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Versions: 00

TSVWG                                                           M. Welzl
Internet-Draft                                        University of Oslo
Intended status: Standards Track                         C. Bormann, Ed.
Expires: November 28, 2019                       Universitaet Bremen TZI
                                                            May 27, 2019


                     LOOPS Generic Information Set
                     draft-welzl-loops-gen-info-00

Abstract

   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.  The
   current version of this document contains sketches of bindings to GUE
   [I-D.ietf-intarea-gue] and Geneve [I-D.ietf-nvo3-geneve].

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 28, 2019.








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Copyright Notice

   Copyright (c) 2019 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
   (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 and restrictions with respect
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   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.

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
     2.5.  Congestion Detection  . . . . . . . . . . . . . . . . . .   8
   3.  Simplifying assumptions . . . . . . . . . . . . . . . . . . .   8
   4.  LOOPS Generic Information Set . . . . . . . . . . . . . . . .   9
     4.1.  Setup Information . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Forward Information . . . . . . . . . . . . . . . . . . .   9
     4.3.  Reverse Information . . . . . . . . . . . . . . . . . . .  10
   5.  LOOPS General Operation . . . . . . . . . . . . . . . . . . .  11
     5.1.  Initial Packet Sequence Number  . . . . . . . . . . . . .  11
     5.2.  Acknowledgement Generation  . . . . . . . . . . . . . . .  11
     5.3.  Measurement . . . . . . . . . . . . . . . . . . . . . . .  12
     5.4.  Loss detection and Recovery . . . . . . . . . . . . . . .  12
       5.4.1.  Local Retransmission  . . . . . . . . . . . . . . . .  12
       5.4.2.  FEC . . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.5.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .  13
   6.  Sketches of Bindings to Tunnel Protocols  . . . . . . . . . .  13
     6.1.  Embedding LOOPS in Geneve . . . . . . . . . . . . . . . .  14
     6.2.  Embedding LOOPS in GUE  . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  15
   Appendix A.  Protocol used in Prototype Implementation  . . . . .  16
     A.1.  Block Code FEC  . . . . . . . . . . . . . . . . . . . . .  17
   Appendix B.  Transparent mode . . . . . . . . . . . . . . . . . .  18
     B.1.  Packet identification . . . . . . . . . . . . . . . . . .  19



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     B.2.  Generic information and protocol operation  . . . . . . .  20
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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
   node.

   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".  These different modes may be applicable
   to different usage scenarios and will be developed in parallel, with
   a view of ultimately standardizing one or both of them.

   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.  The current version of this
   document contains sketches of bindings to GUE [I-D.ietf-intarea-gue]
   and 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

   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
      segment.

   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.

   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.

   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).






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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 avoid 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.)

   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



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   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.

   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.  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.

   If the end-to-end transport is 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.  If LOOPS detects a
   congestion loss for a non-ECT packet, it needs to signal a congestion
   loss event by introducing a packet loss.  This can be done by
   choosing not to retransmit or repair the packet loss locally in this
   case.  Note that one congestion loss per end-to-end RTT is sufficient



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   to provide the rate reduction, so LOOPS may still be able to recover
   most packets, in particular for burst losses.  (As LOOPS does not
   interact with the end-to-end transport, it does not know the end-to-
   end RTT.  Some lower bound derived from configuration and
   measurements could be used instead.)

2.5.  Congestion Detection

   Properly informing the end-to-end transport protocol about congestion
   loss events requires distinguishing these from random losses.  In
   some special cases, distinguishing information may be available from
   a link layer (e.g., see Section 3 of
   [I-D.li-tsvwg-loops-problem-opportunities]).  By enabling ECN inside
   the tunnel, congestion events experienced at ECN-capable routers will
   usually be identified by the CE mark, which clearly rules out a
   random loss.

   In the general case, the segment may be composed of hops without such
   special indications.  In these cases, some detection mechanism is
   required to provide this distinguishing information.  The specific
   mechanism used by an implementation is out of scope of LOOPS, but
   LOOPS will need to provide measurement information for this
   mechanism.  For instance, congestion detection might be based on path
   segment latency information, the proper measurement of which
   therefore requires special attention in 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.



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   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.

4.  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
   specification.

4.1.  Setup Information

   Setup Information might include:

   o  encapsulation protocol in use, and its vital parameters

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

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

   o  maximum retransmission count (*)

   In the data plane, we have forward information (information added to
   each data packet) and reverse information.  The latter can be sent in
   separate packets (e.g., Geneve control-only packets
   [I-D.ietf-nvo3-geneve]) and/or piggybacked like the forward
   information.

4.2.  Forward Information

   In the forward information, we have identified:

   o  tunnel type (a few bits, meaning agreed between Ingress and
      Egress)

   o  packet sequence number PSN (20+ bits), counting the LOOPS packets
      transmitted by the LOOPS ingress (i.e., retransmissions receive a
      new PSN)




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

   o  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 5.3.)

   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.

4.3.  Reverse Information

   For the reverse information, we have identified:

   o  one optional block 1, possibly repeated:

   o  PSN being acknowledged

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

   o  one optional block 2, possibly repeated:

   o  an ACK bitmap (based on PSN), always starting at a multiple of 8

   o  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
   this.

   The latency calculation is done by the sender, who occasionally sets
   "ACK desirable", and notes down the absolute time of transmission for



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   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.

5.  LOOPS General Operation

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

5.1.  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.

   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.)

5.2.  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 a NACK list.




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   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.)

5.3.  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.)

   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.

