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INTERNATIONAL TELECOMMUNICATION |
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TELECOMMUNICATION STUDY PERIOD 2001-2004 |
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Title: |
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LIAISON STATEMENT |
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To: |
IETF Traffic Engineering Working Group
(TEWG), MPLS Working Group, CCAMP Working Group |
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Approval: |
Agreed
to at 26 November- |
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Information |
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Deadline: |
None |
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Gerald Ash AT&T |
Tel: +1 732 420 4578 Fax: +1 732 368 8659 Email: gash@att.com |
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Q.2/2 would like to inform the IETF TEWG, MPLS and CCAMP Working Groups that Q.2/2 has finalized the work on the E.360 Series of 7 Recommendations on ‘QoS Routing & Related Traffic Engineering Methods for IP-, ATM-, & TDM-Based Multiservice Networks’. This Liaison provides a summary of the E.360 Recommendations. A full copy of these Recommendations is available at http://www.research.att.com/~jrex/jerry/.
We would be pleased to receive any comments you have on these documents.
The E.360 Series of Recommendations is relevant to the request from the TEWG for service provider uses, requirements, and desires for traffic engineering best current practices. In the E.360 Series, analysis models are used to demonstrate that currently operational TE/QoS routing methods and best current practices are extensible to QoS routing and Internet traffic engineering (TE). These Qos routing and TE methods include traffic management through control of routing functions, which include call routing, connection routing, QoS resource management, routing table management, and dynamic transport routing. Recommendations E.360 provide a performance analysis of various TE/QoS routing methods which control a network's response to traffic demands and other stimuli, such as link failures or node failures. Essentially all of the methods analyzed are already widely applied in operational networks worldwide, particularly in PSTN networks employing TDM-based technology. However, the methods are shown to be extensible to packet-based technologies, in particular, to IP-based and ATM-based technologies. Results of performance analysis models are presented which illustrate the tradeoffs between various approaches. Based on the results of these studies as well as established practice and experience, methods for dynamic QoS routing and admission control are proposed for consideration in network evolution to IP-based and ATM-based technologies.
1.0
Introduction
QoS routing and related traffic engineering
methods are indispensable network functions which controls a network’s response
to traffic demands and other stimuli, such as network overloads and
failures. Current and future networks
are rapidly evolving to carry a multitude of voice/ISDN services and packet
data services on internet protocol (IP)-based and asynchronous transfer mode
(ATM)-based networks, driven in part by the extremely rapid growth of
packet-based data services. Within
networks and services supported by packet and TDM protocols have evolved
various QoS routing methods. These QoS
routing mechanisms are reviewed in the E.360 Series of 7 Recommendations “QoS Routing & Related Traffic Engineering Methods for IP-, ATM-,
& TDM-Based Multiservice Networks” [E.360].
This Liaison summarizes these
Recommendations, which includes a comparative analysis and performance
evaluation of various QoS routing alternatives.
QoS routing functions include a) call routing, which entails number/name translation to routing address, b) connection or bearer-path routing methods, c) QoS resource management, and d) routing table management. These functions can be a) decentralized and distributed to the network nodes, b) centralized and allocated to a centralized controller such as a QoS-routing processor, or c) performed by a hybrid combination of these approaches. The scope of the QoS routing methods includes the establishment of connections for narrowband, wideband, and broadband multimedia services within multiservice networks and between multiservice networks. Here a multiservice network refers to one in which various classes of service share the transmission, switching, management, and other resources of the network. These classes of services can include constant bit rate (CBR), variable bit rate (VBR), and unassigned bit rate (UBR) traffic classes. There are quantitative performance requirements that the various classes of service normally are required to meet, such as end-to-end blocking, delay, and/or delay-jitter objectives. These objectives are achieved through a combination of QoS routing, traffic management, and capacity management.
The E.360 Series
of Recommendations [E.360] provides a performance analysis of lost/delayed
traffic and control load for various QoS routing methods, which control a
network's response to traffic demands and other stimuli, such as traffic
overloads, link failures, or node failures. Essentially all of the methods analyzed are already
widely applied in operational networks worldwide, particularly in PSTN networks
employing TDM-based technology. Such methods have been analyzed in practice for
TDM-based networks [ASH1], and in modeling studies for IP-based and ATM-based
networks [ASH2, E.360]. In [E.360] these
QoS routing methods are described, and the methods
are shown to be extensible to packet-based technologies, in particular, to
IP/MPLS-based technology. Results of performance analysis models
are presented which illustrate the tradeoffs between various approaches. Based
on the results of these studies as well as established practice and experience,
methods for dynamic QoS routing and admission control are proposed for
consideration in network evolution to IP-based and ATM-based technologies. In particular, we find that aggregated per-virtual-network
bandwidth allocation compares favorably with per-flow allocation. We also find that event-dependent
routing methods for management of label switched paths perform just as well or
better than the state-dependent routing methods with flooding, which means that
event-dependent routing path selection has potential to significantly enhance
network scalability.
Awduche [AWD, RFC3272] gives excellent overviews
of traffic engineering approaches for IP-based networks, and also provides
traffic engineering requirements [RFC2702].
In Section 2 we summarize the E.360 Series of Recommendations, and in ANNEX A we provide a brief summary of the analysis of QoS routing methods given in Recommendations E.360. In particular, ANNEX A discusses the general principles of QoS routing methods, including connection routing methods, QoS resource management, and routing table management. ANNEX A also includes modeling and analysis results, as well as a summary and conclusions.
2.0
Summary of E.360 Series of Recommendations
A new series of seven Recommendations has been approved by the ITU-T Study Group 2, which focus on QoS routing and traffic engineering methods for IP-, ATM-, & TDM-based multiservice networks. The methods addressed include call and connection routing, QoS resource management, routing table management, dynamic transport routing, capacity management, and operational requirements. The Recommendations provide a performance analysis of various QoS routing methods, and based on the results and established practice, methods for dynamic QoS routing and admission control are recommended for consideration in network evolution to ATM- and IP-based technologies.
The following is a brief summary of each of the
seven Recommendations (a full text of the Recommendations is available at http://www.research.att.com/~jrex/jerry/.
2.1 Recommendation E.360.1 “QoS Routing & Related
Traffic Engineering Methods for IP-, ATM-, & TDM-Based Multiservice
Networks – Framework”
The E.360 Series of Recommendation describes, analyzes, and recommends methods which control a network's response to traffic demands and other stimuli, such as link failures or node failures. The methods addressed in the E.360 series include call and connection routing, QoS resource management, routing table management, dynamic transport routing, capacity management, and operational requirements. Analysis models are used to demonstrate that currently operational QoS routing methods and best current practices are extensible to IP-based and ATM-based QoS routing. The Recommendations provide a performance analysis of various QoS routing methods, where essentially all of the methods analyzed are already widely applied in operational networks worldwide, particularly in PSTN networks employing TDM-based technology. However, the methods are shown to be extensible to packet-based technologies, in particular, to IP-based and ATM-based technologies. Results of performance analysis models are presented which illustrate the tradeoffs between various approaches. Based on the results of these studies as well as established practice and experience, methods for dynamic QoS routing and admission control are recommended for consideration in network evolution to IP-based and ATM-based technologies.
2.2 Recommendation E.360.2 “QoS Routing & Related
Traffic Engineering Methods for IP-, ATM-, & TDM-Based Multiservice
Networks – Call Routing & Connection Routing Methods”
Call routing involves the translation of a number or name to a routing address. This Recommendation describes how number (or name) translation should result in the E.164 ATM end-system addresses (AESA), network routing addresses (NRAs), and/or IP addresses. These addresses are used for routing purposes and therefore must be carried in the connection-setup information element. Connection or bearer-path routing involves the selection of a path from the originating node to the destination node in a network. This Recommendation discusses bearer-path selection methods, which are categorized into the following four types: fixed routing (FR), time-dependent routing (TDR), state-dependent routing (SDR), and event-dependent routing (EDR). These methods are associated with routing tables, which consist of a route and rules to select one path from the route for a given connection or bandwidth-allocation request. Recommendations include a) QoS routing methods to be applied, b) sparse-topology multilink-routing networks, c) single-area flat topologies, d) event-dependent-routing (EDR) QoS routing path selection methods, and e) interdomain routing methods which extend the intradomain call routing and connection routing concepts.
2.3 Recommendation E.360.3 “QoS Routing & Related
Traffic Engineering Methods for IP-, ATM-, & TDM-Based Multiservice
Networks – QoS Resource Management Methods”
QoS resource management functions include class-of-service derivation, policy-based routing table derivation, connection admission, bandwidth allocation, bandwidth protection, bandwidth reservation, priority routing, priority queuing, and other related resource management functions. Recommendations include a) QoS resource management to achieve connection-level and packet-level grade-of-service objectives, as well as key service, normal service, and best effort service differentiation, b) admission control, c) bandwidth reservation to achieve stable and efficient performance of QoS routing methods and to ensure the proper operation of multiservice bandwidth allocation, protection, and priority treatment, d) per-virtual network (VNET) bandwidth allocation, and e) application of both MPLS bandwidth management and DiffServ priority queuing management
2.4 Recommendation E.360.4 “QoS Routing & Related
Traffic Engineering Methods for IP-, ATM-, & TDM-Based Multiservice
Networks – Routing Table Management Methods & Requirements”
Routing table management information, such as topology update, status information, or routing recommendations, is used for purposes of applying the routing table design rules for determining path choices in the routing table. This information is exchanged between one node and another node, such as between the originating node and destination node, for example, or between a node and a network element such as a bandwidth-broker processor. This information is used to generate the routing table, and then the routing table is used to determine the path choices used in the selection of a path. Recommendations include a) per-VNET bandwidth allocation, which is preferred to per-flow allocation because of the much lower routing table management overhead requirements, b) EDR QoS routing methods, which can lead to a large reduction in flooding overhead without loss of network throughput performance, and c) larger administrative areas and lower routing table management overhead requirements.
