One document matched: draft-ietf-rsvp-policy-lpm-00.txt
Internet Draft Shai Herzog
Expiration: December 1996 USC/ISI
File: draft-ietf-rsvp-policy-lpm-00.txt
Local Policy Modules (LPM):
Policy Enforcement for Resource Reservation Protocols
June 12, 1996
Status of Memo
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Abstract
This memo describes a set of building blocks for policy based
admission control in RSVP and similar resource reservation protocols.
We describe an interface between RSVP and Local Policy Modules (LPM);
this interface provides RSVP with policy related information, and
allows local policy modules to support various accounting and access
control policies.
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1. Introduction
The current admission process in RSVP uses resource (capacity) based
admission control; we expand this model to include policy based
admission control as well, in one atomic operation. Policy admission
control is enforced at border/policy nodes by Local Policy Modules
(LPMs). LPMs based their admission decision, among other factors, on
the contents of POLICY_DATA objects that are carried inside RSVP
messages. LPMs are responsible for receiving, processing, and
forwarding POLICY_DATA objects. Subject to the applicable bilateral
agreements, and local policies, LPMs may also rewrite and modify the
POLICY_DATA objects as the pass through policy nodes.
In this document, we describe the range of policies that can be
supported, however, we recommend that you read this document along
side with its policy reference document~[HER96b]. This document
describes a generic framework for policy enforcement; we do not
advocate any specific access control policies since we believe that
standardization of policies (as opposed to the framework) may require
significantly more research and better understanding of the
tradeoffs.
Section provides a general description of the RSVP/LPM interface,
Section~ specified the internal representation of POLICY_ELEMENT
objects, Section~ describes the detailed interface between RSVP and
the LPM, and Section~ provides a peek into some of the more important
LPM implementation internals.
2. The RSVP/LPM interface
Unless we are willing to declare a single monolithic access policy we
need to accommodate varying, independent access control mechanisms in
RSVP (e.g., over different regions of the Internet, internal
accounting vs. inter-provider accounting, quota vs. advanced
reservations, etc.). Each mechanism can have its own, type-specific
internal format, can be configured for local needs (e.g., policy data
rewrite (conversion) table, etc.), and can be added and removed from
nodes with little or no impact on other mechanisms.
2.1 POLICY_DATA objects
RSVP messages may carry optional POLICY_DATA objects. Policy data
objects are a general container for policy related information
that could assist local RSVP nodes along the reserved path in
their policy decisions. Policy information may originate from
end-users, however, it can also be created or converted at the
core of the network. POLICY_DATA objects contain an optional list
of FILTER_SPEC objects which identify the flows it is associated
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with: we expect that some access control mechanisms to use session
POLICY_DATA objects (with wildcard FILTER_SPEC) while others may
require the full power of per-flow object semantics. Generally, we
assume that POLICY_DATA objects can be carried by any RSVP
message, (e.g., Path, Resv, ResvErr, etc.).
2.2 Modular Context
Before RSVP accepts a reservation it must check for access
authorization. This is where local policy modules take effect,
verifying access rights to local resources (i.e. links, clouds,
etc.). Figure illustrates the context for the proposed design:
RSVP interfaces to the LPM to handle input and output of
POLICY_DATA objects and to check the status of reservations.
Conceptually, a reservation must be accepted both physically and
administratively; physically, by traditional admission control
(based on congestion) and administratively by the local access
policy enforced by the LPM. This dual admission must be atomic and
this atomicity is represented by the "accept/reject" module. In
this document, we concentrate only on the highlighted modules: the
RSVP and the LPM interfaces. The RSVP interface is defined by
describing the functionality that is expected from RSVP in order
to support access control. It includes the handling of incoming
messages, scheduling outgoing messages, and performing status
checks. The LPM interface describes the services the LPM
provides, through a set of LPM functions. However, we do not
define how RSVP should check the status of reservations (it could
be done by calling the LPM directly, through an accept/reject
module, or in other ways). [Note 1]
_________________________
[Note 1] The RSVP admission process is unidirectional and does not
include upcalls to RSVP, e.g., there is no upcall to notify RSVP that a
previously made reservation was canceled or preempted. We do however
anticipate that once the initial access control architecture is in
place, later changes to the RSVP spec, would define an "accept/reject"
module, and associated status update upcalls to RSVP.
