| Internet-Draft | IP Traffic Flow Security | October 2021 |
| Hopps | Expires 27 April 2022 | [Page] |
This document describes a mechanism for aggregation and fragmentation of IP packets when they are being encapsulated in ESP payload. This new payload type can be used for various purposes such as decreasing encapsulation overhead for small IP packets; however, the focus in this document is to enhance IPsec traffic flow security (IP-TFS) by adding Traffic Flow Confidentiality (TFC) to encrypted IP encapsulated traffic. TFC is provided by obscuring the size and frequency of IP traffic using a fixed-sized, constant-send-rate IPsec tunnel. The solution allows for congestion control as well as non-constant send-rate usage.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 27 April 2022.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
Traffic Analysis ([RFC4301], [AppCrypt]) is the act of extracting information about data being sent through a network. While directly obscuring the data with encryption [RFC4303], the traffic pattern itself exposes information due to variations in its shape and timing ([RFC8546], [AppCrypt]). Hiding the size and frequency of traffic is referred to as Traffic Flow Confidentiality (TFC) per [RFC4303].¶
[RFC4303] provides for TFC by allowing padding to be added to encrypted IP packets and allowing for transmission of all-pad packets (indicated using protocol 59). This method has the major limitation that it can significantly under-utilize the available bandwidth.¶
This document defines an aggregation and fragmentation (AGGFRAG) mode for ESP, and its use for IP Traffic Flow Security (IP-TFS). This solution provides for full TFC without the aforementioned bandwidth limitation. This is accomplished by using a constant-send-rate IPsec [RFC4303] tunnel with fixed-sized encapsulating packets; however, these fixed-sized packets can contain partial, whole or multiple IP packets to maximize the bandwidth of the tunnel. A non-constant send-rate is allowed, but the confidentiality properties of its use are outside the scope of this document.¶
For a comparison of the overhead of IP-TFS with the RFC4303 prescribed TFC solution see Appendix C.¶
Additionally, IP-TFS provides for operating fairly within congested networks [RFC2914]. This is important for when the IP-TFS user is not in full control of the domain through which the IP-TFS tunnel path flows.¶
The mechanisms, such as the AGGFRAG mode, defined in this document are generic with the intent of allowing for non-TFS uses, but such uses are outside the scope of this document.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document assumes familiarity with IP security concepts including TFC as described in [RFC4301].¶
As mentioned in Section 1, AGGFRAG mode utilizes an IPsec [RFC4303] tunnel as its transport. For the purpose of IP-TFS, fixed-sized encapsulating packets are sent at a constant rate on the AGGFRAG tunnel.¶
The primary input to the tunnel algorithm is the requested bandwidth to be used by the tunnel. Two values are then required to provide for this bandwidth use, the fixed size of the encapsulating packets, and rate at which to send them.¶
The fixed packet size MAY either be specified manually or be determined through other methods such as the Packetization Layer MTU Discovery (PLMTUD) ([RFC4821], [RFC8899]) or Path MTU discovery (PMTUD) ([RFC1191], [RFC8201]). PMTUD is known to have issues so PLMTUD is considered the more robust option. For PLMTUD, congestion control payloads can be used as in-band probes (see Section 6.1.2 and [RFC8899]).¶
Given the encapsulating packet size and the requested bandwidth to be used, the corresponding packet send rate can be calculated. The packet send rate is the requested bandwidth to be used divided by the size of the encapsulating packet.¶
The egress (receiving) side of the AGGFRAG tunnel MUST allow for and expect the ingress (sending) side of the AGGFRAG tunnel to vary the size and rate of sent encapsulating packets, unless constrained by other policy.¶
As previously mentioned, one issue with the TFC padding solution in [RFC4303] is the large amount of wasted bandwidth as only one IP packet can be sent per encapsulating packet. In order to maximize bandwidth, IP-TFS breaks this one-to-one association by introducing an AGGFRAG mode for ESP.¶
AGGFRAG mode aggregates as well as fragments the inner IP traffic flow into encapsulating IPsec tunnel packets. For IP-TFS, the IPsec encapsulating tunnel packets are a fixed size. Padding is only added to the the tunnel packets if there is no data available to be sent at the time of tunnel packet transmission, or if fragmentation has been disabled by the receiver.¶
This is accomplished using a new Encapsulating Security Payload (ESP, [RFC4303]) Next Header field value AGGFRAG_PAYLOAD (Section 6.1).¶
Other non-IP-TFS uses of this AGGFRAG mode have been suggested, such as increased performance through packet aggregation, as well as handling MTU issues using fragmentation. These uses are not defined here, but are also not restricted by this document.¶
