DCCP Working Group                                      Gorry Fairhurst
Internet-Draft                                      Arjuna Sathiaseelan
Intended status: Experimental                    University of Aberdeen
Expires: November 31, 2008


Intended status: Experimental                              June 1, 2008


   Quick-Start for Datagram Congestion Control Protocol (DCCP)
                  draft-fairhurst-tsvwg-dccp-qs-03.txt

Status of this Draft

   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
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on November 31, 2008.


Abstract

   This document specifies the use of the Quick-Start mechanism by the
   Datagram Congestion Control Protocol (DCCP).  DCCP is a transport
   protocol that allows the transmission of congestion-controlled,
   unreliable datagrams.  DCCP is intended for applications such as
   streaming media, Internet telephony, and on-line games.  In DCCP, an
   application has a choice of congestion control mechanisms, each
   specified by a Congestion Control Identifier (CCID). This document
   specifies general procedures applicable to all DCCP CCIDs and
   specific procedures for the use of Quick-Start with DCCP CCID-2 and
   CCID-3.  Quick-Start enables a DCCP sender to cooperate with any
   Quick-Start routers along the end-to-end path to determine an
   allowed sending rate at the start and, at times, in the middle of a
   DCCP connection (e.g., after an idle or application-limited period).
   The present specification is provided for use in controlled
   environments, and not as a mechanism that would be intended or
   appropriate for ubiquitous deployment in the global Internet.



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Table of Contents

   1. Introduction
   2. Quick-Start for DCCP
        2.1 Sending a Quick-Start Request for a DCCP flow
        2.2 The Quick-Start Interval
        2.3 Receiving a Quick-Start Request for a DCCP flow
        2.3.1 The Quick-Start Response Option
        2.4 Receiving a Quick-Start Response
        2.5 Procedure when no response to a Quick-Start Request
        2.6 Procedure when a Quick-Start Packet is dropped
        2.7 Interactions with Mobility and Signalled Path Changes
        2.8 Interactions with Path MTU Discovery
        2.9 Interactions with Middle boxes
   3. Mechanisms for Specific CCIDs
        3.1 Quick-Start for CCID-2
        3.1.1 The Quick-Start Request for CCID-2
        3.1.2 Sending a Quick-Start Response with CCID-2
        3.1.3 Using the Quick-Start Response with CCID-2
        3.1.4 Reported Loss during Quick-Start Mod
        3.1.5 CCID-2 Feedback Traffic on the Reverse Path
        3.2 Quick-Start for CCID-3
        3.2.1 The Quick-Start Request for CCID-3
        3.2.2 Sending a Quick-Start Response with CCID-3
        3.2.3 Using the Quick-Start Response with CCID-3
        3.2.4 The Quick-Start Validation Phase
        3.2.5 Reported Loss during Quick-Start Mode or Validation Phase
        3.2.6 An Example Quick-Start Scenario with CCID-3
        3.2.7 CCID-3 Feedback Traffic on the Reverse Path
        3.3 Quick-Start for CCID-4
        3.3.1 The Quick-Start Request for CCID-4
        3.3.2 Sending a Quick-Start Response with CCID-4
        3.3.3 Using the Quick-Start Response with CCID-4
        3.3.4 CCID-4 Feedback Traffic on the Reverse Path
   4. Discussion of Issues
        4.1 Over-run and Quick-Start Validation
        4.2 Experimental Status
   5. IANA Considerations
   6. Acknowledgments
   7. Security Considerations
   8. References
       8.1 Normative References
       8.2 Informative References
   9. Authors' Addresses
   10. IPR Notices
       10.1 Intellectual Property Statement
       10.2 Disclaimer of Validity
   11. Copyright Statement






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1. Introduction

   The Datagram Congestion Control Protocol (DCCP) [RFC4340] is a
   transport protocol for congestion-controlled, unreliable datagrams,
   intended for applications such as streaming media, Internet
   telephony, and on-line games.

   In DCCP, an application has a choice of congestion control
   mechanisms, each specified by a Congestion Control Identifier (CCID)
   [RFC4340]. There are general procedures applicable to all DCCP CCIDs
   that are described in Section 2, and details that relate to how
   individual CCIDs should operate, which are described in Section 3.
   This separation of CCID-specific and DCCP general functions is in
   the spirit of the modular approach adopted by DCCP.

   Quick-Start [RFC4782] is an Experimental mechanism for transport
   protocols specified for use in controlled environments. The current
   specification of this mechanism is not intended or appropriate for
   ubiquitous deployment in the global Internet.

   Quick-Start is designed for use between end hosts within the same
   network or on Internet paths that include IP routers. It works in
   cooperation with any routers, allowing a sender to determine an
   allowed sending rate at the start and at times in the middle of a
   data transfer (e.g., after an idle or application-limited period).

