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Network Working Group                                       S. JackowskiRequest for Comments: 2688                        Deterministic NetworksCategory: Standards Track                                     D. Putzolu                                                 Intel Architecture Labs                                                              E. Crawley                                                          Argon Networks                                                                B. Davie                                                           Cisco Systems                                                          September 1999Integrated Services Mappings for Low Speed NetworksStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.Abstract   A set of companion documents describe an architecture for providing   integrated services over low-bitrate links, such as modem lines, ISDN   B-channels, and sub-T1 links [1,2,3,4]. The main components of the   architecture are: a set of real-time encapsulation formats for   asynchronous and synchronous low-bitrate links, a header compression   architecture optimized for real-time flows, elements of negotiation   protocols used between routers (or between hosts and routers), and   announcement protocols used by applications to allow this negotiation   to take place.   This document defines the service mappings of the IETF Integrated   Services for low-bitrate links, specifically the controlled load [5]   and guaranteed [6] services.  The approach takes the form of a set of   guidelines and considerations for implementing these services, along   with evaluation criteria for elements providing these services.Jackowski, et al.           Standards Track                     [Page 1]

RFC 2688      Integrated Services Mappings Low Speed Nets September 19991. Introduction   In addition to the "best-effort" services the Internet is well-known   for, other types of services ("integrated services") are being   developed and deployed in the Internet. These services support   special handling of traffic based on bandwidth, latency, and other   requirements that cannot usually be met using "best-effort" service.   This document defines the mapping of integrated services "controlled   load" [5] and "guaranteed" [6] services on to low-bandwidth links.   The architecture and mechanisms used to implement these services on   such links are defined in a set of companion documents. The   mechanisms defined in these documents include both compression of   flows (for bandwidth savings) [4,10] and a set of extensions to the   PPP protocol which permit fragmentation [2] or suspension [3] of   large packets in favor of packets from flows with more stringent   service requirements.1.1.  Specification Language   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [11].2. Issues for Providing Controlled and Guaranteed Service   Unlike other link layers, the links referred to in this document   operate only over low speed point to point connections.  Examples of   the kinds of links addressed here include dial-up lines, ISDN   channels, and low-speed (1.5Mbps or less) leased lines.  Such links   can occur at different positions within the end-to-end path:   - host to directly connected host.   - host to/from network access device (router or switch).   - Edge device (subnet router or switch) to/from router or switch.   - In rare circumstances, a link from backbone router to backbone     router.   These links often represent the first or last wide area hop in a true   end to end service.  Note that these links may be the most bandwidth   constrained along the path between two hosts.   The services utilized in mapping integrated services to these links   are only provided if both endpoints on the link support the   architecture and mechanisms referenced above. Support for these   mechanisms is determined during the PPP negotiation.  The non-sharedJackowski, et al.           Standards Track                     [Page 2]

RFC 2688      Integrated Services Mappings Low Speed Nets September 1999   nature of these links, along with the fact that point-to-point links   are typically dual simplex (i.e., the send and receive channels are   separate) allows all admission control decisions to be made locally.   As described in [2] and [3], for systems that can exert real time   control of their transmission at a finer grain than entire HDLC   frames, the suspend/resume approach optimizes the available bandwidth   by minimizing header overhead associated with MLPPP pre-fragmentation   and can provide better delay.  However, this comes at the expense of   preparing all outgoing data and scanning all incoming data for   suspend/resume control information.  The fragmentation approach can   be implemented without additional scanning of the data stream (beyond   bit-/byte-stuffing, which may be in hardware) and is applicable to   systems which provide only frame-oriented transmission control.   Choice of suspend/resume versus fragmentation should be made based on   the level of transmission control, the element's capability to handle   the HDLC-like framing described in [2], and the system overhead   associated with byte by byte scanning (required by suspend/resume).   To provide controlled load or guaranteed service with the   suspend/resume approach, when a packet for an admitted flow (QoS   packet) arrives during transmission of a best effort packet and   continued transmission of the best effort packet would violate delay   constraints of the QoS service flows, the best effort packet is   preempted, the QoS packet/fragments are added to the transmission,   and the best effort packet transmission is then resumed: usually all   in one transmission.  The receiving station separates the best effort   packet from the embedded QoS packet's fragments.  It is also   conceivable that one QoS flow's packet might suspend another flow's   packet if the delivery deadline of the new packet is earlier than the   current packet.   For systems which use fragmentation, any packets longer than the   maximum tolerable delay for packets from enhanced service flows are   fragmented prior to transmission so that a short packet for another   flow can be interleaved between fragments of a larger packet and   still meet the transmission deadline for the flow requiring enhanced   services.   Note that the fragmentation discussed in this document refers to   multilink PPP (MLPPP) fragmentation and associated MCMLPPP   modifications as described in [2], not IP or other layer 3   fragmentation.  MLPPP fragmentation is local to the PPP link, and   does not affect end-to-end (IP) MTU.Jackowski, et al.           Standards Track                     [Page 3]

