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Network Working Group                                            P. Amer
Internet-Draft                                    University of Delaware
Intended status: Experimental                                   M. Becke
Expires: September 11, 2012                                 T. Dreibholz
                                            University of Duisburg-Essen
                                                                 N. Ekiz
                                                  University of Delaware
                                                              J. Iyengar
                                           Franklin and Marshall College
                                                            P. Natarajan
                                                           Cisco Systems
                                                              R. Stewart
                                                          Adara Networks
                                                               M. Tuexen
                                        Muenster Univ. of Appl. Sciences
                                                          March 10, 2012


    Load Sharing for the Stream Control Transmission Protocol (SCTP)
                draft-tuexen-tsvwg-sctp-multipath-04.txt

Abstract

   The Stream Control Transmission Protocol (SCTP) supports multi-homing
   for providing network fault tolerance.  However, mainly one path is
   used for data transmission.  Only timer-based retransmissions are
   carried over other paths as well.

   This document describes how multiple paths can be used simultaneously
   for transmitting user messages.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on September 11, 2012.




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Copyright Notice

   Copyright (c) 2012 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Load Sharing . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Split Fast Retransmissions . . . . . . . . . . . . . . . .  4
     3.2.  Appropriate Congestion Window Growth . . . . . . . . . . .  5
     3.3.  Appropriate Delayed Acknowledgements . . . . . . . . . . .  5
   4.  Non-Renegable SACK . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Negotiation  . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  The New Chunk Type: Non-Renegable SACK (NR-SACK) . . . . .  7
     4.3.  An Illustrative Example  . . . . . . . . . . . . . . . . . 11
     4.4.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . 15
       4.4.1.  Sending an NR-SACK chunk . . . . . . . . . . . . . . . 15
       4.4.2.  Receiving an NR-SACK Chunk . . . . . . . . . . . . . . 17
   5.  Buffer Blocking Mitigation . . . . . . . . . . . . . . . . . . 18
     5.1.  Sender Buffer Splitting  . . . . . . . . . . . . . . . . . 18
     5.2.  Receiver Buffer Splitting  . . . . . . . . . . . . . . . . 18
     5.3.  Problems during Path Failure . . . . . . . . . . . . . . . 18
       5.3.1.  Problem Description  . . . . . . . . . . . . . . . . . 18
       5.3.2.  Solution: Potentially-failed Destination State . . . . 19
     5.4.  Non-Renegable SACK . . . . . . . . . . . . . . . . . . . . 19
       5.4.1.  Problem Description  . . . . . . . . . . . . . . . . . 19
       5.4.2.  Solution: Non-Renegable SACKs  . . . . . . . . . . . . 20
   6.  Handling of Shared Bottlenecks . . . . . . . . . . . . . . . . 20
     6.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 20
     6.2.  Initial Values . . . . . . . . . . . . . . . . . . . . . . 21
     6.3.  Congestion Window Growth . . . . . . . . . . . . . . . . . 21
     6.4.  Congestion Window Decrease . . . . . . . . . . . . . . . . 21
   7.  Chunk Scheduling . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  Socket API Considerations  . . . . . . . . . . . . . . . . . . 21
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
     9.1.  A New Chunk Type . . . . . . . . . . . . . . . . . . . . . 21
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     12.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24












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1.  Introduction

   One of the important features of the Stream Control Transmission
   Protocol (SCTP), which is currently specified in [RFC4960], is
   network fault tolerance.  This feature is for example required for
   Reliable Server Pooling (RSerPool, [RFC5351]).  Therefore,
   transmitting messages over multiple paths is supported, but only for
   redundancy.  So [RFC4960] does not specify how to use multiple paths
   simultaneously.

   This document overcomes this limitation by specifying how multiple
   paths can be used simultaneously.  This has several benefits:

   o  Improved bandwidth usage.

   o  Better availability check with real user messages compared to
      HEARTBEAT-based information.


2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  Load Sharing

   Basic requirement for applying SCTP load sharing is the Concurrent
   Multipath Transfer (CMT) extension of SCTP, which utilises multiple
   paths simultaneously.  We denote CMT-enabled SCTP as CMT-SCTP
   throughout this document.  CMT-SCTP is introduced in [IAS06] and in
   more detail in [I06], some illustrative examples of chunk handling
   are provided in [DBP10a].  CMT-SCTP provides three modifications to
   standard SCTP (split Fast Retransmissions, appropriate congestion
   window growth and delayed SACKs), which are described in the
   following subsections.

3.1.  Split Fast Retransmissions

   Paths with different latencies lead to overtaking of DATA chunks.
   This leads to gap reports, which are handled by Fast Retransmissions.
   However, due to the fact that multiple paths are used simultaneously,
   these Fast Retransmissions are usually useless and furthermore lead
   to a decreased congestion window size.

   To avoid unnecessary Fast Retransmissions, the sender has to keep
   track of the path each DATA chunk has been sent on and consider



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   transmission paths before performing Fast Retransmissions.  That is,
   on reception of a SACK, the sender MUST identify the highest
   acknowledged TSN on each path.  A chunk SHOULD only be considered as
   missing if its TSN is smaller than the highest acknowledged TSN on
   its path.  Section 3.1 of [DBP10a] contains an illustrated example.

3.2.  Appropriate Congestion Window Growth

   The congestion window adaptation algorithm for SCTP [RFC4960] allows
   increasing the congestion window only when a new cumulative ack
   (CumAck) is received by a sender.  When SACKs with unchanged CumAcks
   are generated (due to reordering) and later arrive at a sender, the
   sender does not modify its congestion window.  Since a CMT-SCTP
   receiver naturally observes reordering, many SACKs are sent
   containing new gap reports but not new CumAcks.  When these gaps are
   later acked by a new CumAck, congestion window growth occurs, but
   only for the data newly acked in the most recent SACK.  Data
   previously acked through gap reports will not contribute to
   congestion window growth, in order to prevent sudden increases in the
   congestion window resulting in bursts of data being sent.

