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Versions: 00 01 02 03 RFC 3366

   Internet Engineering Task Force              Gorry Fairhurst
   INTERNET DRAFT                               University of Aberdeen
                                                Lloyd Wood
                                                Cisco Systems Ltd


   draft-ietf-pilc-link-arq-issues-01.txt       March 2001
                                                Expires: September 2001



                  Link ARQ issues for IP traffic


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



ABSTRACT

   This document discusses the use of automatic repeat request (ARQ)
   procedures as an option in the design of a link protocol, and the
   impact of the use of ARQ on IP traffic travelling over the link.
   ARQ is described in a general way that includes its use over a wide
   range of underlying physical media, including cellular wireless,
   wireless LANs, RF links, and other types of bearer channel. This
   document is intended to outline matters for consideration by, and to
   provide guidance to, designers and implementers of future link ARQ
   mechanisms.

   The material presented here is intended to be combined with the PILC
   'Advice for Internet Subnetwork Designers' draft [DRAFTKARN00] to
   provide a single document giving guidance to subnetwork designers.

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CONTENTS

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
      1.1 Link ARQ                                                 3
      1.2 Causes of packet loss on a link . . . . . . . . . . . . .3
      1.3 Why use ARQ?                                             4
      1.4 Commonly-used ARQ techniques . . . . . . . . . . . . . . 5
          1.4.1 Stop-and-wait ARQ                                  5
          1.4.2 Sliding-window ARQ. . . . . . . . . . . . . . . . .5
      1.5 Causes of delay across a link                            6
   2. ARQ persistence. . . . . . . . . . . . . . . . . . . . . . . 6
      2.1 Perfectly-persistent (reliable) ARQ protocols            7
      2.2 High-persistence (highly-reliable) ARQ protocols. . . . .8
      2.3 Low-persistence (partially-reliable) ARQ protocols       9
      2.4 Choosing your persistency. . . . . . . . . . . . . . . .10
   3. Packet ordering                                             11
      3.1 Using link ARQ to support multiple flows. . . . . . . . 12
      3.2 Differentiation of link service classes                 13
   4. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . .14
   5. Security implications                                       16
   6. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . 16
   7. References                                                  16
   Authors' addresses. . . . . . . . . . . . . . . . . . . . . . .19
   Full copyright statement                                       19



1. INTRODUCTION

   Over the years, traffic using the Internet Protocol (IP [RFC791])
   has been carried over a wide variety of links, of vastly different
   capacities, delays and loss characteristics.  IP traffic can be
   expected to be carried over a very wide variety of new and existing
   link designs in the future.

   This document is intended for three distinct groups of readers:

   a. Link designers wishing to configure (or tune) a link for the IP
      traffic that it will carry, using standard link-layer mechanisms
      such as the ISO High-level Data Link Control (HDLC) [ISO4335] or
      its derivatives.

   b. Link implementers who may wish to design new link mechanisms that
      perform well for IP traffic when carrying IP.

   c. The community of people using or developing TCP, UDP and related
      protocols, who may wish to be aware of the ways in which links
      can operate.

   The primary audiences are intended to be groups a. and b.  Group c.
   should not need to be aware of the exact details of an ARQ scheme
   across a single link, and should not have to consider it in their

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   implementations; much of the Internet runs across links that do not
   use any form of ARQ.

   However, the TCP/IP community does need to be aware that the IP
   protocol operates over a diverse range of underlying subnetworks.
   This document may help to raise that awareness.

   Some familiarity with TCP [RFC2581, RFC2488, STE94] is assumed here.
   There are a number of variants of TCP, with differing levels of
   sophistication in their procedures for handling loss recovery and
   congestion avoidance.  Far from being static, the TCP protocol is
   itself subject to ongoing gradual refinement and evolution, e.g.
   [RFC2760].

   This document should be read in conjunction with other documents
   produced by the Performance Implications of Link Characteristics
   (PILC) IETF workgroup [DRAFTDAW00, DRAFTKARN00, DRAFTBOR00].


1.1 LINK ARQ

   At the link layer, ARQ operates on blocks of data, known as frames,
   and attempts to deliver frames from the sender to the receiver over
   a physical link.  Frames often have a small maximum size for
   convenience of processing by media-access control (MAC) and link-
   layer protocols.  This contrasts with the variable lengths of IP
   datagrams, or 'packets'.

   The ARQ link mechanism provides an integrity check and a
   retransmission process to retransmit lost or errored frames.  Link
   ARQ protocols operate within the link layer of the OSI reference
   model [DRAFTKARN00].

   A frame may contain one or more IP datagrams, although transparent
   implicit fragmentation is often used so that each frame will contain
   only a fraction of an IP datagram [KEN88].

