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Versions: 00 01 02 03

Network Working Group                                            A. Falk
Internet-Draft                                               Y. Pryadkin
Expires: July 22, 2007                                               ISI
                                                               D. Katabi
                                                                     MIT
                                                        January 18, 2007


         Specification for the Explicit Control Protocol (XCP)
                       draft-falk-xcp-spec-03.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on July 22, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2007).

Abstract

   This document contains an initial specification for the Explicit
   Control Protocol (XCP), an experimental congestion control protocol.
   XCP is designed to deliver the highest possible end-to-end throughput
   over a broad range of network infrastructure, including links with
   very large bandwidth-delay products, which are not well served by the
   current control algorithms.  XCP is potentially applicable to any



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   transport protocol, although initial testing has applied it to TCP in
   particular.  XCP routers are required to perform a small calculation
   on congestion state carried in each data packet.  XCP routers also
   periodically recalculate the local parameters required to provide
   fairness.  On the other hand, there is no per-flow congestion state
   in XCP routers.

   This version specification (-03) includes protocol changes that move
   per-packet divisions from the router to the sender.


Table of Contents

   1.  Changes Since Last Version . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  XCP Protocol Overview  . . . . . . . . . . . . . . . . . .  5
   3.  The Congestion Header  . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Header placement . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Congestion Header Formats  . . . . . . . . . . . . . . . .  9
     3.3.  IPsec issues . . . . . . . . . . . . . . . . . . . . . . . 12
     3.4.  NAT, middlebox issues  . . . . . . . . . . . . . . . . . . 12
     3.5.  MPLS/Tunneling Issues  . . . . . . . . . . . . . . . . . . 13
   4.  XCP Functions  . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  End-System Functions . . . . . . . . . . . . . . . . . . . 14
       4.1.1.  Sending Packets  . . . . . . . . . . . . . . . . . . . 14
       4.1.2.  Processing Feedback at the Receiver  . . . . . . . . . 16
       4.1.3.  Processing Feedback at the Sender  . . . . . . . . . . 16
     4.2.  Router functions . . . . . . . . . . . . . . . . . . . . . 18
       4.2.1.  Calculations Upon Packet Arrival . . . . . . . . . . . 19
       4.2.2.  Calculations Upon Control Interval Timeout . . . . . . 20
       4.2.3.  Calculations Upon Packet Departure . . . . . . . . . . 22
       4.2.4.  The Control Interval . . . . . . . . . . . . . . . . . 25
       4.2.5.  Obtaining the Persistent Queue . . . . . . . . . . . . 25
   5.  Unresolved Issues  . . . . . . . . . . . . . . . . . . . . . . 28
     5.1.  XCP With Non-XCP Routers . . . . . . . . . . . . . . . . . 28
     5.2.  Variable Rate Links  . . . . . . . . . . . . . . . . . . . 29
     5.3.  XCP as a TCP PEP . . . . . . . . . . . . . . . . . . . . . 29
     5.4.  Sharing resources between XCP and TCP  . . . . . . . . . . 30
     5.5.  A Generalized Router Model . . . . . . . . . . . . . . . . 30
     5.6.  Host back-to-back operation  . . . . . . . . . . . . . . . 30
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 32
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 34
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
   Intellectual Property and Copyright Statements . . . . . . . . . . 39





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1.  Changes Since Last Version

   Changes between version -02 and -03

   o  Re-ordered XCP header fields to allow more cycles for XCP
      calculations in lines 20-22 when packets are streamed 32-bits per
      cycle.  This change improved performance of an experimental FPGA
      implementation.

   o  Incremented version number (to 0x03) to reflect the change in the
      XCP header

   o  Added a subsection on malicious receivers.

   Changes between version -01 and -02

   o  Minor edits and typo fixes.

   Changes between version -00 and -01

   o  Updated protocol to reflect movement of the per-packet division
      from the router to the end-system.

   o  Incremented version number (to 0x02) to reflect change in packet
      header format.

   o  Reordered the Protocol, Length, Version, and Format fields in the
      congestion header (in anticipation of future support of IPv6
      extension headers).

   o  Routers now MUST (from SHOULD) ignore fields other than
      reverse_feedback when minimal header is used.

   o  No longer ignore packets with RTT set to zero.  Senders with
      coarse-grained timer may generate these if the RTT is less than
      the timer precision.

   o  Added 'Open Issues' section on variable-rate links.













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2.  Introduction

   The Van Jacobson congestion control algorithms [Jacobson88] [RFC2581]
   are used by the Internet transport protocols TCP [RFC0793] and SCTP
   [RFC2960].  The Jacobson algorithms are fundamental to stable and
   efficient Internet operation, and they have been highly successful
   over many orders of magnitude of Internet bandwidth and delay.

   However, the Jacobson congestion control algorithms have begun to
   reach their limits.  Gigabit-per-second file transfers, lossy
   wireless links, and high latency connections are all driving current
   TCP congestion control outside of its natural operating regime.  The
   resulting performance problems are of great concern for important
   network applications.

   The original Jacobson algorithm was a purely end-to-end solution,
   requiring no congestion-related state in routers.  More recent
   modifications have backed off from this purity.  Active queue
   management (AQM) in routers (e.g., RED) [RFC2309] improves
   performance by keeping queues small, while Explicit Congestion
   Notification (ECN) [RFC3168] passes one bit of congestion information
   back to senders.  These measures do improve performance, but there is
   a limit to how much can be accomplished without more information from
   routers.  The requirement of extreme scalability together with
   robustness has been a difficult hurdle to accelerating information
   flow.

   This document concerns the Explicit Control Protocol (XCP) developed
   by Dina Katabi of MIT [KHR02].  XCP represents a significant advance
   in Internet congestion control: it extracts congestion information
   directly from routers, without any per-flow state.  XCP should be
   able to deliver the highest possible application performance over a
   broad range of network infrastructure, including extremely high speed
   and very high delay links that are not well served by the current
   control algorithms.  XCP achieves fairness, maximum link utilization,
   and efficient use of bandwidth.  XCP is novel in separating the
   efficiency and fairness policies of congestion control, enabling
   routers to put available capacity to work quickly while
   conservatively managing the allocation of capacity to flows.  XCP is
   potentially applicable to any transport protocol, although initial
   testing has applied it to TCP in particular.

   XCP's scalability is built upon the principle of carrying per-flow
   congestion state in packets.  XCP packets carry a congestion header
   through which the sender requests a desired throughput.  Routers make
   a fair per-flow bandwidth allocation without maintaining any per-flow
   state.  This enables the sender to learn the bottleneck router's
   allocation to a particular flow in a single round trip.