The AGGFRAG_PAYLOAD payload content defined in this document is comprised of a 4 or 24 octet header followed by either a partial datablock, a full datablock, or multiple partial or full datablocks. The following diagram illustrates this payload within the ESP packet. See Section 6.1 for the exact formats of the AGGFRAG_PAYLOAD payload.¶
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Outer Encapsulating Header ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ESP Header... . +---------------------------------------------------------------+ | [AGGFRAG subtype/flags] : BlockOffset | +---------------------------------------------------------------+ : [Optional Congestion Info] : +---------------------------------------------------------------+ | DataBlocks ... ~ ~ ~ ~ | +---------------------------------------------------------------| . ESP Trailer... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The BlockOffset value is either zero or some offset into or past
the end of the DataBlocks data.¶
If the BlockOffset value is zero it means that the DataBlocks
data begins with a new data block.¶
Conversely, if the BlockOffset value is non-zero it points to the
start of the new data block, and the initial DataBlocks data
belongs to the data block that is still being re-assembled.¶
If the BlockOffset points past the end of the DataBlocks data
then the next data block occurs in a subsequent encapsulating packet.¶
Having the BlockOffset always point at the next available data
block allows for recovering the next inner packet in the
presence of outer encapsulating packet loss.¶
An example AGGFRAG mode packet flow can be found in Appendix A.¶
+---------------------------------------------------------------+ | Type | rest of IPv4, IPv6 or pad. +--------
A data block is defined by a 4-bit type code followed by the data
block data. The type values have been carefully chosen to coincide
with the IPv4/IPv6 version field values so that no per-data block
type overhead is required to encapsulate an IP packet. Likewise, the
length of the data block is extracted from the encapsulated IPv4's
Total Length or IPv6's Payload Length fields.¶
Since a data block's type is identified in its first 4-bits, the only
time padding is required is when there is no data to encapsulate. For
this end padding a Pad Data Block is used.¶
In order for a receiver to reassemble fragmented inner-packets, the
sender MUST send the inner-packet fragments back-to-back in the
logical outer packet stream (i.e., using consecutive ESP sequence
numbers). However, the sender is allowed to insert "all-pad" payloads
(i.e., payloads with a BlockOffset of zero and a single pad
DataBlock) in between the packets carrying the inner-packet
fragment payloads. This interleaving of all-pad payloads allows the
sender to always send a tunnel packet, regardless of the
encapsulation computational requirements.¶
When a receiver is reassembling an inner-packet, and it receives an "all-pad" payload, it increments the expected sequence number that the next inner-packet fragment is expected to arrive in.¶
Given the above, the receiver will need to handle out-of-order arrival of outer ESP packets prior to reassembly processing. ESP already provides for optionally detecting replay attacks. Detecting replay attacks normally utilizes a window method. A similar sequence number based sliding window can be used to correct re-ordering of the outer packet stream. Receiving a larger (newer) sequence number packet advances the window, and received older ESP packets whose sequence numbers the window has passed by are dropped. A good choice for the size of this window depends on the amount of re-ordering the user may normally experience.¶
As the amount of reordering that may be present is hard to predict, the window size SHOULD be configurable by the user. Implementations MAY also dynamically adjust the reordering window based on actual reordering seen in arriving packets.¶
Please note when IP-TFS sends a continuous stream of packets, there is no requirement for an explicit drop timer; however, using a drop timer is RECOMMENDED. If an implementation does not use a drop timer and only considers an outer packet lost when the reorder window moves by it, the inner traffic can be delayed by up to the reorder window size times the per packet send rate. This amount of delay could be significant for slower send rates or when larger reorder window sizes are in use.¶
While ESP guarantees an increasing sequence number with subsequently sent packets, it does not actually require the sequence numbers to be generated with no gaps (e.g., sending only even numbered sequence numbers would be allowed as long as they are always increasing). Gaps in the sequence numbers will not work for this document so the sequence number stream MUST increase monotonically by 1 for each subsequent packet.¶
