Differentiated Services and Integrated Services use of MPLS
`
`Nicolas Rouhana
`University Saint-Joseph - Lebanon
`Nicolas. Rouhana@usj. edu. lb
`
`Eric Horlait
`University Pierre et Marie Curie -France
`Eric. Horlait@lip6fr
`
`Abstract
`
`All the new emerging QoS service architectures are
`motivated by
`the desire
`to
`improve
`the overall
`performance of IP networks. Integrated Services
`(Intserv), Differentiated Services (Diffserv), MultiProtocol
`Label Switching (MPLS) and constraint-based routing
`are all technologies starting to coexist together in t o d q s
`Internet to provide means for the delivery of end-to-end
`QoS to applications over heterogeneous networks.
`In this paper, we propose DRUM (Diffserv and
`RSVPhntserv Use of MPLS), an architecture that delivers
`end-to-end service guarantees for both Diffserv and
`Intserv networks, where part of the underr’ying technology
`used for IP
`transport is MPLS using Diffserv-like
`mechanisms for QoS provision. We also show how trafic
`engineering can ameliorate service diflerentiation, and
`illustrate how interoperability can be achieved between
`DRUM and neighboring Diffserv and Intserv networks.
`
`1. Introduction
`
`In the past several years, works on QoS enabled
`networks led to several propositions. The Integrated
`Services (IntServ) architecture [l] was first introduced
`along with
`the RSVP signaling protocol [2] that
`applications used for setting up paths and reserving
`resources towards receivers before sending data. The
`Differentiated Services (DiffServ or DS) architecture [6],
`a more scalable solution, classifies packets into a small
`number of aggregated flows or service classes that
`specified a specific forwarding treatment or Per Hop
`Behavior (PHB). The MultiProtocol Label Switching
`(MPLS) [9] architecture, originally presented as a way of
`improving the forwarding speed of routers, is now
`emerging as a crucial standard technology that offers new
`IP
`QoS
`capabilities
`for
`large-scale
`networks.
`Furthermore,
`traffic
`engineering associated with
`constraint-based routing have the ability to compute
`routes subject to multiple constraints such as bandwidth
`or delay requirement, and constitute important tools used
`by MPLS for arranging how traffic flows through the
`network and improve network utilization.
`
`0-7695-0722-O/OO $10.00 0 2000 IEEE
`
`194
`
`All these service architectures are now viewed as
`the pursuit of end-to-end QoS
`complementary
`in
`provisioning. For example, uneven traffic distribution can
`be a problem for Premium service in a DS domain [8]
`because aggregation of Premium traffic in the core
`network may invalidate the assumption that the arrival
`rate of premium traffic is below the service rate and
`Differentiated Services alone cannot solve this problem;
`traffic engineering is needed to avoid congestion.
`Figure 1 shows how these different technologies
`would fit together in today’s Internet: an MPLS core
`network consisting of Label Switch Routers (LSRs)
`providing transport and QoS guarantees for boundary
`“customer” networks supporting Intserv and Diffserv
`architectures.
`
`Figure 1. End-to-end QoS network
`
`The QoS model depicted in figure 1 considers that
`senders from the Intserv (resp. Diffserv) networks need to
`communicate with receivers in other Intserv (resp.
`Diffserv) networks through the MPLS b-ansit_network.
`This is consequent to the capability of MPLS to provide
`“Virtual Private Networks”
`(VPNs)
`to connect
`organizations to their multiple sites with compatible end-
`to-end QoS needs.
`The DRUM architecture described in this paper
`proposes simple mechanisms for MPLS edge and core
`LSRs to deliver end-to-end service guarantees and
`scalable QoS for both Diffserv and Intserv neighboring
`networks. The next section introduces a service definition
`in DRUM. Section 3 explains the internals of the LSRs.
`Section 4 presents the necessary interworking functions
`between the different networks, and section 5 analyses
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`simulation results.