2.5 Recommendation E.360.5 “QoS Routing & Related
Traffic Engineering Methods for IP-, ATM-, & TDM-Based Multiservice
Networks – Transport Routing Methods”
Dynamic transport routing combines with dynamic traffic routing to shift transport bandwidth among node pairs and services through use of flexible transport switching technology, such as optical cross-connects (OXCs). Dynamic transport routing offers advantages of simplicity of design and robustness to load variations and network failures, and can provide automatic link provisioning, diverse link routing, and rapid link restoration for improved transport capacity utilization and performance under stress. OXCs can reconfigure logical transport capacity on demand, such as for peak day traffic, weekly redesign of link capacity, or emergency restoration of capacity under node or transport failure. MPLS control capabilities are proposed for the setup of layer 2 logical links through OXCs. Recommendations include a) dynamic transport routing, which provides greater network throughput, enhanced revenue, enhanced network performance under failure as well as abnormal and unpredictable traffic load patterns, b) traffic and transport restoration level design, which allows for link diversity to ensure performance under failure, and c) robust routing techniques, which include dynamic traffic routing, multiple ingress/egress routing, and logical link diversity routing; these methods improve response to node or transport failures.
2.6 Recommendation E.360.6 “QoS Routing & Related
Traffic Engineering Methods for IP-, ATM-, & TDM-Based Multiservice
Networks – Capacity Management Methods”
This Recommendation discusses capacity management
principles, which include a) link capacity design models, b) shortest path
selection models, c) multihour network design models, d) day-to-day variation
design models, and e) forecast uncertainty/reserve capacity design models. Recommendations include a) discrete event
flow optimization design models, which are able to capture very complex routing
behavior, b) sparse topology options, which lead to capital cost advantages,
operation simplicity and cost reduction.
Capital cost savings are subject to the particular switching and
transport cost assumptions, c) voice and data integration, d) multilink routing methods, which exhibit greater design efficiencies
in comparison with 2-link routing methods, e) single-area flat topologies,
which exhibit greater design efficiencies in termination and transport
capacity, f) EDR methods, which exhibit comparable design efficiencies to SDR,
and g) dynamic transport routing, which
achieves capital savings by concentrating capacity on fewer, high-capacity
physical fiber links and higher network throughput and enhanced revenue by
their ability to flexibly allocate bandwidth on the logical links serving the
access and inter-node traffic.
2.7 Recommendation E.360.7 “QoS Routing & Related
Traffic Engineering Methods for IP-, ATM-, & TDM-Based Multiservice
Networks – Operational Requirements”
This Recommendation discusses traffic engineering
operational requirements, as follows: a) traffic management requirements for
real-time performance monitoring, network control, and work center functions,
b) capacity management – forecasting requirements for load forecasting,
including configuration database functions, load aggregation, basing, and
projection functions, and load adjustment cycle and view of business adjustment
cycle, c) capacity management – daily and weekly performance monitoring
requirements for daily congestion analysis, study-week congestion analysis, and
study-period congestion analysis, and d)
capacity management – short-term network adjustment requirements for
network design, work center functions, and interfaces to other work centers.
2.8 Analysis of QoS
Routing Methods for MPLS-Based Multiservice Networks
ANNEX A provides a summary of QoS routing methods
analyzed in the E.360 Series of Recommendations. The
ANNEX summarizes the performance analysis of lost/delayed traffic and control
load for various QoS routing methods, which control a network's response to
traffic demands and other stimuli, such as traffic overloads, link failures, or
node failures. Essentially all
of the methods analyzed are already widely applied in operational networks
worldwide, particularly in PSTN networks employing TDM-based technology. However, the methods are shown to be
extensible to packet-based technologies, in particular, to IP-based and
ATM-based technologies. Results of performance analysis models are
presented which illustrate the tradeoffs between various approaches. Based on
the results of these studies as well as established practice and experience,
methods for dynamic QoS routing and admission control are proposed for
consideration in network evolution to IP-based and ATM-based technologies. In particular, we find that aggregated per-virtual-network
bandwidth allocation compares favorably with per-flow allocation. We also find that event-dependent
routing methods for management of label switched paths perform just as well or
better than the state-dependent routing methods with flooding, which means that
event-dependent routing path selection has potential to significantly enhance
network scalability.
3.0
References
[AHM] Ahmadi, H., et. al., Dynamic Routing and Call Control in High-Speed Integrated Networks, Proceedings
of ITC-13, Copenhagen, 1992.
[AKI] Akinpelu, J. M., The Overload Performance of Engineered Networks with Nonhierarchical
& Hierarchical Routing, BSTJ, Vol. 63, 1984.
[APO] Apostolopoulos, G., Intra-Domain QoS Routing in IP Networks: A Feasibility and Cost/Benefit Analysis, IEEE Network, September 1999.
[ASH1] Ash, G. R., Dynamic Routing in Telecommunications Networks, McGraw-Hill, 1998.
[ASH2] Ash, G. R., et. al., Routing Evolution in Multiservice Integrated Voice/Data Networks, Proceeding of ITC-16, Edinburgh, June 1999.
[AWD] Awduche, D., MPLS and Traffic Engineering in IP Networks, IEEE Communications Magazine, December 1999.
[BUR] Burke, P. J., Blocking Probabilities Associated with Directional Reservation,
unpublished memorandum, 1961.
[E.360] ITU-T Recommendations, QoS Routing & Related Traffic
Engineering Methods for Multiservice TDM-, ATM-, & IP-Based Networks.
[ELW] Elwalid, A., et. al., MATE: MPLS Adaptive Traffic Engineering, Proceedings INFOCOM'01, April 2001.
[KRU] Krupp, R. S., Stabilization of Alternate Routing Networks, Proceedings of ICC,
Philadelphia, 1982.
[LIL] Liljenstolpe, C., An Approach to IP Network Traffic Engineering (Cable & Wireless), work in progress.
[MA] Ma, Q., Quality of Service Routing in Integrated Services Networks, Ph.D. Thesis, Carnegie Mellon University, 1998.
[MUM] Mummert, V. S., Network Management and Its Implementation on the No. 4ESS,
International Switching Symposium, Japan, 1976.
[NAK] Nakagome, Y., Mori, H., Flexible Routing in the Global Communication
Network, Proceedings of ITC-7, Stockholm, 1973.
[ODL]
Odlyzko, A., The
economics of the Internet: Utility, utilization, pricing, and Quality of
Service, http://www.dtc.umn.edu/~odlyzko/doc/networks.html.
[PNNI] ATM Forum Technical Committee, Private Network-Network Interface Specification Version 1.0 (PNNI 1.0), af-pnni-0055.000.
[RFC1247] Moy,
J., OSPF Version 2.
[RFC2386]
Crawley, E., et. al., A Framework for
QoS-based Routing in the Internet.
[RFC2475] Blake, S., et. al., Weiss, W., An Architecture for Differentiated Services.
[RFC2702] Awduche,
D., et. al., Requirements for Traffic
Engineering over MPLS.
[RFC3031] Rosen, E., et. al., Multiprotocol Label Switching Architecture.
[RFC3209] Awduche, D., et. al., RSVP-TE: Extension to RSVP for LSP Tunnels.
[RFC3212] Jamoussi, B., et. al., Constraint-Based LSP Setup using LDP.
[RFC3270] Le
Faucheur, F., et. al., MPLS Support of
Differentiated Services.
[RFC3272]
Awduche, D., et. al., Overview &
Principles of Internet Traffic Engineering.
[SPR] Springer, V., et. al., Level3 MPLS Protocol Architecture, work in progress.
[VIL] Villamizar, C., MPLS Optimized Multipath, work in progress.
[XIAO1] Xiao, X., et. al., Internet QoS: A Big Picture, IEEE Network, March/April 1999.
[XIAO2] Xiao, X., et. al., Traffic Engineering with MPLS in the Internet, IEEE Network, March 2000.
[XIAO3] Xiao, X., Providing Quality of Service in the Internet, Ph.D. Thesis,
Michigan State University, 2000.
ANNEX A – Analysis of QoS Routing Methods [E.360]
A.1. QoS Routing Methods
In this ANNEX we
summarize the QoS routing methods discussed
and analyzed in the E.360 Series of Recommendations [E.360], including a)
connection or bearer-path routing methods, b) QoS resource management, and c)
routing table management.
A.1.1 Connection Routing
Connection routing methods are used for establishment of a bearer path for a given service request or session flow, and include fixed routing, time-dependent routing, state-dependent routing, and event-dependent routing methods.
Hierarchical fixed routing (FR) is an important routing method employed in all types of networks, including packet- and TDM-based networks. In IP-based and ATM-based networks, there is often a hierarchical relationship among different “areas”, “peer-groups,” or sub-networks. Hierarchical multi-domain (or multi-area, multi-peer-group, or multi-autonomous-system) topologies are normally used with IP routing protocols (OSPF, BGP) and ATM routing protocols (PNNI), as well as within almost all TDM-based network routing topologies.
Time dependent routing (TDR) methods are a type of dynamic routing in which the routing tables are altered at a fixed point in time during the day or week. TDR routing tables are determined on an off-line, preplanned basis and are implemented consistently over a time period. The TDR routing tables are determined considering the time variation of traffic load in the network, for example based on measured hourly load patterns. Several TDR time periods are used to divide up the hours on an average business day and weekend into contiguous routing intervals sometimes called load set periods. Typically, the TDR routing tables used in the network are coordinated by taking advantage of noncoincidence of busy hours among the traffic loads.