   To overcome the problems described above, the congestion window
   growth has to be handled as follows [IAS06]:

   o  The sender SHOULD keep track of the earliest non-retransmitted
      outstanding TSN per path.

   o  The sender SHOULD keep track of the earliest retransmitted
      outstanding TSN per path.

   o  The in-order delivery per path SHOULD be deduced.

   o  The congestion window of a path SHOULD be increased when the
      earliest non-retransmitted outstanding TSN of this path is
      advanced ('Pseudo CumAck') OR when the earliest retransmitted
      outstanding TSN of this path is advanced ('RTX Pseudo CumAck').

   Section 3.2 of [DBP10a] contains an illustrated example of
   appropriate congestion window handling for CMT-SCTP.

3.3.  Appropriate Delayed Acknowledgements

   Standard SCTP [RFC4960] sends a SACK as soon as an out-of-sequence
   TSN has been received.  Delayed Acknowledgements are only allowed if
   the received TSNs are in sequence.  However, due to the load
   balancing of CMT-SCTP, DATA chunks may overtake each other.  This
   leads to a high number of out-of-sequence TSNs, which have to be
   acknowledged immediately.  Clearly, this behaviour increases the



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   overhead traffic (usually nearly one SACK chunk for each received
   packet containing a DATA chunk).

   Delayed Acknowledgements for CMT-SCTP are handled as follows:

   o  In addition to [RFC4960], delaying of SACKs SHOULD *also* be
      applied for out-of-sequence TSNs.

   o  A receiver MUST maintain a counter for the number of DATA chunks
      received before sending a SACK.  The value of the counter is
      stored into each SACK chunk (FIXME: add details; needs reservation
      of flags bits by IANA).  After transmitting a SACK, the counter
      MUST be reset to 0.  Its initial value MUST be 0.

   o  The SACK handling procedure for a missing TSN M is extended as
      follows:

      *  If all newly acknowledged TSNs have been transmitted over the
         same path:

         +  If there are newly acknowledged TSNs L and H so that L <= M
            <= H, the missing count of TSN M SHOULD be incremented by
            one (like for standard SCTP according to [RFC4960]).

         +  Else if all newly acknowledged TSNs N satisfy the condition
            M <= N, the missing count of TSN M SHOULD be incremented by
            the number of TSNs reported in the SACK chunk.

      *  Otherwise (that is, there are newly acknowledged TSNs on
         different paths), the missing count of TSN M SHOULD be
         incremented by one (like for standard SCTP according to
         [RFC4960]).

   Section 3.3 of [DBP10a] contains an illustrated example of Delayed
   Acknowledgements for CMT-SCTP.


4.  Non-Renegable SACK

4.1.  Negotiation

   Before sending/receiving NR-SACKs, both peer endpoints MUST agree on
   using NR-SACKs.  This agreement MUST be negotiated during association
   establishment.  NR-SACK is an extension to the core SCTP, and SCTP
   extensions that an endpoint supports are reported to the peer
   endpoint in Supported Extensions Parameter during association
   establishment (see Section 4.2.7 of [RFC5061].)  The Supported
   Extensions Parameter consists of a list of non-standard Chunk Types



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   that are supported by the sender.

   An endpoint supporting the NR-SACK extension MUST list the NR-SACK
   chunk in the Supported Extensions Parameter carried in the INIT or
   INIT-ACK chunk, depending on whether the endpoint initiates or
   responds to the initiation of the association.  If the NR-SACK chunk
   type ID is listed in the Chunk Types List of the Supported Extensions
   Parameter, then the receiving endpoint MUST assume that the NR-SACK
   chunk is supported by the sending endpoint.

   Both endpoints MUST support NR-SACKs for either endpoint to send an
   NR-SACK.  If an endpoint establishes an association with a remote
   endpoint that does not list NR-SACK in the Supported Extensions
   Parameter carried in INIT chunk, then both endpoints of the
   association MUST NOT use NR-SACKs.  After association establishment,
   an endpoint MUST NOT renegotiate the use of NR-SACKs.

   Once both endpoints indicate during association establishment that
   they support the NR-SACK extension, each endpoint SHOULD acknowledge
   received DATA chunks with NR-SACK chunks, and not SACK chunks.  That
   is, throughout an SCTP association, both endpoints SHOULD send either
   SACK chunks or NR-SACK chunks, never a mixture of the two.

4.2.  The New Chunk Type: Non-Renegable SACK (NR-SACK)

   Table 1 illustrates a new chunk type that will be used to transfer
   NR-SACK information.

      Chunk Type  Chunk Name
      --------------------------------------------------------------
      0x10        Non-Renegable Selective Acknowledgment    (NR-SACK)

      Table 1: NR-SACK Chunk

   As the NR-SACK chunk replaces the SACK chunk, many SACK chunk fields
   are preserved in the NR-SACK chunk.  These preserved fields have the
   same semantics with the corresponding SACK chunk fields, as defined
   in [RFC4960], Section 3.3.4.  The Gap Ack fields from RFC4960 have
   been renamed as R Gap Ack to emphasize their renegable nature.  Their
   semantics are unchanged.  For completeness, we describe all fields of
   the NR-SACK chunk, including those that are identical in the SACK
   chunk.

   Similar to the SACK chunk, the NR-SACK chunk is sent to a peer
   endpoint to (1) acknowledge DATA chunks received in-order, (2)
   acknowledge DATA chunks received out-of-order, and (3) identify DATA
   chunks received more than once (i.e., duplicate.)  In addition, the
   NR-SACK chunk (4) informs the peer endpoint of non-renegable out-of-



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   order DATA chunks.