When using the AGGFRAG_PAYLOAD in conjunction with replay detection, the window size for both MAY be reduced to the smaller of the two window sizes. This is because packets outside of the smaller window but inside the larger would still be dropped by the mechanism with the smaller window size. However, there is also no requirement to make these values the same. Indeed, in some cases, such as slow tunnels where a very small or zero reorder window size is appropriate, the user may still want a large replay detection window to log replayed packets. Additionally, large replay windows can be implemented with very little overhead compared to large reorder windows.¶
Finally, as sequence numbers are reset when switching SAs (e.g., when re-keying a child SA), senders MUST NOT send initial fragments of an inner packet using one SA and subsequent fragments in a different SA.¶
When the tunnel bandwidth is not being fully utilized, a sender MAY pad-out the current encapsulating packet in order to deliver an inner packet un-fragmented in the following outer packet. The benefit would be to avoid inner-packet fragmentation in the presence of a bursty offered load (non-bursty traffic will naturally not fragment). Senders MAY also choose to allow for a minimum fragment size to be configured (e.g., as a percentage of the AGGFRAG_PAYLOAD payload size) to avoid fragmentation at the cost of tunnel bandwidth. The cost with these methods is complexity and added delay of inner traffic. The main advantage to avoiding fragmentation is to minimize inner packet loss in the presence of outer packet loss. When this is worthwhile (e.g., how much loss and what type of loss is required, given different inner traffic shapes and utilization, for this to make sense), and what values to use for the allowable/added delay may be worth researching, but is outside the scope of this document.¶
While use of padding to avoid fragmentation does not impact interoperability, used inappropriately it can reduce the effective throughput of a tunnel. Senders implementing either of the above approaches will need to take care to not reduce the effective capacity, and overall utility, of the tunnel through the overuse of padding.¶
To support reporting of congestion control information (described later) using a non-AGGFRAG_PAYLOAD enabled SA, it is allowed to send an AGGFRAG_PAYLOAD payload with no data blocks (i.e., the ESP payload length is equal to the AGGFRAG_PAYLOAD header length). This special payload is called an empty payload.¶
Currently this situation is only applicable in non-IKEv2 use cases.¶
[RFC4301] provides some direction on when and how to map various values from an inner IP header to the outer encapsulating header, namely the Don't-Fragment (DF) bit ([RFC0791] and [RFC8200]), the Differentiated Services (DS) field [RFC2474] and the Explicit Congestion Notification (ECN) field [RFC3168]. Unlike [RFC4301], AGGFRAG mode may and often will be encapsulating more than one IP packet per ESP packet. To deal with this, these mappings are restricted further.¶
AGGFRAG mode never maps the inner DF bit as it is unrelated to the AGGFRAG tunnel functionality; AGGFRAG mode never needs to IP fragment the inner packets and the inner packets will not affect the fragmentation of the outer encapsulation packets.¶
The ECN value need not be mapped as any congestion related to the constant-send-rate IP-TFS tunnel is unrelated (by design) to the inner traffic flow. The sender MAY still set the ECN value of inner packets based on the normal ECN specification [RFC3168].¶
By default the DS field SHOULD NOT be copied, although a sender MAY choose to allow for configuration to override this behavior. A sender SHOULD also allow the DS value to be set by configuration.¶
[RFC4301] specifies how to modify the inner packet TTL [RFC0791].¶
Any errors (e.g., ICMP errors arriving back at the tunnel ingress due to tunnel traffic) are handled the same as with non-AGGFRAG IPsec tunnels.¶
Unlike [RFC4301], there is normally no effective MTU (EMTU) on an AGGFRAG tunnel as all IP packet sizes are properly transmitted without requiring IP fragmentation prior to tunnel ingress. That said, a sender MAY allow for explicitly configuring an MTU for the tunnel.¶
If fragmentation has been disabled on the AGGFRAG tunnel, then the tunnel's EMTU and behaviors are the same as normal IPsec tunnels [RFC4301].¶
This document does not specify mixed use of an AGGFRAG_PAYLOAD enabled SA. A sender MUST only send AGGFRAG_PAYLOAD payloads over an SA configured for AGGFRAG mode.¶
Just as with normal IPsec/ESP tunnels, AGGFRAG tunnels are unidirectional. Bidirectional IP-TFS functionality is achieved by setting up 2 AGGFRAG tunnels, one in either direction.¶
An AGGFRAG tunnel used for IP-TFS can operate in 2 modes, a non-congestion controlled mode and congestion controlled mode.¶
In the non-congestion controlled mode, IP-TFS sends fixed-sized packets over an AGGFRAG tunnel at a constant rate. The packet send rate is constant and is not automatically adjusted regardless of any network congestion (e.g., packet loss).¶
