Internet DRAFT - draft-ietf-rsvp-lpm-arch
draft-ietf-rsvp-lpm-arch
Internet Draft Shai Herzog
Expiration: December 1996 USC/ISI
File: draft-ietf-rsvp-lpm-arch-00.txt
March 5, 1996
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Abstract
This memo describes a set of building blocks for policy based
admission control in RSVP. 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
RSVP, by its definition, discriminates between users, by providing
some users with better service at the expense of others. Therefore,
it is reasonable to expect that RSVP be accompanied by mechanisms for
controlling and enforcing access and usage policies. In this
document, we refer to such policies as "access control". The term
"access control" is quite broad; it ranges from simple access
approval to sophisticated accounting and debiting mechanisms (Section
describes a few sample scenarios of access control mechanisms). For
scaling reasons, we concentrate on policies that follow the bilateral
agreements model. The bilateral model assumes that network clouds
(providers) contract with their closest point of contact (neighbor)
to establish ground rules and arrangements for access control and
accounting. These contracts are mostly local and do not rely on
global agreements. The bilateral model has similar scaling properties
to RSVP and is easier to maintain in distributed environments.
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, but leave the specific policies to local LPM
configurations. [Note 1]
We begin (Section ) by describing a few sample scenarios which
provide both motivation and demonstration of possible access control
policies. Section provides a general description of the RSVP/LPM
interface and Section discusses RSVP spec related issues. The
appendices describe the detailed interface (object formats, LPM
calls, etc.), and provide a peek into some of the more important LPM
implementation internals.
_________________________
[Note 1] We do not advocate specific access control policies since we
believe that standardization of specific policies may require
significantly more research and better understanding of the tradeoffs.
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2. Sample scenarios
In this section, we outline a few sample scenarios for access
control; we provide these scenarios as motivation and as needed
context for the LPM architecture proposed in this document.
These scenarios, as well as the LPM architecture as a whole, are
based on two simple assumptions: (1) RSVP would provide the needed
transport service of carrying access control state (POLICY_DATA
objects), hop-by-hop. (2) Access control policies are based on
bilateral agreements between neighboring providers or users, and are
enforced locally by a Local Policy Modules (LPMs). In this document
we do not discuss policies based on global agreements or global
information because of obvious scalability concerns.
2.1 Simple access control
To provide simple access control, the LPM attempts to match
incoming policy objects with one or more of the pre-configured
policies or bilateral agreements, in order to accept or reject the
reservation.
Consider the following network scenario: one receiver from ISI and
two from MIT listen to a PARC seminar. For simplicity of the
scenario, let us limit ourselves to a receiver based access
control scenario.
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........... .............
. *ba* . *ba* . .
. S1------->A--------------->B---+ *mc* .
. . . | .
........... ...C.........
PARC / BARRNet
*mc* /
/
........ .............D..... ........
. . . *ln+ne* / . . .
. *is* . *ln* . / . *ne* . *mi* .
. +----G------F<--------E-------->J------->K----+ .
. | . . *ln* *ne* .| . | .
...H.... ................. | ....L...
Los | MCINet | | Near
Nettos| *is* *sp* / *mi* | Net
.......I.... / .......M...
. | . ......N.. .*r2*/ \*r3*.
. *r1*| . . *r4*| . . / \ .
. R1 . . R4 . . R2 R3 .
............ ......... ...........
ISI Sprint MIT
LEGEND:
*xx* Credential
.... Cloud border
A..N Nodes
Si Sender i
Ri Receiver i
Figure 1: Simple access control
The bilateral agreements between each two neighboring providers
(e.g., R1, R2 with ISI, ISI with LosNettos,... BARRNet with PARC)
are simple: the first provider obtains a permission to make
reservations over the second provider's network. The notation
PD(cr,uid) represents a policy data object of type "cr"
(credential) verifying that the flow belongs to uid. Credentials
can be hierarchical, and may be rewritten on a hop by hop basis
through a locally configured conversion table.
Figure illustrates a reservation scenario. An typical example of
a bilateral agreement could be between MCI and LosNettos: MCI
would allow the LosNettos users to use its backbone. A policy data
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object PD(cr, LosNettos) would be interpreted by MCI as a green
light to accept the reservation. In this scenario, reservations
from R1, R2, R3 carry policy data objects that propagate hop-by-
hop (encapsulated in reservation messages) toward S1. Assuming
all nodes are configured consistently, policy objects are
rewritten in nodes B,D,G,I,K,M, which are entry points to clouds).