     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 = 0x10 |  Chunk Flags  |         Chunk Length          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Cumulative TSN Ack                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Advertised Receiver Window Credit (a_rwnd)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Number of R Gap Ack Blocks = N |Number of NR Gap Ack Blocks = M|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Number of Duplicate TSNs = X  |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | R Gap Ack Block #1 Start      |   R Gap Ack Block #1 End      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  R Gap Ack Block #N Start     |  R Gap Ack Block #N End       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  NR Gap Ack Block #1 Start    |   NR Gap Ack Block #1 End     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   NR Gap Ack Block #M Start   |  NR Gap Ack Block #M End      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Duplicate TSN 1                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Duplicate TSN X                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type: 8 bits

   This field holds the IANA defined chunk type for NR-SACK chunk.  The
   suggested value of this field for IANA is 0x10.

   Chunk Flags: 8 bits




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   Currently not used.  It is recommended a sender set all bits to zero
   on transmit, and a receiver ignore this field.

   Chunk Length: 16 bits (unsigned integer) [Same as SACK chunk]

   This value represents the size of the chunk in bytes including the
   Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields.

   Cumulative TSN Ack: 32 bits (unsigned integer) [Same as SACK chunk]

   The value of the Cumulative TSN Ack is the last TSN received before a
   break in the sequence of received TSNs occurs.  The next TSN value
   following the Cumulative TSN Ack has not yet been received at the
   endpoint sending the NR-SACK.

   Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
   integer) [Same as SACK chunk]

   Indicates the updated receive buffer space in bytes of the sender of
   this NR-SACK, see Section 6.2.1 of [RFC4960] for details.

   Number of (R)enegable Gap Ack Blocks (N): 16 bits (unsigned integer)

   Indicates the number of Renegable Gap Ack Blocks included in this NR-
   SACK.

   Number of (N)on(R)enegable Gap Ack Blocks (M): 16 bits (unsigned
   integer)

   Indicates the number of Non-Renegable Gap Ack Blocks included in this
   NR-SACK.

   Number of Duplicate TSNs (X): 16 bits [Same as SACK chunk]

   Contains the number of duplicate TSNs the endpoint has received.
   Each duplicate TSN is listed following the NR Gap Ack Block list.

   Reserved : 16 bits

   Currently not used.  It is recommended a sender set all bits to zero
   on transmit, and a receiver ignore this field.

   (R)enegable Gap Ack Blocks:

   The NR-SACK contains zero or more R Gap Ack Blocks.  Each R Gap Ack
   Block acknowledges a subsequence of renegable out-of-order TSNs.  By
   definition, all TSNs acknowledged by R Gap Ack Blocks are "greater
   than" the value of the Cumulative TSN Ack.



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   Because of TSN numbering wraparound, comparisons and all arithmetic
   operations discussed in this document are based on "Serial Number
   Arithmetic" as described in Section 1.6 of [RFC4960].

   R Gap Ack Blocks are repeated for each R Gap Ack Block up to 'N'
   defined in the Number of R Gap Ack Blocks field.  All DATA chunks
   with TSNs >= (Cumulative TSN Ack + R Gap Ack Block Start) and <=
   (Cumulative TSN Ack + R Gap Ack Block End) of each R Gap Ack Block
   are assumed to have been received correctly, and are renegable.

   R Gap Ack Block Start: 16 bits (unsigned integer)

   Indicates the Start offset TSN for this R Gap Ack Block.  This number
   is set relative to the cumulative TSN number defined in Cumulative
   TSN Ack field.  To calculate the actual start TSN number, the
   Cumulative TSN Ack is added to this offset number.  The calculated
   TSN identifies the first TSN in this R Gap Ack Block that has been
   received.

   R Gap Ack Block End: 16 bits (unsigned integer)

   Indicates the End offset TSN for this R Gap Ack Block.  This number
   is set relative to the cumulative TSN number defined in the
   Cumulative TSN Ack field.  To calculate the actual TSN number, the
   Cumulative TSN Ack is added to this offset number.  The calculated
   TSN identifies the TSN of the last DATA chunk received in this R Gap
   Ack Block.

   N(on)R(enegable) Gap Ack Blocks:

   The NR-SACK contains zero or more NR Gap Ack Blocks.  Each NR Gap Ack
   Block acknowledges a continuous subsequence of non-renegable out-of-
   order DATA chunks.  If a TSN is nr-gap-acked in any NR-SACK chunk,
   then all subsequently transmitted NR-SACKs with a smaller cum-ack
   value than that TSN SHOULD also nr-gap-ack that TSN.

   NR Gap Ack Blocks are repeated for each NR Gap Ack Block up to 'M'
   defined in the Number of NR Gap Ack Blocks field.  All DATA chunks
   with TSNs >= (Cumulative TSN Ack + NR Gap Ack Block Start) and <=
   (Cumulative TSN Ack + NR Gap Ack Block End) of each NR Gap Ack Block
   are assumed to be received correctly, and are Non-Renegable.

   NR Gap Ack Block Start: 16 bits (unsigned integer)

   Indicates the Start offset TSN for this NR Gap Ack Block.  This
   number is set relative to the cumulative TSN number defined in
   Cumulative TSN Ack field.  To calculate the actual TSN number, the
   Cumulative TSN Ack is added to this offset number.  The calculated



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   TSN identifies the first TSN in this NR Gap Ack Block that has been
   received.

   NR Gap Ack Block End: 16 bits (unsigned integer)

   Indicates the End offset TSN for this NR Gap Ack Block.  This number
   is set relative to the cumulative TSN number defined in Cumulative
   TSN Ack field.  To calculate the actual TSN number, the Cumulative
   TSN Ack is added to this offset number.  The calculated TSN
   identifies the TSN of the last DATA chunk received in this NR Gap Ack
   Block.

