RFC4495: A Resource Reservation Protocol (RSVP) Extension for the Reduction of Bandwidth of a Reservation Flow
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Related keywords:
(rsvpv1)
Network Working Group J. Polk
Request for Comments: 4495 S. Dhesikan
Updates: 2205 Cisco Systems
Category: Standards Track May 2006
A Resource Reservation Protocol (RSVP) Extension for the
Reduction of Bandwidth of a Reservation Flow
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document proposes an extension to the Resource Reservation
Protocol (RSVPv1) to reduce the guaranteed bandwidth allocated to an
existing reservation. This mechanism can be used to affect
individual reservations, aggregate reservations, or other forms of
RSVP tunnels. This specification is an extension of RFC 2205.
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Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................4
2. Individual Reservation Reduction Scenario .......................4
3. RSVP Aggregation Overview .......................................6
3.1. RSVP Aggregation Reduction Scenario ........................8
4. Requirements for Reservation Reduction ..........................9
5. RSVP Bandwidth Reduction Solution ..............................10
5.1. Partial Preemption Error Code .............................11
5.2. Error Flow Descriptor .....................................11
5.3. Individual Reservation Flow Reduction .....................11
5.4. Aggregation Reduction of Individual Flows .................12
5.5. RSVP Flow Reduction Involving IPsec Tunnels ...............12
5.6. Reduction of Multiple Flows at Once .......................13
6. Backwards Compatibility ........................................13
7. Security Considerations ........................................14
8. IANA Considerations ............................................15
9. Acknowledgements ...............................................15
10. References ....................................................15
10.1. Normative References .....................................15
10.2. Informative References ...................................16
Appendix A. Walking through the Solution ..........................17
1. Introduction
This document proposes an extension to the Resource Reservation
Protocol (RSVP) [1] to allow an existing reservation to be reduced in
allocated bandwidth in lieu of tearing that reservation down when
some of that reservation's bandwidth is needed for other purposes.
Several examples exist in which this mechanism may be utilized.
The bandwidth allotted to an individual reservation may be reduced
due to a variety of reasons such as preemption, etc. In such cases,
when the entire bandwidth allocated to a reservation is not required,
the reservation need not be torn down. The solution described in
this document allows endpoints to negotiate a new (lower) bandwidth
that falls at or below the specified new bandwidth maximum allocated
by the network. Using a voice session as an example, this indication
in RSVP could lead endpoints, using another protocol such as Session
Initiation Protocol (SIP) [9], to signal for a lower-bandwidth codec
and retain the reservation.
With RSVP aggregation [2], two aggregate flows with differing
priority levels may traverse the same router interface. If that
router interface reaches bandwidth capacity and is then asked to
establish a new reservation or increase an existing reservation, the
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router has to make a choice: deny the new request (because all
resources have been utilized) or preempt an existing lower-priority
reservation to make room for the new or expanded reservation.
If the flow being preempted is an aggregate of many individual flows,
this has greater consequences. While [2] clearly does not terminate
all the individual flows if an aggregate is torn down, this event
will cause packets to be discarded during aggregate reservation
reestablishment. This document describes a method where only the
minimum required bandwidth is taken away from the lower-priority
aggregated reservation and the entire reservation is not preempted.
This has the advantage that only some of the microflows making up the
aggregate are affected. Without this extension, all individual flows
are affected and the deaggregator will have to attempt the
reservation request with a reduced bandwidth.
RSVP tunnels utilizing IPsec [8] also require an indication that the
reservation must be reduced to a certain amount (or less). RSVP
aggregation with IPsec tunnels is being defined in [11], which should
be able to take advantage of the mechanism created here in this
specification.
Note that when this document refers to a router interface being
"full" or "at capacity", this does not imply that all of the
bandwidth has been used, but rather that all of the bandwidth
available for reservation(s) via RSVP under the applicable policy has
been used. Policies for real-time traffic routinely reserve capacity
for routing and inelastic applications, and may distinguish between
voice, video, and other real-time applications.
Explicit Congestion Notification (ECN) [10] is an indication that the
transmitting endpoint must reduce its transmission. It does not
provide sufficient indication to tell the endpoint by how much the
reduction should be. Hence the application may have to attempt
multiple times before it is able to drop its bandwidth utilization
below the available limit. Therefore, while we consider ECN to be
very useful for elastic applications, it is not sufficient for the
purpose of inelastic application where an indication of bandwidth
availability is useful for codec selection.
