Internet-Draft IP Traffic Flow Security November 2020
Hopps Expires 19 May 2021 [Page]
Network Working Group
Intended Status:
Standards Track
C. Hopps
LabN Consulting, L.L.C.

IP Traffic Flow Security


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 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

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 19 May 2021.

Table of Contents

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 transmission of 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. 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 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 (SA) as it's transport. To provide for full TFC, fixed-sized encapsulating packets are sent 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 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 corresponding 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 MUST allow for and expect the ingress (sending) side of the IP-TFS tunnel to vary the size and rate of sent encapsulating packets, unless constrained by other policy.

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.

IP-TFS aggregates as well as fragments the inner IP traffic flow into fixed-sized encapsulating IPsec tunnel packets. Padding is only added to the 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]) type which is identified by the number IPTFS_PROTOCOL (Section 6.1).

2.2. IPTFS_PROTOCOL Payload Content

The IPTFS_PROTOCOL payload content defined in this document is comprised of a 4 or 24 octet header followed by either a partial, a full or multiple partial or full data blocks. The following diagram illustrates this IPTFS_PROTOCOL payload within the ESP packet. See Section 6.1 for the exact formats of the IPTFS_PROTOCOL payload.

 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
 . Outer Encapsulating Header ...                                .
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
 . ESP Header...                                                 .
 |               ...            :           BlockOffset          |
 :                  [Optional Congestion Info]                   :
 |       DataBlocks ...                                          ~
 ~                                                               ~
 ~                                                               |
 . ESP Trailer...                                                .
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 1: Layout of an IP-TFS IPsec Packet

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 a previous data block that is still being re-assembled.

The BlockOffset can point past the end of the DataBlocks data which indicates that 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 full inner packet in the presence of outer encapsulating packet loss.

An example IP-TFS packet flow can be found in Appendix A.

2.2.1. 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.

2.2.2. No Implicit End Padding Required

It's worth noting that since a data block type is identified by its first octet there is never a need for an implicit pad 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 in the current encapsulating packet, the fact that the length comes at a known location and is guaranteed to be present is enough to fetch the length field from the subsequent encapsulating packet payload. Only when there is no data to encapsulated is end padding required, and then an explicit Pad Data Block would be used to identify the padding.

2.2.3. Fragmentation, Sequence Numbers and All-Pad Payloads

In order for a receiver to be able to reassemble fragmented inner-packets, the sender MUST send the inner-packet fragments back-to-back in the logical IP-TFS packet stream (i.e., using consecutive ESP sequence numbers). However, the sender is allowed to insert "all-pad" IP-TFS packets (i.e., packets having payloads with a BlockOffset of zero and a single pad DataBlock) in between the IP-TFS packets carrying the inner-packet fragment payloads. This possible interleaving of all-pad packets allows the sender to always be able to send an IP-TFS tunnel packet, regardless of the encapsulation computational requirements.

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

2.2.4. Empty Payload

In order to support reporting of congestion control information (described later) on a non-IP-TFS enabled SA, IP-TFS allows for the sending of an IP-TFS payload with no data blocks (i.e., the ESP payload length is equal to the IP-TFS header length). This special payload is called an empty payload.

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], IP-TFS may and often will be encapsulating more than one IP packet per ESP packet. To deal with this, these mappings are restricted further. In particular IP-TFS never maps the inner DF bit as it is unrelated to the IP-TFS tunnel functionality; IP-TFS never IP fragments 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.

2.3. Exclusive SA Use

It is not the intention of this specification to allow for mixed use of an IP-TFS enabled SA. In other words, an SA that has IP-TFS enabled 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 non-empty IP-TFS (IP protocol TBD1) 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.

2.4. Modes of Operation

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

An IP-TFS tunnel 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 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.

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.

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. An example of an implementation of the [RFC5348] algorithm which matches the requirements of IP-TFS (i.e., designed for fixed-size packet and send rate varied based on congestion) is documented in [RFC4342].

