A Packet Eligible Time Calculation Mechanism For

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A Packet Eligible Time Calculation Mechanism forProviding Temporal QoS for Multicast Routing

Ye Ge, Jennifer C. Hou and Hung-Ying TyanDept . of Electrical Engineering

The Ohio State UniversityColumbus, OH 43210-1272

Tel: (614) 292-7290 (voice), (614) 292-7596 (fax)E-mail: {gey,jhou,tyanh}@ee.eng.ohio-state.edu

AbstractIn this paper, we propose a packet el igible t ime calcula-t io n mechani sm and i t s assoc ia ted in form at ion update method

to provide tempora l QoS to mult icast services, in t e r m s of de -lay bound coupled with inter -mess age delay j i t t er bound and /orbounded in ter -des t ina t ion de lay j i t t er bound . W e assume theavailability of a mult icast tree o n which the delay between asource and any des t ina t ion fa l l s wi th in the end- to-end de laybound. W e then exp loi t the idea of artificially delaying trans-m i s s i o n of data packets un t i l th e i r eligible t i m e s [5], and modeleach router as a regulator fol lowed by a packet scheduler. Adata packet is not el igible to be scheduled unti l the current t im eis greater tha n or equal to i ts el igible t im e.For each temporal QoS required, we deriv e the appropriatepacke t e ligible t imes . W e a lso dev ise a n in form at ion updatemethod to collect/update in a decentralized m an ne r the param-eters needed in the calculation of packet el igible t imes. Withall the paramete rs available, a n intermed iate router can calcu-late packet el igible t i mes and th e upper bound o n the buffersneeded in order to ful f i l l the &OS. inally , we validate the pro-posed mechanism in t e r m s of the probabil ity of locating feas i-ble mult icast trees and m essage overheads, and scalabil ity withevent-driven simulations.1 Introduction

Many future applications of computer networks such asdistance education, teleconference and distributed interactivesimulation, rely on the underlying networks to provide multi-cast services with quality-of-service (QoS) guarantees. TheQoS guarantees are usually expressed in te rms of t he end-to-end delay bound, the inter-message delay jitter bound,the inter-destination delay jitter bound, and/or the minimumguaranteed bandwidth. Depending on the natu re of applica-tions, a single Qo S or a combination of several may be required.The main intent of thi s paper is thu s to develop a packet eligi-ble time calculation mechanism and the associated informationupdate strategies to provide Qo S to multicast services. Thi smechanism, coupled with any multicast routing protocol thatgives the end-to-end delay bound guarantee, can provide tem-poral Qo S to t he following four categories of applications:C1: Applications with end-to-end delay bound requirements.The work reported in this paper was supported in part by DARPAunder Contract No. N66001-97-C-8526, by NS F under GrantNo. NCR-9625064, and by the OSU Graduate School Fellowship.0-7803-5284-X/99/$10.001999 IEEE. 721

c2:

c3:

c4:

Applications with end-to-end delay bound and inter-message delay jitter bound requirements.Applications with end-to-end delay bound, inter-messagedelay jitter bound, and inter-destination delay jitterbound requirements.Applications which require the delay experienced by eachpacket from a source node to every destination node fallsin [D- ,D] ,where D s the user-specified group-specificdelay bound and 6 is the user-specified group-specific de-lay jitter bound.

We assume the availability of a multicast routing protocolthat generates multicast trees which satisfy th e end-to-end de-lay bound. (For ease of exposition, we use the QoS-enhancedCBT protocol proposed in [3] as an example protocol.) Tofulfill the various delay jitter constraints, we then exploit themethod of artificially delaying transmission of dat a packets un-til their eligible times [ 5 ] ,and model each node as a regulatorfollowed by a packet scheduler. A da ta packet is not eligible tobe scheduled until the current time is greater than or equal toits eligible time. The key issue is then to derive, with respectto t he temporal QoS requirements, appropr iate packet eligibletimes.

The packet eligible time under the inter-message delayjitter constraint ((C2)) has been derived in the jitter-EDDscheme [ 5 ] . In th is pa per, we focus on deriving packet eligibletimes for applications in the categories of (C3) and (C4).