The MCI cloud is interesting. E is not a border/policy node, but
still, it receives the following policy data objects: F->E:
PD(cr,LosNettos) and J->E: PD(cr,NearNet). Assuming E has no
authority to merge or rewrite these credentials, it must
concatenate the two objects and send PD(cr,LosNettos) +
PD(cr,NearNet) to D. Let us further assume that D is configured
with the following conversion table:
PD(cr, LosNettos) -> PD(cr, MCI)
PD(cr, NearNet) -> PD(cr, MCI)
D's LPM first checks if LosNettos and NearNet are authorized to
reserve on their corresponding links and responds accordingly.
Assuming authorization is cleared, it merges and rewrites these
policy objects as PD(cr, MCI) and forwards the reservation to C.
To complicate the example, assume the conversion table was:
PD(cr, LosNettos) -> PD(cr, MCI1)
PD(cr, NearNet) -> PD(cr, MCI2)
Then D's LPM would forward PD(cr, MCI1) + PD(cr, MCI2) to C
instead.
Local policies can also reject reservations:
In figure we see that a reservation made by R4 is rejected
because it arrives with insufficient credentials: the local policy
in node J accepts only traffic marked as PD(cr, NearNet), and R4's
reservation arrives with PD(cr, Sprint).
2.2 Advanced reservation and preemption control
Advanced reservation can be built on top of simple access control:
consider the case where every advanced reservation consists of a
set of bilateral agreements between different service providers,
reserving network capacity at some future period of time. When
advanced reservations are not public (i.e., only authorized users
can use them), three classes of reservations exist: (1) walk-ins
(where the conference itself does not have advanced reservations,
(2) advanced reservation with unauthorized users, and (3) advanced
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reservation with authorized users. These numbers (1..3) can define
a "preemption priority" (i.e., walk-ins are preempted first,
unauthorized pre-reserved second, and authorized pre-reserved are
never preempted).
The advanced reservation scenario is almost identical to the
simple access control: let us assume that each bilateral pre-
registration is identified by a PRID (Pre-Registration
confirmation ID). Policy data objects of type AR (Advanced
Reservation) would take the following form: PD(ar, prid ,uid).
When an AR object arrives, the LPM verifies the existence of pre-
reservation prid, and checks that uid is permitted to use it.
Finally, the flow is classified to one of the above three
preemptive priorities and RSVP is notifies.
2.3 Quota enforcement/accounting/debiting
The next step is to allow for more sophisticated access control
that is based on usage feedback. Here we add two additional
mechanisms which (1) determine how much should be debited for a
reservation and (2) what debiting mechanism should be used (if
any). The following scenarios assume a pre-existing set of local
accounts. These accounts are established by bilateral agreements
that pre-purchase network capacity and set applicable debiting
rules. The role of accounting mechanism is to verify the
availability of funds/quotas in these accounts for maintaining the
reservation. We consider several accounting schemes and briefly
describe three: simple debiting, limited debiting, Edge Pricing,
and MultiCost (MCost).
Simple debiting
Consider the following example: lets assume that LosNettos and
Nearnet each have a debit account (pre-purchased capacity) with
MCI for their traffic. When E's LPM receives the following
PD(cr,LosNettos) and PD(cr,NearNet) for flow f, it must decide the
following: (1) How much should be debited for flow f, and (2) how
would that debit be shared between the account of LosNettos and
NearNet. These are local configuration issues left for service
providers. In this scenario, the LPM would attempt to perform the
debiting, and would notify RSVP on success or failure. The other
aspects of the scenario (Merging policy data objects and
forwarding them) is identical to that of simple access control.
Limited debiting (willingness to pay)
Although we do not have a full understanding of the dynamics of
willingness-to-pay and its properties, we can outline the basic
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scenario, as an extension of the simple debiting model.
Willingness to pay is manifested as a limit on the policy object
that authorizes the debit. For instance, PD(crwp,ISI,10% of
unicast) would represent a policy data object of type crwp
(Credential, Willingness to Pay), that authorizes debiting the ISI
account up to 10% of the unicast cost. Here, the basic idea is
that market forces would be the driving force behind what users
specify as their willingness to pay.
Edge Pricing
Edge Pricing was presented in [SHE95]. This paradigm is based on the
assumption that network costs can be estimated and approximated at
the edge of the network, based on purely local information. Edge
Pricing is an extension of simple debiting: Edge Pricing can
determine how much is to be debited, and the set of credentials
associated with the reservation determines who (which account)
should be debited.
MultiCost (MCost)
MCost is an accounting scheme (and mechanism) that was introduced
in [HER95]. MCost has a unique feature: it takes into account the
benefits of sharing a multicast tree and distributes these savings
among the members of the multicast group, according to
configurable policies, basic fairness, and equality.
MCost computes the cost allocated to each user, and that cost can
be the basis for debiting. MCost can be combined with simple
debiting in a similar manner to Edge Pricing.
