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IP Traffic Flow Security
draft-hopps-ipsecme-iptfs-00

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author Christian Hopps
Last updated 2019-03-11
Replaced by draft-ietf-ipsecme-iptfs, draft-ietf-ipsecme-iptfs, draft-ietf-ipsecme-iptfs, draft-ietf-ipsecme-iptfs, RFC 9347
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Nov 2020
Traffic Flow Confidentiality document to IESG
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draft-hopps-ipsecme-iptfs-00
Network Working Group                                           C. Hopps
Internet-Draft                                   LabN Consulting, L.L.C.
Intended status: Standards Track                          March 11, 2019
Expires: September 12, 2019

                        IP Traffic Flow Security
                      draft-hopps-ipsecme-iptfs-00

Abstract

   This document describes a mechanism to enhance IPsec traffic flow
   security by adding traffic flow confidentiality to encrypted IP
   encapsulated traffic.  Traffic flow confidentiality 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.

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 September 12, 2019.

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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   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 & Concepts  . . . . . . . . . . . . . . . . .   3
   2.  The IP-TFS Tunnel . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Tunnel Content  . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  IPSec/ESP Payload . . . . . . . . . . . . . . . . . .   4
       2.1.2.  Data-Blocks . . . . . . . . . . . . . . . . . . . . .   5
       2.1.3.  No Implicit Padding . . . . . . . . . . . . . . . . .   5
       2.1.4.  IP Header Value Mapping . . . . . . . . . . . . . . .   6
     2.2.  Exclusive SA Use  . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Initiation of TFS mode  . . . . . . . . . . . . . . . . .   6
     2.4.  Example of an encapsulated IP packet flow . . . . . . . .   7
     2.5.  Modes of operation  . . . . . . . . . . . . . . . . . . .   7
       2.5.1.  Non-Congestion Controlled Mode  . . . . . . . . . . .   7
       2.5.2.  Congestion Controlled Mode  . . . . . . . . . . . . .   8
   3.  Congestion Information  . . . . . . . . . . . . . . . . . . .   9
     3.1.  ECN Support . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Configuration . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Bandwidth . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Fixed Packet Size . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Congestion Information Configuration  . . . . . . . . . .  10
   5.  Packet and Data Formats . . . . . . . . . . . . . . . . . . .  11
     5.1.  IPSec . . . . . . . . . . . . . . . . . . . . . . . . . .  11
       5.1.1.  Payload Format  . . . . . . . . . . . . . . . . . . .  11
       5.1.2.  Data Blocks . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  IKEv2 . . . . . . . . . . . . . . . . . . . . . . . . . .  13
       5.2.1.  IKEv2 Congestion Information Configuration Attribute   13
       5.2.2.  IKEv2 Congestion Information Notification Data  . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Comparisons of IP-TFS  . . . . . . . . . . . . . . .  17
     A.1.  Comparing Overhead  . . . . . . . . . . . . . . . . . . .  17
       A.1.1.  IP-TFS Overhead . . . . . . . . . . . . . . . . . . .  17
       A.1.2.  ESP with Padding Overhead . . . . . . . . . . . . . .  18
     A.2.  Overhead Comparison . . . . . . . . . . . . . . . . . . .  19
     A.3.  Comparing Available Bandwidth . . . . . . . . . . . . . .  19
       A.3.1.  Ethernet  . . . . . . . . . . . . . . . . . . . . . .  20
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  22
   Appendix C.  Contributors . . . . . . . . . . . . . . . . . . . .  22
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  22

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1.  Introduction

   Traffic Analysis ([RFC4301], [AppCrypt]) is the act of extracting
   information about data being sent through a network.  While one may
   directly obscure the data through the use of encryption [RFC4303],
   the traffic pattern itself exposes information due to variations in
   it's shape and timing ([I-D.iab-wire-image], [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 sending all-pad packets
   (indicated using protocol 59).  This method has the major limitation
   that it can significantly under-utilize the available bandwidth.