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+--------------------+
| RSVP |
+--------------------+
/|\ /|\
Resv. status | | In/Outgoing objects
\|/ \|/
+---------------+ +---------------+
| Accept/Reject |<---->| LPM |
+---------------+ +---------------+
/|\
|
\|/
+---------------+
| Ad. Control |
+---------------+
Figure 1: The modular context of access control
2.3 Local Policy Modules
Local Policy Modules (LPMs) can be configured locally, to a
particular access policy. LPMs have three basic functions: first,
to receive incoming policy data objects, second, to update the
access/accounting status of reservations, and third, to build
accounting/policy data objects for outgoing RSVP messages (The LPM
message flow outline is illustrated in figure ). LPMs maintain
local access state for supporting the LPM operations, and this
state must remain consistent with RSVP's state.
2.3.1 Processing incoming messages
RSVP calls the LPM for object processing each time it receives
a POLICY_DATA object. The LPM processes, stores the object's
information, and returns a status code to RSVP. The status code
reports the success/failure of object processing, but does not
reflect the acceptance of the reservation. The status of a
reservation must be checked separately (see Section for more
details).
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+----------------------------------------------+
| RSVP |
| |
************** ************************************>
<=============*========*======== =====================
| * * || || |
| * * ***||******||******************>
| * * * || || ===============
+--------*--------*--*----||------||----||-----+
* * * || || ||
\*/ ** || \||/ \||/
+--------*--------*-------||------||----||-----+
| ********** +==============+
| LPM: Common Layer |
+----------------------------------------------+
/|\ /|\ /|\
| | |
\|/ \|/ \|/
+-----------+ +-----------+ +-----------+
| Handler 0 | | Handler 1 |<----+ Handler 2 |
+-----------+ +-----------+ +-----------+
Figure 2: LPM and RSVP: message flow outline
2.3.2 Processing outgoing messages
When RSVP generates an outgoing message it calls the LPM. The
LPM assembles the outgoing policy data objects and hands them
to RSVP for placing inside the outgoing message.
2.3.3 Reservation status updates
The concept of access control assumes that even previously
admitted reservations are conditional, in a sense that changes
in access status may trigger some action against the associated
reservation (i.e., cancel it, allow its preemption, etc.).
Therefore, the access control mechanism must periodically check
for reservation status changes (like quota exhaustion) and take
the appropriate measures. Reservation status should also be
checked when system events require it, (e.g., the arrival of a
new policy data object with updated information). Status
checks may be limited to the scope of the change (e.g., only
the interface from which the new RSVP message arrived).
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2.3.4 Optional debiting for Reservations
The simplest form of access control performs a binary task:
accept or reject a reservation. More advanced policies may
require the LPM to perform book keeping (i.e., usage quota
enforcement or even cost recovery). To achieve such tasks, the
LPM can be configured to perform debiting. Debiting is not
part of the LPM interface, and can be configured as an option
into the status update: when RSVP queries the LPM about the
status of a reservation, the LPM may perform debiting, and
update the status of the reservation according to the debiting
result. The debiting process is based on two separate
functions: determining "cost", and actual debiting. These two
functions can be fully independent from each other, and most
likely be carried out by different handlers.
In multicast environments, with upstream merging, it is very
likely that a reservation will be debited against multiple
network entities that represent the aggregated credentials of
the downstream receivers. This raises the issue of the "sharing
model". The sharing model defines how the reservation is
shared among the different policy data objects. [Note 2]
The sharing model, and the selection of cost allocation and
actual debiting mechanisms is an issue of LPM local
configuration, and is not discussed in this document.
2.3.5 Security issues
Hop-by-hop authentication mechanism:
The RSVP security mechanism proposed in [BAK96] relies on hop-
by-hop authentication. This form of authentication creates
a chain of trust that is only as strong as its weakest
element (in our case, the weakest router). As long as we
believe that all RSVP nodes are policy nodes as well, then
RSVP security is sufficient for the entire RSVP message,
including POLICY_DATA objects. This however is not the
case when policy is only enforced at boundary nodes.
_________________________
[Note 2] Sharing model examples: (1) Each policy object is allocated the
full cost, (2) The cost is divided equally between the different objects
(3) The cost is attributed to an arbitrary object (4) The cost allocated
relative to some criteria like the number of downstream receivers, the
size of the organization, the amount of pre-purchased capacity
(remaining quota), etc.
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Security over clouds:
If policies are only enforced at cloud entry and exit
points, then RSVP's security is insufficient to protect
policy objects, since from a policy enforcement
perspective, the in-cloud nodes are unsecured. We propose
a "policy data tunneling" approach, where the logical
policy topology is discovered automatically, and security
is enforced over the logical topology. When policy
objects are created at border routers, they are
encapsulated in a security envelope (described in Sections
and ref security-issues). The envelop is forwarded as-is
over the cloud, and is only removed by the cloud border
(exit) node.
