Internet DRAFT - draft-ietf-dccp-quickstart

draft-ietf-dccp-quickstart



DCCP Working Group                                         G. Fairhurst 
Internet-Draft                                          A. Sathiaseelan 
Intended status: Experimental                    University of Aberdeen 
Expires: November 31, 2009                       
                                                                        
                                                                        
Intended status: Experimental                             03 June, 2009 
 
    
   Quick-Start for Datagram Congestion Control Protocol (DCCP) 
                  draft-ietf-dccp-quickstart-05.txt 
    
Status of this Draft 
    
   This Internet-Draft is submitted to IETF in full conformance with 
   the provisions of BCP 78 and BCP 79. 
    
   Internet-Drafts are working documents of the Internet Engineering 
   Task Force (IETF), its areas, and its working groups.  Note that 
   other groups may also distribute working documents as Internet- 
   Drafts. 
    
   Internet-Drafts are draft documents valid for a maximum of six 
   months and may be updated, replaced, or obsoleted by other documents 
   at any time.  It is inappropriate to use Internet-Drafts as 
   reference material or to cite them other than as "work in progress." 
    
   The list of current Internet-Drafts can be accessed at 
   http://www.ietf.org/ietf/1id-abstracts.txt. 
    
   The list of Internet-Draft Shadow Directories can be accessed at 
   http://www.ietf.org/shadow.html. 
    
   This Internet-Draft will expire on November 31, 2009. 
 
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, 
   CCID 3 and CCID 4.  Quick-Start enables a DCCP sender to cooperate 
   with Quick-Start routers along the end-to-end path to determine an 
   allowed sending rate at the start of a connection 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 
        1.1 Terminology 
    
   2. Quick-Start for DCCP 
        2.1 Sending a Quick-Start Request for a DCCP flow 
        2.1.1 The Quick-Start Interval 
        2.2 Receiving a Quick-Start Request for a DCCP flow 
        2.2.1 The Quick-Start Response Option 
        2.3 Receiving a Quick-Start Response 
        2.3.1 The Quick-Start Mode  
        2.3.2 The Quick-Start Validation Phase 
        2.4 Procedure when no response to a Quick-Start Request 
        2.5 Procedure when a Quick-Start Packet is dropped 
        2.6 Interactions with Mobility and Signalled Path Changes 
        2.7 Interactions with Path MTU Discovery 
        2.8 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 Quick-Start Validation Phase for CCID 2 
        3.1.5 Reported loss or congestion while using Quick-Start 
        3.1.6 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 Quick-Start Validation Phase for CCID 3 
        3.2.5 Reported loss during Quick-Start Mode or Validation Phase 
        3.2.6 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 Reported loss or congestion while using Quick-Start 
        3.3.5 CCID 4 Feedback Traffic on the Reverse Path 
   4. Discussion of Issues 
        4.1 Over-run and the Quick-Start Validation Phase  
        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 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 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 
   provides answers to 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 (possibly with a reduced 
   rate) 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 a period 
   determined by the method specified for each DCCP CCID, and not 
   exceeding three round-trip times.  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. 
    
   DCCP receivers are not required to acknowledge individual packets 
   (or pairs of segments) as in TCP. CCID 2 [RFC4341] allows much less 
   frequent feedback. Rate-based protocols (e.g. TFRC [RFC5348], CCID 3 
  
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   [RFC4342]) have a different feedback mechanism than that of TCP. 
   With rate-based protocols, feedback may be sent less frequently 
   (e.g. once per RTT). In such cases, a sender using Quick-Start needs 
   to implement a different mechanism to determine whether the Quick-
   Start sending rate has been sustained by the network. This 
   introduces a new mechanism called the Quick-Start Validation Phase 
   (Section 2.5). 
    
   In addition, this document defines two more general enhancements 
   that refine the use of Quick-Start after a flow has started 
   (expected to be more common in applications using DCCP). These are 
   the Quick-Start Interval (Section 2.2), and the reaction to mobility 
   triggers (Section 2.7). 
    
1.1 Terminology 
    
   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]. 
    