   This document assumes that the reader is familiar with RFC4782
   [RFC4782], which specifies the use of Quick-Start with IP and with
   TCP. Section 7 of RFC4782 also provides guidelines for the use of
   Quick-Start with other transport protocols, including DCCP. This
   document answers some of the issues that were raised by RFC4782 and
   provides a definition of how Quick-Start must be used with DCCP.

   In using Quick-Start, the sending DCCP end host indicates the
   desired sending rate in bytes per second, using a Quick-Start option
   in the IP header of a DCCP packet.  Each Quick-Start capable router
   along the path could, in turn, either approve the requested rate,
   reduce the requested rate, or indicate that the Quick-Start Request
   is not approved.

   If the Quick-Start Request is approved by all the routers along the
   path, then the DCCP receiver returns an appropriate Quick-Start
   Response. On receipt of this, the sending end host can send at up to
   the approved rate for one round-trip time.  Subsequent transmissions
   will be governed by the default CCID congestion control mechanisms
   for the connection. If the Quick-Start Request is not approved, then
   the sender must use the default congestion control mechanisms.







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2. Quick-Start for DCCP

   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 RFC 2119 [RFC2119].

   Unless otherwise specified, DCCP end hosts follow the procedures
   specified in Section 4 of [RFC4782], following the use specified for
   Quick-Start with TCP.

2.1 Sending a Quick-Start Request for a DCCP flow

   A DCCP sender MAY use a Quick-Start Request during the start of a
   connection, when the sender would prefer to have a larger initial
   rate than allowed by standard mechanisms (e.g. [RFC-to-be-3448.bis]
   or [RFC3390]).

   A Quick-Start Request MAY be also used once a DCCP flow is connected
   (in the middle of a DCCP flow). In standard operation, DCCP CCIDs
   can constrain the sending rate (or window) to less than that desired
   (e.g. when an application increases the rate at which it wishes to
   send). A DCCP sender that has data to send after an idle period or
   application-limited period (i.e. where the sender transmits at less
   than the allowed sending rate) can send a Quick-Start Request using
   the procedures defined in Section 3.

   Quick-Start Requests will be more effective if the Quick-Start Rate
   is not larger than necessary. Each requested Quick-Start Rate that
   has been approved, but was not fully utilized, takes away from the
   bandwidth pool maintained by Quick-Start routers that would be
   otherwise available for granting successive requests [RFC4782].

   In contrast to most TCP applications, many DCCP applications have
   the notion of a natural media rate that they wish to achieve. For
   example, during the initial connection, a host may request a Quick-
   Start rate equal to the media rate of the application.

   When sending a Quick-Start Request, the DCCP sender SHOULD send the
   Quick-Start Request using a packet that requires an acknowledgement,
   such as a DCCP-Request, DCCP-Response, or DCCP-Data.

2.2 The Quick-Start Interval

   Excessive use of the Quick-Start mechanism is undesirable. This
   document therefore introduces the concept of the Quick-Start
   Interval. End hosts therefore MUST NOT make a subsequent Quick-Start
   Request within a period specified as the Quick-Start Interval.

   When a connection is established, the Quick-Start Interval is
   initialized to a value of 6 seconds, and the previous Quick-Start
   Interval is set to 0 seconds. This value is chosen to be
   sufficiently large to prevent excessive router processing over
   typical Internet paths.

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   Whenever a Quick-Start Request is sent, the Quick-Start Interval is
   then recalculated as:

   Quick-Start Interval = max(6 seconds, QSPrev_Interval*2, 4*RTT)

   where the QSPrev_Interval is the value of the previous Quick-Start
   Interval.

   Each unsuccessful Quick-Start Request, therefore results in the
   Quick-Start Interval being doubled (resulting in an exponential
   back-off). The maximum time the sender can back-off is 64 seconds,
   after which a sender ceases using Quick-Start and MUST NOT send any
   further Quick-Start Requests for the remainder of the DCCP
   connection.

   Whenever a Quick-Start Request is approved, the previous Quick-Start
   Interval and QSPrev_Interval are reset to their initial value.

2.3 Receiving a Quick-Start Request for a DCCP flow

   The procedure for processing a received Quick-Start Request is
   normatively defined in [RFC4782], and summarised in this paragraph.
   An end host that receives an IP packet containing a Quick-Start
   Request passes the Quick-Start Request, along with the value in the
   IP TTL field, to the receiving DCCP layer. If the receiving host is
   willing to permit the Quick-Start Request, it SHOULD respond
   immediately by sending a packet that carries the Quick-Start
   Response option in the DCCP header of the corresponding feedback
   packet (e.g. using a DCCP-Ack packet or in a DCCP-DataAck packet).

