LOOPS Generic Information Set

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Authors Michael Welzl  , Carsten Bormann 
Last updated 2020-07-13
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TSVWG                                                           M. Welzl
Internet-Draft                                        University of Oslo
Intended status: Standards Track                         C. Bormann, Ed.
Expires: 14 January 2021                          Universit├Ąt Bremen TZI
                                                            13 July 2020

                     LOOPS Generic Information Set


   LOOPS (Local Optimizations on Path Segments) aims to provide local
   (not end-to-end but in-network) recovery of lost packets to achieve
   better data delivery in the presence of losses.
   [I-D.li-tsvwg-loops-problem-opportunities] provides an overview over
   the problems and optimization opportunities that LOOPS could address.

   The present document is a strawman for the set of information that
   would be interchanged in a LOOPS protocol, without already defining a
   specific data packet format.

   The generic information set needs to be mapped to a specific
   encapsulation protocol to actually run the LOOPS optimizations.  A
   companion document contains a sketch of a binding to the tunnel
   encapsulation protocol Geneve [I-D.ietf-nvo3-geneve].

Status of This Memo

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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Challenges  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  No Access to End-to-End Transport Information . . . . . .   6
     2.2.  Path Asymmetry  . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Reordering vs. Spurious Retransmission  . . . . . . . . .   6
     2.4.  Informing the End-to-End Transport  . . . . . . . . . . .   7
   3.  Simplifying assumptions . . . . . . . . . . . . . . . . . . .   8
   4.  LOOPS Architecture  . . . . . . . . . . . . . . . . . . . . .   9
   5.  LOOPS Generic Information Set . . . . . . . . . . . . . . . .  10
     5.1.  Setup Information . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Forward Information . . . . . . . . . . . . . . . . . . .  10
     5.3.  Reverse Information . . . . . . . . . . . . . . . . . . .  11
   6.  LOOPS General Operation . . . . . . . . . . . . . . . . . . .  12
     6.1.  Initial Packet Sequence Number  . . . . . . . . . . . . .  12
       6.1.1.  Minimizing collisions . . . . . . . . . . . . . . . .  12
       6.1.2.  Optional Initial PSN procedure  . . . . . . . . . . .  12
     6.2.  Acknowledgement Generation  . . . . . . . . . . . . . . .  13
     6.3.  Measurement . . . . . . . . . . . . . . . . . . . . . . .  14
       6.3.1.  Ingress-relative timestamps . . . . . . . . . . . . .  14
       6.3.2.  ACK generation  . . . . . . . . . . . . . . . . . . .  15
     6.4.  Loss detection and Recovery . . . . . . . . . . . . . . .  15
       6.4.1.  Local Retransmission  . . . . . . . . . . . . . . . .  15
       6.4.2.  FEC . . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.5.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Sketches of Bindings to Tunnel Protocols  . . . . . . . . . .  16
     7.1.  Embedding LOOPS in Geneve . . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
     9.1.  Threat model  . . . . . . . . . . . . . . . . . . . . . .  17
     9.2.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .  18
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  Protocol used in Prototype Implementation  . . . . .  21
     A.1.  Block Code FEC  . . . . . . . . . . . . . . . . . . . . .  22
   Appendix B.  Transparent mode . . . . . . . . . . . . . . . . . .  22

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     B.1.  Packet identification . . . . . . . . . . . . . . . . . .  24
     B.2.  Generic information and protocol operation  . . . . . . .  25
     B.3.  A hybrid mode . . . . . . . . . . . . . . . . . . . . . .  25
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   Today's networks exhibit a wide variety of data rates and, relative
   to those, processing power and memory capacities of nodes acting as
   routers.  For instance, networks that employ tunneling to build
   overlay networks may position powerful virtual router nodes in the
   network to act as tunnel endpoints.  The capabilities available in
   the more powerful cases provide new opportunities for optimizations.

   LOOPS (Local Optimizations on Path Segments) aims to provide local
   (not end-to-end but in-network) recovery of lost packets to achieve
   better data delivery.  [I-D.li-tsvwg-loops-problem-opportunities]
   provides an overview over the problems and optimization opportunities
   that LOOPS could address.  One simplifying assumption (Section 3) in
   the present document is that LOOPS segments operate independently
   from each other, each as a pair of a LOOPS Ingress and a LOOPS Egress

   The present document is a strawman for the set of information that
   would be interchanged in a LOOPS protocol between these nodes,
   without already defining a specific data packet format.  The main
   body of the document defines a mode of the LOOPS protocol that is
   based on traditional tunneling, the "tunnel mode".  Appendix B is an
   even rougher strawman of a radically different, alternative mode that
   we call "transparent mode", as well as a slightly more conventional
   "hybrid mode" (Appendix B.3).  These different modes may be
   applicable to different usage scenarios and will be developed in
   parallel, with a view of ultimately standardizing one or more of

   For tunnel mode, the generic information set needs to be mapped to a
   specific encapsulation protocol to actually run the LOOPS
   optimizations.  LOOPS is not tied to any specific overlay protocol,
   but is meant to run embedded into a variety of tunnel protocols.
   LOOPS information is added as part of a tunnel protocol header at the
   LOOPS ingress as shown in Figure 1.  A companion document
   [I-D.bormann-loops-geneve-binding] contains a sketch of a binding to
   the tunnel encapsulation protocol Geneve [I-D.ietf-nvo3-geneve].