   Explicit network-layer IP fragmentation is undesirable for a variety
   of reasons, and should be avoided [KEN88].  Its use can be minimised
   with use of Path MTU discovery for TCP connections [RFC1191,
   RFC1435].


1.2 CAUSES OF PACKET LOSS ON A LINK

   Not all packets sent to a link are necessarily received successfully
   by the receiver at the other end of the link.  There are a number of
   possible causes of packet loss that may occur as frames travel
   across a link.  These include:

   a.  Loss due to channel noise, often characterised by random frame
       loss.

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   b.  Frame loss due to channel interference.  This interference can
       be random, structured, and in some cases even periodic.

   c.  A link outage, causing loss due to unavailability of the
       physical link for a period of time. This is a common
       characteristic of some types of link, e.g. mobile cellular
       radio. In the outage, the link loses all or virtually all frames
       for a period of time, until the link is restored.

   Other forms of packet loss are not related to channel conditions,
   but include:

   i.  Packet discards due to congestion.  Queues will eventually
      overflow as the arrival rate of new packets to send continues to
      exceed the outgoing packet transmission rate over the link.

   ii. Loss due to implementation errors, including hardware faults and
      software errors.

   The levels of loss and patterns of loss experienced are functions of
   the design of the link's physical and link layers.  These vary
   significantly across different link configurations.  The performance
   of a specific implementation may also vary considerably across the
   same link configuration when operated over different types of
   physical channel.


1.3 WHY USE ARQ?

   Reasons that encourage considering the use of ARQ include:

   a. ARQ across a single link has a shorter control loop than the end-
      to-end path over which TCP must operate.  ARQ can therefore
      provide more rapid retransmission for TCP segments lost on the
      link, at least for a reasonable number of retries [DRAFTDAW00,
      SAL81].

   b. Link ARQ can operate on individual frames, which are much smaller
      than IP datagrams and may therefore permit more efficient
      recovery in terms of repetition of data after occasional losses.
      For any specific target error rate there is an optimal trade-off
      between header overhead and frame size.

   c. A link ARQ procedure may be able to use local knowledge that is
      not available to endhosts, including information about the state
      of the link and physical layer techniques, in order to optimise
      delivery performance for the current link conditions.  This
      information can include knowledge of the current available
      transmission rate, the prevailing error environment, or available
      transmit power in wireless links.


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1.4 COMMONLY-USED ARQ TECHNIQUES

   Two basic categories of ARQ retransmission process are widely used:

1.4.1 STOP-AND-WAIT ARQ

   A stop-and-wait ARQ sender transmits a single frame and then waits
   for an acknowledgement from the receiver for that frame, before
   either continuing transmission with the next frame, or repeating
   transmission of the same frame if the original frame is errored or
   lost.

   Stop-and-wait ARQ is simple for protocol designers to implement,
   e.g. tftp [RFC1350]. However, when stop-and-wait ARQ is used in the
   link layer, it is well-suited only to links with low bandwidth-delay
   products.  This technique is not discussed further in this document.

1.4.2 SLIDING-WINDOW ARQ

   A sliding-window link ARQ protocol numbers every frame with a unique
   sequence number, according to a modulus.  TCP is itself a sliding-
   window protocol, so similarities between this link-interface-to-
   link-interface protocol and end-to-end TCP may be recognisable.  A
   sliding-window link protocol is much more complex in implementation
   than a simpler stop-and-wait protocol, particularly if per-flow
   ordering is preserved.

   At any time the link sender may have a number of frames outstanding
   and awaiting acknowledgement, up to the space available in its
   transmission window.  A sufficiently large link sender window
   (equivalent to or greater than the number of frames sent, or larger
   than the bandwidth*delay capacity of the link) enables continuous
   transmission of new frames.  A smaller link sender window causes the
   sender to pause transmission of new frames until a timeout or a
   control frame, such as an acknowledgement, is received.  When frames
   are lost, a larger window, i.e. more than the link's bandwidth*delay
   product, is needed to allow continuous operation while frame
   retransmission takes place.

   The modulus numbering space determines the size of the frame header
   sequence number field.  This sequence space needs to be larger than
   the link window size, and if using selective frame repeat, larger
   than twice the link window size. For continuous operation, the
   sequence space should be larger than the bandwidth-persistence
   product, so that in-flight frames can be identified uniquely.

   As with TCP, which provides sliding-window delivery across an entire
   path, rather than a single link, there are a number of variations on
   the basic sliding-window implementation, with increased complexity
   and sophistication to make them suitable for various conditions.



