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IP-TFS: Aggregation and Fragmentation Mode for ESP and its Use for IP Traffic Flow Security
draft-ietf-ipsecme-iptfs-13

Document Type Active Internet-Draft (ipsecme WG)
Author Christian Hopps
Last updated 2022-08-10 (Latest revision 2022-06-05)
Replaces draft-hopps-ipsecme-iptfs
Stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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INTDIR Telechat Review Incomplete, due 2022-08-20
Stream WG state Submitted to IESG for Publication
Document shepherd Tero Kivinen
Shepherd write-up Show Last changed 2022-03-24
IESG IESG state IESG Evaluation - Defer
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Consensus boilerplate Yes
Telechat date On agenda of 2022-08-25 IESG telechat
Has a DISCUSS. Needs 9 more YES or NO OBJECTION positions to pass.
Responsible AD Roman Danyliw
Send notices to kivinen@iki.fi
IANA IANA review state IANA OK - Actions Needed
IANA expert review state Expert Reviews OK
draft-ietf-ipsecme-iptfs-13
Network Working Group                                           C. Hopps
Internet-Draft                                   LabN Consulting, L.L.C.
Intended status: Standards Track                             5 June 2022
Expires: 7 December 2022

 IP-TFS: Aggregation and Fragmentation Mode for ESP and its Use for IP
                         Traffic Flow Security
                      draft-ietf-ipsecme-iptfs-13

Abstract

   This document describes a mechanism for aggregation and fragmentation
   of IP packets when they are being encapsulated in ESP payloads.  This
   new payload type can be used for various purposes such as decreasing
   encapsulation overhead for small IP packets; however, the focus in
   this document is to enhance IPsec traffic flow security (IP-TFS) by
   adding Traffic Flow Confidentiality (TFC) to encrypted IP
   encapsulated traffic.  TFC is provided by obscuring the size and
   frequency of IP traffic using a fixed-sized, constant-send-rate IPsec
   tunnel.  The solution allows for congestion control as well as non-
   constant send-rate usage.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 7 December 2022.

Copyright Notice

   Copyright (c) 2022 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.

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology & Concepts  . . . . . . . . . . . . . . . . .   4
   2.  The AGGFRAG Tunnel  . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Tunnel Content  . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Payload Content . . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Data Blocks . . . . . . . . . . . . . . . . . . . . .   6
       2.2.2.  End Padding . . . . . . . . . . . . . . . . . . . . .   6
       2.2.3.  Fragmentation, Sequence Numbers and All-Pad
               Payloads  . . . . . . . . . . . . . . . . . . . . . .   6
       2.2.4.  Empty Payload . . . . . . . . . . . . . . . . . . . .   8
       2.2.5.  IP Header Value Mapping . . . . . . . . . . . . . . .   9
       2.2.6.  IP Time-To-Live (TTL) and Tunnel errors . . . . . . .   9
       2.2.7.  Effective MTU of the Tunnel . . . . . . . . . . . . .   9
     2.3.  Exclusive SA Use  . . . . . . . . . . . . . . . . . . . .  10
     2.4.  Modes of Operation  . . . . . . . . . . . . . . . . . . .  10
       2.4.1.  Non-Congestion-Controlled Mode  . . . . . . . . . . .  10
       2.4.2.  Congestion-Controlled Mode  . . . . . . . . . . . . .  10
     2.5.  Summary of Receiver Processing  . . . . . . . . . . . . .  12
   3.  Congestion Information  . . . . . . . . . . . . . . . . . . .  13
     3.1.  ECN Support . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Configuration of AGGFRAG Tunnels for IP-TFS . . . . . . . . .  14
     4.1.  Bandwidth . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Fixed Packet Size . . . . . . . . . . . . . . . . . . . .  15
     4.3.  Congestion Control  . . . . . . . . . . . . . . . . . . .  15
   5.  IKEv2 . . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  USE_AGGFRAG Notification Message  . . . . . . . . . . . .  15
   6.  Packet and Data Formats . . . . . . . . . . . . . . . . . . .  16
     6.1.  AGGFRAG_PAYLOAD Payload . . . . . . . . . . . . . . . . .  16
       6.1.1.  Non-Congestion Control AGGFRAG_PAYLOAD Payload
               Format  . . . . . . . . . . . . . . . . . . . . . . .  16
       6.1.2.  Congestion Control AGGFRAG_PAYLOAD Payload Format . .  17
       6.1.3.  Data Blocks . . . . . . . . . . . . . . . . . . . . .  19
       6.1.4.  IKEv2 USE_AGGFRAG Notification Message  . . . . . . .  21
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     7.1.  AGGFRAG_PAYLOAD Sub-Type Registry . . . . . . . . . . . .  21
     7.2.  USE_AGGFRAG Notify Message Status Type  . . . . . . . . .  22
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  23

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   Appendix A.  Example of An Encapsulated IP Packet Flow  . . . . .  25
   Appendix B.  A Send and Loss Event Rate Calculation . . . . . . .  26
   Appendix C.  Comparisons of IP-TFS  . . . . . . . . . . . . . . .  27
     C.1.  Comparing Overhead  . . . . . . . . . . . . . . . . . . .  27
       C.1.1.  IP-TFS Overhead . . . . . . . . . . . . . . . . . . .  27
       C.1.2.  ESP with Padding Overhead . . . . . . . . . . . . . .  27
     C.2.  Overhead Comparison . . . . . . . . . . . . . . . . . . .  28
     C.3.  Comparing Available Bandwidth . . . . . . . . . . . . . .  29
       C.3.1.  Ethernet  . . . . . . . . . . . . . . . . . . . . . .  29
   Appendix D.  Acknowledgements . . . . . . . . . . . . . . . . . .  31
   Appendix E.  Contributors . . . . . . . . . . . . . . . . . . . .  31
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   Traffic Analysis ([RFC4301], [AppCrypt]) is the act of extracting
   information about data being sent through a network.  While directly
   obscuring the data with encryption [RFC4303], fully, the patterns in
   the message traffic may expose information due to variations in its
   shape and timing ([RFC8546], [AppCrypt]).  Hiding the size and
   frequency of traffic is referred to as Traffic Flow Confidentiality
   (TFC) per [RFC4303].

   [RFC4303] provides for TFC by allowing padding to be added to
   encrypted IP packets and allowing for transmission of all-pad packets
   (indicated using protocol 59).  This method has the major limitation
   that it can significantly under-utilize the available bandwidth.

   This document defines an aggregation and fragmentation (AGGFRAG) mode
   for ESP, and its use for IP Traffic Flow Security (IP-TFS).  This
   solution provides for full TFC without the aforementioned bandwidth
   limitation.  This is accomplished by using a constant-send-rate IPsec
   [RFC4303] tunnel with fixed-sized encapsulating packets; however,
   these fixed-sized packets can contain partial, whole or multiple IP
   packets to maximize the bandwidth of the tunnel.  A non-constant
   send-rate is allowed, but the confidentiality properties of its use
   are outside the scope of this document.

   For a comparison of the overhead of IP-TFS with the RFC4303
   prescribed TFC solution see Appendix C.

   Additionally, IP-TFS provides for operating fairly within congested
   networks [RFC2914].  This is important for when the IP-TFS user is
   not in full control of the domain through which the IP-TFS tunnel
   path flows.