2.4 Default handling of policy data objects
Because we do not expect (or desire) that every RSVP node will be
capable of processing all types of policy data objects, it is
essential that RSVP define default handling of such unrecognized
objects, and that this default handling be required from any
RSVP/LPM implementation. The general concept is that RSVP play
the role of a repeater (or a tunnel) by forwarding the received
objects without modification. Implementation details are an part
of the internal LPM architecture, described in Section .
3. POLICY_ELEMENT objects: internal representation
The contents of the POLICY_ELEMENT is opaque to RSVP; the format we
describe here is only visible to the LPM. POLICY_ELEMENT objects are
made of a list of policy particles. Policy particles have a length, a
policy type (PType) and a type specific format.
+-------------+-------------+-------------+-------------+
| Length | 20 | CType |
+---------------------------+-------------+-------------+
| Policy Particles (list) |
+-------------------------------------------------------+
Individual policy particle has the following format
+---------------------------+---------------------------+
| Length | PType |
+---------------------------+---------------------------+
| Ptype specific format |
+-------------------------------------------------------+
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4. LPM calls
The LPM maintains access control state per flow. This state is
complementary to the RSVP state, and both are semantically attached
by flow handles, for all the LPM calls.
4.1 Success codes
All the LPM calls report success/failure status. This report is
made of three components: (1) a return code of the lpm function,
that reports the general success of the call (2) a global variable
"lpm_errno" that reports specific reason code (similar to the
errno in Unix), and (3) a global variable "lpm_eflgs" used for
flags set by the LPM call.
4.2 Flow handles (fh)
The LPM uses Flow Handles (fh) to associate RSVP flows with LPM
state. RSVP obtains flow handles by calling "lpm_open()", which
is called only once for each session or flow, upon the first
arrival of a POLICY_DATA object associated with that flow or
session. RSVP obtains the flow handle and stores it in the flow's
data structures, for future lpm calls.
When an RSVP message is fragmented, POLICY_DATA objects may be out
of order, and may reside in separate packets. The responsibility
of associating a POLICY_DATA object with a particular flow (and
its flow handles (fh)) lies "always" with RSVP. The FILTER_SPEC
object inside the POLICY_DATA object is visible to RSVP, and
should be used by it to aid in this classification. [Note 3]
It is important to note that under no circumstances should this
classification be left to the LPM.
4.3 Associating source and receiver objects
The access status of a reservation may depend on policy data
objects originating from the source, receivers or both. For
instance, a lecture can be sponsored by the source that would
provide the necessary credentials. If the LPM architecture is to
support source based policies, it must be able to associate source
objects with reservation state. Some associations are trivial
_________________________
[Note 3] The FILTER_SPEC object is opaque to the LPM and the only reason
it is included inside the POLICY_DATA object is to allow RSVP to
associate the object with its corresponding flow.
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(like in the case of fixed filter (FF) reservation style) but some
are more complicated (as in WF reservations). Since the LPM
architecture associates flow handles with individual source state,
it is the responsibility of RSVP to map reservations to their list
of associated sources. The list takes the form of a list of flow
handles, and can be passed on to LPM functions through a pair of
parameters, "int fh_num" and "int *fn_vec").
4.4 LPM calls format
lpm_open (int *fh)
When RSVP first encounters POLICY_DATA objects, it calls the LPM's
"lpm_open" routine. The LPM builds internal control blocks and
places the flow handle value in fh, for future reference.
All incoming POLICY_DATA objects are passed by RSVP to the LPM:
lpm_in (int fh_num, int *fh_vec, int vif, RSVP_HOP *hop, int
mtype, POLICY_DATA *polp, int ttd)
Parameter "vif" describes the input virtual interface [Note 4]
from which the RSVP message was received, "hop" describes the
node that sent the RSVP message (previous hop/next hop), and
"mtype" describes the type (and implicitly, the direction) of the
RSVP message (i.e., Path, Resv etc.). Parameter "polp" points to
the policy data object, and "ttd" provides a timeout (time to die)
value for the policy data object.
When RSVP is ready for output, it queries the LPM:
lpm_out (int fh_num, int *fh_vec, int vif, RSVP_HOP *hop, int
mtype, POLICY_DATA **polp)
The parameters are similar to those for "lpm_in". A successful
call places a pointer to the outgoing POLICY_DATA object in
"polp"; Notice that the output process is performed separately for
each outgoing RSVP message, but is required to maintain
_________________________
[Note 4] The term Virtual Interface (vif) is borrowed from DVMRP
terminology, although, for LPM purposes it can be any integer index that
RSVP associates with specific interfaces, independently from any routing
protocol.
