QoS with TCP/IP and shared media LANs Heikki Kokkinen Nokia Research Center Tel.

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1 QoS wth TCP/IP and shared meda LANs Hekk Kokknen Noka Research Center Tel Abstract The QoS over shared meda LANs has three major elements: mprovng the current LANs for QoS, developng QoS aware LANs for future purposes, and utlzng the capabltes of QoS aware LANs optmally. Followng the categorzaton, ths paper dscusses how TCP/IP s currently carred over the most popular shared meda LAN, Ethernet, and how t s developed. An ntroducton to dfferent LAN and packet based access networks s gven. In the context of mappng the IP QoS to lnk layer, the work of IETF ISSLL group s referred. The shared meda MAC protocols are dscussed and an example of MAC protocol analyss s carred out by studyng Slotted Aloha throughput and delay characterstcs. 1 Introducton The Qualty of Servce (QoS) s nterpreted as a capablty to provde the servces to the customer n the way that he or she expects t. The expectatons change fast as the nformaton technology develops. So, n order to keep track on the QoS of a network also quanttatve measures are used. These measures nclude at least throughput, packet loss, delay and delay varaton. Transmsson Control Protocol (TCP) and Internet Protocol (IP) are the most mportant layer 3 and 4 protocols n the data networks. The Local Area Networks (LAN) locate n the customer premses. The customer, organzaton or socety typcally shares computng and storage resources and accessores n that network. The local loop s the network connectng the local exchange, headend, pont of presence or other operaton pont of an network operator to the customer premses. The dfference between LAN and local loop may not reman clear n the future, at least from the lnk layer technology perspectve. The networks whch allow practcally smultaneous access of several termnals on the same physcal transmsson medum are called shared. The access may be centrally controlled by a master or the control may be dstrbuted. The QoS wth TCP/IP n shared meda LANs has three aspects: tunng the exstng, not QoS aware lnk layers towards QoS, developng QoS capable lnk layer for new systems and sgnalng the QoS. The secton 2 dscusses The Insttute of Electrcal and Electroncs Engneers, Inc. (IEEE) 802 shared meda LANs and how TCP/IP s carred over them. Tunng exstng networks and the sgnalng are dscussed n the secton 3. The lnk layer QoS s manly determned by the Medum (or Multple) Access Control (MAC) protocol. The major categores of MAC protocols are lsted n the secton 4. An ntroducton to analyss of MAC protocols s gven by usng Slotted Aloha as an example n the secton 5. 2 Shared meda LANs A good overvew on the areas where shared meda LANs are used, s the collecton of IEEE 802 standards [6], whch are collected n the Table 1. They nclude Collson Sense Multple Access (CSMA) Collson Detect (CD), token bus, token rng, Dstrbuted Queue Dual Bus (DQDB), wreless and cable televson access methods. The varous standards dffer at the physcal layer and MAC sublayer, but are compatble at the data lnk layer. The acceptance of these standards vary n the ndustry. For example combnaton of and defnes a dfferent frame format from the one whch s used n the true Ethernet s practcally not mplemented, because of ETS and Data Over Cable Servce Interface Specfcaton (DOCSIS). Addtonally, European Telecommuncaton Standards Insttute (ETSI) Hperlan wll compete wth Table 1: IEEE 802 standards and drafts [6] Number IEEE standard or draft LAN/MAN Brdgng & Management Logcal Lnk Control CSMA/CD Access Method Token-Passng Bus Access Method Token Rng Access Method DQDB Access Method Broadband LAN Fber Optcs Integrated Servces LAN/MAN Securty Wreless Demand Prorty Access Method Cable TV