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1.5 CAUSES OF DELAY ACROSS A LINK

   Links and link protocols contribute to the total path delay
   experienced between communicating applications on hosts.  Delay has
   a number of causes, including:

   a. Input packet queuing and frame buffering at the link head before
      transmission over the link.

   b. Waiting for access to the allocated channel when the physical
      medium is shared.  There may be processing or protocol-induced
      delay before transmission takes place [FER99, PAR00].

   c. Frame serialisation and transmission processing.  These are
      functions of frame size and the transmission speed of the link.

   d. Physical link propagation time, caused by the speed of light in
      the channel medium.

   e. Processing, including buffering frame contents for packet
      reassembly, at the receiver before onward transmission of the
      packet.

   When link ARQ is used, steps b., c. and d. may be repeated together
   at intervals a number of times after a frame is lost, increasing
   overall delay for the packet that the frame partially transmits.

   It is important to understand that applications at the endhosts are
   unaware of the individual delays contributed by each link in the
   path, and only see the overall path delay.  Application performance
   is determined by the cumulative delay of the entire end-to-end
   Internet path.  This path may include an arbitrary or even widely-
   varying number of links, where each link may or may not use ARQ.



2. ARQ PERSISTENCE

   ARQ protocols may be characterised by their persistence.
   Persistence is the willingness of the protocol to retransmit lost
   frames to ensure reliable delivery of the stream of traffic across
   the link.

   A link's retransmission persistency defines how long the link is
   allowed to delay an IP packet, in an attempt to transmit all the
   frames carrying the packet and its content over the link, before
   giving up and discarding the packet.  This persistency is normally
   measured in milliseconds, but may, if the link propagation delay is
   specified, also be expressed in terms of the maximum number of link
   retransmission attempts permitted.



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   An example of a reliable link protocol that is perfectly persistent
   is the ISO HDLC protocol in the Asynchronous Balanced Mode (ABM)
   [ISO4335].  A protocol that only retransmits a fixed number of times
   before giving up is less persistent, e.g. Ethernet [FER99] or GSM
   RLP [RFC2757].  Here, lower persistence ensures stability and fair
   sharing of the shared subnetwork, even when many senders are
   attempting retransmissions.

   In a broadcast wireless environment, the interval between repeat
   attempts will be set to be greater than the link round trip delay,
   and frequently increases with the number of retransmission attempts.
   This is similar to having increasing backoffs in Ethernet / CSMA to
   decrease offered channel load in the presence of impairments.

   A wireless receiver may experience lost or errored frames due to
   fading, shadowing or interference on its channel from the wireless
   sender.  In this situation, it is wasteful for the sender to consume
   shared capacity when retransmissions are likely to experience the
   same conditions; other receivers can benefit from that capacity.
   One simple solution is to delay retransmissions to that receiver in
   the hope that the channel conditions will improve.  A complementary
   approach is to adapt the way in which retransmitted information is
   sent over the channel, e.g. by adapting modulation or coding.  The
   interval or delay between retransmission attempts may be varied as a
   function of the perceived channel state and the number of previous
   retransmission attempts.

   Perfect reliability is not a requirement for IP subnetworks
   [DRAFTKARN00]. IP networks may discard packets due to a variety of
   reasons entirely unrelated to link errors, including lack of queuing
   space, congestion management, faults, and route changes.  It has
   long been widely understood that perfect reliability can be obtained
   only at the transport layer [SAL81].

   TCP [RFC2488, RFC2581, STE94] and a number of applications using UDP
   implement their own end-to-end reliable delivery mechanisms. Many
   TCP and UDP applications, e.g. streaming multimedia, benefit from
   timely delivery from lower layers with sufficient reliability,
   rather than perfect reliability with increased link delays.


2.1 PERFECTLY-PERSISTENT (RELIABLE) ARQ PROTOCOLS

   Most well-known link protocols are reliable protocols, which use
   perfectly-persistent ARQ to ensure that all frames (and therefore
   all packets) are received by the other side of the link.  The
   perfectly-persistent ARQ protocol will repeat a lost or errored
   frame an indefinite number of times until the frame is successfully
   received.

   If traffic is going no further than across one link, this ensures
   reliability between the two link ends without requiring higher-

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   layers protocols.  This reliability can be counterproductive for
   traffic traversing multiple links, as it interacts with protocol
   mechanisms at higher layers.

   Arguments against perfect persistence for IP traffic include:

   a. Variable link delay; the impact of ARQ introduces a degree of
      jitter, a function of the link's physical delay and frame
      serialisation and transmission times, to all flows sharing the
      link performing frame retransmission.

   b. High persistence does not provide a clear upper bound on the
      maximum retransmission delay for the link.  Significant changes
      in path delay caused by excessive link retransmissions may lead
      to timeout of TCP retransmission timers, although a high variance
      in link delay and the resulting overall path delay may also cause
      a large TCP Retransmission Timeout (RTO) value to be used
      [LUD99a, PAR00].  Future TCP implementations may use techniques
      that reduce premature expiry of this timer, e.g. Eifel [LUD00]
      and Rapid Packet Loss Detection [SAM98], but further research is
      required in this area.

   c. Applications needing perfectly-reliable delivery can implement a
      form of perfectly-persistent ARQ themselves, or use a reliable
      transport protocol within the endhosts.  Implementing perfect
      persistence at each link along the path between the endhosts is
      redundant, and cannot guarantee the same reliability [SAL81]. It
      may also affect other end-to-end protocol mechanisms, e.g. using
      Explicit Congestion Notification (ECN).