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consistency and atomicity even if some LPM status had changed in
between outputs of different outgoing RSVP messages. Notice that
there is no formal limit on the size of the resulting POLICY_DATA
object. If the resulting object is too large to be sent in a
single RSVP message it is RSVP's responsibility to perform
semantic fragmentation because it has the unique knowledge about
available message space. An alternative solution would be to
provide an lpm_fragment() service to help RSVP in this task.
Checking the status of an existing reservation is done by calling:
lpm_status (int fh_session, int fh_num, int *fh_vec, int vif, int
cur_time, int phy_resv_handle, Object_header
*phy_resv_flwspec, int ind)
Status is checked individually for each outgoing (reserved) link.
Parameter "fh_session" specifies the flow handle associated with
the session, "phy_resv_handle" identifies the physical reservation
(e.g., ISPS, etc.), and "phy_resv_flwspec" describes the current,
merged FlowSpec of the reservation. The value of "cur_time"
describe the current RSVP time, which allows the LPM to timeout
old state (state with earlier time to die values). Parameter
"ind" is used to have different flavors of status checks:
"LPM_STATF_AGE": setting this flag ages (and times
out) LPM state associated with the specified fh. Status checks may
be periodic or event driven; this flag is set only for periodic
status checks. "LPM_STATF_RECALC": Status checks may involve
calculations over multiple outgoing interfaces, and thus need only
be done once for all interfaces before individual per-interface
status is reported. This bit is set on for the first vif checked
and is reset for the rest. [Note 5]
Status checks with "ind" set to 0 simply report values that were
already calculated before and do not age the LPM state.
If RSVP prunes branches from the reservation tree, it must notify
the LPM by calling:
lpm_prune (int fh_num, int *fh_vec, int vif, RSVP_HOP *hop, int
mtype)
_________________________
[Note 5] This is an optimization. While useless, there should be no harm
in recalculating status parameters, for each outgoing interface.
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(The details of this call is described in Section ).
When RSVP deletes an entire flow state, it must notify the LPM:
lpm_close (int fh)
Upon this notification, the LPM finishes its accounting for this
reservation (final debits/credits) and deletes all internal state
associated with fh.
Initializing the LPM is done once only, in the initialization
phase of RSVP, by calling.
lpm_config (void)
4.5 State Maintenance
LPM state must remain consistent with the corresponding RSVP
state. State is created when POLICY_DATA objects are passed to the
LPM and can be updated or removed through several possible
mechanisms that correspond to RSVP's state management mechanisms:
Timeout:
When new POLICY_DATA objects cease to arrive (as a result of
either change of policy or fragmentation loss) the locally
stored state begins to age. Each POLICY_ELEMENT/FILTER_SPEC
pair is subject to a timer, and when the timer goes off, the
state should be deleted. The timer mechanism should be
similar to that of RSVP and both should remained synchronized
in the following way: each time RSVP hands over a policy
object to the LPM (lpm_in()) it provides the LPM with time-
to-die value ("current-timer + time-to-live) ". Each time
RSVP verifies the status of a reservation (lpm_status()), it
provides the current timer value, forcing all pieces of
information with an earlier timeout value to be purged.
Teardown
From a network security standpoint, creating new policy state
requires the similar integrity protection as tearing it down.
We propose a very simple mechanism for tearing down state:
the state created by sending POLICY_ELEMENT Pe_i is torn down
by sending -Pe_i (the same object marked as teardown). In
this case, the LPM would locate the original state, compare
it with the teardown object, if a match is found, tear it
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down. We define each POLICY_ELEMENT as a pair of two CTypes,
thus effectively splitting the CType range of POLICY_ELEMENT
objects in two. Given a POLICY_ELEMENT i, Pe_i represents an
updated state, while Pe_i+1 represents teardown state of
CType i (-Pe_i).
Pruning When the shape of the reserved tree changes due to routing
updates or RSVP teardown messages, RSVP purges the state of
the pruned link, and must also call "lpm_prune()" to purge
the corresponding LPM state.
Closing: The call "lpm_close(fh)" purges all the state associated
with the handle fh. Closing a flow handle is done when RSVP
no longer maintains any state associated with that flow (a
sender quits, the session is over, etc.).
5. LPM internals
This section describes the current internal design of the LPM. While
this design is not part of the mandatory specification we recommend
following it.
5.1 LPM configurations
LPM configuration can be general, for all handlers, but can also
be type/handler specific. (e.g., a specific handler's rewrite
conversion table for policy data objects). Configuration may be
expressed in a simple configuration file or even through a
configuration language.