Section 2 discusses the individual reservation flow problem, while
Section 3 discusses the aggregate reservation flow problem space.
Section 4 lists the requirements for this extension. Section 5
details the protocol changes necessary in RSVP to create a
reservation reduction indication. And finally, the appendix provides
a walk-through example of how this extension modifies RSVP
functionality in an aggregate scenario.
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This document updates RFC 2205 [1], as this mechanism affects the
behaviors of the ResvErr and ResvTear indications defined in that
document.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [4].
2. Individual Reservation Reduction Scenario
Figure 1 is a network topology that is used to describe the benefit
of bandwidth reduction in an individual reservation.
+------------+ +------------+
| |Int 1 | |Int 7 | |
Flow 1===> | +----- | |------+ | Flow 1===>
| R1 |Int 2 |===========>|Int 8 | R2 |
| | |:::::::::::>| | |
Flow 2:::> | +----- | |------+ | Flow 2:::>
| |Int 3 | |Int 9 | |
+------------+ +------------+
Figure 1. Simple Reservation Flows
Legend/Rules:
- Flow 1 priority = 300
- Flow 2 priority = 100
- Both flows are shown in the same direction (left to
right). Corresponding flows in the reverse direction are
not shown for diagram simplicity
RSVP is a reservation establishment protocol in one direction only.
This split-path philosophy is because the routed path from one device
to the other in one direction might not be the routed path for
communicating between the same two endpoints in the reverse
direction. End-systems must request 2 one-way reservations if that
is what is needed for a particular application (like voice calls).
Please refer to [1] for the details on how this functions. This
example only describes the reservation scenario in one direction for
simplicity's sake.
Figure 1 depicts 2 routers (R1 and R2) initially with only one flow
(Flow 1). The flows are forwarded from R1 to R2 via Int 2. For this
example, let us say that Flow 1 and Flow 2 each require 80 units of
bandwidth (such as for the codec G.711 with no silence suppression).
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Let us also say that the RSVP bandwidth limit for Int 2 of R1 is 100
units.
As described in [3], a priority indication is established for each
flow. In fact, there are two priority indications:
1) one to establish the reservation, and
2) one to defend the reservation.
In this example, Flow 1 and Flow 2 have an 'establishing' and a
'defending' priority of 300 and 100, respectively. Flow 2 will have
a higher establishing priority than Flow 1 has for its defending
priority. This means that when Flow 2 is signaled, and if no
bandwidth is available at the interface, Flow 1 will have to
relinquish bandwidth in favor of the higher-priority request of Flow
2. The priorities assigned to a reservation are always end-to-end,
and not altered by any routers in transit.
Without the benefit of this specification, Flow 1 will be preempted.
This specification makes it possible for the ResvErr message to
indicate that 20 units are still available for a reservation to
remain up (the interface's 100 units maximum minus Flow 2's 80
units). The reservation initiating node (router or end-system) for
Flow 1 has the opportunity to renegotiate (via call signaling) for
acceptable parameters within the existing and available bandwidth for
the flow (for example, it may decide to change to using a codec such
as G.729)
The problems avoided with the partial failure of the flow are:
- Reduced packet loss, which results as Flow 1 attempts to
reestablish the reservation for a lower bandwidth.
- Inefficiency caused by multiple attempts until Flow 1 is able to
request bandwidth equal to or lower than what is available. If
Flow 1 is established with much less than what is available then it
leads to inefficient use of available bandwidth.
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3. RSVP Aggregation Overview
The following network overview is to help visualize the concerns that
this specification addresses in RSVP aggregates. Figure 2 consists
of 10 routers (the boxes) and 11 flows (1, 2, 3, 4, 5, 9, A, B, C, D,
and E). Initially, there will be 5 flows per aggregate (Flow 9 will
be introduced to cause the problem we are addressing in this
document), with 2 aggregates (X and Y); Flows 1 through 5 in
aggregate X and Flows A through E in aggregate Y. These 2 aggregates
will cross one router interface utilizing all available capacity (in
this example).
RSVP aggregation (per [2]) is no different from an individual
reservation with respect to being unidirectional.