   The Rate Request in the Quick-Start Response option is set to the
   received value of the Rate Request in the Quick-Start option or to a
   lower value if the DCCP receiver is only willing to allow a lower
   Rate Request. Where information is available (e.g. knowledge of the
   local layer 2 interface speed), a QS receiver SHOULD verify that the
   received rate does not exceed its expected receive link capacity.
   The TTL Diff in the Quick-Start Response is set to the difference
   between the IP TTL value and the Quick-Start TTL value.  The Quick-
   Start Nonce in the Response is set to the received value of the
   Quick-Start Nonce in the Quick-Start option.

   If an end host receives an IP packet with a Quick-Start Request with
   a request rate of zero, then this host SHOULD NOT send a Quick-Start
   Response [RFC4782].

   The Quick-Start Response MUST NOT be resent if it is lost in the
   network [RFC4782]. Packet loss could be an indication of congestion
   on the return path, in which case it is better not to approve the
   Quick-Start Request.

2.3.1 The Quick-Start Response Option


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   The Quick-Start Response message must be carried by the transport
   protocol using Quick-Start.  This section defines a DCCP Header
   option used to carry the Quick-Start response. This header option is
   REQUIRED for end hosts to utilise the Quick-Start mechanism with
   DCCP flows. The format resembles that defined for TCP [RFC4782].

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type=xQSOx   |  Length=8     | Resv. | Rate  |   TTL Diff    |
   |               |               |       |Request|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Quick-Start Nonce                     | R |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 1.  The Quick-Start Response option.

   ### IANA ACTION, PLEASE REPLACE xQSOx with the assigned value in the
   figure above.###


   ### IANA ACTION, PLEASE ALSO REPLACE xQSOx with the assigned value
   in the paragraph below.###

   The first byte of the Quick-Start Response option contains the
   option kind, identifying the DCCP option (xQSOx).

   The second byte of the Quick-Start Response option contains the
   option length in bytes.  The length field MUST be set to 8 bytes.

   The third byte of the Quick-Start Response option contains a four-
   bit Reserved field, and the four-bit allowed Rate Request, formatted
   as in the IP Quick-Start Rate Request option [RFC4782].

   The fourth byte of the DCCP Quick-Start Response option contains the
   TTL Diff.  The TTL Diff contains the difference between the IP TTL
   and Quick-Start TTL fields in the received Quick-Start Request
   packet, as calculated in [RFC4782].

   Bytes 5-8 of the DCCP option contain the 30-bit Quick-Start Nonce
   and a two-bit Reserved field.

2.4 Receiving a Quick-Start Response

   Reception of a Quick-Start Response packet results in the sender
   entering the Quick-Start Mode.  The procedure following reception of
   a Quick-Start Response packet is CCID-specific and described in
   Section 3.

2.5 Procedure when no response to a Quick-Start Request

   As in TCP, if a Quick-Start Request is dropped (i.e., the Request or
   Response is not delivered by the network) the DCCP sender MUST

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   revert to the congestion control mechanisms it would have used if
   the Quick-Start Request had not been approved. The connection is not
   permitted to send a subsequent Quick-Start Request before expiry of
   the current Quick-Start Interval (section 2.1).

2.6 Procedure when a Quick-Start Packet is dropped

   While the sender is in the Quick-Start Mode, all sent packets are
   known as Quick-Start Packets [RFC4782].  Loss of a Quick-Start
   Packet is an indication of potential network congestion.  The
   behaviour of a DCCP sender following the loss of a Quick-Start
   Packet is specific to a particular CCID (see section 3).

2.7 Interactions with Mobility and Signalled Path Changes

   The use of Quick-Start may assist hosts in determining when it is
   appropriate to increase their rate following an explicitly signalled
   change of the network path. Senders must ensure this does not
   generate an excessive rate of Quick-Start Requests by using the
   method below.

   A sender that has explicit information that the network path has
   changed (e.g. a mobile IP binding update [RFC3344], [RFC3775]) MAY
   reset the Quick-Start Interval and QSPrev_Interval to their initial
   values (specified in section 2.1).

   The sender MAY also send a Quick-Start Request to determine a new
   safe transmission rate, but must observe the following rules:

     . It MUST NOT send a Quick-Start Request within a period less
        than the initial Quick-Start Interval (i.e., 6 seconds) since
        it previously sent a Quick-Start Request. That is, it must wait
        for at least a period of 6 seconds after the previous request,
        before sending a new Quick-Start Request.