The rest of the paper is organized as follows. In Section 2 ,we first describe the network model under consideration andformally define th e four categories of applications to be sup-ported . We present the packet eligible time calculation mech-anism in Section 3, followed by a discussion on the associatedinformation update strategy in Section 4 . We briefly discussthe event-driven simulation results in Section 5 and concludethe paper in Section 6.2 Background Materials2.1 Network Model

We represent a network by a weighed digraph G = (V ,E )where V denotes the set of nodes (routers and end hosts) an dE the set of arcs. The later corresponds to the set of networkcommunication links connecting th e nodes. Withou t loss ofgenerality, we only consider digraphs in which there exists atmost one a rc between a pair of nodes. Let a multicast groupM c V be a set of hosts involved in the group communication.
mailto:gey,jhou,tyanh%[email protected]:gey,jhou,tyanh%[email protected]


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-d-Each node v E M is a group member. We consider the mostgeneral many-to-many multicast paradigm in which each hostv E M can be a source node as well as a destination nodein the multicast group. Packets originated from node v aredelivered to a set of destination nodes M - U } .

A multicast tree T s a subgraph of G that spans all therouters along the path(s) in the tre e from a source node to somedestination node(s). For ease of exposition, we assume tha t thedelay between an on-tree router and any group member on itsdirectly atta ched subn et is negligible. jFrom the perspectiveof data tr.ansfer, each source node U, E M may view T as a

"ULt .1

LY *

LR;:

nodes in 1M and has no cycles. In addition, T includes relay L", -

tree rooted at itself. We denote the tree rooted a t node U, asTu8.ll the group members in the set M - U, } are the leafnodes of Tu,. e use PT(~,,~)o denote the path from a sourcenode vs to an on-tree router or an end host, U , on the tree Tu,(and I PT(V,,u)the hop count of the path ). We also useD$(,,-.,,, o denote the delay which th e lcth packet experiences

Figure 1: The internal queuing structure of an cln-treeICUI&T.2.3

--QoS-Based Multicast Routing That Pro-vides End-to-End Delay Bound

- < - - . - ,the path P T ( v ~ 7 ~ )rom a Source node ' 5 to an on-treenode U. We assume that a group member may not Join/leave amulticast group during the transmission of a message session,but is otherwise free to join/leave t he multicast group betweentransmissions of message sessions.

The proposed packet eligible time calculation mechanismassumes the availability of a QoS-driven multicast routing pro-tocol that provides the end-to-end delay bound betveen a

node and any destination node. ~ h ~ ts, the multi-cast tree located by the multicast routing protocol satisfies

2.2 Formal Statement of Application Re- Eqs. ( 2 .1 ) . The QoS-driven CBT Protocol Proposed in [3], forexample, is such a protocol. Due t o the limited space, we referthe interested readers to [3] fo r a detailed account. IVe alsouirements

be formally stated as follows.C1: Given the end-to-end delay bound D , this category of 3 Packet Eligible Time CalculationMechanismapplications requires

Vk E N, V d E M - vs}, an d Vu, E M .C2: Given the end-to-end delay bound D and the inter-message delay jitter bound 6,, this category of appli-

cations requires thatD$(udJd)I , ( 2 . 2 )

and

V k , i E N , V V d E M - us } ,and Vu , E M .C3: Given the end-to-end delay bound D, the inter-

destination delay jitter bound & , and the inter-messagedelay jitter bound a thi s category of applications re-quires tha t

I G ( v 8 , U d ) - !r (Us,Ud)l 5 am , (2 .3 )

D$(Ua,Ud,) 5 D l ( 2 . 4 )~ ~ + ( u ~ , u d ~ )!r(Us,Ud])l I 6m ,I D + ( v s , v d l ) - $ ( V s , U d z ) l 5 ' d l

( 2 . 5 )(2 .6 )

and

V k , i E N , V d l , V d 2 E M - U S } , and V U ,E M .C4: Given the end-to-end delay bound D and th e delay jitter

bound 6, this category of applications requires tha t thedelay experienced by the kth packet from a source nodeU, to every destination node v d satisfies

3.1 Nodal Structure of On-Tree NodesThe internal queuing structure of an on-tree router, nk, s

depicted in Fig. 1 . When a packet leaves an on-tree router,it is stamped with a holding time which indicates hcw longthe packet should be held in the regulator of the i m m e d i a t edownstream router before it can be scheduled. T he regulatorholds the packets from its upstream link until their eligibletimes (when they will be delivered to a scheduler). Ncte thata regulator is associated with each incoming link. If routern k has e on-tree outgoing links, L z y t , 1 5 j 5 e, each ofthe e outgoing links, becomes the incoming link of theimmediate downstream router at which Lzy:" s incident, anda regulator is equipped at that immediate downstream router.