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Aggregator of X Deaggregator of X
| |
V V
+------+ +------+ +------+ +------+
Flow 1-->| | | | | | | |-->Flow 1
Flow 2-->| | | | | | | |-->Flow 2
Flow 3-->| |==>| | | |==>| |-->Flow 3
Flow 4-->| | ^ | | | | ^ | |-->Flow 4
Flow 5-->| | | | | | | | | |-->Flow 5
Flow 9 | R1 | | | R2 | | R3 | | | R4 | Flow 9
+------+ | +------+ +------+ | +------+
| || || |
Aggregate X-->|| Aggregate X ||<--Aggregate X
|| | ||
+--------------+ | +--------------+
| |Int 7 | | |Int 1 | |
| +----- | V |------+ |
| R10 |Int 8 |===========>|Int 2 | R11 |
| | |:::::::::::>| | |
| +----- | ^ |------+ |
| |Int 9 | | |Int 3 | |
+--------------+ | +--------------+
.. | ..
Aggregate Y--->.. Aggregate Y ..<---Aggregate Y
| .. .. |
+------+ | +------+ +------+ | +------+
Flow A-->| | | | | | | | | |-->Flow A
Flow B-->| | V | | | | V | |-->Flow B
Flow C-->| |::>| | | |::>| |-->Flow C
Flow D-->| | | | | | | |-->Flow D
Flow E-->| R5 | | R6 | | R7 | | R8 |-->Flow E
+------+ +------+ +------+ +------+
^ ^
| |
Aggregator of Y Deaggregator of Y
Figure 2. Generic RSVP Aggregate Topology
Legend/Rules:
- Aggregate X priority = 100
- Aggregate Y priority = 200
- All boxes are routers
- Both aggregates are shown in the same direction (left to
right). Corresponding aggregates in the reverse direction
are not shown for diagram simplicity.
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The path for aggregate X is:
R1 => R2 => R10 => R11 => R3 => R4
where aggregate X starts in R1, and deaggregates in R4.
Flows 1, 2, 3, 4, 5, and 9 communicate through aggregate A.
The path for aggregate Y is:
R5 ::> R6 ::> R10 ::> R11 ::> R7 ::> R8
where aggregate Y starts in R5, and deaggregates in R8.
Flows A, B, C, D, and E communicate through aggregate B.
Both aggregates share one leg or physical link: between R10 and R11,
thus they share one outbound interface: Int 8 of R10, where
contention of resources may exist. That link has an RSVP capacity of
800 kbps. RSVP signaling (messages) is outside the 800 kbps in this
example, as is any session signaling protocol like SIP.
3.1. RSVP Aggregation Reduction Scenario
Figure 2 shows an established aggregated reservation (aggregate X)
between the routers R1 and R4. This aggregated reservation consists
of 5 microflows (Flows 1, 2, 3, 4, and 5). For the sake of this
discussion, let us assume that each flow represents a voice call and
requires 80 kb (such as for the codec G.711 with no silence
suppression). Aggregate X request is for 400 kbps (80 kbps * 5
flows). The priority of the aggregate is derived from the individual
microflows that it is made up of. In the simple case, all flows of a
single priority are bundled as a single aggregate (another priority
level would be in another aggregate, even if traversing the same path
through the network). There may be other ways in which the priority
of the aggregate is derived, but for this discussion it is sufficient
to note that each aggregate contains a priority (both hold and
defending priority). The means of deriving the priority is out of
scope for this discussion.
Aggregate Y, in Figure 2, consists of Flows A, B, C, D, and E and
requires 400 kbps (80 kbps * 5 flows), and starts at R5 and ends R8.
This means there are two aggregates occupying all 800 kbps of the
RSVP capacity.
When Flow 9 is added into aggregate X, this will occupy 80 kbps more
than Int 8 on R10 has available (880k offered load vs. 800k capacity)
[1] and [2] create a behavior in RSVP to deny the entire aggregate Y
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and all its individual flows because aggregate X has a higher
priority. This situation is where this document focuses its
requirements and calls for a solution. There should be some means to
signal to all affected routers of aggregate Y that only 80 kbps is
needed to accommodate another (higher priority) aggregate. A
solution that accomplishes this reduction instead of a failure could:
- reduce significant packet loss of all flows within aggregate Y
During the re-reservation request period of time no packets will
traverse the aggregate until it is reestablished.