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             |           Outer header             |
           / |         Tunnel Base Header         |
         /   +------------------------------------+\
    Tunnel   |    +-------------------------+     | \
    Header   ~    |    LOOPS Information    |     ~  Tunnel Header
         \   |    +-------------------------+     |  Extensions
           \ +------------------------------------+ /
             |           Data packet              |

             Figure 1: Packet in Tunnel with LOOPS Information

   Figure 2 is extracted from the LOOPS problems and opportunities
   document [I-D.li-tsvwg-loops-problem-opportunities].  It illustrates
   the basic architecture and terms of the applicable scenario of LOOPS.
   Not all of the concepts introduced in the problems and opportunities
   document are actually used in the current strawman specification;
   Section 3 lays out some simplifying assumptions that the present
   proposal makes.

                                                      ON=overlay node
                                                      UN=underlay node

   +---------+                                               +---------+
   |   App   | <---------------- end-to-end ---------------> |   App   |
   +---------+                                               +---------+
   |Transport| <---------------- end-to-end ---------------> |Transport|
   +---------+                                               +---------+
   |         |                                               |         |
   |         |        +--+  path  +--+  path segment2  +--+  |         |
   |         |        |  |<-seg1->|  |<--------------> |  |  |         |
   | Network |  +--+  |ON|  +--+  |ON|  +--+   +----+  |ON|  | Network |
   |         |--|UN|--|  |--|UN|--|  |--|UN|---| UN |--|  |--|         |
   +---------+  +--+  +--+  +--+  +--+  +--+   +----+  +--+  +---------+
     End Host                                                  End Host
                        LOOPS domain: path segments enabling
                        optimization for local in-network recovery

                       Figure 2: LOOPS Usage Scenario

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1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document makes use of the terminology defined in
   [I-D.li-tsvwg-loops-problem-opportunities].  This section defines
   additional terminology used by this document.

   Data packets:  The payload packets that enter and exit a LOOPS

   LOOPS Segment:  A part of an end-to-end path covered by a single
      instance of the LOOPS protocol, the sub-path between the LOOPS
      Ingress and the LOOPS Egress.  Several LOOPS segments may be
      encountered on an end-to-end path, with or without intervening

   LOOPS Ingress:  The node that forwards data packets and forward
      information into the LOOPS segment, potentially performing
      retransmission and forward error correction based on
      acknowledgements and measurements received from the LOOPS Egress.

   LOOPS Egress:  The node that receives the data packets and forward
      information from the LOOPS ingress, sends acknowledgements and
      measurements back to the LOOPS ingress (reverse information),
      potentially recovers data packets from forward error correction
      information received.

   LOOPS Nodes:  Collective term for LOOPS Ingress and LOOPS Egress in a
      LOOPS Segment.

   Forward Information:  Information that is added to the stream of data
      packets in the forward direction by the LOOPS Ingress.

   Reverse Information:  Information that flows in the reverse
      direction, from the LOOPS Egress back to the LOOPS Ingress.

   Setup Information:  Information that is not transferred as part of
      the Forward or Reverse Information, but is part of the setup of
      the LOOPS Nodes.

   PSN:  Packet Sequence Number, a sequence number identifying a data
      packet between the LOOPS Ingress and Egress.

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   Sender:  Original sender of a packet on an end-to-end path that
      includes one or more LOOPS segment(s).

   Receiver:  Ultimate receiver of a packet on an end-to-end path that
      includes one or more LOOPS segment(s).

2.  Challenges

   LOOPS has to perform well in the presence of some challenges, which
   are discussed in this section.

2.1.  No Access to End-to-End Transport Information

   LOOPS is defined to be independent of the content of the packets
   being forwarded: there is no dependency on transport-layer or higher
   information.  The intention is to keep LOOPS useful with a traffic
   mix that may contain encrypted transport protocols such as QUIC as
   well as encrypted VPN traffic.

2.2.  Path Asymmetry

   A LOOPS segment is defined as a unidirectional forwarding path.  The
   tunnel might be shared with a LOOPS segment in the inverse direction;
   this then allows to piggyback Reverse Information on encapsulated
   packets on that segment.  But there is no guarantee that the inverse
   direction of any end-to-end-path crosses that segment, so the LOOPS
   optimizations have to be useful on their own in each direction.

2.3.  Reordering vs. Spurious Retransmission

   The end-to-end transport layer protocol may have its own
   retransmission mechanism to recover lost packets.  When LOOPS
   recovers a loss, ideally this local recovery would replace the
   triggering of a retransmission at the end-to-end sender.

   Whether this is possible depends on the specific end-to-end mechanism
   used for triggering retransmission.  When end-to-end retransmission
   is triggered by receiving a sequence of duplicate acknowledgements
   (DUPACKs), and with more than a few packets in flight, the recovered
   packet is likely to be too late to fill the hole in the sequence
   number space that triggers the DUPACK detection.

   (Given a reasonable setting of parameters, the local retransmission
   will still arrive earlier than the end-to-end retransmission and will
   possibly unblock application processing earlier; with spurious
   retransmission detection, there also will be little long-term effect
   on the send rate.)

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   While LOOPS makes no requirements on end-to-end protocols, it is
   worth noting that the waste of bandwidth caused by a DUPACK-based
   end-to-end retransmission can be avoided when the end-to-end loss
   detection is based on time instead of sequence numbers, e.g., with
   RACK [I-D.ietf-tcpm-rack].  This requires a limit on the additional
   latency that LOOPS will incur in its attempt to recover the loss
   locally.  In the present version of this document, opportunity to set
   such a limit is provided in the Setup Information.  The limit can be
   used to compute a deadline for retransmission, but also can be used
   to choose FEC parameters that keep extra latency low.

2.4.  Informing the End-to-End Transport

   Congestion control at the end-to-end sender is used to adapt its
   sending rate to the network congestion status.  In typical TCP
   senders, packet loss implies congestion and leads to a reduction in
   sending rate.  With LOOPS operating, packet loss can be masked from
   the sender as the loss may have been locally recovered.  In this
   case, rate reduction may not be invoked at the sender.  This is a
   desirable performance improvement if the loss was a random loss, but
   it is hard to ascertain that.