   Examples of ARQ protocols that are perfectly persistent include
   ISO/ITU-T LAP-B [ISO7776] and ISO HDLC in the Asynchronously
   Balanced Mode (ABM) [ISO4335], e.g. using Multiple Selective Reject
   (MSREJ [ISO4335a]).  These protocols will retransmit a frame an
   unlimited number of times until receipt of the frame is
   acknowledged.


2.2 HIGH-PERSISTENCE (HIGHLY-RELIABLE) ARQ PROTOCOLS

   High-persistence ARQ protocols attempt only a finite, large, number
   of transmissions of each frame before giving up.  Arguments against
   high persistence are similar to those against perfect persistence,
   except that infinite delay and perfectly reliable delivery are
   traded off against imperfect reliability and an upper bound on link
   delay.

   It has been recommended that a single IP datagram should never be
   delayed by more than the Maximum Segment Lifetime (MSL) of 120
   seconds defined for TCP [RFC1122].  Failure to limit the maximum
   datagram lifetime can result in sequence number wrapping at high TCP
   transmission rates [RFC1323], where old data segments may be

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   confused with newer segments where the sequence number space has
   been exhausted and reused in the interim.

   Some TCP implementations use timestamps to remove this ambiguity
   [RFC1323].

   (One case where segment lifetime may be significant is where
   alternate paths of different delays exist between hosts.  Some TCP
   segments may follow a short path, while others follow a much longer
   path, e.g. using persistent ARQ over a link outage.)

   In practice, the MSL is usually very large compared to the typical
   TCP RTO.  The calculation of RTO is based on estimated round-trip
   path delay.  If the number of link retransmissions causes a path
   delay larger than the value of RTO, the TCP retransmission timer
   will expire, leading to a timeout and retransmission by the TCP
   sender.

   Although high persistency may benefit bulk flows, the additional
   delay (and variations in delay) may be highly undesirable for other
   types of flows.  Being able to treat flows separately with different
   classes of link service is useful, and is discussed in section 3.2.

   Examples of highly-persistent ARQ include [BHA97, ECK98, LUD99,
   MEY99].


2.3 LOW-PERSISTENCE (PARTIALLY-RELIABLE) ARQ PROTOCOLS

   The characteristics of a link using a low-persistence ARQ protocol
   may be summarised as:

   a. The link is not perfectly reliable and does not provide an
      absolute guarantee of delivery; i.e. the transmitter will discard
      some frames as it 'gives up' before receiving an acknowledgement
      of successful transmission across the link.

   b. There is a lowered limit on the maximum added delay that IP
      datagrams will experience when using the link.  This reduces
      interaction with TCP timers or with UDP-based error-control
      schemes.

   c. The link bounds the time that retransmission for any one frame
      can block completed transmission and assembly of other correctly-
      received datagrams originally sent before the missing frame.
      Limiting delay avoids aggravating contention and interaction
      between different packet flows (see also 3.2).

   Selection of even a quite high persistence, e.g. allowing tens of
   attempts to deliver a single frame, would usually satisfy a bound
   for b. for low-delay links.  However, a packet may traverse a number


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   of successive links on its total end-to-end path.  This is an
   argument for much lower persistency on individual links.


2.4 CHOOSING YOUR PERSISTENCY

   The TCP Maximum RTO is an upper limit on the maximum time that TCP
   will wait until it performs a retransmission.  Most TCP
   implementations will generally have a TCP RTO of typically a few
   times the path delay.

   Setting a lower link persistency (of the order 2-5 retransmission
   attempts) reduces interaction with the RTO timer, and may therefore
   reduce the probability of duplicate copies of the same packet being
   present in the link transmit buffer under some patterns of loss.

   The following table is calculated from the probability of loss,
   assuming the 'ideal' case of random loss.  It shows the probability
   of successful frame transmission using an idealised ARQ procedure,
   i.e. one in which the control frames are not corrupted.

