Internet DRAFT - draft-herbert-sub-path-ps



INTERNET-DRAFT                                                T. Herbert
Intended Status: Informational                                Quantonium
Expires: March 2019                                                     
                                                      September 25, 2018

               Sub-path Transport Layer Problem Statement


   This document presents the problem statement and use cases for a sub-
   path transport layer. A sub-path is a portion of a network path that
   has localized characteristics that are of interest to an operator to
   manage. The  structure for managing a sub-path is the "sub-path
   transport layer". A sub-path transport layer can provide transport
   layer mechanisms, such as reliability and congestion control, over
   the aggregate of packets traversing the sub-path. Additionally, a
   sub-path transport layer can provide performance measurements of
   traffic over a sub-path which in turn can be input to operations and
   management. The sub-path transport layer implies the possibility that
   traffic may be subject to two transport layer control loops, that of
   the sub-path and that of a higher layer transport protocol, any such
   solution must consider the ramifications of that.

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Copyright and License Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1 Passive measurement  . . . . . . . . . . . . . . . . . . . .  3
     1.2 Transport layer mechanisms . . . . . . . . . . . . . . . . .  3
   2  Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1 Lossy access network . . . . . . . . . . . . . . . . . . . .  4
     2.2 Low latency handover . . . . . . . . . . . . . . . . . . . .  4
     2.3 Congestion control for nest of mice  . . . . . . . . . . . .  5
   3  Existing solutions  . . . . . . . . . . . . . . . . . . . . . .  6
     3.1 Layer 2 protocols with transport layer functionality . . . .  6
     3.2 TCP proxies  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3 In-situ OAM  . . . . . . . . . . . . . . . . . . . . . . . .  7
   4  Sub-path transport layer solution . . . . . . . . . . . . . . .  7
     4.1 Sub-path tunnels . . . . . . . . . . . . . . . . . . . . . .  7
     4.2 Nested control loop problem  . . . . . . . . . . . . . . . .  7
       4.2.1 Avoiding conflict with higher layer transports . . . . .  8
       4.2.2 Work in concert with higher layer transports . . . . . .  8
     4.3 Advanced features  . . . . . . . . . . . . . . . . . . . . .  8
   5  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1  Normative References  . . . . . . . . . . . . . . . . . . .  9
     5.2  Informative References  . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  9


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1  Introduction

   This document presents the problem statement that motivates defining
   a sub-path transport layer. A sub-path is a portion a network path
   for a flow that is typically contained within some network. For
   instance, a sub-path may constitute a path from an ingress to an
   egress point of a transit network. A sub-path may be common to many
   flows where the aggregate of packets traversing the sub-path could be
   termed a "sub-path flow".

   We propose a "sub-path transport layer" that can be used to manage
   traffic over a sub-path layer with transport layer semantics. There
   are two goals in this:

      o Perform passive measurement of packets traversing a sub-path

      o Implement transport layer mechanisms, like reliability and
        congestion control, on the sub-path flow

1.1 Passive measurement

   A network operator may be interested in performance metrics of
   packets flowing over a sub-path. The motivation for measurement is
   operations and management (OAM), and the data might be used for such
   things as traffic engineering and route selection. A sub-path
   transport layer would provide measurements and statistics for a sub-
   path flow that are analogous to that provided by a canonical
   transport layer for a transport flow. The data would include packet
   and byte counts, latency and latency variance, packet loss, and out
   of order packets.

1.2 Transport layer mechanisms

   Sub-paths may exhibit radically different characteristics compared to
   the rest of the path for a flow. For instance, a radio network
   portion of a path may be much more susceptible to congestion and
   packet loss relative to the rest of a flow's path.  End-to-end
   mechanisms for congestion control and reliability don't take this
   into account, so transport layer end points are often far removed
   from problems. The result is that congestion control and reliability
   is sub-optimal and have high response times to problems originating
   in a sub-path.

   The proposed solution is to run a congestion control and reliability
   protocol over sub-paths. This would constitute an in-network
   transport layer below the end-to-end transport (e.g. TCP) that
   applies transport layer algorithms to manage a sub-path flow.


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2  Use cases

   This section describes some potential use cases of a sub-path
   transport layer.

2.1 Lossy access network

   Certain radio technologies, such as mmWave (millimeter wave) cellular
   systems, offer the opportunity to use an untapped frequency spectrum
   to achieve high data rates. However, the harsh propagation conditions
   of mmWave present a considerable challenge. A mmWave link may be both
   lossy and have high variability in data rates that adversely affects
   performance of end to end transport protocols like TCP.