     . If it has not sent a Quick-Start Request within the previous 6
        seconds, it SHOULD defer sending a Quick-Start Request for a
        randomly chosen period between 0 and 6 seconds. The random
        period should be statistically independent between different
        hosts and between different connections on the same host. This
        delay is to mitigate the effect on router load of synchronised
        responses by multiple connections in response to a path change
        that effects multiple connections.

2.8 Interactions with Path MTU Discovery

   DCCP implementations are encouraged to support Path MTU Discovery
   (PMTUD) when applications are able to use a DCCP packet size that
   exceeds the default Path MTU [RFC4340], [RFC4821]. Quick-Start
   Requests SHOULD NOT be sent with packets that are used as a PMTUD
   Probe Packet, since these packets could be lost in the network
   increasing the probability of loss. It may therefore be preferable
   to separately negotiate the PMTU and the use of Quick-Start.

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   The DCCP protocol is datagram-based and therefore the size of the
   segments that are sent is a function of application behaviour as
   well as being constrained by the largest supported Path MTU.

2.9 Interactions with Middle boxes

   A Quick-Start Request is carried in an IP packet option [RFC4782].
   Interactions with network devices (middleboxes) that inspect or
   modify IP options could therefore led to discard, ICMP error, or
   DCCP-Reset when attempting to forward packets carrying a Quick-Start
   Request.

   If a DCCP sender sends a DCCP-Request that also carries a Quick-
   Start Request, and does not receive a DCCP-Response to the packet,
   the DCCP sender SHOULD resend the DCCP-Request packet without
   including a Quick-Start Request.

   Similarly, if a DCCP sender receives a DCCP-Reset in response to a
   DCCP-Request packet that also carries a Quick-Start Request, then
   the DCCP sender SHOULD resend DCCP-Request packet without the
   Quick-Start Request.

   The DCCP sender then ceases to use the Quick-Start Mechanism for the
   remainder of the connection.

   A DCCP sender that uses a Quick-Start Request within an established
   connection, and does not receive a response will treat this as non-
   approval of the request.  Successive unsuccessful attempts will
   result in an exponential increase in the Quick-Start Interval
   (section 2.1). If this grows to a value exceeding 64 seconds the
   DCCP sender ceases to use the Quick-Start Mechanism for the
   remainder of the connection.


3. Mechanisms for Specific CCIDs

   This section specifies the use of Quick-Start with DCCP CCID-2,
   CCID-3, and CCID-4.

3.1 Quick-Start for CCID-2

   This section describes the Quick-Start mechanism to be used with
   DCCP CCID-2 [RFC4341]. CCID-2 uses a TCP-like congestion control
   mechanism.

3.1.1 The Quick-Start Request for CCID-2

   A Quick-Start Request MAY be sent to allow the sender to determine
   if it is safe to use a larger initial cwnd. This permits a faster
   start-up of a new DCCP CCID-2 flow.


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   A Quick-Start Request MAY also be sent for an established connection
   to request a higher sending rate after an idle period or
   application-limited period (described in section 2.1). This allows a
   receiver to use a larger cwnd than allowed with standard operation.

   A Quick-Start Request SHOULD NOT be sent within a period less than
   the Quick-Start Interval following a previous Quick-Start Request
   (section 2.2), unless it is sent as a result of a network path
   change (section 2.7). A Quick-Start Request that follows a loss or
   congestion event MUST NOT request a Quick-Start rate that exceeds
   the largest congestion window achieved by the CCID-2 connection
   since the last packet drop (translated to a sending rate).

3.1.2 Sending a Quick-Start Response with CCID-2

   A receiver processing a Quick-Start Request uses the method
   described in Section 2.2. On receipt of a Quick-Start Request, the
   receiver MUST send a Quick-Start Response (even if a receiver is
   constrained by the ACK Ratio).

3.1.3 Using the Quick-Start Response with CCID-2

   On receipt of a valid Quick-Start Response option, the sender MUST
   send a Quick-Start Approved option [RFC4782] as an IP option using
   the first Quick-Start Packet or send this as an option using a DCCP
   control packet if there are no DCCP-Data packets pending
   transmission.

   If the approved Quick-Start rate is less than current sending rate,
   the sender does not enter the Quick-Start Mode, and continues using
   the procedure defined in CCID-2.

   If the approved Quick-Start rate at the sender exceeds the current
   sending rate, the sender enters the Quick-Start Mode and continues
   in the Quick-Start Mode for a maximum period of 1 RTT. While in the
   Quick-Start Mode, all DCCP packets that it sends are known as Quick-
   Start Packets.

   The sender sets its Quick-Start cwnd (QS_cwnd) as follows:

             QS_cwnd = (R * T) / (s + H)                          (1)

   where R is the Rate Request in bytes per second, s is the packet
   size, and H is the estimated DCCP/IP header size in bytes (e.g., 32
   bytes for DCCP layered directly over IPv4).