   Frame loss (random)     Persistence (Link RTT)    Average frame loss
     after link ARQ

    1%                             x3                      0.0001%
    1%                             x5                      0.00000001%

    3%                             x3                      0.003%
    3%                             x5                      0.000002%

   10%                             x3                      0.1%
   10%                             x5                      0.001%
   10%                             x7                      0.00001%

   30%                             x3                      3%
   30%                             x5                      0.2%
   30%                             x7                      0.02%
   30%                             x9                      0.002%

   50%                             x3                     12%
   50%                             x5                      3%
   50%                             x7                      0.8%
   50%                             x9                      0.2%

   When interpreting these results, remember that:

   a. Link protocols normally employ transparent fragmentation and all
      frames containing fragments must be delivered to reassemble an IP
      packet.

   b. Channel error conditions are seldom characterised by random loss.


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   c. In these calculations, link persistence is expressed in terms of
      multiples of the Link Round Trip Time (Link RTT).

   Some implementers have chosen a lower persistence, falling back on
   the judgement of TCP or a UDP application to retransmit any packets
   that are not recovered by the link ARQ protocol. Examples of low-
   persistence ARQ protocols include [SAM96, WAR95, CHE00].



3. PACKET ORDERING

   A common debate is whether a link should be allowed to forward
   packets in an order different to that in which they were originally
   received at the link's transmit interface.

   IP networks are not required to deliver all datagrams in order,
   although generally networks do deliver most datagrams in their
   original transmission order. Routers supporting class-based queuing
   do reorder packets received, by reordering packets in different
   flows -- but these usually retain per-flow ordering.

   Policy-based queuing, allowing fairer access to the link, may
   reorder packets.  There is still much debate on optimal algorithms,
   and on optimal queue sizes for particular link speeds.  This,
   however, is not related to use of link ARQ and applies to any
   (potential) bottleneck router.

   Although small amounts of reordering are common in IP networks
   [BEN00], significant re-ordering within a flow is undesirable, as it
   can have several effects:

   a. reordering may cause unwanted side effects in poorly-implemented
      IP fragment reassembly code.

   b. reordering may cause unwanted side effects in poorly-implemented
      IP demultiplexing (filter) code.

   c. reordering may cause unwanted side effects in poorly-implemented
      UDP applications.

   d. reordering will increase packet jitter for real-time
      applications.  This may lead to application data loss if a small
      play-out buffer is used by the receiving application.

   e. reordering may impact congestion avoidance and may negatively
      impact overall throughput on paths.

   f. reordering will interleave arrival of TCP segments, leading to
      generation of duplicate ACKs (dupacks [STE94, BAL97]). A sequence
      of three identical dupacks causes the TCP sender to trigger fast
      retransmission and recovery, a form of congestion avoidance,

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      since TCP always presumes loss due to congestion [RFC2581,
      STE94]. This reduces TCP throughput efficiency as far as the
      application is concerned, but it should not impact data
      integrity.

   Ordering effects must be considered when breaking the end-to-end
   paradigm and evaluating transport-level relays such as split-TCP
   implementations or Protocol Enhancing Proxies [DRAFTBOR00].

   As with total path delay, reordering effects accumulate over
   multiple links.  Do not assume that your link is the only link
   undertaking packet reordering, as some level of reordering may be
   introduced by other links along the same path, or by router
   processing within the network [BEN00].

   TCP and UDP flows are impacted by the cumulative effect of
   reordering along the entire path.  The link protocol should ideally
   eliminate reordering within a flow, or at least ensure that it does
   not significantly increase the level of reordering within the flow.

   A number of ARQ protocols have allowed out-of-order delivery [BAL95,
   SAM96, WAR95].


3.1 USING LINK ARQ TO SUPPORT MULTIPLE FLOWS

   Most links can be expected to carry more than one IP traffic flow at
   a time.  Some high-capacity links are expected to carry a very large
   number of simultaneous flows, often from and to a large number of
   different hosts. With use of multiple applications at a host,
   multiple flows can be considered the norm rather than the exception,
   even for last-hop links.

   When packets from several flows are simultaneously in transit within
   a link ARQ protocol, ARQ may cause a number of additional effects:

   a. ARQ introduces jitter to a TCP flow sharing a link with another
      flow experiencing loss.  In-sequence link-level delivery of
      packets may adversely impact other applications sharing the link,
      and can increase the duration of the initial slow-start period
      for TCP flows for these applications. This is significant for
      short-lived TCP flows (e.g. those used by HTTP/1.0 and earlier),
      which spend most of their lives in slow-start.

   b. High-persistence ARQ may cause premature timeout of another TCP
      flow's RTO timer, although modern TCP implementations should not
      experience this since their computed RTO values should leave
      sufficient margin over path RTTs to cope with such jitter.

   c. ARQ introduces jitter to UDP flows.  An end-to-end protocol may
      not require reliable delivery, particularly if it is delay-
      sensitive.