   Note:

   NR Gap Ack Blocks and R Gap Ack Blocks in an NR-SACK chunk SHOULD
   acknowledge disjoint sets of TSNs.  That is, an out-of-order TSN
   SHOULD be listed in either an R Gap Ack Block or an NR Gap Ack Block,
   but not the both.  R Gap Ack Blocks and NR Gap Ack Blocks together
   provide the information as do the Gap Ack Block of a SACK chunk, plus
   additional information about non-renegability.

   If all out-of-order data acked by an NR-SACK are renegable, then the
   Number of NR Gap Ack Blocks MUST be set to 0.  If all out-of-order
   data acked by an NR-SACK are non-renegable, then the Number of R Gap
   Ack Blocks SHOULD be set to 0.  TSNs listed in R Gap Ack Block will
   be referred as r-gap-acked.

   Duplicate TSN: 32 bits (unsigned integer) [Same as SACK chunk]

   Indicates a duplicate TSN received since the last NR-SACK was sent.
   Exactly 'X' duplicate TSNs SHOULD be reported where 'X' was defined
   in Number of Duplicate TSNs field.

   Each duplicate TSN is listed in this field as many times as the TSN
   was received since the previous NR-SACK was sent.  For example, if a
   data receiver were to get the TSN 19 three times, the data receiver
   would list 19 twice in the outbound NR-SACK.  After sending the NR-
   SACK if the receiver received one more TSN 19, the receiver would
   list 19 as a duplicate once in the next outgoing NR-SACK.

4.3.  An Illustrative Example

   Assume the following DATA chunks have arrived at the receiver.








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                      --------------------------------
                      | TSN=16| SID=2 | SSN=N/A| U=1 |
                      --------------------------------
                      | TSN=15| SID=1 | SSN= 4 | U=0 |
                      --------------------------------
                      | TSN=14| SID=0 | SSN= 4 | U=0 |
                      --------------------------------
                      | TSN=13| SID=2 | SSN=N/A| U=1 |
                      --------------------------------
                      |                              |
                      --------------------------------
                      | TSN=11| SID=0 | SSN= 3 | U=0 |
                      -------------------------------
                      |                              |
                      --------------------------------
                      |                              |
                      --------------------------------
                      | TSN=8 | SID=2 | SSN=N/A| U=1 |
                      --------------------------------
                      | TSN=7 | SID=1 | SSN= 2 | U=0 |
                      --------------------------------
                      | TSN=6 | SID=1 | SSN= 1 | U=0 |
                      --------------------------------
                      | TSN=5 | SID=0 | SSN= 1 | U=0 |
                      --------------------------------
                      |                              |
                      --------------------------------
                      | TSN=3 | SID=1 | SSN= 0 | U=0 |
                      --------------------------------
                      | TSN=2 | SID=0 | SSN= 0 | U=0 |
                      --------------------------------


   The above figure shows the list of DATA chunks at the receiver.  TSN
   denotes the transmission sequence number of the DATA chunk, SID
   denotes the stream id to which the DATA chunk belongs, SSN denotes
   the sequence number of the DATA chunk within its stream, and the U
   bit denotes whether the DATA chunk requires ordered(=0) or
   unordered(=1) delivery [RFC4960].  Note that TSNs 4,9,10, and 12 have
   not arrived.

   This data can be viewed as three separate streams as follows (assume
   each stream begins with SSN=0.)  Note that in this example, the
   application uses stream 2 for unordered data transfer.  By
   definition, SSN fields of unordered DATA chunks are ignored.

   Stream-0:




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   SSN:         0     1     2     3     4
   TSN:      |  2  |  5  |     |  11 |  14 |
   U-Bit:    |  0  |  0  |     |  0  |   0 |

   Stream-1:

   SSN:         0     1     2     3     4
   TSN:      |  3  |  6  |  7  |     |  15 |
   U-Bit:    |  0  |  0  |  0  |     |   0 |

   Stream-2:

   SSN:        N/A   N/A   N/A
   TSN:      |  8  |  13 |  16 |
   U-Bit:    |  1  |   1 |   1 |

   The NR-SACK to acknowledge the above data SHOULD be constructed as
   follows for each of the three cases described below (the a_rwnd is
   arbitrarily set to 4000):

   CASE-1: Minimal Data Receiver Responsibility - no out-of-order
   deliverable data yet delivered

   None of the deliverable out-of-order DATA chunks have been delivered,
   and the receiver of the above data does not take responsibility for
   any of the received out-of-order DATA chunks.  The receiver reserves
   the right to renege any or all of the out-of-order DATA chunks.

       +-----------------------------+-----------------------------+
       | Type = 0x10  |   00000000   |      Chunk Length = 32      |
       +-----------------------------+-----------------------------+
       |                    Cumulative TSN Ack = 3                 |
       +-----------------------------+-----------------------------+
       |                        a_rwnd = 4000                      |
       +-----------------------------+-----------------------------+
       | Num of R Gap Ack Blocks = 3 |Num of NR Gap Ack Blocks = 0 |
       +-----------------------------+-----------------------------+
       |    Num of Duplicates = 0    |            0x00             |
       +-----------------------------+-----------------------------+
       |R Gap Ack Block #1 Start = 2 | R Gap Ack Block #1 End = 5  |
       +-----------------------------+-----------------------------+
       |R Gap Ack Block #2 Start = 8 | R Gap Ack Block #2 End = 8  |
       +-----------------------------+-----------------------------+
       |R Gap Ack Block #3 Start = 10| R Gap Ack Block #3 End = 13 |
       +-----------------------------+-----------------------------+


   CASE-2: Minimal Data Receiver Responsibility - all out-of-order



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   deliverable data delivered

   In this case, the NR-SACK chunk is being sent after the data receiver
   has delivered all deliverable out-of-order DATA chunks to its
   receiving application(i.e., TSNs 5,6,7,8,13, and 16.)  The receiver
   reserves the right to renege on all undelivered out-of-order DATA
   chunks(i.e., TSNs 11,14, and 15.)