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. The lack of receiving this information over multiple consecutive RTT intervals should be considered a congestion event that causes the sender to adjust it's 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 it's IP-TFS 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 an implementation is choosing a congestion control algorithm (or a selection of algorithms) one should remember 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 variable send-rate of a congestion controlled IP-TFS 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. 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.

3. Congestion Information

In order to support the congestion control mode, the sender needs to know the loss event rate and also be able to approximate the RTT ([RFC5348]). In order to obtain these values the receiver sends congestion control information on it's SA back to the sender. Thus, in order 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-IP-TFS enabled SA then an IPTFS_PROTOCOL 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 it's 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 it's CC header it places this latest recorded value 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 it's half of the IP-TFS tunnel). When the sender receives back it's TVal in the TEcho header field it calculates 2 RTT estimates. The first is the actual delay found by subtracting the TEcho value from it's 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 senders own transmission delay (i.e., the average time between sending packets on it's half of the IP-TFS tunnel). The larger of these 2 RTT estimates SHOULD be used as the RTT value. The two estimates are required to handle different combinations of faster or slow tunnel packet paths with fast or slow 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 tunnel rate (the second estimate), as well as never being considered faster than the actual RTT along the tunnel packet path (the first estimate).

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 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 considers that packet as being dropped, although it does not drop it. The receiver MUST set the E bit in any IPTFS_PROTOCOL 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

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

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 Control

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

5. IKEv2

5.1. USE_TFS Notification Message

When using IKEv2, a new "USE_IPTFS" Notification Message is used to enable operation of IP-TFS on a child SA pair. The method used is similar to how USE_TRANSPORT_MODE is negotiated, as described in [RFC7296].

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

The USE_IPTFS notification MUST NOT be sent, and MUST be ignored, during a CREATE_CHILD_SA rekeying exchange as it is not allowed to change IP-TFS operation during rekeying.

The USE_IPTFS notification contains a 1 octet payload of flags that specify any requirements from the sender of the message. If any requirement flags are not understood or cannot be supported by the receiver then the receiver should not enable IP-TFS mode (either by not responding with the USE_IPTFS notification, or in the case of the initiator, by deleting the child SA if the now established non-IP-TFS operation is unacceptable).

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

6. Packet and Data Formats

6.1. IP-TFS Payload

ESP Payload Type: 0x5

An IP-TFS payload is identified by the ESP payload type IPTFS_PROTOCOL which has the value 0x5. The first octet of this payload indicates the format of the remaining payload data.

  0 1 2 3 4 5 6 7
 |   Sub-type    | ...
An 8 bit value indicating the payload format.

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

6.1.1. Non-Congestion Control IPTFS_PROTOCOL Payload Format

The non-congestion control IPTFS_PROTOCOL payload is comprised 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 ...
An octet indicating the payload format. For this non-congestion control format, the value is 0.
An octet set to 0 on generation, and ignored on receipt.
A 16 bit unsigned integer counting the number of octets of DataBlocks data before the start of a new data block. BlockOffset can count past the end of the DataBlocks data in which case all the DataBlocks data belongs to the previous data block being re-assembled. If the BlockOffset extends into subsequent packets it continues to only count subsequent DataBlocks data (i.e., it does not count subsequent packets non-DataBlocks octets).
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 IPTFS_PROTOCOL Payload Format