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   The gains of XCP come with some pain.  XCP is more difficult to
   deploy than other proposed Internet congestion control improvements,
   since it requires changes in the routers as well as in end systems.
   It will be necessary to develop and test XCP with real user traffic
   and in real environments, to gain experience with real router and
   host implementations and to collect data on performance.  Providing
   specifications is an important step towards enabling experimentation
   which, in turn, will lead to deployment.  XCP deployment issues will
   be addressed in more detail in a subsequent version of this document.

   This document contains an initial specification of the protocol and
   algorithms used by XCP, as an experimental protocol.  The XCP
   algorithms defined here are based upon Katabi's SIGCOMM paper
   [KHR02], her MIT thesis [Katabi03], and her ns simulation.  However,
   this document includes algorithmic modifications and clarifications
   that have arisen from early experience with implementing and testing
   XCP at the USC Information Sciences Institute.  (See
   http://www.isi.edu/isi-xcp for our project page.)  This document is
   intended to provide a baseline for further engineering and testing of
   XCP.

   This document is organized as follows.  The remainder of Section 2
   provides an overview of the XCP protocol, Section 3 discusses the
   format of the congestion header, Section 4 describes the functions
   occurring in the end-systems and routers, and Section 5 lists some
   unresolved issues.

   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 in [RFC2119].

2.1.  XCP Protocol Overview

   The participants in the XCP protocol include sender hosts, receiver
   hosts, and intermediate nodes in which queuing occurs along the path
   from the sender to the receiver.  The intermediate nodes are
   generally routers, but link-layer switches may also contain packet
   queues.

   XCP supplies feedback from the network to the sender on the maximum
   rate (throughput) for injecting data into the network.  XCP feedback
   is acquired through the use of a congestion header on each packet
   that is sent.  Routers along the path may update the congestion
   header as it moves from the sender to the receiver.  The receiver
   copies the network feedback into outbound packets of the same flow.
   An end-system may function as both a sender and a receiver in the
   case of a bidirectional flow.




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   The figure below illustrates four entities participating in XCP.  The
   sender initializes the congestion header, two routers along the way
   may update it, and the receiver copies the feedback from the network
   into a returning packet in the same flow.

   +----------+        +--------+     +--------+        +----------+
   |          |------->| Router |---->| Router |------->|          |
   |  Sender  |        +--------+     +--------+        | Receiver |
   |          |<----------------------------------------|          |
   +----------+                                         +----------+

   The congestion header contains four pieces of data:


   o  RTT: Set by the sender to its current estimate of the round-trip
      time.


   o  X: Set by the sender to its current estimate of inter-packet time
      gap.  This quantity is used in place of Throughput in earlier
      drafts of these specifications to avoid per-packet division in the
      router.  See Section 4.1.1 for more about X.


   o  Delta_Throughput: Initialized to the amount which the sender would
      like to change (increase or decrease) its throughput, and updated
      by the routers along the path to be the network's allocated change
      in throughput.  This value will be a negative number if an XCP-
      capable queue along the path wants the sender to slow down.


   o  Reverse_Feedback: When a data packet reaches the receiver, its
      Delta_Throughput value is returned to the sender in the
      Reverse_Feedback field of a congestion header of a returning
      packet (e.g., in an ACK packet).


   An XCP-capable router calculates a fair capacity re-allocation for
   each packet.  A flow only receives this re-allocation from a
   particular router if that router is the bottleneck for that flow.
   For XCP, a bottleneck router is defined to be a router that has
   insufficient capacity to accept a flow's current or desired
   throughput.

   Based on current conditions, an XCP-capable router generates positive
   or negative feedback each time a packet arrives and compares it the
   packet's Delta_Throughput field.  Delta_Throughput is reduced if the
   current value exceeds this calculated feedback allocation.  Each XCP-



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   capable router along the path from sender to receiver performs this
   processing.  A packet reaching the receiver therefore contains the
   minimal feedback allocation from the network, i.e., the capacity
   reallocation from the bottleneck router.

   The receiver copies this value into the Reverse_Feedback field of a
   returning packet in the same flow (e.g., an ACK or DATA-ACK for TCP)
   and, in one round-trip, the sender learns the flow's per-packet
   throughput allocation.

   The sender uses the reverse feedback information to adjust its
   allowed sending rate.  For the transport protocol TCP [RFC0793], for
   example, this may be accomplished by adjusting the congestion window,
   or cwnd, that limits the amount of unacknowledged data in the
   network.  (Cwnd is defined for Van Jacobson congestion control in
   [RFC2581].)

   Additionally, it is possible to use XCP's explicit notification of
   the bottleneck capacity allocation for other types of applications.
   For example, XCP may be implemented to support multimedia streams
   over DCCP [I-D.ietf-dccp-spec] or other transport protocols.

   An XCP-capable router maintains two control algorithms on each output
   port: a congestion controller and a fairness controller.  The
   congestion controller is responsible for making maximal use of the
   outbound link while at the same time draining any standing queues.
   The fairness controller is responsible for fairly allocating
   bandwidth to flows sharing the link.  These two algorithms are
   executed only periodically, at an interval known as the "control
   interval".  The algorithms defined below set this interval to the RTT
   averaged across all flows.  Further work on choosing an appropriate
   value for the control interval may be required.

   Each port-specific instance of XCP is independent of every other, and
   references to an "XCP router" should be considered an instance of XCP
   running on a particular output port.

      Actually, it is an oversimplification to say that congestion in
      routers only appears at output ports.  Routers are complex devices
      which may experience resource contention in many forms and
      locations.  Correctly expressing congestion which doesn't occur at
      the router output port is a topic for further study.  Even so, it
      is important to correctly identify where the queue will build up
      in a router.  The XCP algorithm will drain a standing queue;
      however it is necessary to measure that queue in order for correct
      operation.  For more discussion of this issue see Section
      Section 5.5.




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   More context, analysis, and background can be found in [KHR02] and
   [Katabi03].

















































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3.  The Congestion Header

   The congestion control data required for XCP are placed in a new
   header which is called the Congestion Header.

3.1.  Header placement

   The Congestion Header is located between the IP and transport
   headers.  This is consistent with the fact that XCP is neither hop-
   by-hop communication -- as in IP -- nor end-to-end communication --
   as in TCP or UDP -- but is rather end-system-to-network
   communication.  It should allow a router to "easily" locate the
   congestion header on a packet with no IP options.

      Other choices were considered for header location.  For example,
      making the Congestion Header a TCP option was suggested.  This
      made sense as the congestion information is related to the
      transport protocol.  However, it requires that routers be aware of
      the header format for every new transport protocol that might ever
      use XCP.  This seemed like an unreasonable burden to place on the
      routers and would impede deployment of new transport protocols
      and/or XCP.


      It has also been suggested that the Congestion Header be an IPv4-
      style option.  While this proposal is transport protocol
      independent, it would generally force XCP packets to take the slow
      path on non-XCP routers along a path.  This could severely impact
      performance.