A scheduler is responsible for multiplexing packe1.s fromdifferent regulators (and hence from different connectioris). Wedo not specify any specific scheduling discipline here, becausethe proposed mechanism is well-suited for a n y schedul ng dis-cipline with a finite per-node delay bound. Because regula-tors are inserted before schedulers in the nodal stru ctur e, itsuffices to consider work-conserving message schedulers withfinite per-node delay bounds.

To facilitate analysis, for each on-tree link Lin incident atan on-tree router n k , we define (refer to Fig. 1)

where d z z p is the propagation delay which a packet experi-ences when it traverses link L p , iski is the per-node delaybound corresponding to the ith scheduler at router 7 i k , and

k- 6I $(u.3ud,I , (2 .7 )V k E N , V V d E M - U , } , and Vu , E M .

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Figure 2: A path PT(~,,,~)nd the regulators and sched-ulers along the path.D F Z is equal to the value of D T n x calculated at router n k l simmediate downstream rout er, n k + l , that is incident at L$Yt.

The parameter D T n x represents the maximum delay expe-rienced by packets that depart from router 7 2 k - 1 to router n k ' sdownstream leaf routers.3.2

Figure 3: Calculation of holding time for applications inthe category Of (c3) 'i k+ l

k Derivation of holding times for (C2): To providemulticast applications in (C2) with both the delay and inter-message delay jitter guarantees, we exploit the result r eportedin the jitter-EDD scheme [5], and set

(3.4)Calculation of Packet Eligible Times

Let n k be the kth intermediate router along the uniqueon-tree Path, PT(v,,v~)Irom a Source node 21s to a destinationnode vd . w e first define the following terms t o facilitate th ecalculation of packet eligible times:

HT i , V s = ET i - l , V s + S k - l - i - l , V afor IC 2 1. Note that ET;-l,vs + d , , - , is the time at which theith packet is scheduled to leave router nkl l if it has experi-enced th e maximum scheduling delay at all upstream routers,an d (ETL- I , ~ ,- d S k - l - L - I , ~ ~ )s the amount of time bywhich the ith packet arrives early at router n k from routern k - 1 . Hence, router n k absorbs the delay jitter from routern k - l by holding the packet for ET;-^,,* + d , , - , - L - l , v 8 )time units.Derivation of holding times for (c3): o fulfill thedelay bound and inter-destination delay jitt er bound require-ments, we set the holding time for the regulator at an on-tree

1. ET;,,* is the eligible time of the it h packet (sent by a

2. GT& is the time at which the ith packet is generated at

3. d s k j is the Per-node delay bound of the jth ~hedulerS k j at router n k . If it is clear from the context whichscheduler a t router n k is referred to, the index j may bedropped. router n k as

source node v,) at router n k ' s regulator.node v,) arrives at router n k .

node v,) departs from router n k .

source node v,) at router n k .

a source node v,.

4. HTL,v8 is the holding time for the it h packet (from a HTL ,V s = ( ETL - l , V s + S k - l - ; - l , V , ) + (Dn","_", -DFr)4(dr-x1S k - l ) (3.5)5 . is the time a t which the it h packet (from a source6. f i , u 3 is the time a t which the it h packet (from a source

For all the four categories of applications, the relation be-

= ( E T ; - I , ~ ~d r - x l - i - l ,va) (DZC:l - D F T ) ,where DZC:l = maxj DFZZ and dr-zl = maxj d s k - - l . j sthe maximum per-node scheduling bound a t router n k - 1 . Th efirst term in Eq. (3 .5 ) enforces each packet to experience themaximum delay over the upstream routers. The second termin Eq. (3.5) absorbs the difference in the delays among dif-ferent downstream interfaces (whose rationale is depicted inFig. 3 ) . Suppose router n k - 1 has C outgoing on-tree links,L o ut L o ut L o utand k - l , l , k - - 1, 2 , . ., k - l , e and that ~ : " _ t , , ~ L? for somej . Then, DZE:l = rnaxl


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source node U , and any of th e on-tree destination nodes. Then,Eq. (3 .5 ) ensures that packets all experience the delay of D F xbetween a. source node v s and any on-tree destination node.