      +------------------------------+------------------------------+
      | Type = 0x10  |     0x00      |       Chunk Length = 40      |
      +------------------------------+------------------------------+
      |                     Cumulative TSN Ack = 3                  |
      +------------------------------+------------------------------+
      |                        a_rwnd = 4000                        |
      +------------------------------+------------------------------+
      | Num of R Gap Ack Blocks = 2  | Num of NR Gap Ack Blocks = 3 |
      +------------------------------+------------------------------+
      |    Num of Duplicates = 0     |              0x00            |
      +------------------------------+------------------------------+
      | R Gap Ack Block #1 Start = 8 | R Gap Ack Block #1 End = 8   |
      +------------------------------+------------------------------+
      | R Gap Ack Block #2 Start = 11| R Gap Ack Block #2 End = 12  |
      +------------------------------+------------------------------+
      |NR Gap Ack Block #1 Start = 2 | NR Gap Ack Block #1 End = 5  |
      +------------------------------+------------------------------+
      |NR Gap Ack Block #2 Start = 10| NR Gap Ack Block #2 End = 10 |
      +------------------------------+------------------------------+
      |NR Gap Ack Block #3 Start = 13| NR Gap Ack Block #3 End = 13 |
      +------------------------------+------------------------------+

   CASE-3: Maximal Data Receiver Responsibility

   In this special case, all out-of-order data blocks acknowledged are
   non-renegable.  This case would occur when the data receiver is
   programmed never to renege, and takes responsibility to deliver all
   DATA chunks that arrive out-of-order.  In this case Num of R Gap Ack
   Blocks is zero indicating all reported out-of-order TSNs are nr-gap-
   acked.












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     +--------------------------------+-------------------------------+
     |  Type = 0x10   |     0x00      |     Chunk Length = 32         |
     +--------------------------------+-------------------------------+
     |                      Cumulative TSN Ack = 3                    |
     +--------------------------------+-------------------------------+
     |                          a_rwnd = 4000                         |
     +--------------------------------+-------------------------------+
     |  Num of R Gap Ack Blocks = 0   |  Num of NR Gap Ack Blocks = 3 |
     +--------------------------------+-------------------------------+
     |     Num of Duplicates = 0      |             0x00              |
     +--------------------------------+-------------------------------+
     | NR Gap Ack Block #1 Start = 2  | NR Gap Ack Block #1 End = 5   |
     +--------------------------------+-------------------------------+
     | NR Gap Ack Block #2 Start = 8  | NR Gap Ack Block #2 End = 8   |
     +--------------------------------+-------------------------------+
     | NR Gap Ack Block #3 Start = 10 | NR Gap Ack Block #3 End = 13  |
     +--------------------------------+-------------------------------+

4.4.  Procedures

   The procedures regarding "when" to send an NR-SACK chunk are
   identical to the procedures regarding when to send a SACK chunk, as
   outlined in Section 6.2 of [RFC4960].

4.4.1.  Sending an NR-SACK chunk

   All of the NR-SACK chunk fields identical to the SACK chunk MUST be
   formed as described in Section 6.2 of [RFC4960].

   It is up to the data receiver whether or not to take responsibility
   for delivery of each out-of-order DATA chunk.  An out-of-order DATA
   chunk that has already been delivered, or that the receiver takes
   responsibility to deliver (i.e., guarantees not to renege) is Non
   Renegable(NR), and SHOULD be included in an NR Gap Ack Block field of
   the outgoing NR-SACK.  All other out-of-order data is (R)enegable,
   and SHOULD be included in R Gap Ack Block field of the outgoing NR-
   SACK.

   Consider three types of data receiver:

   CASE-1:  Data receiver takes no responsibility for delivery of any
      out-of-order DATA chunks


   CASE-2:  Data receiver takes responsibility for all out-of-order DATA
      chunks that are "deliverable" (i.e., DATA chunks in-sequence
      within the stream they belong to, or DATA chunks whose (U)nordered
      bit is 1)



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   CASE-3:  Data receiver takes responsibility for delivery of all out-
      of-order DATA chunks, whether deliverable or not deliverable


   The data receiver SHOULD follow the procedures outlined below for
   building the NR-SACK.

   CASE-1:

   1A)  Identify the TSNs received out-of-order.


   1B)  For these out-of-order TSNs, identify the R Gap Ack Blocks.
      Fill the Number of R Gap Ack Blocks (N) field, R Gap Ack Block #i
      Start, and R Gap Ack Block #i End where i goes from 1 to N.


   1C)  Set the Number of NR Gap Ack Blocks (M) field to 0.


   CASE-2:

   2A)  Identify the TSNs received out-of-order.


   2B)  For the received out-of-order TSNs, check the (U)nordered bit of
      each TSN.  Tag unordered TSNs as NR.


   2C)  For each stream, also identify the TSNs received out-of-order
      but are in-sequence within that stream.  Tag those in-sequence
      TSNs as NR.


   2D)  Tag all out-of-order data that is not NR as (R)enegable.


   2E)  For those TSNs tagged as (R)enegable, identify the (R)enegable
      Blocks.  Fill the Number of R Gap Ack Blocks(N) field, R Gap Ack
      Block #i Start, and R Gap Ack Block #i End where i goes from 1 to
      N.


   2F)  For those TSNs tagged as NR, identify the NR Blocks.  Fill the
      Number of NR Gap Ack Blocks(M) field, NR Gap Ack Block #i Start,
      and NR Gap Ack Block #i End where i goes from 1 to M.





