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Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
Use is subject to license terms.

Architectural Overview for the DHCP agent
Peter Memishian
ident	"%Z%%M%	%I%	%E% SMI"

INTRODUCTION
============

The Solaris DHCP agent (dhcpagent) is a DHCP client implementation
compliant with RFCs 2131, 3315, and others.  The major forces shaping
its design were:

	* Must be capable of managing multiple network interfaces.
	* Must consume little CPU, since it will always be running.
	* Must have a small memory footprint, since it will always be
	  running.
	* Must not rely on any shared libraries outside of /lib, since
	  it must run before all filesystems have been mounted.

When a DHCP agent implementation is only required to control a single
interface on a machine, the problem is expressed well as a simple
state-machine, as shown in RFC2131.  However, when a DHCP agent is
responsible for managing more than one interface at a time, the
problem becomes much more complicated.

This can be resolved using threads or with an event-driven model.
Given that DHCP's behavior can be expressed concisely as a state
machine, the event-driven model is the closest match.

While tried-and-true, that model is subtle and easy to get wrong.
Indeed, much of the agent's code is there to manage the complexity of
programming in an asynchronous event-driven paradigm.

THE BASICS
==========

The DHCP agent consists of roughly 30 source files, most with a
companion header file.  While the largest source file is around 1700
lines, most are much shorter.  The source files can largely be broken
up into three groups:

	* Source files that, along with their companion header files,
	  define an abstract "object" that is used by other parts of
	  the system.  Examples include "packet.c", which along with
	  "packet.h" provide a Packet object for use by the rest of
	  the agent; and "async.c", which along with "async.h" defines
	  an interface for managing asynchronous transactions within
	  the agent.

	* Source files that implement a given state of the agent; for
	  instance, there is a "request.c" which comprises all of
	  the procedural "work" which must be done while in the
	  REQUESTING state of the agent.  By encapsulating states in
	  files, it becomes easier to debug errors in the
	  client/server protocol and adapt the agent to new
	  constraints, since all the relevant code is in one place.

	* Source files, which along with their companion header files,
  	  encapsulate a given task or related set of tasks.  The
	  difference between this and the first group is that the
	  interfaces exported from these files do not operate on
	  an "object", but rather perform a specific task.  Examples
	  include "defaults.c", which provides a useful interface
	  to /etc/default/dhcpagent file operations.

OVERVIEW
========

Here we discuss the essential objects and subtle aspects of the
DHCP agent implementation.  Note that there is of course much more
that is not discussed here, but after this overview you should be able 
to fend for yourself in the source code.

For details on the DHCPv6 aspects of the design, and how this relates
to the implementation present in previous releases of Solaris, see the
README.v6 file.

Event Handlers and Timer Queues
-------------------------------

The most important object in the agent is the event handler, whose
interface is in libinetutil.h and whose implementation is in
libinetutil.  The event handler is essentially an object-oriented
wrapper around poll(2): other components of the agent can register to
be called back when specific events on file descriptors happen -- for
instance, to wait for requests to arrive on its IPC socket, the agent
registers a callback function (accept_event()) that will be called
back whenever a new connection arrives on the file descriptor
associated with the IPC socket.  When the agent initially begins in
main(), it registers a number of events with the event handler, and
then calls iu_handle_events(), which proceeds to wait for events to
happen -- this function does not return until the agent is shutdown
via signal.

When the registered events occur, the callback functions are called
back, which in turn might lead to additional callbacks being
registered -- this is the classic event-driven model.  (As an aside,
note that programming in an event-driven model means that callbacks
cannot block, or else the agent will become unresponsive.)

A special kind of "event" is a timeout.  Since there are many timers
which must be maintained for each DHCP-controlled interface (such as a
lease expiration timer, time-to-first-renewal (t1) timer, and so
forth), an object-oriented abstraction to timers called a "timer
queue" is provided, whose interface is in libinetutil.h with a
corresponding implementation in libinetutil.  The timer queue allows
callback functions to be "scheduled" for callback after a certain
amount of time has passed.