- reduces the chances that the reestablishment of the aggregate
will reserve an inefficient amount of bandwidth, causing the
likely preemption of more individual flows at the aggregator
than would be necessary had the aggregator had more information
(that RSVP does not provide at this time)
During reestablishment of the aggregation in Figure 2 (without any
modification to RSVP), R8 would guess at how much bandwidth to ask
for in the new RESV message. It could request too much bandwidth,
and have to wait for the error that not that much bandwidth was
available; it could request too little bandwidth and have that
aggregation accepted, but this would mean that more individual flows
would need to be preempted outside the aggregate than were necessary,
leading to inefficiencies in the opposite direction.
4. Requirements for Reservation Reduction
The following are the requirements to reduce the bandwidth of a
reservation. This applies to both individual and aggregate
reservations:
Req#1 - MUST have the ability to differentiate one reservation from
another. In the case of aggregates, it MUST distinguish one
aggregate from other flows.
Req#2 - MUST have the ability to indicate within an RSVP error
message (generated at the router with the congested
interface) that a specific reservation (individual or
aggregate) is to be reduced in bandwidth.
Req#3 - MUST have the ability to indicate within the same error
message the new maximum amount of bandwidth that is available
to be utilized within the existing reservation, but no more.
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Req#4 - MUST NOT produce a case in which retransmitted reduction
indications further reduce the bandwidth of a reservation.
Any additional reduction in bandwidth for a specified
reservation MUST be signaled in a new message.
RSVP messages are unreliable and can get lost. This specification
should not compound any error in the network. If a reduction message
were lost, another one needs to be sent. If the receiver ends up
receiving two copies to reduce the bandwidth of a reservation by some
amount, it is likely the router will reduce the bandwidth by twice
the amount that was actually called for. This will be in error.
5. RSVP Bandwidth Reduction Solution
When a reservation is partially failed, a ResvErr (Reservation Error)
message is generated just as it is done currently with preemptions.
The ERROR_SPEC object and the PREEMPTION_PRI object are included as
well. Very few additions/changes are needed to the ResvErr message
to support partial preemptions. A new error subcode is required and
is defined in Section 5.1. The ERROR_SPEC object contained in the
ResvErr message indicates the flowspec that is reserved. The
bandwidth indication in this flowspec SHOULD be less than the
original reservation request. This is defined in Section 5.2.
A comment about RESV messages that do not use reliable transport:
This document RECOMMENDS that ResvErr messages be made reliable by
implementing mechanisms in [6].
The current behavior in RSVP requires a ResvTear message to be
transmitted upstream when the ResvErr message is transmitted
downstream (per [1]). This ResvTear message terminates the
reservation in all routers upstream of the router where the failure
occurred. This document requires that the ResvTear is only generated
when the reservation is to be completely removed. In cases where the
reservation is only to be reduced, routers compliant with this
specification require that the ResvTear message MUST NOT be sent.
The appendix has been written to walk through the overall solution to
the problems presented in Sections 2 and 3. There is mention of this
ResvTear transmission behavior in the appendix.
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5.1. Partial Preemption Error Code
The ResvErr message generated due to preemption includes the
ERROR_SPEC object as well as the PREEMPTION_PRI object. The format
of ERROR_SPEC objects is defined in [1]. The error code listed in
the ERROR_SPEC object for preemption [5] currently is as follows:
Errcode = 2 (Policy Control Failure) and
ErrSubCode = 5 (ERR_PREEMPT)
The following error code is suggested in the ERROR_SPEC object for
partial preemption:
Errcode = 2 (Policy Control Failure) and
ErrSubCode = 102 (ERR_PARTIAL_PREEMPT)
There is also an error code in the PREEMPTION-PRI object. This error
code takes a value of 1 to indicate that the admitted flow was
preempted [3]. The same error value of 1 may be used for the partial
preemption case as well.
5.2. Error Flow Descriptor
The error flow descriptor is defined in [1] and [7]. In the case of
partial failure, the flowspec contained in the error flow descriptor
indicates the highest average and peak rates that the preempting
system can accept in the next RESV message. The deaggregator must
reduce its reservation to a number less than or equal to that,
whether by changing codecs, dropping reservations, or some other
mechanism.
5.3. Individual Reservation Flow Reduction
When a router requires part of the bandwidth that has been allocated
to a reservation be used for another flow, the router engages in the
partial reduction of bandwidth as described in this document. The
router sends a ResvErr downstream to indicate the partial error with
the error code and subcode as described in section 5.1. The flowspec
contained in the ResvErr message will be used to indicate the
bandwidth that is currently allocated.