2 a) MAC LLC SNAP destnaton addr source addr length DSAP AA SSAP AA cntl 03 org c data crc b) 0800 IP datagram destnaton addr source addr data crc IP datagram IP header 20 TCP header 20 applcaton data c) destnaton addr source addr ETPID TCI, CFI=C data crc IP datagram Fgure 1: Encapsulaton of TCP and IP accordng to a) RFC 1042, b) RFC 894 [13], and c) one of the extended Ethernet frames [7] 2.1 TCP/IP over Ethernet In the TCP/IP world, the encapsulaton of IP datagrams s defned n RFC 894 [5] for Ethernet and n RFC 1042 [10] for IEEE 802 networks. It s requred that every Internet host connected to 10 Mb/s Ethernet cable has to be able to transmt and receve accordng to RFC 894 and t has to be able to receve accordng to RFC The Fgure 1 shows the encapsulaton of TCP/IP packet n the Ethernet frame. The new felds n the extended Ethernet frame are Ethernet-encoded Tag Protocol Identfer (ETPID), Tag Control Informaton (TCI), and Canoncal Format Identfer (CFI). The PID feld vares dependng on used Ethernet encapsulaton.e. wth or wthout SNAP. The TCI feld s two octets n length, and contans user_prorty, CFI and Vrtual LAN Identfer (VID). The user_prorty s a three bt long feld. It represents eght prorty levels, 0 through 7. 3 IETF ISSLL The Internet Engneerng Task Force (IETF) Integrated Servces over Specfc Lnk Layers (ISSLL) Workng Group tres to add more servce levels than just the current IP best-effort wthn subnetwork technologes. The followng tems are specfed for each network technology: - Servce mappngs defne how the lnk layer provdes ntegrated servce management. - Setup protocol mappngs defnes how setup protocol such as RSVP s mapped onto the lnk layer technology. - Adaptaton protocols utlze the natve QoS capabltes of the lnk-layer technology. - Statements of non-applcablty descrbe the lmtatons of the lnk layer for ntegrated servces. The group tres to utlze the current lnk layer technology as well as possble, to follow the ongong standardzaton on lnk layer protocols and to nfluence the development of the future lnk layers. 3.1 Framework [4] There are several general requrements for the work: - Resource reservaton - Admsson control - Flow separaton and schedulng - Polcng/shapng - Soft state nformaton s mantaned soft state about reservatons - Both centralzed or dstrbuted mplementatons are possble. - Scalablty - Error recovery and fault tolerance - Co-exstence wth exstng resource management

3 MAC capabltes IEEE 802.1D [8] defnes a consstent way to carry user_prorty value over a brdged network consstng of Ethernet, Token Rng, Demand Prorty, FDDI or other MAC layer meda usng an extended frame format. It labels the packets accordng to the classes. The IEEE 802 specfcatons make no assumptons about how user_prorty s to be used by end statons or by the network. Although IEEE 802.1D defnes statc prorty queung as the default mode of operaton of swtches that mplement multple queues, the user_prorty s really a prorty only n a loose sense snce t depends on the number of traffc classes actually mplemented by a swtch. There s no explct traffc class or user_prorty feld carred n Ethernet packets. Ths means that user_prorty must be regenerated at a downstream recever or swtch accordng to some defaults or by parsng further nto hgher layer protocol felds n the packet. Alternatvely, IEEE P802.1Q encapsulaton [7] may be used whch provdes an explct user_prorty feld on top of the basc MAC frame format Bandwdth Manager (5, 6, 7) The Bandwdth Manager (BM) s responsble for provdng mechansms for an applcaton or hgher layer protocol to request QoS from the network. The BM conssts of Requester Module (RM), Bandwdth allocator (BA) and related communcaton protocols. The RM resdes n every end staton n the subnet. One of ts functons s to provde an nterface between applcatons or hgher layer protocols such as RSVP, ST2, SNMP, etc. and the BM. The RM utlzes followng parameters: desred class of servce, the traffc descrptors contaned n the TSpec, and a RSpec specfyng the amount of resources to be reserved. The ISSLL group concentrates on the lnk layer capablty to provde two classes of servce. The Controlled Load servce provdes a loose guarantee, nformally stated as "the same as best effort would be on an unloaded network", and the Guaranteed Servce provdes an upper bound on the transt delay of any packet. The BA s responsble for performng admsson control and mantanng state about the allocaton of resources n the subnet lke bandwdth reservaton, modfcaton of an exstng reservaton, queres about resource avalablty. The communcaton between the end staton and the BA takes place through the RM. The BA can be centralzed or dstrbuted. The BA s also responsble for labelng the flows. The centralzed BA archtecture s shown n the Fgure 2. App RM Layer 2 RSVP Host / Router BA Layer 2 Layer 2 Layer 2 Intermedate Brdge / Swtch Intermedate Brdge / Swtch App RM Layer 2 RSVP Host / Router Fgure 2: Bandwdth Manager wth centralzed