Network Working Group                                       S. Jackowski
Request for Comments: 2688                        Deterministic Networks
Category: Standards Track                                     D. Putzolu
                                                 Intel Architecture Labs
                                                              E. Crawley
                                                          Argon Networks
                                                                B. Davie
                                                           Cisco Systems
                                                          September 1999

          Integrated Services Mappings for Low Speed Networks

Status 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.


   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.

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1. 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",
   document are to be interpreted as described in RFC 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

   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-shared

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   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

   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.

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2.1 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.

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   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 Services

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   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

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.

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Time(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 arrives
10     2kbit fragment from A sent, cls 1 (8kbit flow A packet start)
11     8kbit packet from flow B arrives
12     2kbit fragment from B sent, cls 2 (8kbit flow B packet start)
13     8kbit packets from flows C, D, and E arrive
14     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 1
20     2kbit fragment from B sent, cls 2
22     2kbit fragment from A sent, cls 1
24     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 arrives
28     2kbit fragment from B sent, cls 2
30     2kbit fragment from C sent, cls 3
32     2kbit fragment from E sent, cls 1
34     2kbit fragment from B sent, cls 2 (8kbit flow B packet done)
36     2kbit fragment from E sent, cls 1
38     2kbit fragment flow A sent, cls 2 (8kbit flow A packet start)

   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.

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3.4 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 Implementers

4.1. 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 on

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   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.
   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.

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  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
  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 to Appendix 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
      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.

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4.4 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 of Section 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 of Section 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.

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5. Security Considerations

   General security considerations for MLPPP and PPP links are addressed
   in RFC 1990 [12] and RFC 1661 [13], respectively.  Security
   considerations relevant to RSVP, used as the signaling protocol for
   integrated services, are discussed in RFC 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", RFC
        2686, 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.

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   [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

   [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

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7. Authors' Addresses

   Steve Jackowski
   Deterministic Networks, Inc.
   245M Mt Hermon Rd, #140
   Scotts Valley, CA  95060

   Phone: +1 (408) 813 6294
   EMail: stevej@DeterministicNetworks.com

   David Putzolu
   Intel Architecture Labs (IAL)
   2111 NE 25th Avenue
   Hillsboro, OR 97124-5961

   Phone: +1 (503) 264 4510
   EMail: David.Putzolu@intel.com

   Eric S. Crawley
   Argon Networks, Inc.
   25 Porter Road
   Littleton, MA 01460

   Phone: +1 (978) 486-0665
   EMail: esc@argon.com

   Bruce Davie
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824

   Phone: +1 (978) 244 8921
   EMail: bdavie@cisco.com


   This document draws heavily on the work of the ISSLL WG of the IETF.

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RFC 2688      Integrated Services Mappings Low Speed Nets September 1999

Appendix 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

   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.

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RFC 2688      Integrated Services Mappings Low Speed Nets September 1999

9.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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