`
`2. Service support in DRUM
`
`MPLS being a core technology, the focus of QoS
`support in MPLS networks is scalability, which
`is
`achieved by flow aggregation that ensures individual end-
`to-end QoS guarantees without maintaining awareness of
`individual flows on each segment of their path. Diffserv
`mechanisms are therefore a good candidate to provide
`QoS within MPLS networks because services are based on
`a per-hop model and aggregate forwarding resources
`(buffer space, bandwidth, scheduling policy) that are pre-
`allocated in the LSRs for each service type. Functions
`such as classification, marking and policing are only
`needed at the edge LSRs of the network while core LSRs
`need only to have PHB classification, hence the scalability
`at the core.
`To support service differentiation in DRUM, labeled
`packets are divided into separate traffic classes. The
`inherent characteristics of MPLS and Label Switched
`Paths (LSPs) make it easy to support aggregated flows.
`When an aggregation of flows is placed inside an LSP, the
`result is a traffic trunk [13]. Many different trunks, each
`with its own traffic class, may share an LSP and the 3-bit
`Exp field of the MPLS packet header could then be used
`to indicate the service class of each packet. In that case,
`no more than eight Behavior Aggregates (BA) can be
`defined within the MPLS network. If more than 8 BAS are
`required, the service class should then be inferred from
`both the MPLS label and the Exp fields. This latter
`scheme yields in less scalability than the former [ 101 and
`is not considered in our case.
`DRUM proposes
`the
`inspired from [7]:
`A Gold class, consisting of a low loss, low latency and
`low jitter service for delay-sensitive traffic. The
`network commits to deliver user datagrams at a rate of
`a Peak Data Rate (PDR) with minimum delay
`requirements. Datagrams in excess of PDR are
`discarded.
`A Silver class and a Bronze class for throughput-
`sensitive traffic. Packets in the Silver class experience
`lighter load (and thus have greater probability for
`timely forwarding) than packets assigned to the Bronze
`class. Packets within each class are further separated
`by two drop precedence levels (high and low). Within
`each class, the network commits to deliver with high
`probability user datagrams at a rate of at least a
`Committed Data Rate (CDR). The user may transmit at
`a rate higher than CDR but datagrams in excess of
`CDR have a lower probability of being delivered.
`A default Best Effort (BE) service class with no
`expected guarantees from the network. A Less than
`
`following service classes,
`
`195
`
`Best-Effort (LBE) class can also be considered for
`background trafic or “demoted” traffic that is out-of-
`profile. This latter case remains to be firther studied as
`it re-orders flows which may be undesirable.
`
`Network ctrl
`Gold
`Silver
`
`The mapping between the Exp-field values and the BA
`are defined by the network operator and are MPLS
`network specific. Table 1 proposes mappings of the Exp
`field value to a pair <FCI,DPI>, where the FCI
`(Forwarding Class Indicator) value indicates an MPLS
`forwarding class and the DPI (Drop Precedence Indicator)
`value indicates a level of drop precedence, used by the
`congestion avoidance mechanisms in DRUM described
`later.
`Tablel. EXD to MPLS service class mamina
`MPLS
`MPLS EXP
`Drop
`service class
`field value
`precedence
`FCI
`DPI
`NIA
`11
`1
`0 N/A
`11
`10
`1
`Low
`0 High
`10
`01
`1
`Low
`0 High
`01
`1
`00
`Low
`0 High (less than BE)
`00
`
`Bronze
`
`Best Effort
`
`3. The LSRs internals
`
`Figures 2 and 3 show the internal architecture of a core
`LSR and an edge LSR respectively used in DRUM.
`
`Figure 2. Functional elements of a core LSR
`
`Both types of LSRs include functions for constraint-
`based routing LSP (CR-LSP) setup with link-admission
`control and scheduling behaviors.. In addition, the ingress
`LSR is also responsible for classification, policing and
`shaping rules, LSP admission control, and interworking
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`functions (IWF) with the neighboring Diffserv and Intserv
`domains.