   A CCID-2 sender MAY then increase its cwnd to the QS_cwnd.  The cwnd
   should not be reduced (i.e., a QS_cwnd lower than cwnd should be
   ignored, since the CCID-2 congestion control method already permits
   this rate). CCID-2 is not a rate-paced protocol. Therefore, if the
   QS_cwnd is used, the sending host MUST implement a suitable method
   to pace the rate at which the Quick-Start Packets are sent until it

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   receives an ACK for a packet sent during the Quick-Start Mode
   [RFC4782]. The sending host SHOULD also record the previous cwnd and
   note that the new cwnd has been determined by Quick-Start, rather by
   other means (e.g. by setting a flag to indicate that it is in Quick-
   Start Mode).

   When the sender receives the first ACK to a packet sent in the
   Quick-Start Mode, the sender MUST reduce the cwnd to the actual
   flight size (the current amount of unacknowledged data sent)
   [RFC4782].

3.1.4 Reported Loss during Quick-Start Mode

   A sender in the Quick-Start Mode or Validation Phase that detects
   congestion (e.g. receives a feedback packet that reports new packet
   loss or a packet with a congestion marking), MUST immediately leave
   the Quick-Start Mode or Validation Phase and enter the congestion
   avoidance phase [RFC4341].

3.1.5 CCID-2 Feedback Traffic on the Reverse Path

   A CCID-2 receiver sends feedback for groups of received packets
   [RFC4341]. Approval of a higher transmission rate using Quick-Start
   will increase control traffic on the reverse path. A return path
   that becomes congested could have a transient negative impact on
   other traffic flows sharing the return link. The lower rate of
   feedback will then limit the achievable rate in the forward
   direction.

3.2 Quick-Start for CCID-3

   This section describes the Quick-Start mechanism to be used with
   DCCP CCID-3 [RFC4342]. The rate-based congestion control mechanism
   used by CCID-3, leads to specific issues that are addressed by
   Quick-Start in this section, and include the introduction of a
   Quick-Start Validation Phase.

3.2.1 The Quick-Start Request for CCID-3

   A Quick-Start Request MAY be sent to allow the sender to determine
   if it is safe to use a larger initial sending rate. This permits a
   faster start-up of a new DCCP flow.

   A Quick-Start Request MAY also be sent to request a higher sending
   rate after an idle period (in which the nofeedback timer expires
   [RFC-to-be-3448bis]) or an application-limited period (described in
   section 2.1). This allows a receiver to increase the sending rate
   faster than allowed with standard operation (i.e. faster than twice
   the rate reported by a CCID-3 receiver in the most recent feedback
   message).

   The requested rate specified in a Quick-Start Request must consider
   the current loss event rate (if any), either from calculation at the

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   sender or from feedback received from the receiver.  CCID-3 requires
   that a sender must not send more than the upper bound dictated by
   the loss event rate. This rate offers a safe response in the
   presence of expected congestion.

3.2.2 Sending a Quick-Start Response with CCID-3

   When processing a received Quick-Start Request, the receiver uses
   the method described in Section 2.3. In addition, if a CCID-3
   receiver uses the window counter to send periodic feedback messages,
   then the receiver sets its local variable last_counter to the value
   of the window counter reported by the segment containing the Quick-
   Start Request. The next feedback message would then be sent when the
   window_counter is greater or equal to last_counter + 4. If the CCID-
   3 receiver uses a feedback timer to send period feedback messages,
   then the DCCP receiver MUST reset the CCID-3 feedback timer, causing
   the feedback to be sent as soon as possible. This helps to align the
   timing of feedback to the start and end of the period in which
   Quick-Start Packets are sent, and will normally result in feedback
   at a time that is approximately the end of the period when Quick-
   Start Packets are received.

   A Quick-Start Request SHOULD NOT be sent within a period less than
   the Quick-Start Interval following a previous Quick-Start Request
   (section 2.2), unless it is sent as a result of a network path
   change (section 2.7).

3.2.3 Using the Quick-Start Response with CCID-3

   On receipt of a valid Quick-Start Response option, the sender enters
   the Quick-Start Mode. The sender MUST send a Quick-Start Approved
   option [RFC4782] as an IP option using the first Quick-Start Packet
   or send this as an option using a DCCP control packet if there are
   no DCCP-Data packets pending transmission.

   If the approved Quick-Start rate is less than current sending rate,
   the sender does not enter the Quick-Start Mode, and continues using
   the procedure defined in CCID-3. While in the Quick-Start Mode, all
   DCCP packets that it sends are known as Quick-Start Packets.