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   The mechanisms, such as the AGGFRAG mode, defined in this document
   are generic with the intent of allowing for non-TFS uses, but such
   uses are outside the scope of this document.

1.1.  Terminology & Concepts

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 assumes familiarity with IP security concepts including
   TFC as described in [RFC4301].

2.  The AGGFRAG Tunnel

   As mentioned in Section 1, AGGFRAG mode utilizes an IPsec [RFC4303]
   tunnel as its transport.  For the purpose of IP-TFS, fixed-sized
   encapsulating packets are sent at a constant rate on the AGGFRAG
   tunnel.

   The primary input to the tunnel algorithm is the requested bandwidth
   to be used by the tunnel.  Two values are then required to provide
   for this bandwidth use, the fixed size of the encapsulating packets,
   and rate at which to send them.

   The fixed packet size MAY either be specified manually or be
   determined through other methods such as the Packetization Layer MTU
   Discovery (PLMTUD) ([RFC4821], [RFC8899]) or Path MTU discovery
   (PMTUD) ([RFC1191], [RFC8201]).  PMTUD is known to have issues so
   PLMTUD is considered the more robust option.  For PLMTUD, congestion
   control payloads can be used as in-band probes (see Section 6.1.2 and
   [RFC8899]).

   Given the encapsulating packet size and the requested bandwidth to be
   used, the corresponding packet send rate can be calculated.  The
   packet send rate is the requested bandwidth to be used divided by the
   size of the encapsulating packet.

   The egress (receiving) side of the AGGFRAG tunnel MUST allow for and
   expect the ingress (sending) side of the AGGFRAG tunnel to vary the
   size and rate of sent encapsulating packets, unless constrained by
   other policy.

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2.1.  Tunnel Content

   As previously mentioned, one issue with the TFC padding solution in
   [RFC4303] is the large amount of wasted bandwidth as only one IP
   packet can be sent per encapsulating packet.  In order to maximize
   bandwidth, IP-TFS breaks this one-to-one association by introducing
   an AGGFRAG mode for ESP.

   AGGFRAG mode aggregates as well as fragments the inner IP traffic
   flow into encapsulating IPsec tunnel packets.  For IP-TFS, the IPsec
   encapsulating tunnel packets are a fixed size.  Padding is only added
   to the tunnel packets if there is no data available to be sent at the
   time of tunnel packet transmission, or if fragmentation has been
   disabled by the receiver.

   This is accomplished using a new Encapsulating Security Payload (ESP,
   [RFC4303]) Next Header field value AGGFRAG_PAYLOAD (Section 6.1).

   Other non-IP-TFS uses of this AGGFRAG mode have been suggested, such
   as increased performance through packet aggregation, as well as
   handling MTU issues using fragmentation.  These uses are not defined
   here, but are also not restricted by this document.

2.2.  Payload Content

   The AGGFRAG_PAYLOAD payload content defined in this document consists
   of a 4 or 24 octet header followed by either a partial datablock, a
   full datablock, or multiple partial or full datablocks.  The
   following diagram illustrates this payload within the ESP packet.
   See Section 6.1 for the exact formats of the AGGFRAG_PAYLOAD payload.

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    . Outer Encapsulating Header ...                                .
    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    . ESP Header...                                                 .
    +---------------------------------------------------------------+
    |   [AGGFRAG sub-type/flags]   :           BlockOffset          |
    +---------------------------------------------------------------+
    :                  [Optional Congestion Info]                   :
    +---------------------------------------------------------------+
    |       DataBlocks ...                                          ~
    ~                                                               ~
    ~                                                               |
    +---------------------------------------------------------------|
    . ESP Trailer...                                                .
    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

              Figure 1: Layout of an AGGFRAG mode IPsec Packet

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   The BlockOffset value is either zero or some offset into or past the
   end of the DataBlocks data.

   If the BlockOffset value is zero it means that the DataBlocks data
   begins with a new data block.

   Conversely, if the BlockOffset value is non-zero it points to the
   start of the new data block, and the initial DataBlocks data belongs
   to the data block that is still being re-assembled.

   If the BlockOffset points past the end of the DataBlocks data then
   the next data block occurs in a subsequent encapsulating packet.

   Having the BlockOffset always point at the next available data block
   allows for recovering the next inner packet in the presence of outer
   encapsulating packet loss.

   An example AGGFRAG mode packet flow can be found in Appendix A.

2.2.1.  Data Blocks

    +---------------------------------------------------------------+
    | Type  | rest of IPv4, IPv6 or pad.
    +--------

                      Figure 2: Layout of a DataBlock

   A data block is defined by a 4-bit type code followed by the data
   block data.  The type values have been carefully chosen to coincide
   with the IPv4/IPv6 version field values so that no per-data block
   type overhead is required to encapsulate an IP packet.  Likewise, the
   length of the data block is extracted from the encapsulated IPv4's
   Total Length or IPv6's Payload Length fields.

2.2.2.  End Padding

   Since a data block's type is identified in its first 4-bits, the only
   time padding is required is when there is no data to encapsulate.
   For this end padding a Pad Data Block is used.

2.2.3.  Fragmentation, Sequence Numbers and All-Pad Payloads

   In order for a receiver to reassemble fragmented inner packets, the
   sender MUST send the inner packet fragments back-to-back in the
   logical outer packet stream (i.e., using consecutive ESP sequence
   numbers).  However, the sender is allowed to insert "all-pad"
   payloads (i.e., payloads with a BlockOffset of zero and a single pad
   DataBlock) in between the packets carrying the inner packet fragment

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   payloads.  This interleaving of all-pad payloads allows the sender to
   always send a tunnel packet, regardless of the encapsulation
   computational requirements.

   When a receiver is reassembling an inner packet, and it receives an
   "all-pad" payload, it increments the expected sequence number that
   the next inner packet fragment is expected to arrive in.

   Given the above, the receiver will need to handle out-of-order
   arrival of outer ESP packets prior to reassembly processing.  ESP
   already provides for optionally detecting replay attacks.  Detecting
   replay attacks normally utilizes a window method.  A similar sequence
   number based sliding window can be used to correct re-ordering of the
   outer packet stream.  Receiving a larger (newer) sequence number
   packet advances the window, and received older ESP packets whose
   sequence numbers the window has passed by are dropped.  A good choice
   for the size of this window depends on the amount of misordering the
   user may normally experience.

   As the amount of misordering that may be present is hard to predict,
   the window size SHOULD be configurable by the user.  Implementations
   MAY also dynamically adjust the reordering window based on actual
   misordering seen in arriving packets.

   Please note, when IP-TFS sends a continuous stream of packets, there
   is no requirement for an explicit lost packet timer; however, using a
   lost packet timer is RECOMMENDED.  If an implementation does not use
   a lost packet timer and only considers an outer packet lost when the
   reorder window moves by it, the inner traffic can be delayed by up to
   the reorder window size times the per packet send rate.  This delay
   could be significant for slower send rates or when larger reorder
   window sizes are in use.  As the lost packet timer affects delay of
   inner packet delivery, an implementation or user could choose to set
   it proportionate to the tunnel rate.