`
`I
`
`I
`
`I
`
`i~ pa (11s
`
`I
`
`d
`
`ta
`
`core LSR. If the LSP setup fails due to insufficient
`resources along the explicit path, an error message is sent
`back to the OSR “tearing” down all reservations, and the
`administrator would then try another path. Once the LSP
`is setup, the desired requested bandwidth would then be
`available end-to-end on the explicit route for the “sum” of
`all aggregate traffic in all the classes.
`For example, figure 4 shows an LSP between LSRl
`and LSR2 of a reserved lOOkbps aggregate end-to-end
`capacity, and an LSP between LSRl and LSR3 of a
`reserved 5Okbps aggregate end-to-end capacity. The
`remaining 1.85Mbps would still be available from the
`2Mbps link on the outgoing interface of LSRl .
`
`Figure3. Functional elements of an edge LSR
`
`The interactions between these modules are detailed in
`the next sub-sections.
`
`3.1 LSP setup and admission control
`
`control
`Figure 4. End-to-end link admission control
`
`The management console at each edge LSR is used by
`the network manager to setup the Constraint-Based
`Routing LSPs by specifying the Explicit Routes (ER) and
`the associated Traffic Parameters, which, for the sake of
`simplicity, are considered in the form of token buckets
`values (r,b). These parameters should be chosen sufficient
`to accommodate the traffic of all classes to be forwarded
`on that LSP, i.e. must reflect at least the “sum” of all the
`traffic parameters of the flows to be reserved traversing
`the LSP. The CR-LSP setup module uses either the
`generic Label Distribution Protocol (LDP) [ 1 11, either an
`extension of RSVP [5]. Labels are allocated in a control-
`driven, downstream-on-demand approach which
`is a
`scheme providing more network control (e.g. all LSRs
`belonging to the same LSP perform the label binding in an
`ordered manner) and better scalability
`in resource
`conservation.
`If LDP is used, the CR-LSP setup module first checks
`the link admission control module of its outgoing interface
`to the next hop in the ER to try reserving the required
`bandwidth. If successful, the remaining capacity of the
`link is diminished by (r,b) and a Label Request (LR)
`message is sent to the next hop in the ER of that LSP. The
`next hop LSR (a core LSR) also checks its link admission
`control to setup a reservation on its outgoing interface and
`so forth until the egress LSR of the ER is reached. The
`egress LSR then sends a Label Mapping (LM) message
`back to the originating LSR (OSR) - following the reverse
`explicit route path - with the label information that is
`stored in the Incoming Label Map (ILM) table within each
`
`At the ingress LSR, table 2 shows the bandwidth
`characteristics associated with each LSP.
`
`Table 2. Example of LSPs and their corresponding
`
`The network administrator can now start allocating
`bandwidth statically for each service class within these
`LSPs, much like VPs in ATM.
`
`3.2 Packet classification
`
`Packet classification is a function required at the edge
`of the MPLS network. Its goal is to provide identification
`of the packets belonging to a traffic stream to a Forward
`Equivalence Class (FEC) [9]. The classifier can be a
`Multi-Field (MF) classifier, which performs packet
`selection based on the combination of one or more header
`fields in the incoming IP packet (source address IP and/or
`port, destination address IP and/or port).
`Once the CR-LSPs have been setup, the next task is to
`configure the classifier to bind a particular flow and its
`traffic parameters (r,b) to an LSP and assign the flow to a
`particular service class. For example, if an organization
`wishes to reserve a certain amount of bandwidth to
`interconnect its two sites across an MPLS network, the
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`network manager “tunnels” the customer’s flow on an
`already established LSP across his network with
`remaining characteristics satisfying the customer’s QoS
`requirements. An LSP admission control module at the
`ingress LSR provides control-load support on that LSP at
`the ingress. This module measures whether bandwidth is
`still available on that LSP for the traffic parameters
`requested by the new flow being added to that path and
`stores the information in the FTN (FEC to Next Hop
`Label Forwarding Entry) of the edge LSR. Various
`admission control algorithms [I21 are used in DRUM to
`provide control-load support on that LSP.
