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Internet Engineering Task Force (IETF) C. Hopps
Request for Comments: 9347 LabN Consulting, L.L.C.
Category: Standards Track January 2023
ISSN: 2070-1721
Aggregation and Fragmentation Mode for Encapsulating Security Payload
(ESP) and Its Use for IP Traffic Flow Security (IP-TFS)
Abstract
This document describes a mechanism for aggregation and fragmentation
of IP packets when they are being encapsulated in Encapsulating
Security Payload (ESP). 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 IP
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-size,
constant-send-rate IPsec tunnel. The solution allows for congestion
control, as well as nonconstant send-rate usage.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9347.
Copyright Notice
Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Terminology & Concepts
2. The AGGFRAG Tunnel
2.1. Tunnel Content
2.2. Payload Content
2.2.1. DataBlocks
2.2.2. End Padding
2.2.3. Fragmentation, Sequence Numbers, and All-Pad Payloads
2.2.4. Empty Payload
2.2.5. IP Header Value Mapping
2.2.6. IPv4 Time To Live (TTL), IPv6 Hop Limit, and ICMP
Messages
2.2.7. Effective MTU of the Tunnel
2.3. Exclusive SA Use
2.4. Modes of Operation
2.4.1. Non-Congestion-Controlled Mode
2.4.2. Congestion-Controlled Mode
2.5. Summary of Receiver Processing
3. Congestion Information
3.1. ECN Support
4. Configuration of AGGFRAG Tunnels for IP-TFS
4.1. Bandwidth
4.2. Fixed Packet Size
4.3. Congestion Control
5. IKEv2
5.1. USE_AGGFRAG Notification Message
6. Packet and Data Formats
6.1. AGGFRAG_PAYLOAD Payload
6.1.1. Non-Congestion-Control AGGFRAG_PAYLOAD Payload Format
6.1.2. Congestion Control AGGFRAG_PAYLOAD Payload Format
6.1.3. Data Blocks
6.1.4. IKEv2 USE_AGGFRAG Notification Message
7. IANA Considerations
7.1. ESP Next Header Value
7.2. AGGFRAG_PAYLOAD Sub-Types
7.3. USE_AGGFRAG Notify Message Status Type
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Example of an Encapsulated IP Packet Flow
Appendix B. A Send and Loss Event Rate Calculation
Appendix C. Comparisons of IP-TFS
C.1. Comparing Overhead
C.1.1. IP-TFS Overhead
C.1.2. ESP with Padding Overhead
C.2. Overhead Comparison
C.3. Comparing Available Bandwidth
C.3.1. Ethernet
Acknowledgements
Contributors
Author's Address
1. Introduction
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 patterns in the
message traffic may expose 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 underutilize the available bandwidth.
This document defines an aggregation and fragmentation (AGGFRAG) mode
for ESP, as well as ESP's 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-size encapsulating packets;
however, these fixed-size packets can contain partial, whole, or
multiple IP packets to maximize the bandwidth of the tunnel. A
nonconstant 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 TFC solution
prescribed in [RFC4303], 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.
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
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].
2. The AGGFRAG Tunnel
As mentioned in Section 1, the AGGFRAG mode utilizes an IPsec
[RFC4303] tunnel as its transport. For the purpose of IP-TFS, fixed-
size 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 the 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, which is then
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.
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 by introducing
an AGGFRAG mode for ESP.
The AGGFRAG mode aggregates and 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 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.
2.2. Payload Content
The AGGFRAG_PAYLOAD payload content defined in this document consists
of a 4- or 24-octet header, followed by either a partial data block,
a full data block, or multiple partial or full data blocks. 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 sub-type/flags] : BlockOffset |
+---------------------------------------------------------------+
: [Optional Congestion Info] :
+---------------------------------------------------------------+
| DataBlocks ... ~
~ ~
~ |
+---------------------------------------------------------------|
. ESP Trailer... .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 1: Layout of an AGGFRAG Mode 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 the data block that is still being reassembled.
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.
2.2.1. DataBlocks
+---------------------------------------------------------------+
| Type | rest of IPv4, IPv6, or pad...
+--------
Figure 2: Layout of a 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's
Total Length or IPv6's Payload Length fields.
2.2.2. End Padding
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.