   While ESP guarantees an increasing sequence number with subsequently
   sent packets, it does not actually require the sequence numbers to be
   generated consecutively (e.g., sending only even numbered sequence
   numbers would be allowed as long as they are always increasing).
   Gaps in the sequence numbers will not work for this document so the
   sequence number stream MUST increase monotonically by 1 for each
   subsequent packet.

   When using the AGGFRAG_PAYLOAD in conjunction with replay detection,
   the window size for both MAY be reduced to the smaller of the two
   window sizes.  This is because packets outside of the smaller window
   but inside the larger would still be dropped by the mechanism with
   the smaller window size.  However, there is also no requirement to

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   make these values the same.  Indeed, in some cases, such as slow
   tunnels where a very small or zero reorder window size is
   appropriate, the user may still want a large replay detection window
   to log replayed packets.  Additionally, large replay windows can be
   implemented with very little overhead compared to large reorder
   windows.

   Finally, as sequence numbers are reset when switching SAs (e.g., when
   re-keying a child SA), senders MUST NOT send initial fragments of an
   inner packet using one SA and subsequent fragments in a different SA.

2.2.3.1.  Optional Extra Padding

   When the tunnel bandwidth is not being fully utilized, a sender MAY
   pad-out the current encapsulating packet in order to deliver an inner
   packet un-fragmented in the following outer packet.  The benefit
   would be to avoid inner packet fragmentation in the presence of a
   bursty offered load (non-bursty traffic will naturally not fragment).
   Senders MAY also choose to allow for a minimum fragment size to be
   configured (e.g., as a percentage of the AGGFRAG_PAYLOAD payload
   size) to avoid fragmentation at the cost of tunnel bandwidth.  The
   cost with these methods is complexity and added delay of inner
   traffic.  The main advantage to avoiding fragmentation is to minimize
   inner packet loss in the presence of outer packet loss.  When this is
   worthwhile (e.g., how much loss and what type of loss is required,
   given different inner traffic shapes and utilization, for this to
   make sense), and what values to use for the allowable/added delay may
   be worth researching but is outside the scope of this document.

   While use of padding to avoid fragmentation does not impact
   interoperability, used inappropriately it can reduce the effective
   throughput of a tunnel.  Senders implementing either of the above
   approaches will need to take care to not reduce the effective
   capacity, and overall utility, of the tunnel through the overuse of
   padding.

2.2.4.  Empty Payload

   To support reporting of congestion control information (described
   later) using a non-AGGFRAG_PAYLOAD-enabled SA, it is allowed to send
   an AGGFRAG_PAYLOAD payload with no data blocks (i.e., the ESP payload
   length is equal to the AGGFRAG_PAYLOAD header length).  This special
   payload is called an empty payload.

   Currently this situation is only applicable in non-IKEv2 use cases.

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2.2.5.  IP Header Value Mapping

   [RFC4301] provides some direction on when and how to map various
   values from an inner IP header to the outer encapsulating header,
   namely the Don't-Fragment (DF) bit ([RFC0791] and [RFC8200]), the
   Differentiated Services (DS) field [RFC2474] and the Explicit
   Congestion Notification (ECN) field [RFC3168].  Unlike [RFC4301],
   AGGFRAG mode may and often will be encapsulating more than one IP
   packet per ESP packet.  To deal with this, these mappings are
   restricted further.

2.2.5.1.  DF bit

   AGGFRAG mode never maps the inner DF bit as it is unrelated to the
   AGGFRAG tunnel functionality; AGGFRAG mode never needs to IP fragment
   the inner packets and the inner packets will not affect the
   fragmentation of the outer encapsulation packets.

2.2.5.2.  ECN value

   The ECN value need not be mapped as any congestion related to the
   constant-send-rate IP-TFS tunnel is unrelated (by design) to the
   inner traffic flow.  The sender MAY still set the ECN value of inner
   packets based on the normal ECN specification [RFC3168].

2.2.5.3.  DS field

   By default, the DS field SHOULD NOT be copied, although a sender MAY
   choose to allow for configuration to override this behavior.  A
   sender SHOULD also allow the DS value to be set by configuration.

2.2.6.  IP Time-To-Live (TTL) and Tunnel errors

   [RFC4301] specifies how to modify the inner packet TTL [RFC0791].

   Any errors (e.g., ICMP errors arriving back at the tunnel ingress due
   to tunnel traffic) are handled the same as with non-AGGFRAG IPsec
   tunnels.

2.2.7.  Effective MTU of the Tunnel

   Unlike [RFC4301], there is normally no effective MTU (EMTU) on an
   AGGFRAG tunnel as all IP packet sizes are properly transmitted
   without requiring IP fragmentation prior to tunnel ingress.  That
   said, a sender MAY allow for explicitly configuring an MTU for the
   tunnel.

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   If fragmentation has been disabled on the AGGFRAG tunnel, then the
   tunnel's EMTU and behaviors are the same as normal IPsec tunnels
   [RFC4301].

2.3.  Exclusive SA Use

   This document does not specify mixed use of an AGGFRAG_PAYLOAD-
   enabled SA.  A sender MUST only send AGGFRAG_PAYLOAD payloads over an
   SA configured for AGGFRAG mode.

2.4.  Modes of Operation

   Just as with normal IPsec/ESP tunnels, AGGFRAG tunnels are
   unidirectional.  Bidirectional IP-TFS functionality is achieved by
   setting up 2 AGGFRAG tunnels, one in either direction.

   An AGGFRAG tunnel used for IP-TFS can operate in 2 modes, a non-
   congestion-controlled mode and congestion-controlled mode.

2.4.1.  Non-Congestion-Controlled Mode

   In the non-congestion-controlled mode, IP-TFS sends fixed-sized
   packets over an AGGFRAG tunnel at a constant rate.  The packet send
   rate is constant and is not automatically adjusted regardless of any
   network congestion (e.g., packet loss).

   For similar reasons as given in [RFC7510] the non-congestion-
   controlled mode should only be used where the user has full
   administrative control over the path the tunnel will take.  This is
   required so the user can guarantee the bandwidth and also be sure as
   to not be negatively affecting network congestion [RFC2914].  In this
   case, packet loss should be reported to the administrator (e.g., via
   syslog, YANG notification, SNMP traps, etc.) so that any failures due
   to a lack of bandwidth can be corrected.

   Non-congestion-controlled mode is also appropriate if ESP over TCP is
   in use [RFC8229].

2.4.2.  Congestion-Controlled Mode

   With the congestion-controlled mode, IP-TFS adapts to network
   congestion by lowering the packet send rate to accommodate the
   congestion, as well as raising the rate when congestion subsides.
   Since overhead is per packet, by allowing for maximal fixed-size
   packets and varying the send rate, transport overhead is minimized.

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   The output of the congestion control algorithm will adjust the rate
   at which the ingress sends packets.  While this document does not
   require a specific congestion control algorithm, best current
   practice RECOMMENDS that the algorithm conform to [RFC5348].
   Congestion control principles are documented in [RFC2914] as well.
   [RFC4342] provides an example of the [RFC5348] algorithm which
   matches the requirements of IP-TFS (i.e., designed for fixed-size
   packets and send rate varied based on congestion).