Bandwdth Allocator [4] The communcaton protocols are specfed between the followng enttes: between the hgher layer protocols and the RM; between the RM and the BA, between peer BAs Lnk layer support The brdges and swtches have dfferent capabltes of supportng QoS. The most basc brdge has a sngle queue per output port. Networks constructed from ths knd of devce cannot be expected to provde servce guarantees of any knd because of the complete lack of traffc solaton. The next level brdges/swtches are those, whch conform to the more recently revsed IEEE 802.1D specfcaton. It wll nclude support for queung up to eght traffc classes separately. The level of traffc solaton provded s coarse because all flows correspondng to a partcular traffc class are aggregated. A next step above these devces are brdges/swtches whch mplement optonal parts of the IEEE 802.1D specfcaton such as mappng the receved user_prorty to some nternal set of canoncal values on a per-nputport bass. It may also support the mappng of these nternal canoncal values onto transmtted user_prorty on a per-output-port bass. Other entrely optonal features that some brdges/swtches may support nclude classfcaton of IntServ flows usng felds n the network layer header, per-flow polcng and/or reshapng whch s essental for supportng Guaranteed Servce, and more sophstcated schedulng algorthms such as varants of weghted far queung to lmt the bandwdth consumed by a traffc class. The number of traffc classes supported and access methods of the technology under consderaton wll determne how many and what servces may be supported. Natve Token Rng/IEEE 802.5, for nstance, supports eght prorty levels, whch may be mapped to one or more traffc classes. Ethernet/IEEE has no support for sgnalng prortes wthn frames. However, the IEEE 802 standards commttee has recently developed a new standard for brdges/swtches related to multmeda traffc expedtng and dynamc multcast flterng. A packet format for carryng a user_prorty feld on all IEEE 802 LAN meda s s now defned. The standards allow for up to eght traffc classes on all

4 meda. The user_prorty bts carred n the frame are mapped to a partcular traffc class wthn a brdge/swtch. The user_prorty s sgnaled on an endto-end bass, unless overrdden by brdge/swtch management. 3.2 SBM [15] Subnet Bandwdth Manager (SBM) s a sgnalng protocol for RSVP-based admsson control over IEEE 802-style networks. SBM provdes a method for mappng an nternet-level setup protocol such as RSVP onto IEEE 802- style networks. In partcular, t descrbes the operaton of RSVP-enabled hosts/routers and lnk layer devces (swtches, brdges) to support reservaton of LAN resources for RSVP-enabled data flows. A protocol entty called "Desgnated SBM" (DSBM) exsts for each managed segment and s responsble for admsson control over the resource reservaton requests orgnatng from the DSBM clents n that segment. The Fgure 3 gves an example of SBM n a LAN sender. From IP Address mappng 802 header Classfer From RSVP SBM clent Packet scheduler Fgure 3: SBM of a LAN sender [3] SBM sgnalng Local Admn. control The basc DSBM-based admsson control procedure has the followng phases: - DSBM Intalzaton - DSBM Clent Intalzaton - DSBM-based Admsson Control The admsson control s carred out n the followng steps: - When a DSBM clent sends or forwards a RSVP PATH message over an nterface attached to a managed segment, t sends the PATH message to the segment's DSBM nstead of sendng t to the RSVP sesson destnaton address (as s done n conventonal RSVP processng). - When an applcaton on host A wshes to make a reservaton for the RSVP sesson, host A follows the standard RSVP message processng rules and sends a RSVP RESV message to the prevous hop L2/L3 address (the DSBMs address) obtaned from the PHOP object(s) n the prevously receved PATH message. Data - The DSBM processes the RSVP RESV message based on the bandwdth avalable. 