   The XCP protocol uses protocol number [TBD], assigned by IANA.  IP
   packets containing XCP headers will use this protocol number in the
   IP header's Protocol field [RFC0791] to indicate to routers and end-
   systems that an XCP congestion header follows the IP header.

3.2.  Congestion Header Formats

   This section defines the XCP Congestion Header formats.  This holds
   for IPv4; the corresponding header for IPv6 is a subject for further
   study.










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   XCP-capable IPv4 packets carry the following Congestion Header:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Protocol    |    Length     |Version|Format |    unused     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               X                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              RTT                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Reverse_Feedback                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Delta_Throughput                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Protocol: 8 bits

      This field indicates the next-level protocol used in the data
      portion of the packet.  The values for various protocols are
      specified by IANA.

   Length: 8 bits

      This field indicates the length of the congestion header, measured
      in bytes.  Length in this version of XCP will always be 20 bytes,
      or 0x14.

   Version: 4 bits

      This field indicates the version of XCP that is in use.  The
      version of XCP described in this document corresponds to a value
      of 0x03.  Future values will be assigned by IANA
      (http://www.iana.org).  See note in the IANA Considerations
      section.

   Format: 4 bits

      This field contains a code to indicate the congestion header
      format.  The current format codes are defined below.











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                        +-----------------+------+
                        |      Format     | Code |
                        +-----------------+------+
                        | Standard Format |  0x1 |
                        |                 |      |
                        |  Minimal Format |  0x2 |
                        +-----------------+------+

                                  Table 1

   Standard Format

      The standard format includes the X, Delta_Throughput, and RTT
      fields shown above.  This format is used by XCP in data packets
      flowing from sender to receiver.

   Minimal Format

      The X, Delta_Throughput, and RTT fields are unused and SHOULD be
      set to zero.  A router MUST not perform any processing on a
      minimal format header.  This format is intended for use in empty
      ACK packets, to return congestion information from receiver to
      sender.

   Other formats may be defined in the future, to define different
   representation formats for the X, Delta_Throughput, and/or RTT
   fields, for example.  The corresponding format values format values
   will be assigned by IANA.  See IANA Considerations section below.

   unused: 8 bits

      This field is unused and MUST be set to zero in this version of
      XCP.

   RTT: 32bits

      This field indicates the round-trip time measured by the sender,
      in fixed point format with 28 bits after the binary point, in
      seconds.  Thus, the value of 1 corresponds to 2^(-28) of a second.
      This field is an unsigned integer.

      The minimum value expressible in this field is 0s.  A value of
      zero in the RTT field is legal and indicates that the sender
      either does not yet know the round-trip time, or operates at a
      coarse-grained timer granularity.  The maximum value expressible
      in this field is 15.9999999963 seconds, in steps of 3.7ns.





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   X: 32 bits

      This field indicates the inter-packet time of the flow as
      calculated by the sender, in fixed point format with 28 bits after
      the binary point, in seconds.  This is the same format as used by
      the RTT field.

   Delta_Throughput: 32 bits

      This field indicates the desired or allocated change in
      throughput.  It is set by the sender to indicate the amount by
      which the sender would like to adjust its throughput, and it may
      be subsequently reduced by routers along the path (See
      Section 4.2).  It is measured in bytes per second and is a signed,
      2's complement value.

      The minimum throughput change expressible in this field is -17
      Gbps.  The maximum value expressible in this field is 17 Gbps, in
      steps of 8 bits per second.

   Reverse_Feedback: 32bits

      This field indicates the value of Delta_Throughput received by the
      data receiver.  The receiver copies the field Delta_Throughput
      into the Reverse_Feedback field of the next outgoing packet in the
      same flow.  See Section 4.1.2.

3.3.  IPsec issues

   IPsec [RFC2401] must be slightly modified to accommodate use of XCP.
   The specifications for the IP Authenticated Header (AH) [RFC2402] and
   IP Encapsulating Security Payload (ESP) [RFC2406] state that the
   IPsec headers immediately follow the IP header.  This would be a
   problem for XCP in that a) it would make the XCP headers harder to
   find by the routers, b) ESP encryption would make it impossible for
   routers along the path to read and write congestion header
   information and c) AH authentication would fail if any router along
   the path had modified a congestion header.  Therefore, the XCP
   congestion header should immediately follow the IP header and precede
   any AH.

3.4.  NAT, middlebox issues

   Middleboxes that attempt to perform actions invisibly on flows must
   preserve the congestion header.  Middleboxes that terminate the TCP
   connection should terminate the XCP connection.  Middleboxes that
   insert queues into the forwarding path should participate in XCP.




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3.5.  MPLS/Tunneling Issues

   When a flow enters an IP tunnel [RFC2003], IPsec ESP tunnel
   [RFC2406], or MPLS [RFC3031], network ingress point, the congestion
   header should be replicated on the "front" of the outer IP header.
   For example, when a packet enters an IP tunnel, the following
   transformation should occur:

                [IP2]  \_ outer header
         __-->  [XCP]  /
   [IP1]/       [IP1]  \
   [XCP] --->   [XCP]   |_ inner header
   [TCP]        [TCP]   |
   ...          ...    /

   Here the XCP header appended to the front of the outer header is
   copied from the inner header, with the appropriate change to the
   Protocol field to indicate that the next protocol is IP.

   When the packet exits the tunnel, the congestion header, which may
   have been modified by routers along the tunneled path, is copied from
   the outer header into the inner header.





























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4.  XCP Functions

   XCP is concerned with the sender and receiver end-systems and the
   routers along the packet's path.  This section describes the XCP-
   related algorithms to be implemented in each of these entities.  The
   specific case of TCP as transport protocol is also described.  The
   emphasis in this section is on explicit and detailed definition of
   the XCP algorithms.  The theoretical derivation and analysis of the
   algorithms can found in [Katabi03].

4.1.  End-System Functions

4.1.1.  Sending Packets

   The sender is responsible for maintaining five parameters, or their
   equivalent: (1) a desired throughput value, (2) a current estimate of
   the actual throughput, (3) the maximum throughput allowed by XCP, (4)
   a current estimate of inter-packet time, denoted X, and (5) a current
   estimate of the round-trip time

   A sender may choose to use any reasonable value, i.e., any achievable
   value, for desired throughput.  An application may supply this value
   via an API, or it might be the speed of the local interface.

   When sending a packet, the sender fills in the fields of the
   congestion header as follows:


   o  The sender sets the RTT field to a scaled smoothed round-trip time
      estimate, or to zero if the round-trip time is not yet known.


   o  The sender sets the X field to the current inter-packet time
      estimate, or to zero if an estimate is not yet available.  Packets
      carrying zero X field can receive negative feedback, but not
      positive.  Since XCP requires only a single round trip for a flow
      to gain an estimate of RTT, this is expected to have negligible
      effect.  The value of X may be estimated as the smoothed round-
      trip time estimate divided by the number of outstanding packets
      (or congestion window size in packets, for window-based
      protocols).  Alternatively, it may be derived from the ratio of
      the packet size to the current throughput estimate.  Using
      instantaneous RTT estimates in the calculation of X may yield
      better results than using the smoothed RTT, especially for senders
      with coarse-grained timers, i.e., timers with precision less than
      the RTT.  This is a subject for further study.  Also,see Section
      Section 4.1.3.3 for a discussion of RTT estimation.