   The required inputs for the TCP friendly rate control algorithm
   described in [RFC5348] are the receiver's loss event rate and the
   sender's estimated round-trip time (RTT).  These values are provided
   by IP-TFS using the congestion information header fields described in
   Section 3.  In particular, these values are sufficient to implement
   the algorithm described in [RFC5348].

   At a minimum, the congestion information MUST be sent, from the
   receiver and from the sender, at least once per RTT.  Prior to
   establishing an RTT the information SHOULD be sent constantly from
   the sender and the receiver so that an RTT estimate can be
   established.  Not receiving this information over multiple
   consecutive RTT intervals should be considered a congestion event
   that causes the sender to adjust its sending rate lower.  For
   example, [RFC4342] calls this the "no feedback timeout" and it is
   equal to 4 RTT intervals.  When a "no feedback timeout" has occurred
   [RFC4342] halves the sending rate.

   An implementation MAY choose to always include the congestion
   information in its AGGFRAG payload header if sending on an IP-TFS-
   enabled SA.  Since IP-TFS normally will operate with a large packet
   size, the congestion information should represent a small portion of
   the available tunnel bandwidth.  An implementation choosing to always
   send the data MAY also choose to only update the LossEventRate and
   RTT header field values it sends every RTT though.

   When choosing a congestion control algorithm (or a selection of
   algorithms), note that IP-TFS is not providing for reliable delivery
   of IP traffic, and so per packet ACKs are not required and are not
   provided.

   It is worth noting that the variable send-rate of a congestion-
   controlled AGGFRAG tunnel, is not private; however, this send-rate is
   being driven by network congestion, and as long as the encapsulated
   (inner) traffic flow shape and timing are not directly affecting the
   (outer) network congestion, the variations in the tunnel rate will
   not weaken the provided inner traffic flow confidentiality.

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2.4.2.1.  Circuit Breakers

   In additional to congestion control, implementations MAY choose to
   define and implement circuit breakers [RFC8084] as a recovery method
   of last resort.  Enabling circuit breakers is also a reason a user
   may wish to enable congestion information reports even when using the
   non-congestion-controlled mode of operation.

2.5.  Summary of Receiver Processing

   An AGGFRAG-enabled SA receiver has a few tasks to perform.

   The receiver MAY process incoming AGGFRAG_PAYLOAD payloads as soon as
   they arrive as much as it can.  I.e., if the incoming AGGFRAG_PAYLOAD
   packet contains complete inner packet(s), the receiver should extract
   and transmit them immediately.  For partial packets, the receiver
   needs to keep the partial packets in the memory until they fall out
   from the reordering window, or until the missing parts of the packets
   are received, in which case it will reassemble and transmit them.  If
   the AGGFRAG_PAYLOAD payload contains multiple packets they SHOULD be
   sent out in the order they are in the AGGFRAG_PAYLOAD (i.e., keep the
   original order they were received on the other end).  The cost of
   using this method is that an amplification of out-of-order delivery
   of inner packets can occur due to inner packet aggregation.

   Instead of the method described in the previous paragraph, the
   receiver MAY reorder out-of-order AGGFRAG_PAYLOAD payloads received
   into in-sequence-order AGGFRAG_PAYLOAD payloads (Section 2.2.3), and
   only after it has an in-order AGGFRAG_PAYLOAD payload stream would
   the receiver transmits the inner packets.  Using this method will
   ensure the inner packets are sent in order.  The cost of this method
   is that a lost packet will cause a delay of up to the lost packet
   timer interval (or the full reorder window if no lost packet timer is
   used).  Additionally, there can be extra burstiness in the output
   stream.  This burstiness can happen when a lost packet is dropped
   from the re-order window, and the remaining outer packets in the re-
   order window are immediately processed and sent out back to back.

   Additionally, if congestion control is enabled, the receiver sends
   congestion control data (Section 6.1.2) back to the sender as
   described in Section 2.4.2 and Section 3.

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3.  Congestion Information

   In order to support the congestion-controlled mode, the sender needs
   to know the loss event rate and to approximate the RTT [RFC5348].  In
   order to obtain these values, the receiver sends congestion control
   information on its SA back to the sender.  Thus, to support
   congestion control the receiver MUST have a paired SA back to the
   sender (this is always the case when the tunnel was created using
   IKEv2).  If the SA back to the sender is a non-AGGFRAG_PAYLOAD
   enabled SA then an AGGFRAG_PAYLOAD empty payload (i.e., header only)
   is used to convey the information.

   In order to calculate a loss event rate compatible with [RFC5348],
   the receiver needs to have a round-trip time estimate.  Thus the
   sender communicates this estimate in the RTT header field.  On
   startup this value will be zero as no RTT estimate is yet known.

   In order for the sender to estimate its RTT value, the sender places
   a timestamp value in the TVal header field.  On first receipt of this
   TVal, the receiver records the new TVal value along with the time it
   arrived locally.  Subsequent receipt of the same TVal MUST NOT update
   the recorded time.

   When the receiver sends its congestion control header it places this
   latest recorded TVal in the TEcho header field, along with 2 delay
   values, Echo Delay and Transmit Delay.  The Echo Delay value is the
   time delta from the recorded arrival time of TVal and the current
   clock in microseconds.  The second value, Transmit Delay, is the
   receiver's current transmission delay on the tunnel (i.e., the
   average time between sending packets on its half of the AGGFRAG
   tunnel).

   When the sender receives back its TVal in the TEcho header field it
   calculates 2 RTT estimates.  The first is the actual delay found by
   subtracting the TEcho value from its current clock and then
   subtracting Echo Delay as well.  The second RTT estimate is found by
   adding the received Transmit Delay header value to the sender's own
   transmission delay (i.e., the average time between sending packets on
   its half of the AGGFRAG tunnel).  The larger of these 2 RTT estimates
   SHOULD be used as the RTT value.

   The two RTT estimates are required to handle different combinations
   of faster or slower tunnel packet paths with faster or slower fixed
   tunnel rates.  Choosing the larger of the two values guarantees that
   the RTT is never considered faster than the aggregate transmission
   delay based on the IP-TFS send rate (the second estimate), as well as
   never being considered faster than the actual RTT along the tunnel
   packet path (the first estimate).

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   The receiver also calculates, and communicates in the LossEventRate
   header field, the loss event rate for use by the sender.  This is
   slightly different from [RFC4342] which periodically sends all the
   loss interval data back to the sender so that it can do the
   calculation.  See Appendix B for a suggested way to calculate the
   loss event rate value.  Initially this value will be zero (indicating
   no loss) until enough data has been collected by the receiver to
   update it.

3.1.  ECN Support

   In additional to normal packet loss information AGGFRAG mode supports
   use of the ECN bits in the encapsulating IP header [RFC3168] for
   identifying congestion.  If ECN use is enabled and a packet arrives
   at the egress (receiving) side with the Congestion Experienced (CE)
   value set, then the receiver considers that packet as being dropped,
   although it does not drop it.  The receiver MUST set the E bit in any
   AGGFRAG_PAYLOAD payload header containing a LossEventRate value
   derived from a CE value being considered.

   As noted in [RFC3168] the ECN bits are not protected by IPsec and
   thus may constitute a covert channel.  For this reason, ECN use
   SHOULD NOT be enabled by default.