   If the approved Quick-Start rate exceeds the current sending rate,
   the sender enters the Quick-Start Mode and continues in the Quick-
   Start Mode for a maximum period of 1 RTT. The sender sets its Quick-
   Start sending rate (QS_sendrate) as follows:

       QS_sendrate = R * s/(s + H)                                (2)
   where R the Rate Request in bytes per second, s is the packet size,
   and H the estimated DCCP/IP header size in bytes (e.g., 32 bytes).
   A CCID-3 host MAY then increase its sending rate (sendrate) to the
   QS_sendrate. The rate should not be reduced.

   CCID-3 is a rate-paced protocol. Therefore, if the QS_sendrate is
   used, the sending host MUST pace the rate at which the Quick-Start

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   Packets are sent over the next RTT. The sending host SHOULD also
   record the previous congestion-controlled rate and note that the new
   rate has been determined by Quick-Start rather by other means (e.g.
   by setting a flag to indicate that it is in Quick-Start Mode).

   The sender exits the Quick-Start Mode after either:

   * Receipt of a feedback packet acknowledging one or more Quick-Start
   Packets,
   * A period of 1 RTT after receipt of a Quick-Start Response,
   or
   * Detection of a loss or congestion event (see Section 3.2.5).

3.2.4 The Quick-Start Validation Phase

   After transmitting a set of Quick-Start Packets (and providing that
   no loss or ECN marking is reported), the sender enters the Quick-
   Start Validation Phase. This phase persists for a period during
   which the sender seeks to affirm that the capacity used by the
   Quick-Start Packets did not introduce congestion. (This phase is
   introduced, because unlike TCP (and CCID-2), CCID-3 does not receive
   frequent feedback that would indicate the congestion state of the
   forward path). While in the Quick-Start Validation Phase, the sender
   is tentatively permitted to continue sending at the QS_sendrate. On
   conclusion of the Validation Phase, the sender expects to receive
   assurance that it may safely use the current rate.

   A sender that receives feedback that reports a loss or congestion
   event MUST follow the procedures described in Section 3.2.5.

   The sender SHOULD exit the Quick-Start Validation Phase on receipt
   of feedback that acknowledges all packets sent in the Quick-Start
   Mode (i.e. all Quick-Start Packets). It MUST exit this phase on
   expiry of the Quick-Start validation time. The Quick-Start
   Validation Phase is limited to the Quick-Start Validation Time (a
   maximum of 1.5 RTTs).

   A sender that completes the Quick-Start Validation phase with no
   reported packet loss or congestion, stops using the QS_sendrate and
   MUST recalculate a suitable sending rate using the standard
   congestion control mechanisms [RFC4342].  For example, if the DCCP
   sender was in slow-start prior to the Quick-Start Request, and no
   packets were lost or marked since that time, then the sender
   continues in slow-start after exiting Quick-Start Mode until the
   sender sees a packet loss, or congestion is reported.

   If no feedback is received within the Quick-Start Validation Phase,
   the sender MUST return to the minimum of the original rate (at the
   start of the Quick-Start Mode) and one half of the QS_sendrate.

3.2.5 Reported Loss during Quick-Start Mode or Validation Phase



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   A sender in the Quick-Start Mode or Validation Phase that detects
   congestion (e.g. receives a feedback packet that reports new packet
   loss or a packet with a congestion marking) MUST immediately leave
   the Quick-Start Mode or Validation Phase and enter the congestion
   avoidance phase [RFC4342].  This implies re-calculating the send
   rate, X, as required by RFC4342:

        X = max(min(X_calc, min(2*X_recv, 2* QS_recv-rate)), s/t_mbi);

   where X_recv is the previously cached receiver rate and QS recv-rate
   is the receiver rate reported by the feedback due to the arrival of
   Quick-Start Packets.

   The current specification of TFRC [RFC-to-be-3448bis], which
   obsoletes RFC 3448, uses a set of X_recv values and uses the maximum
   of the set during data-limited intervals. This calculates the send
   rate, X as:

        X = max(min(X_calc, min(recv_limit, 2* QS_recv-rate)),
            s/t_mbi);

   where recv_limit could be max(X_recv_set) or 2*max(X_recv_set)
   dependent on whether there was a new loss event during a data-
   limited interval, or no loss event during a data-limited interval
   respectively. When the sender is not data-limited, recv_limit is set
   to 2*max(X_recv_set).

   When RFC4342 adopts [RFC-to-be-3448bis], the send rate, X would also
   be calculated using the above formula from [RFC-to-be-3448bis].