3.3 Servce mappngs [12] The servce mappngs the mappng of the traffc classes and the characterzaton parameters Learnng the mappng In the aggregated flow model, each arrvng flow s assgned to one of the avalable layer-2 classes and t traverses the 802 cloud n ths class. The classes could be unversally defned. The meanngs of a set of classes; e.g. 1 = best effort, 2 = 100 ms peak delay target, 3 = 10 ms peak delay target, 4 = 1 ms peak delay target, etc. would be set. They could then be encoded drectly n end statons, and the flow-to-class mappngs computed drectly n these devces. Ths unversal defnton approach would be smple to mplement, but s too rgd to map the wde range of possble user requrements onto the lmted number of avalable 802.1D classes. In a more flexble mappng clents ask the network whch user_prorty traffc class to use for a gven traffc flow, as categorzed by ts flow-spec and layer-2 endponts. The network provdes a value back to the requester Characterzaton Parameters The ntegrated servce model requres that each network element that supports ntegrated servces computes and makes avalable certan characterzaton parameters descrbng the behavor of the elements. - Ingress lnk - Egress lnks and ther MTUs, framng overheads and mnmum packet szes - Avalable path bandwdth: updated hop-by-hop by any devce along the path of the flow. - Mnmum latency A network element supportng the Guaranteed Servce must be able to determne the followng parameters: - Constant delay bound through the devce - Rate-proportonal delay bound - Receve resources that would need to be assocated wth the flow e.g. bufferng, bandwdth - Transmt resources that would need to be assocated wth the flow e.g. bufferng, bandwdth, constantand rate-proportonal delay bounds A network element mplementng the Controlled Load servce must be able to determne the followng: - Receve resources that would need to be assocated wth the flow e.g. bufferng - Transmt resources that would need to be assocated wth the flow e.g. bufferng

5 4 MAC protocols The lnk layer QoS characterstcs are ted to the MAC protocol and to the physcal layer framng structure. On the physcal layer delay s caused for example due to propagaton delay, error correcton, nterleavng, and large frames. All frame nformaton related to functonaltes, whch requre a transmsson round trp, lke acknowledgement, should occur fast enough. Also the segmentaton of the large packets s an mportant feature. The MAC protocol controls how the termnals fll the physcal frames. The MAC protocol can be n a centrally controlled pont-to-multpont or n a dstrbuted multpont-to-multpont archtecture. It s possble to obtan a determnstc delay by reservng capacty to the termnal on the transmsson medum. Many of the recent MAC protocols nclude the reservaton capabltes, but wthout defned QoS related sgnalng on the upper layers, ther fancy features are useless. The general multple access schemes are Tme Dvson Multple Access (TDMA), Frequency DMA (FDMA), and Code DMA (CDMA). In ths context only TDMA approach s dscussed but wth lttle magnaton the same prncples can be appled to other access schemes, as well. The Fgure 4 roughly shows the optmal applcaton areas of dfferent access s. The fgure has ntutvely been formed keepng the throughput delay characterstcs on mnd. The best opton for a partcular case depends also on other requrements lke delay tolerance, mnmum throughput, stablty, and complexty. The TDMA gves a fxed tme slot for each termnal also when the termnal has nothng to send. TDMA s at ts best when there s no burstness n the traffc or when the number of users s relatvely small. The opposte of TDMA s random access. Any user can send at anytme and collsons between termnals are resolved by watng a random tme before retransmsson. Collson Sense Multple Access (CSMA), Slotted Aloha, and splttng algorthms belong to ths category. For a large number of users havng large packets compared to tme slot sze, arrvng n a bursty manner, the random reservaton access s the most sutable access mode. The reservaton request s transmtted usng a small random access tme slot and the channel capacty s reserved for the actual transmsson. If the number of users s small, the best performance s acheved by allocatng a small amount of TDMA capacty for reservaton requests and reservng capacty for the actual transmsson. TDMA reservaton s a verson of pollng. In some applcatons you can also use random access to reserve a TDMA channel, but t could be consdered to be a varaton of random reservaton. Token passng could also ft nto random reservaton or TDMA reservaton dependng on the method how the token s passed. No. of users TDMA Random reservaton Random TDMA reservaton Burstness Fgure 4: Applcaton areas of dfferent access modes 5 Slotted Aloha analyss The Slotted Aloha s one of the basc random access algorthms. It