The event handler and timer queue objects work hand-in-hand: the event
handler is passed a pointer to a timer queue in iu_handle_events() --
from there, it can use the iu_earliest_timer() routine to find the
timer which will next fire, and use this to set its timeout value in
its call to poll(2).  If poll(2) returns due to a timeout, the event
handler calls iu_expire_timers() to expire all timers that expired
(note that more than one may have expired if, for example, multiple
timers were set to expire at the same time).

Although it is possible to instantiate more than one timer queue or
event handler object, it doesn't make a lot of sense -- these objects
are really "singletons".  Accordingly, the agent has two global
variables, `eh' and `tq', which store pointers to the global event
handler and timer queue.

Network Interfaces
------------------

For each network interface managed by the agent, there is a set of
associated state that describes both its general properties (such as
the maximum MTU) and its connections to DHCP-related state (the
protocol state machines).  This state is stored in a pair of
structures called `dhcp_pif_t' (the IP physical interface layer or
PIF) and `dhcp_lif_t' (the IP logical interface layer or LIF).  Each
dhcp_pif_t represents a single physical interface, such as "hme0," for
a given IP protocol version (4 or 6), and has a list of dhcp_lif_t
structures representing the logical interfaces (such as "hme0:1") in
use by the agent.

This split is important because of differences between IPv4 and IPv6.
For IPv4, each DHCP state machine manages a single IP address and
associated configuration data.  This corresponds to a single logical
interface, which must be specified by the user.  For IPv6, however,
each DHCP state machine manages a group of addresses, and is
associated with DUID value rather than with just an interface.

Thus, DHCPv6 behaves more like in.ndpd in its creation of "ADDRCONF"
interfaces.  The agent automatically plumbs logical interfaces when
needed and removes them when the addresses expire.

The state for a given session is stored separately in `dhcp_smach_t'.
This state machine then points to the main LIF used for I/O, and to a
list of `dhcp_lease_t' structures representing individual leases, and
each of those points to a list of LIFs corresponding to the individual
addresses being managed.

One point that was brushed over in the preceding discussion of event
handlers and timer queues was context.  Recall that the event-driven
nature of the agent requires that functions cannot block, lest they
starve out others and impact the observed responsiveness of the agent.
As an example, consider the process of extending a lease: the agent
must send a REQUEST packet and wait for an ACK or NAK packet in
response.  This is done by sending a REQUEST and then returning to the
event handler that waits for an ACK or NAK packet to arrive on the
file descriptor associated with the interface.  Note however, that
when the ACK or NAK does arrive, and the callback function called
back, it must know which state machine this packet is for (it must get
back its context).  This could be handled through an ad-hoc mapping of
file descriptors to state machines, but a cleaner approach is to have
the event handler's register function (iu_register_event()) take in an
opaque context pointer, which will then be passed back to the
callback.  In the agent, the context pointer used depends on the
nature of the event: events on LIFs use the dhcp_lif_t pointer, events
on the state machine use dhcp_smach_t, and so on.

Note that there is nothing that guarantees the pointer passed into
iu_register_event() or iu_schedule_timer() will still be valid when
the callback is called back (for instance, the memory may have been
freed in the meantime).  To solve this problem, all of the data
structures used in this way are reference counted.  For more details
on how the reference count scheme is implemented, see the closing
comments in interface.h regarding memory management.

Transactions
------------

Many operations performed via DHCP must be performed in groups -- for
instance, acquiring a lease requires several steps: sending a
DISCOVER, collecting OFFERs, selecting an OFFER, sending a REQUEST,
and receiving an ACK, assuming everything goes well.  Note however
that due to the event-driven model the agent operates in, these
operations are not inherently "grouped" -- instead, the agent sends a
DISCOVER, goes back into the main event loop, waits for events
(perhaps even requests on the IPC channel to begin acquiring a lease
on another state machine), eventually checks to see if an acceptable
OFFER has come in, and so forth.  To some degree, the notion of the
state machine's current state (SELECTING, REQUESTING, etc) helps
control the potential chaos of the event-driven model (for instance,
if while the agent is waiting for an OFFER on a given state machine,
an IPC event comes in requesting that the leases be RELEASED, the
agent knows to send back an error since the state machine must be in
at least the BOUND state before a RELEASE can be performed.)