RFC 2688      Integrated Services Mappings Low Speed Nets September 19992.1 Calculating "Acceptable Delay" for Int-serv flows   A router which provides Controlled Load or Guaranteed Service over a   low speed serial link needs to have some notion of the "acceptable   delay" for packets that belong to int-serv flows. If using   fragmentation, a router needs to know what size to fragment packets   to; if using suspend/resume, it needs to know when it is appropriate   to suspend one packet to meet the delay goals of another.   Unfortunately, there is no hard and fast way for a single delay bound   to be determined for a particular flow; while the end-points of a   flow have enough information to determine acceptable end-to-end delay   bounds and to make reservation requests of the network to meet those   bounds, they do not communicate a "per-hop" delay to routers.   In the case of Guaranteed Service [6], one approach is to let the   network operator configure parameters on the router that will   directly affect its delay performance. We observe that guaranteed   service allows routers to deviate from the ideal fluid flow model and   to advertise the extent of the deviation using two error terms C and   D, the rate-dependent and rate-independent error terms, defined in   [6]. A network operator can configure parameters of the low speed   link in such a way that D is set to a value of her choice.   If link-level fragmentation is used, the router controlling a low-   speed link can be configured with a certain fragment size. This will   enable a component of the error term D to be calculated based on the   time to send one fragment over the link. (Note that D may have other   components such as the speed of light delay over the link.)  Details   of the calculation of D are described below. Similarly, if   suspend/resume is used, the router may be configured with a delay   parameter, which would enable it to decide when it was appropriate to   suspend a packet.   For Controlled Load, there are no error terms, and the router must   decide how best to meet the requirements of the admitted reservations   using only the information in their TSpecs. Since the definition of   Controlled Load states that a CL flow with Tspec rate r should   receive treatment similar to an unloaded network of capacity r, CL   packets should not generally experience end-to-end delays   significantly greater than b/r + propagation delays. Clearly a router   connected to a low speed link should not introduce a delay greater   than b/r due to transmission of other fragments; ideally it should   introduce substantially less delay than b/r, since other hops on the   end-to-end path may introduce delay as well. However, this may be   difficult for flows with very small values of b.Jackowski, et al.           Standards Track                     [Page 4]

