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26
27 The DHCP server cache implementation
28 Zhenghui.Xie@sun.com
29
30
31 #ident "%Z%%M% %I% %E% SMI"
32
33 INTRODUCTION
34 ============
35
36 The Solaris DHCP server implements a caching mechanism to:
37
38 * Ensure the ACK is consistent with the original OFFER, so that
39 subsequent requests get the same answer.
40 * Ensure the same IP address isn't offered to a different client.
41 * Improve performance by reducing the frequency of datastore lookups.
42
43 IMPLEMENTATION OVERVIEW
44 =======================
45
46 The cache implementation consists of a number of hash tables and lists,
47 enumerated below, along with a timeout and refresh mechanism:
48
49 * A global DHCP table hash (ntable):
50
51 Each active network table (dvsc_dnet_t) is hashed by subnet number
52 into a global hash called 'ntable'. The dsvc_dnet_t itself
53 contains a variety of information about each subnet, an access
54 handle to the underlying datastore, and a variety of caches which
55 we describe next.
56
57 An ntable hash entry will be removed after DHCP_NET_THRESHOLD
58 seconds of inactivity.
59
60 * A per-network client hash (ctable):
61
62 Each client structure (dsvc_clnt_t) is hashed by client identifier
63 into the per-network ctable. This table is used by the interface
64 worker thread to get client information.
65
66 A ctable hash entry will be removed if the client does not communicate
67 with the server for DHCP_CLIENT_THRESHOLD seconds.
68
69 * A per-network offer hash (itable):
70
71 The IP address associated with each pending OFFER is hashed into
72 the per-network itable. This table is used to reserve the offered
73 and in-use IP addresses on the given network.
74
75 An itable hash entry will be removed if more than DSVC_CV_OFFER_TTL
76 seconds elapse without update_offer() being called on it.
77
78 * A per-network free record list (freerec):
79
80 This is a cache of free records, populated with any unused records
81 retrieved from previous datastore lookups or from IP addresses that
82 have been explicitly released by DHCP clients. This is the first
83 list select_offer() consults.
84
85 An entry in the freerec list expires after DSVC_CV_CACHE_TTL seconds.
86
87 * A per-network least recently used record list (lrurec)
88
89 This is a cache of least recently used records, populated with any
90 unused records retrieved from a previous datastore lookup for
91 LRU records. This is the second list select_offer() consults, after
92 checking freerec.
93
94 An entry in the lrurec list expires after DSVC_CV_CACHE_TTL seconds.
95
96 The concurrency between the datastore and cached records is handled by the
97 underlying datastore implementation using perimeters and is transparent to
98 in.dhcpd. $SRC/lib/libdhcpsvc/private/{private.c,public.c} implement the
99 functions used by in.dhcpd and DHCP server admin tools to lookup and modify
100 the underlying datastore records.
101
102 TRANSACTIONS
103 ============
104
105 When in.dhcpd receives a packet from a client, the interface thread first
106 calls open_dnet() to retrieve the dsvc_dnet_t which describes the network.
107 The dsvc_dnet_t is either in the ntable hash, in which case it is returned,
108 or a new dsvc_dnet_t is allocated and inserted to ntable. Then, using the
109 dsvc_dnet_t, the interface thread calls open_clnt(), which searches the
110 dsvc_dnet_t's ctable hash and returns the client structure (dsvc_clnt_t) if
111 found. Otherwise, a new dsvc_clnt_t is allocated and inserted into the
112 ctable. Finally, the packet is put to the client's packet list so that the
113 client thread can process it.
114
115 The client thread then processes the packet according to whether it's a
116 DISCOVER, REQUEST, RELEASE, or DECLINE.
117
118 For a DISCOVER:
119
120 1. If there is a pre-assigned IP for this client and the cached offer
121 is not timed out, then use the IP and the record in the
122 dsvc_clnt_t and make an OFFER.
123
124 2. If there is a pre-assigned IP but the cached offer is expired,
125 remove the cached OFFER from the itable, and try to find a new
126 record for the client (see below).
127
128 3. If there is no pre-assigned IP for this client, just try to find
129 a new record for the client (see following text).
130
131 To find a new record, the client thread first searches the datastore
132 for a record matching the client identifier provided in the packet.
133 If none is found, then the free record list (freerec) and least
134 recently used record list (lrurec) are searched, in that order. If
135 either freerec or lrurec is empty, or the head record on either list
136 is expired, in.dhcpd removes any existing records and attempts to
137 repopulate them by performing datastore lookups. Finally, any
138 unused records are cached for later use.
139
140 If a usable record is found, the server generates and sends an OFFER
141 to the client. Once sent, the client's dsvc_clnt_t is inserted to
142 the dsvc_dnet_t's itable if it is a new IP, or the itable is refreshed
143 if it is a pre-assigned IP.
144
145 For a REQUEST:
146
147 1. If the REQUEST is a reply to a previous OFFER, it checks if the
148 OFFER has expired. If not, the itable timer is reset, the client
149 record is updated, and the ACK is sent. If it is expired and the
150 address cannot be confirmed to still be free, the REQUEST is
151 silently ignored (and the client should eventually drop back
152 to DISCOVER).
153
154 2. If the REQUEST is associated with a client INIT-REBOOT or a client
155 extending a lease, then the client thread does a datastore lookup
156 by client identifier. If a match is found, the record is updated
157 and an ACK is sent.
158
159 If the packet is a RELEASE, the client thread modifies the record in the
160 datastore so that it is marked free, and, if successful, puts the record onto
161 the free record list (freerec).
162
163 Finally, if the packet is a DECLINE, the client thread modifies the record in
164 the datastore so that it is marked unusable.