In state dependent routing (SDR), illustrated in Figure 1, the routing tables are altered automatically according to the state of the network. For a given SDR method, the routing table rules are implemented to determine the path choices in response to changing network status, and are used over a relatively short time period. Information on network status may be collected at a central QoS-routing processor or distributed to nodes in the network. The information exchange may be performed on a periodic or on-demand basis. SDR methods use the principle of routing connections on the best available path based on network state information. For example, in the least loaded routing method, the residual capacity of candidate paths is calculated, and the path having the largest residual capacity is selected for the connection. Various relative levels of link occupancy can be used to define link load states, such as lightly-loaded, heavily-loaded, or bandwidth-not-available states. In general, SDR methods calculate a path cost for each connection request based on various factors such as the load-state or congestion state of the links in the network.
In SDR, the routing tables are
designed on-line, in real time, by the originating node (ON) or a central
QoS-routing processor through the use of network status and topology information
obtained through information exchange with other nodes and/or a centralized
QoS-routing processor. There are various
implementations of SDR distinguished by
- whether the computation of the routing tables is distributed
among the network nodes or centralized and done in a centralized QoS-routing
processor, and
- whether the computation of the routing tables is done
periodically or connection by connection.
This leads to three different
implementations of SDR (see Figure 1):
- centralized periodic SDR (CP-SDR) -- here the centralized
QoS-routing processor obtains link status and traffic status information from
the various nodes on a periodic basis (e.g., every 10 seconds) and performs a
computation of the optimal routing table on a periodic basis. To determine the optimal routing table, the
QoS-routing processor executes a particular routing table optimization
procedure such as least-loaded routing and transmits the routing tables to the
network nodes on a periodic basis (e.g., every 10 seconds). Typically, if the shortest path is busy (e.g., bandwidth is unavailable
on one or more links), the second path is selected from the list of feasible
paths on the basis of having the greatest level of idle bandwidth at the time.
- distributed periodic SDR (DP-SDR) -- here each node in the SDR
network obtains link status and traffic status information from all the other
nodes on a periodic basis (e.g., every 5 minutes) and performs a computation of
the optimal routing table on a periodic basis (e.g., every 5 minutes). Flooding is a common technique for distributing the status and traffic
data, however other techniques with less overhead are also available, such as a
query-for-status method. To determine the optimal routing table, the ON executes a
particular routing table optimization procedure such as least-loaded routing.
- distributed connection-by-connection SDR (DC-SDR) -- here an ON
in the SDR network obtains link status and traffic status information from the
destination node (DN), and perhaps from selected via nodes (VNs), on a
connection by connection basis and performs a computation of the optimal
routing table for each connection. Typically, the ON first tries the primary path and if it is not available
finds an optimal alternate path by querying the DN and perhaps several VNs
through query-for-status network signaling for the busy-idle load status of all
links connected on the alternate paths to the DN. To determine the
optimal routing table, the ON executes a particular routing table optimization
procedure such as least-loaded routing.
In event
dependent routing (EDR), the routing tables are updated locally on the basis of
whether connections succeed or fail on a given path choice. In the
EDR learning approaches, the path last tried, which is also successful, is
tried again until blocked, at which time another path is selected at random and
tried on the next connection request. EDR path choices can also be changed with
time in accordance with changes in traffic load patterns. Success-to-the-top (STT) EDR path selection,
illustrated in Figure 1, is a decentralized, on-line path selection method with
update based on random routing. STT-EDR
uses a simplified decentralized learning method to achieve flexible adaptive
routing. The primary path path-p is used first if sufficient resources are
available, and if not a currently successful alternate path path-s is used
until it is blocked (i.e., sufficient resources are not available, such as
bandwidth not available on one or more links). In the case that path-s is
blocked, a new alternate path path-n is selected at random as the alternate
path choice for the next connection request overflow from the primary
path. In the EDR learning approaches,
the current alternate path choice can be updated randomly, cyclically
(round-robin), or by some other means, and may be maintained as long as a
connection can be established successfully on the path. Hence the routing table is constructed with the
information determined during connection setup, and no additional information
is required by the ON.
There are features commonly applied in all the connection routing methods. With TDR, SDR, and EDR, dynamically activated bandwidth reservation is typically used under congestion conditions to protect traffic on the primary path, as discussed in Section 2.2.2. Crankback may be used when an alternate path is blocked at a VN, and the connection request advances to a new path choice. Many path choices can be tried by a given connection request before the request is blocked. Paths in the routing table may consist of the direct link, a 2-link path through a single VN, or a multiple-link path through multiple VNs. Paths in the routing table are subject to allowed load state restrictions on each link. For either SDR or EDR, as in TDR, the alternate path choices for a connection request may be changed in a time-dependent manner considering the time-variation of the traffic load.
A.1.2 QoS Resource
Management
QoS resource management functions include class-of-service
identification, routing table derivation, connection admission, bandwidth
allocation, bandwidth protection, bandwidth reservation, priority routing, and
packet-level control (e.g., priority queuing) functions. QoS resource management methods have been
applied successfully in TDM-based networks [ASH1], and are being extended to
IP-based and ATM-based networks. In an
illustrative QoS resource management method, bandwidth is allocated to each of
several virtual networks (VNETs), which are each assigned a priority corresponding
to either high-priority services, normal-priority services, or best-effort
priority services. Examples of services
within these VNET categories include
- high-priority services such as emergency telecommunication
service,
- normal-priority services such as constant rate voice, variable
rate IP-telephony, and WWW file transfer, and
- low-priority best-effort services such as voice mail, email, and
file transfer.
Bandwidth changes in VNET bandwidth capacity can be determined by edge nodes on a per-flow (per-connection) basis, or based on an overall aggregated bandwidth demand, or “bandwidth pipe” concept, for VNET capacity (not on a per-connection demand basis). In the latter case of per-VNET bandwidth allocation, based on the aggregated bandwidth demand, edge nodes make periodic discrete changes in bandwidth allocation, that is, either increase or decrease bandwidth, such as on the multiprotocol label switching (MPLS) [RFC3031] constraint-based routing label switched paths (CRLSPs) constituting the VNET bandwidth capacity.
In the illustrative QoS resource management method for per-VNET bandwidth allocation, which we assume is MPLS-based, the bandwidth allocation control for each VNET CRLSP is based on estimated bandwidth needs, bandwidth use, and status of links in the CRLSP. The edge node, or ON, determines when VNET bandwidth needs to be increased or decreased on a CRLSP, and uses an MPLS CRLSP bandwidth modification procedure to execute needed bandwidth allocation changes on VNET CRLSPs. In the bandwidth allocation procedure the constraint-based routing label distribution protocol [RFC3212] or the resource reservation protocol [RFC3209] could be used, for example, to specify appropriate parameters in the label request message a) to request bandwidth allocation changes on each link in the CRLSP, and b) to determine if link bandwidth can be allocated on each link in the CRLSP. If a link bandwidth allocation is not allowed, a notification message with a crankback parameter allows the ON to search out possible bandwidth allocation on another CRLSP. We illustrate an allowed load state (ALS) parameter in the label request message to control the bandwidth allocation on individual links in a CRLSP. In addition, we illustrate a modify parameter in the label request message to allow dynamic modification of the assigned traffic parameters (such as peak data rate, committed data rate, etc.) of an already existing CRLSP.
In addition to
controlling bandwidth allocation, the QoS resource management procedures can
check end-to-end transfer delay, delay variation, and transmission quality
considerations such as loss, echo, and noise.
QoS resource management provides integration of services on a shared
network, for many classes-of-service such as:
- CBR services including voice, 64- and 384-kbps ISDN switched
digital data, virtual private network, 800/free-phone, and other services.
- Real-time VBR services including IP-telephony, compressed video,
and other services .
- Non-real-time VBR services including WWW file transfer, credit
card check, and other services.
- UBR services including voice mail, email, file transfer, and
other services.
A.1.2.1
Class-of-Service Identification & QoS Resource Management Steps
QoS resource management entails identifying class-of-service and class-of-service parameters, which may include, for example:
service-identity,
virtual network (VNET) (with associated priority, QoS, & traffic parameters), and
link-capability.
The service-identity describes the actual service
associated with the connection or flow.
The VNET describes the bandwidth allocation and routing table parameters
to be used by the connection. The
link-capability describes the link hardware capabilities such as fiber, radio,
satellite, and digital circuit multiplexing equipment, that the connection may
require, prefer, or avoid. The combination of service-identity, VNET, and
link-capability constitute the class-of-service, which together with the
network node number is used to access routing table data.
Determination of class-of-service begins with
translation at the ON of the number or name identifying the destination
end-user, to determine the routing address of the DN. If multiple ingress/egress routing is used,
multiple possible DN addresses are derived for the connection. Class-of-service parameters are derived
through application of policy-based routing, which involves the application of
rules against the input parameters to derive a routing table and its associated
parameters. Input parameters for
applying policy-based rules to derive service-identity, VNET, and
link-capability could include numbering plan, type of origination/destination
network, and type of service.
Policy-based routing rules may then be applied to the derived
service-identity, VNET, and link-capability to derive the routing table and
associated parameters.
The
illustrative QoS resource management method consists of the following steps:
- The ON determines the DN address,
service-identity, VNET, and link capability through the number/name translation
and other service information available at the ON.
- The ON accesses the VNET priority,
QoS/traffic parameters, and routing table between the ON and DN.
- The ON sets up the connection
request over the first available path in the routing table based on the QoS
resource management rules.
In the first step, the
connection request for an individual service is allocated an equivalent
bandwidth equal to EQBW to be routed on a particular VNET. For CBR services the equivalent bandwidth
EQBW is equal to the average or sustained bit rate. For VBR services the equivalent bandwidth
EQBW is a function of the sustained bit rate, peak bit rate, and perhaps other
parameters. For example, EQBW equals 64
kbps of bandwidth for CBR voice connections, 64 kbps of bandwidth for CBR ISDN
switched digital 64-kbps connections, and 384-kbps of bandwidth for CBR ISDN
switched digital 384-kbps connections.