RFC 2688      Integrated Services Mappings Low Speed Nets September 1999   It is expected that implementers will make their own tradeoffs as to   how low to make the delay for Controlled Load flows. Similarly, it   may not be possible or desirable to configure the parameters   affecting D to arbitrarily small values, since there is a cost in   overhead in fragmenting packets to very small sizes. Conversely, if D   is too large, some applications may find that they cannot make a   reservation that will meet their delay objectives.   For the remainder of this document, we assume that a router has some   notion of the acceptable delay that it may introduce before beginning   transmission of a packet. This delay is in addition to any delay that   a packet might be subjected to as a result of the "ideal" queuing   algorithm that the router uses to schedule packets.3. Controlled Load and Guaranteed Service Class Mapping   Supporting integrated services over PPP links which implement MCML or   RTF can be accomplished in several ways.  Guidelines for mapping   these services to PPP links and to the classes provided by the   suspend/resume and fragmentation mechanisms are presented below.   Note that these guidelines assume that some sort of signaling   protocol is used to indicate desired quality of service to both the   sender and receiver of a flow over a PPP link.3.1 Predefined Class Mappings   A relatively simple method of class mapping that MAY be used is one   where class values correspond to predefined levels of service.  In   this arrangement, all admitted flows are grouped into one of several   buckets, where each bucket roughly corresponds to the level of   service desired for the flows placed in it. An example set of   mappings appears below:   MCML Short   MCML Long  RTF     Service     0b00        0b0000    0b000   Best Effort     NA          0b0001    0b001   Reserved     0b01        0b0010    0b010   Delay Sensitive, no bound     NA          0b0011    0b011   Reserved     NA          0b0100    0b100   Reserved     0b10        0b0101    0b101   Delay Sensitive, 500ms bound     NA          0b0110    0b110   Delay Sensitive, 250ms bound     0b11        0b0111    0b111   Network Control   Table 1: Example Mappings of Classes to ServicesJackowski, et al.           Standards Track                     [Page 5]

RFC 2688      Integrated Services Mappings Low Speed Nets September 1999   Note that MCML has two formats, short sequence numbers, and long   sequence numbers, that allow for 2 and 4 bits of class identification.   RTF allows for 3 bits of class identification in all formats.   Using a default-mapping method of assigning classes to flows in a   fixed fashion comes with certain limitations. In particular, all flows   which fall within a particular bucket (are assigned to a particular   class) will be scheduled against each other at the granularity of   packets, rather than at the finer grained level of fragments.  This   can result in overly conservative admission control when the number of   available classes is small such as in MCML short sequence number   format.3.2 Predefined Class Mappings and Prefix Elision   In the case where fewer reservations are expected than the total   number of classes negotiated for a PPP link, it is possible to assign   individual flows to fixed class numbers. This assignment is useful in   the case where the protocol identifier associated with one or more   flows is known at LCP negotiation time and the bandwidth of the   connection is relatively small. If these conditions hold true, then   for those flows that are known, a specific class can optionally be   assigned to them and the prefix elision PPP option [2] can be used for   those classes to achieve a small bandwidth savings.3.3 Dynamic Class Mappings   In the case where predefined class mappings are not satisfactory, an   implementer MAY map class values to individual packets rather than   assigning flows to fixed classes.  This can be done due to the fact   that the classes that MCML and RTF provide can be viewed purely as   PPP-specific segmentation/fragmentation mechanisms. That is, while the   class number MUST remain constant on an intra-packet basis, it MAY   vary on an inter-packet basis for all flows transiting a PPP   link. Actual assignment of particular flows to fixed classes is   unnecessary, as the class numbers are NOT REQUIRED to have any meaning   other than in the context of identifying the membership of   fragments/segments as part of a single packet.  This point is   sufficiently important that an example is provided below.   Consider a PPP link using the MCML short sequence number fragment   format (that is, four classes are provided).  Assume that in addition   to carrying best effort traffic, this link is carrying five guaranteed   service flows, A, B, C, D, and E. Further assume that the link   capacity is 100kbit/s and the latency is 100ms. Finally, assume the BE   traffic is sufficient to keep the pipe full at all times and that GS   flows A-E are each 10kbit/s and all have delay bounds of 145ms.Jackowski, et al.           Standards Track                     [Page 6]