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   CASE-3:

   3A)  Identify the TSNs received out-of-order.  All of these TSNs
      SHOULD be nr-gap-acked.


   3B)  Set the Number of R Gap Ack Blocks (N) field to 0.


   3C)  For these out-of-order TSNs, identify the NR Gap Ack Blocks.
      Fill the Number of NR Gap Ack Blocks (M) field, NR Gap Ack Block
      #i Start, and NR Gap Ack Block #i End where i goes from 1 to M.


   RFC4960 states that the SCTP endpoint MUST report as many Gap Ack
   Blocks as can fit in a single SACK chunk limited by the current path
   MTU.  When using NR-SACKs, the SCTP endpoint SHOULD fill as many R
   Gap Ack Blocks and NR Gap Ack Blocks starting from the Cumulative TSN
   Ack value as can fit in a single NR-SACK chunk limited by the current
   path MTU.  If space remains, the SCTP endpoint SHOULD fill as many
   Duplicate TSNs as possible starting from Cumulative TSN Ack value.

4.4.2.  Receiving an NR-SACK Chunk

   When an NR-SACK chunk is received, all of the NR-SACK fields
   identical to a SACK chunk SHOULD be processed and handled as in SACK
   chunk handling outlined in Section 6.2.1 of [RFC4960].

   The NR Gap Ack Block Start(s) and NR Gap Ack Block End(s) are offsets
   relative to the cum-ack.  To calculate the actual range of nr-gap-
   acked TSNs, the cum-ack MUST be added to the Start and End.

   For example, assume an incoming NR-SACK chunk's cum-ack is 12 and an
   NR Gap Ack Block defines the NR Gap Ack Block Start=5, and the NR Gap
   Ack Block End=7.  This NR Gap Ack block nr-gap-acks TSNs 17 through
   19 inclusive.

   Upon reception of an NR-SACK chunk, all TSNs listed in either R Gap
   Ack Block(s) or NR Gap Ack Block(s) SHOULD be processed as would be
   TSNs included in Gap Ack Block(s) of a SACK chunk.  All TSNs in all
   NR Gap Ack Blocks SHOULD be removed from the data sender's
   retransmission queue as their delivery to the receiving application
   has either already occurred, or is guaranteed by the data receiver.

   Although R Gap Ack Blocks and NR Gap Ack Blocks SHOULD be disjoint
   sets, NR-SACK processing SHOULD work if an NR-SACK chunk has a TSN
   listed in both an R Gap Ack Block and an NR Gap Ack Block.  In this
   case, the TSN SHOULD be treated as Non-Renegable.



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   Implementation Note:

   Most of NR-SACK processing at the data sender can be implemented by
   using the same routines as in SACK that process the cum ack and the
   gap ack(s), followed by removal of nr-gap-acked DATA chunks from the
   retransmission queue.  However, with NR-SACKs, as out-of-order DATA
   is sometimes removed from the retransmission queue, the gap ack
   processing routine should recognize that the data sender's
   retransmission queue has some transmitted data removed.  For example,
   while calculating missing reports, the gap ack processing routine
   cannot assume that the highest TSN transmitted is always at the tail
   (right edge) of the retransmission queue.


5.  Buffer Blocking Mitigation

   TBD.  See [ADB11], [DBR10].

5.1.  Sender Buffer Splitting

   TBD.  See [ADB11], [DBR10].

5.2.  Receiver Buffer Splitting

   TBD.  See [ADB11], [DBR10].

5.3.  Problems during Path Failure

   This section discusses CMT's receive buffer related problems during
   path failure, and proposes a solution for the same.

5.3.1.  Problem Description

   Link failures arise when a router or a link connecting two routers
   fails due to link disconnection, hardware malfunction, or software
   error.  Overloaded links caused by flash crowds and denial-of-service
   (DoS) attacks also degrade end-to-end communication between peer
   hosts.  Ideally, the routing system detects link failures, and in
   response, reconfigures the routing tables and avoids routing traffic
   via the failed link.  However, existing research highlights problems
   with Internet backbone routing that result in long route convergence
   times.  The pervasiveness of path failures motivated us to study
   their impact on CMT, since CMT achieves better throughput via
   simultaneous data transmission over multiple end-to-end paths.

   CMT is an extension to SCTP, and therefore retains SCTP's failure
   detection process.  A CMT sender uses a tunable failure detection
   threshold called Path.Max.Retrans (PMR).  When a sender experiences



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   more than PMR consecutive timeouts while trying to reach an active
   destination, the destination is marked as failed.  With PMR=5, the
   failure detection takes 6 consecutive timeouts or 63s.  After every
   timeout, the CMT sender continues to transmit new data on the failed
   path increasing the chances of receive buffer (rbuf) blocking and
   degrading CMT performance during permanent and short-term path
   failures [NEA08].

5.3.2.  Solution: Potentially-failed Destination State

   To mitigate the rbuf blocking, we introduce a new destination state
   called 'potentially-failed' state in SCTP (and CMT's) failure
   detection process [I-D.nishida-tsvwg-sctp-failover].  This solution
   is based on the rationale that loss detected by a timeout implies
   either severe congestion or failure en route.  After a single timeout
   on a path, a sender is unsure, and marks the corresponding
   destination as 'potentially-failed' (PF).  A PF destination is not
   used for data transmission or retransmission.  CMT's retransmission
   policies are augmented to include the PF state.  Performance
   evaluations prove that the PF state significantly reduces rbuf
   blocking during failure detection [NEA08].

5.4.  Non-Renegable SACK

   This section discusses problems with SCTP's SACK mechanism and how it
   affects the send buffer and CMT performance.