2.2.3. Fragmentation, Sequence Numbers, and All-Pad Payloads
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
data block ) 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
reordering of the outer packet stream. Receiving a larger (newer)
sequence number packet advances the window, and if any older ESP
packets whose sequence numbers the window has passed by are received,
then the packets are dropped. A good choice for the size of this
window depends on the amount of misordering the user is experiencing;
however, a value of 3 has been suggested as a default when no more
informed choice exists.
As the amount of misordering 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
misordering seen in arriving packets.
Please note, when IP-TFS sends a continuous stream of packets, there
is no requirement for an explicit lost packet timer; however, using a
lost packet timer is RECOMMENDED. If an implementation does not use
a lost packet 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 delay
could be significant for slower send rates or when larger reorder
window sizes are in use. As the lost packet timer affects the delay
of inner packet delivery, an implementation or user could choose to
set it proportionate to the tunnel rate.
While ESP guarantees an increasing sequence number with subsequently
sent packets, it does not actually require the sequence numbers to be
generated consecutively (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 window 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 Security
Associations (SAs) (e.g., when rekeying a Child SA), senders MUST NOT
send initial fragments of an inner packet using one SA and subsequent
fragments in a different SA.
| A note on BlockOffset values: Senders MUST encode the
| BlockOffset consistently with the immediately preceding non-
| all-pad payload packet. Specifically, if the immediately
| preceding non-all-pad payload packet ended with a Pad Data
| Block, this BlockOffset MUST be zero, as Pad Data Blocks are
| never fragmented. The BlockOffset MUST be consistent with the
| remaining size implied by the length field from the fragmented
| inner packet.
2.2.3.1. Optional Extra Padding
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 unfragmented 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
costs with these methods are complexity and an 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, if padding is 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.
2.2.4. Empty Payload
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 use cases without
Internet Key Exchange Protocol Version 2 (IKEv2).
2.2.5. IP Header Value Mapping
[RFC4301] provides some direction on when and how to map various
values from an inner IP header to the outer encapsulating header,
namely the Don't Fragment (DF) bit [RFC0791], the Differentiated
Services (DS) field [RFC2474], and the Explicit Congestion
Notification (ECN) field [RFC3168]. Unlike in [RFC4301], the 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.
2.2.5.1. DF Bit
The AGGFRAG mode never maps the inner DF bit, as it is unrelated to
the AGGFRAG tunnel functionality; the AGGFRAG mode never needs to IP
fragment the inner packets, and the inner packets will not affect the
fragmentation of the outer encapsulation packets.
2.2.5.2. ECN Value
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] [RFC4301]
[RFC6040].
2.2.5.3. DS Field
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.
2.2.6. IPv4 Time To Live (TTL), IPv6 Hop Limit, and ICMP Messages
How to modify the inner packet IPv4 TTL [RFC0791] or IPv6 Hop Limit
[RFC8200] is specified in [RFC4301].
[RFC4301] specifies how to apply policy to authenticated and
unauthenticated ICMP error packets (e.g., Destination Unreachable)
arriving at or being forwarded through the endpoint, in particular,
whether to process, ignore, or forward said packets. With the one
exception that this document does not change the handling of these
packets, they should be handled as specified in [RFC4301].
The one way in which an AGGFRAG tunnel differs in ICMP error packet
mechanics is with PMTU. When fragmentation is enabled on the AGGFRAG
tunnel, then no ICMP "Too Big" errors need to be generated for
arriving ingress traffic, as the arriving inner packets will be
naturally fragmented by the AGGFRAG encapsulation.
Otherwise, when fragmentation has been disabled on the AGGFRAG
tunnel, then the treatment of arriving inner traffic exactly maps to
that of a non-AGGFRAG ESP tunnel. Explicitly, IPv4 with DF set and
IPv6 packets that cannot fit in its own outer packet payload will
generate the appropriate ICMP "Too Big" error, as described in
[RFC4301], and IPv4 packets without DF set will be IP fragmented, as
described in [RFC4301].
Packets egressing the tunnel continue to be handled as specified in
[RFC4301].
All other aspects of PMTU and the handling of ICMP "Too Big" messages
(i.e., with regards to the outer AGGFRAG/ESP tunnel packet size) also
remain unchanged from [RFC4301].
2.2.7. Effective MTU of the Tunnel
Unlike in [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].
2.3. Exclusive SA Use
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.
2.4. Modes of Operation
Just as with normal IPsec/ESP SAs, AGGFRAG SAs are unidirectional.
Bidirectional IP-TFS functionality is achieved by setting up 2
AGGFRAG SAs, 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.