3.2.6  An Example Quick-Start Scenario with CCID-3

                      DCCP Sender                     DCCP Receiver

   Quick-Start      +----------------------------------------------+
   Request/Response | Quick-Start Request -->                      |
                    |                    <-- Quick-Start Response  |
                    | Quick-Start Approve -->                      |
                    +----------------------------------------------+
                    +----------------------------------------------+
   Quick-Start      | Quick-Start Packets -->                      |
   Mode             |                                              |
                    |                    <-- Feedback from Receiver|
                    +----------------------------------------------+
                    +----------------------------------------------
   Quick-Start      | Packets -->                                  |
   Validation Phase |                                              |
                    |                    <-- Feedback from Receiver|
                    +----------------------------------------------+
   CCID-3           | Packets -->                                  |
   Congestion       |                                              |
   Control          |                   <-- Feedback from Receiver |
                    |                                              |

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          Figure 2.  The Quick-Start Mode and Validation Phase.

   Figure 2 shows an example use of Quick-Start with CCID-3.

3.2.7 CCID-3 Feedback Traffic on the Reverse Path

   A CCID-3 receiver sends feedback at least once each RTT [RFC4342].
   Use of Quick-Start is therefore not expected to significantly
   increase control traffic on the reverse path.

3.3 Quick-Start for CCID-4

   This section describes the Quick-Start mechanism to be used with
   DCCP CCID-4 [ID.CCID4]. CCID-4 is similar to CCID-3 except that a
   sender using CCID-4 is limited to a maximum of 100 packets/second.
   The Quick-Start procedure defined below therefore resembles that for
   CCID-3.

3.3.1 The Quick-Start Request for CCID-4

   The procedure for sending a Quick-Start Request using CCID-4 is the
   same as for CCID-3, defined in section 3.2.1. In addition, the
   requested rate MUST be less than or equal to the equivalent of a
   sending rate of 100 packet per second [ID.CCID4].

3.3.2 Sending a Quick-Start Response with CCID-4

   This procedure is the same as for CCID-3, defined in section 3.2.2.

3.3.3 Using the Quick-Start Response with CCID-4

   This procedure is the same as for CCID-3, defined in sections 3.2.3,
   3.2.4, and 3.2.5, except that the congestion control procedures
   defined in [ID.CCID4] and used in place of those defined in
   [RFC4342]

3.3.4 CCID-4 Feedback Traffic on the Reverse Path

   A CCID-4 receiver sends feedback at least once each RTT (defined in
   [RFC4342]). Use of Quick-Start is therefore not expected to
   significantly increase control traffic on the reverse path.

4. Discussion of Issues

   The considerations for using Quick-Start with DCCP are not
   significantly different to those for Quick-Start with TCP.

4.1 Over-run and Quick-Start Validation

   CCID-3 raises an issue in that a sender using Quick-Start may
   continue to use the rate specified by a Quick-Start Response for a
   period that exceeds one path round trip time (i.e., that which TCP

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   would have used). This over-run is a result of the less frequent
   feedback interval used by TFRC (i.e., CCID-3 and CCID-4 provide
   feedback once per RTT, rather than once for a few packets). In the
   method specified by this document, the Quick-Start Validation Time
   bounds this over-run to be not more than an additional 1.5 RTTs.

   The currently selected method is chosen as a compromise that
   reflects the need to terminate quickly following the loss of a
   feedback packet, and the need to allow sufficient time for end host
   and router processing as well as the different perceptions of the
   path RTT held at the sender and receiver. Any reported loss or
   congestion results in immediate action without waiting for
   completion of the Quick-Start Validation period.

4.2 Experimental Status

   There are many cases in which Quick-Start Requests would not be
   approved [RFC4782].  These include communication over paths
   containing routers, IP tunnels, MPLS paths, and the like, that do
   not support Quick-Start.  These cases also include paths with
   routers or middleboxes that drop packets containing IP options.
   Quick-Start Requests could be difficult to approve over paths that
   include multi-access layer-two networks.

   Transient effects could arise when the transport protocol packets
   associated with a connection are multiplexed over multiple parallel
   (sometimes known as alternative) link or network-layer paths, and
   Quick-Start is used, since it will be effective on only one of the
   paths, but could lead to increased traffic on all paths.

   A CCID-2 sender using Quick-Start can increase the control traffic
   on the reverse path, which could have a transient negative impact on
   other traffic flows sharing the return link (section 3.1.5). The
   lower rate of feedback will then limit the achievable rate in the
   forward direction.

   [RFC4782] also describes environments where the Quick-Start
   mechanism could fail with false positives, with the sender
   incorrectly assuming that the Quick-Start Request had been approved
   by all of the routers along the path.  As a result of these
   concerns, and as a result of the difficulties and seeming absence of
   motivation for routers, such as core routers, to deploy Quick-Start,
   Quick-Start has been proposed as a mechanism that could be of use in
   controlled environments, and not as a mechanism that would be
   intended or appropriate for ubiquitous deployment in the global
   Internet.