   If LOOPS successfully conceals congestion losses from the end-to-end
   transport protocol, that might increase the rate to a level that
   congests the LOOPS segment, or that causes excessive queueing at the
   LOOPS ingress.  What LOOPS should be able to achieve is to let the
   end host sender invoke the rate reduction mechanism when there is a
   congestion loss no matter if the lost packet was recovered locally.

   As with any tunneling protocol, information about congestion events
   inside the tunnel needs to be exported to the end-to-end path the
   tunnel is part of.  See e.g., [RFC6040] for a discussion of how to do
   this in the presence of ECN.  A more recent draft,
   [I-D.ietf-tsvwg-tunnel-congestion-feedback], proposes to activate ECN
   for the tunnel regardless of whether the end-to-end protocol signals
   the use of an ECN-capable transport (ECT), which requires more
   complicated action at the tunnel egress.

   A sender that interprets reordering as a signal of packet loss
   (DUPACKs) initiates a retransmission and reduces the sending rate.
   When spurious retransmission detection (e.g., via F-RTO [RFC5862] or
   DSACK [RFC3708]) is enabled by the TCP sender, it will often be able
   undo the unnecessary window reduction shortly afterwards.  As LOOPS
   recovers lost packets locally, in most cases the end host sender will
   eventually find out its reordering-based retransmission (if any) is
   spurious.  This is an appropriate performance improvement if the loss
   was a random loss.  For congestion losses, a congestion event needs
   to be signaled to the end-to-end transport.

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   The present version of LOOPS requires the end-to-end transport to be
   ECN-capable (which is visible at the IP level).  Congestion loss
   events can easily be signaled to them by setting the CE (congestion
   experienced) mark.  Effectively, LOOPS converts a packet loss (which
   would be a congestion indication) to a CE mark (which also is a
   congestion indication).

   In effect, LOOPS can be used to convert a path segment that does not
   yet use CE marks for congestion indication, and drops packets
   instead, into a segment that marks for congestion and does not drop
   packets except in extreme cases, incurring the benefits of Using
   Explicit Congestion Notification (ECN) [RFC8087].  We speak about the
   "drop-to-mark" function of LOOPS.

3.  Simplifying assumptions

   The above notwithstanding, Implementations may want to make use of
   indicators such as transport layer port numbers to partition a tunnel
   flow into separate application flows, e.g., for active queue
   management (AQM).  Any such functionality is orthogonal to the LOOPS
   protocol itself and thus out of scope for the present document.

   One observation that simplifies the design of LOOPS in comparison to
   that of a reliable transport protocol is that LOOPS does not _have_
   to recover every packet loss.  Therefore, probabilistic approaches,
   and simply giving up after some time has elapsed, can simplify the
   protocol significantly.

   For now, we assume that LOOPS segments that may line up on an end-to-
   end path operate independently of each other.  Since the objective of
   LOOPS ultimately is to assist the end-to-end protocol, it is likely
   that some cooperation between them would be beneficial, e.g., to
   obtain some measurements that cover a larger part of the end-to-end
   path.  For instance, cooperating LOOPS segments could try to divide
   up permissible increases to end-to-end latency between them.  This is
   out of scope for the present version.

   Another simplifying assumption is that LOOPS nodes have reasonably
   precise absolute time available to them, so there is no need to
   burden the LOOPS protocol with time synchronization.  How this is
   achieved is out of scope.

   LOOPS nodes are created and set up (information about their peers,
   parameters) by some control plane mechanism that is out of scope for
   this specification.  This means there is no need in the LOOPS
   protocol itself to manage setup information.

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4.  LOOPS Architecture

   From the above, the following architecture is derived for LOOPS.

   LOOPS governs the segment from an ingress node to an egress node,
   which is part of one or more end-to-end paths.  Often, a LOOPS
   segment will operate on aggregate traffic from many such end-to-end

   The LOOPS protocol itself does not define how a LOOPS segment and the
   protocol entities in the ingress and egress node are set up.  We
   expect that a _setup protocol_ on the control plane will provide some
   _setup information_ to the two nodes, including when to start and to
   tear down processing.

   Each LOOPS segment governs traffic on one direction in the segment.
   The LOOPS ingress adds _forward information_ to that traffic; the
   LOOPS egress removes the forward information and sends some _reverse
   information_ to inform the behavior of the ingress.

   Hence, in the data plane, forward information is added to each data
   packet.  Reverse information can be sent in separate packets (e.g.,
   Geneve control-only packets [I-D.ietf-nvo3-geneve]) and/or
   piggybacked on a related, reverse-direction LOOPS flow, similar to
   the way the forward information for that flow is carried.  The setup
   protocol is used to provide the relationship between the LOOPS
   segments in the two directions that is used for piggybacking reverse

   The above describes the "tunnel mode".  A transparent mode is
   described in Appendix B, which does not modify the data packets and
   therefore needs to send any forward information (if needed, e.g., for
   FEC) in separate packets, usually aggregated.

   The LOOPS _generic information set_ defines what information is
   provided as setup information, forward information, and reverse
   information. _Bindings_ map this information set to specific control
   plane and data plane protocols, including defining the specific
   encoding being used.  Where separate packets (outside the data plane
   protocols being used) need to be sent, a special UDP-based protocol
   needs to be defined as well.  The various bindings aim for some
   commonality, so that an implementation for multiple bindings does not
   need to support gratuitous variety between them.