   As shown in the diagram below, a sub-path transport protocol could be
   used over a mmWave link that provides fast retransmissions for packet
   loss on the link, as well as responsive congestion control that
   adapts quickly to changing data rates. In this case, the endpoints of
   the sub-path transport layer (denoted SPEP in this document) are the
   end device (UE) and a node in a provider network (PGW).
                 _______                  ______
                (       )                (      )
    +------+   (  Radio  )   +------+   (        )   +--------+
    | UE   |--(  Network  )--| PGW  |--( Internet )--| Server |
    | Host |   (         )   | SPEP |   (        )   +--------+
    +------+    (_______)    +------+    (______)

           Sub-path transport

                             End-to-end transport

2.2 Low latency handover

   In a cellular network, handover is the process of transferring
   communications from one channel to another. Handover is a very common
   operation as mobile devices move from one cell to another, so that
   the point of attachment changes from one cellular base station to
   another. Handover is not instantaneous, especially considering that
   most devices can only connect to one channel at a time. Handover
   latency is defined as the time between the last reception of data to
   when the device starts receiving packets at the new point of
   attachment. Handover latency can be in the hundreds of milliseconds.
   Handover of UE (user equipment) is illustrated below.


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       +--  +-----+       ________                   ______
       |    | gNB +--+   (        )                 (      )
    +----+  +-----+   \ ( Provider )    +-----+    (        )   +------+
    | UE |             (    RAN     )---+ PGW +---( Internet )--+ Peer |
    +----+  +-----+   / (          )    +-----+    (        )   +------+
       |    | gNB +--+   (________)                 (______)
       +->  +-----+
                  Sub-path flow over RAN

   When a handover occurs, packets for the device that has moved need to
   be forwarded to new point of attachment. The timing of events to
   complete negotiation and handover is non-deterministic so that the
   network does not know the precise time at which events occur. A
   proposed optimization is that during the handover period, packets are
   duplicated and forwarded to both the old and new point of attachment.
   This potentially reduces the handover latency. A sub-path transport
   protocol facilitates this with sequence numbers to avoid delivery of
   duplicate packets as well as to maintain ordering during a handover.

2.3 Congestion control for nest of mice

   Large IoT networks may contain millions of devices and billions of
   flows that periodically do request/response communications with small
   amounts of data. Such a collection of flows could be called a "nest
   of mice". The nest of mice may share a critical link that can become
   congested due to the aggregate of traffic. End-to-end congestion
   control is ineffective since each connection generates little data so
   there is little input to any particular transport control loop.

   A sub-transport layer could aggregate all the mice flows together and
   treat them as one single large flow. The control loop for the
   aggregate flow has sufficient input to perform effective congestion
   control over sub-path. Aggregation of mice flows over a sub-path
   transport layer is illustrated in the diagram below.

    +------+               _____                  ______
    | Host |              (     )                (      )
    +------+  +------+   (       )   +------+   (        )   +--------+
    ...       | SPEP |--( Network )--| SPEP |--( Internet )--| Server |
    +------+  +------+   (       )   +------+   (        )   +--------+
    | Host |              (_____)                (______)

     FLOW \   <-------------------------->
     .... /      Aggregated sub-path flow

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3  Existing solutions

   This sections surveys some related solutions.

3.1 Layer 2 protocols with transport layer functionality

   A number of link layer protocols implement transport layer semantics
   such as reliability.

   Link-layer retransmission is a feature of the IEEE 802.11 standard
   that aims to increase the reliability of data communications.
   [LLRETRANS] analyzes the impact link layer retransmission on video
   streaming over a wireless mesh. The conclusion of that work is that
   link layer retranmission provides the most benefit when the number of
   retransmissions is limited (to one or two), and the traffic over the
   mesh network is far below network capacity (in other words there is
   no risk that retransmitted packets contribute to congestion).

3.2 TCP proxies

   TCP proxies are often used in mobile networks to enhance performance
   by running proxy connections over radio sub-paths. [MILLIPROXY] is a
   proposal to use TCP proxies to optimize communications in 5G mmWave.
   That paper describes the unique characteristics of high bandwidth
   mmWave links, and why end-to-end TCP gives sub-optimal performance in
   that environment. A TCP proxy can split the path over two TCP
   connections, where one connection is overlaid on the radio network.
   That connection is then very close to the sub-path of interest (the
   mmWave link), thus both congestion control and reliability are
   isolated and are tailored to the characteristics of the link.

   While TCP proxies can optimize traffic in specific situations like
   this, as a general solution to the problem they have a number of

      * They only work with TCP and no other transport protocol. In
        particular, they don't work with QUIC, SCTP, etc.

      * Proxies break the end-to-end model. This is most evident in that
        they break transport layer security like the TCP authentication
        option. They also ossify TCP since the options that can be used
        become the least common denominator of what the endpoints and
        the proxy supports.