    
 
 
 
 
 



























  
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2. Quick-Start for DCCP 
    
   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. [RFC5348] or 
   [RFC3390]).  
    
   A Quick-Start Request MAY also be used once a DCCP flow is connected 
   (i.e., 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 has 
   transmitted 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, appropriately 
   increased to account for the size of packet headers. (Note that 
   Quick-Start only provides a course-grain indication of the  
   desired rate that is expected to be sent in the next RTT.) 
 
   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.  
    
2.1.1 The Quick-Start Interval 
    
   Excessive use of the Quick-Start mechanism is undesirable. This 
   document defines an enhancement to RFC4782 to update the use of 
   Quick-Start after a DCCP flow has started, by introducing the 
   concept of the Quick-Start Interval. The Quick-Start Interval 
   specifies a period of time during which a Quick-Start Request SHOULD 
   NOT be sent. The Quick-Start Interval is measured from the time of 
   transmission of the previous Quick-Start Request (section 2.1). The 
   Quick-Start Interval MAY be overridden as a result of a network path 
   change (section 2.6). 
 
  
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   When a connection is established, the Quick-Start Interval is 
   initialized to the Initial_QSI. The Initial_QSI MUST be at least 6 
   Seconds (larger values are permitted). This value was chosen so that 
   it is sufficiently large to prevent excessive router processing over 
   typical Internet paths. Quick-Start routers that track per-flow 
   state MAY penalise senders by suspending Quick-Start processing of 
   flows that make Quick-Start Requests for the same flow with an 
   interval less than 6 seconds.  
 
   When the first Quick-Start Request is sent, the Quick-Start Interval 
   is set to: 
 
   Quick-Start Interval = Initial_QSI; 
    
   After sending each subsequent Quick-Start Request, the Quick-Start 
   Interval is then recalculated as: 
 
   Quick-Start Interval = max(Quick-Start Interval *2, 4*RTT); 
    
 
   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. 
   When the back-off calculation results in a larger value, the sender 
   MUST NOT send any further Quick-Start Requests for the remainder of 
   the DCCP connection (i.e. the sender ceases to use Quick-Start).   
    
   Whenever a Quick-Start Request is approved (at any rate), the Quick-
   Start Interval is reset to the Initial_QSI.  
 
2.2 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 field 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 Quick-Start receiver SHOULD 
   verify that the received rate does not exceed its expected receive 
   link capacity. The TTL Diff field in the Quick-Start Response is set 
   to the difference between the received IP TTL value (Hop Limit field 
   in IPv6) and the Quick-Start TTL value.  The Quick-Start Nonce in 

  
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   the Response is set to the received value of the Quick-Start Nonce 
   in the Quick-Start option (or IPv6 Header Extension). 
  
   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. 
 
   If an end host receives an IP packet with a Quick-Start Request with  
   a requested rate of zero, then this host SHOULD NOT send a Quick-
   Start Response [RFC4782].  
    
    
2.2.1 The Quick-Start Response Option 
    
   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]. 
  
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   Bytes 5-8 of the DCCP option contain the 30-bit Quick-Start Nonce 
   and a two-bit Reserved field [RFC4782]. 
    
2.3 Receiving a Quick-Start Response  
    
   On reception of a Quick-Start Response packet, the sender MUST 
   report the approved rate, by sending a Quick-Start Report of 
   Approved Rate [RFC4782]. This report includes the Rate Report field 
   set to the Approved Rate, and the QS Nonce set to the QS Nonce value 
   sent in the Quick-Start Request.  
    
   The Quick-Start Report of Approved Rate is sent as an IPv4 option or 
   IPv6 header extension using the first Quick-Start Packet or sent as 
   an option using a DCCP control packet if there are no DCCP-Data 
   packets pending transmission.  
    
   The Quick-Start Interval is also reset (as described in section 
   2.1.1). 
    
   Reception of a Quick-Start Response packet that approves a rate 
   higher than the current rate results in the sender entering the 
   Quick-Start Mode.  
    