In the second step, the
service-identity value is used to derive the VNET. Bandwidth is allocated to individual VNETs,
which is protected as needed but otherwise shared. Under normal non-blocking/delay network
conditions, all services fully share all available bandwidth. When blocking/delay occurs for a particular
VNET-i, bandwidth reservation acts to prohibit alternate-routed traffic and
traffic from other VNETs from seizing the allocated capacity for VNET-i. Associated with each VNET are average
bandwidth (BWavg) and maximum bandwidth (BWmax) parameters to govern bandwidth
allocation and protection, which are discussed further in the next
Section. Link-capability selection
allows connection requests to be routed on specific transmission links that
have the particular characteristics required by a connection request.
In the third step, the VNET routing
table determines which network capacity is allowed to be selected for each
connection request. In using the VNET
routing table, for example, the ON selects a first choice path based on the
routing table selection rules. Whether
or not bandwidth can be allocated to the connection request on the first choice
path is determined by the QoS resource management rules given in the next
Section. If a first choice path cannot
be accessed, the ON may then try alternate paths determined by FR, TDR, SDR, or
EDR path selection rules, and again applies the QoS resource management rules
now described.
A.1.2.2
Dynamic Bandwidth Allocation, Protection, & Reservation
Through the use of bandwidth allocation, protection, and reservation mechanisms, QoS resource management can provide good network performance under normal and abnormal operating conditions for all services sharing the integrated network. Such methods have been analyzed in practice for TDM-based networks [ASH1], and in modeling studies for IP-based and ATM-based networks [ASH2, E.360]. In this Section we discuss these mechanisms.
Two approaches to bandwidth allocation are considered in [E.360]: per-VNET bandwidth allocation and per-flow bandwidth allocation. In the per-VNET method, aggregated MPLS CRLSP bandwidth is managed to meet the overall bandwidth requirements of VNET service needs. Individual flows are allocated bandwidth within the CRLSPs accordingly, as CRLSP bandwidth is available. In the per-flow method, bandwidth is allocated to each individual flow from the overall pool of bandwidth, as the total pool bandwidth is available. A fundamental principle applied in these bandwidth allocation methods is the use of bandwidth reservation techniques. We first review bandwidth reservation principles and then discuss per-VNET and per-flow bandwidth allocation and protection.
Bandwidth reservation (the TDM-network
terminology is “trunk reservation”) gives preference to the preferred traffic
by allowing it to seize any idle bandwidth on a link, while allowing the
non-preferred traffic to only seize bandwidth if there is a minimum level of
idle bandwidth available, where the minimum-bandwidth threshold is called the
reservation level. P. J. Burke [BUR]
first analyzed bandwidth reservation behavior from the solution of the birth-death
equations for the bandwidth reservation model.
Burke’s model showed the relative lost-traffic level for preferred
traffic, which is not subject to bandwidth reservation restrictions, as
compared to non-preferred traffic, which is subject to the restrictions.
Bandwidth reservation protection is robust to traffic variations and provides
significant dynamic protection of particular streams of traffic.
Bandwidth reservation is a crucial technique used
in nonhierarchical networks to prevent "instability," which can
severely reduce throughput in periods of congestion, perhaps by as much as 50
percent of the traffic-carrying capacity of a network. Bandwidth reservation is
used to prevent this unstable behavior by having the preferred traffic on a link
be the traffic on the primary, shortest path, and the non-preferred traffic,
subjected to bandwidth reservation restrictions as described above, be the
alternate-routed traffic on longer paths. In this way the alternate-routed
traffic is inhibited from selecting longer alternate paths when sufficient idle
trunk capacity is not available on all links of an alternate-routed connection,
which is the likely condition under network and link congestion.
Mathematically, the studies of bistable network behavior have shown that
bandwidth reservation used in this manner to favor primary shortest connections
eliminates the bistability problem in nonhierarchical networks and allows such
networks to maintain efficient utilization under congestion by favoring
connections completed on the shortest path [AKI, KRU, NAK]. For this reason, dynamic bandwidth
reservation is universally applied in nonhierarchical TDM-based networks, and
often in hierarchical networks [MUM].
It is beneficial for bandwidth reservation
techniques to be included in IP-based and ATM-based routing methods, in order
to ensure the efficient use of network resources especially under congestion
conditions. Currently proposed
path-selection methods, such as methods for optimized multipath in IP-based
MPLS networks [VIL], or path selection in ATM-based PNNI networks [PNNI], give
no guidance on the necessity for using bandwidth-reservation techniques. Such guidance is essential for acceptable
network performance.
Figure 2 illustrates multi-service, QoS resource management, in which bandwidth is allocated on an aggregated basis to the individual VNETs (high-priority, normal-priority, and best-effort priority services VNETs). This allocated bandwidth is protected by bandwidth reservation methods, as needed, but otherwise shared. Each ON monitors VNET bandwidth use on each VNET CRLSP, and determines when VNET CRLSP bandwidth needs to be increased or decreased. In Figure 2, bandwidth changes in VNET bandwidth capacity are determined by ONs based on an overall aggregated bandwidth demand for VNET capacity (not on a per-connection demand basis). Based on the aggregated bandwidth demand, ONs make periodic discrete changes in bandwidth allocation, that is, either increase or decrease bandwidth on the CRLSPs constituting the VNET bandwidth capacity. For example, if connection requests are made for VNET CRLSP bandwidth that exceeds the current CRLSP bandwidth allocation, the ON initiates a bandwidth modification request on the appropriate CRLSP(s). For example, this bandwidth modification request may entail increasing the current CRLSP bandwidth allocation by a discrete increment of bandwidth denoted here as delta-bandwidth (DBW). DBW, for example, could be the additional amount needed by the current connection request. In any case, DBW is a large enough bandwidth change so that modification requests are made relatively infrequently. Also, the ON periodically monitors CRLSP bandwidth use, such as once each minute, and if bandwidth use falls below the current CRLSP allocation the ON initiates a bandwidth modification request to decrease the CRLSP bandwidth allocation, for example, down to the current level of bandwidth utilization.
In making a VNET
bandwidth allocation modification, the ON determines the VNET priority (high,
normal, or best-effort), VNET bandwidth-in-use, VNET bandwidth allocation
thresholds, and whether the CRLSP is a
first choice CRLSP or alternate CRLSP.
These parameters are used to access a VNET table (illustrated below in Table 1) to determine the allowed load
state threshold (ALSi) to which network capacity can be allocated for the VNET
bandwidth modification request. In using the ALS
threshold to allocate VNET bandwidth capacity, the ON selects a first choice
CRLSP based on the routing table selection rules, or alternate paths if the
first choice path is not available.
Path selection may use open shortest path first (OSPF) [RFC1247] for intra-domain routing, which provides at each node a topology database that may also include, for example, available bandwidth on each link. From the topology database, ON A in Figure 3 could determine a list of shortest paths by using, for example, Dijkstra’s algorithm. This path list could be determined based on administrative weights of each link, which are communicated to all nodes within the routing domain. These administrative weights may be set, for example, to [1 + epsilon x distance], where epsilon is a factor giving a relatively smaller weight to the distance in comparison to the hop count. The ON selects a path from the list based on, for example, FR, TDR, SDR, or EDR path selection.
For example, in using the first CRLSP A-B-E in Figure 3, ON A sends an MPLS label request message to VN B, which in turn forwards the label request message to DN E. VN B and DN E are passed in the explicit-routing parameter contained in the label request message. Each node in the CRLSP reads the explicit-routing information, and passes the label request message to the next node listed in the explicit-routing parameter. The connection admission control for each link in the path is performed based on the status of the link. The ON may select any path for which the first link is allowed according to QoS resource management criteria. If the first path is blocked at any of the links in the path, an MPLS notification message with a crankback parameter is returned to ON A, which can then attempt the next path. If FR is used, then this path is the next path in the shortest path list, for example path A-C-D-E. If TDR is used, then the next path is the next path in the routing table for the current time period. If SDR is used, OSPF implements a distributed method of flooding link status information, which is triggered either periodically and/or by crossing load state threshold values. This method of distributing link status information can be resource intensive and may not be any more efficient than simpler path selection methods such as EDR. If EDR is used, then the next path is the last successful path, and if that path is unsuccessful another alternate path is searched out according to the EDR path selection method. EDR path selection, which entails the use of the release with crankback mechanism to search for an available path, is an alternative to SDR path selection, which may entail flooding of frequently changing link state parameters such as available-link-bandwidth. With EDR path selection, the reduction in the frequency of such link-state parameter flooding allows for increased scalability. This is because link-state flooding can consume substantial processor and link resources, in terms of message processing by the processors and link bandwidth consumed by messages on the links.
Hence in using the selected CRLSP, the ON sends the explicit route, the requested traffic parameters (peak data rate, committed data rate, etc.), an ALS threshold, and a modify-parameter in the MPLS label request message to each VN and the DN in the selected CRLSP. Whether or not bandwidth can be allocated to the bandwidth modification request on the first choice CRLSP is determined by each VN applying the QoS resource management rules. That is, the VN determines the CRLSP link states, based on bandwidth use, and compares the link load state to the ALS threshold ALSi sent in the MPLS signaling parameters, as further explained below. If the first choice CRLSP cannot admit the bandwidth change, a VN or DN returns control to the ON through the use of the crankback-parameter in the MPLS notification message. At that point the ON may then try an alternate CRLSP. Whether or not bandwidth can be allocated to the bandwidth modification request on the alternate path again is determined by the use of the ALS threshold compared to the CRLSP link load state at each VN. Priority queuing is used during the time the CRLSP is established, and at each link the queuing discipline is maintained such that the packets are given priority according to the VNET traffic priority, which is discussed in Section 2.2.3.
Hence determination of the
CRLSP link load states is necessary for QoS resource management to select
network capacity on either the first choice CRLSP or alternate CRLSPs. Three link load states are distinguished:
available (non-reserved) bandwidth (ABW), reserved-bandwidth (RBW), and
bandwidth-not-available (BNA).