For similar reasons as given in [RFC7510] the non-congestion controlled mode should only be used where the user has full administrative control over the path the tunnel will take. This is required so the user can guarantee the bandwidth and also be sure as to not be negatively affecting network congestion [RFC2914]. In this case packet loss should be reported to the administrator (e.g., via syslog, YANG notification, SNMP traps, etc) so that any failures due to a lack of bandwidth can be corrected.¶
Non-congestion control mode is also appropriate if ESP over TCP is in use [RFC8229].¶
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. [RFC4342] provides an example 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.¶
The required inputs for the TCP friendly rate control algorithm described in [RFC5348] are the receiver's loss event rate and the sender's estimated round-trip time (RTT). These values are provided by IP-TFS using the congestion information header fields described in Section 3. In particular, these values are sufficient to implement the algorithm described in [RFC5348].¶
At a minimum, the congestion information MUST be sent, from the receiver and from the sender, at least once per RTT. Prior to establishing an RTT the information SHOULD be sent constantly from the sender and the receiver so that an RTT estimate can be established. Not receiving this information over multiple consecutive RTT intervals should be considered a congestion event that causes the sender to adjust its sending rate lower. For example, [RFC4342] calls this the "no feedback timeout" and it is equal to 4 RTT intervals. When a "no feedback timeout" has occurred [RFC4342] halves the sending rate.¶
An implementation MAY choose to always include the congestion
information in its AGGFRAG payload header if sending on an IP-TFS
enabled SA. Since IP-TFS normally will operate with a large packet
size, the congestion information should represent a small portion of
the available tunnel bandwidth. An implementation choosing to always
send the data MAY also choose to only update the LossEventRate
and RTT header field values it sends every RTT though.¶
When choosing a congestion control algorithm (or a selection of algorithms) note that IP-TFS is not providing for reliable delivery of IP traffic, and so per packet ACKs are not required and are not provided.¶
It is worth noting that the variable send-rate of a congestion controlled AGGFRAG tunnel, is not private; however, this send-rate is being driven by network congestion, and as long as the encapsulated (inner) traffic flow shape and timing are not directly affecting the (outer) network congestion, the variations in the tunnel rate will not weaken the provided inner traffic flow confidentiality.¶
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.¶
An AGGFRAG enabled SA receiver has a few tasks to perform.¶
The receiver first reorders, possibly out-of-order ESP packets received on an SA into in-sequence-order AGGFRAG_PAYLOAD payloads (Section 2.2.3). If congestion control is enabled, the receiver considers a packet lost when it's sequence number is abandoned (e.g., pushed out of the re-ordering window, or timed-out) by the reordering algorithm. As an optional optimization (e.g., to handle very lossy and/or reordered tunnel paths), the receiver MAY transmit any fully formed inner packets contained within the AGGFRAG_PAYLOADs prior to re-ordering the outer packets.¶
Additionally, if congestion control is enabled, the receiver sends congestion control data (Section 6.1.2) back to the sender as described in Section 2.4.2 and Section 3.¶
Finally, the receiver processes the now in-order AGGFRAG_PAYLOAD payload stream to extract the inner-packets (Section 2.2.3, Section 6.1).¶
In order to support the congestion control mode, the sender needs to know the loss event rate and to approximate the RTT [RFC5348]. In order to obtain these values, the receiver sends congestion control information on it's SA back to the sender. Thus, to support congestion control the receiver must have a paired SA back to the sender (this is always the case when the tunnel was created using IKEv2). If the SA back to the sender is a non-AGGFRAG_PAYLOAD enabled SA then an AGGFRAG_PAYLOAD empty payload (i.e., header only) is used to convey the information.¶
In order to calculate a loss event rate compatible with [RFC5348], the
receiver needs to have a round-trip time estimate. Thus the sender
communicates this estimate in the RTT header field. On startup this
value will be zero as no RTT estimate is yet known.¶
In order for the sender to estimate its RTT value, the sender
places a timestamp value in the TVal header field. On first receipt
of this TVal, the receiver records the new TVal value along with
the time it arrived locally, subsequent receipt of the same TVal
MUST NOT update the recorded time.¶
When the receiver sends its CC header it places this latest recorded
TVal in the TEcho header field, along with 2 delay values, Echo
Delay and Transmit Delay. The Echo Delay value is the time delta
from the recorded arrival time of TVal and the current clock in
microseconds. The second value, Transmit Delay, is the receiver's
current transmission delay on the tunnel (i.e., the average time