RFC 2688      Integrated Services Mappings Low Speed Nets September 1999Time(ms)        Action 0     BE traffic is queued up 0     2kbit fragment from 10kbit packet of BE traffic sent, cls 0 (...) 8     2kbit fragment from BE sent, cls 0 (10kbit BE packet done) 9     8kbit packet from flow A arrives10     2kbit fragment from A sent, cls 1 (8kbit flow A packet start)11     8kbit packet from flow B arrives12     2kbit fragment from B sent, cls 2 (8kbit flow B packet start)13     8kbit packets from flows C, D, and E arrive14     2kbit fragment from C sent, cls 3 (8kbit flow C packet start)16     2kbit fragment from D sent, cls 0 (8kbit flow D packet start)18     2kbit fragment from A sent, cls 120     2kbit fragment from B sent, cls 222     2kbit fragment from A sent, cls 124     2kbit fragment from A sent, cls 1 (8kbit flow A packet done)26     2kbit fragment from E sent, cls 1 (8kbit flow E packet start)27     8kbit packet from flow A arrives28     2kbit fragment from B sent, cls 230     2kbit fragment from C sent, cls 332     2kbit fragment from E sent, cls 134     2kbit fragment from B sent, cls 2 (8kbit flow B packet done)36     2kbit fragment from E sent, cls 138     2kbit fragment flow A sent, cls 2 (8kbit flow A packet start)       (etc.)   This example shows several things. First, multiple flows MAY share   the same class, particularly in the case where there are more flows   than classes. More importantly, there is no reason that a particular   flow must be assigned to a fixed class - the only requirement is that   each packet, when fragmented, MUST have the same class value assigned   to all fragments.  Beyond this requirement the link scheduler may   assign individual to changing class numbers as necessary to meet   reservation requirements.   One suggestion to implementers of integrated services on MCML and RTF   links using dynamic mappings is that all BE traffic SHOULD be   logically separated from QoS traffic, and mapped to a fragmentable   (MCML classes 0-3 in short sequence number fragment format, 0-15 in   long sequence number fragment format) or suspendable (RTF classes 0-   6) class. Since BE traffic will in most implementations not be   scheduled for transmission except when a link is empty (that is, no   CL or GS traffic is ready for transmission), implementers MAY choose   to make use of class number 0 for BE traffic.Jackowski, et al.           Standards Track                     [Page 7]

RFC 2688      Integrated Services Mappings Low Speed Nets September 19993.4 Non-Conformant Traffic   Treatment of non-conformant QoS traffic is largely determined by the   appropriate service specifications, but the detailed implementation   in the context of this draft allows for some flexibility.  Policing   of flows containing non-conformant traffic SHOULD always be done at   the level of granularity of individual packets rather than at a finer   grained level.  In particular, in those cases where a network element   scheduling flows for transmission needs to drop non-conformant   traffic, it SHOULD drop entire packets rather than dropping   individual fragments of packets belonging to non-conformant traffic.   In those cases where a network element forwards non-conformant   traffic when link bandwidth is available rather than dropping the   traffic, the implementation SHOULD fragment packets of such traffic   as if it were best effort traffic.   Whether BE and non-conformant traffic are treated differently in   regards to transmission (e.g., BE is given priority access over non-   conformant traffic to the link) or whether within each type of   traffic special treatment is afforded to individual flows (e.g., WFQ,   RED, etc.) is service dependent.4. Guidelines for Implementers4.1. PPP Bit and Byte Stuffing Effects on Admission Control   An important consideration in performing admission control for PPP   links is reductions in effective link rate due to bit stuffing.   Typical bit stuffing algorithms can result in as much as 20%   additional overhead. Thus, admission control implementations for   guaranteed service over links where bit stuffing is used SHOULD take   the RSpec rate of all flows and multiply by 1.2, to account for the   20% overhead from bit stuffing, when determining whether a new flow   can be admitted or not. Admission control implementations for   controlled load reservations may use a similar algorithm using the   TSpec peak rate or may attempt to measure the actual degree of   expansion occurring on a link due to bit stuffing. This   characterization can then be used to adjust the calculated remaining   link capacity. Such measurements must be used cautiously, in that the   degree of bit stuffing that occurs may vary significantly, both in an   inter- and intra-flow fashion.   Byte stuffing is also used on many PPP links, most frequently on POTS   modems when using the v.42 protocol. Byte stuffing poses a difficult   problem to admission control, particularly in the case of guaranteed   service, due to its highly variable nature. In the worse case, byte   stuffing can result in a doubling of frame sizes. As a consequence, a   strict implementation of admission control for guaranteed load onJackowski, et al.           Standards Track                     [Page 8]