   The IP-TFS solution provides for full TFC without the aforementioned
   bandwidth limitation.  To do this we use a constant-send-rate IPsec
   [RFC4303] tunnel with fixed-sized encapsulating packets; however,
   these fixed-sized packets can contain partial, full or multiple IP
   packets to maximize the bandwidth of the tunnel.

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

   Additionally, IP-TFS provides for dealing with network congestion
   [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.

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
   [RFC2119] [RFC8174] when, and only when, they appear in all capitals,
   as shown here.

   This document assumes familiarity with IP security concepts described
   in [RFC4301].

2.  The IP-TFS Tunnel

   As mentioned in Section 1 IP-TFS utilizes an IPsec [RFC4303] tunnel
   as it's transport.  To provide for full TFC we send fixed-sized
   encapsulating packets at a constant rate on the tunnel.

   The primary input to the tunnel algorithm is the requested bandwidth
   of the tunnel.  Two values are then required to provide for this

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   bandwidth, the fixed size of the encapsulating packets, and rate at
   which to send them.

   The fixed packet size may either be specified manually or can be
   determined through the use of Path MTU discovery [RFC1191] and
   [RFC8201].

   Given the encapsulating packet size and the requested tunnel
   bandwidth, the correct packet send rate can be calculated.  The
   packet send rate is the requested bandwidth divided by the payload
   size of the encapsulating packet.

   The egress of the IP-TFS tunnel SHOULD NOT impose any restrictions on
   tunnel packet size or arrival rate.  Packet size and send rate is
   entirely the function of the ingress (sending) side of the IP-TFS
   tunnel.  Indeed, the ingress (sending) side of the IP-TFS tunnel MUST
   be allowed by the egress side to vary the size and rate at which it
   sends encapsulating packets, including sending them larger, smaller,
   faster or slower than the requested size and rate.

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.

   With IP-TFS we fragment as well as aggregate the inner IP traffic
   flow into fixed-sized encapsulating IP tunnel packets.  We only pad
   the tunnel packets if there is no data available to be sent at the
   time of tunnel packet transmission.

   In order to do this we create a new payload data type identified with
   a new IP protocol number IPTFS_PROTOCOL (TBD).  A payload of
   IPTFS_PROTOCOL type is comprised of a 32 bit header followed by
   either a partial, a full or multiple partial or full data-blocks.

2.1.1.  IPSec/ESP Payload

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    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    . Outer Encapsulating Header ...                                  .
    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    . ESP Header...                                                   .
    +-----------------------------------------------------------------+
    |               ...            :           BlockOffset            |
    +-----------------------------------------------------------------+
    |       Data Blocks Payload ...                                   ~
    ~                                                                 ~
    ~                                                                 |
    +-----------------------------------------------------------------|
    . ESP Trailer...                                                  .
    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

                  Figure 1: Layout of IP-TFS IPSec Packet

   The BlockOffset value is either zero or some offset into or past the
   end of the data blocks payload data.  If the value is zero it means
   that a new data-block immediately follows the fixed header (i.e., the
   BlockOffset value).  Conversely, if the BlockOffset value is non-zero
   it points at the start of the next data block.  The BlockOffset can
   point past the end of the data block payload data, this means that
   the next data-block occurs in a subsequent encapsulating packet.
   When the BlockOffset is non-zero the data immediately following the
   header belongs to the previous data-block that is still being re-
   assembled.

2.1.2.  Data-Blocks

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

                   Figure 2: Layout of IP-TFS data block

   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 or
   IPv6 packet's length field.

2.1.3.  No Implicit Padding

   It's worth noting that there is no need for implicit pads at the end
   of an encapsulating packet.  Even when the start of a data block
   occurs near the end of a encapsulating packet such that there is no
   room for the length field of the encapsulated header to be included

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   in the current encapsulating packet, the fact that the length comes
   at a known location and as is guaranteed to be present is enough to
   fetch the length field from the subsequent encapsulating packet
   payload.