between sending packets on its half of the AGGFRAG tunnel).¶
When the sender receives back its TVal in the TEcho header field
it calculates 2 RTT estimates. The first is the actual delay found by
subtracting the TEcho value from its current clock and then
subtracting Echo Delay as well. The second RTT estimate is found by
adding the received Transmit Delay header value to the senders own
transmission delay (i.e., the average time between sending packets on
its half of the AGGFRAG tunnel). The larger of these 2 RTT estimates
SHOULD be used as the RTT value.¶
The two RTT estimates are required to handle different combinations of
faster or slower tunnel packet paths with faster or slower fixed
tunnel rates. Choosing the larger of the two values guarantees that
the RTT is never considered faster than the aggregate transmission
delay based on the IP-TFS send rate (the second estimate), as well
as never being considered faster than the actual RTT along the tunnel
packet path (the first estimate).¶
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.¶
In additional to normal packet loss information AGGFRAG mode supports use
of the ECN bits in the encapsulating IP header [RFC3168] for
identifying congestion. If ECN use is enabled and a packet arrives at
the egress (receiving) side with the Congestion Experienced (CE) value set,
then the receiver considers that packet as being dropped, although it
does not drop it. The receiver MUST set the E bit in any
AGGFRAG_PAYLOAD payload header containing a LossEventRate value
derived from a CE value being considered.¶
As noted in [RFC3168] the ECN bits are not protected by IPsec and thus may constitute a covert channel. For this reason, ECN use SHOULD NOT be enabled by default.¶
IP-TFS is meant to be deployable with a minimal amount of configuration. All IP-TFS specific configuration should be specified at the unidirectional tunnel ingress (sending) side. It is intended that non-IKEv2 operation is supported, at least, with local static configuration.¶
Bandwidth is a local configuration option. For non-congestion controlled mode, the bandwidth SHOULD be configured. For congestion controlled mode, the bandwidth can be configured or the congestion control algorithm discovers and uses the maximum bandwidth available. No standardized configuration method is required.¶
The fixed packet size to be used for the tunnel encapsulation packets MAY be configured manually or can be automatically determined using other methods such as PLMTUD ([RFC4821], [RFC8899]) or PMTUD ([RFC1191], [RFC8201]). As PMTUD is known to have issues, PLMTUD is considered the more robust option. No standardized configuration method is required.¶
Congestion control is a local configuration option. No standardized configuration method is required.¶
As mentioned previously AGGFRAG tunnels utilize ESP payloads of type AGGFRAG_PAYLOAD.¶
When using IKEv2, a new "USE_AGGFRAG" Notification Message enables the AGGFRAG_PAYLOAD payload on a child SA pair. The method used is similar to how USE_TRANSPORT_MODE is negotiated, as described in [RFC7296].¶
To request use of the AGGFRAG_PAYLOAD payload on the Child SA pair, the initiator includes the USE_AGGFRAG notification in an SA payload requesting a new Child SA (either during the initial IKE_AUTH or during CREATE_CHILD_SA exchanges). If the request is accepted then the response MUST also include a notification of type USE_AGGFRAG. If the responder declines the request the child SA will be established without AGGFRAG_PAYLOAD payload use enabled. If this is unacceptable to the initiator, the initiator MUST delete the child SA.¶
As the use of the AGGFRAG_PAYLOAD payload is currently only defined for non-transport mode tunnels, the USE_AGGFRAG notification MUST NOT be combined with USE_TRANSPORT notification.¶
The USE_AGGFRAG notification contains a 1 octet payload of flags that specify requirements from the sender of the notification. If any requirement flags are not understood or cannot be supported by the receiver then the receiver SHOULD NOT enable use of AGGFRAG_PAYLOAD (either by not responding with the USE_AGGFRAG notification, or in the case of the initiator, by deleting the child SA if the now established non-AGGFRAG_PAYLOAD using SA is unacceptable).¶
The notification type and payload flag values are defined in Section 6.1.4.¶
The packet and data formats defined below are generic with the intent of allowing for non-IP-TFS uses, but such uses are outside the scope of this document.¶
ESP Next Header value: 0x5¶
An AGGFRAG payload is identified by the ESP Next Header value AGGFRAG_PAYLOAD which has the value 0x5. The value 5 was chosen to not conflict with other used values. The first octet of this payload indicates the format of the remaining payload data.¶
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+-+-+- | Sub-type | ... +-+-+-+-+-+-+-+-+-+-+-¶
This document defines 2 payload sub-types. These payload formats are defined in the following sections.¶
The non-congestion control AGGFRAG_PAYLOAD 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 ...