RFC 2688      Integrated Services Mappings Low Speed Nets September 1999   byte stuffed PPP links SHOULD double the RSpec of link traffic in   making flow admission decisions. As with bit stuffing,   implementations of controlled load service admission control   algorithms for links with byte stuffing MAY attempt to determine   average packet expansion via observation or MAY use the theoretical   worst case values.4.2. Compression Considerations   The architecture for providing integrated services over low bandwidth   links uses several PPP options to negotiate link configuration as   described in [4,8,10].  When deciding whether to admit a flow,   admission control MUST compute the impact of the following on MTU   size, rate, and fragment size:   Header compression: Van Jacobson or Casner-Jacobson [4,8,10].   Prefix Elision.   CCP.   Fragment header option used.   Fragmentation versus suspend/resume approach.   If any of the compression options are implemented for the connection,   the actual transmission rate, and thus the bandwidth required of the   link, will be reduced by the compression method(s) used.   Prefix elision can take advantage of mapping flows to MLPPP classes   to elide prefixes which cannot be compressed at higher layers.  By   establishing agreement across the link, the sender may elide a prefix   for a certain class of traffic and upon receiving packets in that   class, the receiver can restore the prefix.   Both compression gain and elision gain MUST be included as described   in the admission control section below. Note that the ability to   perform compression at higher layers (e.g. TCP or RTP/UDP) may depend   on the provision of a hint by the sender, as described in [9].4.3. Admission Control   Admission control MUST decide whether to admit a flow based on rate   and delay.  Assume the following:  LinkRate is the rate of the link.  MTU is the maximum transmission unit from a protocol.  MRU is the maximum receive unit for a particular link.  CMTU is the maximum size of the MTU after compression is applied.  eMTU is the effective size at the link layer of an MTU-sized packet    after link layer fragmentation and addition of the fragment headers.  FRAG is the fragment size including MLPPP header/trailers.Jackowski, et al.           Standards Track                     [Page 9]

RFC 2688      Integrated Services Mappings Low Speed Nets September 1999  Header is the size of the header/trailers/framing for MLPPP/Fragments.  pHeader is the additional header/framing overhead associated with    suspend/resume.  This should include FSE and worst case stuffing    overhead.  pDelay is the time take to suspend a packet already "in flight",    e.g. due to the delay to empty the output FIFO.  b is the bucket depth in bytes  R is the requested Rate.  Dlink is the fixed overhead delay for the link (Modem, DSU,    speed-of-light, etc).  eRate is the effective rate after compression and fragmentation.   The Dlink term MAY be configured by an administrative tool once the   network is installed; it may be determined by real-time measurement   means; or it MAY be available from hardware during link setup and/or   PPP negotiation.  Refer toAppendix A for more considerations on PPP   link characteristics and delays.   Admission control MUST compute CMTU, eMTU, and eRate for Controlled   Load Service, and it MUST compute CMTU, eMTU, eRate, and D for   Guaranteed Service:   To determine whether the requested rate is available, Admission   Control MUST compute the effective rate of the request (eRate) -   worst case - as follows:   #_of_Fragments = CMTU div (FRAG-Header) [Integer divide]   Last_Frag_Size = CMTU mod (FRAG-Header   If Last_Frag_Size != 0      eMTU = (#_of_Fragments) * FRAG + Last_Frag_Size + Header   Else      eMTU = (#_of_Fragments) * FRAG   eRate = eMTU/CMTU * R [floating point divide]   Admission control SHOULD compare the eRate of the request against the   remaining bandwidth available to determine if the requested rate can   be delivered.   For Controlled Load Service, a flow can be admitted as long as there   is sufficient bandwidth available (after the above computation) to   meet the rate requirement, and if there is sufficient buffer space   (sum of the token bucket sizes does not exceed the buffer capacity).   While some statistical multiplexing could be done in computing   admissibility, the nature of the low-bitrate links could make this   approach risky as any delay incurred to address a temporary   overcommitment could be difficult to amortize.Jackowski, et al.           Standards Track                    [Page 10]