`When a packet is received fiom a neighboring
`network, the edge LSR “encapsulates” the packet in an
`MPLS header with the appropriate label and Exp value.
`Table 3 shows the binding of two Gold flows to the same
`LSP that are each using 2Okbps of the bandwidth of the
`LSP with LSP-ID 4.
`
`Table 3. Example of an FTN table for LSP-ID 4
`and its remainin ca acit of 60 kb s
`
`flow1
`flow2
`
`value
`
`110
`
`ca acit
`20kb s
`20kb s
`
`3.3 Traffic conditioners
`
`Traffic conditioners are a vital part of a differentiated
`services architecture as they perform the necessary
`policing actions on incoming packets at the edge of the
`network. They act on the classified packets, and, as shown
`in figure 5, consist of leaky buckets associated with each
`incoming “Gold” traffic, and a token bucket for each
`“Silver” and “Bronze” traffic. Packets that are out-of-
`profile are either discarded (in the Gold class), either
`given a high drop precedence (in the Silver/Bronze/BE
`class).
`
`incoming
`
`-
`
`Figure 5. Traffic conditioners
`
`3.4 Per-Hop scheduling classes
`
`Several types of scheduling behaviors and drop
`policies may be used to deliver the forwarding behavior
`described in section 2. One simple example is given in
`
`figure 6, which consists of four different queues, one
`queue per traffic class, with a simple priority scheduler
`serving
`the queues. MPLS packets are classified
`according to the Exp field and forwarded to the
`appropriate queue.
`
`Figure 6. Output scheduler in the LSRs
`
`EXP
`Since all the LSRs support the same PHB
`mapping, LSPs are merged implicitly, and traffic of the
`same class from different LSPs are statically multiplexed
`together in the same queue. The different admission
`control mechanisms and traffic conditioners described
`earlier protect low priority queues from being starved by
`the high priority queues. The Gold class is the one with
`the highest priority, with tail-drop discard giving the
`minimum service delay for the packets. Each of the other
`classes uses a separate queue managed by the congestion
`management scheme Random Early Detection (RED) with
`In and Out (RIO) [ 141.
`
`4. Interworking with Diffserv and Intserv
`domains
`
`In this section, we show how the MPLS service
`architecture can support QoS for Intserv and Diffserv
`networks and provide interoperability at the boundary. In
`order for a customer to receive services from the network,
`he must have a service level agreement (SLA) with the
`provider. SLAs can be static or dynamic; static SLAs are
`negotiated on a regular (e.g., monthly or yearly) basis,
`while dynamic SLAs require the use of some signaling
`protocol (e.g., RSVP) to request service on demand.
`These dynamic requests are taken into account by the IWF
`module at the ingress LSR, which “tunnels” them through
`the MPLS network to the destination, thus ensuring that
`flow reservation happens end-to-end [13]. Also, in order
`to support both types of SLAs and minimize the LSP
`setup delay, the Service Provider can provision his
`network by statically allocating the necessary resources
`and setting up all the constraint-based LSPs between the
`MPLS end-points, based on customers’ needs and
`anticipated incoming traffic patterns through the SLAs.
`
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`For instance, in table 5 , one choice might be to give
`the classes AFx2 and AFx3 (x=1,2,3,4) the same drop
`precedence within each service class ‘x’. For Controlled-
`Load service requests incoming from Intserv networks, the
`Exp value can vary between Silver or Bronze service
`depending on the network provider’s pre-configured
`settings based on per customer criteria.
`
`5. Validation
`
`In this section, we briefly describe a simulation setup
`used to validate DRUM in providing service and delay
`differentiation for Intserv and Diffserv networks. We used
`the network simulator ns-2 [ 181 for our analysis with new
`modules supporting the DRUM architecture described
`earlier. The sample network topology is given in figure 7,
`and consists of seven edge LSRs (nl, n6, n7, n8, n9, nlO),
`four core LSRs (n2, n3, n4, n5) and links at 2 Mbps.