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   o  The sender calculates a desired change (typically, an increase) in
      throughput.  This is normally the difference between the current
      estimated throughput and the desired throughput.  However, if the
      sender does not have sufficient data to send at the current
      allowed throughput, the desired change in throughput SHOULD be
      zero.


   o  The sender then divides the desired throughput change by the
      number of packets in one round-trip time, and puts the result in
      the Delta_Throughput field of the Congestion Header.  This per-
      packet distribution of the throughput change is necessary because
      an XCP router does not maintain per-flow congestion state; it must
      treat each packet independently of others in the same flow.  The
      number of packets in an RTT may be estimated by the product of the
      current throughput and the RTT, divided by the Maximum Segment
      Size (MSS).


      The Delta_Throughput (in bytes/second) can be calculated as:

                              desired_throughput - Throughput
           Delta_Throughput = -----------------------------------
                                      Throughput * ( RTT/MSS )

           where:
                 desired_throughput is measured in bytes/second
                 Throughput is measured in bytes/second
                 RTT is measured in seconds
                 MSS is measured in bytes

      However, Delta_Throughput should be set to zero if, for any
      reason, no additional capacity is needed, e.g., there is
      insufficient data to maintain Throughput for the next RTT, as
      discussed above.

   o  An issue for future consideration is how to treat the case when
      Delta_Throughput is calculated to be < 1 Bps. Since an integer
      representation is passed in the Congestion Header, the result will
      appear as zero.  It would be possible to send a fraction of the
      packets in a round trip time with non-zero Delta_Throughput
      values.

   o  For TCP, the throughput estimate can be obtained by dividing the
      congestion window cwnd (in bytes) by RTT (in seconds), for
      example.  Alternatively, it could be measured.  The ratio cwnd/RTT
      differs from true throughput in two respects.  First, cwnd doesn't
      account for header size.  This may become significant should XCP



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      be applied to real-time flows that send large numbers of small
      packets, but it is probably not much worry for TCP flows that tend
      to use the largest possible packet size.  Second, cwnd represents
      permission for the sender to transmit data.  If the application
      doesn't use all of the available cwnd, the advertised throughput
      will be larger than the true throughput.  This may result in an
      XCP router creating an unfair allocation of negative feedback to a
      flow.


4.1.2.  Processing Feedback at the Receiver

   An XCP receiver is responsible for copying the Delta_Throughput data
   it sees on arriving packets into the Reverse_Feedback field of
   outgoing packets.  In TCP, outgoing packets would normally be ACK-
   only segments.

   In some cases returning packets are sent less frequently than
   arriving packets, e.g., with delayed acknowledgments [RFC1122].  The
   receiver is responsible for calculating the sum of the arriving
   Delta_Throughput fields for placement in outgoing Reverse_Feedback
   fields.

4.1.2.1.  Feedback from the Receiver

   The receiver end-system returns XCP congestion feedback from the
   network to the sender, by copying the Delta_Throughput information
   from arriving packets into the Reverse_Feedback field of Congestion
   Headers in outgoing packets (possibly aggregating data as described
   in Section 4.1.2).

   It is possible that even empty ACK packets may create or encounter
   congestion in the reverse direction.  Although TCP implementations
   generally do not perform congestion-based pacing of empty ACK
   segments, some transport protocols (e.g., DCCP) may be.  Such a
   transport protocol may choose to use XCP congestion control on the
   returning ACKs as well as on the data.

   In the normal case of a unidirectional data flow with XCP applied
   only to that data flow, the feedback can be sent in a Minimal format
   Congestion Header, in which the RTT, X, and Delta_Throughput fields
   are set to zero.

4.1.3.  Processing Feedback at the Sender

   When packets arrive back to the sender carrying reverse feedback, the
   sender must adjust its sending rate accordingly.




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   As noted earlier, this throughput adjustment may be made in TCP by
   updating the sender's congestion window, cwnd.  This should use the
   formula:

           cwnd = max(cwnd + feedback * RTT, MSS)

           where:
               cwnd     = current congestion window (bytes)
               feedback = Reverse_Feedback field from received packet,
                          (bytes/sec, may be +/-)
               RTT      = Sender's current round-trip time estimate
                          (seconds)
               MSS      = maximum segment size (bytes)

   The value of cwnd has a minimum of MSS to avoid the "Silly Window
   Syndrome" [RFC0813].

4.1.3.1.  Aging the Allowed Throughput

   When a sending application does not send data fast enough to fully
   utilize the allowed throughput, XCP should reduce the allowed
   throughput as time passes, to avoid sudden bursts of data into the
   network if the application starts to send data later.

   We present a slight modification of the algorithm for aging the
   allowed throughput below.  It is based on Section 4.5 of [Katabi03].
   Each RTT in which the sender sends with actual throughput which is
   less than the allowed throughput, the allowed throughput MUST be
   reduced by the following exponential averaging formula:

           Allowed_Throughput = Allowed_Throughput*(1-p) +
                                Actual_Throughput * p

               where: p is a parameter controlling the speed of aging,
                      ranged between 0 and 1.

   Using p = 0.5 is suggested.  Consideration of values of p or other
   algorithms is a research topic.

4.1.3.2.  Response to Packet Loss

   When the transport protocol is TCP, a packet drop or detection of an
   ECN notification [RFC3168] should trigger a transition to standard
   TCP congestion control behavior[RFC2581].  In other words, cwnd
   should be halved and Jacobson's fast retransmission/fast recovery,
   slow start, and congestion avoidance algorithms should be applied for
   the remainder of the connection or until the congestion event is
   known to have passed (see Section 5.1 for discussion of alternative



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   approaches to this issue.  The assumption is that the packet drop
   reveals the presence of a congested non-XCP router in the path.
   Transitioning to standard TCP behavior is a conservative response.

   Note also the following:


   o  The change in congestion control algorithm should be delayed until
      the three DUPACKs have arrived, according to the Fast
      Retransmission/Fast Recovery algorithm[RFC2581].


   o  Once the change to standard TCP congestion control has occurred,
      cwnd should be managed using the RFC2581 algorithm.


   o  The X field in outgoing packets should continue to reflect the
      current inter-packet time.  This allows the XCP processes in the
      routers along the path to continue to monitor the flow's
      utilization.


   o  Further study is needed to determine whether it will be possible
      to return a connection to XCP congestion control, once it has
      transitioned to Van Jacobson mode.


   o  For transport protocols other than TCP, the response to a packet
      loss or ECN notification is a subject for further study.