s selected as an example, because the analyss s smple. The analyss method can be appled to other protocols, as well. The analyss for fnte number of termnals follows the references [9] and [11]. The channel s slotted; n ths example we consder further that t s tme slotted. The transmsson of a packet takes T tme unts and at the same tme T s the duraton of a tmeslot. Each termnal can have two states: thnkng or backlogged. When a termnal has nothng to transmt t s n the thnkng state. When a packet arrves to the termnal, t transmts the packet mmedately. If the packet was receved successfully the termnal enters the thnkng state agan, otherwse t goes to backlogged state. When n ths state, the termnal transmts a packet n a slot wth probablty σ and t does not transmt wth a probablty 1 - σ. 5.1 Markov chan Let the slots of the system be numbered sequentally k = 0, 1, and let Ñ(k) denote the number of backlogged termnals at the begnnng of the kth slot. Snce statetranston of a termnal s ndependent of the actvtes n any prevous slot, the process s a Markov chan. Whle the number of backlogged termnals cannot exceed the number of termnals M, ths chan s fnte. The transton dagram for the system s shown n Fgure 5. Upward transton are possble between every state and all the hgher numbered states, snce a collson among any number of packets s possble. Downward transtons are possble only to the adjacent state snce only one packet can be successfully transmtted n a slot. The dagram shows that all states communcate, so ths Markov chan s ergodc, meanng that a steady-state dstrbuton exsts.

6 0 1 2 M From To - 1 To M From + 1 Fgure 5: State transtons of fnte populaton Aloha [11] 5.2 Steady-state probabltes Let π be the steady-state probablty of the system beng n state, that s π = lm k Pr[ N( k) = ]. Further let p j be the steady-state transton probablty,.e., pj = lm k Pr[ N( k) = j N ( k 1) = ]. Fnally denote by P the matrx whose elements are p j and by π the row vector whose elements are. From the above argumentaton t follows that the steady-state probablty vector s the soluton to the fnte set of lnear equatons. π = πp π = (1) 1 to whch the exstence of a unque soluton s guaranteed. We must therefore construct the matrx P and derve the desred soluton. Let us assume that the retransmsson of every termnal s an ndependent geometrc process. Then the probablty that out of the j backlogged termnals wll schedule a retransmsson n a gven slot s bnomally dstrbuted: Pr[ backlogged termnals transmt n a slot j n backlog] j j 1 = ν ). (2) In a smlar manner, for the thnkng termnals Pr[ thnkng termnals transmt n a slot j n backlog] M j M j 1 = σ (1 σ ). (3) By studyng the state transton dagram the followng probabltes can be formulated: p j 0 = M σ j 1 M [ ν ) ](1 σ ) 1 M [ 1 ν ) ](1 σ ) + M--1 [(M - ) σ (1-σ ) ]( ν ) M--1 [(M - ) σ (1-σ ) ][ 1 ) ] j (1 σ ) M j j < -1 j = -1 j = j = + 1 j > Throughput and delay analyss For a slot to be successful only a sngle transmsson must take place wthn t. Ether all backlogged termnals reman slent and a sngle new termnal transmts, or a sngle backlogged termnal transmts whle no new packets arrve. Gven that there are backlogged termnals ths can be stated as P suc () = Pr[Successful slot termnals n backlog] = M 1 (1 υ) ( M ) σ (1 σ ) (5) 1 M + ν ) (1 σ ) If we do not dstngush new arrvals from the backlogged packets, ν = σ, we get Psuc ( ) = Mσ (1 σ ), (6) whch s ndependent of. The throughput of Slotted Aloha becomes [ ] S = E Psuc ( ) = Mσ (1 σ ). (7) When the system s n state there are M thnkng termnals each generatng packets n every slot wth probablty σ. Thus, the average rate of new packet generaton when n state s (M )σ. Usng ths we obtan (4) S = E[ ( M ) σ ] = ( M N ) σ, (8) where Ñ s the average number of backlogged termnals. Denote by b the average rate at whch packets jon the backlog. Accordng to Lttle's formula, the average amount of tme spent n the backlog s the rato of the average number of backlogged termnals to the average rate of jonng or Ñ / b. Some of the packets go through on the frst try and some of the packets must go through the backlogged state. Ths s reflected n the average delay of the protocol: S b b N D = 1+ S S + 1 b. (9) N 1 M = 1+ = 1 + S σ S For the specal case ν = σ, we get 1 (1 σ ) D = 1+. (10) σ (1 σ ) The Fgure 6 shows the delay behavor as a functon of throughput.