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   Reordering of packets belonging to different flows may be desirable
   to achieve fair sharing of the link between established bulk data
   transfer sessions and sessions that transmit less data but would
   benefit from lower link transit delay.  Preserving ordering within
   each flow, to avoid the effects of reordering described earlier, is
   worthwhile.


3.2 DIFFERENTIATION OF LINK SERVICE CLASSES

   High ARQ persistency is generally considered unsuitable for many
   applications using UDP, where reliable delivery is not always
   required and where it may introduce unacceptable jitter, but may
   benefit bulk data transfers under certain link conditions.  A scheme
   that differentiates packet flows into two or more classes, to
   provide different service to each class, is therefore desirable.

   Observation of flow behaviour can tell you which flows are
   controlled and congestion-sensitive, or uncontrolled and not, so
   that you can treat them differently and ensure fairness.  However,
   this cannot tell you whether a flow is intended as reliable or
   unreliable by its application, or what the application requires for
   best operation.

   Supporting different link services for different classes of flows
   therefore needs an explicit indication of the class of each IP
   packet.

   Some potential schemes for indicating the class of a packet include:

   a. Using the Type of Service (ToS) bits in the IP header [RFC791].
      The IETF has replaced these globally-defined bits, which were
      rarely used, with a much more complex identification scheme for
      use with differentiated services (diffserv [RFC2475]).  Diffserv
      defines classes of service that are mapped to various Per Hop
      Behaviours (PHBs) as the packet is processed within the network.
      Its uses include policy-based routing, class-based queuing, and
      support for other QoS metrics, including packet priority, delay,
      reliability, cost, etc.

   b. Inspecting the network packet header and viewing the IP protocol
      type [RFC791], to gain an idea of the transport protocol used and
      thus guess its needs.  This is not possible when carrying an
      encrypted payload, e.g. using the IP security extensions (IPSec)
      with the Encapsulation Security Payload (ESP) [RFC1827].

   c. By inspecting the transport packet header information to view the
      TCP or UDP headers and port numbers (e.g. [PAR00, BAL95]).  This
      is not possible when using payload encryption, e.g. IPSec with
      ESP [RFC1827], and incurs processing overhead in routers.


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   There are, however, some drawbacks to these schemes:

   i.   The ToS/Differentiated Services Code Point (DSCP) values
        [RFC2475] may not be set reliably, and may be overwritten by
        intermediate routers along the packet's path. These values may
        be set by an ISP, and do not necessarily indicate the level of
        reliability required by the end application.

   ii.  Tunnelling of traffic (e.g. GRE, MPLS, L2TP, IP-in-IP
        encapsulation) can aggregate flows of different transport
        classes, complicating individual flow classification with
        schemes b. and c. and incurring further header processing if
        tunnel contents are inspected.

   iii. Use of the port number makes assumptions about application
        behaviour and requirements. New applications or protocols can
        invalidate these assumptions.

   iv.  In IPv6, locating the transport layer protocol type requires
        parsing the entire IPv6 header, adding complexity to header
        inspection.  Again, this assumes that IPSec encryption is not
        used.

   Flow management is desirable; clear flow identification increases
   the number of tools available for the link designer to use, and
   permits more complex ARQ strategies that may otherwise make
   misassumptions about traffic requirements and behaviour without flow
   identification.

   Despite the difficulties in providing a framework for accurate flow
   identification, [RFC3002] advocates approach (a), and urges against
   adding optimisations to future links that are triggered by
   inspecting the contents of specific IP packets.

   Links that are unable to distinguish clearly and safely between
   delay-sensitive flows, e.g. real-time multimedia, DNS queries or
   telnet, and delay-insensitive flows, e.g. bulk ftp transfers or
   reliable multicast file transfer, cannot separate link service
   classes safely. All flows should therefore experience the same link
   behaviour.

   In general, if separation of flows according to class is not
   practicable, a low persistency is best for link ARQ.



4. CONCLUSIONS

   It is currently difficult to make simple statements about arbitrary
   patterns of loss that cover a wide range of link speeds and
   propagation delays.  Some general statements follow, summarising the
   preceding discussion.  In each case, it is recommended that specific

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   details of the link characteristics and mechanisms are also
   considered; solutions vary with conditions.

   ARQ has been and continues to be used on links that carry IP
   traffic.  At low error rates, many of the details of ARQ, such as
   degree of persistence or resulting out-of-order delivery, become
   unimportant.  Most frame losses will be resolved in one or two
   retransmission attempts, and this is generally unlikely to cause
   significant impact to e.g. TCP.  However, on shared high-delay
   links, the impact of ARQ on other UDP or TCP flows may lead to
   unwanted jitter.

   In a link outage, interactions between ARQ and multiple flows are
   less significant; the ARQ protocol is likely to be equally
   unsuccessful in retransmitting frames for all flows.  High
   persistence may also be desirable to enable prompt recovery once the
   channel is restored.