4.  Configuration of AGGFRAG Tunnels for IP-TFS

   IP-TFS is meant to be deployable with a minimal amount of
   configuration.  All IP-TFS specific configuration should be specified
   at the unidirectional tunnel ingress (sending) side.  It is intended
   that non-IKEv2 operation is supported, at least, with local static
   configuration.

   YANG and MIB documents have been defined for IP-TFS in
   [I-D.ietf-ipsecme-yang-iptfs] and [I-D.ietf-ipsecme-mib-iptfs].

4.1.  Bandwidth

   Bandwidth is a local configuration option.  For non-congestion-
   controlled mode, the bandwidth SHOULD be configured.  For congestion-
   controlled mode, the bandwidth can be configured or the congestion
   control algorithm discovers and uses the maximum bandwidth available.
   No standardized configuration method is required.

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4.2.  Fixed Packet Size

   The fixed packet size to be used for the tunnel encapsulation packets
   MAY be configured manually or can be automatically determined using
   other methods such as PLMTUD ([RFC4821], [RFC8899]) or PMTUD
   ([RFC1191], [RFC8201]).  As PMTUD is known to have issues, PLMTUD is
   considered the more robust option.  No standardized configuration
   method is required.

4.3.  Congestion Control

   Congestion control is a local configuration option.  No standardized
   configuration method is required.

5.  IKEv2

5.1.  USE_AGGFRAG Notification Message

   As mentioned previously AGGFRAG tunnels utilize ESP payloads of type
   AGGFRAG_PAYLOAD.

   When using IKEv2, a new "USE_AGGFRAG" Notification Message enables
   the AGGFRAG_PAYLOAD payload on a child SA pair.  The method used is
   similar to how USE_TRANSPORT_MODE is negotiated, as described in
   [RFC7296].

   To request use of the AGGFRAG_PAYLOAD payload on the Child SA pair,
   the initiator includes the USE_AGGFRAG notification in an SA payload
   requesting a new Child SA (either during the initial IKE_AUTH or
   during CREATE_CHILD_SA exchanges).  If the request is accepted then
   the response MUST also include a notification of type USE_AGGFRAG.
   If the responder declines the request the child SA will be
   established without AGGFRAG_PAYLOAD payload use enabled.  If this is
   unacceptable to the initiator, the initiator MUST delete the child
   SA.

   As the use of the AGGFRAG_PAYLOAD payload is currently only defined
   for non-transport mode tunnels, the USE_AGGFRAG notification MUST NOT
   be combined with USE_TRANSPORT notification.

   The USE_AGGFRAG notification contains a 1 octet payload of flags that
   specify requirements from the sender of the notification.  If any
   requirement flags are not understood or cannot be supported by the
   receiver then the receiver SHOULD NOT enable use of AGGFRAG_PAYLOAD
   (either by not responding with the USE_AGGFRAG notification, or in
   the case of the initiator, by deleting the child SA if the now
   established non-AGGFRAG_PAYLOAD using SA is unacceptable).

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   The notification type and payload flag values are defined in
   Section 6.1.4.

6.  Packet and Data Formats

   The packet and data formats defined below are generic with the intent
   of allowing for non-IP-TFS uses, but such uses are outside the scope
   of this document.

6.1.  AGGFRAG_PAYLOAD Payload

   ESP Next Header value: 0x5

   An AGGFRAG payload is identified by the ESP Next Header value
   AGGFRAG_PAYLOAD which has the value 0x5.  The value 5 was chosen to
   not conflict with other used values.  The first octet of this payload
   indicates the format of the remaining payload data.

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+-+-+-
    |   Sub-type    | ...
    +-+-+-+-+-+-+-+-+-+-+-

                  Figure 3: AGGFRAG_PAYLOAD payload format

   Sub-type:
      An 8-bit value indicating the payload format.

   This document defines 2 payload sub-types.  These payload formats are
   defined in the following sections.

6.1.1.  Non-Congestion Control AGGFRAG_PAYLOAD Payload Format

   The non-congestion control AGGFRAG_PAYLOAD payload consists of a
   4-octet header followed by a variable amount of DataBlocks data as
   shown below.

                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Sub-Type (0) |   Reserved    |          BlockOffset          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       DataBlocks ...
    +-+-+-+-+-+-+-+-+-+-+-

              Figure 4: Non-congestion control payload format

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   Sub-type:
      An octet indicating the payload format.  For this non-congestion
      control format, the value is 0.

   Reserved:
      An octet set to 0 on generation and ignored on receipt.

   BlockOffset:
      A 16-bit unsigned integer counting the number of octets of
      DataBlocks data before the start of a new data block.  If the
      start of a new data block occurs in a subsequent payload the
      BlockOffset will point past the end of the DataBlocks data.  In
      this case all the DataBlocks data belongs to the current data
      block being assembled.  When the BlockOffset extends into
      subsequent payloads it continues to only count DataBlocks data
      (i.e., it does not count subsequent packets non-DataBlocks data
      such as header octets).

   DataBlocks:
      Variable number of octets that begins with the start of a data
      block, or the continuation of a previous data block, followed by
      zero or more additional data blocks.

6.1.2.  Congestion Control AGGFRAG_PAYLOAD Payload Format

   The congestion control AGGFRAG_PAYLOAD payload consists of a 24 octet
   header followed by a variable amount of DataBlocks data as shown
   below.

                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Sub-type (1) |  Reserved |P|E|          BlockOffset          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          LossEventRate                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      RTT                  |   Echo Delay ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         ... Echo Delay   |           Transmit Delay                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              TVal                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             TEcho                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       DataBlocks ...
    +-+-+-+-+-+-+-+-+-+-+-

                Figure 5: Congestion control payload format

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   Sub-type:
      An octet indicating the payload format.  For this congestion
      control format, the value is 1.

   Reserved:
      A 6-bit field set to 0 on generation and ignored on receipt.

   P:
      A 1-bit value that if set indicates that PLMTUD probing is in
      progress.  This information can be used to avoid treating missing
      packets as loss events by the CC algorithm when running the PLMTUD
      probe algorithm.

   E:
      A 1-bit value that if set indicates that Congestion Experienced
      (CE) ECN bits were received and used in deriving the reported
      LossEventRate.

   BlockOffset:
      The same value as the non-congestion-controlled payload format
      value.

   LossEventRate:
      A 32-bit value specifying the inverse of the current loss event
      rate as calculated by the receiver.  A value of zero indicates no
      loss.  Otherwise the loss event rate is 1/LossEventRate.

   RTT:
      A 22-bit value specifying the sender's current round-trip time
      estimate in microseconds.  The value MAY be zero prior to the
      sender having calculated a round-trip time estimate.  The value
      SHOULD be set to zero on non-AGGFRAG_PAYLOAD-enabled SAs.  If the
      RTT is equal to or larger than 0x3FFFFF the value MUST be set to
      0x3FFFFF.

   Echo Delay:
      A 21-bit value specifying the delay in microseconds incurred
      between the receiver first receiving the TVal value which it is
      sending back in TEcho.  If the delay is equal to or larger than
      0x1FFFFF the value MUST be set to 0x1FFFFF.