+-+-+-+-+-+-+-+-+-+-+-
¶
DataBlocks data before the start of a
new data block. If the start of a new data block
occurs in a subsequent payload the BlockOffset
will point past the end of the DataBlocks data.
In this case all the DataBlocks data belongs to
the current data block being assembled. When the
BlockOffset extends into subsequent payloads it
continues to only count DataBlocks data (i.e.,
it does not count subsequent packets
non-DataBlocks data such as header octets).¶
The congestion control AGGFRAG_PAYLOAD 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 |P|E| BlockOffset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LossEventRate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTT | Echo Delay ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Echo Delay | Transmit Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TVal |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TEcho |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DataBlocks ...
+-+-+-+-+-+-+-+-+-+-+-
¶
LossEventRate.¶
1/LossEventRate.¶
0x3FFFFF it MUST be set to 0x3FFFFF.¶
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.¶
0x1FFFFF it MUST
be set to 0x1FFFFF.¶
TEcho field, along with
an Echo Delay value of how long that echo took.¶
TVal
field. The received TVal is placed in TEcho along with
an Echo Delay value indicating how long it has been since
receiving the TVal value.¶
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...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
¶
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.¶
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.¶
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 ...
+-+-+-+-+-+-+-+-+-+-+-
¶
As discussed in Section 5.1, a notification message USE_AGGFRAG is used to negotiate use of the ESP AGGFRAG_PAYLOAD Next Header value.¶
The USE_AGGFRAG Notification Message State Type is (TBD2).¶
The notification payload contains 1 octet of requirement flags. There are currently 2 requirement flags defined. This may be revised by later specifications.¶
+-+-+-+-+-+-+-+-+ |0|0|0|0|0|0|C|D| +-+-+-+-+-+-+-+-+¶
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_AGGFRAG notification.¶
This document requests IANA create a registry called "AGGFRAG_PAYLOAD Sub-Type Registry" under a new category named "ESP AGGFRAG_PAYLOAD Parameters". The registration policy for this registry is "Expert Review" ([RFC8126] and [RFC7120]).¶
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
¶
This document requests a status type USE_AGGFRAG be allocated from the "IKEv2 Notify Message Types - Status Types" registry.¶
This document describes an aggregation and fragmentation mechanism and it use to add TFC to IP traffic. The use described 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 in (Section 3.1) the ECN bits are not protected by IPsec and thus may constitute a covert channel. For this reason, ECN use SHOULD NOT be enabled by default.¶
As noted previously in Section 2.4.2, for TFC to be 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.¶
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) ] [--800--][--800--][60][-240-][--4000----------------------][pad]
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 AGGFRAG 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 AGGFRAG 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 [RFC5348]. For this IP-TFS use case (as with [RFC4342]) the (fixed) packet size is used as the segment size for the algorithm. The main formula in the algorithm for the send rate is then as follows:¶
1
X = -----------------------------------------------
R * (sqrt(2*p/3) + 12*sqrt(3*p/8)*p*(1+32*p^2))
¶
Where X is the send rate in packets per second, R is the