    
2.3.1 The Quick-Start Mode  
 
   While a sender is in the Quick-Start Mode, all sent packets are 
   known as Quick-Start Packets [RFC4782]. The Quick-Start Packets MUST 
   be sent at a rate not greater than the rate specified in the Quick-
   Start Response. The Quick-Start Mode continues for a period up to 
   one RTT (shorter, if a feedback message arrives acknowledging the 
   receipt of one or more Quick-Start Packets).   
    
   The procedure following exit of the Quick-Start Mode is specified in 
   the following paragraphs. Note that this behaviour is CCID-specific 
   and the details for each current CCID are described in Section 3.  
 
2.3.2 The Quick-Start Validation Phase  
    
   After transmitting a set of Quick-Start Packets in the Quick-Start 
   Mode (and providing that no loss or congestion 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, DCCP 
   senders do not necessarily 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 using the Quick-Start rate. This phase 
   normally concludes when the sender receives feedback that includes 
   an acknowledgment that all Quick-Start Packets were received. 
  
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   However, the duration of the Quick-Start Validation Phase MUST NOT 
   exceed the Quick-Start Validation Time (a maximum of 2 RTTs). 
   Implementations may set a timer (initialised to the Quick-Start 
   Validation Time) to detect the end of this phase. There may be scope 
   for optimisation of timer resources in an implementation, since the 
   Quick-Start Validation period temporarily enforces more strict 
   monitoring of acknowledgements than normally used in a CCID (e.g. an 
   implementation may consider using a common timer resource for Quick-
   Start Validation and a nofeedback timer)._  
    
   An example sequence of packet exchanges showing Quick-Start with 
   DCCP is shown in Figure 2. 
 
                      DCCP Sender                     DCCP Receiver 
   Quick-Start      +----------------------------------------------+ 
   Request/Response | Quick-Start Request -->                      | 
                    |                    <-- Quick-Start Response  | 
                    | Quick-Start Approve -->                      | 
                    +----------------------------------------------+ 
                    +----------------------------------------------+ 
   Quick-Start      | Quick-Start Packets -->                      | 
   Mode             | Quick-Start Packets -->                      | 
                    |                  <-- Feedback A from Receiver| 
                    |               (acknowledging first QS Packet)| 
                    +----------------------------------------------+ 
                    +---------------------------------------------- 
   Quick-Start      | Packets -->                                  | 
   Validation Phase |                  <-- Feedback B from Receiver| 
                    |                (acknowledging all QS Packets)| 
                    +---------------------------------------------- 
                    +----------------------------------------------+ 
   DCCP             | Packets -->                                  | 
   Congestion       |                  <-- Feedback C from Receiver| 
   Control          |                                              | 
    
    
          Figure 2.  The Quick-Start Mode and Validation Phase. 
    
   On conclusion of the Validation Phase (Feedback B in the above 
   figure), the sender expects to receive assurance that it may safely 
   use the current rate. A sender that completes the Quick-Start 
   Validation Phase with no reported packet loss or congestion stops 
   using the Quick-Start rate and continues to adjust its rate using 
   the standard congestion control mechanisms.  For example, if the 
   DCCP sender was in slow-start prior to the Quick-Start Request, and 
   no packets were lost or ECN 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.  
    
2.4 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 2.1.1). 
 
2.5 Procedure when a packet is dropped while using Quick-Start 
    
   A lost or ECN-marked packet is an indication of potential network 
   congestion.  The behaviour of a DCCP sender following a lost or ECN-
   marked Quick-Start Packet or a lost feedback packet is specific to a 
   particular CCID (see section 3).  
 
2.6 Interactions with Mobility and Signalled Path Changes 
    
   The use of Quick-Start may assist end hosts in determining when it 
   is appropriate to increase their rate following an explicitly 
   signalled change of the network path.  
    
   When an end host receives a signal from an upstream link/network 
   notifying it of a path change, the change could simultaneously 
   impact more than one flow, and may affect flows between multiple 
   endpoints. Senders should avoid responding immediately, since this 
   could result in unwanted synchronisation of signalling messages, and 
   control loops (e.g. a synchronised attempt to probe for a larger 
   congestion window), which may negatively impact the performance of 
   the network and transport sessions. In Quick-Start, this could 
   increase the rate of Quick-Start Requests, possibly incurring 
   additional router load, and may result in some requests not being 
   granted. A sender 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]) 
   SHOULD reset the Quick-Start Interval to its initial value 
   (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 (Initial_QSI) since it 
        previously sent a Quick-Start Request. That is, it must wait 
        for at least a period of Initial_QSI after the previous 
        request, before sending a new Quick-Start Request. 
    