7 Fgure 6: Slotted Aloha delay as a functon of throughput 6 Concluson The majorty of exstng shared meda LANs do not provde QoS. There s development gong on to enhance the capabltes nto QoS aware drecton. The IEEE has revsed the 802 specfcatons for QoS support. In order to utlze those capabltes the products have to support those features. The shared meda LANs under development nclude some capabltes on the lnk and MAC layer. In that case a mappng of QoS on IP level to lnk layer needs to be specfed. IETF ISSLL group s workng on the area of supportng RSVP over IEEE 802 networks. The work s on IETF draft level. The IP QoS sgnalng wth non IEEE 802 lnk layers s a bg queston mark. In the recently developed lnk layers, there are three payload framng strateges: Ethernet brdgng brngs the Ethernet problems wth t, ATM support may be or may not be a practcal soluton for QoS sgnalng, from-the-scratch lnk layer has the hghest potental, but t also requres most specfcaton. For that, the sgnalng of QoS s a totally open ssue. A trval soluton to mprove the QoS of an arbtrary traffc load s just to ncrease the lnk layer bandwdth. [5] Horng, Charles. A Standard for the Transmsson of IP Datagrams over Ethernet Networks, Aprl 1984, RFC 894. [6] [7] IEEE Standards for Local and Metropoltan Area Networks: Draft Standard for Vrtual Brdged Local Area Networks, P802.1Q/D11, July 30, [8] IEEE Standards for Local and Metropoltan Area Networks: Meda Access Control (MAC) brdges, P802.1D/D17, May 25, [9] Klenrock, L. & Lam, S. S. Packet swtchng n a multaccess broadcast channel: Performance evaluaton. IEEE Transactons on communcatons, Vol. 23, No. 4, Pp [10] Postel, J. & Reynolds, J. A Standard for the Transmsson of IP Datagrams over IEEE 802 Networks, February 1988, RFC [11] Rom, Raphael & Sd, Moshe. Multple access protocols. Performance and analyss. Sprnger- Verlag, New York, NY, ISBN [12] Seaman, M. & Smth, A. & Crawley, E. & Wroclawsk, J. Integrated Servce Mappngs on IEEE 802 Networks. November draft-etfssll-s802-svc-mappng-03.txt. [13] Stevens, Rchard W. TCP/IP Illustrated, Volume 1. The protocols. Addson-Wesley, Readng, MA, ISBN [14] Wroclawsk, J. The Use of RSVP wth IETF Integrated Servces, RFC 2210, September [15] Yavatkar, Raj & Hoffman, Don & Bernet, Yoram & Baker, Fred & Speer, Mchael. SBM (Subnet Bandwdth Manager). November draft-etfssll-s802-sbm-07.txt. 7 References [1] Braden, B. & Zhang, L. & Berson, S. & Herzog, S. & Jamn, S. Resource Reservaton Protocol (RSVP) -Verson 1 Functonal Specfcaton, RFC 2205, September [2] Braden, R. & Clark, D. & Shenker, S. Integrated Servces n the Internet Archtecture: An Overvew, RFC 1633, June [3] Ferguson, Paul & Huston, Geoff. Qualty of Servce. John Wley & Sons, Inc. New York, NY, ISBN [4] Ghanwan, Anoop & Pace, Wayne J. & Srnvasan, Vjay & Smth, Andrew & Seaman, Mck. A framework for provdng ntegrated servces over shared and swthced IEEE 802 LAN technologes. May draft-etf-ssll-s802-framework-05.txt.