3. 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.
3.1 POLICY_DATA objects
RSVP messages may carry optional POLICY_DATA objects. Each
individual POLICY_DATA object includes a FILTER_SPEC object which
identifies the flow it is associated with. We expect some access
control mechanisms to use session POLICY_DATA objects (with
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wildcard FILTER_SPEC) while others may require the full power of
per-flow object semantics. Generally, we assume that POLICY_DATA
objects may be carried by any RSVP message, (e.g., Path, Resv,
ResvErr, etc.).
3.2 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 2]
_________________________
[Note 2] 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 2: The modular context of access control
3.3 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.
3.3.1 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 3: LPM and RSVP: message flow outline
3.3.2 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.
3.3.3 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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3.3.4 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 3]
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.
3.3.5 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 the policy data objects. This however is not
the case when policy is enforced at boundary nodes only.
_________________________
[Note 3] 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.
3.4 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 appendix .
4. RSVP spec issues
This section presents changes to the RSVP specifications, required to
support the LPM architecture.
4.1 New RSVP message: Reservation Report
The basic building blocks of access control and accounting must be
bi-directional in order to allow both source and receiver based
policy data objects and both advertising and feedback. Which RSVP
messages should encapsulate these upstream and downstream objects?
The choice for upstream message is natural; the reservation
message. The downstream direction, however, is more problematic:
Path messages flow downstream, but are routed according to the
multicast group membership, and therefore cannot be accurately
delivered to a specific next hop. [Note 4]
_________________________
[Note 4] The same problem existed for the original design of ResvErr,
until it was changed to a unicast delivery along the multicast tree.
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This makes Path messages less likely to be used for access
control, and especially for accounting.
We proposed a new RSVP message type: "Reservation Report" (Rept).
Reservation Report messages are sent unicast, downstream,
according to the Next_Hop object carried by Resv messages;
although Reservation Report messages follow the multicast tree,
their unicast delivery provides accurate delivery to the
appropriate next hop nodes and only to these nodes [Note 5]
Although we propose this new message for supporting the LPM
architecture, it may prove useful for other, more general
functions of the RSVP protocol. A reservation may have different
forms of responses to it: A negative response (ResvErr), a
positive response (ack), and a more advanced form of a Reservation
Report, like the one proposed here. An integrated approach may
incorporate all three responses in the same message type while
leaving room for future types.
4.2 List of proposed changes to the RSVP spec
o LPM interface (LPM calls, error codes and response to errors)
o API modifications.
o Reservation Report messages (Either in a general form, or
specific to the LPM architecture).
o Default handling of policy data objects.
5. 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.
[HER95] S. Herzog, S. Shenker and D. Estrin, Sharing the Cost of
_________________________
[Note 5] Consider the case of multipoint links or network clouds: a
single copy of a Path message may be delivered to an unknown number of
next hops, while the copy of a Report message is guaranteed to reach
only the targeted node.
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Multicast Trees: An Axiomatic Analysis, "Proceedings of ACM/SIGCOMM
'95", Cambridge, MA, Aug. 1995
[SHE95] S. Shenker, D. Clark, D. Estrin, and S. Herzog Pricing in
Computer Networks: Reshaping the Research Agenda,
"Telecommunications Policy", Vol. 20, No. 1, 1996 also published in
"Proceedings of the Twenty-Third Annual Telecommunications Policy
Research Conference", 1995.
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7. Appendix: object format
POLICY_DATA objects are built from basic building blocks (sub-objects),
with the following format:
+-------------+-------------+-------------+-------------+
| length | PType |
+-------------+-------------+-------------+-------------+
| PType specific format |
+-------------------------------------------------------+
The header of the POLICY_DATA object is defined by the RSVP spec, while
the body format is hidden from RSVP, and is only known to the LPM.
POLICY_DATA object include the standard RSVP object header, with Class =
class_POLICY_DATA, and a CType value. Currently, the CType value selects
from three versions of POLICY_DATA objects: "POLICY_SIMPLE",
"POLICY_INTEGRITY", and "POLICY_ENCAP".
POLICY_SIMPLE:
+-------------+-------------+-------------+-------------+
| length | POLICY_DATA | 1 |
+-------------+-------------+-------------+-------------+
| policy data sub-object 1 |
+-------------------------------------------------------+
....
+-------------------------------------------------------+
| policy data sub-object n |
+-------------------------------------------------------+
POLICY_INTEGRITY:
The object format is similar to POLICY_SIMPLE, with the added integrity
envelope.