      * Proxies are a single point of failure and potential network

      * Proxies require considerable transport state to be maintained in

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        the network. This is very hard to scale especially in light of
        5G scaling goals of a trillion devices attached to the Internet.

3.3 In-situ OAM

   [IOAM] defines a scheme to measure per hop latencies and queue delay
   for packets. Whereas in-situ OAM performs low level measurements that
   assume participation by intermediate nodes to provide data, the sub-
   path transport layer provides end-to-end measurements over the sub-
   path. Both mechanisms measure performance of the network, but at
   different levels. In this sense they may be considered complementary
   techniques, and it is conceivable that the two techniques could be
   used in concert to fully characterize the performance of a sub-path.

4  Sub-path transport layer solution

   Sub-paths within a network can be defined arbitrarily by the network
   operator. Generally, sub-paths consist of two network nodes, denoted
   "sub-path end points", and the path between them in both directions.
   Sub-paths may segregate packets by class or other convenient
   characteristics; for instance, a sub-path for high priority packets
   and one for low priority packets could be defined. There is no one to
   one correspondence between sub-path flows and higher layer transport
   flows, neither is there any requirement that all packets of a
   transport flow traverse the same sub-path or sub-path transport

4.1 Sub-path tunnels

   To achieve a generic, well layered solution, a sub-path transport
   layer protocol could be specified and implemented for use over
   network tunnels. That is to use an network overlay to traverse a sub-
   path. The tunnel endpoints are the sub-path endpoints, and the wire
   protocol for the sub-path transport protocol can be contained in
   optional data of an encapsulation protocol for the tunnel. The
   solution should be generic so that it can be used with any
   encapsulation protocol that is reasonably extensible: GUE, Geneve,
   and GTP-U are candidates. It is also plausible that an IPv6
   destination option could be used so that sub-path transport layer
   could generally work with any IPv6 encapsulation.

4.2 Nested control loop problem

   An important consideration is how a sub-path transport layer
   interacts with end-to-end transport protocols. Packets of a flow may
   be subject to two (possibly more) transport control loops, that of
   the sub-path and that of a higher layer transport protocol. Previous
   experience has shown this to be problematic and yield poor transport

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   behavior. To this end, there are two levels of interaction that could
   be targeted in a sub-path transport protocol:

      1) avoid conflict with higher layer transport protocols

      2) work in concert with higher layer transport protocols

4.2.1 Avoiding conflict with higher layer transports

   To avoid conflict with higher layer transport protocols, one must
   take into account the effect of an algorithm at a higher layer. For
   instance, consider that a sub-path transport layer implements
   retransmissions. If a packet is lost and can quickly be retransmitted
   within the RTT variance of the end-to-end flow, then the algorithm is
   potentially beneficial. If a sub-path retransmission timer is long,
   or multiple retransmissions are required, then the algorithm might
   interfere with the higher layer algorithms. In the worst case, an
   endpoint sender might retransmit a packet that is still in the
   process of retransmitted over a sub-path needlessly increasing
   congestion in the sub-path. This motivates "best effort" approach to
   a sub-path transport layer. For instance, a sub-path transport layer
   might retransmit only once (as motivated by data in [LLRETRANS]).

4.2.2 Work in concert with higher layer transports

   Creating a solution that works in concert with upper layer transport
   protocols might be compelling. This would necessitate some signaling
   between the end-to-end transport and the sub-path transport layer.
   Explicit congestion notification (ECN) is a signal that could be used
   for the sub-path transport to signal congestion conditions to the
   higher transport layer.

   Some intermediate nodes will do deep packet inspection (DPI) into TCP
   in order to extract sequence numbers, round trip times (RTT), and
   acknowledgements in order to deduce things like retransmissions.
   Parsing into the TCP headers is not a generic solution and generally
   contributes to protocol ossification, however there are emerging
   techniques such as in-situ OAM in IPv6 Hop-by-Hop headers that may be
   useful. If a sub-path transport layer knew the round trip latency and
   variance of a flow, then it might be able to use that information to
   determine the number of retransmissions that could be tolerated over
   the sub-path before a packet is retransmitted at a higher layer.

4.3 Advanced features

   In addition to the typical transport features of reliability and
   congestion control, a sub-path transport layer may also allow
   features and optimizations that that are more specific to

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   characteristics of a particular sub-path.

   These might include:

      o Forward error correction

      o Purposely duplicating packets (for low latency handover)

      o Random packet spraying across a sub-paths

      o Optimized path selection and routing

5  References

5.1  Normative References

5.2  Informative References

Author's Address

   Tom Herbert
   Santa Clara, CA


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