   For links with highly-variable error rates, high ARQ persistence may
   provide good performance for a single TCP flow.  However, lower
   persistence may in some cases have merit, particularly when many
   flows share and compete for capacity on the same link.  The
   reasoning here is that the link can perform useful work forwarding
   some complete packets, and that blocking all flows by retransmitting
   the frames of a single packet with high persistence is undesirable.

   Low ARQ persistence is particularly useful where it is difficult or
   impossible to classify traffic flows, and hence treat each flow
   independently, and where the link capacity can accommodate a large
   number of simultaneous flows.

   In summary, low persistence is generally desirable; preserving
   packet ordering is generally desirable.  Extremely high persistence
   is generally undesirable.

   There is currently insufficient experience to recommend a specific
   ARQ scheme for any class of link.  It is also important to realise
   that link ARQ is just one method of error recovery, and that other
   complementary physical-layer techniques may be used instead of, or
   together with, ARQ to improve overall link throughput for IP
   traffic.

   The choice of potential schemes includes adapting the data rate,
   adapting the signal bandwidth, adapting the transmission power,
   adaptive modulation, and adaptive information redundancy / forward
   error control (FEC).  All of these schemes can be used to improve
   the received signal energy per bit, and hence reduce error, frame
   loss and resulting packet loss rates given specific physical channel
   conditions.

   There is a need for more research to more clearly identify the
   importance of and trade-offs between the above issues over various

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   types of link.  It would be useful if researchers and implementers
   clearly indicated the loss model, link capacity and characteristics,
   link and end-to-end path delays, details of TCP, and the number (and
   details) of flows sharing a link when describing their experiences.



5. SECURITY IMPLICATIONS

   No security implications have been identified as directly impacting
   IP traffic.  However, an unreliable link service may adversely
   impact some link-layer key management distribution protocols if
   encryption is used over such a link.

   Denial-of-service attacks exploiting the behaviour of the link
   protocol, e.g. using knowledge of its retransmission behaviour and
   propagation delay to cause a particular form of jamming, may be
   specific to an individual link scenario.



6. ACKNOWLEDGMENTS

   Much of what is described here has been developed from a summary of
   a subset of the discussions on the archived IETF PILC mailing list.
   We thank the contributors to PILC for vigorous debate.



7. REFERENCES

   References of the form RFCnnnn are Internet Request for Comments
   (RFC) documents available online at http://www.rfc-editor.org/.

   [BAL95] H. Balakrishnan, S. Seshan and R. H. Katz, Improving
   Reliable Transport and Handoff Performance in Cellular Wireless
   Networks, ACM MOBICOM, Berkeley, 1995.

   [BAL97] H. Balakrishnan, V. N. Padmanabhan, S. Seshan and R. H.
   Katz, A Comparison of Mechanisms for Improving TCP Performance over
   Wireless Links, IEEE/ACM Transactions on Networking, 5(6),
   pp. 756-759, 1997.

   [BEN00] J.C. Bennett, C. Partridge, and N. Schectman, Packet
   Reordering is Not Pathological Network Behaviour, IEEE/ACM
   Transactions on Networking, 7(6), pp. 789-798, 2000.

   [BHA97] P. Bhagwat, P. Bhattacharya, A. Krishna and S. K. Tripathi,
   Using channel state dependent packet scheduling to improve TCP
   throughput over wireless LANs, ACM/Baltzer Wireless Networks
   Journal, (3)1, 1997.


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   [CHE00] H. S. Cheng, G. Fairhurst et al., An Efficient Partial
   Retransmission ARQ Strategy with Error Codes by Feedback Channel,
   IEE Proceedings - Communications, (147)5, pp. 263-268, 2000.

   [DRAFTBOR00] J. Border, M. Kojo, J. Griner et al., Performance
   Enhancing Proxies, draft-ietf-pilc-pep-nn.txt, work in progress as
   internet-draft.

   [DRAFTDAW00] S. Dawkins, G. Montenegro, M. Kojo, V. Magret et al.,
   End-to-end Performance Implications of Links with Errors,
   draft-ietf-pilc-error-nn.txt, work in progress as internet-draft.

   [DRAFTKARN00] P. Karn, A. Falk, J. Touch et al., Advice for Internet
   Subnetwork Designers, draft-ietf-pilc-link-design-nn.txt, work in
   progress as internet-draft.

   [ECK98] D. A. Eckhardt and P. Steenkiste, Improving Wireless LAN
   Performance via Adaptive Local Error Control, IEEE ICNP, 1998.

   [FER99] A. Ferrero, The Eternal Ethernet, Addison-Wesley, 1999.

   [ISO4335]  HDLC Procedures: Specification for Consolidation of
   Elements of Procedures, ISO 4335 and AD/1, International
   Standardization Organization, 1985.

