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The DHCP server cache implementation

Zhenghui.Xie@sun.com

#ident "%Z%%M% %I% %E% SMI"

INTRODUCTION

============

The Solaris DHCP server implements a caching mechanism to:

* Ensure the ACK is consistent with the original OFFER, so that

subsequent requests get the same answer.

* Ensure the same IP address isn't offered to a different client.

* Improve performance by reducing the frequency of datastore lookups.

IMPLEMENTATION OVERVIEW

=======================

The cache implementation consists of a number of hash tables and lists,

enumerated below, along with a timeout and refresh mechanism:

* A global DHCP table hash (ntable):

Each active network table (dvsc_dnet_t) is hashed by subnet number

into a global hash called 'ntable'. The dsvc_dnet_t itself

contains a variety of information about each subnet, an access

handle to the underlying datastore, and a variety of caches which

we describe next.

An ntable hash entry will be removed after DHCP_NET_THRESHOLD

seconds of inactivity.

* A per-network client hash (ctable):

Each client structure (dsvc_clnt_t) is hashed by client identifier

into the per-network ctable. This table is used by the interface

worker thread to get client information.

A ctable hash entry will be removed if the client does not communicate

with the server for DHCP_CLIENT_THRESHOLD seconds.

* A per-network offer hash (itable):

The IP address associated with each pending OFFER is hashed into

the per-network itable. This table is used to reserve the offered

and in-use IP addresses on the given network.

An itable hash entry will be removed if more than DSVC_CV_OFFER_TTL

seconds elapse without update_offer() being called on it.

* A per-network free record list (freerec):

This is a cache of free records, populated with any unused records

retrieved from previous datastore lookups or from IP addresses that

have been explicitly released by DHCP clients. This is the first

list select_offer() consults.

An entry in the freerec list expires after DSVC_CV_CACHE_TTL seconds.

* A per-network least recently used record list (lrurec)

This is a cache of least recently used records, populated with any

unused records retrieved from a previous datastore lookup for

LRU records. This is the second list select_offer() consults, after

checking freerec.

An entry in the lrurec list expires after DSVC_CV_CACHE_TTL seconds.

The concurrency between the datastore and cached records is handled by the

underlying datastore implementation using perimeters and is transparent to

in.dhcpd. $SRC/lib/libdhcpsvc/private/{private.c,public.c} implement the

functions used by in.dhcpd and DHCP server admin tools to lookup and modify

the underlying datastore records.

TRANSACTIONS

============

When in.dhcpd receives a packet from a client, the interface thread first

calls open_dnet() to retrieve the dsvc_dnet_t which describes the network.

The dsvc_dnet_t is either in the ntable hash, in which case it is returned,

or a new dsvc_dnet_t is allocated and inserted to ntable. Then, using the

dsvc_dnet_t, the interface thread calls open_clnt(), which searches the

dsvc_dnet_t's ctable hash and returns the client structure (dsvc_clnt_t) if

found. Otherwise, a new dsvc_clnt_t is allocated and inserted into the

ctable. Finally, the packet is put to the client's packet list so that the

client thread can process it.

The client thread then processes the packet according to whether it's a

DISCOVER, REQUEST, RELEASE, or DECLINE.

For a DISCOVER:

1. If there is a pre-assigned IP for this client and the cached offer

is not timed out, then use the IP and the record in the

dsvc_clnt_t and make an OFFER.

2. If there is a pre-assigned IP but the cached offer is expired,

remove the cached OFFER from the itable, and try to find a new

record for the client (see below).

3. If there is no pre-assigned IP for this client, just try to find

a new record for the client (see following text).

To find a new record, the client thread first searches the datastore

for a record matching the client identifier provided in the packet.

If none is found, then the free record list (freerec) and least

recently used record list (lrurec) are searched, in that order. If

either freerec or lrurec is empty, or the head record on either list

is expired, in.dhcpd removes any existing records and attempts to

repopulate them by performing datastore lookups. Finally, any

unused records are cached for later use.

If a usable record is found, the server generates and sends an OFFER

to the client. Once sent, the client's dsvc_clnt_t is inserted to

the dsvc_dnet_t's itable if it is a new IP, or the itable is refreshed

if it is a pre-assigned IP.

For a REQUEST:

1. If the REQUEST is a reply to a previous OFFER, it checks if the

OFFER has expired. If not, the itable timer is reset, the client

record is updated, and the ACK is sent. If it is expired and the

address cannot be confirmed to still be free, the REQUEST is

silently ignored (and the client should eventually drop back

to DISCOVER).

2. If the REQUEST is associated with a client INIT-REBOOT or a client

extending a lease, then the client thread does a datastore lookup

by client identifier. If a match is found, the record is updated

and an ACK is sent.

If the packet is a RELEASE, the client thread modifies the record in the

datastore so that it is marked free, and, if successful, puts the record onto

the free record list (freerec).

Finally, if the packet is a DECLINE, the client thread modifies the record in

the datastore so that it is marked unusable.