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5.  LOOPS Generic Information Set

   This section sketches a generic information set for the LOOPS
   protocol.  Entries marked with (*) are items that may not be
   necessary and probably should be left out of an initial

5.1.  Setup Information

   Setup Information might include:

   *  encapsulation protocol in use, and its vital parameters

   *  identity of LOOPS ingress and LOOPS egress; information relevant
      for running the encapsulation protocol such as port numbers

   *  target maximum latency increase caused by the operation of LOOPS
      on this segment

   *  maximum retransmission count (*)

5.2.  Forward Information

   In the forward information, we have identified:

   *  tunnel type (a few bits, meaning agreed between Ingress and

   *  packet sequence number PSN (20+ bits), counting the data packets
      forwarded transmitted by the LOOPS ingress (i.e., retransmissions
      re-use the PSN)

   *  an "ACK desirable" flag (one bit, usually set for a certain
      percentage of the data packets only)

   *  an optional blob, to be echoed by the egress

   *  anything that the FEC scheme needs.

   The first four together (say, 3+24+4+1) might even fit into 32 bits,
   but probably need up to 48 bits total.  FEC info of course often
   needs more space.

   (Note that in this proposal there is no timestamp in the forward
   information; see Section 6.3.)

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   24 bits of PSN, minus one bit for sequence number arithmetic, gives 8
   million packets (or 2.4 GB at typical packet sizes) per worst-case
   RTT.  So if that is, say, 30 seconds, this would be enough to fill
   640 Mbit/s.

5.3.  Reverse Information

   For the reverse information, we have identified:

   *  one optional block 1, possibly repeated:

      -  PSN being acknowledged

      -  absolute time of reception for the packet acknowledged (PSN)

      -  the blob, if present, echoed back

   *  one optional block 2, possibly repeated:

      -  an ACK bitmap (based on PSN), always starting at a multiple of

      -  a delta indicating the end PSN of the bitmap (actually the
         first PSN that is beyond it), using (Acked-PSN & ~7) +
         8*(delta+1) as the end of the bitmap.  Acked-PSN in that
         formula is the previous block 1 PSN seen in this packet, or 0
         if none so far.

   Block 1 and Block 2 can be interspersed and repeated.  They can be
   piggybacked on a reverse direction data packet or sent separately if
   none occurs within some timeout.  They will usually be aggregated in
   some useful form.  Block 1 information sets are only returned for
   packets that have "ACK desirable" set.  Block 2 information is sent
   by the receiver based on some saturation scheme (e.g., at least three
   copies for each PSN span over time).  Still, it might be possible to
   go down to 1 or 2 amortized bytes per forward packet spent for all

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   The latency calculation is done by the sender, who occasionally sets
   "ACK desirable", and notes down the absolute time of transmission for
   this data packet (the timekeeping can be done quite efficiently as
   deltas).  Upon reception of a block 1 ACK, it can then subtract that
   from the absolute time of reception indicated.  This assumes time
   synchronization between the nodes is at least as good as the
   precision of latency measurement needed, which should be no problem
   with IEEE 1588 PTP synchronization (but could be if using NTP-based
   synchronization only).  A sender can freely garbage collect noted
   down transmission time information; doing this too early just means
   that the quality of the RTT sampling will reduce.

6.  LOOPS General Operation

   In the Tunnel Mode described in the main body of this document, LOOPS
   information is carried by some tunnel encapsulation.

6.1.  Initial Packet Sequence Number

   There is no connection establishment procedure in LOOPS.  The initial
   PSN is assigned unilaterally by the LOOPS Ingress.

   Because of the short time that is usually set in the maximum latency
   increase, there is little damage from a collision of PSNs with
   packets still in flight from previous instances of LOOPS.

6.1.1.  Minimizing collisions

   If desired, collisions can be minimized by assigning initial PSNs
   randomly, or using stable storage.  Random assignment is more useful
   for longer PSNs, where the likelihood of overlap will be low.  The
   specific way a LOOPS ingress uses stable storage is a local matter
   and thus out of scope.  (Implementation note: this can be made to
   work similar to secure nonce generation with write attenuation: Say,
   every 10000 packets, the sender notes down the PSN into stable
   storage.  After a reboot, it reloads the PSN and adds 10000 in
   sequence number arithmetic [RFC1982], plus maybe another 10000 so the
   sender does not have to wait for the store operation to succeed
   before sending more packets.)

6.1.2.  Optional Initial PSN procedure

   As a potential option (to be discussed), an initial packet sequence
   number could be communicated using a simple two-bit protocol, based
   on an I flag (Initial PSN) carried in the forward information and an
   R flag (Initial PSN Received) in the reverse information.  This
   procedure essentially clears the egress of any previous state,
   however, the benefits of this procedure are limited.

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   The initial PSN is assigned unilaterally by the LOOPS ingress,
   selected randomly.  The ingress will keep setting the I flag to one
   when it starts to send packets from a new beginning or whenever it
   believes there is a need to notify the egress about a new initial
   PSN.  The ingress will stop setting the I flag when it receives an
   acknowledgement with the R flag set from the egress.

   When the LOOPS egress receives a packets with the I flag set, it
   stops performing services that assume a sequential PSN.  The egress
   will no longer provide acknowledgement information for the packets
   with PSN smaller than this new initial PSN (per sequence number
   arithmetic [IEN74]).  The egress sends acknowledgement information
   back without any delay by echoing the value of the I flag in the R
   flag.  This also means the egress unsets the R flag in subsequent
   acknowledgements for packets with the I flag unset.

