Internet Draft Stefano Salsano Expiration: February 2001 CoRiTeL File: draft-salsano-simpler-ef-00.txt A simpler EF PHB Redefinition August 1, 2000 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. Abstract This document proposes text aiming at providing clarification to RFC 2598. As explained in [CHA], the primary motivation is that the definition of EF PHB given in RFC 2598 does not match the intuition of the EF PHB, and has been determined to be unimplementable in a manner compliant with the definition. [CHA] gives a new rigorous definition of EF PHB, using a recurrence equation that determines a bound for time at the which a packet must exit the node, given that the arriving times and the exit times of all packets are recorded. Salsano Expires February 2001 1 A simpler EF PHB Redefinition Aug-00 This draft proposes a (hopefully) simpler definition of EF PHB, which captures the original intention of RFC 2598 and is also adequate to verify if a node implements the EF PHB. Similarly to RFC 2598 this draft defines a lower bound to the rate of EF aggregate exiting a node during a given interval. The lower bound depends on the configured rate, on the input traffic and on a latency term representing the router behavior. 1. Introduction The motivations for the redefinition of EF PHB with respect to RFC 2598 are well described in the Introduction section of [CHA]. Basically the definition provided in RFC 2598 seems to be not implementable, because it is impossible to provide the EF rate "over any time interval equal to or longer than the time it takes to send an output link MTU sized packet at the configured rate" [RFC 2598]. It is apparent from the discussion on the Diffserv mailing list that at least some clarifications are needed with respect to the EF definition given in RFC 2598. It should be possible to define methods to check if a node provides the EF PHB or not. The missing clarification could be a set of additional conditions on the starting time of the measurement interval and on its duration. Also the outgoing measured traffic should be somehow related with the traffic arrival process. The difficulties in providing such a new definition are described in section 1.3 of [CHA]. The redefinition which is proposed in this draft relates the amount of outgoing traffic measured on an arbitrary interval with the amount of incoming traffic in the same interval, with the EF configured rate and with a latency term that is introduced to represent the router behavior. 2. Redefinition of EF PHB The EF PHB is defined as a forwarding treatment for a particular diffserv aggregate where the amount of the aggregate's traffic departing from the router in a given time interval MUST equal or exceed a quantifiable lower bound which is given below. The lower bound depends on the amount of the aggregate input traffic in that time interval, on the configured rate R for EF traffic and on a latency term which takes into account the router behavior. Let I(t1, t2) be the amount of EF traffic directed to a given outgoing port that entered the router in the interval (t1, t2) and let O(t1,t2) the amount of EF traffic that exited the given port in the interval (t1, t2). A packet has entered the router when the last bit of the packet has entered the router and it has exited the router when its last bit has left the router. Salsano Expires February 2001 2 A simpler EF PHB Redefinition Aug-00 Let R be the configured rate for EF traffic, R <= C where C is the link capacity. A node supports the EF PHB at a rate R with a latency term E if for each t1 and t2, where t2 > t1 + E O (t1, t2) >= min (R*(t2-t1-E), I(t1,t2-E)) The above condition applies under the hypothesis that no loss occurs for the EF traffic under observation. Note that actually the lower bound on the amount of output traffic in the interval (t1,t2) depends on the amount of input traffic in the interval (t1,t2-E) 3. Comments The lower bound basically says that the rate of outgoing EF traffic is always greater or equal than the configured EF rate if there is enough EF traffic to be transmitted. Otherwise the outgoing rate will follow the incoming rate. A latency term must be included to define the interval in which the rates can be measured. This latency term takes into account the short term behavior of the router, including processing delays and scheduling delays. In fact, the formal expression of lower bound shows that the amount of outgoing traffic in a given time interval of duration (t1, t2) is greater than or equal to the minimum of two terms. The first term represents the amount of traffic contained in an interval (t1,t2-E) at the configured rate. The second term represent the amount of traffic that entered the router in the interval (t1,t2-E). In both terms the latency E takes into account the overall behavior of the router, including the processing times (e.g route look-up), possible input queuing time, and the output queuing time (i.e. due to the scheduling algorithm). There are two possibilities with respect to the incoming traffic in the interval (t1,t2-E): - If in the (t1,t2-E) interval the amount of incoming traffic has been lower than the amount of traffic in the interval at the configured rate, the correct behavior for the EF router is to forward all this entered traffic (within the admitted latency E). Therefore the minimum operator will select the second term. - If in the (t1,t2-E) interval the amount of incoming traffic has exceeded the amount of traffic in that interval at the configured rate, the minimum operator will select the first term: the correct EF behavior is to forward the EF aggregate providing at least the configured EF rate. Salsano Expires February 2001 3 A simpler EF PHB Redefinition Aug-00 The second possibility is just a transient state that cannot last for long time intervals if no loss has to be provided. It takes into account the burstiness that cannot be avoided in packet switched networks. If loss have to be taken into account the lower bound has to be properly modified. Note that in [CHA] the loss events are not taken into account as well. 4. Application to the Priority Queuing scheduler If the scheduling algorithm is Priority Queuing and the router behaves as a pure FIFO output queue (i.e. there are no processing delay, no input queuing delay), the latency term E can be simply expressed as E(PQ) = 2 * MTU / C and the following expression is valid for any value of R <= C. O (t1, t2) >= min (R*(t2-t1-E(PQ)), I(t1,t2-E(PQ))) 5. References [CHA] A. Charny, ed. et al. "EF PHB Redefined" (draft-charny-ef-definition.txt), July 2000, work in progress. [RFC2598]V. Jacobson, K. Nichols, K. Poduri, "An Expedited Forwarding PHB", RFC 2598, June 1999 6. Author Information Stefano Salsano CoRiTeL - Consorzio di Ricerca sulle Telecomunicazioni Via di Tor Vergata, 135 00133 Roma - ITALY email: salsano@coritel.it 7. Full Copyright Copyright (C) The Internet Society 2000. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be Salsano Expires February 2001 4 A simpler EF PHB Redefinition Aug-00 followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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