4.1.3.3.  RTT Estimates

   Having a good estimate of the round trip time is more important in
   XCP than in Van Jacobson congestion control.  There is evidence that
   small errors in the RTT estimate can result in larger errors in the
   throughput and X estimates.  The current cwnd divided by SRTT is only
   an approximation of the actual throughput.  Likewise, SRTT divided by
   cwnd in packets is only an approximation of the highly variable
   inter-packet time, X. The RTT used in the ns-2 code in [KHR02] used a
   smoothed floating-point RTT estimator, rather than instantaneous
   measurements.  Additional research is needed to develop
   recommendations for RTT estimation.

4.2.  Router functions

   The router calculations for XCP are divided into those that occur
   upon packet arrival, those that occur upon control interval timeout,
   those that occur upon packet departure, and the assessment of the



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   persistent queue, which uses a separate timer.  The calculations are
   presented in the following sections as annotated pseudo-code.

4.2.1.  Calculations Upon Packet Arrival

   When a packet arrives at a router, several parameters used by XCP
   need to be updated.  The steps are described in the following pseudo-
   code.

   ========================================================
   On packet arrival do:

   1. input_traffic += Pkt_size

   2. sum_x += X

   3. if (Rtt < MAX_INTERVAL) then

   4.   sum_xrtt += X * Rtt

   5. else

   6.   sum_xrtt += X * MAX_INTERVAL
   ========================================================

   Line 1: The variable input_traffic accumulates the volume of data
      that have arrived during a control interval.  When a packet
      arrives, the packet size is taken from the IP header and is added
      to the ongoing count.


   Line 2: The variable sum_x is used in the control interval
      calculation (see equation 4.2 of [Katabi03]) and in capacity
      allocation.  For each packet, values of X from the XCP header is
      accumulated.  It is recommended that sum_x is stored in a 64-bit
      unsigned integer variable.


   Lines 3 and 5: A test is performed to check whether the round trip
      time of the flow exceeds the maximum allowable control interval.
      If so, MAX_INTERVAL, the maximum allowable control interval, is
      used in the subsequent calculations.  Too large a control interval
      will delay new flows from acquiring their fair allocation of
      capacity.  See Section 4.2.4 for a discussion of the recommended
      value for MAX_INTERVAL.






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   Lines 4 and 6: As in Line 2, the variable sum_xrtt is used in the
      control interval calculation.  It is recommended that it is stored
      in a 96-bit unsigned variable.


4.2.2.  Calculations Upon Control Interval Timeout

   When the control timer expires, several variables need to be updated
   as shown below.

   Note that several calculations show divisions.  These divisions
   should either be accomplished using floating-point arithmetic or
   integer arithmetic and appropriate scaling to avoid over- or under-
   flow.

   ======================================================== On
   estimation-control timeout do:

    7. avg_rtt = sum_xrtt / sum_x

    8. input_bw = input_traffic / ctl_interval

    9. F = a * (capacity - input_bw) - b * queue / avg_rtt

   10. shuffled_traffic = shuffle_function(...)

   11. residue_pos_fbk = shuffled_traffic + max(F,0)

   12. residue_neg_fbk = shuffled_traffic + max(-F,0)

   13. Cp = residue_pos_fbk / sum_x

   14. Cn = residue_neg_fbk / input_traffic

   15. input_traffic = 0

   16. sum_x = 0

   17. sum_xrtt = 0

   18. ctl_interval = max(avg_rtt, MIN_INTERVAL)

   19. timer.reschedule(ctl_interval)
   ========================================================







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   Line 7: Update avg_rtt by taking the ratio of the two sums
      accumulated in the previous section.  This value is used to
      determine the control interval (line 17).


   Line 8: The average bandwidth of arriving traffic is calculated by
      dividing the bytes received in the previous control interval by
      the duration of the previous control interval.


   Line 9: The aggregate feedback, F, is calculated.  The variable
      'capacity' is the ability of the outbound link to carry IP
      packets, in bytes/second.  The variable 'avg_rtt' was calculated
      in line 7.  The variable 'queue' is the persistent queue and is
      defined in section Section 4.2.5.  The values a and b are constant
      parameters.  According to [Katabi03], the constant a may be any
      positive number such that a < (pi/4*sqrt(2)).  A nominal value of
      0.4 is recommended.  The constant b is defined to be b =
      a^2*sqrt(2).  (If the nominal value of a is used, the value for b
      would be 0.226.)  Note that F may be positive or negative.


   Line 10: This line establishes the amount of capacity that will be
      shuffled in the next control interval through the use of the
      shuffle_function.  Shuffling takes a small amount of the available
      capacity and redistributes it by adding it to both the positive
      and negative feedback pools.  This allows new flows to acquire
      capacity in a full loaded system.


      The recommended shuffle_function is as follows:


                    shuffled_traffic = max(0, 0.1 * input_bw - |F|)


      The variable 'input_bw' is defined above in Line 8.  Implementers
      may choose other functions.  It is important to consider that more
      shuffled traffic decreases the time for new flows to acquire
      capacity and converge to fairness.  However, too much shuffling
      may impede flows from acquiring their fair share of available
      capacity.  (For example, consider a setup of N flows bottlenecked
      downstream from the given router and another flow, not limited as
      those, trying to acquire its fair share.  In this case shuffling
      leads to under-utilization of the available bandwidth and impedes
      the unlimited flow.)  Shuffled_traffic is always a positive value.

   The objective of the feedback calculations is to obtain a per-packet



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   feedback allocation from the router.  Lines 13 and 14 obtain factors
   in this calculation that, unfortunately, have no physical meaning.
   One might view them as per-flow capacity allocations that have some
   additional processing to prepare them for per-packet allocation.
   Note that, with the use of shuffled_traffic, a non-idle router will
   always start a control interval with non-zero values for both Cn and
   Cp.

   Line 11: The variable 'residue_pos_fbk' keeps track of the pool of
      available positive capacity a router has to allocate.  It is
      initialized to the positive aggregate feedback.


   Line 12: The variable 'residue_neg_fbk' keeps track of the pool of
      available negative capacity a router has to allocate.  It is
      initialized to the negative aggregate feedback.  This variable is
      always positive.


   Line 13: This line calculates the positive feedback scale factor, Cp.
      The variables residue_pos_fbk, and sum_x are defined above.


   Line 14: This line calculates the negative feedback scale factor, Cn.
      This is a positive value.  The definitions for residue_neg_fbk,
      and input_traffic are given above.


   Line 15-17: Reset various counters for the next control interval.


   Line 18: Set the next control interval.  The use of MIN_INTERVAL is
      important to establish a reasonable control interval when the
      router is idle.