     . If it has not sent a Quick-Start Request within the previous 
        Initial_QSI period, it SHOULD defer sending a Quick-Start 
        Request for a randomly chosen period between 0 and the 
        Initial_QSI value in 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 

  
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        multiple connections in response to a path change that affects 
        multiple connections. 
    
   Hosts do not generally have sufficient information to choose an 
   appropriate randomisation interval. This value was selected to 
   ensure randomisation of requests over the Quick-Start Interval. In 
   networks where a large number of senders may potentially be impacted 
   by the same signal, a larger value may be desirable (or methods may 
   be used to control this effect in the path change signalling).  
               
2.7 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 of the request. It may therefore 
   be preferable to separately negotiate the PMTU and the use of Quick-
   Start. 
    
   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.  
    
2.8 Interactions with Middle boxes 
    
   A Quick-Start Request is carried in an IPv4 packet option or IPv6 
   extension header [RFC4782]. Interactions with network devices 
   (middle boxes) that inspect or modify IP options could therefore 
   lead 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 2.2). 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. 
 
 
  
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3. Mechanisms for Specific CCIDs 
 
   The following sections specify the use of Quick-Start with DCCP CCID 
   2, CCID 3, and for DCCP with TFRC-SP (as proposed for 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 CCID 2 flow.   
        
   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 that follows a reported 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 2.3. 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] (see Section 2.3).  
    
   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.  
    
   The sender sets its Quick-Start cwnd (QS_cwnd) as follows:  
 
             QS_cwnd = (R * T) / (s + H)                          (1)  
  

  
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   where R is the Rate Request in bytes per second, T is the measured 
   round-trip path delay (RTT), 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, but larger when using IPsec or IPv6).  
     
   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 
   receives a DCCP-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 DCCP-ACK to a packet sent in the 
   Quick-Start Mode, it leaves the Quick-Start Mode and enters the 
   Validation Phase.  
    
3.1.4 Quick-Start Validation Phase for CCID 2 
    
   A CCID 2 sender MAY continue to send at the paced Quick-Start Rate 
   while in the Validation Phase. It leaves the Validation Phase on 
   receipt of an ACK that acknowledges the last Quick-Start Packet, or 
   if the validation phase persists for a period that exceeds the 
   Quick-Start Validation Time of 1 RTT. It MUST then reduce the cwnd 
   to the actual flight size (the current amount of unacknowledged data 
   sent) [RFC4782], and uses the congestion control methods specified 
   for CCID 2. 
    
3.1.5 Reported loss or congestion while using Quick-Start 
    
   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).  It then resets the cwnd 
   to half the recorded previous cwnd and enters the congestion 
   avoidance phase described in [RFC4341].  
    
   In the absence of any feedback at the end of the Validation period, 
   the sender resets the cwnd to half the recorded previous cwnd and 
   enters the congestion avoidance phase. 
    
3.1.6 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 

  
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   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. 
    
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 CCID 3 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 
   [RFC5348]) 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 NOT 
   exceed the TFRC-controlled sending rate [RFC4342] when this is 
   bounded by the current loss event rate (if any), either from 
   calculation at the sender or from feedback received from the 
   receiver.  CCID 3 considers this rate is 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. 
    
3.2.3 Using the Quick-Start Response with CCID 3 
    
   On receipt of a valid Quick-Start Response option, the sender MUST 
   send a Quick-Start Approved option [RFC4782] (see Section 2.3).  
 
  
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   If the approved Quick-Start rate is greater than the current sending 
   rate, the sender enters the Quick-Start Mode, otherwise it does not 
   enter the Quick-Start Mode and continues using the procedure defined 
   in CCID 3.  
    