RFC 2688      Integrated Services Mappings Low Speed Nets September 19994.4 Error Term Calculations   Guaranteed Service requires the calculation of C and D error terms. C   is a rate-dependent error term and there are no special   considerations affecting its calculation in the low-speed link   environment. The D term is calculated from the inherent link delay   (Dlink) plus the potential worst case delay due to transmission of   another fragment or suspend/resume overhead. Thus, D should be   calculated as   D = Dlink + FRAG/LinkRate   in the case of a fragementing implementation and   D = Dlink + pHeader + pDelay   for a suspend/resume implementation.4.5 Scheduling Considerations   We may think of the link scheduler as having two parts, the first of   which schedules packets for transmission before passing them to the   second part of the scheduler -- the link level scheduler -- which is   responsible for fragmenting packets, mapping them to classes, and   scheduling among the classes.   In the dynamic class mapping mode ofSection 3.3, when deciding which   class to assign a packet to, the link level scheduler should take   account of the sizes of other packets currently assigned to the same   class. In particular, packets with the tightest delay constraints   should not be assigned to classes for which relatively large packets   are in the process of being transmitted.   In either the dynamic or the static class mapping approach, note that   the link-level scheduler SHOULD control how much link bandwidth is   assigned to each class at any instant. The scheduler should assign   bandwidth to a class according to the bandwidth reserved for the sum   of all flows which currently have packets assigned to the class. Note   that in the example ofSection 3.3, when packets from flows A and E   were assigned to the same class (class 1), the scheduler assigned   more bandwidth to class 1, reflecting the fact that it was carrying   traffic from reservations totaling 20kbit/s while the other classes   were carrying only 10kbit/s.Jackowski, et al.           Standards Track                    [Page 11]

RFC 2688      Integrated Services Mappings Low Speed Nets September 19995. Security Considerations   General security considerations for MLPPP and PPP links are addressed   inRFC 1990 [12] andRFC 1661 [13], respectively.  Security   considerations relevant to RSVP, used as the signaling protocol for   integrated services, are discussed inRFC 2209 [14].   A specific security consideration relevant to providing quality of   service over PPP links appears when relying on either observed or   theoretical average packet expansion during admission control due to   bit- or byte-stuffing.  Implementations based on these packet-   expansion values contain a potential vulnerability to denial of   service attacks.  An adversary could intentionally send traffic that   will result in worst case bit- or byte stuffing packet expansion.   This in turn could result in quality of service guarantees not being   met for other flows due to overly permissive admission control. This   potential denial of service attack argues strongly for using a worst   case expansion factor in admission control calculations, even for   controlled load service.   Beyond the considerations documented above, this document introduces   no new security issues on top of those discussed in the companion   ISSLL documents [1], [2] and [3] and AVT document [4].  Any use of   these service mappings assumes that all requests for service are   authenticated appropriately.6. References   [1]  Bormann, C., "Providing Integrated Services over Low-bitrate        Links",RFC 2689, September 1999.   [2]  Bormann, C., "The Multi-Class Extension to Multi-Link PPP",RFC2686, September 1999.   [3]  Bormann, C., "PPP in a Real-time Oriented HDLC-like Framing",RFC 2687, September 1999.   [4]  Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for        Low-Speed Serial Links",RFC 2508, February 1999.   [5]  Wroclawski, J., "Specification of the Controlled-Load Network        Element Service",RFC 2211, September 1997.   [6]  Partridge, C. and  R. Guerin, "Specification of Guaranteed        Quality of Service",RFC 2212, September 1997.Jackowski, et al.           Standards Track                    [Page 12]