2.1.4.  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] with
   IP-TFS we may and often will be encapsulating more than 1 IP packet
   per ESP packet.  To deal with this we further restrict these
   mappings.  In particular we never map the inner DF bit as it is
   unrelated to the IP-TFS tunnel functionality; we never directly
   fragment the inner packets and the inner packets will not affect the
   fragmentation of the outer encapsulation packets.  Likewise, 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.  Finally, by default the DS field SHOULD NOT be copied
   although an implementation MAY choose to allow for configuration to
   override this behavior.  An implementation SHOULD also allow the DS
   value to be set by configuration.

2.2.  Exclusive SA Use

   It is not the intention of this specification to allow for mixed use
   of an IPsec SA.  In other words, an SA that is created for IP-TFS is
   exclusively for IP-TFS use and MUST NOT have non-IP-TFS payloads such
   as IP (IP protocol 4), TCP transport (IP protocol 6), or ESP pad
   packets (protocol 59) intermixed with IP-TFS (IP protocol TBD)
   payloads.  While it's possible to envision making the algorithm work
   in the presence of sequence number skips in the IP-TFS payload
   stream, the added complexity is not deemed worthwhile.  Other IPsec
   uses can configure and use their own SAs.

2.3.  Initiation of TFS mode

   While normally a user will configure their IPsec tunnel to operate in
   IP-TFS mode to start, we also allow IP-TFS mode to be enabled post-SA
   creation.  This may be useful for debugging or other purposes.  In
   this late enabled mode the receiver would switch to IP-TFS mode on
   receipt of the first ESP payload with the IPTFS_PROTOCOL indicated as
   the payload type.

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2.4.  Example of an encapsulated IP packet flow

   Below we show an example inner IP packet flow within the
   encapsulating tunnel packet stream.  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: 2900    Offset: 1400
    [ ESP1  (1500) ][ ESP2  (1500) ][ ESP3  (1500) ][ ESP4  (1500) ]
    [--800--][--800--][60][-240-][--4000----------------------][pad]

                   Figure 3: Inner and Outer Packet Flow

   The encapsulated IP packet flow (lengths include IP header and
   payload) is as follows: an 800 octet packet, an 800 octet packet, a
   60 octet packet, a 240 octet packet, a 4000 octet packet.

   The BlockOffset values in the 4 IP-TFS payload headers for this
   packet flow would thus be: 0, 100, 2900, 1400 respectively.  The
   first encapsulating packet ESP1 has a zero BlockOffset which points
   at the IP data block immediately following the IP-TFS header.  The
   following packet ESP2s 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 4000 octet data block so the
   offset points past its end and into the forth encapsulating packet.
   The fourth packet ESP4s offset is 1400 pointing at the padding which
   follows the completion of the continued 4000 octet packet.

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

2.5.  Modes of operation

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

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

2.5.1.  Non-Congestion Controlled Mode

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

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   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.

2.5.2.  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.

   If congestion were handled in the network on a octet level we might
   consider lowering the IPsec (encapsulation) packet size to adapt;
   however, as congestion is normally handled in the network by dropping
   packets we instead choose to lower the frequency we send our fixed
   sized packets.  This choice also minimizes transport overhead.

   The output of a congestion control algorithm SHOULD adjust the
   frequency that ingress sends packets until the congestion is
   accommodated.  While this document does not standardize the
   congestion control algorithm, the algorithm used by an implementation
   SHOULD conform to the guidelines in [RFC2914].

   When an implementation is choosing a congestion control algorithm it
   is worth noting 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's worth noting that the adjustable rate of sending over the
   congestion controlled IP-TFS tunnel is being controlled by the
   network congestion.  As long as the encapsulated traffic flow shape
   and timing are not directly affecting the network congestion, the
   variations in the tunnel rate will not weaken the provided traffic
   flow confidentiality.

2.5.2.1.  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.  The definition of
   circuit breakers are outside the scope of this document.

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3.  Congestion Information

   In order to support the congestion control mode, the receiver (egress
   tunnel endpoint) MUST send regular packet drop reports to the sender
   (ingress tunnel endpoint).  These reports indicate the number of
   packet drops during a sequence of packets.  The sequence or range of
   packets is identified using the start and end ESP sequence numbers of
   the packet range.