   Line 19: Set timer.

4.2.3.  Calculations Upon Packet Departure

   An XCP router processes each packet using the feedback parameters
   calculated above.  As stated earlier, each packet indicates the
   current inter-packet time (X) and a throughput adjustment,
   Delta_Throughput.  The router calculates a per-packet capacity change
   which will be compared to the Delta_Throughput field in the packet
   header.  Using the AIMD rule, positive feedback is applied equally
   per-flow, while negative feedback is made proportional to each flow's
   capacity.



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   To accommodate high-speed routers, XCP uses a fixed-point numeric
   representation for the Congestion Header fields.  This means that the
   per-packet calculations defined below result in residual error that
   is less than 1 Bps per packet.  These errors accumulate across all
   the packets in a control interval, resulting in an inaccuracy in
   XCP's allocation of available bandwidth to flows.  Further work is
   needed to understand whether this will be a significant problem and,
   if so, whether there is any solution short of using 64 bit precision
   or floating point.

   Processing should be done according to the pseudo-code below.

   ========================================================
   On packet departure:

   20. pos_fbk = Cp * X

   21. neg_fbk = Cn * Pkt_size

   22. feedback = pos_fbk - neg_fbk

   23. if(Delta_Throughput > feedback) then

   24.   Delta_Throughput = feedback

   25. else

   26.   neg_fbk = min(residue_neg_fbk, neg_fbk +
                            (feedback - Delta_Throughput))

   27.   pos_fbk = Delta_Throughput + neg_fbk

   28. residue_pos_fbk = max(0, residue_pos_fbk - pos_fbk)

   29. residue_neg_fbk = max(0, residue_neg_fbk - neg_fbk)

   30. if (residue_pos_fbk <= 0) then Cp = 0

   31. if (residue_neg_fbk <= 0) then Cn = 0

   ========================================================

   Line 20: The contribution of positive feedback for the current packet
      is calculated using Cp, defined in line 13, and X (the flow's
      advertised inter-packet time) from the Congestion Header.  Note
      that if Cp (and Cn in Line 21) is implemented as a floating point
      number, this calculation would be implemented by multiplying the
      Cp-mantissa by the value of X, then shifting the result by the



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      amount of the Cp-exponent.


   Line 21: The contribution of negative feedback for the current packet
      is calculated using Cn, defined in line 14, and Pkt_size from the
      IP header.  This value of neg_fbk is positive.


   Line 22: The router's allocated feedback for the packet is the
      positive per-packet feedback minus the negative per-packet
      feedback.  This value may be positive or negative.


   Line 23-24: Line 23 tests whether the packet is requesting greater
      capacity increase (via the packet's Delta_Throughput field) than
      the router has allocated.  If so, this means the the sender's
      desired throughput needs to be reduced to be the router's
      allocation.  In line 24 the Delta_Throughput field in the packet
      header updated with the router feedback allocation.


   Line 25: This branch is executed when the packet is requesting a
      smaller throughput increase than the router's allocation.  In this
      branch, and the rest of this pseudo-code, the packet header is not
      updated and the remaining code is to correctly update the feedback
      pool variables.


   Line 26: In this line, the packet's negative feedback contribution,
      neg_fbk, is set to be the smaller of two terms.  The first term,
      residue_neg_fbk, is the pool of negative feedback, i.e., this
      drains the remaining negative feedback in the pool.  The second
      term increases the nominal negative feedback from the router by
      the amount which the Delta_Throughput is less than net router
      allocation.  This allows the router to capture feedback which is
      allocated by an upstream bottleneck.


   Line 27: The positive allocation, pos_fbk, is adjusted to be the sum
      of Delta_Throughput and neg_fbk, from Line 26.  This is required
      for the sum of pos_fbk and neg_fbk to equal Delta_Throughput.


   Line 28-29: In these two lines, the feedback pools, residue_pos_fbk
      and residue_neg_fbk, are reduced by the values of pos_fbk and
      neg_fbk accordingly, but prevented from going negative.





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   Line 30-31: When a feedback pool becomes empty, set the scale factor
      to zero, i.e., stop handing out associated feedback.


4.2.4.  The Control Interval

   The capacity allocation algorithm in XCP router updates several
   parameters every Control Interval.  The Control Interval is currently
   defined to be the average RTT of the flows passing through the
   router, i.e., avg_rtt calculated in Line 7 above.  Other possible
   choices for the control interval are under study.

   Notes on avg_rtt:

   o  In this document, the quantity 'avg_rtt' refers to the last
      calculated value.  In other words, the avg_rtt calculated based on
      packets arriving in the previous control interval.


   o  The avg_rtt calculation should ignore packets with an RTT of zero
      in the header.


   o  avg_rtt MUST have a minimum value.  This is to allow flows to
      acquire bandwidth from a previously idle router.  The default
      minimum value, MIN_INTERVAL, should be max(5-10ms, propagation
      delay on attached link).


   o  avg_rtt MUST have a maximum value.  The default maximum value,
      MAX_INTERVAL, should be max(0.5-1 sec, propagation delay on
      attached link).


4.2.5.  Obtaining the Persistent Queue

   In Section 4.2.2 the variable 'queue' contains the persistent queue
   over the control interval.  This is intended to be the minimum
   standing queue over the queue estimation interval.

   The following pseudo-code describes how to obtain the minimum
   persistent queue:









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   ========================================================
   On packet departure do:

   32. min_queue = min(min_queue, inst_queue)

   ========================================================

   When the queue-computation timer expires do:

   33. queue = min_queue

   34. min_queue = inst_queue

   35. Tq = max(ALLOWED_QUEUE, (avg_rtt - inst_queue/capacity)/2)

   36. queue_timer.reschedule(Tq)

   ========================================================

   Line 32: The current instantaneous queue length is checked each time
      a packet departs compute the minimum queue size.

   If avg_rtt is being used as the Control Interval, it MUST NOT be used
   as the interval for measuring the minimum persistent queue.  Doing so
   can result in a feed-forward loop.  For example, if a queue develops
   the average RTT will increase.  If the avg_rtt increases, it takes
   longer to react to the growing queue and the queue gets larger,
   leading to instability.

   Line 33: Upon expiration of the queue estimation timer, Tq, the
      variable queue, the persistent queue, is set to be the minimum
      queue occupancy over the last Tq.


   Line 34: Upon expiration of the queue estimation timer, reset the
      running estimate of the minimum queue to be the current queue
      occupancy.


   Line 35: The first term in the max function, ALLOWED_QUEUE, is the
      time to drain a standing queue that you are willing to tolerate.
      (A nominal value of 2ms worth of queuing is recommended but this
      may be tuned by implementers.)  The second term is an estimate of
      the propagation delay.  In other words the persistent queue is a
      queue that does not drain in a propagation delay. the division by
      2 is a conservative factor to avoid overestimating the propagation
      delay.