round trip time estimate and p is the loss event rate (the inverse
of which is provided by the receiver).¶
In addition the algorithm in [RFC5348] also uses an X_recv value (the
receiver's receive rate). For IP-TFS one MAY set this value according to
the sender's current tunnel send-rate (X).¶
The IP-TFS receiver, having the RTT estimate from the sender can use the
same method as described in [RFC5348] and [RFC4342] to collect the loss
intervals and calculate the loss event rate value using the weighted
average as indicated. The receiver communicates the inverse of this
value back to the sender in the AGGFRAG_PAYLOAD payload header field
LossEventRate.¶
The IP-TFS sender now has both the R and p values and can calculate
the correct sending rate. If following [RFC5348] the sender should also
use the slow start mechanism described therein when the IP-TFS SA is
first established.¶
For comparing overhead the overhead of ESP for both normal and AGGFRAG tunnel packets must be calculated, and so an algorithm for encryption and authentication must be chosen. For the data below AES-GCM-256 was selected. This leads to an IP+ESP overhead of 54.¶
54 = 20 (IP) + 8 (ESPH) + 2 (ESPF) + 8 (IV) + 16 (ICV)¶
Additionally, for IP-TFS, non-congestion control AGGFRAG_PAYLOAD headers were chosen which adds 4 octets for a total overhead of 58.¶
For comparison the overhead of AGGFRAG payload is 58 octets per outer packet. Therefore the octet overhead per inner packet is 58 divided by the number of outer packets required (fractional allowed). The overhead as a percentage of inner packet size is a constant based on the Outer MTU size.¶
OH = 58 / Outer Payload Size / Inner Packet Size OH % of Inner Packet Size = 100 * OH / Inner Packet Size OH % of Inner Packet Size = 5800 / Outer Payload Size¶
Type IP-TFS IP-TFS IP-TFS
MTU 576 1500 9000
PSize 518 1442 8942
-------------------------------
40 11.20% 4.02% 0.65%
576 11.20% 4.02% 0.65%
1500 11.20% 4.02% 0.65%
9000 11.20% 4.02% 0.65%
The overhead per inner packet for constant-send-rate padded ESP (i.e., traditional IPsec TFC) is 36 octets plus any padding, unless fragmentation is required.¶
When fragmentation of the inner packet is required to fit in the outer IPsec packet, overhead is the number of outer packets required to carry the fragmented inner packet times both the inner IP overhead (20) and the outer packet overhead (54) minus the initial inner IP overhead plus any required tail padding in the last encapsulation 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
NF = CEILING(NF0)
OH = NF * (IP Overhead + IPsec Overhead)
- IP Overhead
+ NF * (Outer Payload Size - IP Overhead)
- Inner Payload Size
OH = NF * (IPsec Overhead + Outer Payload Size)
- (IP Overhead + Inner Payload Size)
OH = NF * (IPsec Overhead + Outer Payload Size)
- Inner Packet Size
¶
The following tables collect the overhead values for some common L3 MTU sizes in order to compare them. The first table is the number of octets of overhead for a given L3 MTU sized packet. The second table is the percentage of overhead in the same MTU sized packet.¶
Type ESP+Pad ESP+Pad ESP+Pad IP-TFS IP-TFS IP-TFS
L3 MTU 576 1500 9000 576 1500 9000
PSize 522 1446 8946 518 1442 8942
-----------------------------------------------------------
40 482 1406 8906 4.5 1.6 0.3
128 394 1318 8818 14.3 5.1 0.8
256 266 1190 8690 28.7 10.3 1.7
518 4 928 8428 58.0 20.8 3.4
576 576 870 8370 64.5 23.2 3.7
1442 286 4 7504 161.5 58.0 9.4
1500 228 1500 7446 168.0 60.3 9.7
8942 1426 1558 4 1001.2 359.7 58.0
9000 1368 1500 9000 1007.7 362.0 58.4
Type ESP+Pad ESP+Pad ESP+Pad IP-TFS IP-TFS IP-TFS
MTU 576 1500 9000 576 1500 9000
PSize 522 1446 8946 518 1442 8942