The congestion control IPTFS_PROTOCOL payload is comprised 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   |E|          BlockOffset          |
 |                          LossEventRate                        |
 |                      RTT                  |   Echo Delay ...
      ... Echo Delay   |           Transmit Delay                |
 |                              TVal                             |
 |                             TEcho                             |
 |       DataBlocks ...
An octet indicating the payload format. For this congestion control format, the value is 1.
A 7 bit field set to 0 on generation, and ignored on receipt.
A 1 bit value if set indicates that Congestion Experienced (CE) ECN bits were received and used in deriving the reported LossEventRate.
The same value as the non-congestion controlled payload format value.
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.
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-IP-TFS enabled SAs. If the value is equal to or larger than 0x3FFFFF it 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 value is equal to or larger than 0x1FFFFF it 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 value is equal to or larger than 0x1FFFFF it MUST be set to 0x1FFFFF.
An opaque 32 bit value that will be echoed back by the receiver in later packets in the TEcho field, along with a Delay value of how long that echo took.
The opaque 32 bit value from a received packet's TVal field. The received TVal is placed in TEcho along with a Delay value indicating how long it has been since receiving the TVal value.
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 an non-IP-TFS enabled SA this value 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...
A 4 bit field where 0x0 identifies a pad data block, 0x4 indicates an IPv4 data block, and 0x6 indicates an IPv6 data block. 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 ...

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.

A 4 bit value of 0x4 indicating IPv4 (i.e., first nibble of the IPv4 packet).
The 16 bit unsigned integer "Total Length" field of the IPv4 inner packet. 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 ...

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.

A 4 bit value of 0x6 indicating IPv6 (i.e., first nibble of the IPv6 packet).
The 16 bit unsigned integer "Payload Length" field of the inner IPv6 inner packet. 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 ...
A 4 bit value of 0x0 indicating a padding data block.
extends to end of the encapsulating packet.

6.1.4. IKEv2 USE_IPTFS Notification Message

As discussed in Section 5.1 a notification message USE_IPTFS is used to negotiate IP-TFS operation in IKEv2.

The USE_IPTFS 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.

6 bits - reserved, MUST be zero on send, unless defined by later specifications.
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.
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 it's USE_IPTFS notification.

7. 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 payload.

An IP-TFS payload.
This document

7.2. IPTFS_PROTOCOL Sub-Type Registry

This document requests IANA create a registry called "IPTFS_PROTOCOL Sub-Type Registry" under "IPTFS_PROTOCOL Parameters" IANA registries. The registration policy for this registry is "Standards Action" ([RFC8126] and [RFC7120]).

IPTFS_PROTOCOL Sub-Type Registry
IPTFS_PROTOCOL Payload Formats.
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

7.3. USE_IPTFS Notify Message Status Type

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

This document

8. 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 2.4.2, 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.

9. References

9.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

9.2. Informative References

Schneier, B., "Applied Cryptography: Protocols, Algorithms, and Source Code in C", .
Trammell, B. and M. Kuehlewind, "The Wire Image of a Network Protocol", Work in Progress, Internet-Draft, draft-iab-wire-image-01, , <>.
Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, , <>.
Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, , <>.
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, , <>.
Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, DOI 10.17487/RFC2914, , <>.
Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, , <>.
Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, , <>.
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, , <>.
Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP Friendly Rate Control (TFRC): Protocol Specification", RFC 5348, DOI 10.17487/RFC5348, , <>.
Cotton, M., "Early IANA Allocation of Standards Track Code Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, , <>.
Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black, "Encapsulating MPLS in UDP", RFC 7510, DOI 10.17487/RFC7510, , <>.
Fairhurst, G., "Network Transport Circuit Breakers", BCP 208, RFC 8084, DOI 10.17487/RFC8084, , <>.
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <>.
Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, , <>.
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, , <>.

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: 2900    Offset: 1400
 [ ESP1  (1500) ][ ESP2  (1500) ][ ESP3  (1500) ][ ESP4  (1500) ]
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.

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:

   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 IPTFS_PROTOCOL 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.

Appendix C. Comparisons of IP-TFS

C.1. Comparing Overhead

C.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
                     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

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 (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 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


  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      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

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 and 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     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
 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.

Appendix D. Acknowledgements

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

Appendix E. Contributors

The following people made significant contributions to this document.

   Lou Berger
   LabN Consulting, L.L.C.


Author's Address

Christian Hopps
LabN Consulting, L.L.C.