5.4.  Loss detection and Recovery

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

5.4.1.  Local Retransmission

   When retransmission is used as recovery mechanism, the LOOPS ingress
   detects a packet loss by receiving a NACK or by local timeout (using
   a RTO value that might be calculated as in [RFC6298]).  It might
   employ a DUPACK-like or a RACK-like mechanism for delayed reaction to
   a NACK.

   When a retransmission is desired (see Section 2.4 for why it might
   not be), the LOOPS ingress performs the local in-network recovery by
   retransmitting the packet.  Further retransmissions may be desirable
   if the NACK is persistent beyond an RTO, as long as the maximum
   latency increase is not reached.





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5.4.2.  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.

5.5.  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
   information.

   To enable resequencing at the LOOPS Egress, a packet numbering scheme
   is needed that allows the LOOPS Egress to reconstruct the sequence at
   the LOOPS ingress.  This could be done by reverting to a traditional
   packet sequence number counting incoming data packets, possibly
   combined with a "retransmission" bit that indicates that the specific
   LOOPS packet is a retransmission and not the original transmission.
   (The acknowledgement/measurement ambiguity could be further reduced
   by adding transmission counter TC that counts transmission/
   retransmission for this PSN; a few bits should be enough for the
   limited retransmission envisaged.)

6.  Sketches of Bindings to Tunnel Protocols

   The LOOPS information defined above in a generic way can be mapped to
   specific tunnel encapsulation protocols.  Sketches for two tunnel
   protocols are given below: Geneve (Section 6.1), and GUE
   (Section 6.2).






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6.1.  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.

   For Geneve, a new LOOPS TLV needs to be defined and its format needs
   to be consistent with LOOPS generic information in Section 4.  When
   the Geneve LOOPS TLV is put in forward information, NVEs should be
   able to process it.  Any settings needed can be provided in the Setup
   Information.

   In the reverse direction, when no data packets are available for
   piggybacking, a control only packet will be used to carry the LOOPS
   reverse information.  Such a control only packet sets the 'O' bit in
   the Geneve header and has no real user data.

   VNI is a mandatory field in Geneve base header.  The LOOPS TLV should
   function on the tunnel between two NVEs without looking at the VNI
   value.  The LOOPS PSN number space is local to the overlay tunnel
   regardless of the VNI inside.  At the ingress NVE, there are
   different ways to decide whether a packet should go to LOOPS enabled
   tunnel, e.g. by protocol number (TCP/UDP certain ports) or by VNI.

6.2.  Embedding LOOPS in GUE

   GUE [I-D.ietf-intarea-gue] is an extensible overlay protocol which
   can embed LOOPS functions.  GUE uses flags to indicate the presence
   of fixed length header extensions.  It also allows variable length
   extensions to be put in "Private data" field.  A new LOOPS data block
   in the "private data" field needs to be defined based on the LOOPS
   generic information in Section 4.

   In the reverse direction, when no data packets are available for
   piggybacking, LOOPS reverse information is carried in a control
   message with the C-bit set in the GUE header.  The Proto/ctype field
   contains a control message type when C bit is set.  Hence a new
   control message type should be defined for such LOOPS reverse
   information.

7.  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
   protocol.




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8.  Security Considerations

   To be defined.

9.  Informative References

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,
              <https://www.rfc-editor.org/info/rfc1982>.

   [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,
              <https://www.rfc-editor.org/info/rfc3708>.

   [RFC5052]  Watson, M., Luby, M., and L. Vicisano, "Forward Error
              Correction (FEC) Building Block", RFC 5052,
              DOI 10.17487/RFC5052, August 2007,
              <https://www.rfc-editor.org/info/rfc5052>.

   [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,
              <https://www.rfc-editor.org/info/rfc5862>.

   [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,
              <https://www.rfc-editor.org/info/rfc6298>.

   [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>.

   [I-D.ietf-tcpm-rack]
              Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
              a time-based fast loss detection algorithm for TCP",
              draft-ietf-tcpm-rack-05 (work in progress), April 2019.





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   [I-D.ietf-nvo3-geneve]
              Gross, J., Ganga, I., and T. Sridhar, "Geneve: Generic
              Network Virtualization Encapsulation", draft-ietf-
              nvo3-geneve-13 (work in progress), March 2019.

   [I-D.ietf-intarea-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-ietf-intarea-gue-07 (work in
              progress), March 2019.

   [I-D.li-tsvwg-loops-problem-opportunities]
              Yizhou, L. and X. Zhou, "LOOPS (Localized Optimizations of
              Path Segments) Problem Statement and Opportunities",
              draft-li-tsvwg-loops-problem-opportunities-02 (work in
              progress), May 2019.

   [I-D.ietf-tsvwg-tunnel-congestion-feedback]
              Wei, X., Yizhou, L., Boutros, S., and L. Geng, "Tunnel
              Congestion Feedback", draft-ietf-tsvwg-tunnel-congestion-
              feedback-07 (work in progress), May 2019.

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
   preconfiguration):

   o  Retransmission mode

   o  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:

   o  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.

   o  Timestamp (32 bits): Information, in a format local to the LOOPS
      ingress, that provides the time when the packet was sent.  In the



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      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.

   o  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.

   The reverse information includes:

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

   o  NACK 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.

   o  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:

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

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

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

   o  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




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   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
   configuration.

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".)

   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.





























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

                     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.

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.




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   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.

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.

Acknowledgements

   Sami Boutros helped with sketching the use of Geneve (Section 6.1),
   and Tom Herbert helped with sketching the use of GUE (Section 6.2).

Authors' Addresses







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

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


   Carsten Bormann (editor)
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

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

































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