Network Working Group C. Hopps
Internet-Draft LabN Consulting, L.L.C.
Intended status: Standards Track June 5, 2019
Expires: December 7, 2019

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.

Status of This Memo

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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. 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, whole 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 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 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 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.

With IP-TFS we aggregate as well as fragment the inner IP traffic flow into fixed-sized encapsulating IPsec tunnel packets. We only pad 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.

In order to do this we use a new Encapsulating Security Payload (ESP, [RFC4303]) payload type which is the new IP protocol number IPTFS_PROTOCOL (TBD1).

2.2. IPTFS_PROTOCOL Payload Content

The IPTFS_PROTOCOL ESP payload is comprised a 4 or 16 octet header followed by either a partial, a full or multiple partial or full data blocks. The following diagram illustrates the IPTFS_PROTOCOL ESP 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 quick recovery with minimal inner packet loss 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 Padding Required

It's worth noting that 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 encapsulate is padding required, and then an explicit Pad Data Block would be used to identify the padding.

2.2.3. 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.

2.2.4. 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 IP 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.

2.3. 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. Initiating IP-TFS Operation On The SA.

While a user will normally configure their IPsec tunnel (SA) to operate using IP-TFS to start, we also allow IP-TFS operation to be enabled post-SA creation and use. This late-enabling may be useful for debugging or other purposes. To support this late-enabled operation the receiver switches to IP-TFS operation on receipt of the first ESP payload with the IPTFS_PROTOCOL indicated as the payload type which also contains a data block (i.e., a non-empty IP-TFS payload). The the receipt of an empty IPTFS_PROTOCOL payload (i.e., one without any data blocks) is used to communicate congestion control information from the receiver back to the sender on a non-IP-TFS enabled SA, and MUST NOT cause IP-TFS to be enabled on that SA.

2.5. 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.

2.5.1. 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 (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.

2.5.2. 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. Since overhead is per packet, by allowing for maximal fixed-size packets and varying the send rate we minimize transport overhead.

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 receivers loss event rate and the senders 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 as well as 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 could 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.

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 to allow the sender to calculate the RTT value, the receiver communicates the last sequence number it has seen to the sender in the LastSeqNum header field. In addition to the LastSeqNum value, the receiver sends an estimate of the amount of time between receiving the LastSeqNum packet and transmitting the LastSeqNum value back to the sender in the congestion information. It places this time estimate in the Delay header field along with the LastSeqNum.

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. TFS Type Transform Type

When IP-TFS is used with IKEv2 a new "TFS Type" Transform Type (TBD2) is used to negotiate (as defined in [RFC7296]) the possible operation of IP-TFS on a child SA pair. This document defines 3 "TFS Type" Transform IDs for the new "TFS Type" Transform Type: None (0), TFS_IPTFS_CC (1) for congestion-controlled IP-TFS mode or TFS_IPTFS_NOCC (2) for non-congestion controlled IP-TFS mode. The selection of a proposal with a "TFS Type" Transform ID TFS_IPTFS_CC or TFS_IPTFS_NOCC does not mandate the use of IP-TFS, rather it indicates a willingness or intent to use IP-TFS on the SA pair. In addition, a new Notify Message Status Type IPTFS_REQUIREMENTS (TBD3) MAY be used by the initiator as well as the responder to further refine any operational requirements.

Additional "TFS Type" Transform IDs may be defined in the future, and so readers are referred to [IKEV2IANA] for the most up to date list.

5.2. IPTFS_REQUIREMENTS Status Notification

As mentioned in the previous section, a new Notify Message Status Type IPTFS_REQUIREMENTS (TBD3) MAY be sent by the initiator and/or the responder to further refine what will be supported. This notification is sent during IKE_AUTH and new CREATE_CHILD_SA exchanges; however, it MUST NOT be sent, and MUST be ignored, during a CREATE_CHILD_SA rekeying exchange as the requirements are not allowed to change during rekeying.

The IPTFS_REQUIREMENTS notification contains a 1 octet payload of flags that specify any extra requirements from the sender of the message. The flag values (currently a single flag) are defined below. If the IPTFS_REQUIREMENTS notification is not sent then it implies that all the flag bits are clear.