`Three classes of traffic were used: Gold traffic at CBR
`rate of 300 kbps and 600 bytes packet sizes, Silver traffic
`at CBR rate of 200 kbps and 500 bytes packet size, and
`Best-Effort traffic at 500 kbps CBR rate with 1000 bytes
`packet size. Nodes nl and n6 each generates one Gold,
`one Silver and one Best-Effort flow towards nodes n7 and
`n10 respectively, whilst node 8 sends to node 9 one Gold
`and one Best-Effort flow.
`
`Ingress LSRs map the incoming QoS requests from the
`neighboring Diffserv and
`Intserv networks
`to
`the
`corresponding service class within the MPLS network. In
`this model, each incoming flow is assigned to one of the
`available classes for the duration of the flow and traverses
`the MPLS cloud in this class. The service mappings given
`in table 4 follow most naturally from the service
`definitions: Guaranteed Service [3] and Expedited
`Forwarding [SI map to the Gold MPLS class, and
`Controlled-Load service [4] and Assured Forwarding [7]
`map to Silver and Bronze classes.
`
`Incoming service requirements
`IntServ
`DiffServ
`Guaranteed
`Expedited
`Forwarding (EF)
`Service (GS)
`Controlled-
`Assured
`Load (CL)
`Forwarding (AF)
`Best Effort
`Best Effort
`
`MPLS matching
`service
`Gold
`
`Silver/Bronze
`
`Best Effort
`
`For completeness, we propose in table 5 possible
`mappings for the service combinations and identify how
`the Exp field can be used in the MPLS header of packets
`across the MPLS network to obtain the equivalent service.
`Taking into account that the 8 BAS defined within the
`MPLS network offer less service granularity than the
`Diffserv classes (one EF and four AF with three possible
`drop precedence levels in each class [7]), the network
`administrator can choose to group together traffic flows
`requiring similar service into a single MPLS service class,
`and configures the mappings of the DSCP values that
`Diffserv QoS uses, as well as the incoming Intserv service
`request, to the Exp value for output port scheduling and
`congestion avoidance mechanisms.
`
`Intserv
`service
`type
`GS
`
`Control
`load
`
`Diffserv class MPLS
`EXP
`field
`110
`101
`100
`100
`101
`100
`
`DSCP
`PHB
`101110
`EF
`AFl1 001010
`AF12 001100
`AF13 001110
`AF21 010010
`AF22 010100
`
`}AF23 010110 I VI;
`
`pings
`MPLS service
`class
`
`Gold
`
`Silver
`
`Figure 7. Sample network topology
`
`A first test case was to mix all the flows on the same
`path n2-n3-n4. Figure 8 shows rate guarantees for the
`Gold and Silver traffic at node 10 with no loss since the
`flows send at the subscribed rate, while the Best Effort
`traffic used up the remaining bandwidth and was subject
`to packet loss.
`
`045
`
`0 .
`
`.
`
`.
`
`.
`
`,
`
`.
`
`/
`
`.
`
`BE
`
`.
`
`1
`
`AF31 011010
`
`IAF43 100110 1 010
`)OOOOOO 1 000
`IDF
`
`BE
`
`Bronze
`
`Best Effort
`
`Figure 8. Throughput characteristics
`
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`Figures 8.a and 9.a show the impact of jitter on the
`Gold and Silver traffic at node 10 respectively using the
`same path for the flows. The simple priority scheduler
`favors the Gold flow at the expense of the Silver flow
`resulting in more jitter for the latter.
`
`state information or complex scheduling is required
`leading to increased scalability. We also proposed how
`this service architecture can interoperate with neighboring
`regions supporting Intserv and Diffserv QoS mechanisms.
`Nevertheless, the effects of DRUM firther aggregating
`Diffserv classes and drop precedence is for future study.
`Works are already underway to include in DRUM
`dynamic constraint-based routing using QOSPF [ 171.
`7. References
`
`
`
`
`
`[I] R. Braden, et al., “Integrated Services in the Internet
`Architecture: an Overview”, Internet RFC 1633, June 1994
`[2] R. Braden, et al., “Resource Reservation Protocol (RSVP)
`Version 1 Functional Specification”, RFC 2205, Proposed
`Standard, September 1997
`[3] S. Shenker, et al., “Specification of the guaranteed quality
`of service”, RFC 22 12, September 1997.