2.4.1. Non-Congestion-Controlled Mode
In the non-congestion-controlled mode, IP-TFS sends fixed-size
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 MUST only be used where the user has full
administrative control over any path the tunnel will take and MUST
NOT be used if this is not the case. 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. The use of circuit breakers is also
RECOMMENDED (Section 2.4.2.1).
Users that choose the non-congestion-controlled mode need to
understand that this mode will send packets at a constant rate,
utilizing a constant, fixed bandwidth, and will not adjust based on
congestion. Thus, if they do not guarantee the bandwidth required by
the tunnel, the tunnel's operation, as well as the rest of their
network, may be negatively impacted.
One expected use case for the non-congestion-controlled mode is to
guarantee the full tunnel bandwidth is available and preferred over
other non-tunnel traffic. In fact, a typical site-to-site use case
might have all of the user traffic utilizing the IP-TFS tunnel.
The non-congestion-controlled mode is also appropriate if ESP over
TCP is in use [RFC9329]. However, the use of TCP is considered a
fallback-only solution for IPsec; it is highly not preferred. This
is also one of the reasons that TCP was not chosen as the
encapsulation for IP-TFS instead of AGGFRAG.
2.4.2. Congestion-Controlled Mode
With the congestion-controlled mode, IP-TFS adapts to network
congestion by lowering the packet send rate to accommodate the
congestion, as well as raising the rate when congestion subsides.
Since overhead is per packet, by allowing for maximal fixed-size
packets and varying the send rate, transport overhead is minimized.
The output of the congestion control algorithm will adjust the rate
at which the ingress sends packets. While this document does not
require a specific congestion control algorithm, best current
practice RECOMMENDS that the algorithm conform to [RFC5348].
Congestion control principles are documented in [RFC2914] as well.
There is an example in [RFC4342] of the algorithm in [RFC5348], which
matches the requirements of IP-TFS (i.e., designed for fixed-size
packets 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, this is called the "no feedback timeout" in [RFC4342], and
it is equal to 4 RTT intervals. When a "no feedback timeout" has
occurred, the sending rate is halved, as per [RFC4342].
An implementation MAY choose to always include the congestion
information in its AGGFRAG payload header if it is sending it 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 through.
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 acknowledgements (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.
2.4.2.1. Circuit Breakers
In addition to congestion control, implementations that support the
non-congestion-control mode SHOULD implement circuit breakers
[RFC8084] as a recovery method of last resort. When circuit breakers
are enabled, an implementation SHOULD also enable congestion control
reports so that circuit breakers have information to act on.
The pseudowire congestion considerations [RFC7893] are equally
applicable to the mechanisms defined in this document, notably the
text on inelastic traffic.
One example of a simple, slow-trip circuit breaker that an
implementation may provide would utilize 2 values: the amount of
persistent loss rate required to trip the circuit breaker and the
required length of time this persistent loss rate must be seen to
trip the circuit breaker. These 2 value are required configurations
from the user. When the circuit breaker is tripped, the tunnel
traffic is disabled and an appropriate log message or other
management type alarm is triggered, indicating operation intervention
is required.
2.5. Summary of Receiver Processing
An AGGFRAG-enabled SA receiver has a few tasks to perform.
The receiver MAY process incoming AGGFRAG_PAYLOAD payloads as soon as
they arrive, as much as it can, i.e., if the incoming AGGFRAG_PAYLOAD
packet contains complete inner packet(s), the receiver should extract
and transmit them immediately. For partial packets, the receiver
needs to keep the partial packets in the memory until they fall out
from the reordering window or until the missing parts of the packets
are received, in which case, it will reassemble and transmit them.
If the AGGFRAG_PAYLOAD payload contains multiple packets, they SHOULD
be sent out in the order they are in the AGGFRAG_PAYLOAD (i.e., keep
the original order they were received on the other end). The cost of
using this method is that an amplification of out-of-order delivery
of inner packets can occur due to inner packet aggregation.
Instead of the method described in the previous paragraph, the
receiver MAY reorder out-of-order AGGFRAG_PAYLOAD payloads received
into in-sequence-order AGGFRAG_PAYLOAD payloads (Section 2.2.3), and
only after it has an in-order AGGFRAG_PAYLOAD payload stream would
the receiver transmit the inner packets. Using this method will
ensure the inner packets are sent in order. The cost of this method
is that a lost packet will cause a delay of up to the lost packet
timer interval (or the full reorder window if no lost packet timer is
used). Additionally, there can be extra burstiness in the output
stream. This burstiness can happen when a lost packet is dropped
from the reorder window, and the remaining outer packets in the
reorder window are immediately processed and sent out back to back.