5.4.1.  Problem Description

   Gap-acks acknowledge DATA chunks that arrive out-of-order to a
   transport layer data receiver.  A gap-ack in SCTP is advisory, in
   that, while it notifies a data sender about the reception of
   indicated DATA chunks, the data receiver is permitted to later
   discard DATA chunks that it previously had gap-acked.  Discarding a
   previously gap-acked DATA chunk is known as 'reneging'.  Because of
   the possibility of reneging in SCTP, any gap-acked DATA chunk MUST
   NOT be removed from the data sender's retransmission queue until the
   DATA chunk is later CumAcked.

   Situations exist when a data receiver knows that reneging on a
   particular out-of-order DATA chunk will never take place, such as
   (but not limited to) after an out-of-order DATA chunk is delivered to
   the receiving application.  With current SACKs in SCTP, it is not
   possible for a data receiver to inform a data sender if or when a
   particular out-of-order 'deliverable' DATA chunk has been 'delivered'
   to the receiving application.  Thus the data sender MUST keep a copy
   of every gap-acked out-of-order DATA chunk(s) in the data sender's
   retransmission queue until the DATA chunk is CumAcked.  This use of



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   the data sender's retransmission queue is wasteful.  The wasted
   buffer often degrades CMT performance; the degradation increases when
   a CMT flow traverses via paths with disparate end-to-end properties
   [NEY08].

5.4.2.  Solution: Non-Renegable SACKs

   Non-Renegable Selective Acknowledgments (NR-SACKs) Section 4 are a
   new kind of acknowledgements, extending SCTP's SACK chunk
   functionalities.  The NR-SACK chunk is an extension of the existing
   SACK chunk.  Several fields are identical, including the Cumulative
   TSN Ack, the Advertised Receiver Window Credit (a_rwnd), and
   Duplicate TSNs.  These fields have the same semantics as described in
   [RFC4960].

   NR-SACKs also identify out-of-order DATA chunks that a receiver
   either: (1) has delivered to its receiving application, or (2) takes
   full responsibility to eventually deliver to its receiving
   application.  These out-of-order DATA chunks are 'non-renegable.'
   Non-Renegable data are reported in the NR Gap Ack Block field of the
   NR-SACK chunk as described Section 4.  We refer to non-renegable
   selective acknowledgements as 'nr-gap-acks.'

   When an out-of-order DATA chunk is nr-gap-acked, the data sender no
   longer needs to keep that particular DATA chunk in its retransmission
   queue, thus allowing the data sender to free up its buffer space
   sooner than if the DATA chunk were only gap-acked.  NR-SACKs improve
   send buffer utilization and throughput for CMT flows [NEY08].


6.  Handling of Shared Bottlenecks

6.1.  Introduction

   CMT-SCTP assumes all paths to be disjoint.  Since each path
   independently uses a TCP-like congestion control, an SCTP association
   using N paths over the same bottleneck acquires N times the bandwidth
   of a concurrent TCP flow.  This is clearly unfair.  A reliable
   detection of shared bottlenecks is impossible in arbitrary networks
   like the Internet.  Therefore, [DBA11], [DBP10b] apply the idea of
   Resource Pooling to CMT-SCTP.  Resource Pooling (RP) denotes 'making
   a collection of resources behave like a single pooled resource'
   [WHB09].  The modifications of RP-enabled CMT-SCTP, further denoted
   as CMT/RP-SCTP, are described in the following subsections.  A
   detailed description of CMT/RP-SCTP, including congestion control
   examples, can be found in [DBA11], [DBP10b].





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6.2.  Initial Values

   TDB.

6.3.  Congestion Window Growth

   TDB.  See [DBA11].

6.4.  Congestion Window Decrease

   TDB.  See [DBA11].


7.  Chunk Scheduling

   TDB.  See [DST10].


8.  Socket API Considerations

   See [I-D.dreibholz-tsvwg-sctpsocket-multipath] and
   [I-D.dreibholz-tsvwg-sctpsocket-sqinfo].


9.  IANA Considerations

   [NOTE to RFC-Editor:

      "RFCXXXX" is to be replaced by the RFC number you assign this
      document.

   ]

   [NOTE to RFC-Editor:

      The suggested values for the chunk type and the chunk parameter
      types are tentative and to be confirmed by IANA.

   ]

   This document (RFCXXXX) is the reference for all registrations
   described in this section.  The suggested changes are described
   below.

9.1.  A New Chunk Type

   A chunk type has to be assigned by IANA.  It is suggested to use the
   values given in Section 4.  IANA should assign this value from the



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   pool of chunks with the upper two bits set to '00'.

   This requires an additional line in the "Chunk Types" registry for
   SCTP:

   Chunk Types

   ID Value    Chunk Type                                     Reference
   -----       ----------                                     ---------
   16          Non-Renegable SACK (NR-SACK)                   [RFCXXXX]

   The registration table as defined in [RFC6096] for the chunk flags of
   this chunk type is empty.


10.  Security Considerations

   This document does not add any additional security considerations in
   addition to the ones given in [RFC4960].


11.  Acknowledgments

   The authors wish to thank Phillip Conrad, Jonathan Leighton, and
   Ertugrul Yilmaz for their invaluable comments.


12.  References

12.1.  Normative References

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

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              September 2007.

   [RFC5351]  Lei, P., Ong, L., Tuexen, M., and T. Dreibholz, "An
              Overview of Reliable Server Pooling Protocols", RFC 5351,
              September 2008.

   [RFC6096]  Tuexen, M. and R. Stewart, "Stream Control Transmission
              Protocol (SCTP) Chunk Flags Registration", RFC 6096,



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              January 2011.

   [I-D.nishida-tsvwg-sctp-failover]
              Nishida, Y., Natarajan, P., and A. Caro, "Quick Failover
              Algorithm in SCTP", draft-nishida-tsvwg-sctp-failover-04
              (work in progress), September 2011.