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   Line 36: The queue computation timer is set.


















































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5.  Unresolved Issues

   XCP is a work-in-progress.  This section describes some known issues
   that need to be resolved.

5.1.  XCP With Non-XCP Routers

   Obviously, non-XCP routers will exist in networks before XCP becomes
   ubiquitously deployed and we expect other non-XCP systems to continue
   to be in the network indefinitely.  Long term non-XCP network
   elements include any sort of link-level switches with queuing, e.g.
   ATM switches and sophisticated Ethernet switches.  Even simple
   multiplexers are non-XCP queues with very little buffering.

   Sources and the network care about these non-XCP elements because any
   one of them can be a site of network congestion, and if an XCP
   endpoint is bottlenecked at one of these non-XCP elements, no router
   feedback will inform the endpoint to slow down.  If nothing is done,
   such an element will probably collapse under congestion.

   Although exactly how XCP sources will operate in this environment is
   an open issue, a current promising direction is for endpoints to run
   a traditional end-to-end congestion detection algorithm in parallel
   with the XCP algorithm and switch over to using that algorithm for
   control when congestion is detected that XCP is not controlling.  For
   example, an XCP source that detects 3 duplicate acknowledgments would
   fall back to TCP Reno behavior.

   An endpoint that is limited by its end-to-end congestion algorithm
   would indicate so to XCP routers by setting a bit in the packet
   header.  A router may process such packets differently than packets
   from endpoints that are being controlled by XCP.  For example, the
   router might allocate end-to-end controlled packets less feedback or
   not reduce its feedback pools by the full amount when assigning
   feedback to those packets.

   Though using its end-to-end algorithm to control its sending rate, an
   endpoint will also monitor the XCP feedback and if the source
   discovers that the XCP feedback would be more restrictive than the
   end-to-end control over a round trip time, the endpoint will revert
   to following XCP feedback.  XCP feedback that is more restrictive
   over a round trip time is an indication that the endpoint's
   bottleneck is once again at an XCP router and the endpoint should
   take advantage of the more precise XCP information.

   Evaluation of these algorithms is ongoing work





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5.2.  Variable Rate Links

   As discussed in [Zhang05], XCP may perform poorly over shared links.
   When a link is shared, such as in CSMA ethernet or 802.11 wireless
   networks, a single queue's drain rate is often a function of the load
   in the shared medium.  So, using a constant value for the variable
   'capacity' in the routing control algorithm may not work well.  For
   correct operation, the XCP router's notion of capacity needs to
   reflect how the link capacity is shared.

5.3.  XCP as a TCP PEP

   In the Internet today TCP performance-enhancing proxies (PEPs) are
   sometimes used to improve application performance over certain
   networks.  TCP PEPs, and the issues surrounding their use are
   described in [RFC3135].  A common mechanism used in TCP PEPs is to
   split a TCP connection into three parts where the first and last run
   TCP and a more aggressive transport protocol is run in the middle
   (across the path which generates poor TCP performance).  This
   improves performance over the "problematic" portion of the path and
   does not require changing the protocol stacks on the end systems.
   For example, if a high-speed satellite link was used to connect a LAN
   to the Internet, a TCP PEP may be placed on either side of the
   satellite link.

   It is not unusual today to find TCP PEPs which, to get high data
   rates, do not use congestion control at all.  Of course, this limits
   the environments in which they can be used.  However, XCP may be used
   in between two TCP PEPs to get high transfer rates and still respond
   to congestion in a correct and scalable way.

   Work on using XCP as a TCP PEP is just beginning [Kapoor05].
   Objectives for such a mechanism would be:

   o  preserve end-to-end TCP semantics as much as possible

   o  enable some form of discovery for one PEP to determine that
      another PEP was in the path

   o  allow for recovery should one half of a PEP pair fail or the route
      change so that one or both PEPs are not on the path

   o  enable aggregation of multiple flows between two PEPs.

   A system which met the above objectives could also be used for
   incremental deployment of XCP.  A network operator could deploy XCP-
   capable routers and use PEPs at the periphery of the network to
   convert from traditional TCP to XCP congestion control.  This may



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   result in smaller queues and improved link utilization within the XCP
   network.  (This may be of more interest to wireless network providers
   than to over-provisioned fiber backbones.)

5.4.  Sharing resources between XCP and TCP

   Katabi describes a system for sharing router bandwidth between XCP
   and TCP flows that is based on sharing the output capacity based on
   the average throughputs of XCP and TCP sources using the
   router.[Katabi03] Two queues are installed at the router and they are
   served with different weights by the forwarding engine.  The
   algorithm is work-conserving; the forwarding engine is never idle
   when either queue has packets in it.

   A TFRC-like algorithm estimates the average congestion window of TCP
   sources and the XCP algorithm estimates the average throughput of XCP
   sources.  These averages are used to dynamically weight the time the
   processor spends on each queue.  Initial experiments indicate that
   this system can provide fairness between the two flow classes (XCP/
   TCP).[Katabi03]

   Open issues remain, however; for example there are questions about
   how well the TFRC-like algorithm can estimate TCP throughput with
   only access to local drop rates, convergence time of the weighting
   algorithm has never been explored, and no system for buffer
   allocation to complement the link capacity allocation has been put
   forward.  These open issues are under study.

5.5.  A Generalized Router Model

   The XCP algorithm described here and in [Katabi03] manages congestion
   at a single point in a router, most likely an output queue.  However,
   resource contention can occur at many points in a router.  Input
   queues, backplanes, computational resources can 'congest' in addition
   to output buffers.  There is a need to develop a general model and a
   variety of mechanisms to identify and manage resource contention
   throughout the router.

5.6.  Host back-to-back operation

   XCP hosts should be capable of back-to-back operation, i.e., with no
   router in the path.  Nominally, this should not be a problem.  A
   sender initializes delta_throughput to the desired value, no router
   modifies it and, thus, it is automatically granted.  However, it has
   not yet been decided whether an XCP receiver should be capable of (or
   require) adjusting the delta_throughput to request flow control from
   the receiver to the sender.




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   At this point, XCP offers no mechanism for flow control.  (Open
   question: Should it?)  It is believed that running XCP on the output
   queue of a host would solve this problem.  However, it isn't clear
   that the complexity is justified by the need to solve this situation.















































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6.  Security Considerations

   The presence of a header which may be read and written by entities
   not participating in the end-to-end communication opens some
   potential security vulnerabilities.  This section describes them and
   tries to give enough context so that users can understand the risks.

   Man-in-the-Middle Attacks

      There is a man-in-the-middle attack where a malicious user can
      force a sender to stop sending by inserting negative feedback into
      flow.  This is little different from a malicious user discarding
      packets belonging to a flow using VJ congestion control or setting
      ECN bits.  One question worth investigating further is whether the
      XCP attack is harder to diagnose.