Management of CRLSP capacity uses the link state model and the ALS
threshold to determine if a bandwidth modification request can be accepted on a
given CRLSP. The allowed load state
threshold ALSi determines if a bandwidth modification request can be accepted
on a given link to an available bandwidth “depth.” In setting up the bandwidth modification
request, the ON encodes the ALS threshold allowed on each link in the
ALS-parameter, which is carried in the MPLS label request. If a CRLSP link is encountered at a VN in
which the idle link bandwidth and link load state are below the allowed load
state threshold ALSi, then the VN sends an MPLS
notification message with the crankback-parameter to the ON, which can then route the bandwidth
modification request to an alternate CRLSP choice. For example, in Figure 3, CRLSP A-B-E may be
the first path tried where link A-B is in the ABW state and link B-E is in the
RBW state. If the ALS load state allowed
is ALSi=ABW, then the CRLSP bandwidth modification request in the MPLS label
request message is routed on link A-B but will not be admitted on link B-E, and
the CRLSP bandwidth modification request will be cranked back in the MPLS
notification message to the ON A to try alternate CRLSP A-C-D-E. Here the CRLSP bandwidth modification request
again does not succeed since link CD is in the RBW state. At this point node A can search for a new
successful CRLSP-n among the candidate choices.
Here we discuss a sparse network
example of per-VNET bandwidth allocation/reservation. Methods are similar for meshed-network and
per-flow bandwidth allocation, with differences being a) bandwidth reservation
is triggered rather not fixed in meshed networks, so as not to over-reserve
bandwidth since there is a large number of links on which to reserve bandwidth,
and b) bandwidth allocation is triggered on a per-flow rather than per-VNET
basis in per-flow bandwidth allocation. For the sparse network case of bandwidth reservation, a
simpler method is illustrated which takes advantage of the concentration of
traffic onto fewer, higher capacity backbone links. That is, a small, fixed level of bandwidth
reservation is used and permanently enabled on each link, and the ALS threshold
is a simple function of bandwidth-in-progress, VNET priority, and bandwidth
allocation thresholds, as follows:
Table 1
Determination of Allowed Load State (ALS) Threshold
(Per-VNET Bandwidth Allocation, Sparse Network)
Allowed |
High- |
Normal-Priority VNET |
Best-Effort |
|
Load Statei |
Priority VNET |
First Choice CRLSP |
Alternate CRLSP |
Priority VNET |
RBW |
If BWIPi Ł |
If BWIPi Ł BWavgi |
Not Allowed |
Note 1 |
ABW |
If 2 ´ BWmaxi < BWIPi |
If BWavgi < BWIPi |
If BWavgi < BWIPi |
Note 1 |
where
BWIPi = bandwidth-in-progress on VNET-i
BWavgi =
minimum guaranteed bandwidth
required for VNET-i to carry the
average offered
bandwidth load
BWmaxi =
the bandwidth required for
VNET-i to meet the blocking/delay probability
grade-of-service
objective for CRLSP bandwidth allocation requests
= 1.1
x BWavgi
Note 1 = CRLSPs for the best effort priority VNET are allocated zero bandwidth;
Diffserv queuing admits best effort packets only if there is available
bandwidth on a link
The corresponding load
state table for the sparse network case is as follows:
Table 2
Determination of Link Load State (Sparse Network)
Link
Load State |
|
Condition |
Bandwidth-Not-Available |
BNA |
ILBWk < DBW |
Reserved-Bandwidth |
RBW |
ILBWk - RBWrk <
DBW |
Available-Bandwidth |
ABW |
DBW Ł ILBWk - RBWrk |
where
ILBWk = idle link bandwidth on link k
DBW = delta bandwidth requirement for a
bandwidth allocation
request
RBWrk = reserved bandwidth for link k
= .01
x TLBWk
TLBWk = the total link bandwidth on link k
Figure 3 summarizes the operation of
STT-EDR path selection and admission control combined with per-VNET bandwidth
allocation. ON
A monitors VNET bandwidth use on each VNET CRLSP, and determines when VNET
CRLSP bandwidth needs to be increased or decreased. Based on the aggregated bandwidth demand, ON
A makes periodic discrete changes in bandwidth allocation, that is, either
increase or decrease bandwidth on the CRLSPs constituting the VNET bandwidth
capacity. If connection requests are made for VNET CRLSP bandwidth that exceeds
the current CRLSP bandwidth allocation, ON A initiates a bandwidth modification
request on the appropriate CRLSP(s). The
STT-EDR QoS routing algorithm used is adaptive and distributed in nature
and uses learning models to find good paths.
For example, in Figure 3 if the LSR-A to LSR-E bandwidth needs to be
modified, say increased by DBW, the primary CRLSP-p (A-B-E) is tried
first. If DBW is not available on one or
more links of CRLSP-p, then the currently successful CRLSP-s (A-C-D-E) is tried
next. If DBW is not available on one or
more links of CRLSP-s, then a new CRLSP is searched by trying additional
candidate paths (not shown) until a new successful CRLSP-n is found or the
candidate paths are exhausted. CRLSP-n
is then marked as the currently successful path for the next time bandwidth
needs to be modified. DBW, for example, can be set to the additional amount of
bandwidth required by the connection request.
Also, ON A periodically monitors CRLSP bandwidth use, such as once each
minute, and if bandwidth use falls below the current CRLSP allocation the ON
initiates a bandwidth modification request to decrease the CRLSP bandwidth
allocation down to the currently used bandwidth level. In the models discussed in Section 3, the
per-VNET bandwidth allocation and admission control method compares favorably
with the per-flow method, and STT-EDR path selection method compares favorable
to the SDR method.
A.1.2.3
Packet-Level Control
Packet level traffic control encompasses the procedures which allow packet level grade-of-service objectives to be met. Once a flow is admitted through the connection admission control functions, packet level control a) ensures through traffic shaping that the traffic conforms to the declared traffic parameters, and b) ensures through packet priority and queue management that the network provides the requested quality of service in conformity with the declared traffic and allocated resources.
Traffic controls may be distinguished according to whether their function is to enable quality of service guarantees at the connection level (e.g. connection blocking probability) or at the packet level (e.g. packet loss ratio). As discussed in Section 2.2.2, connection admission control (CAC) determines if a link or path is capable of handling the requested connection with its associated traffic and QoS requirements. When CAC is applied, the network decides if it has sufficient resources to accept it without infringing packet level grade-of-service requirements for all established connections as well as the new connection. This decision is made by allocating resources to specific connections and refusing new requests when insufficient resources are available, where the resources in question are typically bandwidth and buffer space. A connection request is specified by traffic and QoS requirements, where end-to-end performance objectives relevant to QoS routing include a) maximum end-to-end queuing delay, b) delay variation, and c) packet loss ratio. These performance objectives must be apportioned to the various network elements contributing to the performance degradation of a given connection so that the end-to-end QoS criteria are satisfied.
If the connection is accepted, there is a traffic contract whereby the network provides the requested quality of service on condition that the traffic conforms to the declared traffic descriptor. This has led to a definition of traffic parameters: peak packet rate, sustainable packet rate, and intrinsic burst tolerance allowing traffic conformance to be determined by the generic packet rate algorithm. In this method, supplementary packet delays may be introduced to shape the characteristics of a given flow. Various scheduling mechanisms, such as priority queuing, may be used. The priority of service parameter may be included in the differentiated services (DiffServ) [RFC2475] parameter in the IP packet header or MPLS header [RFC3270]. DiffServ does not require that a particular queuing mechanism be used to achieve needed QoS behavior. Therefore the queuing implementation used for DiffServ could be weighted fair queuing, priority queuing, or other queuing mechanism, depending on the choice in the implementation. In the analysis priority queuing is used for illustration, however the same or comparable results would be obtained with weighted fair queuing or other queuing mechanisms. These scheduling and shaping mechanisms compliment the connection admission mechanisms described in the previous Section to appropriately allocate bandwidth on links in the network.
A.1.3
Routing Table Management
Routing table management information, such as topology update, status information, or routing recommendations, is used for purposes of applying the routing table design rules for determining path choices in the routing table. This information is exchanged, for example, between one node and another node, such as between the ON and DN, or between a node and a network element such as a QoS-routing processor. This information is used to generate the routing table, and then the routing table is used to determine the path choices used in the selection of a path.
IP networks typically run the OSPF protocol for intra-domain routing [RFC2328], which provides each node a link-state topology exchange mechanism to construct its topology database and from that it constructs shortest path routing tables. OSPF provides for a) exchange of node information, link state information, and reachable address information, b) automatic update and synchronization of topology databases, and c) fixed and/or dynamic route selection based on topology and status information. For topology database synchronization the link state advertisement (LSA) is used to automatically provision nodes, links, and reachable addresses in the topology database. For topology database synchronization, each node exchanges status information with its immediate neighbors and then bundles its state information in LSAs, which are reliably flooded throughout the routing domain.
Some of the topology state information is static and some is dynamic, and for network scalability it is important to minimize the amount of dynamic topology state information flooding, such as available link bandwidth. Query for status methods allow efficient determination of status information, as compared to flooding mechanisms, and are provided in TDM-based networks [ASH1]. Routing recommendation methods provide for a QoS-routing processor, for example, to advertise recommended paths to network nodes based on status information available in the database. Such routing recommendation methods are provided in TDM-based networks [ASH1].
Different
routing table management techniques are employed to achieve a) per-VNET
bandwidth allocation and per-flow allocation, and b) EDR versus SDR QoS routing
methods. These tradeoffs have
significantly different routing table management overhead requirements, which
are investigated in Section 3. EDR QoS
routing methods are distinct from SDR QoS routing methods in how the paths are
selected. In the SDR QoS routing case,
the available link bandwidth (based on LSA flooding of available-link-bandwidth
information) is typically used to compute the path. In the EDR QoS routing case, the
available-link-bandwidth information is not needed to compute the path,
therefore the available-link-bandwidth flooding does not need to take place,
reducing the overhead. As discussed in
Section 2.1, EDR QoS routing algorithms are adaptive and distributed in nature
and typically use learning models to find good paths for QoS routing in a
network, such as in the STT method.