-----------------------------------------------------------
40 1205.0% 3515.0% 22265.0% 11.20% 4.02% 0.65%
128 307.8% 1029.7% 6889.1% 11.20% 4.02% 0.65%
256 103.9% 464.8% 3394.5% 11.20% 4.02% 0.65%
518 0.8% 179.2% 1627.0% 11.20% 4.02% 0.65%
576 100.0% 151.0% 1453.1% 11.20% 4.02% 0.65%
1442 19.8% 0.3% 520.4% 11.20% 4.02% 0.65%
1500 15.2% 100.0% 496.4% 11.20% 4.02% 0.65%
8942 15.9% 17.4% 0.0% 11.20% 4.02% 0.65%
9000 15.2% 16.7% 100.0% 11.20% 4.02% 0.65%
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.¶
Size E + P E + P E + P IPTFS IPTFS IPTFS Enet ESP
MTU 590 1514 9014 590 1514 9014 any any
OH 92 92 92 96 96 96 38 74
------------------------------------------------------------
40 614 1538 9038 47 42 40 84 114
128 614 1538 9038 151 136 129 166 202
256 614 1538 9038 303 273 258 294 330
518 614 1538 9038 614 552 523 574 610
576 1228 1538 9038 682 614 582 614 650
1442 1842 1538 9038 1709 1538 1457 1498 1534
1500 1842 3076 9038 1777 1599 1516 1538 1574
8942 11052 10766 9038 10599 9537 9038 8998 9034
9000 11052 10766 18076 10667 9599 9096 9038 9074
Size E + P E + P E + P IPTFS IPTFS IPTFS Enet ESP
MTU 590 1514 9014 590 1514 9014 any any
OH 92 92 92 96 96 96 38 74
--------------------------------------------------------------
40 2.0M 0.8M 0.1M 26.4M 29.3M 30.9M 14.9M 11.0M
128 2.0M 0.8M 0.1M 8.2M 9.2M 9.7M 7.5M 6.2M
256 2.0M 0.8M 0.1M 4.1M 4.6M 4.8M 4.3M 3.8M
518 2.0M 0.8M 0.1M 2.0M 2.3M 2.4M 2.2M 2.1M
576 1.0M 0.8M 0.1M 1.8M 2.0M 2.1M 2.0M 1.9M
1442 678K 812K 138K 731K 812K 857K 844K 824K
1500 678K 406K 138K 703K 781K 824K 812K 794K
8942 113K 116K 138K 117K 131K 138K 139K 138K
9000 113K 116K 69K 117K 130K 137K 138K 137K
Size E + P E + P E + P IPTFS IPTFS IPTFS Enet ESP
590 1514 9014 590 1514 9014 any any
92 92 92 96 96 96 38 74
----------------------------------------------------------------------
40 6.51% 2.60% 0.44% 84.36% 93.76% 98.94% 47.62% 35.09%
128 20.85% 8.32% 1.42% 84.36% 93.76% 98.94% 77.11% 63.37%
256 41.69% 16.64% 2.83% 84.36% 93.76% 98.94% 87.07% 77.58%
518 84.36% 33.68% 5.73% 84.36% 93.76% 98.94% 93.17% 87.50%
576 46.91% 37.45% 6.37% 84.36% 93.76% 98.94% 93.81% 88.62%
1442 78.28% 93.76% 15.95% 84.36% 93.76% 98.94% 97.43% 95.12%
1500 81.43% 48.76% 16.60% 84.36% 93.76% 98.94% 97.53% 95.30%
8942 80.91% 83.06% 98.94% 84.36% 93.76% 98.94% 99.58% 99.18%
9000 81.43% 83.60% 49.79% 84.36% 93.76% 98.94% 99.58% 99.18%
A sometimes unexpected result of using an AGGFRAG tunnel (or any packet aggregating tunnel) is that, for small to medium sized packets, the available bandwidth is actually greater than native Ethernet. This is due to the reduction in Ethernet framing overhead. This increased bandwidth is paid for with an increase in latency. This latency is the time to send the unrelated octets in the outer tunnel frame. The following table illustrates the latency for some common values on a 10G Ethernet link. The table also includes latency introduced by padding if using ESP with padding.¶
ESP+Pad ESP+Pad IP-TFS IP-TFS
1500 9000 1500 9000
------------------------------------------
40 1.12 us 7.12 us 1.17 us 7.17 us
128 1.05 us 7.05 us 1.10 us 7.10 us
256 0.95 us 6.95 us 1.00 us 7.00 us
518 0.74 us 6.74 us 0.79 us 6.79 us
576 0.70 us 6.70 us 0.74 us 6.74 us
1442 0.00 us 6.00 us 0.05 us 6.05 us
1500 1.20 us 5.96 us 0.00 us 6.00 us
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. We would also like to thank Michael Richardson, Sean Turner, Valery Smyslov and Tero Kivinen for reviews and many suggestions for improvements, as well as Joseph Touch for the transport area review and suggested improvements.¶
The following people made significant contributions to this document.¶
Lou Berger LabN Consulting, L.L.C. Email: lberger@labn.net¶