However, states are not enough -- for instance, suppose that the agent
begins trying to renew a lease.  This is done by sending a REQUEST
packet and waiting for an ACK or NAK, which might never come.  If,
while waiting for the ACK or NAK, the user sends a request to renew
the lease as well, then if the agent were to send another REQUEST,
things could get quite complicated (and this is only the beginning of
this rathole).  To protect against this, two objects exist:
`async_action' and `ipc_action'.  These objects are related, but
independent of one another; the more essential object is the
`async_action', which we will discuss first.

In short, an `async_action' represents a pending transaction (aka
asynchronous action), of which each state machine can have at most
one.  The `async_action' structure is embedded in the `dhcp_smach_t'
structure, which is fine since there can be at most one pending
transaction per state machine.  Typical "asynchronous transactions"
are START, EXTEND, and INFORM, since each consists of a sequence of
packets that must be done without interruption.  Note that not all
DHCP operations are "asynchronous" -- for instance, a DHCPv4 RELEASE
operation is synchronous (not asynchronous) since after the RELEASE is
sent no reply is expected from the DHCP server, but DHCPv6 Release is
asynchronous, as all DHCPv6 messages are transactional.  Some
operations, such as status query, are synchronous and do not affect
the system state, and thus do not require sequencing.

When the agent realizes it must perform an asynchronous transaction,
it calls async_async() to open the transaction.  If one is already
pending, then the new transaction must fail (the details of failure
depend on how the transaction was initiated, which is described in
more detail later when the `ipc_action' object is discussed).  If
there is no pending asynchronous transaction, the operation succeeds.

When the transaction is complete, either async_finish() or
async_cancel() must be called to complete or cancel the asynchronous
action on that state machine.  If the transaction is unable to
complete within a certain amount of time (more on this later), a timer
should be used to cancel the operation.

The notion of asynchronous transactions is complicated by the fact
that they may originate from both inside and outside of the agent.
For instance, a user initiates an asynchronous START transaction when
he performs an `ifconfig hme0 dhcp start', but the agent will
internally need to perform asynchronous EXTEND transactions to extend
the lease before it expires.  Note that user-initiated actions always
have priority over internal actions: the former will cancel the
latter, if necessary.

This leads us into the `ipc_action' object.  An `ipc_action'
represents the IPC-related pieces of an asynchronous transaction that
was started as a result of a user request, as well as the `BUSY' state
of the administrative interface.  Only IPC-generated asynchronous
transactions have a valid `ipc_action' object.  Note that since there
can be at most one asynchronous action per state machine, there can
also be at most one `ipc_action' per state machine (this means it can
also conveniently be embedded inside the `dhcp_smach_t' structure).

One of the main purposes of the `ipc_action' object is to timeout user
events.  When the user specifies a timeout value as an argument to
ifconfig, he is specifying an `ipc_action' timeout; in other words,
how long he is willing to wait for the command to complete.  When this
time expires, the ipc_action is terminated, as well as the
asynchronous operation.

The API provided for the `ipc_action' object is quite similar to the
one for the `async_action' object: when an IPC request comes in for an 
operation requiring asynchronous operation, ipc_action_start() is
called.  When the request completes, ipc_action_finish() is called.
If the user times out before the request completes, then
ipc_action_timeout() is called.

Packet Management
-----------------

Another complicated area is packet management: building, manipulating,
sending and receiving packets.  These operations are all encapsulated
behind a dozen or so interfaces (see packet.h) that abstract the
unimportant details away from the rest of the agent code.  In order to
send a DHCP packet, code first calls init_pkt(), which returns a
dhcp_pkt_t initialized suitably for transmission.  Note that currently
init_pkt() returns a dhcp_pkt_t that is actually allocated as part of
the `dhcp_smach_t', but this may change in the future..  After calling
init_pkt(), the add_pkt_opt*() functions are used to add options to
the DHCP packet.  Finally, send_pkt() and send_pkt_v6() can be used to
transmit the packet to a given IP address.