   It may happen that the first few packets are lost in an initial PSN
   assignment process.  In this case, the loss of these packets is not
   detectable by the LOOPS ingress since the first received PSN will be
   treated as an initial PSN at the egress.  This is an acceptable
   temporary performance degradation: LOOPS does not intend to provide
   perfect reliability, and LOOPS usually applies to the aggregated
   traffic over a tunnel so that the initial PSN assignment happens

6.2.  Acknowledgement Generation

   A data packet forwarded by the LOOPS ingress always carries PSN
   information.  The LOOPS egress uses the largest newly received PSN
   with the "ACK desired" bit as the ACK number in the block 1 part of
   the acknowledgement.  This means that the LOOPS ingress gets to
   modulate the number of acknowledgement sent by the LOOPS egress.
   However, whenever an out-of-order packet arrives while there still
   are "holes" in the PSNs received, the LOOPS receiver should generate
   a block 2 acknowledgement immediately that the LOOPS sender can use
   as an ACK list.

   Reverse information can be piggybacked in a reverse direction data
   packet.  When the reverse direction has no user data to be sent, a
   pure reverse information packet needs to be generated.  This may be
   based on a short delay during which the LOOPS egress waits for a data
   packet to piggyback on.  (To reduce MTU considerations, the egress
   could wait for less-than-full data packets.)

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6.3.  Measurement

   When sending a block 1 acknowledgement, the LOOPS egress indicates
   the absolute time of reception of the packet.  The LOOPS ingress can
   subtract the absolute time of transmission that it still has
   available, resulting in one high quality latency sample.  (In an
   alternative design, the forward information could include the
   absolute time of transmission as well, and block1 information would
   echo it back.  This trades memory management at the ingress for
   increased bandwidth and MTU reduction.)

   When a data packet has been transmitted, it may not be clear which
   specific copy is acknowledged in a block 1 acknowledgement: the
   acknowledgement for the initial (or, more generally, an earlier) copy
   may have been delayed (ACK ambiguity)).  The LOOPS ingress therefore
   SHOULD NOT base its measurements on acknowledgements for
   retransmitted data packets.  One way to achieve this is by not
   setting the "ACK desired" bit on retransmissions in the first place.

   The LOOPS ingress can also use the time of reception of the block 1
   acknowledgement to obtain a segment RTT sample.  Note that this will
   include any wait time the LOOPS egress incurs while waiting for a
   piggybacking opportunity -- this is appropriate, as all uses of an
   RTT will be for keeping a retransmission timeout.

   To maintain quality of information during idle times, the LOOPS
   ingress may send keepalive packets, which are discarded at the LOOPS
   egress after sending acknowledgements.  The indication that a packet
   is a keepalive packet is dependent on the encapsulation protocol.

6.3.1.  Ingress-relative timestamps

   As an optional procedure, the ingress node can attach a small blob of
   data to a forward packet that carries an ACK desired flag; this blob
   is then echoed by the egress in its block 1 acknowledgement.  This is
   typically used to attach a timestamp on a time scale defined by the
   ingress; we speak of an ingress-relative timestamp.  Alternatively,
   the ingress can keep a timestamp in its local storage, associated
   with the PSN of the packet that carries an ACK desired flag; it can
   then retrieve this timestamp when the block 1 acknowledgement

   In either case, the LOOPS ingress keeps track of the local segment
   round trip time (LRTT) based on the (saved or received) timestamp and
   the arrival time of the block 1 acknowledgement, by setting the ACK
   Desired flag (D flag) occasionally (several times per RTT) and
   saving/including a sending timestamp for/in the packet.

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   As the egress will send block 1 acknowledgement information right
   away when it receives a packet with the D flag set, the measurement
   of LRTT is more accurate for such packets.  A smoothed local segment
   round trip time S_LRTT can be computed in a similar way as defined by
   [RFC0793].  A recent minimum value of LRTT is also kept as min_LRTT.
   S_LRTT is used as a basis for the overall timing of retransmission
   and state management.

   Retransmitted packets MUST NOT be used for local segment round trip
   time (LRTT) calculation.

6.3.2.  ACK generation

   A block 1 acknowledgement is generated based on receiving a forward
   packet with a D flag.

   The way block 2 acknowledgement information is sent is more subject
   to control by the egress.  Generally, the egress will aggregate ACK
   bits for at least K packets before sending a block 2; this can be
   used to amortize the overhead to close to a couple of bits per ACK.
   In order to counter loss of reverse information packets, an egress
   will also want to send an ACK bit more than once -- a saturation
   value of 3 or more may be chosen based on setup information.
   Typically, ACK bits already sent will be prepended to ACK bits that
   are new in this block 2 information set.  If K packets do not
   accumulate for a while, the egress will send one or more packets with
   block 2 information that covers the unsent ACK bits it has so far.

   (Discussion: This works best if the egress has information both about
   the S_RTT and min_RTT that the ingress uses and the reverse packet
   loss rate.)

6.4.  Loss detection and Recovery

   There are two ways for LOOPS local recovery, retransmission and FEC.

6.4.1.  Local Retransmission

   When retransmission is used as recovery mechanism, the LOOPS ingress
   detects a packet loss by not receiving an ACK for the packet within
   the time expected based on an RTO value (which might be calculated as
   in [RFC6298]).  Packet retransmission should then not be performed
   more than once within an LRTT.

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   When a retransmission is desired, the LOOPS ingress performs the
   local in-network recovery by retransmitting the packet.  Further
   retransmissions may be desirable if the lack of ACK is persistent
   beyond an RTO, as long as the maximum latency increase is not

6.4.2.  FEC

   FEC is another way to perform local recovery.  When FEC is in use, a
   FEC header is sent with data packets as well as with special repair
   packets added to the flow.  The specific FEC scheme used could be
   defined in the Setup Information, using a mechanism like [RFC5052].
   The FEC rate (amount of redundancy added) and possibly the FEC scheme
   could be unilaterally adjusted by the LOOPS ingress in an adaptive
   mechanism based on the measurement information.