`[4] J. Wroclawski, et al., “Specification of the controlled-load
`network element service”, RFC 22 1 1, September 1997.
`[5] Awduche et al, “Extensions to RSVP for LSP Tunnels”,
`draft-ietf-mpls-rsvp-Isp-tunnel-03 .txt, September 1999
`[6] Blake, S., et al., “An Architecture for Differentiated
`Services”, RFC 2475, December 1998.
`[7] Heinanen et al., “Assured Forwarding PHB Group”, RFC-
`2597, June 1999.
`[8] Jacobson et al., “An Expedited Forwarding PHB’, RFC-
`2598, June 1999.
`[9] Rosen et al., “Multiprotocol label switching Architecture”,
`drafi-ietf-mpls-arch-06.txt, August 1999.
`[lo] Le Faucheur et al., “MPLS Support of Differentiated
`Services”, drafi-ietf-mpls-diff-ext-04.txt, March 2000
`[ 1 I] Jamoussi et al., “Constraint-Based LSP Setup using LDP”,
`draft-ietf-mpls-cr-ldp-03.txt, October 1999
`[ 121 Jamin, S., et al., “Comparison of measurement-based
`admission control algorithms for controlled-load service”,
`Proc. IEEE INFOCOM 97, April 1997.
`131 Li, Rekhter, “A provider architecture for differentiated
`services and traffic engineering (PASTE)”, RFC 2430,
`October 1998.
`141 D. Clark, et al., “An approach to service allocation in the
`Internet”, draft-clark-different-svc-alloc-OO.txt, July 1997.
`151 H. Zhang, “Service disciplines for guaranteed performance
`service in packet-switching networks”, Proc IEEE, vol. 83,
`no. 10, October 1995
`[16] S. J. Golestani, “Network delay analysis of a class of fair
`queuing algorithms”, IEEE J. Select. Areas in Commun.,
`vol. 13, no. 6, pp. 1057-1070, August 1995.
`[I71 R. Guerin et al., “QoS Routing Mechanisms and OSPF
`Extensions”, draft-guerin-QoS-routing-ospf-05.txt, 1999
`[ 181 “The ns-2 simulator”, http://www-mash.cs.berkeley.edu/ns.
`
`I M a m . , , r l
`
`1
`
`7
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`snxilam.,uI,
`
`*
`
`7
`
`1
`
`(b) Separate paths
`(a) Same path
`Figure 8. Jitter on Gold flow at node 10
`
`001
`2
`
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`1
`
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`,
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`1
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`(a) Same path
`(b) Separate paths
`Figure 9. Jitter on Silver flow at node 10
`
`l
`
`I
`O,mY.,al,m <,,c,
`
`I
`
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`
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`
`O
`
`In order to further differentiate between the flows,
`another test was considered using different routes per
`traffic class: path n2-n3-n4 for Best Effort flows, path n2-
`n4 for Gold flows, and path n2-n5-n4 for Silver flows.
`Gold trafic now follows a much shorter path resulting in
`less delay and jitter (figure 8.b) through the network.
`Results are also noticeably better for the jitter of Silver
`traffic
`(figure
`9.b).
`Furthermore,
`a
`significant
`improvement affects throughput for the Best Effort flows.
`These preliminary results show that service and delay
`differentiation can greatly benefit from traffic engineering
`associated with simple priority scheduling at the core and
`admission control/policing at the edge, without the use of
`complex per-class weight scheduling to control delays
`such as WFQ [15] or SCFQ [16]. Isolating the various
`flows on disjoint paths as long as possible inside the
`network yields in a better quality of service to the various
`classes.
`
`6. Conclusion
`
`In this paper, we showed how MPLS combined with
`differentiated services and constraint-based routing can
`form a simple and efficient Internet model capable of
`providing applications with differential QoS. N o per-flow
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