The requesting endpoint that receives the ResvErr can then negotiate
with the transmitting endpoint to lower the bandwidth requirement (by
selecting another lower bandwidth codec, for example). After the
negotiations, both endpoints will issue the RSVP PATH and RESV
message with the new, lowered bandwidth.
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5.4. Aggregation Reduction of Individual Flows
When a partial failure occurs in an aggregation scenario, the
deaggregator receives the ResvErr message with the reduction
indication from a router in the path of the aggregate. It then
decides whether one or more individual flows from the aggregate are
to be affected by this ResvErr message. The following choices are
possible:
o If that (deaggregator) router determines that one or more
individual flow(s) are to partially failed, then it sends a
ResvErr message with a reduced bandwidth indication to those
individual flow(s). This is as per the descriptions in the
previous section (5.3).
o If that (deaggregator) router determines that one individual flow
is to be preempted to satisfy the aggregate ResvErr, it determines
which flow is affected. That router transmits a new ResvErr
message downstream per [3]. That same router transmits a ResvTear
message upstream. This ResvTear message of an individual flow
does not tear down the aggregate. Only the individual flow is
affected.
o If that (deaggregator) router determines that multiple individual
flows are to be preempted to satisfy the aggregate ResvErr, it
chooses which flows are affected. That router transmits a new
ResvErr message downstream as per [3] to each individual flow.
The router also transmits ResvTear messages upstream for the same
individual flows. These ResvTear messages of an individual flow
do not tear down the aggregate. Only the individual flows are
affected.
In all cases, the deaggregator lowers the bandwidth requested in the
Aggregate Resv message to reflect the change.
Which particular flow or series of flows within an aggregate are
picked by the deaggregator for bandwidth reduction or preemption is
outside the scope of this document.
5.5. RSVP Flow Reduction Involving IPsec Tunnels
RFC 2207 (per [8]) specifies how RSVP reservations function in IPsec
data flows. The nodes initiating the IPsec flow can be an end-system
like a computer, or it can router between two end-systems, or it can
be an in-line bulk encryption device immediately adjacent to a router
interface; [11] directly addresses this later scenario.
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The methods of identification of an IPsec with reservation flow are
different from non-encrypted flows, but how the reduction mechanism
specified within this document functions is not.
An IPsec with reservation flow is, for all intents and purposes,
considered an individual flow with regard to how to reduce the
bandwidth of the flow. Obviously, an IPsec with reservation flow can
be a series of individual flows or disjointed best-effort packets
between two systems. But to this specification, this tunnel is an
individual RSVP reservation.
Anywhere within this specification that mentions an individual
reservation flow, the same rules of bandwidth reduction and
preemption MUST apply.
5.6. Reduction of Multiple Flows at Once
As a cautionary note, bandwidth SHOULD NOT be reduced across multiple
reservations at the same time, in reaction to the same reduction
event. A router not knowing the impact of reservation bandwidth
reduction on more than one flow may cause more widespread ill effects
than is necessary.
This says nothing to a policy where preemption should or should not
occur across multiple flows.
6. Backwards Compatibility
Backwards compatibility with this extension will result in RSVP
operating as it does without this extension, and no worse. The two
routers involved in this extension are the router that had the
congested interface and the furthest downstream router that
determines what to do with the reduction indication.
In the case of the router that experiences congestion or otherwise
needs to reduce the bandwidth of an existing reservation:
- If that router supports this extension:
#1 - it generates the ResvErr message with the error code
indicating the reduction in bandwidth.
#2 - it does not generate the ResvTear message.
- If that router does not support this extension, it generates both
ResvErr and ResvTear messages according to [1].
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In the case of the router at the extreme downstream of a reservation
that receives the ResvErr message with the reduction indication:
- If that router does support this extension:
#1 - it processes this error message and applies whatever local
policy it is configured to do to determine how to reduce the
bandwidth of this designated flow.
- If the router does not support this extension:
#1 - it processes the ResvErr message according to [1] and all
extensions it is able to understand, but not this extension
from this document.
Thus, this extension does not cause ill effects within RSVP if one or
more routers support this extension, and one or more routers do not
support this extension.
7. Security Considerations
This document does not lessen the overall security of RSVP or of
reservation flows through an aggregate.