6.5.  Discussion

   Without progress in the way that end-host transport protocols handle
   reordering, LOOPS will be unable to prevent end-to-end
   retransmissions that duplicate effort that is spent in local
   retransmissions.  It depends on parameters of the path segment
   whether this wasted effort is significant or not.

   One remedy against this waste could be the introduction of
   resequencing at the LOOPS Egress node.  This increases overall mean
   packet latency, but does not always increase actual end-to-end data
   stream latency if a head-of-line blocking transport such as TCP is in
   use.  For applications with a large percentage of legacy TCP end-
   hosts and sufficient processing capabilities at the LOOPS Egress
   node, resequencing may be a viable choice.  Note that resequencing
   could be switched off and on depending on some measurement

   The packet numbering scheme chosen by LOOPS already provides the
   necessary information for the LOOPS Egress to reconstruct the
   sequence of data packets at the LOOPS ingress.

7.  Sketches of Bindings to Tunnel Protocols

   The LOOPS information defined above in a generic way can be mapped to
   specific tunnel encapsulation protocols.  A sketch for the tunnel
   protocol Geneve is given below (Section 7.1).  The actual
   encapsulation can be designed in a "native" way by putting each of
   the various elements into the TLV format of the encapsulation
   protocol, or it can be achieved by providing single TLVs for forward
   and reverse information and using some generic encoding of both kinds
   of information as shown in Appendix B.3.

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7.1.  Embedding LOOPS in Geneve

   Geneve [I-D.ietf-nvo3-geneve] is an extensible overlay protocol which
   can embed LOOPS functions.  Geneve uses TLVs to carry optional
   information between NVEs.  NVE is logically the same entity as the
   LOOPS node.

   The Geneve header has a mandatory Virtual Network Identifier (VNI)
   field.  The specific VNI value to be used is part of the setup
   information for the LOOPS tunnel.

   More details for a Geneve binding for LOOPS can be found in

8.  IANA Considerations

   No IANA action is required at this stage.  When a LOOPS
   representation is designed for a specific tunneling protocol, new
   codepoints will be required in the registries that pertain to that

9.  Security Considerations

   The security of a specific LOOPS segment will depend both on the
   properties of the generic information set described here and those of
   the encapsulation protocol employed.  The security considerations of
   the encapsulation protocol will apply, as will the protection
   afforded by any security measures provided by the encapsulation
   protocol.  Any LOOPS encapsulation specification is expected to
   provide information about preferred configurations of the
   encapsulation protocol employed, including security mechanisms, and
   to provide a security considerations section discussing the
   combination.  The following discussion aims at discussing security
   considerations that will be common between different encapsulations.

9.1.  Threat model

   Attackers might attempt to perturb the operation of a LOOPS segment
   for a number of purposes:

   *  Denial of Service: Damaging the ability of LOOPS to recover
      packets, or damaging packet forwarding through the LOOPS segment
      in general.

   *  Attacks on Confidentiality or Integrity: Obtaining the content of
      data packets, modifying them, injecting new or suppressing
      specific data packets.

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   For the purposes of these security considerations, we can distinguish
   three classes of attackers:

   1.  on-path read-write: The attacker sees packets under way on the
       segment and can modify, inject, or suppress them.

       In this case there is really nothing LOOPS can do, except for
       acting as a full security protocol on its own, which would be the
       task of the encapsulation protocol.  Without that, attackers
       already can manipulate the packet stream as they wish.  This
       class of attackers is considered out of scope for these security

   2.  on-path read + inject: The attacker sees packets under way on the
       segment and can inject new packets.

       For this case, LOOPS itself similarly cannot add to the
       confidentiality of the data stream.  However, LOOPS could protect
       against denial of service against its own protocol operation and,
       in a limited fashion, against attacks on integrity that wouldn't
       already have been possible by packet injection without LOOPS.

   3.  off-path inject: The attacker can inject new packets, but cannot
       see existing packets under way on the segment.

       Similar considerations apply as for class 2, except that the
       "blind" class 3 attacker might need to guess information it could
       have extracted from the packet stream in class 2.

9.2.  Discussion

   Class 2 attackers can see e.g. sequence numbers and can inject, but
   not modify traffic.  Attacks might include injecting false ACKs,
   initial PSN flags, ... (TBD)

   Class 3 ("blind") attackers might still be able to fake initial PSN
   bits + false ACKs, but will have a harder time otherwise as it would
   need to guess the PSN range in which it can wreak havoc.  Even random
   guesses will sometimes hit, though, so the protocol needs to be
   robust to such injection attacks. ... (TBD)

10.  References

10.1.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,

   [RFC3708]  Blanton, E. and M. Allman, "Using TCP Duplicate Selective
              Acknowledgement (DSACKs) and Stream Control Transmission
              Protocol (SCTP) Duplicate Transmission Sequence Numbers
              (TSNs) to Detect Spurious Retransmissions", RFC 3708,
              DOI 10.17487/RFC3708, February 2004,

   [RFC5052]  Watson, M., Luby, M., and L. Vicisano, "Forward Error
              Correction (FEC) Building Block", RFC 5052,
              DOI 10.17487/RFC5052, August 2007,

   [RFC5862]  Yasukawa, S. and A. Farrel, "Path Computation Clients
              (PCC) - Path Computation Element (PCE) Requirements for
              Point-to-Multipoint MPLS-TE", RFC 5862,
              DOI 10.17487/RFC5862, June 2010,

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <https://www.rfc-editor.org/info/rfc6040>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

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   [RFC6330]  Luby, M., Shokrollahi, A., Watson, M., Stockhammer, T.,
              and L. Minder, "RaptorQ Forward Error Correction Scheme
              for Object Delivery", RFC 6330, DOI 10.17487/RFC6330,
              August 2011, <https://www.rfc-editor.org/info/rfc6330>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,

              Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
              a time-based fast loss detection algorithm for TCP", Work
              in Progress, Internet-Draft, draft-ietf-tcpm-rack-08, 9
              March 2020, <http://www.ietf.org/internet-drafts/draft-

              Gross, J., Ganga, I., and T. Sridhar, "Geneve: Generic
              Network Virtualization Encapsulation", Work in Progress,
              Internet-Draft, draft-ietf-nvo3-geneve-16, 7 March 2020,

              Yizhou, L., Zhou, X., Boucadair, M., Wang, J., and F. Qin,
              "LOOPS (Localized Optimizations on Path Segments) Problem
              Statement and Opportunities for Network-Assisted
              Performance Enhancement", Work in Progress, Internet-
              Draft, draft-li-tsvwg-loops-problem-opportunities-05, 6
              July 2020, <http://www.ietf.org/internet-drafts/draft-li-

              Wei, X., Yizhou, L., Boutros, S., and L. Geng, "Tunnel
              Congestion Feedback", Work in Progress, Internet-Draft,
              draft-ietf-tsvwg-tunnel-congestion-feedback-07, 5 May
              2019, <http://www.ietf.org/internet-drafts/draft-ietf-

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              Bormann, C., "Embedding LOOPS in Geneve", Work in
              Progress, Internet-Draft, draft-bormann-loops-geneve-
              binding-01, 12 June 2020, <http://www.ietf.org/internet-

   [IEN74]    Plummer, W.W., "Sequence Number Arithmetic", Internet
              Experiment Note 74, September 1978.

Appendix A.  Protocol used in Prototype Implementation

   This appendix describes, in a somewhat abstracted form, the protocol
   as used in a prototype implementation, as described by Yizhou Li, and
   Xingwang Zhou.

   The prototype protocol can be run in one of two modes (defined by

   *  Retransmission mode

   *  Forward Error Correction (FEC) mode

   Forward information is piggybacked in data packets.

   Reverse information can be carried in a pure acknowledgement packet
   or piggybacked when carrying packets for the inverse direction.

   The forward information includes:

   *  Packet Sequence Number (PSN) (32 bits): This identifies a packet
      over a specific overlay segment from a specific LOOPS Ingress.  If
      a packet is retransmitted by LOOPS, the retransmission uses the
      original PSN.

   *  Timestamp (32 bits): Information, in a format local to the LOOPS
      ingress, that provides the time when the packet was sent.  In the
      current implementation, a 32-bit unsigned value specifying the
      time delta in some granularity from the epoch time to the sending
      time of the packet carrying this timestamp.  The granularity can
      be from 1 ms to 1 second.  The epoch time follows the current TCP
      practice which is 1 January 1970 00:00:00 UTC.  Note that a
      retransmitted packet uses its own Timestamp.

   *  FEC Info for Block Code (56 bits): This header is used in FEC
      mode.  It currently only provides for a block code FEC scheme.  It
      includes the Source Block Number (SBN), Encoding Symbol ID (ESI),
      number of symbols in a single source block and symbol size.
      Appendix A.1 gives more details on FEC.

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   The reverse information includes:

   *  ACK Number (32 bits): The largest (in sequence number arithmetic
      [RFC1982]) PSN received so far.

   *  ACK List (variable): This indicates an array of PSN numbers to
      describe the PSN "holes" preceding the ACK number.  It
      conceptually lists the PSNs of every packet perceived as lost by
      the LOOPS egress.  In actual use, it is truncated.

   *  Echoed Timestamp (32 bits): The timestamp received with the packet
      being acknowledged.

A.1.  Block Code FEC

   The prototype currently uses a block code FEC scheme (RaptorQ
   [RFC6330]).  The fields in the FEC Info forward information are:

   *  Source Block Number (SBN): 16 bits.  An integer identifier for the
      source block that the encoding symbols within the packet relate

   *  Encoding Symbol ID (ESI): 16 bits.  An integer identifier for the
      encoding symbols within the packet.

   *  K: 8 bits.  Number of symbols in a single source block.

   *  T: 16 bits.  Symbol size in bytes.

   The LOOPS Ingress uses the data packet in Figure 1 to generate the
   encoding packet.  Both source packets and repair packets carry the
   FEC header information; the LOOPS Egress reconstructs the data
   packets from both kinds of packets.  The LOOPS Egress currently
   resequences the forwarded and reconstructed packets, so they are
   passed on in-order when the lost packets are recoverable within the
   source block.

   The LOOPS Nodes need to agree on the use of FEC block mode and on the
   specific FEC Encoding ID to use; this is currently done by

Appendix B.  Transparent mode

   This appendix defines a very different way to provide the LOOPS
   services, "transparent mode".  (We call the protocol described in the
   main body of the document "encapsulated mode".)

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   In transparent mode, the idea is that LOOPS does not meddle with the
   forward transmission of data packets, but runs on the side exchanging
   additional information.

   An implementation could be based on conventional forwarding switches
   that just provide a copy of the ingress and egress packet stream to
   the LOOPS implementations.  The LOOPS process would occasionally
   inject recovered packets back into the LOOPS egress node's forwarding
   switch, see Figure 3.

      |       |                                           |
      |  +----+--------+   +-------------------+          |
      |  |    | copy   |   |                   |          |
      |  |    |----------------> LOOPS ingress |          |
      |  |    |        |   |     |     ^       |          |
      |  +----+--------+   +-----|-----|-------+          |
      |   data|packets    forward|     |reverse           |
      |       |              info|     |info              |
              |                  |     |
      |       |                  |     |                  |
      |  +----+---------+   +----|-----|----------+       |
      |  |    | copy    |   |    v     |          |       |
      |  |    |---------|---|---> LOOPS egress    |       |
      |  |    |         |   |                     |       |
      |  |    |<--------|---|---- inject          |       |
      |  +----+---------+   +---------------------+       |
      |       |                                           |

                      Figure 3: LOOPS Transparent Mode

   The obvious advantage of transparent mode is that no encapsulation is
   needed, reducing processing requirements and keeping the MTU
   unchanged.  The obvious disadvantage is that no forward information
   can be provided with each data packet, so a replacement needs to be
   found for the PSN (packet sequence number) employed in encapsulated
   mode.  Any forward information beyond the data packets is sent in
   separate packets exchanged directly between the LOOPS nodes.