Note that the above mechanism does not work (in the sensethat the inter-message delay jitter constraint may be violated)if group members may join a nd leave during the transmissionof a message session. Hence, we impose the restriction th at nomember may join/leave a multicast group during the trans-mission of a message session (but may be otherwise free tojoin/leave the multicast group between transmissions of mes-sage sessions).Derivation of holding times for (C4): To fulfill the"fixe# d'elay requirement imposed in Eq. (2.7), we set theholding time at an on-tree router nk asHTL,,, =

where the term (ET;-l,V, + d S k - l - fi - l ,vs)nforces each packetto experience the maximum delay over the upstream routers, andsTk,,s is the slack time that should be absorbed at router n k inorder to make all packets arrive at their destinations D - 6 timesunits after their departure from the source. sTk,vs is calculated byevenly dividing the current total slack time among router nk andits downstream routers. Specifically, let

s T k , p s > k = 0 ,( E T i - l , v a + d s k - i - . f ~ - l , v s ) + s s T k , v , , I s r c < ?

(3.6){

the ith packet departs early ( ETL- l , v ,+drdXl- L - l , t s ) , (ii)On","_", , and (iii) dr-xl -d sk b1 . I tems (i ) and (iii) 'can be locallycalculated by nk-1 and passed to nk. To calculate item (ii),each router nk-1 has to collect and update D T Z , for everyoutgoing interface e (asDFi:l = maxj D9" ).

On the other hand, the third item in Eq. (3.5), DT:,should be maintained and updat ed by rou ter nk itself. Specif-ically, recall that DYkx can be expressed as (Eq. (3 .1 ) )

( k - l ) , C

k - 1 , j

E

The parameter dpr","can be collected by the underlying unicastrouting protocol, the parameter d s k i depends on the schedul-ing discipline used a t th e it h scheduler of rout er nk and isassumed to be bounded and available, and DrE Z ' s have to beon-line collected and updat ed.

Similarly, to facilitate calculation of holding tiines forapplications in the category of (C4), each on-tree routernk--1 should calculate (i) (ETi-l ,vs+ dr-zl -- fi-.l,,8),ii)D&ve,nk-l), nd (iii) N k - 1 , and provide them (in the headerof a packet) to its immediate downstream router nk. In ad-dition, router nk has to maintain and updat e DTF by itself

Lk

k . t

k = 1,using the same method as that for ((33).+ d ~ p - lro p f d S k - l + STk-l,u,, k > 1, In summary , the only information to be on-line collected,( 3 . 7 ).denote the accumulative delay the ith packet experiences on

the path from the source U , to router nk-1, Mnk-l,vs enotethe set of router nk- 1' ~downstream members (with respect tosource U,), and

denote the maximum number of hops between router nk-1 andall of router nk-1'~downstream group members. Then , STk , , ,can be expressed as

max(D -D&(v,,nk-l) DTF - ,O). (3 .9 )s T k , v , -Nk-iNote thai max(D -D&(,,,nk-ll - DFF - ,O ) represents theslack time tha t should be absorbed by router nk an d its down-stream routers.4 Information Update Procedures

In this section, we first identify t he parameters needed forthe eligible time calculation mechanism (which we term as thestat e information). The n, we discuss how the parameter s areon-line maintained and updated by each on-tree router as thestate information.4.1 State Information Maintained by EachOn-Tree Router

To facilitate calculation of holding times for applications inthe category of ( C 3 ) , each on-tree router nk-1 should calculateand pas:; (in the header of a packet) parameter s needed for eachimmediate downstream router nk to calculate the holding time.jFrom Eq. (3 .5 ) , we know that router nk-1 has to provide tonk the following information: (i) the amount of time by which

maintained, and updated by each on-tree router nl: when-ever group membership changes is the maximum delay, D T Z ,which packets experience between router nkk outgoing inter-face t? and router nk's downstream leaf nodes, Ve. In the caseof supporting applications in the category of (C4), each on-tree router also needs to maintain and update the maximumnumber, Nk, of hops between router nk and all of router nk'sdownstream group members.