   Covert Data Channels

      IPsec needs to be modified, as discussed in Section 3.3, to allow
      routers to read the entire congestion header and write the
      delta_feedback field.  This could become a covert data-channel,
      i.e., a way in which an end-system can make data viewable to
      observers in the network, on a compromised end-system.


   Malicious Sources

      The XCP algorithms rely on senders to advertise information about
      their current RTT and X and correctly respond to feedback
      delivered from the network.  Naturally, the possibility occurs
      that a sender won't perform these functions correctly.  Chapter 7
      of [Katabi03] and [Katabi04] examine these issues.


      A source which lies about its values of X and hence throughput
      cannot affect the link utilization and, in the worst case, can
      unfairly acquire capacity.  However, this is equivalent to a
      sender opening up multiple TCP flows.  So, there is an incentive
      to lie about X. However, because X is explicitly stated in each
      packet header, it is a simpler matter to police it at the edge of
      the network than, say, for TCP.


      A source which lies about its RTT can disrupt the router control
      algorithm, particularly when a large number of sources lie about
      their RTT and the router control interval is adaptive and uses the
      average RTT.  However, there is little incentive to lie as it will



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      not affect the fair allocation of capacity and the liar will
      experience the same degradation as the non-lying flows.  Lying
      about RTT should be considered a weak denial-of-service attack.


      A flow may also ignore negative feedback from the router.  Such a
      flow can obtain unfair throughput in a congested router.  However,
      as with lying, the explicit nature of XCP makes it possible to
      verify that flows are responding to feedback.  For example, a
      policing function in the path (presumably near the edge so that
      the load is manageable and it can be expected to see packet flow
      in both directions) may inspect congestion headers for a flow in
      both directions.  If the policer sees negative feedback heading
      towards a source and no reduction in throughput it may, e.g.,
      punish the flow by severely restricting the throughput.  Note that
      this can be applied on a probabilistic basis, sampling flows only
      occasionally.


   Malicious Receivers

      An XCP receiver is required to return XCP congestion feedback
      unmodified to the sender.  It is possible for a receiver to lie
      about the feedback by sending a greater value than XCP actually
      allocated thus causing the flow to acquire an unfair share of
      bandwidth.  It is worth noting that a similar problem exists in
      TCP [Sherwood05] whereby TCP receivers can ACK data before it
      arrives (optimistic ACKing), however its technical implementation
      in XCP is much easier.

      The explicit nature of XCP though makes it possible to verify that
      the receiver does a truthful job.  For example, a policer at the
      edge can monitor the feedback going in both directions and may
      punish flows whose feedback values have been tampered with;
      another possibility is for a bottleneck router to punish packets
      that report an unfairly large throughput.















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7.  IANA Considerations

   XCP requires the assignment of an IP protocol number.  Once this
   value has been assigned, the number may be inserted (by the RFC
   Editor) into Section 3.1 and this paragraph may be removed prior to
   publication.













































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8.  Acknowledgements

   The authors would like to acknowledge the many contributors who have
   assisted in this work.  Bob Braden applied his usual sage guidance to
   the project and to the spec, in particular.  Ted Faber wrote the
   initial implementation framework and provided much wisdom on kernel
   development and congestion control.  John Wroclawski advised on
   project priorities and strategy.  Eric Coe developed the initial
   implementation and testbed.  Aman Kapoor performed supporting
   simulations and debugged kernel code.  Padma Haldar ported ns-2
   simulation code to ns-2 distribution.  Jasmeet Bagga and Anuraag
   Mittal conducted simulations on various aspects of XCP performance.
   On the XCP mailing list, Tim Shepherd, Tom Henderson, and Matt Mathis
   made valuable contributions to the effort.  To all the above go our
   sincere thanks.

9.  References

   [I-D.ietf-dccp-spec]
              Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
              draft-ietf-dccp-spec-04 (work in progress), July 2003.

   [Jacobson88]
              Jacobson, V., "Congestion Avoidance and Control", ACM
              Computer Communication Review Proceedings of the Sigcomm
              '88 Symposium, August 1988.

   [KHR02]    Katabi, D., Handley, M., and C. Rohr, "Internet Congestion
              Control for Future High Bandwidth-Delay Product
              Environments", ACM Computer Communication
              Review Proceedings of the Sigcomm '02 Symposium,
              August 2002.

   [Kapoor05]
              Kapoor, A., Falk, A., Faber, T., and Y. Pryadkin,
              "Achieving Faster Access to Satellite Link Bandwidth",
              IEEE 8th IEEE Global Internet Symposium, Miami, FL, March
              2005, 2005.

   [Katabi03]
              Katabi, D., "Decoupling Congestion Control and Bandwidth
              Allocation Policy With Application to High Bandwidth-Delay
              Product Networks", MIT PhD. Thesis, March 2003.

   [Katabi04]
              Katabi, D., "XCP's Performance in the Presence of
              Malicious Flows", Second International Workshop on
              Protocols for Fast Long-Distance Networks, Presentation,



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              February 2004.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC0813]  Clark, D., "Window and Acknowledgement Strategy in TCP",
              RFC 813, July 1982.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, April 1998.

   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

   [RFC2402]  Kent, S. and R. Atkinson, "IP Authentication Header",
              RFC 2402, November 1998.

   [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.

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

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.



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              Shelby, "Performance Enhancing Proxies Intended to
              Mitigate Link-Related Degradations", RFC 3135, June 2001.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [Sherwood05]
              Sherwood, R., Bhattacharjee, B., and R. Braud,
              "Misbehaving TCP receivers can cause internet-wide
              congestion collapse", ACM Proceedings of the 12th ACM
              Conference on Computer and Communications Security (CCS
              2005) pp. 383-392, Alexandria, VA, 2005.

   [Zhang05]  Zhang, Y. and T. Henderson, "An Implementation and
              Experimental Study of the eXplicit Control Protocol
              (XCP)", IEEE Proceedings of the 24th IEEE International
              Conference on Computer Communications (INFOCOM 2005), pp.
              1037-1048, Miami, Florida, Mar 2005., 2005.
































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Authors' Addresses

   Aaron Falk
   USC Information Sciences Institute
   4676 Admiralty Way
   Suite 1001
   Marina Del Rey, CA  90292

   Phone: 310-448-9327
   Email: falk@isi.edu
   URI:   http://www.isi.edu/~falk


   Yuri Pryadkin
   USC Information Sciences Institute
   4676 Admiralty Way
   Suite 1001
   Marina Del Rey, CA  90292

   Phone: 310-448-8417
   Email: yuri@isi.edu


   Dina Katabi
   Massachusetts Institute of Technology
   200 Technology Square
   Cambridge, MA  02139

   Phone: 617-324-6027
   Email: dk@mit.edu
   URI:   http://www.ana.lcs.mit.edu/dina/




















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