   Transmit Delay:
      A 21-bit value specifying the transmission delay in microseconds.
      This is the fixed (or average) delay on the receiver between it
      sending packets on the IPTFS tunnel.  If the delay is equal to or
      larger than 0x1FFFFF the value MUST be set to 0x1FFFFF.

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   TVal:
      An opaque 32-bit value that will be echoed back by the receiver in
      later packets in the TEcho field, along with an Echo Delay value
      of how long that echo took.

   TEcho:
      The opaque 32-bit value from a received packet's TVal field.  The
      received TVal is placed in TEcho along with an Echo Delay value
      indicating how long it has been since receiving the TVal value.

   DataBlocks:
      Variable number of octets that begins with the start of a data
      block, or the continuation of a previous data block, followed by
      zero or more additional data blocks.  For the special case of
      sending congestion control information on a non-IP-TFS enabled SA
      this field MUST be empty (i.e., be zero octets long).

6.1.3.  Data Blocks

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Type  | IPv4, IPv6 or pad...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                        Figure 6: Data Block format

   Type:
      A 4-bit field where 0x0 identifies a pad data block, 0x4 indicates
      an IPv4 data block, and 0x6 indicates an IPv6 data block.

6.1.3.1.  IPv4 Data Block

                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  0x4  |  IHL  |  TypeOfService  |         TotalLength         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Rest of the inner packet ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                      Figure 7: IPv4 Data Block format

   These values are the actual values within the encapsulated IPv4
   header.  In other words, the start of this data block is the start of
   the encapsulated IP packet.

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   Type:
      A 4-bit value of 0x4 indicating IPv4 (i.e., first nibble of the
      IPv4 packet).

   TotalLength:
      The 16-bit unsigned integer "Total Length" field of the IPv4 inner
      packet.

6.1.3.2.  IPv6 Data Block

                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  0x6  | TrafficClass  |               FlowLabel               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         PayloadLength         | Rest of the inner packet ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                      Figure 8: IPv6 Data Block format

   These values are the actual values within the encapsulated IPv6
   header.  In other words, the start of this data block is the start of
   the encapsulated IP packet.

   Type:
      A 4-bit value of 0x6 indicating IPv6 (i.e., first nibble of the
      IPv6 packet).

   PayloadLength:
      The 16-bit unsigned integer "Payload Length" field of the inner
      IPv6 inner packet.

6.1.3.3.  Pad Data Block

                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  0x0  | Padding ...
    +-+-+-+-+-+-+-+-+-+-+-

                      Figure 9: Pad Data Block format

   Type:
      A 4-bit value of 0x0 indicating a padding data block.

   Padding:
      Extends to end of the encapsulating packet.

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6.1.4.  IKEv2 USE_AGGFRAG Notification Message

   As discussed in Section 5.1, a notification message USE_AGGFRAG is
   used to negotiate use of the ESP AGGFRAG_PAYLOAD Next Header value.

   The USE_AGGFRAG Notification Message State Type is (TBD2).

   The notification payload contains 1 octet of requirement flags.
   There are currently 2 requirement flags defined.  This may be revised
   by later specifications.

    +-+-+-+-+-+-+-+-+
    |0|0|0|0|0|0|C|D|
    +-+-+-+-+-+-+-+-+

                  Figure 10: USE_AGGFRAG requirement flags

   0:
      6 bits - Reserved MUST be zero on send, unless defined by later
      specifications.

   C:
      Congestion Control bit.  If set, then the sender is requiring that
      congestion control information MUST be returned to it periodically
      as defined in Section 3.

   D:
      Don't Fragment bit.  If set, indicates the sender of the notify
      message does not support receiving packet fragments (i.e., inner
      packets MUST be sent using a single Data Block).  This value only
      applies to what the sender is capable of receiving; the sender MAY
      still send packet fragments unless similarly restricted by the
      receiver in its USE_AGGFRAG notification.

7.  IANA Considerations

7.1.  AGGFRAG_PAYLOAD Sub-Type Registry

   This document requests IANA create a registry called "AGGFRAG_PAYLOAD
   Sub-Type Registry" under a new category named "ESP AGGFRAG_PAYLOAD
   Parameters".  The registration policy for this registry is "Expert
   Review" ([RFC8126] and [RFC7120]).

   Name:
      AGGFRAG_PAYLOAD Sub-Type Registry

   Description:
      AGGFRAG_PAYLOAD Payload Formats.

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   Reference:
      This document

   This initial content for this registry is as follows:

    Sub-Type  Name                           Reference
   --------------------------------------------------------
           0  Non-Congestion Control Format  This document
           1  Congestion Control Format      This document
       3-255  Reserved

7.2.  USE_AGGFRAG Notify Message Status Type

   This document requests a status type USE_AGGFRAG be allocated from
   the "IKEv2 Notify Message Types - Status Types" registry.

   Value:
      TBD2

   Name:
      USE_AGGFRAG

   Reference:
      This document

8.  Security Considerations

   This document describes an aggregation and fragmentation mechanism to
   efficiently implement TFC for IP traffic.  This approach is expected
   to reduce the efficacy of traffic analysis on IPsec communication.
   Other than the additional security afforded by using this mechanism,
   IP-TFS utilizes the security protocols [RFC4303] and [RFC7296] and so
   their security considerations apply to IP-TFS as well.

   As noted in Section 3.1, the ECN bits are not protected by IPsec and
   thus may constitute a covert channel.  For this reason, ECN use
   SHOULD NOT be enabled by default.

   As noted previously in Section 2.4.2, for TFC to be maintained, the
   encapsulated traffic flow should not be affecting network congestion
   in a predictable way, and if it would be, then non-congestion-
   controlled mode use should be considered instead.

9.  References

9.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [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>.

9.2.  Informative References

   [AppCrypt] Schneier, B., "Applied Cryptography: Protocols,
              Algorithms, and Source Code in C", 1 November 2017.

   [I-D.ietf-ipsecme-mib-iptfs]
              Fedyk, D. and E. Kinzie, "Definitions of Managed Objects
              for IP Traffic Flow Security", Work in Progress, Internet-
              Draft, draft-ietf-ipsecme-mib-iptfs-03, 18 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ipsecme-mib-
              iptfs-03.txt>.

   [I-D.ietf-ipsecme-yang-iptfs]
              Fedyk, D. and C. Hopps, "A YANG Data Model for IP Traffic
              Flow Security", Work in Progress, Internet-Draft, draft-
              ietf-ipsecme-yang-iptfs-06, 17 May 2022,
              <https://www.ietf.org/archive/id/draft-ietf-ipsecme-yang-
              iptfs-06.txt>.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

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   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <https://www.rfc-editor.org/info/rfc2914>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4342]  Floyd, S., Kohler, E., and J. Padhye, "Profile for
              Datagram Congestion Control Protocol (DCCP) Congestion
              Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
              DOI 10.17487/RFC4342, March 2006,
              <https://www.rfc-editor.org/info/rfc4342>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, DOI 10.17487/RFC5348, September 2008,
              <https://www.rfc-editor.org/info/rfc5348>.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <https://www.rfc-editor.org/info/rfc7120>.

   [RFC7510]  Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
              "Encapsulating MPLS in UDP", RFC 7510,
              DOI 10.17487/RFC7510, April 2015,
              <https://www.rfc-editor.org/info/rfc7510>.