Additionally, if congestion control is enabled, the receiver sends
congestion control data (Section 6.1.2) back to the sender, as
described in Sections 2.4.2 and 3.
Finally, a note on receiving incorrect BlockOffset values: To account
for misbehaving senders, a receiver SHOULD gracefully handle the case
where the BlockOffset of consecutive packets, and/or the inner packet
they share, do not agree. It MAY drop the inner packet or one or
both of the outer packets.
3. Congestion Information
In order to support the congestion-controlled 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 its 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 an RTT 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 congestion control 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 the Echo Delay as well. The second RTT estimate is found
by adding the received Transmit Delay header value to the sender's
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.
3.1. ECN Support
In addition to normal packet loss information, the 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.
In [RFC6040], which updates [RFC3168] and [RFC4301], behaviors for
marking the outer ECN field value based on the ECN field of the inner
packet are defined. As the AGGFRAG mode may have multiple inner
packets present in a single outer packet, and there is no obvious
correct way to map these multiple values to the single outer packet
ECN field value, the tunnel ingress endpoint SHOULD operate in the
"compatibility" mode, rather than the "default" mode from [RFC6040].
In particular, this means that the ingress (sending) endpoint of the
tunnel always sets the newly constructed outer encapsulating packet
header ECN field to Not-ECT [RFC6040].
4. Configuration of AGGFRAG Tunnels for IP-TFS
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.
YANG and MIB documents have been defined for IP-TFS in [RFC9348] and
[RFC9349].
4.1. Bandwidth
Bandwidth is a local configuration option. For the non-congestion-
controlled mode, the bandwidth SHOULD be configured. For the
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.
4.2. Fixed Packet Size
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.
4.3. Congestion Control
Congestion control is a local configuration option. No standardized
configuration method is required.
5. IKEv2
5.1. USE_AGGFRAG Notification Message
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 the 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.
6. Packet and Data Formats
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.
6.1. AGGFRAG_PAYLOAD Payload
ESP Next Header value: 144
An AGGFRAG payload is identified by the ESP Next Header value
AGGFRAG_PAYLOAD, which has the value 144, which has been reserved in
the IP protocol numbers space. The first octet of the payload
indicates the format of the remaining payload data.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-
| Sub-type | ...
+-+-+-+-+-+-+-+-+-+-+-
Figure 3: AGGFRAG_PAYLOAD Payload Format
Sub-type:
An 8-bit value indicating the payload format.
This document defines 2 payload sub-types. These payload formats are
defined in the following sections.
6.1.1. Non-Congestion-Control AGGFRAG_PAYLOAD Payload Format
The non-congestion-control AGGFRAG_PAYLOAD payload consists 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 ...
+-+-+-+-+-+-+-+-+-+-+-
Figure 4: Non-Congestion-Control Payload Format
Sub-type:
An octet indicating the payload format. For this non-congestion-
control format, the value is 0.
Reserved:
An octet set to 0 on generation and ignored on receipt.
BlockOffset:
A 16-bit unsigned integer counting the number of octets of
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 of the non-DataBlocks
data, such as header octets).
DataBlocks:
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 AGGFRAG_PAYLOAD Payload Format
The congestion control AGGFRAG_PAYLOAD payload consists 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 ...
+-+-+-+-+-+-+-+-+-+-+-
Figure 5: Congestion Control Payload Format
Sub-type:
An octet indicating the payload format. For this congestion
control format, the value is 1.
Reserved:
A 6-bit field set to 0 on generation and ignored on receipt.
P:
A 1-bit value that, if set, indicates that PLMTUD probing is in
progress. This information can be used to avoid treating missing
packets as loss events by the congestion control algorithm when
running the PLMTUD probe algorithm.
E:
A 1-bit value that, if set, indicates that Congestion Experienced
(CE) ECN bits were received and used in deriving the reported
LossEventRate.
BlockOffset:
The same value as the non-congestion-controlled payload format
value.
LossEventRate:
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.