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+-----------------------------------------------------------+
| RSVP |
| Incoming Resv: Resv-header,LPM-header,P1,P2,P3,P4 |
| | |
+-----------------------------------------+-----------------+
| LPM: Common Layer \|/ |
| lpm_in() +-------- LPM-header,P1,P2,P3,P4 |
| / / | \ |
+-----------+-----+-----+-----+-----+-----+-----+-----+-----+
| | P1| | P2| | P3| | P4| |
| | \|/ | \|/ | \|/ | \|/ |
| | | | | |
| Handler 0 | Handler 1 | Handler 2 | Handler 4 | Handler 5 |
+-----------+-----------+-----------+-----------+-----------+
Figure 3: Disassembly of an incoming Resv message with POLICY_DATA
objects
5.2 The LPM layered Design
The internal format of POLICY_DATA objects is PType specific,
allowing up to 65535 independent types. Our design allow each
specific PType to be handled by a separate handler, and allow such
handlers to be added and configured independently. Clearly,
handlers are allowed to handler more than one PTypes.
The LPM is divided into two layers: a PType specific layer and a
common layer (figure ). The PType specific layer provides a set
of locally configured independent handlers, one for each PType
supported by the local node. The common layer provides the glue
between RSVP and the PType specific layer by multiplexing RSVP's
lpm calls into individual, PType specific calls.
On input, the common layer disassembles the incoming POLICY_DATA
object, dispatches the internal objects to their PType specific
handlers, and aggregates the return code status (figure ). On
output, it collects the internal objects from all active handlers,
and assembles them into a single POLICY_DATA object (figure ).
On status queries, the common layer queries all the active
handlers, and combines their individual status responses into a
single status result. We use the following rule: a reservation is
approved by the common layer, if there is at least one handler
that approves it, and none other rejects it. PType specific
handlers can accept, reject or be neutral in their responses.
[Note 6]
_________________________
[Note 6] A policy data object that determines cost is a good example for
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+-----------------------------------------------------------+
| RSVP |
| Outgoing Resv: Resv-header,LPM-header,P1,P2,P3,P4 |
| /|\ |
+-----------------------------------------+-----------------+
| LPM: Common Layer | |
| lpm_out() +-------> LPM-header,P1,P2,P3,P4 |
| / / /|\ \ |
+-----------+-----+-----+-----+-----+-----+-----+-----+-----+
| | P1| | P2| | P3| | P4| |
| | | | | | | | | |
| | | | | |
| Handler 0 | Handler 1 | Handler 2 | Handler 4 | Handler 5 |
+-----------+-----------+-----------+-----------+-----------+
Figure 4: Assembly of POLICY_DATA objects for an outgoing Resv message
5.3 Interaction between handlers
It is reasonable to assume that independent PTypes may require
some interaction between their handlers. Consider the case where
policy object type-1 is a credential type (defines a user
identity) and a type-2 is an accounting type (determines cost), a
possible interaction could be to let type-2 determine the cost,
and let type-1 perform the actual debiting according to the user
identity. Such interaction has two basic requirements: order
dependency and export capability. Order dependency is required
because type-2 must calculate the cost before type-1. Export
capability is needed to allow type-2 to export the calculation
results to type-1. Our implementation allows the ordering or
handlers to be expressed as part of local LPM configuration. It
also provides internal support for function calls between
independent handlers (in order to obtain exported state).
Consider the case where type-3 and type-4 also perform accounting.
The proposed architecture is flexible enough to allow local
configuration to select the handler that determines the debited
cost: type-2, type-3 or type-4.
_________________________
a neutral handler. It provide information about how much the flow costs,
but does not perform actual debiting.
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5.4 Default handling of policy data objects
In~[HER96c] we define the default handling of unrecognized POLICY_DATA
objects. If an RSVP node is LPM capable, it may be more beneficial
for the LPM to take that burden off from RSVP and perform it
itself. We propose the use of CType 0 for default handling: In a
policy node, only unrecognized objects would be handled by handler
PType 0. In a non-policy node, all objects are unrecognized, and
therefore should all are handled as PType 0, regardless of their
actual PType. PType 0 is regarded as a reserved type.
6. Acknowledgment
This document incorporates inputs from Deborah Estrin, Scott Shenker
and Bob Braden and feedback from RSVP collaborators.
References
[BAK96] F. Baker. RSVP Cryptographic Authentication "Internet-Draft",
draft-ietf-rsvp-md5-02.txt, 1996.
[HER96c] RSVP Extensions for Policy Control. "Internet-Draft", draft-
ietf-rsvp-policy-ext-00.[ps,txt].
[HER96b] Accounting and Access Control Policies for Resource
Reservation Protocols. "Internet-Draft", draft-ietf-rsvp-policy-
arch-00.[ps,txt].
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