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B.1.  Packet identification

   Retransmission mode and FEC mode differ in their needs for packet
   identification.  For retransmission mode, a somewhat probabilistic
   accuracy of the packet identification is sufficient, for FEC mode,
   packet identification should not make mistakes (as these would lead
   to faultily reconstructed packets).

   In Retransmission mode, misidentification of a packet could lead to
   measurement errors as well as missed retransmission opportunities.
   The latter will be fixed end-to-end.  The tolerance for measurement
   errors would influence the degree of accuracy that is aimed for.

   Packet identification can be based on a cryptographic hash of the
   packet, computed in LOOPS ingress and egress using the same algorithm
   (excluding fields that can change in transit, such as TTL/hop limit).
   The hash can directly be used as a packet number, or it can be sent
   in the forward information together with a packet sequence number,
   establishing a mapping.

   For probabilistic packet identification, it is almost always
   sufficient to hash the first few (say, 64) bytes of the packet; all
   known transport protocols keep sufficient identifying information in
   that part (and, for encrypted protocols, the entropy will be
   sufficient).  Any collisions of the hash could be used to disqualify
   the packet for measurement purposes, minimizing the measurement
   errors; this could allow rather short packet identifiers in
   retransmission mode.

   For FEC mode, the packet identification together with the per-packet
   FEC information needs to be sent in the (separate) forward
   information, so that a systematic code can be reconstructed.  For
   retransmission mode, there is no need to send any forward information
   for most packets, or a mapping from packet identifiers to packet
   sequence numbers could be sent in the forward information (probably
   in some aggregated form).  The latter would allow keeping the
   acknowledgement form described in the main body (with aggregate
   acknowledgement); otherwise, packet identifiers need to be
   acknowledged.  With this change, the LOOPS egress will send reverse
   information as in the encapsulating LOOPS protocol.

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B.2.  Generic information and protocol operation

   With the changes outlined above, transparent mode operates just as
   encapsulated mode.  If packet sequence numbers are not used, there is
   no use for block2 reverse information; if they are used, a new block3
   needs to be defined that provides the mapping from packet identifiers
   to packet sequence numbers in the forward information.  To avoid MTU
   reduction, some mechanism will be needed to encapsulate the actual
   FEC information (additional packets) in the forward information.

B.3.  A hybrid mode

   Figure 3 can be modified by including a GRE encapsulator into the top
   left corner and a GRE decapsulator in the bottom left corner.  This
   provides more defined ingress and egress points, but it also provides
   an opportunity to add a packet sequence number at the ingress.  The
   copies to the top right and bottom right corners are the encapsulated
   form, i.e., include the sequence number.

   The GRE packet header then has the form:

      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
     |0|0|0|1|    000000000    | 000 |         Protocol Type         |
     |                         Sequence Number                       |

   The forward and reverse information can be designed closer to the
   approach in the main body of the document, to be exchanged using UDP
   packets between top right ingress and bottom right egress using a
   port number allocated for this purpose.

   Rough ideas for both directions are given below in CDDL [RFC8610].
   This information set could be encoded in CBOR or in a bespoke
   encoding; details such as this can be defined later.

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   forward-information = [
     [rel-psn, ack-desired, ? fec-info] /

   rel-psn = uint; relative packet sequence number
   ; always given as a delta from the previous one in the array
   ; starting out with a "previous value" of 0

   ack-desired = bool

   fec-info = [
       sbn: uint, ; Source Block Number
       esi: uint, ; Encoding Symbol ID
       ? (
         nsssb: uint; number of symbols in a single source block
         ss: uint; symbol size

   fec-repair-data = [
       repair-data: bytes
       ? (
         sbn: uint, ; Source Block Number
         esi: uint, ; Encoding Symbol ID

   If left out for a sequence number, the fec-info block is constructed
   by adding one to the previous one.  fec-repair-data contain repair
   symbols for the sbn/esi given (which, again, are reconstructed from
   context if not given).

   reverse-information = [
       block1 / block2

   block1 = [rel-psn, timestamp]
   block2 = [end-psn-delta: uint, acked-bits: bytes]

   The acked-bits in a block2 is a bitmap that gives acknowledgments for
   received data packets.  The bitmap always comes as a multiple of 8
   bits (all bytes are filled in with 8 bits, each identifying a PSN).
   The end PSN of the bitmap (actually the first PSN that would be
   beyond it) is computed from the current PSN as set by rel-psn,
   rounded down to a multiple of 8, and adding 8*(end-psn-delta+1) to
   that value.

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   Sami Boutros helped with sketching the use of Geneve (Section 7.1).

   Michael Welzl has been supported by the Research Council of Norway
   under its "Toppforsk" programme through the "OCARINA" project.

Authors' Addresses

   Michael Welzl
   University of Oslo
   PO Box 1080 Blindern
   N-0316 Oslo

   Phone: +47 22 85 24 20
   Email: michawe@ifi.uio.no

   Carsten Bormann (editor)
   Universit├Ąt Bremen TZI
   Postfach 330440
   D-28359 Bremen

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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