+-------------+-------------+-------------+-------------+
| length | POLICY_DATA | 2 |
+-------------+-------------+-------------+-------------+
| RSVP_HOP object |
+-------------------------------------------------------+
| INTEGRITY object |
+-------------------------------------------------------+
| policy data sub-object 1 |
+-------------------------------------------------------+
....
+-------------------------------------------------------+
| policy data sub-object n |
+-------------------------------------------------------+
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Encapsulation provides an optional security envelope for policy data
objects; it ensures that all the policy sub-objects were created by the
node described by the RSVP_HOP object, and were not compromised. In
this document, we do not define how the INTEGRITY object is to be
computed. However, we would like to note that it may be computed over
other RSVP objects like SESSION, SCOPE etc., in order to guarantee that
the POLICY_DATA object is associated with the right flow/reservation.
POLICY_ENCAP:
This object is the external (visible) representation of POLICY_DATA
object, representing the full format.
+-------------+-------------+-------------+-------------+
| length | POLICY_DATA | 255 |
+-------------+-------------+-------------+-------------+
| FILTER_SPEC object |
+-------------+-----------------------------------------+
| Flags | Reserved |
+-------------+-----------------------------------------+
| POLICY_ENCAP or POLICY_SIMPLE object 1 |
+-------------------------------------------------------+
....
+-------------------------------------------------------+
| POLICY_ENCAP or POLICY_SIMPLE object n |
+-------------------------------------------------------+
Note: The FILTER_SPEC object is opaque to the LPM, however, it is
included in the POLICY_OBJECT to assist RSVP with fragmentation.
There is currently only one flag in the flags field,
POLICYD_FLAG_REPORT. This flag can be specified only for Resv messages,
and tells the LPM (and RSVP) that the reservation requires a Reservation
Report message. [Note 6]
8. Appendix: 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.
_________________________
[Note 6] This may become obsolete if/when a Report Request bit is added
to the Resv message format.
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8.1 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.
8.2 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 7]
It is important to note that under no circumstances should this
classification be left to the LPM.
8.3 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 (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
_________________________
[Note 7] 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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*fn_vec").
8.4 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)
Parameter "vif" describes the input virtual interface [Note 8]
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.
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 consistency and atomicity
even if some LPM status had changed in between outputs of different
outgoing RSVP messages.
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
phy_resv_handle, Object_header *phy_resv_flwspec, int ind)
_________________________
[Note 8] 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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Status is checked individually for each outgoing (reserved) link.
Parameter "fh_session" specifies the flow handle associated with the
session, and "phy_resv_handle" identifies the physical reservation
(e.g., ISPS, etc.), "phy_resv_flwspec" describes the current, merged
FlowSpec of the reservation. "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 9]
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)
(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)
_________________________
[Note 9] This is an optimization. While useless, there should be no harm
in recalculating status parameters, for each outgoing interface.
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8.5 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:
Atomic object management: Every new POLICY_DATA object is self
contained and its content overrides all previous state: existing
state that is not listed in the newly arriving POLICY_DATA
object is purged.
Aging: When new POLICY_DATA objects cease to arrive (either because
RSVP messages cease to arrive, or because they arrive without
policy data objects), the stored state begins to age. Aging is
done in a similar manner to the way RSVP ages reservations: When
a policy data object arrives, a timer is set to TTL_FACTOR.
Every call to "lpm_status" decreases the timer by 1. When the
timer reaches 0, the state is purged.
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.).
9. Appendix: LPM internals
This appendix describes the current internal design of the LPM. While
this design is not part of the mandatory specification we recommend
following it.
9.1 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 4: Disassembly of an incoming Resv message with POLICY_DATA
objects
9.2 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 10]
_________________________
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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 5: Assembly of POLICY_DATA objects for an outgoing Resv message
9.3 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. (See the
scenario 3 Section ). 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.
_________________________
[Note 10] A policy data object that determines cost is a good example
for a neutral handler. It provide information about how much the flow
costs, but does not perform actual debiting.
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9.4 Default handling of policy data objects
Default handling of policy data objects is needed in two cases:
first, when the RSVP node is not a policy node at all, and second,
when the arriving POLICY_DATA object includes objects of an unknown
type. Both cases are handled in a similar manner: the policy object
is stored and forwarded without modification, merging or any other
operation. In our implementation we dedicate PType 0 for default
handling: Unrecognized objects are handled by handler of PType 0. In
a non-policy node, all objects are unrecognized, and thus all are
handled as PType 0, regardless of their actual PType. PType 0 is
regarded as a reserved type.
Notice that the internal format of POLICY_DATA objects is a list of
objects; If a node is a merging point in the multicast tree, the
default handler output is simply a concatenation of the lists of
incoming objects encapsulated in a single POLICY_DATA object, of type
POLICY_ENCAP.
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