RTT:
A 22-bit value specifying the sender's current RTT estimate in
microseconds. The value MAY be zero prior to the sender having
calculated an RTT estimate. The value SHOULD be set to zero on
non-AGGFRAG_PAYLOAD-enabled SAs. If the RTT is equal to or larger
than 0x3FFFFF, the value MUST be set to 0x3FFFFF.
Echo Delay:
A 21-bit value specifying the delay in microseconds incurred
between the receiver first receiving the TVal value, which it is
sending back in TEcho. If the delay is equal to or larger than
0x1FFFFF, the value MUST be set to 0x1FFFFF.
Transmit Delay:
A 21-bit value specifying the transmission delay in microseconds.
This is the fixed (or average) delay on the receiver between it
sending packets on the IP-TFS tunnel. If the delay is equal to or
larger than 0x1FFFFF, the value MUST be set to 0x1FFFFF.
TVal:
An opaque, 32-bit value that will be echoed back by the receiver
in later packets in the TEcho field, along with an Echo Delay
value of how long that echo took.
TEcho:
The opaque, 32-bit value from a received packet's 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.
DataBlocks:
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 a non-IP-TFS-enabled SA,
this field 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...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Figure 6: Data Block Format
Type:
A 4-bit field where 0x0 identifies a Pad Data Block, 0x4 indicates
an IPv4 data block, and 0x6 indicates an IPv6 data block.
6.1.3.1. IPv4 Data Block
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x4 | IHL | TypeOfService | TotalLength |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rest of the inner packet ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Figure 7: IPv4 Data Block Format
These values are the actual values within the encapsulated IPv4
header. In other words, the start of this data block is the start of
the encapsulated IP packet.
Type:
A 4-bit value of 0x4 indicating IPv4 (i.e., first nibble of the
IPv4 packet).
TotalLength:
The 16-bit unsigned integer "Total Length" field of the IPv4 inner
packet.
6.1.3.2. IPv6 Data Block
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x6 | TrafficClass | FlowLabel |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PayloadLength | Rest of the inner packet ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Figure 8: IPv6 Data Block Format
These values are the actual values within the encapsulated IPv6
header. In other words, the start of this data block is the start of
the encapsulated IP packet.
Type:
A 4-bit value of 0x6 indicating IPv6 (i.e., first nibble of the
IPv6 packet).
PayloadLength:
The 16-bit unsigned integer "Payload Length" field of the inner
IPv6 inner packet.
6.1.3.3. Pad Data Block
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0 | Padding ...
+-+-+-+-+-+-+-+-+-+-+-
Figure 9: Pad Data Block Format
Type:
A 4-bit value of 0x0 indicating a padding data block.
Padding:
Extends to end of the encapsulating packet.
6.1.4. IKEv2 USE_AGGFRAG Notification Message
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 16442.
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|
+-+-+-+-+-+-+-+-+
Figure 10: USE_AGGFRAG Requirement Flags
0:
6 bits - Reserved MUST be zero on send, unless defined by later
specifications.
C:
Congestion Control bit. If set, then the sender is requiring that
congestion control information MUST be returned to it
periodically, as defined in Section 3.
D:
Don't Fragment bit. If set, it 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 its USE_AGGFRAG notification.
7. IANA Considerations
7.1. ESP Next Header Value
IANA has allocated an IP protocol number from the "Protocol Numbers -
Assigned Internet Protocol Numbers" registry as follows.
Decimal: 144
Keyword: AGGFRAG
Protocol: AGGFRAG encapsulation payload for ESP
Reference: RFC 9347
7.2. AGGFRAG_PAYLOAD Sub-Types
IANA has created a registry called "AGGFRAG_PAYLOAD Sub-Types" under
a new category named "ESP AGGFRAG_PAYLOAD". The registration policy
for this registry is "Expert Review" [RFC8126] [RFC7120].
Name: AGGFRAG_PAYLOAD Sub-Types
Description: AGGFRAG_PAYLOAD Payload Formats
Reference: RFC 9347
This initial content for this registry is as follows:
+==========+===============================+===========+
| Sub-Type | Name | Reference |
+==========+===============================+===========+
| 0 | Non-Congestion-Control Format | RFC 9347 |
+----------+-------------------------------+-----------+
| 1 | Congestion Control Format | RFC 9347 |
+----------+-------------------------------+-----------+
| 3-255 | Reserved | |
+----------+-------------------------------+-----------+
Table 1: AGGFRAG_PAYLOAD Sub-Types
7.3. USE_AGGFRAG Notify Message Status Type
IANA has allocated a status type USE_AGGFRAG from the "IKEv2 Notify
Message Types - Status Types" registry.