If this specification is implemented poorly - which is never
intended, but is a consideration - the following issues may arise:
1) If the ResvTear messages are transmitted initially (at the same
time as the ResvErr messages indicating a reduction in bandwidth
is necessary), all upstream routers will tear down the entire
reservation. This will free up the total amount of bandwidth of
this reservation inadvertently. This may cause the re-
establishment of an otherwise good reservation to fail. This has
the most severe affects on an aggregate that has many individual
flows that would have remained operational.
2) Just as RSVP has the vulnerability of premature termination of
valid reservations by rogue flows without authentication [12, 13],
this mechanism will have the same vulnerability. Usage of RSVP
authentication mechanisms is encouraged.
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8. IANA Considerations
The IANA has assigned the following from RFC 4495 (i.e., this
document):
The following error code has been defined in the ERROR_SPEC object
for partial reservation failure under "Errcode = 2 (Policy Control
Failure)":
ErrSubCode = 102 (ERR_PARTIAL_PREEMPT)
The behavior of this ErrSubCode is defined in this document.
9. Acknowledgements
The authors would like to thank Fred Baker for contributing text and
guidance in this effort and to Roger Levesque and Francois Le
Faucheur for helpful comments.
10. References
10.1. Normative References
[1] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[2] Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
"Aggregation of RSVP for IPv4 and IPv6 Reservations", RFC 3175,
September 2001.
[3] Herzog, S., "Signaled Preemption Priority Policy Element", RFC
3181, October 2001.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Herzog, S., "RSVP Extensions for Policy Control", RFC 2750,
January 2000.
[6] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F., and S.
Molendini, "RSVP Refresh Overhead Reduction Extensions", RFC
2961, April 2001.
[7] Wroclawski, J., "The Use of RSVP with IETF Integrated Services",
RFC 2210, September 1997.
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[8] Berger, L. and T. O'Malley, "RSVP Extensions for IPSEC Data
Flows", RFC 2207, September 1997.
10.2. Informative References
[9] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[10] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[11] Le Faucheur, F., Davie, B., Bose, P., Christou, C., and M.
Davenport, "Generic Aggregate RSVP Reservations", Work in
Progress, October 2005.
[12] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
[13] Braden, R. and L. Zhang, "RSVP Cryptographic Authentication --
Updated Message Type Value", RFC 3097, April 2001.
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Appendix A. Walking through the Solution
Here is a concise explanation of roughly how RSVP behaves with the
solution to the problems presented in Sections 2 and 3 of this
document. There is no normative text in this appendix.
Here is a duplicate of Figure 2 from section 3 of the document body
(to bring it closer to the detailed description of the solution).
Aggregator of X Deaggregator of X
| |
V V
+------+ +------+ +------+ +------+
Flow 1-->| | | | | | | |-->Flow 1
Flow 2-->| | | | | | | |-->Flow 2
Flow 3-->| |==>| | | |==>| |-->Flow 3
Flow 4-->| | ^ | | | | ^ | |-->Flow 4
Flow 5-->| | | | | | | | | |-->Flow 5
Flow 9-->| R1 | | | R2 | | R3 | | | R4 |-->Flow 9
+------+ | +------+ +------+ | +------+
| || || |
Aggregate X--->|| Aggregate X ||<--Aggregate X
|| | ||
+--------------+ | +--------------+
| |Int 7 | | |Int 1 | |
| +----- | V |------+ |
| R10 |Int 8 |===========>|Int 2 | R11 |
| | |:::::::::::>| | |
| +----- | ^ |------+ |
| |Int 9 | | |Int 3 | |
+--------------+ | +--------------+
.. | ..
Aggregate Y--->.. Aggregate Y ..<---Aggregate Y
| .. .. |
+------+ | +------+ +------+ | +------+
Flow A-->| | | | | | | | | |-->Flow A
Flow B-->| | V | | | | V | |-->Flow B
Flow C-->| |::>| | | |::>| |-->Flow C
Flow D-->| | | | | | | |-->Flow D
Flow E-->| R5 | | R6 | | R7 | | R8 |-->Flow E
+------+ +------+ +------+ +------+
^ ^
| |
Aggregator of Y Deaggregator of Y
Duplicate of Figure 2. Generic RSVP Aggregate Topology
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RFC 4495 RSVP Bandwidth Reduction May 2006
Looking at Figure 2, aggregate X (with five 80 kbps flows) traverses:
R1 ==> R2 ==> R10 ==> R11 ==> R3 ==> R4
And aggregate Y (with five 80 kbps flows) traverses:
R5 ::> R6 ::> R10 ::> R11 ::> R7 ::> R8
Both aggregates are 400 kbps. This totals 800 kbps at Int 7 in R10,
which is the maximum bandwidth that RSVP has access to at this
interface. Signaling messages still traverse the interface without
problem. Aggregate X is at a higher relative priority than aggregate
Y. Local policy in this example is for higher relative priority
flows to preempt lower-priority flows during times of congestion.