RFC 2688      Integrated Services Mappings Low Speed Nets September 1999   [7]  Shenker, S. and J. Wroclawski, "General Characterization        Parameters for Integrated Service Network Elements",RFC 2215,        September 1997.   [8]  Jacobson, V., "TCP/IP Compression for Low-Speed Serial Links",RFC 1144, February 1990.   [9]  B. Davie et al. "Integrated Services in the Presence of        Compressible Flows", Work in Progress (draft-davie-intserv-compress-00.txt), Feb. 1999.   [10] Engan, M., Casner, S. and C. Bormann, "IP Header Compression        over PPP",RFC 2509, February 1999.   [11] Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [12] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T.        Coradettim, "The PPP Multilink Protocol (MP)",RFC 1990, August        1996.   [13] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD        51,RFC 1661, July 1994.   [14] Braden, R. and L. Zhang, "Resource ReSerVation Protocol (RSVP)        -- Version 1 Message Processing Rules",RFC 2209, September        1997.Jackowski, et al.           Standards Track                    [Page 13]

RFC 2688      Integrated Services Mappings Low Speed Nets September 19997. Authors' Addresses   Steve Jackowski   Deterministic Networks, Inc.   245M Mt Hermon Rd, #140   Scotts Valley, CA  95060   USA   Phone: +1 (408) 813 6294   EMail: stevej@DeterministicNetworks.com   David Putzolu   Intel Architecture Labs (IAL)   JF3-206-H10   2111 NE 25th Avenue   Hillsboro, OR 97124-5961   USA   Phone: +1 (503) 264 4510   EMail: David.Putzolu@intel.com   Eric S. Crawley   Argon Networks, Inc.   25 Porter Road   Littleton, MA 01460   USA   Phone: +1 (978) 486-0665   EMail: esc@argon.com   Bruce Davie   Cisco Systems, Inc.   250 Apollo Drive   Chelmsford, MA, 01824   USA   Phone: +1 (978) 244 8921   EMail: bdavie@cisco.comAcknowledgements   This document draws heavily on the work of the ISSLL WG of the IETF.Jackowski, et al.           Standards Track                    [Page 14]

RFC 2688      Integrated Services Mappings Low Speed Nets September 1999Appendix A. Admission Control Considerations for POTS Modems   The protocols used in current implementations of POTS modems can   exhibit significant changes in link rate and delay over the duration   of a connection. Admission control and link scheduling algorithms   used with these devices MUST be prepared to compensate for this   variability in order to provide a robust implementation of integrated   services.   Link rate on POTS modems is typically reported at connection time.   This value may change over the duration of the connection. The v.34   protocol, used in most POTS modems, is adaptive to link conditions,   and is able to recalibrate transmission rate multiple times over the   duration of a connection. Typically this will result in a small   (~10%) increase in transmission rate over the initial connection   within the first minute of a call. It is important to note, however,   that other results are possible as well, including decreases in   available bandwidth. Admission control algorithms MUST take such   changes into consideration as they occur, and implementations MUST be   able to gracefully handle the pathological case where link rate   actually drops below the currently reserved capacity of a link.   Delay experienced by traffic over POTS modems can vary significantly   over time.  Unlike link rate, the delay often does not converge to a   stable value.  The v.42 protocol is used in most POTS modems to   provide link-layer reliability. This reliability, which is   implemented via retransmission, can cause frames to experience   significant delays.  Retransmissions also implicitly steal link   bandwidth from other traffic. These delays and reductions in link   bandwidth make it extremely difficult to honor a guaranteed service   reservation. On a link that is actually lightly or moderately loaded,   a controlled load service can to some extent accept such events as   part of the behavior of a lightly loaded link. Unfortunately, as   actual link utilization increases, v.42 retransmissions have the   potential of stealing larger and larger fractions of available link   bandwidth; making even controlled load service difficult to offer at   high link utilization when retransmissions occur.Jackowski, et al.           Standards Track                    [Page 15]

RFC 2688      Integrated Services Mappings Low Speed Nets September 19999.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  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   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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Jackowski, et al.           Standards Track                    [Page 16]