   [RFC8084]  Fairhurst, G., "Network Transport Circuit Breakers",
              BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
              <https://www.rfc-editor.org/info/rfc8084>.

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   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [RFC8229]  Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
              of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
              August 2017, <https://www.rfc-editor.org/info/rfc8229>.

   [RFC8546]  Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
              2019, <https://www.rfc-editor.org/info/rfc8546>.

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/info/rfc8899>.

Appendix A.  Example of An Encapsulated IP Packet Flow

   Below, an example inner IP packet flow within the encapsulating
   tunnel packet stream is shown.  Notice how encapsulated IP packets
   can start and end anywhere, and more than one or less than 1 may
   occur in a single encapsulating packet.

     Offset: 0        Offset: 100    Offset: 2000    Offset: 600
    [ ESP1  (1404) ][ ESP2  (1404) ][ ESP3  (1404) ][ ESP4  (1404) ]
    [--750--][--750--][60][-240-][--3000----------------------][pad]

                   Figure 11: Inner and outer packet flow

   Each outer encapsulating ESPupayload space is a fixed-size of 1404
   octets the first 4 octets of which contains the AGGFRAG header.  The
   encapsulated IP packet flow (lengths include IP header and payload)
   is as follows: a 750-octet packet, a 750-octet packet, a 60-octet
   packet, a 240-octet packet, a 3000-octet packet.

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   The BlockOffset values in the 4 AGGFRAG payload headers for this
   packet flow would thus be: 0, 100, 2000, 600 respectively.  The first
   encapsulating packet (ESP1) has a zero BlockOffset which points at
   the IP data block immediately following the AGGFRAG header.  The
   following packet's (ESP2) BlockOffset points inward 100 octets to the
   start of the 60-octet data block.  The third encapsulating packet
   (ESP3) contains the middle portion of the 3000-octet data block so
   the offset points past its end and into the fourth encapsulating
   packet.  The fourth packet's (ESP4) offset is 600, pointing at the
   padding which follows the completion of the continued 3000-octet
   packet.

Appendix B.  A Send and Loss Event Rate Calculation

   The current best practice indicates that congestion control SHOULD be
   done in a TCP-friendly way.  A TCP-friendly congestion control
   algorithm is described in [RFC5348].  For this IP-TFS use case (as
   with [RFC4342]), the (fixed) packet size is used as the segment size
   for the algorithm.  The main formula in the algorithm for the send
   rate is then as follows:

                                 1
      X = -----------------------------------------------
          R * (sqrt(2*p/3) + 12*sqrt(3*p/8)*p*(1+32*p^2))

   Where X is the send rate in packets per second, R is the round trip
   time estimate and p is the loss event rate (the inverse of which is
   provided by the receiver).

   In addition, the algorithm in [RFC5348] also uses an X_recv value
   (the receiver's receive rate).  For IP-TFS one MAY set this value
   according to the sender's current tunnel send-rate (X).

   The IP-TFS receiver, having the RTT estimate from the sender can use
   the same method as described in [RFC5348] and [RFC4342] to collect
   the loss intervals and calculate the loss event rate value using the
   weighted average as indicated.  The receiver communicates the inverse
   of this value back to the sender in the AGGFRAG_PAYLOAD payload
   header field LossEventRate.

   The IP-TFS sender now has both the R and p values and can calculate
   the correct sending rate.  If following [RFC5348], the sender should
   also use the slow start mechanism described therein when the IP-TFS
   SA is first established.

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Appendix C.  Comparisons of IP-TFS

C.1.  Comparing Overhead

   For comparing overhead, the overhead of ESP for both normal and
   AGGFRAG tunnel packets must be calculated, and so an algorithm for
   encryption and authentication must be chosen.  For the data below
   AES-GCM-256 was selected.  This leads to an IP+ESP overhead of 54.

     54 = 20 (IP) + 8 (ESPH) + 2 (ESPF) + 8 (IV) + 16 (ICV)

   Additionally, for IP-TFS, non-congestion control AGGFRAG_PAYLOAD
   headers were chosen which adds 4 octets for a total overhead of 58.

C.1.1.  IP-TFS Overhead

   For comparison, the overhead of an AGGFRAG payload is 58 octets per
   outer packet.  Therefore, the octet overhead per inner packet is 58
   divided by the number of outer packets required (fractions allowed).
   The overhead as a percentage of inner packet size is a constant based
   on the Outer MTU size.

      OH = 58 / Outer Payload Size / Inner Packet Size
      OH % of Inner Packet Size = 100 * OH / Inner Packet Size
      OH % of Inner Packet Size = 5800 / Outer Payload Size

                        Type  IP-TFS  IP-TFS  IP-TFS
                         MTU     576    1500    9000
                       PSize     518    1442    8942
                      -------------------------------
                          40  11.20%   4.02%   0.65%
                         576  11.20%   4.02%   0.65%
                        1500  11.20%   4.02%   0.65%
                        9000  11.20%   4.02%   0.65%

       Figure 12: IP-TFS Overhead as Percentage of Inner Packet Size

C.1.2.  ESP with Padding Overhead

   The overhead per inner packet for constant-send-rate padded ESP
   (i.e., traditional IPsec TFC) is 36 octets plus any padding, unless
   fragmentation is required.

   When fragmentation of the inner packet is required to fit in the
   outer IPsec packet, overhead is the number of outer packets required
   to carry the fragmented inner packet times both the inner IP overhead
   (20) and the outer packet overhead (54) minus the initial inner IP
   overhead plus any required tail padding in the last encapsulation

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   packet.  The required tail padding is the number of required packets
   times the difference of the Outer Payload Size and the IP Overhead
   minus the Inner Payload Size.  So:

     Inner Payload Size = IP Packet Size - IP Overhead
     Outer Payload Size = MTU - IPsec Overhead

                   Inner Payload Size
     NF0 = ----------------------------------
            Outer Payload Size - IP Overhead

     NF = CEILING(NF0)

     OH = NF * (IP Overhead + IPsec Overhead)
          - IP Overhead
          + NF * (Outer Payload Size - IP Overhead)
          - Inner Payload Size

     OH = NF * (IPsec Overhead + Outer Payload Size)
          - (IP Overhead + Inner Payload Size)

     OH = NF * (IPsec Overhead + Outer Payload Size)
          - Inner Packet Size

C.2.  Overhead Comparison

   The following tables collect the overhead values for some common L3
   MTU sizes in order to compare them.  The first table is the number of
   octets of overhead for a given L3 MTU sized packet.  The second table
   is the percentage of overhead in the same MTU sized packet.