The following points describe the flow when aggregate A is increased
to include Flow 9.
o When Flow 9 (at 80 kbps) is added to aggregate X, R1 will initiate
the PATH message towards the destination endpoint of the flow.
This hop-by-hop message will take it through R2, R10, R11, R3, and
R4, which is the aggregate X path (that was built per [2] from the
aggregate's initial setup) to the endpoint node.
o In response, R4 will generate the RESV (reservation) message
(defined behavior per [1]). This RESV from the deaggregator
indicates an increase bandwidth sufficient to accommodate the
existing 5 flows (1, 2, 3, 4, and 5) and the new flow (9), as
stated in [2].
o As mentioned before, in this example, Int 8 in R10 can only
accommodate 800 kbps, and aggregates X and Y have each already
established 400 kbps flows comprised of five 80 kbps individual
flows. Therefore, R10 (the interface that detects a congestion
event in this example) must make a decision about this new
congestion generating condition in regard to the RESV message
received at Int 8.
o Local policy in this scenario is to preempt lower-priority
reservations to place higher-priority reservations. This would
normally cause all of aggregate Y to be preempted just to
accommodate aggregate X's request for an additional 80 kbps.
o This document defines how aggregate Y is not completely preempted,
but reduced in bandwidth by 80 kbps. This is contained in the
ResvErr message that R10 generates (downstream) towards R11, R7,
and R8. See section 5 for the details of the error message.
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RFC 4495 RSVP Bandwidth Reduction May 2006
o Normal operation of RSVP is to have the router that generates a
ResvErr message downstream to also generate a ResvTear message
upstream (in the opposite direction, i.e., towards R5). The
ResvTear message terminates an individual flow or aggregate flow.
This document calls for that message not to be sent on any partial
failure of reservation.
o R8 is the deaggregator of aggregate Y. The deaggregator controls
all the parameters of an aggregate reservation. This will be the
node that reduces the necessary bandwidth of the aggregate as a
response to the reception of an ResvErr message (from R10)
indicating such an action is called for. In this example,
bandwidth reduction is accomplished by preempting an individual
flow within the aggregate (perhaps picking on Flow D for
individual preemption by generating a ResvErr downstream on that
individual flow).
o At the same time, a ResvTear message is transmitted upstream on
that individual flow (Flow D) by R8. This will not affect the
aggregate directly, but is an indication to the routers (and the
source end-system) which individual flow is to be preempted.
o Once R8 preempts whichever individual flow (or 'bandwidth' at the
aggregate ingress), it transmits a new RESV message for that
aggregate (Y), not for a new aggregate. This RESV from the
deaggregator indicates a decrease in bandwidth sufficient to
accommodate the remaining 4 flows (A, B, C, and E), which is now
320 kbps (in this example).
o This RESV message travels the entire path of the reservation,
resetting all routers to this new aggregate bandwidth value. This
should be what is necessary to prevent a ResvTear message from
being generated by R10 towards R6 and R5.
R5 will not know through this RESV message which individual flow was
preempted. If in this example, R8 was given more bandwidth to keep,
it might have transmitted a bandwidth reduction ResvErr indication
towards the end-system of Flow D. In that case, a voice signaling
protocol (such as SIP) could have attempted a renegotiation of that
individual flow to a reduced bandwidth (say, but changing the voice
codec from G.711 to G. 729). This could have saved Flow D from
preemption.
Polk & Dhesikan Standards Track [Page 19]
RFC 4495 RSVP Bandwidth Reduction May 2006
Authors' Addresses
James M. Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, Texas 75082 USA
EMail: jmpolk@cisco.com
Subha Dhesikan
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134 USA
EMail: sdhesika@cisco.com
Polk & Dhesikan Standards Track [Page 20]
RFC 4495 RSVP Bandwidth Reduction May 2006
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Polk & Dhesikan Standards Track [Page 21]