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Keywords: RSVP, resource reservation protocol, QoS, Quality of Service
Network Working Group Y. Bernet
Request for Comments: 2996 Microsoft
Category: Standards Track November 2000
Format of the RSVP DCLASS Object
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 (2000). All Rights Reserved.
Abstract
Resource Reservation Protocol (RSVP) signaling may be used to request
Quality of Service (QoS) services and enhance the manageability of
application traffic's QoS in a differentiated service (diff-serv or
DS) network. When using RSVP with DS networks it is useful to be
able to carry carry Differentiated Services Code Points (DSCPs) in
RSVP message objects. One example of this is the use of RSVP to
arrange for the marking of packets with a particular DSCP upstream
from the DS network's ingress point, at the sender or at a previous
network's egress router.
The DCLASS object is used to represent and carry DSCPs within RSVP
messages. This document specifies the format of the DCLASS object
and briefly discusses its use.
1. Introduction
This section describes the mechanics of using RSVP [RSVP] signaling
and the DCLASS object for effecting admission control and applying
QoS policy within a Differentiated Service network [DS]. It assumes
standard RSVP senders and receivers, and a diff-serv network
somewhere in the path between sender and receiver. At least one RSVP
aware network element resides in the diff-serv network. This network
element may be a policy enforcement point (PEP) [RAP] or may simply
act as an admission control agent for the network, admitting or
denying resource requests based on the availability of resources. In
either case, this network element interacts with RSVP messages
arriving from outside the DS network, accepting resource requests
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from RSVP-aware senders and receivers, and conveying the DS network's
admission control and resource allocation decisions to the higher-
level RSVP. The network element is typically a router and will be
considered to be so for the purpose of this document. This model is
described fully in [INTDIFF].
1.1 Use of the DCLASS Object to Carry Upstream Packet Marking
Information
A principal usage of the DCLASS object is to carry DSCP information
between a DS network and upstream nodes that may wish to mark packets
with DSCP values. Briefly, the sender composes a standard RSVP PATH
message and sends it towards the receiver. At some point the PATH
message reaches the DS network. The PATH message traverses one or
more network elements that are PEPs and/or admission control agents
for the diff-serv network. These elements install appropriate state
and forward the PATH message towards the receiver. If admission
control is successful downstream of the diff-serv network, then a
RESV message will arrive from the direction of the receiver. As this
message arrives at the PEPs and/or admission control agents that are
RSVP enabled, each of these network elements must make a decision
regarding the admissibility of the signaled flow to the diff-serv
network.
If the network element determines that the request represented by the
PATH and RESV messages is admissible to the diff-serv network, the
appropriate diff-serv service level (or behavior aggregate) for the
traffic represented in the RSVP request is determined. Next, a
decision is made to mark arriving data packets for this traffic
locally using MF classification, or to request upstream marking of
the packets with the appropriate DSCP(s). This upstream marking
could occur anywhere before the DS network's ingress point. Two
likely candidates are the originating sender and the egress boundary
router of some upstream (DS or non-DS) network. The decision about
where the RSVP request's packets should be marked can be made by
agreement or through a negotiation protocol; the details are outside
the scope of this document.
If the packets for this RSVP request are to be marked upstream,
information about the DSCP(s) to use must be conveyed from the RSVP-
aware network element to the upstream marking point. This
information is conveyed with the DCLASS object. To do this, the
network element adds a DCLASS object containing one or more DSCPs
corresponding to the behavior aggregate, to the RESV message. The
RESV message is then sent upstream towards the RSVP sender.
If the network element determines that the RSVP request is not
admissible to the diff-serv network, it sends a RESV error message
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towards the receiver. No DCLASS is required.
1.1 Additional Uses of the DCLASS Object
The DCLASS object is intended to be a general tool for conveying DSCP
information in RSVP messages. This may be useful in a number of
situations. We give one further example here as motivation.
In this example, we assume that the decision about the appropriate
behavior aggregate for a RSVP-mediated traffic flow is made at the DS
network egress router (or a related Policy Decision Point) by
observing RSVP PATH and RESV messages and other necessary
information. However, the actual packet marking must be done at the
ingress of the network. The DCLASS object can be used to carry the
needed marking information between egress and ingress routers.
2. Format of the DCLASS Object
The DCLASS object has the following format:
0 | 1 | 2 | 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (>= 8) | C-Num (225) | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | 1st DSCP | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | 2nd DSCP | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | . . . . | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first word contains the standard RSVP object header (the Class
Num for the DCLASS object is 225). The length field indicates the
total object length in bytes. The object header is followed by one
or more 32-bit words, each containing a DSCP in the six high-order
bits of the least significant byte. The length field in the object
header indicates the number of DSCPs included in the object.
Specifically, the number of DCLASS objects present is equal to
(Length - 4) / 4.
The network may return multiple DSCPs in the DCLASS object in order
to enable the host to discriminate sub-flows within a behavior
aggregate. For example, in the case of the AF PHB group [AF], the
network may return the DSCPs 001010, 001100, and 001110 corresponding
to increasing levels of drop precedence within Class 1 of the AF PHB
group. Note that this document makes no statements regarding the
significance of the order of the returned DSCPs. Further
interpretation of DSCP sets is dependent on the specific service
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requested by the host and is beyond the scope of this document.
Note that the Class-Num for the DCLASS object is chosen from the
space of unknown class objects that should be ignored and forwarded
by nodes that do not recognize it. This is to assure maximal
backward compatibility.
3. Admission Control Functionality
From a black-box perspective, admission control and policy
functionality amounts to the decision whether to accept or reject a
request and the determination of the DSCPs that should be used for
the corresponding traffic. The specific details of admission control
are beyond the scope of this document. In general the admission
control decision is based both on resource availability and on
policies regarding the use of resources in the diff-serv network.