   [ISO4335a] HDLC Procedures: Elements of Procedures, Amendment 4:
   Multi-Selective Reject Option, ISO 4335/4, International Standards
   Organization, 1991.

   [ISO7776] Specification for X.25 LAPB-Compatible DTE Data Link
   Procedures, ISO 4335/4, International Standards Organization, 1985.

   [KEN88] C. A. Kent and J. C. Mogul, Fragmentation Considered
   Harmful, Proceedings of ACM SIGCOMM, pp. 390-401, 1988.

   [LUD99] R. Ludwig, B. Rathonyi, A. Konrad, K. Oden and A. Joseph,
   Multi-Layer Tracing of TCP over a Reliable Wireless Link, ACM
   SIGMETRICS, pp. 144-154, 1999.

   [LUD99a] R. Ludwig, A. Konrad, A. Joseph, and R. Katz, Optimizing
   the End-to-End Performance of Reliable Flows over Wireless Links,
   ACM MobiCOM, 1999.

   [LUD00] R. Ludwig and R. Katz, The Eifel Algorithm: Making TCP
   Robust Against Spurious Retransmissions, ACM Computer Communication
   Review, (30)1, January 2000.

   [MEY99] M. Meyer, TCP Performance over GPRS, IEEE WCNC, 1999.

   [PAR00] C. Parsa and J. J. Garcia-Luna-Aceves, Improving TCP
   Performance over Wireless Networks at the Link Layer, Mobile
   Networks and Applications, (1)5, pp. 57-71, 2000.

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   [RFC791] J. Postel, Internet Protocol, RFC 791, 1981.

   [RFC1122] R. Braden et al., Requirements for Internet Hosts --
   Communication Layers, RFC 1122, 1989.

   [RFC1191] J. Mogul and S. Deering, Path MTU Discovery, RFC 1191,
   1990.

   [RFC1323] V. Jacobson and R. Braden, TCP Extensions for High
   Performance, RFC 1323, 1992.

   [RFC1350] K. Solins, The TFTP Protocol (Revision 2), IETF RFC 1350,
   1992.

   [RFC1435] S. Knowles, IESG Advice from Experience with Path MTU
   Discovery, RFC 1435, 1993.

   [RFC1827] R. Atkinson, IP Encapsulating Security Payload (ESP),
   RFC 1827, 1995.

   [RFC2475] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang and W.
   Weiss, An Architecture for Differentiated Services, RFC 2475, 1998.

   [RFC2488] M. Allman, D. Glover and L. Sanchez, Enhancing TCP Over
   Satellite Channels using Standard Mechanisms, RFC 2488, 1999.

   [RFC2581] M. Allman, V. Paxson and W. Stevens, TCP Congestion
   Control, RFC 2581, 1999.

   [RFC2757] S. Dawkins, M. Kojo, V. Magret and N. Vaidya, Long Thin
   Networks, RFC 2757, 2000.

   [RFC2760] M. Allman, S. Dawkins, D. Glover, J. Griner et al.,
   Ongoing TCP Research Related to Satellites, RFC 2760, 1999.

   [RFC3002] D. Mitzel, Overview of 2000 IAB Wireless Internetworking
   Workshop, RFC 3002, 2000.

   [SAL81] J. H. Saltzer, D. P. Reed and D. Clark, End-to- End
   Arguments in System Design, Second International Conference on
   Distributed Computing Systems, pp. 509-512, 1988.  Published with
   minor changes in ACM Transactions in Computer Systems (2)4,
   pp. 277-288, 1984.

   [SAM96] N. Samaraweera and G. Fairhurst, Robust Data Link Protocols
   for Connection-less Service over Satellite Links, International
   Journal of Satellite Communications, 14(5), pp. 427-437, 1996.

   [SAM98] N. Samaraweera and G. Fairhurst, Reinforcement of TCP/IP
   Error Recovery for Wireless Communications, ACM CCR, 28(2),
   pp. 30-38, 1998.

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   [STE94] W. R. Stevens, TCP/IP Illustrated, Volume 1, Addison-Wesley,
   1994.

   [WAR95] C. Ward et al., A Data Link Control Protocol for LEO
   Satellite Networks Providing a Reliable Datagram Service, IEEE/ACM
   Transactions on Networking, 3(1), 1995.



AUTHORS' ADDRESSES

   Gorry Fairhurst (gorry@erg.abdn.ac.uk)
   Department of Engineering, University of Aberdeen,
   Aberdeen AB24 3UE, United Kingdom.

   Lloyd Wood (lwood@cisco.com)
   Cisco Systems Ltd, 4 The Square, Stockley Park,
   Uxbridge UB11 1BY, United Kingdom.



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