. The default is two packets.
118	
119	4) Message types
120	----------------
121	
122	a) XFRM_MSG_GETAE issued by user-->kernel.
123	XFRM_MSG_GETAE does not carry any TLVs.
124	The response is a XFRM_MSG_NEWAE which is formatted based on what
125	XFRM_MSG_GETAE queried for.
126	The response will always have XFRMA_LTIME_VAL and XFRMA_REPLAY_VAL TLVs.
127	*if XFRM_AE_RTHR flag is set, then XFRMA_REPLAY_THRESH is also retrieved
128	*if XFRM_AE_ETHR flag is set, then XFRMA_ETIMER_THRESH is also retrieved
129	
130	b) XFRM_MSG_NEWAE is issued by either user space to configure
131	or kernel to announce events or respond to a XFRM_MSG_GETAE.
132	
133	i) user --> kernel to configure a specific SA.
134	any of the values or threshold parameters can be updated by passing the
135	appropriate TLV.
136	A response is issued back to the sender in user space to indicate success
137	or failure.
138	In the case of success, additionally an event with
139	XFRM_MSG_NEWAE is also issued to any listeners as described in iii).
140	
141	ii) kernel->user direction as a response to XFRM_MSG_GETAE
142	The response will always have XFRMA_LTIME_VAL and XFRMA_REPLAY_VAL TLVs.
143	The threshold TLVs will be included if explicitly requested in
144	the XFRM_MSG_GETAE message.
145	
146	iii) kernel->user to report as event if someone sets any values or
147	thresholds for an SA using XFRM_MSG_NEWAE (as described in #i above).
148	In such a case XFRM_AE_CU flag is set to inform the user that
149	the change happened as a result of an update.
150	The message will always have XFRMA_LTIME_VAL and XFRMA_REPLAY_VAL TLVs.
151	
152	iv) kernel->user to report event when replay threshold or a timeout
153	is exceeded.
154	In such a case either XFRM_AE_CR (replay exceeded) or XFRM_AE_CE (timeout
155	happened) is set to inform the user what happened.
156	Note the two flags are mutually exclusive.
157	The message will always have XFRMA_LTIME_VAL and XFRMA_REPLAY_VAL TLVs.
158	
159	Exceptions to threshold settings
160	--------------------------------
161	
162	If you have an SA that is getting hit by traffic in bursts such that
163	there is a period where the timer threshold expires with no packets
164	seen, then an odd behavior is seen as follows:
165	The first packet arrival after a timer expiry will trigger a timeout
166	aevent; i.e we dont wait for a timeout period or a packet threshold
167	to be reached. This is done for simplicity and efficiency reasons.
168	
169	-JHS
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