   These congestion information reports MAY also be sent when in the
   non-congestion controlled mode to allow for reporting from the
   sending device or to implement Circuit Breakers [RFC8084].

   The congestion information is sent using an IKEv2 INFORMATION
   notifications [RFC7296].  These notifications are sent at a
   configured interval (which can be configured to 0 to disable the
   sending of the reports).

3.1.  ECN Support

   In additional to normal packet loss information IP-TFS 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 endpoint with the Congestion Experienced (CE) value
   set, then the receiver records that packet as being dropped, although
   it does not drop it.  When the CE information is used to calculate
   the packet drop count the receiver also sets the E bit in the
   congestion information notification data.  In order to respond
   quickly to the congestion indication the receiver MAY immediately
   send a congestion information notification to the sender upon
   receiving a packet with the CE indication.  This additional immediate
   send SHOULD only be done once per normal congestion information
   sending interval though.

   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

   IP-TFS is meant to be deployable with a minimal amount of
   configuration.  All IP-TFS specific configuration (i.e., in addition
   to the underlying IPsec tunnel configuration) should be able to be
   specified at the tunnel ingress (sending) side alone (i.e., single-
   ended provisioning).

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4.1.  Bandwidth

   Bandwidth is a local configuration option.  For non-congestion
   controlled mode the bandwidth SHOULD be configured.  For congestion
   controlled mode one can configure the bandwidth or have no
   configuration and let congestion control discover the maximum
   bandwidth available.  No standardized configuration method is
   required.

4.2.  Fixed Packet Size

   The fixed packet size to be used for the tunnel encapsulation packets
   can be configured manually or can be automatically determined using
   Path MTU discovery (see [RFC1191] and [RFC8201]).  No standardized
   configuration method is required.

4.3.  Congestion Information Configuration

   If congestion control mode is to be used, or if the user wishes to
   receive congestion information on the sender for circuit breaking or
   other operational notifications in the non-congestion controlled
   mode, IP-TFS will need to configure the egress tunnel endpoint to
   send congestion information periodically.

   In order to configure the sending interval of periodic congestion
   information on the egress tunnel endpoint, we utilize the IKEv2
   Configuration Payload (CP) [RFC7296].  Implementations MAY also allow
   for manual (or default) configuration of this interval; however,
   implementations of IP-TFS MUST support configuration using the IKEv2
   exchange described below.

   We utilize a new IKEv2 configuration attribute TFS_INFO_INTERVAL
   (TBD) to configure the sending interval from the egress endpoint of
   the tunnel.  This value is configured using a CFG_REQUEST payload and
   is acknowledge by the receiver using a CFG_REPLY payload.  This
   configuration exchange SHOULD be sent during the IKEv2 configuration
   exchanges occurring as the tunnel is first brought up.  The sending
   interval value MAY also be changed at any time afterwards using a
   similar CFG_REQUEST/CFG_REPLY payload inside an IKEv2 INFORMATIONAL
   exchange.

   In the absence of a congestion information configuration exchange the
   sending interval is up to the receiving device configuration.

   The sending interval value is given in milliseconds and is 16 bits
   wide; however, it is not recommended that values below 1/10th of a
   second are used as this could lead to early exhaustion of the Message

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   ID field used in the IKEv2 INFORMATIONAL exchange to send the
   congestion information.

   {{question: Could we get away with sending the info using the same
   message ID each time?  We have a timestamp that would allow for
   duplicate detection, and the payload will be authenticated by IKEv2.
   }}

   A sending interval value of 0 disables sending of the congestion
   information.

5.  Packet and Data Formats

5.1.  IPSec

5.1.1.  Payload Format

                         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 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |V|          Reserved           |          BlockOffset            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       DataBlocks ...
    +-+-+-+-+-+-+-+-+-+-+-

   V:
      A 1 bit version field that MUST be set to zero.  If received as
      one the packet MUST be dropped.