The send_pkt() function handles the details of packet timeout and
retransmission.  The last argument to send_pkt() is a pointer to a
"stop function."  If this argument is passed as NULL, then the packet
will only be sent once (it won't be retransmitted).  Otherwise, before
each retransmission, the stop function will be called back prior to
retransmission.  The callback may alter dsm_send_timeout if necessary
to place a cap on the next timeout; this is done for DHCPv6 in
stop_init_reboot() in order to implement the CNF_MAX_RD constraint.

The return value from this function indicates whether to continue
retransmission or not, which allows the send_pkt() caller to control
the retransmission policy without making it have to deal with the
retransmission mechanism.  See request.c for an example of this in
action.

The recv_pkt() function is simpler but still complicated by the fact
that one may want to receive several different types of packets at
once.  The caller registers an event handler on the file descriptor,
and then calls recv_pkt() to read in the packet along with meta
information about the message (the sender and interface identifier).
				
For IPv6, packet reception is done with a single socket, using
IPV6_PKTINFO to determine the actual destination address and receiving
interface.  Packets are then matched against the state machines on the
given interface through the transaction ID.

For IPv4, due to oddities in the DHCP specification (discussed in
PSARC/2007/571), a special IP_DHCPINIT_IF socket option must be used
to allow unicast DHCP traffic to be received on an interface during
lease acquisition.  Since the IP_DHCPINIT_IF socket option can only
enable one interface at a time, one socket must be used per interface.

Time
----

The notion of time is an exceptionally subtle area.  You will notice
five ways that time is represented in the source: as lease_t's,
uint32_t's, time_t's, hrtime_t's, and monosec_t's.  Each of these
types serves a slightly different function.

The `lease_t' type is the simplest to understand; it is the unit of
time in the CD_{LEASE,T1,T2}_TIME options in a DHCP packet, as defined
by RFC2131. This is defined as a positive number of seconds (relative
to some fixed point in time) or the value `-1' (DHCP_PERM) which
represents infinity (i.e., a permanent lease).  The lease_t should be
used either when dealing with actual DHCP packets that are sent on the
wire or for variables which follow the exact definition given in the
RFC.

The `uint32_t' type is also used to represent a relative time in
seconds.  However, here the value `-1' is not special and of course
this type is not tied to any definition given in RFC2131.  Use this
for representing "offsets" from another point in time that are not
DHCP lease times.

The `time_t' type is the natural Unix type for representing time since
the epoch.  Unfortunately, it is affected by stime(2) or adjtime(2)
and since the DHCP client is used during system installation (and thus
when time is typically being configured), the time_t cannot be used in
general to represent an absolute time since the epoch.  For instance,
if a time_t were used to keep track of when a lease began, and then a
minute later stime(2) was called to adjust the system clock forward a
year, then the lease would appeared to have expired a year ago even
though it has only been a minute.  For this reason, time_t's should
only be used either when wall time must be displayed (such as in
DHCP_STATUS ipc transaction) or when a time meaningful across reboots
must be obtained (such as when caching an ACK packet at system
shutdown).

The `hrtime_t' type returned from gethrtime() works around the
limitations of the time_t in that it is not affected by stime(2) or
adjtime(2), with the disadvantage that it represents time from some
arbitrary time in the past and in nanoseconds.  The timer queue code
deals with hrtime_t's directly since that particular piece of code is
meant to be fairly independent of the rest of the DHCP client.

However, dealing with nanoseconds is error-prone when all the other
time types are in seconds.  As a result, yet another time type, the
`monosec_t' was created to represent a monotonically increasing time
in seconds, and is really no more than (hrtime_t / NANOSEC).  Note
that this unit is typically used where time_t's would've traditionally
been used.  The function monosec() in util.c returns the current
monosec, and monosec_to_time() can convert a given monosec to wall
time, using the system's current notion of time.