   [I-D.dreibholz-tsvwg-sctpsocket-multipath]
              Dreibholz, T., Becke, M., and H. Adhari, "SCTP Socket API
              Extensions for Concurrent Multipath Transfer",
              draft-dreibholz-tsvwg-sctpsocket-multipath-03 (work in
              progress), March 2012.

   [I-D.dreibholz-tsvwg-sctpsocket-sqinfo]
              Dreibholz, T., Seggelmann, R., and M. Becke, "Sender Queue
              Info Option for the SCTP Socket API",
              draft-dreibholz-tsvwg-sctpsocket-sqinfo-03 (work in
              progress), March 2012.

12.2.  Informative References

   [I06]      Iyengar, J., "End-to-End Concurrent Multipath Transfer
              Using Transport Layer Multihoming", PhD
              Dissertation Computer Science Dept., University of
              Delaware, April 2006.

   [IAS06]    Iyengar, J., Amer, P., and R. Stewart, "Concurrent
              Multipath Transfer Using SCTP Multihoming Over Independent
              End-to-End Paths", Journal IEEE/ACM Transactions on
              Networking, October 2006.

   [NEA08]    Natarajan, P., Ekiz, N., Iyengar, J., Amer, P., and R.
              Stewart, "Concurrent Multipath Transfer Using Transport
              Layer Multihoming: Introducing the Potentially-failed
              Destination State", Proceedings of the IFIP Networking,
              May 2008.

   [NEY08]    Natarajan, P., Ekiz, N., Yilmaz, E., Amer, P., Iyengar,
              J., and R. Stewart, "Non-Renegable Selective
              Acknowledgments (NR-SACKs) for SCTP", Proceedings of the
              16th IEEE International Conference on Network Protocols
              (ICNP) , October 2008.

   [WHB09]    Wischik, D., Handley, M., and M. Braun, "The Resource
              Pooling Principle", Journal ACM SIGCOMM Computer
              Communication Review, October 2009.

   [DBP10a]   Dreibholz, T., Becke, M., Pulinthanath, J., and E.



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              Rathgeb, "Implementation and Evaluation of Concurrent
              Multipath Transfer for SCTP in the INET Framework",
              Proceedings of the 3rd ACM/ICST OMNeT++ Workshop,
              March 2010.

   [DBP10b]   Dreibholz, T., Becke, M., Pulinthanath, J., and E.
              Rathgeb, "Applying TCP-Friendly Congestion Control to
              Concurrent Multipath Transfer", Proceedings of the IEEE
              24th International Conference on Advanced Information
              Networking and Applications (AINA), April 2010.

   [YEN10]    Yilmaz, E., Ekiz, N., Natarajan, P., Amer, P., Leighton,
              J., Baker, F., and R. Stewart, "Throughput analysis of
              Non-Renegable Selective Acknowledgments (NR-SACKs) for
              SCTP", Comput. Commun. (2010), doi:10.1016/
              j.comcom.2010.06.028 , 2010.

   [DST10]    Dreibholz, T., Seggelmann, R., Tuexen, M., and E. Rathgeb,
              "Transmission Scheduling Optimizations for Concurrent
              Multipath Transfer", Proceedings of the 8th International
              Workshop on Protocols for Future, Large-Scale and Diverse
              Network Transports (PFLDNeT) , November 2010.

   [DBR10]    Dreibholz, T., Becke, M., Rathgeb, E., and M. Tuexen, "On
              the Use of Concurrent Multipath Transfer over Asymmetric
              Paths", Proceedings of the IEEE Global Communications
              Conference (GLOBECOM), December 2010.

   [ADB11]    Adhari, H., Dreibholz, T., Becke, M., Rathgeb, E., and M.
              Tuexen, "Evaluation of Concurrent Multipath Transfer over
              Dissimilar Paths", Proceedings of the 1st International
              Workshop on Protocols and Applications with Multi-Homing
              Support (PAMS), March 2011.

   [DBA11]    Dreibholz, T., Becke, M., Adhari, H., and E. Rathgeb, "On
              the Impact of Congestion Control for Concurrent Multipath
              Transfer on the Transport Layer", Proceedings of the 11th
              IEEE International Conference on
              Telecommunications (ConTEL), June 2011.












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Authors' Addresses

   Paul D. Amer
   University of Delaware, Computer and Information Sciences Department
   Newark, DE  19716
   US

   Phone: +1-302-831-1944
   Email: amer@cis.udel.edu


   Martin Becke
   University of Duisburg-Essen, Institute for Experimental Mathematics
   Ellernstrasse 29
   45326 Essen, Nordrhein-Westfalen
   DE

   Phone: +49-201-183-7667
   Fax:   +49-201-183-7673
   Email: martin.becke@uni-due.de


   Thomas Dreibholz
   University of Duisburg-Essen, Institute for Experimental Mathematics
   Ellernstrasse 29
   45326 Essen, Nordrhein-Westfalen
   DE

   Phone: +49-201-183-7637
   Fax:   +49-201-183-7673
   Email: dreibh@iem.uni-due.de
   URI:   http://www.iem.uni-due.de/~dreibh/


   Nasif Ekiz
   University of Delaware, Computer and Information Sciences Department
   Newark, DE  19716
   US

   Email: nekiz@udel.edu











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   Janardhan Iyengar
   Franklin and Marshall College, Mathematics and Computer Science
   PO Box 3003
   Lancaster, Pennsylvania  17604-3003
   US

   Phone: +1-717-358-4774
   Email: jiyengar@fandm.edu
   URI:   http://www.fandm.edu/jiyengar/


   Preethi Natarajan
   Cisco Systems
   425 East Tasman Drive
   San Jose, California  95134
   US

   Email: prenatar@cisco.com


   Randall R. Stewart
   Adara Networks
   Chapin, SC  29036
   US

   Email: randall@lakerest.net


   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt
   DE

   Email: tuexen@fh-muenster.de
















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