Available-link-bandwidth flooding can be very resource intensive, since
it requires link bandwidth to carry LSAs, processor capacity to process LSAs,
and the overhead can impact network scalability and stability. Modeling results in Section 3 show EDR QoS
routing methods can lead to a large reduction in available-link-bandwidth
flooding overhead without loss of network throughput performance.
A.2. QoS Routing Modeling & Analysis
We now provide a performance analysis of lost/delayed traffic and control load for various QoS routing methods developed in [E.360]. A full-scale model of a national network is used together with a multiservice traffic demand model to study various QoS routing scenarios and tradeoffs. The 135-node model is illustrated in Figure 4.
Typical voice/ISDN traffic loads are used to
model the various network alternatives, which are based on 72 hours of actual
traffic loads on the national network used for the model. Table 3 summarizes
the multiservice traffic model used for the QoS routing studies. Three levels of traffic priority – high,
normal, and best-effort -- are given to the various class-of-service
categories, or VNETs, illustrated in Table 3.
The voice/ISDN loads are further segmented in the model into eight CBR
VNETs, including business voice, consumer voice, international voice in and
out, high-priority voice, normal and high-priority 64-kbps ISDN data, and
384-kbps ISDN data. For the CBR voice
services, the mean data rate is assumed to be 64 kbps for all VNETs except the
384-kbps ISDN data VNET-8, for which the mean data rate is 384 kbps.
Table 3
Virtual Network (VNET)
Traffic Model used for QoS routing Studies
Virtual Network Index |
Virtual Network
Name
|
Service Identity
Examples
|
Virtual Network
Traffic Priority
& Traffic Characteristics |
VNET-1 (CBR) |
BUSINESS VOICE |
VIRTUAL PRIVATE NETWORK (VPN), DIRECT CONNECT 800, 800 SERVICE, 900
SERVICE |
NORMAL-PRIORITY; 64 KBPS CBR; 72 HOURS TRAFFIC LOAD DATA (SATURDAY, SUNDAY, MONDAY) |
VNET-2 (CBR) |
CONSUMER VOICE |
LONG DISTANCE SERVICE (LDS) |
NORMAL-PRIORITY; 64 KBPS CBR; 72 HOURS TRAFFIC LOAD DATA (SATURDAY, SUNDAY, MONDAY) |
VNET-3 (CBR) |
INTL VOICE OUTBOUND |
INTL LDS OUTBOUND, INTL 800 OUTBOUND, GLOBAL VPN OUTBOUND, INTL
TRANSIT |
NORMAL-PRIORITY; 64 KBPS CBR; 72 HOURS TRAFFIC LOAD DATA (SATURDAY, SUNDAY, MONDAY) |
VNET-4 (CBR) |
INTL VOICE INBOUND (HIGH-PRIORITY) |
INTL LDS INBOUND, INTL 800 INBOUND, GLOBAL VPN INBOUND, INTL TRANSIT
INBOUND |
HIGH-PRIORITY; 64 KBPS CBR; 72 HOURS TRAFFIC LOAD DATA (SATURDAY, SUNDAY, MONDAY) |
VNET-5 (CBR) |
800-GOLD (HIGH-PRIORITY) |
DIRECT CONNECT 800 GOLD, VPN-HIGH-PRIORITY |
HIGH-PRIORITY; 64 KBPS CBR; 72 HOURS TRAFFIC LOAD DATA (SATURDAY, SUNDAY, MONDAY) |
VNET-6 (CBR) |
64 KBPS ISDN |
64 KBPS SWITCHED DIGITAL SERVICE (SDS), 64 KBPS SWITCHED DIGITAL INTL (SDI) |
NORMAL-PRIORITY; 64 KBPS CBR; 72 HOURS TRAFFIC LOAD DATA (SATURDAY, SUNDAY, MONDAY) |
VNET-7 (CBR) |
64 KBPS ISDN (HIGH-PRIORITY) |
64 KBPS SDS & SDI (HIGH-PRIORITY) |
HIGH-PRIORITY; 64 KBPS CBR; 72 HOURS TRAFFIC LOAD DATA (SATURDAY, SUNDAY, MONDAY) |
VNET-8 (CBR) |
384 KBPS ISDN |
384 KBPS SDS, 384 KBPS SDI |
NORMAL-PRIORITY; 384 KBPS CBR; 72 HOURS TRAFFIC LOAD DATA (SATURDAY, SUNDAY, MONDAY) |
VNET-9 (VBR-RT) |
IP TELEPHONY VARIABLE RATE, EQUIV-BW ALLOCATION, INTERACTIVE & DELAY SENSITIVE |
IP TELEPHONY, COMPRESSED VOICE |
NORMAL-PRIORITY; VARIABLE RATE, EQUIV-BW ALLOCATION, INTERACTIVE & DELAY
SENSITIVE; VBR-RT: 10% OF VNET1+VNET2+VNET3+ VNET4+VNET5 TRAFFIC LOAD, CALL DATA RATE VARIES FROM 6.4 KBPS TO 51.2 KBPS (25.6
KBPS MEAN) |
VNET-10 (VBR-NRT) |
IP MULTIMEDIA VARIABLE RATE, EQUIV-BW ALLOCATION, NON-INTERACTIVE & NOT DELAY SENSITIVE |
IP MULTIMEDIA, WWW, CREDIT CARD CHECK |
NORMAL-PRIORITY; VARIABLE RATE, EQUIV-BW ALLOCATION, NON-INTERACTIVE
& NOT DELAY SENSITIVE; VBR-NRT: 30% OF VNET2 TRAFFIC LOAD, CALL DATA RATE VARIES FROM 38.4 KBPS TO 64 KBPS (51.2 KBPS
MEAN) |
VNET-11 (UBR) |
UBR BEST EFFORT VARIABLE RATE, NO BW ALLOCATION, NON-INTERACTIVE & NOT DELAY SENSITIVE |
VOICE MAIL, EMAIL, FILE TRANSFER |
BEST-EFFORT PRIORITY; VARIABLE RATE, NO BW ALLOCATION, NON-INTERACTIVE & NOT DELAY
SENSITIVE; UBR: 30% OF VNET1 TRAFFIC LOAD, CALL DATA RATE VARIES FROM 6.4 KBPS TO 3072 KBPS (1536
KBPS MEAN) |
The data
services traffic model incorporates typical traffic load patterns and comprises
three additional VNET load patterns.
These data services VNETs include
- variable
bit rate real-time (VBR-RT) VNET-9, representing services such as IP-telephony
and compressed voice,
- variable
bit rate non-real-time (VBR-NRT) VNET-10, representing services such as WWW
multimedia and credit card check, and
- unassigned
bit rate (UBR) VNET-11, representing services such as email, voice mail, and
file transfer multimedia applications.
For the VBR-RT
connections, the data rate varies from 6.4 to 51.2 kbps with a mean of 25.6
kbps. The VBR-RT connections are assumed to be interactive and delay
sensitive. For the VBR-NRT connections,
the data rate varies from 38.4 to 64 kbps with a mean of 51.2 kbps, and the
VBR-NRT flows are assumed to be non-delay sensitive. For the UBR connections, the data rate varies
from 6.4 to 3072 kbps with a mean of 1536 kbps. The UBR flows are assumed to be
best-effort priority and non-delay sensitive.
For modeling purposes, the service and link bandwidth is segmented into
6.4 kbps slots, that is, 10 slots per 64 kbps channel.
In addition to the QoS bandwidth management procedure for bandwidth allocation requests, a QoS priority of service queuing capability is used during the time connections are established on each of the VNETs. At each link, a queuing discipline is maintained such that the packets being served are given priority in the following order: high-priority, normal-priority, and best-effort priority VNET services. This queuing model quantifies the level of delayed traffic for each VNET.
The cost model represents typical switching and
transport costs, and illustrates the economies-of-scale for costs projected for
high capacity network elements in the future.
Table 4 gives the model used for average switching and transport costs
allocated per 64 kbps unit of bandwidth, as follows:
Table 4
Cost Assumptions (average
cost per equivalent 64 kbps bandwidth, dollars)
Data Rate |
Average Transport Cost |
Average Switching/Cross-Connect Cost |
DS3 |
0.19 x miles + 8.81 |
26.12 |
OC3 |
0.17 x miles + 9.76 |
19.28 |
OC12 |
0.15 x miles + 7.03 |
9.64 |
OC48 |
0.05 x miles + 2.77 |
3.92 |
A discrete event network design model is used in
the design and analysis of the QoS routing methods: 2-link STT-EDR path routing
in a meshed logical network, 2-link DC-SDR routing in a meshed logical network,
and multilink STT-EDR, DC-SDR, and DP-SDR routing in a sparse logical
network. We also model the case where no
QoS routing methods are applied.
A.2.1 Performance
Comparisons of QoS Routing Methods
The network models for the 2-link and
multilink networks are now described.
Links in the 2-link models are assumed to have fine-grained (1.536
mbpsT1-level) logical transport link bandwidth allocation, and a meshed network
topology design results in which links exist between most (90 percent or more)
of the nodes. In the 2-link models, one and 2-link routing with crankback is
used with both EDR and
SDR path selection. In routing a connection with 2-link STT-EDR routing, the ON
checks the equivalent bandwidth and ALS threshold first on the direct path,
then on the current successful 2-link via path, and then sequentially on all
candidate 2-link paths. In routing a
connection with 2-link DC-SDR, the ON checks the equivalent bandwidth and
allowed ALS threshold first on the direct path, and then on the least-loaded
path that meets the equivalent bandwidth and ALS requirements. Each VN checks the equivalent bandwidth and
ALS threshold provided in the setup message, and uses crankback to the ON if the
equivalent bandwidth or ALS threshold are not met.