One additional limitation of the `hrtime_t' and `monosec_t' types is
that they are unaware of the passage of time across checkpoint/resume
events (e.g., those generated by sys-suspend(1M)).  For example, if
gethrtime() returns time T, and then the machine is suspended for 2
hours, and then gethrtime() is called again, the time returned is not
T + (2 * 60 * 60 * NANOSEC), but rather approximately still T.

To work around this (and other checkpoint/resume related problems),
when a system is resumed, the DHCP client makes the pessimistic
assumption that all finite leases have expired while the machine was
suspended and must be obtained again.  This is known as "refreshing"
the leases, and is handled by refresh_smachs().

Note that it appears like a more intelligent approach would be to
record the time(2) when the system is suspended, compare that against
the time(2) when the system is resumed, and use the delta between them
to decide which leases have expired.  Sadly, this cannot be done since
through at least Solaris 10, it is not possible for userland programs
to be notified of system suspend events.

Configuration
-------------

For the most part, the DHCP client only *retrieves* configuration data
from the DHCP server, leaving the configuration to scripts (such as
boot scripts), which themselves use dhcpinfo(1) to retrieve the data
from the DHCP client.  This is desirable because it keeps the mechanism
of retrieving the configuration data decoupled from the policy of using
the data.

However, unless used in "inform" mode, the DHCP client *does*
configure each IP interface enough to allow it to communicate with
other hosts.  Specifically, the DHCP client configures the interface's
IP address, netmask, and broadcast address using the information
provided by the server.  Further, for IPv4 logical interface 0
("hme0"), any provided default routes are also configured.

For IPv6, only the IP addresses are set.  The netmask (prefix) is then
set automatically by in.ndpd, and routes are discovered in the usual
way by router discovery or routing protocols.  DHCPv6 doesn't set
routes.

Since logical interfaces cannot be specified as output interfaces in
the kernel forwarding table, and in most cases, logical interfaces
share a default route with their associated physical interface, the
DHCP client does not automatically add or remove default routes when
IPv4 leases are acquired or expired on logical interfaces.

Event Scripting
---------------

The DHCP client supports user program invocations on DHCP events.  The
supported events are BOUND, EXTEND, EXPIRE, DROP, RELEASE, and INFORM
for DHCPv4, and BUILD6, EXTEND6, EXPIRE6, DROP6, LOSS6, RELEASE6, and
INFORM6 for DHCPv6.  The user program runs asynchronous to the DHCP
client so that the main event loop stays active to process other
events, including events triggered by the user program (for example,
when it invokes dhcpinfo).

The user program execution is part of the transaction of a DHCP command.
For example, if the user program is not enabled, the transaction of the
DHCP command START is considered over when an ACK is received and the
interface is configured successfully.  If the user program is enabled,
it is invoked after the interface is configured successfully, and the
transaction is considered over only when the user program exits.  The
event scripting implementation makes use of the asynchronous operations
discussed in the "Transactions" section.

An upper bound of 58 seconds is imposed on how long the user program
can run. If the user program does not exit after 55 seconds, the signal
SIGTERM is sent to it. If it still does not exit after additional 3
seconds, the signal SIGKILL is sent to it.  Since the event handler is
a wrapper around poll(), the DHCP client cannot directly observe the
completion of the user program.  Instead, the DHCP client creates a
child "helper" process to synchronously monitor the user program (this
process is also used to send the aformentioned signals to the process,
if necessary).  The DHCP client and the helper process share a pipe
which is included in the set of poll descriptors monitored by the DHCP
client's event handler.  When the user program exits, the helper process
passes the user program exit status to the DHCP client through the pipe,
informing the DHCP client that the user program has finished.  When the
DHCP client is asked to shut down, it will wait for any running instances
of the user program to complete.

Indexes created Thu Nov 26 20:41:12 UTC 2009

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