An intuitively straigh tforward (but expensive) method forinformation updat e is for each on-tree router nk to updateDT","t's and Nk whenever a group member joins/le.ives themulticast tree. In wha t follows, we present a much less expen-sive information update method.4.2 Information Update in the Case of Mem-ber Join

k . t

Consider the situat ion in which a join-request trzre ls hop-by-hop toward the core until i t reaches an on-t ree router nk.For clarity of no tation , let the path the join-request xaversesbe denoted as P = (v1,v2,v3, . . . , v j = nk). The join requestcarries (in addition t o t he interface information) (i) 1,he accu-mulative delay information, (ii) the per-node delay 'Jound ofthe previous router, and (iii) the cumulative hop co mt infor-mation. When a join-request arrives at router U ; on interfacee, it carries DE::, an d N;-1. Router vi updates itsD T Z and N;, respectively, as

v i t

When t he join-request arrives at the on-tree router v j = nk ,router nk conducts the first eligibility test (as summarizedin Section 2.3) to check whether or not the end-to-end de-lay bound of the new member, as well as the existing guar-antees to the other on-tree members, can be fulfilled. If the

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Figure 4: Performance comparison between the CBT protocol and the QoS-enhanced CB T protocol with the proposedpacket eligible time calculation mechanism for applications in the category of (Cl).join-request survives the first eligibility test, router n k thenconducts the second eligibility test and checks whether or notthe new branch P replaces some of the tree branches incidentat router nk and become the branch with the maximal delayor the maximal hop count with respect to any of nk's incom-ing interfaces, C;. If so, th e delay information and /or the hopcount information a t some on-tree nodes reachable on interfaceC; may have to be updated as a result of the member join.

Take the two on-tree routers n k - 1 and nk in Fig. 3 as anexample. Suppose a new branch P = (VI, Z , 3 , .. , j = nk)joins the multicast tree at router nk through interface C' + 1.Let DF;" = maxlSjlp D Y Z denote the maximum outgoingdelay with respect to the incoming interface L F , router n kchecks' whether or not

k , j

or 1 P > the currently maintained value of N k , with respectto L;I". If either case is true, router n k updates its DF;" asn k - 1 (and perhaps further upstream routers) of the memberjoin and the possibility of updating DFf:;,j , OFET,,and N k - 1 .(In this case, an information-update message is composed thatcontains the new values of LIT,", N k , and t he per-node delaybound d , , and forwarded upstream on interface Lin ) As in acore-based multicast t ree any group member may be a source(i.e., many-to-many multicast paradigm), the check has to beperformed for every incoming interface of node nk's.

d P T o Pou t + d s j - l + DGY or Nk as I P , and notifies routerk , t ' + l

'in addition to the first eligibility test.

4.3 Information Update in the Case of Mem-ber LeaveWhen a group member leaves the multicast group, if the

local router does not have downstream on-tree nodes (routersand/or directly attached end hosts), the router sends a quit-noti f ication message to its parent router on th e tree and deletesthe corresponding forwarding cache. The process repeats untilth e quit-noti f ication message arrives on interface C at a router(say router nk) that has other downstream on-tree nodes (withrespect to any source). If the leaving branch is the branch withthe maximal delay (i.e., DL"."-"t= D:rX)or the maximum num-ber of hop counts, with respect to any of nk's incoming inter-faces C i , router nk 's upstream routers reachable on interface

should be notified of the member leave. An i n f o r m a t i o n -update message is again composed t ha t contains the new valueof D:;" = maxjge DYZ."-"t,he per-node delay bound d , , andNk , and forwarded upstream.

k. L

k , j

5 Simulation ResultsWe have developed a customized Java-integrated network

simulation tool, N e t S i m Q , to evaluate t he QoS control capa-bility for systems equipped with different admission control,resource reservation, QoS unicast/multicast routing, and mes-sage scheduling mechanisms [4].