   Further experimentation would be required to confirm the deployment
   of Quick-Start and to investigate performance issues that may arise,
   prior to any recommendation for use over the general Internet.


5. IANA Considerations

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   This document requires IANA involvement for the assignment of a DCCP
   Option Type from the DCCP Option Types Registry. This Option is
   applicable to all CCIDs and is known as the "Quick-Start Response"
   Option and is defined in Section 2.2.1. It specifies a length value
   in the format used for options numbered 32-128.


6. Acknowledgments

   The author gratefully acknowledges the previous work by Sally Floyd
   to identify issues that impact Quick-Start for DCCP, and her
   comments to improve this document. We also acknowledge comments and
   corrections from Pasi Sarolahti, Mark Allman and others in the IETF
   DCCP WG.


7. Security Considerations

   Security issues are discussed in [RFC4782].  Middlebox deployment
   issues are also highlighted in section 2.7. No new security issues
   are raised within this document.


8. References


8.1 Normative References

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

   [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
   Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
   Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion
   Control", RFC 4341, March 2006.

   [RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
   Datagram Congestion Control Protocol (DCCP) Congestion Control ID 3:
   TCP-Friendly Rate Control (TFRC)", RFC 4342, March 2006.

   [RFC4782] Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
   Start for TCP and IP", RFC 4782, January 2007.

   [ID.CCID4] Floyd, S., Kohler, E., "Profile for Datagram Congestion
   Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate Control
   for Small Packets (TFRC-SP)", IETF Work In Progress, 2007.

   [RFC-to-be-3448bis] Floyd, S., Padhye, J., Widmer, J., "TCP Friendly
   Rate Control (TFRC): Protocol Specification", IETF Work In Progress,
   2008.

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8.2 Informative References

   [RFC3344] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC
   3344, August 2002.

   [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
   in IPv6", RFC 3775, June 2004.

   [RFC3390] Allman, M., Floyd, S., Partridge, C., "Increasing TCP's
   Initial Window", RFRC 3390, October 2002.

   [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
   Discovery", RFC 4821, March 2007.


9. Authors' Addresses

   Godred Fairhurst
   School of Engineering
   University of Aberdeen
   Aberdeen, AB24 3UE
   UK
   Email: gorry@erg.abdn.ac.uk
   Web: http://www.erg.abdn.ac.uk/users/gorry

   Arjuna Sathiaseelan
   School of Engineering
   University of Aberdeen
   Aberdeen, AB24 3UE
   UK
   Email: arjuna@erg.abdn.ac.uk
   Web: http://www.erg.abdn.ac.uk/users/arjuna


10. IPR Notices


10.1 Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology described
   in this document or the extent to which any license under such
   rights might or might not be available; nor does it represent that
   it has made any independent effort to identify any such rights.
   Information on the procedures with respect to rights in RFC
   documents can be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this

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   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.


10.2 Disclaimer of Validity

   This document and the information contained herein are provided on
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
   IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
   WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
   WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
   ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
   FOR A PARTICULAR PURPOSE.

11. Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   Acknowledgment

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





















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-------------------------------------------------------------------

   [RFC EDITOR NOTE:
   This section must be deleted prior to publication]

   DOCUMENT HISTORY

   Individual Draft 00
   This is the first presentation of this document.

   Individual Draft 01
   This includes fixes for NiTs (thanks Pasi)
   It also includes a note on initial rates in 2.1
   All mention of packet loss now qualified with loss/congestion.
   It adds supports for CCID-2.
   It also defines the Quick-Start Interval as a way of controlling the
   rate at which hosts may issue Quick-Start requests.

   Individual Draft 02 - Draft intended for more general review
   Resolution of many minor outstanding editorial issues.
   Includes feedback on a longer Quick-Start period from Mark Allman.
   Includes new section on the interaction with middleboxes.
   CCID-2 and CCID-3 text now use the same style.
   Added description for CCID-4, based on CCID-3.
   Added clarification of PMTUD interaction.
   Reorganised to create a section on the QS Interval
   Rewritten sections on what to do after loss/congestion
   Clarified path change triggers (e.g. from mobility binding updates)
   There are no currently known remaining issues to be addressed.

   Individual Draft 03
   This includes fixes for NiTs, especially to shorten some parts of
   text.
   It includes some additional clarification based on the progress of
   RFC3448.bis.
   Replaced reference to Faster Restart.
   Change to paragraph on mobility usage.


   [END of RFC EDITOR NOTE]














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