           Type  ESP+Pad  ESP+Pad  ESP+Pad  IP-TFS  IP-TFS  IP-TFS
         L3 MTU      576     1500     9000     576    1500    9000
          PSize      522     1446     8946     518    1442    8942
        -----------------------------------------------------------
             40      482     1406     8906     4.5     1.6     0.3
            128      394     1318     8818    14.3     5.1     0.8
            256      266     1190     8690    28.7    10.3     1.7
            518        4      928     8428    58.0    20.8     3.4
            576      576      870     8370    64.5    23.2     3.7
           1442      286        4     7504   161.5    58.0     9.4
           1500      228     1500     7446   168.0    60.3     9.7
           8942     1426     1558        4  1001.2   359.7    58.0
           9000     1368     1500     9000  1007.7   362.0    58.4

                  Figure 13: Overhead comparison in octets

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          Type  ESP+Pad  ESP+Pad   ESP+Pad  IP-TFS  IP-TFS  IP-TFS
           MTU      576     1500      9000     576    1500    9000
         PSize      522     1446      8946     518    1442    8942
        -----------------------------------------------------------
            40  1205.0%  3515.0%  22265.0%  11.20%   4.02%   0.65%
           128   307.8%  1029.7%   6889.1%  11.20%   4.02%   0.65%
           256   103.9%   464.8%   3394.5%  11.20%   4.02%   0.65%
           518     0.8%   179.2%   1627.0%  11.20%   4.02%   0.65%
           576   100.0%   151.0%   1453.1%  11.20%   4.02%   0.65%
          1442    19.8%     0.3%    520.4%  11.20%   4.02%   0.65%
          1500    15.2%   100.0%    496.4%  11.20%   4.02%   0.65%
          8942    15.9%    17.4%      0.0%  11.20%   4.02%   0.65%
          9000    15.2%    16.7%    100.0%  11.20%   4.02%   0.65%

           Figure 14: Overhead as Percentage of Inner Packet Size

C.3.  Comparing Available Bandwidth

   Another way to compare the two solutions is to look at the amount of
   available bandwidth each solution provides.  The following sections
   consider and compare the percentage of available bandwidth.  For the
   sake of providing a well-understood baseline normal (unencrypted)
   Ethernet as well as normal ESP values are included.

C.3.1.  Ethernet

   In order to calculate the available bandwidth the per packet overhead
   is calculated first.  The total overhead of Ethernet is 14+4 octets
   of header and CRC plus an additional 20 octets of framing (preamble,
   start, and inter-packet gap), for a total of 38 octets.
   Additionally, the minimum payload is 46 octets.

         Size  E + P  E + P  E + P  IPTFS  IPTFS  IPTFS  Enet   ESP
          MTU    590   1514   9014    590   1514   9014   any   any
           OH     92     92     92     96     96     96    38    74
        ------------------------------------------------------------
           40    614   1538   9038     47     42     40    84   114
          128    614   1538   9038    151    136    129   166   202
          256    614   1538   9038    303    273    258   294   330
          518    614   1538   9038    614    552    523   574   610
          576   1228   1538   9038    682    614    582   614   650
         1442   1842   1538   9038   1709   1538   1457  1498  1534
         1500   1842   3076   9038   1777   1599   1516  1538  1574
         8942  11052  10766   9038  10599   9537   9038  8998  9034
         9000  11052  10766  18076  10667   9599   9096  9038  9074

                      Figure 15: L2 Octets Per Packet

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        Size  E + P  E + P  E + P  IPTFS  IPTFS  IPTFS  Enet   ESP
         MTU  590    1514   9014   590    1514   9014   any    any
          OH  92     92     92     96     96     96     38     74
       --------------------------------------------------------------
          40  2.0M   0.8M   0.1M   26.4M  29.3M  30.9M  14.9M  11.0M
         128  2.0M   0.8M   0.1M   8.2M   9.2M   9.7M   7.5M   6.2M
         256  2.0M   0.8M   0.1M   4.1M   4.6M   4.8M   4.3M   3.8M
         518  2.0M   0.8M   0.1M   2.0M   2.3M   2.4M   2.2M   2.1M
         576  1.0M   0.8M   0.1M   1.8M   2.0M   2.1M   2.0M   1.9M
        1442  678K   812K   138K   731K   812K   857K   844K   824K
        1500  678K   406K   138K   703K   781K   824K   812K   794K
        8942  113K   116K   138K   117K   131K   138K   139K   138K
        9000  113K   116K   69K    117K   130K   137K   138K   137K

               Figure 16: Packets Per Second on 10G Ethernet

   Size   E + P   E + P   E + P   IPTFS   IPTFS   IPTFS    Enet     ESP
            590    1514    9014     590    1514    9014     any     any
             92      92      92      96      96      96      38      74
  ----------------------------------------------------------------------
     40   6.51%   2.60%   0.44%  84.36%  93.76%  98.94%  47.62%  35.09%
    128  20.85%   8.32%   1.42%  84.36%  93.76%  98.94%  77.11%  63.37%
    256  41.69%  16.64%   2.83%  84.36%  93.76%  98.94%  87.07%  77.58%
    518  84.36%  33.68%   5.73%  84.36%  93.76%  98.94%  93.17%  87.50%
    576  46.91%  37.45%   6.37%  84.36%  93.76%  98.94%  93.81%  88.62%
   1442  78.28%  93.76%  15.95%  84.36%  93.76%  98.94%  97.43%  95.12%
   1500  81.43%  48.76%  16.60%  84.36%  93.76%  98.94%  97.53%  95.30%
   8942  80.91%  83.06%  98.94%  84.36%  93.76%  98.94%  99.58%  99.18%
   9000  81.43%  83.60%  49.79%  84.36%  93.76%  98.94%  99.58%  99.18%

            Figure 17: Percentage of Bandwidth on 10G Ethernet

   A sometimes unexpected result of using an AGGFRAG tunnel (or any
   packet aggregating tunnel) is that, for small- to medium-sized
   packets, the available bandwidth is actually greater than native
   Ethernet.  This is due to the reduction in Ethernet framing overhead.
   This increased bandwidth is paid for with an increase in latency.
   This latency is the time to send the unrelated octets in the outer
   tunnel frame.  The following table illustrates the latency for some
   common values on a 10G Ethernet link.  The table also includes
   latency introduced by padding if using ESP with padding.

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                        ESP+Pad  ESP+Pad  IP-TFS   IP-TFS
                        1500     9000     1500     9000

                 ------------------------------------------
                    40  1.12 us  7.12 us  1.17 us  7.17 us
                   128  1.05 us  7.05 us  1.10 us  7.10 us
                   256  0.95 us  6.95 us  1.00 us  7.00 us
                   518  0.74 us  6.74 us  0.79 us  6.79 us
                   576  0.70 us  6.70 us  0.74 us  6.74 us
                  1442  0.00 us  6.00 us  0.05 us  6.05 us
                  1500  1.20 us  5.96 us  0.00 us  6.00 us

                          Figure 18: Added Latency

   Notice that the latency values are very similar between the two
   solutions; however, whereas IP-TFS provides for constant high
   bandwidth, in some cases even exceeding native Ethernet, ESP with
   padding often greatly reduces available bandwidth.

Appendix D.  Acknowledgements

   We would like to thank Don Fedyk for help in reviewing and editing
   this work.  We would also like to thank Michael Richardson, Sean
   Turner, Valery Smyslov and Tero Kivinen for reviews and many
   suggestions for improvements, as well as Joseph Touch for the
   transport area review and suggested improvements.

Appendix E.  Contributors

   The following people made significant contributions to this document.

      Lou Berger
      LabN Consulting, L.L.C.

      Email: lberger@labn.net

Author's Address

   Christian Hopps
   LabN Consulting, L.L.C.
   Email: chopps@chopps.org

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