The admission control decision made by RSVP aware network elements
represents both considerations.
In order to decide whether the RSVP request is admissible in terms of
resource availability, one or more network elements within or at the
boundary of the diff-serv network must understand the impact that
admission would have on specific diff-serv resources, as well as the
availability of these resources along the relevant data path in the
diff-serv network.
In order to decide whether the RSVP request is admissible in terms of
policy, the network element may use identity objects describing users
and/or applications that may be included in the request. The router
may act as a PEP/PDP and use data from a policy database or directory
to aid in this decision.
See Appendix A for a simple mechanism for configurable resource based
admission control.
4. Security Considerations
The DCLASS object conveys information that can be used to request
enhanced QoS from a DS network, so inappropriate modification of the
object could allow traffic flows to obtain a higher or lower level of
QoS than appropriate. Particularly, modification of a DCLASS object
by a third party inserted between the DS network ingress node and the
upstream marker constitutes a possible denial of service attack.
This attack is subtle because it is possible to reduce the received
QoS to an unacceptably low level without completely cutting off data
flow, making the attack harder to detect.
The possibility of raising the received level of QoS by inappropriate
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modification of the DCLASS object is less significant because it a
subclass of a larger class of attacks that must already be detected
by the system. Protection must already be in place to prevent a host
raising its received level of QoS by simply guessing "good" DSCP's
and marking packets accordingly. If this protection is at the
boundary of the DS network, it will detect inappropriate marking of
arriving packets caused by modified DCLASS objects as well. If,
however, the protection function as well as the marking function has
been pushed upstream (perhaps to a trusted third party or
intermediate node), correct transmission of the DCLASS object must be
ensured to prevent a possible theft of service attack.
Simple observation of the DCLASS object in a RSVP message raises
several issues which may be seen as security concerns. Correlation
of observed DCLASS object values with RSVP requests or MF
classification parameters allows the observer to determine that
different flows are receiving different levels of QoS, which may be
knowledge that should be protected in some environments. Similarly,
observation of the DCLASS object can allow the observer to determine
that a single flow's QoS has been promoted or demoted, which may
signal significant events in the life of that flow's application or
user. Finally, observation of the DCLASS object may reveal
information about the internal operations of a DS network that could
be useful to observers interested in theft-of-services attacks.
5. References
[INTDIFF] Bernet, Y., Yavatkar, R., Ford, P., Baker, F., Zhang, L.,
Speer, M., Braden, R., Davie, B. and J. Wroclawski, "A
Framework for Integrated Services Operation over Diffserv
Networks", RFC 2998, November 2000.
[DS] Blake, S., Carlson, M., Davies, D., Wang, Z. and W. Weiss,
"An Architecture for Differentiated Services", RFC 2475,
December 1998.
[RSVP] Braden, R., Zhang, L., Berson, S., Herzog, S. and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RAP] Yavatkar, R., Pendarakis, D. and R. Guerin, "A Framework
for Policy Based Admission Control", RFC 2753, January
2000.
[AF] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.
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6. Acknowledgments
Thanks to Fred Baker and Carol Iturralde for reviewing this document.
Thanks to Ramesh Pabbati, Tim Moore, Bruce Davie and Kam Lee for
input.
7. Author's Address
Yoram Bernet
Microsoft
One Microsoft Way,
Redmond, WA 98052
Phone: (425) 936-9568
EMail: yoramb@microsoft.com
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Appendix A - Simple Configurable Resource Based Admission Control
Routers may use quite sophisticated mechanisms in making the
admission control decision, including policy considerations, various
intra-domain signaling protocols, results of traffic monitoring and
so on. It is recommended that the following basic functionality be
provided to enable simple resource based admission control in the
absence of more sophisticated mechanisms. This functionality can be
used with configurable, standalone routers. It applies to standard
RSVP/Intserv requests. This minimal functionality assumes only a
single DSCP is included in the DCLASS object, but may readily be
extended to support multiple DSCPs.
It must be possible to configure two tables in the router. These are
described below.
A.1 Service Type to DSCP Mapping
One table provides a mapping from the intserv service-type specified
in the RSVP request to a DSCP that can be used to obtain a
corresponding service in the diff-serv network. This table contains
a row for each intserv service type for which a mapping is available.
Each row has the following format:
Intserv service type : DSCP
The table would typically contain at least three rows; one for
Guaranteed service, one for Controlled Load service and one for Best-
Effort service. (The best-effort service will typically map to DSCP
000000, but may be overridden). It should be possible to add rows
for as-yet-undefined service types.
This table allows the network administrator to statically configure a
DSCP that the router will return in the DCLASS object for an admitted
RSVP request. In general, more sophisticated and likely more dynamic
mechanisms may be used to determine the DSCP to be returned in the
DCLASS object. Also, it is likely that a real mapping for some
services would use more than one DSCP, with the DSCP depending on the
invocation parameters of a specific service request. In this case,
these mechanisms may override or replace the static table based
mapping described here.
A.2 Quantitative Resource Availability
Standard intserv requests are quantitative in nature. They include
token bucket parameters describing the resources required by the
traffic for which admission is requested. The second table enables
the network administrator to statically configure quantitative
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parameters to be used by the router when making an admission control
decision for quantitative service requests. Each row in this table
has the following form:
DSCP : Token bucket profile
The first column specifies those DSCPs for which quantitative
admission control is applied. The second column specifies the token
bucket parameters which represent the total resources available in
the diff-serv network to accommodate traffic in the service class
specified by the DSCP.
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