   Reserved:
      A 15 bit field set to 0 and ignored on receipt.

   BlockOffset:
      A 16 bit unsigned integer counting the number of octets following
      this 32 bit header before the next data block.  It can also point
      past the end of the containing packet in which case the data
      entirely belongs to the previous data block.  If the offset
      extends into subsequent packets the subsequent 32 bit IP-TFS
      headers are not counted by this value.

   DataBlocks:
      Variable number of octets that constitute the start or
      continuation of a previous data block.

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5.1.2.  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 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Type  | IPv4, IPv6 or pad...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   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.

5.1.2.1.  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 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  0x4  |  IHL  |  TypeOfService  |         TotalLength           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Rest of the inner packet ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   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.

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

   TotalLength:
      The 16 bit unsigned integer length field of the IPv4 inner packet.

5.1.2.2.  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 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  0x6  | TrafficClass  |               FlowLabel                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          TotalLength          | Rest of the inner packet ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   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:

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

   TotalLength:
      The 16 bit unsigned integer length field of the inner IPv6 inner
      packet.

5.1.2.3.  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 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  0x0  | Padding ...
    +-+-+-+-+-+-+-+-+-+-+-

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

   Padding:
      extends to end of the encapsulating packet.

5.2.  IKEv2

5.2.1.  IKEv2 Congestion Information Configuration Attribute

   The following defines the configuration attribute structure used in
   the IKEv2 [RFC7296] configuration exchange to set the congestion
   information report sending interval.

                         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 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |R|       Attribute Type        |             Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Interval           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   R:
      1 bit set to 0.

   Attribute Type:
      15 bit value set to TFS_INFO_INTERVAL (TBD).

   Length:
      2 octet length set to 2.

   SendInterval:
      A 2 octet unsigned integer.  The sending interval in milliseconds.

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5.2.2.  IKEv2 Congestion Information Notification Data

   We utilize a send only (i.e., no response expected) IKEv2
   INFORMATIONAL exchange (37) to transmit the congestion information
   using a notification payload of type TFS_CONGEST_INFO (TBD).  The The
   Response bit should be set to 0.  As no response is expected the only
   payload should be the congestion information in the notification
   payload.  The following diagram defines the notification payload
   data.

                         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 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |E|  Reserved   |                  DropCount                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Timestamp                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          AckSeqStart                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          AckSeqEnd                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   E:
      A 1 bit value that if set indicates that packet[s] with Congestion
      Experienced (CE) ECN bits set were received and used in
      calculating the DropCount value.

   Reserved:
      A 7 bit field set to 0 ignored on receipt.

   DropCount:
      A 24 bit unsigned integer count of the drops that occurred between
      AckSeqStart and AckSeqEnd.  If the drops exceed the resolution of
      the counter then set to the maximum value (i.e., 0xFFFFFF).

   AckSeqStart:
      A 32 bit unsigned integer containing the first ESP sequence number
      (as defined in [RFC4303]) of the packet range that this
      information relates to.

   AckSeqEnd:
      A 32 bit unsigned integer containing the last ESP sequence number
      (as defined in [RFC4303]) of the packet range that this
      information relates to.

   Timestamp:
      A 32 bit unsigned integer containing the lower 32 bits of a
      running monotonic millisecond timer of when this notification data

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      was created/sent.  This value is used to determine duplicates and
      drop counts of this information.  Implementations should deal with
      wrapping of this timer value.

6.  IANA Considerations

   This document requests a protocol number IPTFS_PROTOCOL be allocated
   by IANA from "Assigned Internet Protocol Numbers" registry for
   identifying the IP-TFS ESP payload format.

   Type: TBD Description: IP-TFS ESP payload format.  Reference: This
   document

   Additionally this document requests an attribute value
   TFS_INFO_INTERVAL (TBD) be allocated by IANA from "IKEv2
   Configuration Payload Attribute Types" registry.

   Type: TBD Description: The sending rate of congestion information
   from egress tunnel endpoint.  Reference: This document

   Additionally this document requests a notify message status type
   TFS_CONGEST_INFO (TBD) be allocated by IANA from "IKEv2 Notify
   Message Types - Status Types" registry.