MUST be zero on send and MUST be ignored on receive.

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 IPTFS_REQUIREMENTS notification.

6. Packet and Data Formats

6.1. ESP IP-TFS Payload

An ESP IP-TFS payload is identified by the IP protocol number IPTFS_PROTOCOL (TBD1). This payload begins with a fixed 4 or 16 octet header followed by a variable amount of DataBlocks data. The exact payload format and fields 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
 |V|C|        Reserved           |          BlockOffset          |
 |       DataBlocks ...


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

A 1 bit value that MUST be set to 0 to indicate no congestion control information is present.

A 14 bit field set to 0 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 16 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
 |V|C|E|        Reserved         |          BlockOffset          |
 |              RTT              |             Delay             |
 |                          LossEventRate                        |
 |                           LastSeqNum                          |
 |       DataBlocks ...


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

A 1 bit value that MUST be set to 1 which indicates the presence of the congestion information header fields RTT, Delay, LossEventRate and LastSeqNum.

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

A 13 bit field set to 0 and ignored on receipt.

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

A 16 bit value specifying the sender's current round-trip time estimate in milliseconds. 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.

A 16 bit value specifying the delay in milliseconds incurred between the receiver receiving the LastSeqNum packet and the sending of this acknowledgement of it.

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 32 bit value containing the lower 32 bits of the largest sequence number last received. This is the latest in the sequence not necessarily the most recent (in the case of re-ordering of packets it may be less recent). When determining largest and 64 bit extended sequence numbers are in use, the upper 32 bits should be used during the comparison.

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 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               |
 |          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.


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

The 16 bit unsigned integer 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.

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 ESP payload format.



IP-TFS ESP payload format.

This document

7.2. IKEv2 Transform Type TFS Type

This document requests an IKEv2 Transform Type "TFS Type" be allocated by IANA from the "Transform Type Values" registry.



TFS Type
Used In:

(optional in ESP)

This document

7.3. TFS Type Transform IDs Registry

This document requests a "Transform Type TBD3 - TFS Type Transform IDs" registry be created. The registration procedure is Expert Review. The initial values are as follows:

  Number  Name            Reference     
       0  NONE            This document 
       1  TFS_IPTFS_CC    This document 
       2  TFS_IPTFS_NOCC  This document 
 3-65535  Reserved        This document 

7.4. IPTFS_REQUIREMENTS Notify Message Status Type

This document requests a status type IPTFS_REQUIREMENTS 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 2.5.2, 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.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005.
[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.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

9.2. Informative References

[AppCrypt] Schneier, B., "Applied Cryptography: Protocols, Algorithms, and Source Code in C", November 2017.
[I-D.iab-wire-image] Trammell, B. and M. Kuehlewind, "The Wire Image of a Network Protocol", Internet-Draft draft-iab-wire-image-01, November 2018.
[IKEV2IANA] IANA, "Internet Key Exchange Version 2 (IKEv2) Parameters"
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, September 1981.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990.
[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.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, DOI 10.17487/RFC2914, September 2000.
[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.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, December 2005.
[RFC4342] Floyd, S., Kohler, E. and J. Padhye, "Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342, DOI 10.17487/RFC4342, March 2006.
[RFC5348] Floyd, S., Handley, M., Padhye, J. and J. Widmer, "TCP Friendly Rate Control (TFRC): Protocol Specification", RFC 5348, DOI 10.17487/RFC5348, September 2008.
[RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R. and D. Black, "Encapsulating MPLS in UDP", RFC 7510, DOI 10.17487/RFC7510, April 2015.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers", BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017.
[RFC8201] McCann, J., Deering, S., Mogul, J. and R. Hinden, "Path MTU Discovery for IP version 6", STD 87, RFC 8201, DOI 10.17487/RFC8201, July 2017.

Appendix A. 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) ]

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 our use case (as with [RFC4342]) we consider our (fixed) packet size the segment size for the algorithm. The formula for the send rate is then as follows:

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

Where X_Pps 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).

The IP-TFS receiver, having the RTT estimate from the sender MAY use the same method as described in [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 (X_Pps). 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 we will also include normal (unencrypted) Ethernet as well as normal ESP values.

C.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

 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 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. EMail: