LabN Consulting, L.L.C.

chopps@chopps.org

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.

Traffic Analysis (, ) 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 , the traffic pattern itself exposes information due to variations in it's shape and timing (, ). Hiding the size and frequency of traffic is referred to as Traffic Flow Confidentiality (TFC) per . 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 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 . Additionally, IP-TFS provides for dealing with network congestion . 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.

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 when, and only when, they appear in all capitals, as shown here. This document assumes familiarity with IP security concepts described in .

As mentioned in IP-TFS utilizes an IPsec 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 and . 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.

As previously mentioned, one issue with the TFC padding solution in 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, ) type which is identified by the IP protocol number IPTFS_PROTOCOL (TBD1).

The IPTFS_PROTOCOL payload content defined in this document is comprised of a 4 or 16 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 for the exact formats of the IPTFS_PROTOCOL payload.

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 .

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.

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.

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.

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.

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 ( and ), the Differentiated Services (DS) field and the Explicit Congestion Notification (ECN) field . Unlike , 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.

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.

Receive-side operation of IP-TFS does not require any per-SA configuration on the receiver; as such, an IP-TFS implementation SHOULD support the option of switching to IP-TFS receive-side operation on receipt of the first IP-TFS payload.

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.

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 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 . 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.

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 . Congestion control principles are documented in as well. An example of an implementation of the 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 . The required inputs for the TCP friendly rate control algorithm described in 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 . In particular these values are sufficient to implement the algorithm described in . 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, calls this the "no feedback timeout" and it is equal to 4 RTT intervals. When a "no feedback timeout" has occurred 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.

In additional to congestion control, implementations MAY choose to define and implement circuit breakers 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.

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 (). 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 , 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 which periodically sends all the loss interval data back to the sender so that it can do the calculation. See 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.

In additional to normal packet loss information IP-TFS supports use of the ECN bits in the encapsulating IP header 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 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.

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.

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.

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 and ). No standardized configuration method is required.

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

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 . 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 .

An IP-TFS payload is identified by the IP protocol number IPTFS_PROTOCOL (TBD1). The first octet of this payload indicates the format of the remaining payload data.

An 8 bit value indicating the payload format. This specification defines 2 payload sub-types. These payload formats are defined in the following sections.

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.

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.

The congestion control IPTFS_PROTOCOL payload is comprised of a 16 octet header followed by a variable amount of DataBlocks data as shown below.

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 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).

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

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.

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.

A 4 bit value of 0x0 indicating a padding data block. extends to end of the encapsulating packet.

As discussed in 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 . 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.

This document requests a protocol number IPTFS_PROTOCOL be allocated by IANA from "Assigned Internet Protocol Numbers" registry for identifying the IP-TFS payload. TBD1 An IP-TFS payload. This document

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" ( and ). IPTFS_PROTOCOL Sub-Type Registry IPTFS_PROTOCOL Payload Formats. This document This initial content for this registry is as follows:

This document requests a status type USE_IPTFS be allocated from the "IKEv2 Notify Message Types - Status Types" registry. TBD2 USE_IPTFS This document

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 and and so their security considerations apply to IP-TFS as well. As noted previously in , 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.

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes an updated version of the Encapsulating Security Payload (ESP) protocol, which is designed to provide a mix of security services in IPv4 and IPv6. ESP is used to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality. This document obsoletes RFC 2406 (November 1998). [STANDARDS-TRACK] This document describes version 2 of the Internet Key Exchange (IKE) protocol. IKE is a component of IPsec used for performing mutual authentication and establishing and maintaining Security Associations (SAs). This document obsoletes RFC 5996, and includes all of the errata for it. It advances IKEv2 to be an Internet Standard. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This memo describes a technique for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. It specifies a small change to the way routers generate one type of ICMP message. For a path that passes through a router that has not been so changed, this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by current practice. [STANDARDS-TRACK] This document defines the IP header field, called the DS (for differentiated services) field. [STANDARDS-TRACK] The goal of this document is to explain the need for congestion control in the Internet, and to discuss what constitutes correct congestion control. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This memo specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header. [STANDARDS-TRACK] This document describes an updated version of the "Security Architecture for IP", which is designed to provide security services for traffic at the IP layer. This document obsoletes RFC 2401 (November 1998). [STANDARDS-TRACK] This document contains the profile for Congestion Control Identifier 3, TCP-Friendly Rate Control (TFRC), in the Datagram Congestion Control Protocol (DCCP). CCID 3 should be used by senders that want a TCP-friendly sending rate, possibly with Explicit Congestion Notification (ECN), while minimizing abrupt rate changes. [STANDARDS-TRACK] This document specifies TCP Friendly Rate Control (TFRC). TFRC is a congestion control mechanism for unicast flows operating in a best-effort Internet environment. It is reasonably fair when competing for bandwidth with TCP flows, but has a much lower variation of throughput over time compared with TCP, making it more suitable for applications such as streaming media where a relatively smooth sending rate is of importance.This document obsoletes RFC 3448 and updates RFC 4342. [STANDARDS-TRACK] This memo describes the process for early allocation of code points by IANA from registries for which "Specification Required", "RFC Required", "IETF Review", or "Standards Action" policies apply. This process can be used to alleviate the problem where code point allocation is needed to facilitate desired or required implementation and deployment experience prior to publication of an RFC, which would normally trigger code point allocation. The procedures in this document are intended to apply only to IETF Stream documents. This document specifies an IP-based encapsulation for MPLS, called MPLS-in-UDP for situations where UDP (User Datagram Protocol) encapsulation is preferred to direct use of MPLS, e.g., to enable UDP-based ECMP (Equal-Cost Multipath) or link aggregation. The MPLS- in-UDP encapsulation technology must only be deployed within a single network (with a single network operator) or networks of an adjacent set of cooperating network operators where traffic is managed to avoid congestion, rather than over the Internet where congestion control is required. Usage restrictions apply to MPLS-in-UDP usage for traffic that is not congestion controlled and to UDP zero checksum usage with IPv6. This document explains what is meant by the term "network transport Circuit Breaker". It describes the need for Circuit Breakers (CBs) for network tunnels and applications when using non-congestion- controlled traffic and explains where CBs are, and are not, needed. It also defines requirements for building a CB and the expected outcomes of using a CB within the Internet. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.This is the third edition of this document; it obsoletes RFC 5226. This document specifies version 6 of the Internet Protocol (IPv6). It obsoletes RFC 2460. This document describes Path MTU Discovery (PMTUD) for IP version 6. It is largely derived from RFC 1191, which describes Path MTU Discovery for IP version 4. It obsoletes RFC 1981. This document defines the wire image, an abstraction of the information available to an on-path non-participant in a networking protocol. This abstraction is intended to shed light on the implications on increased encryption has for network functions that use the wire image.

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.

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.

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 . For this IP-TFS use case (as with ) the (fixed) packet size is used as the segment size for the algorithm. The formula for the send rate is then as follows:

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 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 the sender SHOULD also use the slow start mechanism described therein when the IP-TFS SA is first established.

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.

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:

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.

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.

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.

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.

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.

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

The following people made significant contributions to this document.