   Type: TBD Description: The sending rate of congestion information
   from egress tunnel endpoint.  Reference: This document

7.  Security Considerations

   This document describes a mechanism to add Traffic Flow
   Confidentiality to IP traffic.  Use of this mechanism is expected to
   increase the security of the traffic being transported.  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 previously in Section 2.5.2, for TFC to be fully 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.

8.  References

8.1.  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>.

8.2.  Informative References

   [AppCrypt]
              Schneier, B., "Applied Cryptography: Protocols,
              Algorithms, and Source Code in C", 11 2017.

   [I-D.iab-wire-image]
              Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", draft-iab-wire-image-01 (work in
              progress), November 2018.

   [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>.

   [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>.

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   [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>.

   [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>.

   [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>.

Appendix A.  Comparisons of IP-TFS

A.1.  Comparing Overhead

A.1.1.  IP-TFS Overhead

   The overhead of IP-TFS is 40 bytes per outer packet.  Therefore the
   octet overhead per inner packet is 40 divided by the number of outer
   packets required (fractional allowed).  The overhead as a percentage
   of inner packet size is a constant based on the Outer MTU size.

      OH = 40 / Outer Payload Size / Inner Packet Size
      OH % of Inner Packet Size = 100 * OH / Inner Packet Size
      OH % of Inner Packet Size = 4000 / Outer Payload Size

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                        Type  IP-TFS  IP-TFS  IP-TFS
                         MTU     576    1500    9000
                       PSize     536    1460    8960
                      -------------------------------
                          40   7.46%   2.74%   0.45%
                         576   7.46%   2.74%   0.45%
                        1500   7.46%   2.74%   0.45%
                        9000   7.46%   2.74%   0.45%

       Figure 4: IP-TFS Overhead as Percentage of Inner Packet Size

A.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 (36) minus the initial inner IP
   overhead plus any required tail padding in the last encapsulation
   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 the Inner Payload Size.  So:

     Inner Paylaod 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

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A.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      540     1464     8964     536    1460    8960
        -----------------------------------------------------------
             40      500     1424     8924     3.0     1.1     0.2
            128      412     1336     8836     9.6     3.5     0.6
            256      284     1208     8708    19.1     7.0     1.1
            536        4      928     8428    40.0    14.7     2.4
            576      576      888     8388    43.0    15.8     2.6
           1460      268        4     7504   109.0    40.0     6.5
           1500      228     1500     7464   111.9    41.1     6.7
           8960     1408     1540        4   668.7   245.5    40.0
           9000     1368     1500     9000   671.6   246.6    40.2

                  Figure 5: Overhead comparison in octets

          Type  ESP+Pad  ESP+Pad   ESP+Pad  IP-TFS  IP-TFS  IP-TFS
           MTU      576     1500      9000     576    1500    9000
         PSize      540     1464      8964     536    1460    8960
        -----------------------------------------------------------
            40  1250.0%  3560.0%  22310.0%   7.46%   2.74%   0.45%
           128   321.9%  1043.8%   6903.1%   7.46%   2.74%   0.45%
           256   110.9%   471.9%   3401.6%   7.46%   2.74%   0.45%
           536     0.7%   173.1%   1572.4%   7.46%   2.74%   0.45%
           576   100.0%   154.2%   1456.2%   7.46%   2.74%   0.45%
          1460    18.4%     0.3%    514.0%   7.46%   2.74%   0.45%
          1500    15.2%   100.0%    497.6%   7.46%   2.74%   0.45%
          8960    15.7%    17.2%      0.0%   7.46%   2.74%   0.45%
          9000    15.2%    16.7%    100.0%   7.46%   2.74%   0.45%

           Figure 6: Overhead as Percentage of Inner Packet Size

A.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 we will also include
   normal (unencrypted) Ethernet as well as normal ESP values.

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A.3.1.  Ethernet

   In order to calculate the available bandwidth we first calculate the
   per packet overhead in bits.  The total overhead of Ethernet is 14+4
   octets of header and CRC plus and additional 20 octets of framing
   (preamble, start, and inter-packet gap) for a total of 48 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     74     74     74     78     78     78    38    74
        ------------------------------------------------------------
           40    614   1538   9038     45     42     40    84   114
          128    614   1538   9038    146    134    129   166   202
          256    614   1538   9038    293    269    258   294   330
          536    614   1538   9038    614    564    540   574   610
          576   1228   1538   9038    659    606    581   614   650
         1460   1842   1538   9038   1672   1538   1472  1498  1534
         1500   1842   3076   9038   1718   1580   1513  1538  1574
         8960  11052  10766   9038  10263   9438   9038  8998  9034
         9000  11052  10766  18076  10309   9480   9078  9038  9074

                      Figure 7: L2 Octets Per Packet

        Size  E + P  E + P  E + P  IPTFS  IPTFS  IPTFS  Enet   ESP
         MTU  590    1514   9014   590    1514   9014   any    any
          OH  74     74     74     78     78     78     38     74
       --------------------------------------------------------------
          40  2.0M   0.8M   0.1M   27.3M  29.7M  31.0M  14.9M  11.0M
         128  2.0M   0.8M   0.1M   8.5M   9.3M   9.7M   7.5M   6.2M
         256  2.0M   0.8M   0.1M   4.3M   4.6M   4.8M   4.3M   3.8M
         536  2.0M   0.8M   0.1M   2.0M   2.2M   2.3M   2.2M   2.0M
         576  1.0M   0.8M   0.1M   1.9M   2.1M   2.2M   2.0M   1.9M
        1460  678K   812K   138K   747K   812K   848K   834K   814K
        1500  678K   406K   138K   727K   791K   826K   812K   794K
        8960  113K   116K   138K   121K   132K   138K   138K   138K
        9000  113K   116K   69K    121K   131K   137K   138K   137K

               Figure 8: Packets Per Second on 10G Ethernet

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   Size   E + P   E + P   E + P   IPTFS   IPTFS   IPTFS    Enet     ESP
            590    1514    9014     590    1514    9014     any     any
             74      74      74      78      78      78      38      74
  ----------------------------------------------------------------------
     40   6.51%   2.60%   0.44%  87.30%  94.93%  99.14%  47.62%  35.09%
    128  20.85%   8.32%   1.42%  87.30%  94.93%  99.14%  77.11%  63.37%
    256  41.69%  16.64%   2.83%  87.30%  94.93%  99.14%  87.07%  77.58%
    536  87.30%  34.85%   5.93%  87.30%  94.93%  99.14%  93.38%  87.87%
    576  46.91%  37.45%   6.37%  87.30%  94.93%  99.14%  93.81%  88.62%
   1460  79.26%  94.93%  16.15%  87.30%  94.93%  99.14%  97.46%  95.18%
   1500  81.43%  48.76%  16.60%  87.30%  94.93%  99.14%  97.53%  95.30%
   8960  81.07%  83.22%  99.14%  87.30%  94.93%  99.14%  99.58%  99.18%
   9000  81.43%  83.60%  49.79%  87.30%  94.93%  99.14%  99.58%  99.18%

             Figure 9: Percentage of Bandwidth on 10G Ethernet

   A sometimes unexpected result of using IP-TFS (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.

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

                 ------------------------------------------
                    40  1.14 us  7.14 us  1.17 us  7.17 us
                   128  1.07 us  7.07 us  1.10 us  7.10 us
                   256  0.97 us  6.97 us  1.00 us  7.00 us
                   536  0.74 us  6.74 us  0.77 us  6.77 us
                   576  0.71 us  6.71 us  0.74 us  6.74 us
                  1460  0.00 us  6.00 us  0.04 us  6.04 us
                  1500  1.20 us  5.97 us  0.00 us  6.00 us

                         Figure 10: 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.

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Appendix B.  Acknowledgements

   We would like to thank Don Fedyk for help in reviewing this work.

Appendix C.  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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