In the multilink model, high rate OC3/12/48 links
provide highly aggregated link bandwidth allocation and a sparse network
topology design results. That is, high
rate OC3/12/48 links exist between relatively few (10 to 20 percent) of the nodes.
The multilink path selection methods which are modeled include STT-EDR, DC-SDR,
and DP-SDR path selection, in which crankback is used. With STT-EDR, the primary CRLSP-p is tried
first and if bandwidth resources are not available on one or more links of
CRLSP-p, then the currently successful CRLSP-s is tried next. If bandwidth is not available on one or more
links of CRLSP-s, then a new CRLSP is searched by trying additional candidate
paths until a new successful CRLSP-n is found or the candidate paths are
exhausted. CRLSP-n is then marked as the
currently successful path for the next time bandwidth needs to be
modified. In the model of DP-SDR, the
status updates with link-status flooding occur every 10 seconds. Note that the multilink DP-SDR performance
results should also be comparable to the performance of multilink CP-SDR, in
which status updates and path selection updates are made every 10 seconds,
respectively, to and from a QoS-routing processor. With the SDR methods, the available link
bandwidth information in the topology database is used to generate the
shortest, least-congested, paths. In
routing a connection, the ON checks the equivalent bandwidth and ALS threshold
first on the first choice path, then on current successful alternate path (EDR)
or least loaded shortest path (SDR), and then sequentially on all other
candidate alternate paths. Each VN
checks the equivalent bandwidth and ALS threshold provided in the setup
message, and uses crankback to the ON if the equivalent bandwidth or ALS
threshold are not met.
In the models
the logical network design is optimized for each routing alternative, while the
physical transport links and node locations are held fixed. We examine the performance and network design
tradeoffs of
logical topology (sparse,
mesh),
connection routing method
(2-link, multilink, SDR, EDR, etc.), and
bandwidth allocation method (per-VNET,
per-flow)
Generally the
meshed logical topologies are optimized by 1- and 2-link routing, while the
sparse logical topologies are optimized by multilink shortest path
routing. Modeling results include
- designs
for SDR connection routing and EDR connection routing,
- designs
for sparse topology with multilink routing and mesh topology with 2-link
routing,
- designs
for separate voice (VNETs 1-8) &
data (VNETs 9-11) and integrated voice/data (VNETs 1-11)
- designs
for per-VNET bandwidth allocation and per-flow bandwidth allocation
Table 5 gives
a summary of the design comparisons for the above tradeoff categories.
Table 5
Network Design Comparisons (135-Node Model)
Network Design Parameters |
EDR Connection Routing SDR Connection Routing (Ratio) |
Integrated Voice/Data Separate Voice/Data (Ratio) |
Per-Flow BW Alloc. Per-VNET BW Alloc. (Ratio) |
Topology & Routing Design |
Mesh with 2-link EDR or 2-link SDR routing |
Sparse with multilink EDR routing |
Sparse with multilink EDR routing |
Termination Capacity (Equivalent 64-kbps, Millions) |
25.6 25.7 (0.996) |
16.4 17.5 (0.937) |
16.4 16.5 (0.994) |
Transport Capacity (Equivalent 64-kbps-miles, Millions) |
11,630.6 11,629.8 (1.000) |
9285.3 9641.4 (0.963) |
137.7 148.1 (0.930) |
Total Cost ($ Millions) |
1238.4 1238.5 (1.000) |
1267.2 1338.5 (0.946) |
1267.2 1306.2 (0.970) |
Some of the conclusions from the network
design comparisons are as follows:
- EDR connection
routing methods exhibit comparable design efficiencies to SDR routing methods.
- Sparse topology
designs with multilink routing provide switching and transport design
efficiencies in comparison to mesh designs with 2-link routing (however,
overall capital costs are comparable).
- Voice and data integration provides some capital (and operational) cost reduction in comparison to separate voice and data design.
- Per-VNET bandwidth allocation exhibits comparable design efficiencies to per-flow bandwidth allocation.
The performance analyses for overloads and
failures include connection admission control (CAC) with QoS resource
management. Performance comparisons are
presented in Table 6 for the various QoS routing methods, including 2-link and
multilink EDR and SDR approaches, and a baseline case of no QoS routing methods
applied. Table 6 gives performance
results for a six-times overload on a single network node at Oakbrook IL.
Table 6
Performance Comparison for Various QoS routing
Methods & No QoS routing Methods
6X Focused Overload on Oakbrook (% Lost/Delayed Traffic)
Virtual Network |
2-Link STT-EDR |
2-Link DC-SDR |
Multilink STT-EDR |
Multilink DC-SDR |
Multilink DP-SDR |
No QoS Routing Methods Applied |
BUSINESS-VOICE |
5.27 |
2.28 |
0.00 |
0.06 |
0.08 |
9.42 |
CONSUMER-VOICE |
7.29 |
3.50 |
0.00 |
0.20 |
0.23 |
13.21 |
INTL-OUT |
3.43 |
3.36 |
0.00 |
0.00 |
0.04 |
6.03 |
INTL-IN (HIGH-PRIORITY) |
2.19 |
4.21 |
0.00 |
0.00 |
0.00 |
6.55 |
HIGH-PRIORITY VOICE |
0.81 |
1.77 |
0.00 |
0.00 |
0.00 |
8.47 |
64-KBPS ISDN DATA |
0.84 |
0.33 |
0.00 |
0.00 |
0.00 |
2.33 |
64-KBPS ISDN DATA (HIGH-PRIORITY) |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.46 |
384-KBPS ISDN DATA |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
0.00 |
VBR-RT VOICE |
5.42 |
2.59 |
0.00 |
0.39 |
0.49 |
9.87 |
VBR-NRT MULTIMEDIA |
7.12 |
3.49 |
0.00 |
2.75 |
3.18 |
12.88 |
UBR BEST EFFORT |
14.07 |
14.68 |
12.46 |
12.39 |
12.32 |
9.75 |
In all cases of the QoS routing methods being
applied, the performance is always better and usually substantially better than
when no QoS routing methods are applied.
The performance analysis results show that the multilink options (in
sparse topologies) perform somewhat better under overloads than the 2-link
options (in meshed topologies), because of greater sharing of network capacity.
Under failure, the 2-link options perform better for many of the VNET
categories than the multilink options, because they have a richer choice of
alternate routing paths and are much more highly connected than the multilink
networks. Loss of a link in a sparely
connected multilink network can have more serious consequences than in more
highly connected logical networks. The
performance results illustrate that capacity sharing of CBR, VBR, and UBR
traffic classes, when combined with QoS resource management and priority
queuing, leads to efficient use of bandwidth with minimal traffic delay and
loss impact, even under overload and failure scenarios.
The EDR and SDR path selection methods
are quite comparable for the 2-link, meshed-topology network scenarios. However, the EDR path selection method
performs somewhat better than the SDR options in the multilink, sparse-topology
case. In addition, the DC-SDR path
selection option performs somewhat better than the DP-SDR option in the
multilink case, which is a result of the 10-second old status information
causing misdirected paths in some cases.
Hence, it can be concluded that frequently-updated,
available-link-bandwidth state information does not necessarily improve
performance in all cases, and that if available-link-bandwidth state
information is used, it is sometimes better that it is very recent status
information.
Some of the conclusions from
the performance comparisons are as follows:
- QoS routing methods result in
network performance that is always better and usually substantially better than
when no QoS routing methods are applied.
- Sparse-topology multilink-routing networks provide better overall performance under overload than meshed-topology networks, but performance under failure may favor the 2-link meshed-topology options with more alternate routing choices.
- EDR QoS
routing methods exhibit comparable or better network performance compared to
SDR methods.
- State information as used by the SDR
options (such as with link-state flooding) provides essentially equivalent
performance to the EDR options, which typically use distributed routing with
crankback and no flooding.
- Single-area
flat topologies exhibit better network performance in comparison with
multi-area hierarchical topologies.
- Various path selection methods can
interwork with each other in the same network, as required for multi-vendor
network operation.
A.2.2
Performance Comparisons of Bandwidth Allocation, Protection & Reservation
Methods
As discussed in Section 2.2.2, dynamic bandwidth
reservation can be used to favor one category of traffic over another category
of traffic. A simple example of the use
of this method is to reserve bandwidth in order to prefer traffic on the
shorter primary paths over traffic using longer alternate paths. We now give
illustrations of this method, and compare the performance of a network in which
bandwidth reservation is used under congestion to the case when bandwidth
reservation is not used. In the example,
traffic is first routed on the shortest path, and then allowed to alternate
route on longer paths if the primary path in not available. In the case where bandwidth reservation is
used, five percent of the link bandwidth is reserved for traffic on the primary
path when congestion is present on the link.
Table 7 illustrates the performance of bandwidth
reservation methods for a high-day network load pattern. We can see from the results that performance
improves when bandwidth reservation is used.
The reason for the poor performance without bandwidth reservation is due
to the lack of reserved capacity to favor traffic routed on the more direct
primary paths under network congestion conditions. Without bandwidth reservation nonhierarchical
networks can exhibit unstable behavior in which essentially all connections are
established on longer alternate paths as opposed to shorter primary paths,
which greatly reduces network throughput and increases network congestion [AKI,
KRU, NAK]. If we add the bandwidth
reservation mechanism, then performance of the network is greatly
improved. Clearly the use of bandwidth
reservation protects the performance of each VNET class-of-service category.