   If loss or congestion is reported after sending the Quick-Start 
   Request, the sender also does not enter the Quick-Start Mode and 
   continues using the procedure defined in CCID 3. 
    
   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 
   [RFC4342], and H the estimated DCCP/IP header size in bytes (e.g., 
   32 bytes for IPv4). A CCID 3 host MAY then increase its sending rate 
   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 
   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 the 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 Quick-Start Validation Phase for CCID 3 
    
   After transmitting a set of Quick-Start Packets in the Quick Start 
   Mode (and providing that no loss or congestion marking is reported), 
   the sender enters the Quick-Start Validation Phase. A sender that 
   receives feedback that reports a loss or congestion event MUST 
   follow the procedures described in Section 3.2.5. 
 
   The sender MUST 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) or if the Validation Phase persists 
   for a period that exceeds the Quick-Start Validation Time of two 
   RTTs.  
 
   A sender that completes the Quick-Start Validation Phase with no 
   reported packet loss or congestion stops using the QS_sendrate and 
  
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   MUST recalculate a suitable sending rate using the standard 
   congestion control mechanisms [RFC4342].  
 
   If no feedback is received within the Quick-Start Validation Phase, 
   the sender MUST return to the minimum of the recorded original rate 
   (at the start of the Quick-Start Mode) and one half of the 
   QS_sendrate. The nofeedback timer is also reset. 
          
3.2.5 Reported loss or congestion during the Quick-Start Mode or 
Validation Phase 
 
   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 sending 
   rate, X, as required by RFC4342: 
    
        X = max(min(X_calc, 2*X_recv), s/t_mbi); 
    
   where X_calc is the transmit rate calculated by the throughput 
   equation, X_recv is the reported receiver rate, s is the packet size 
   and t_mbi is the maximum backoff interval of 64 seconds. 
    
   The current specification of TFRC [RFC5348], 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 sending rate, X 
   as: 
         
    
        X = max(min(X_calc, recv_limit),s/t_mbi); 
     
   where recv_limit could be max(X_recv_set) or 2*max(X_recv_set) 
   depending 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, the recv_limit is 
   set to 2*max(X_recv_set). 
     
   A sender using RFC4342 updated by [RFC5348], calculates the sending 
   rate, X, using the above formula normatively defined in [RFC5348]. 
 
3.2.6 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 when 
   DCCP uses TFRC-SP [RFC4828] in place of TFRC [RFC5348], as specified 
   in 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. 
  
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   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 packets per second [RFC4828 CCID 4 [RFC4828] 
   specifies that the allowed sending rate derived from the TCP 
   throughput equation is reduced by a factor that accounts for packet 
   header size. 
    
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 is 
   updated to use TFRC-SP [RFC4828].  
    
   A CCID 4 sender does not need to account for headers a second time 
   when translating the approved Quick-Start rate into an allowed 
   sending rate (as described in section 5 of [ID.CCID4].  
    
3.3.4 Reported loss or congestion while using Quick-Start 
    
   This procedure is the same as for CCID 3, defined in 3.2.5, except 
   that the congestion control procedures is updated to use TFRC-SP 
   [RFC4828]. 
 
3.3.5 CCID 4 Feedback Traffic on the Reverse Path 
    
   A CCID 4 receiver sends feedback at least once each RTT. 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. The 
   document does not modify the router behaviour specified for Quick-
   Start. 
    
4.1 Over-run and the Quick-Start Validation Phase 
 
   The less frequent feedback of DCCP 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 would have used). This over-run is a result of 
   the less frequent feedback interval used by DCCP (i.e., CCID 2 may 
  
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   delay feedback by at most one half cwnd and CCID 3 and CCID 4 
   provide feedback at least once per RTT). In the method specified by 
   this document, the Quick-Start Validation Phase bounds this over-run 
   to be not more than an additional 2 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 (or 
   IPv6 extensions).  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) links 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 the 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. 
    
    
    
    
  
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5. IANA Considerations  
    
   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, Vincent Roca, Mark Allman, Michael 
   Scharf, Sally Floyd, and others in the IETF DCCP WG. 
    