Using N e t S i m Q , we validated th e proposed mechanism byincorporating it into the QoS-enhanced CBT protocol, andevaluate it in terms of the probability of locating feasible mul-ticast trees and message overheads with event-driven simula-tions. Due to the limited space, we refer interested readers toour technical report for detailed simulation description[2].

In the simulation, the network size varies from n = 10to 100 routers. The network topology is generated using the

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Figure 5: Performance comparison between the CBT protocol and the QoS-enhanced CBT protocol with the proposedeligible time calculation mechanism for applications in the category of (C3). --method proposed in [6]. The size of the multicast group variesfrom 4 to 16 members (with each member being both a sourceand a destinati on). Group members and the core are randomlypicked up. The Qo S requirement of each multicast session usedin the simulation is the end-to-end delay bound that variesfrom 5.0 to 9.0 units of time and the end-to-end delay jitterbound that varies from 2.0 to 4.0 units of time. The delay ateach link is uniformly distributed with mean 1unit of time, andis collected by the underlying distance vector routing protocol.Totally 100 multicast sessions are generated in each simulationrun. The simulation results are shown in Fig. 4 and Fig. 5

The simulation results indicate that the performance gain(in terms of locating feasible multicast trees) can be as highas 50% for applications in the category of (C3) . Although themessage overhead can be at times four times higher than thatwithout any QoS enhancement, it incurs mainly in the off-treesearch stage (when a join-request searches for a feasible routeto an on-tree rout er), rather tha n in the stage of conductingeligibility tests and information update . Th at is, the mes-sage overhead is contributed, in large, by the QoS-enhancedCBT protocol, but not the information update method in theproposed mechanism. We are curren tly investigating how tofurther reduce the message overhead by devising alternativeoff-tree search approaches for the QoS-enhanced CBT proto-col.6 Conclusion

In this paper, we present a packet eligible time calculationmechanism for providing temporal &OS n multicast routing.Thi s mechanism is well-suited for both source-based and core-based multicast routing protocols as long as the protocol locatemulticast Crees tha t satisfy t he end-to-end delay bound.

We model each node as a regulator followed by a packet

scheduler. Each packet is held in the regulator until its eligibletime is smaller than or equal to th e current real timi:. Weidentify four categories of applications that require differentlevels of &OS.For each category of applications, we then derivethe packet eligible times for fulfilling the Qo S requirements.We also discuss the associated information update method.

Finally, we validate the proposed mechanism by incorpo-rating it into the QoS-enhanced CBT protocol a nd evaluate itsperformance using an event-driven simulation to,ol N e t S i m Q .References

A. Ballardie. Core based trees (CBT Ver 3) multicast rout-ing protocol specification. http://www.ietf.org/internet -drafts/draft- iet f- id mr- cbt-spec-v3-O f . x t , August 1998.Ye Ge, Jennifer C. Hou, and Hung-Ying Tym. A packeteligible time calculation mechanism for provicling temporalqos for multicast routing. Technical Report, July 1!)98.Hung-Ying Tyan, Chao-Ju Hou, and Bin Wang. Cn pro-viding quality-of-service control for core-based mi,lticastrouting. Proc . of I E E E 1 9 t h In t ' l C o n f . o n D i s t ri b ut e dComput ing Sys tems , June 1999.Hung-Ying Tyan, Bin Wang, Yi Ye, and Chao-Jii Hou.NetSimQ: A Java-in tegrated network simulation t Dol forQoS control in point-to-point high speed networks. 3rdN A S A R e s e a r c h a n d E d u c a t i o n N e t w o r k W o r k s h o p , Au-gust 1998.D. C. Verma, H. Zhang, and D. Ferrari. Delay jit ter controlfor real-time communication in a packet switching network.In Proc. Tracomm'91, pages 35-43, April 1991.B. Waxman. SWSL: rout ing of multipoint connections.IE EE Journal on Se lec ted Areas in Comm unica t ions ,6: 1617-1622, December 1988.

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A Packet Eligible Time Calculation Mechanism For