Table 7
Performance of Dynamic Bandwidth Reservation Methods
Percent Lost/Delayed Traffic under 50% General
Overload (Multilink STT-EDR)
Virtual Network |
Without Bandwidth Reservation |
With Bandwidth Reservation |
BUSINESS-VOICE |
2.42 |
0.00 |
CONSUMER-VOICE |
2.33 |
0.02 |
INTL-OUT |
2.46 |
1.33 |
INTL-IN (HIGH-PRIORITY) |
2.56 |
0.00 |
HIGH-PRIORITY VOICE |
2.41 |
0.00 |
64-KBPS ISDN DATA |
2.37 |
0.10 |
64-KBPS ISDN DATA (HIGH-PRIORITY) |
2.04 |
0.00 |
384-KBPS ISDN DATA |
12.87 |
0.00 |
VBR-RT VOICE |
1.25 |
0.07 |
VBR-NRT MULTIMEDIA |
1.90 |
0.01 |
UBR BEST EFFORT |
24.95 |
11.15 |
We use the 135-node model to compare the per-virtual-network methods of QoS resource management and the per-flow methods, as described in Section 2. We look at these two cases in Figure 5, which illustrates the case of per-virtual-network CRLSP bandwidth allocation the case of per-flow CRLSP bandwidth allocation. The two figures compare the performance in terms of lost or delayed traffic under a focused overload scenario on the Oakbrook, IL node (such a scenario might occur, for example, with a radio call-in give-away offer). The size of the focused overload is varied from the normal load (1X case) to a 10 times overload of the traffic to Oakbrook (10X case). The results show that the per-flow and per-virtual-network bandwidth allocation performance is similar; however, the improved performance of the high-priority traffic and normal-priority traffic in relation to the best-effort priority traffic is clearly evident.
We illustrate the operation of MPLS and DiffServ in multiservice network bandwidth allocation with some examples. First suppose there is 10 mbps of normal-priority traffic and 10 mbps of best-effort priority traffic being carried in the network between nodes A and B. Best-effort traffic is treated in the model as UBR traffic and is not allocated any bandwidth. Hence while the best-effort traffic does not get any CRLSP bandwidth allocation, it is always ‘admitted’ by the CAC, and must contend at the lowest priority in the queues. Hence the best-effort traffic cannot be denied ‘admission’ as a means to throttle back such traffic at the edge router, which can be done with the normal-priority and high-priority traffic (i.e., normal and high-priority traffic could be denied bandwidth allocation through connection admission control). The only way that the best-effort traffic gets dropped/lost is to drop it at the queues, therefore it is essential that the traffic that is allocated bandwidth on the CRLSPs have higher priority at the queues than the best-effort traffic. Therefore in the model the three classes of traffic get these DiffServ markings: best-effort traffic gets no marking, which corresponds to best-effort priority queuing treatment. Normal-priority traffic gets a middle priority level of queuing treatment, and high-priority and delay-sensitive traffic gets the highest priority queuing level.
Now suppose that there is 30 mbps of bandwidth
available between A and B and that the traffic for both the normal-priority and
best-effort traffic increases to 20 mbps.
The normal-priority traffic requests and gets a CRLSP bandwidth
allocation increase to 20 mbps on the A to B CRLSP. However, the best-effort traffic, since it
has no CRLSP assigned and therefore no bandwidth allocation, is just sent into
the network at 20 mbps. Since there is
only 30 mbps of bandwidth available from A to B, the network must drop 10 mbps
of best-effort traffic in order to leave room for the 20 mbps of
normal-priority traffic. The way this is
done in the model is through the queuing mechanisms governed by the DiffServ
priority settings on each category of traffic.
Through the DiffServ marking, the queuing mechanisms in the model
discard 10 mbps of the best-effort traffic at the priority queues. If the DiffServ markings were not used, then
the normal-priority and best-effort traffic would compete equally in the
queues, and perhaps 15 mbps of each would get through, which is not the desired
situation.
Taking this example further, if the
normal-priority and best-effort traffic both increase to 40 mbps, then the
normal-priority traffic tries to get a CRLSP bandwidth allocation increase to
40 mbps. However, the most it can get is
30 mbps, so 10 mbps is denied for the normal-priority traffic in the MPLS
constraint-based routing procedure. By
having the DiffServ markings on the normal-priority traffic and none on the
best-effort traffic, essentially all the best-effort traffic is dropped at the
queues since the normal-priority traffic is allocated and gets the full 30 mbps
of A to B bandwidth. If there are no
DiffServ markings, then again perhaps 15 mbps of both normal-priority and
best-effort get through. Or in this
case, perhaps a greater amount of best-effort traffic is carried than
normal-priority traffic, since 40 mbps of best-effort traffic is sent into the
network and only 30 mbps of normal-priority traffic is sent into the network,
and the queues will receive more best-effort pressure than normal-priority
pressure.
In a multiservice network where the
normal-priority and high-priority traffic use CAC with MPLS to receive
bandwidth allocation and there is no best-effort priority traffic, then the
DiffServ/priority queuing becomes less important. This is because the MPLS bandwidth allocation
more-or-less assures that the queues will not overflow, and perhaps therefore
DiffServ would not be needed as much. As
bandwidth gets more and more plentiful/lower-cost and the network is more
‘over-engineered’, the point at which the MPLS and DiffServ mechanisms have a
significant effect under traffic overload goes to a higher and higher
threshold. For example, the models show
that the overload factor at which congestion occurs gets larger as the
bandwidth modules get larger (i.e., OC3 to OC12 to OC48 to OC192, etc.). However, the congestion point will always be
reached with failures and/or large-enough overloads necessitating the
MPLS/DiffServ mechanisms.
Some of the conclusions from the modeling of bandwidth allocation and reservation are as follows:
- QoS resource management is shown to
be effective in achieving connection-level and packet-level grade-of-service
objectives, as well as high-priority, normal-priority, and best-effort priority
service differentiation.
- Bandwidth reservation is critical to the stable and efficient performance of QoS routing methods in a network, and to ensure the proper operation of multiservice bandwidth allocation, protection, and priority treatment.
- Per-VNET bandwidth allocation is essentially equivalent to per-flow bandwidth allocation in network performance and efficiency. Because of the much lower routing table management overhead, per-VNET bandwidth allocation is preferred to per-flow allocation.
- Both CAC with MPLS bandwidth management and DiffServ priority queuing management are important for ensuring that multiservice network performance objectives are met under a range of network conditions. Both mechanisms operate together to ensure QoS resource allocation mechanisms (bandwidth allocation, protection, and priority queuing) are achieved.
- In a multiservice
network environment where high-priority, normal-priority, and best-effort
traffic share the same network, under congestion (e.g., from overloads or
failures), the DiffServ/priority-queuing mechanisms push out the best-effort
priority traffic at the queues so that the normal-priority and high-priority
traffic can get through on the MPLS-allocated CRLSP bandwidth.
A.2.4 Performance Comparisons of Routing Table
Management Methods
Table 8 gives a comparison of the control
overhead performance of a) DP-SDR with LSA flooding and per-flow bandwidth
allocation, b) STT-EDR with per-flow bandwidth allocation, and c) STT-EDR with
per-VNET bandwidth allocation. The
numbers in the table give the total messages of each type needed to do the
indicated QoS routing functions, including flow setup, bandwidth allocation,
crankback, and LSA flooding to update the topology database. The DP-SDR method does
available-link-bandwidth flooding to update the topology database while the EDR
method does not. In the simulation there
is a 6-times focused overload on the Oakbrook node. Clearly the DP-SDR/flooding method is
consuming more message resources, particular LSA flooding messages, than the
STT-EDR method. Also, the per-flow
bandwidth allocation is consuming far more CRLSP bandwidth allocation messages
than per-VNET bandwidth allocation, while the traffic lost/delayed performance
of the three methods is comparable.
Table 8
Routing Table Management Overhead
SDR/Flooding/Per-Flow, EDR/Per-Flow, EDR/Per-VNET (6X
Focused Overload on Oakbrook)
QoS Routing Function |
Message Type |
DP-SDR/Flooding (per-flow bandwidth
allocation) |
DP-SDR/Flooding (per-flow bandwidth
allocation) |
STT-EDR (per-VNET bandwidth
allocation) |
Flow Routing |
Flow Setup |
18,758,992 |
18,758,992 |
18,758,992 |
QoS Resource Management (CRLSP Rtg., BW Alloc., Queue Mgmt.) |
CRLSP Bandwidth Allocation |
18,469,477 |
18,839,216 |
2,889,488 |
Crankback |
30,459 |
12,850 |
14,867 |
|
Topology Database Update |
LSA |
14,405,040 |
|
|
Some of the conclusions from the
comparisons of routing table management overhead are as follows:
- Per-VNET bandwidth allocation is preferred to per-flow allocation because of the much lower routing table management overhead. Per-VNET bandwidth allocation is essentially equivalent to per-flow bandwidth allocation in network performance and efficiency.
- EDR methods provide a large
reduction in flooding overhead without loss of network throughput performance.
Flooding is very resource intensive since it requires link bandwidth to carry
LSAs, processor capacity to process LSAs, and the overhead limits autonomous
system size. EDR
methods therefore can help to increase network scalability.
A.3. Summary & Conclusions
In summary, QoS
routing methods are proposed in [E.360] for consideration in network
evolution. These proposals are based on
results of analysis models, which illustrate the tradeoffs between various QoS
routing approaches, and established best current practices and experience. These QoS routing methods will ensure stable/efficient network
performance and help manage resources for and differentiate high-priority,
normal-priority, and best-effort priority services. Figures 2 and 3 illustrates the proposed QoS
routing methods, which a) allocate bandwidth
to individual VNETs, b) protect allocated bandwidth by bandwidth reservation methods, as
needed, but otherwise fully share bandwidth, c) differentiate high-priority, normal-priority, and best-effort
priority services, d) monitor
VNET bandwidth use and determine when bandwidth needs to be increased or
decreased, e) change VNET bandwidth allocation based on aggregated bandwidth
demand, and f) provide QoS routing
admission control to reject connection requests when needed to meet performance
objectives. In the modeling results, the per-VNET
bandwidth allocation method compares favorably with the per-flow method. Furthermore,
we find that the fully distributed STT-EDR method of CRLSP management performs
just as well or better than the SDR methods with flooding, which means that
STT-EDR path selection has potential to significantly enhance network
scalability.
Figures
_________________