    
7. Security Considerations 
    
   Security issues are discussed in [RFC4782].  Middlebox deployment 
   issues are also highlighted in section 2.9. 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. 
    
   [RFC4828] Floyd, S. and E. Kohler, "TCP Friendly Rate Control 
   (TFRC): The Small-Packet (SP) Variant", RFC 4828, April 2007. 
    
   [RFC5348] Floyd, S., Padhye, J., Widmer, J., "TCP Friendly Rate 
   Control (TFRC): Protocol Specification", RFC 5348, September 2008. 
 
  
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8.2 Informative References 
    
   [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. 
     
   [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", RFC 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 
   Scotland, UK 
   Email: gorry@erg.abdn.ac.uk 
   URI: http://www.erg.abdn.ac.uk/users/gorry 
    
   Arjuna Sathiaseelan 
   School of Engineering 
   University of Aberdeen 
   Aberdeen, AB24 3UE 
   Scotland, UK 
   Email: arjuna@erg.abdn.ac.uk 
   URI: http://www.erg.abdn.ac.uk/users/arjuna 
    
    
    
10. Copyright Notice 
 
   Copyright (c) 2009 IETF Trust and the persons identified as the 
   document authors.  All rights reserved. 
    
   This document is subject to BCP 78 and the IETF Trust's Legal 
   Provisions Relating to IETF Documents in effect on the date of 
   publication of this document (http://trustee.ietf.org/license-info).  
   Please review these documents carefully, as they describe your 
   rights and restrictions with respect to this document. 
    
   This document may contain material from IETF Documents or IETF 
   Contributions published or made publicly available before November 
   10, 2008.  The person(s) controlling the copyright in some of this 
  
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   material may not have granted the IETF Trust the right to allow 
   modifications of such material outside the IETF Standards Process. 
   Without obtaining an adequate license from the person(s) controlling 
   the copyright in such materials, this document may not be modified 
   outside the IETF Standards Process, and derivative works of it may 
   not be created outside the IETF Standards Process, except to format 
   it for publication as an RFC or to translate it into languages other 
   than English. 
    












































  
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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. 
    
    
   Working Group Draft 00 
   Title change only.  
    
   Working Group Draft 01 : Following WG feedback 
   Issued following feedback on Quick-Start issues and various comments 
   from Michael Scharf. CCID 2 feedback expected in 1.5 RTTs (allowing 
   for less frequent ACKs). TFRC-SP is now cited in CCID 4 discussion. 
   Included more on the rationale of the validation phase and the 
   mobility trigger randomisation interval of 6 seconds. 
    
   Note: This I-D must be Last-Called in both TSV and DCCP. 
    
   Working Group Draft 02 
   Clarified text on QS Interval. 
  
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   Harmonised the Quick-Start mode across all CCIDs. 
   Introduced the concept of the Quick_Start Validation Phase for CCID 
   2, as a counter to using a less frequent acknowledgment policy. This 
   behavior is now similar to that of CCID 3 in this regard. 
   A new subsection was added in Section 2 to describe the validation 
   for all CCIDs. 
    
   Working Group Draft 03 
   This revision renumbered some sections to improve organisation. 
   Revised ID following feedback from Vincent Roca. 
   Revised ID following feedback from Sally Floyd. 
    
   Working Group Draft 04 - based on Pasi's comments during WGLC 
 
    
   The constant 6 seconds was replaced by the text constant 
   Initital_QSI. 
    
   QSPrev_Interval has been removed - to avoid the ambiguity concerning 
   when to reset the value. 
    
   The max(Initital_QSI..) term has been removed, since the value can 
   never anyway be less than Initital_QSI, and the text now reflects 
   this. 
    
   The CCID 3 equations have been simplified, by removing a smaller 
   term from a MIN(,) expression - again this value can not change the 
   calculation. 
    
   The QS Approve method was mentioned in section 2 for completeness. 
    
   Working Group Draft 04 - based on Pasi's comments aftre WGLC 
    
   Fixed section number ref in IANA section. 
    
   Note: This I-D must be Last-Called in both TSV and DCCP. 
    
   [END of RFC EDITOR NOTE]  
    
 













  
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