Decimal: 16442
Name: USE_AGGFRAG
Reference: RFC 9347
8. Security Considerations
This document describes an aggregation and fragmentation mechanism to
efficiently implement TFC for IP traffic. This approach is expected
to reduce the efficacy of traffic analysis on IPsec communication.
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 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[AppCrypt] Schneier, B., "Applied Cryptography: Protocols,
Algorithms, and Source Code in C", 1996.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[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,
<https://www.rfc-editor.org/info/rfc4342>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
[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,
<https://www.rfc-editor.org/info/rfc5348>.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <https://www.rfc-editor.org/info/rfc6040>.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.
[RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
"Encapsulating MPLS in UDP", RFC 7510,
DOI 10.17487/RFC7510, April 2015,
<https://www.rfc-editor.org/info/rfc7510>.
[RFC7893] Stein, Y(J)., Black, D., and B. Briscoe, "Pseudowire
Congestion Considerations", RFC 7893,
DOI 10.17487/RFC7893, June 2016,
<https://www.rfc-editor.org/info/rfc7893>.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/info/rfc8084>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
[RFC8546] Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
2019, <https://www.rfc-editor.org/info/rfc8546>.
[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
[RFC9329] Pauly, T. and V. Smyslov, "TCP Encapsulation of Internet
Key Exchange Protocol (IKE) and IPsec Packets", RFC 9329,
DOI 10.17487/RFC9329, November 2022,
<https://www.rfc-editor.org/info/rfc9329>.
[RFC9348] Fedyk, D. and C. Hopps, "A YANG Data Model for IP Traffic
Flow Security", RFC 9348, DOI 10.17487/RFC9348, January
2023, <https://www.rfc-editor.org/info/rfc9348>.
[RFC9349] Fedyk, D. and E. Kinzie, "Definitions of Managed Objects
for IP Traffic Flow Security", RFC 9349,
DOI 10.17487/RFC9349, January 2023,
<https://www.rfc-editor.org/info/rfc9349>.
Appendix A. Example of an Encapsulated IP Packet Flow
Below, an example inner IP packet flow within the encapsulating
tunnel packet stream is shown. Notice how encapsulated IP packets
can start and end anywhere, and more than one or less than one may
occur in a single encapsulating packet.
Offset: 0 Offset: 100 Offset: 2000 Offset: 600
[ ESP1 (1404) ][ ESP2 (1404) ][ ESP3 (1404) ][ ESP4 (1404) ]
[--750--][--750--][60][-240-][--3000----------------------][pad]
Figure 11: Inner and Outer Packet Flow
Each outer encapsulating ESP space is a fixed size of 1404 octets,
the first 4 octets of which contain the AGGFRAG header. The
encapsulated IP packet flow (lengths include the IP header and
payload) is as follows: a 750-octet packet, a 750-octet packet, a
60-octet packet, a 240-octet packet, and a 3000-octet packet.
The BlockOffset values in the 4 AGGFRAG payload headers for this
packet flow would thus be: 0, 100, 2000, and 600, 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's (ESP2) 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 3000-octet data block, so
the offset points past its end and into the fourth encapsulating
packet. The fourth packet's (ESP4) offset is 600, pointing at the
padding that follows the completion of the continued 3000-octet
packet.
Appendix B. A Send and Loss Event Rate Calculation
The current best practice indicates that congestion control SHOULD be
done in a TCP-friendly way. A TCP-friendly congestion control
algorithm is described in [RFC5348]. For this IP-TFS use case (as
with [RFC4342]), the (fixed) packet size is used as the segment size
for the algorithm. The main formula in the algorithm for the send
rate is then as follows:
1
X = -----------------------------------------------
R * (sqrt(2*p/3) + 12*sqrt(3*p/8)*p*(1+32*p^2))
X is the send rate in packets per second, R is the RTT 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.
Appendix C. Comparisons of IP-TFS
C.1. Comparing Overhead
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.
C.1.1. IP-TFS Overhead
For comparison, the overhead of an 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 (fractions 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% |
+-------+--------+--------+--------+
Table 2: 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., original 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 Payload 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
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 | 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 |
+--------+---------+---------+---------+--------+--------+--------+
Table 3: 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 | 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% |
+-------+---------+---------+----------+--------+--------+--------+
Table 4: Overhead as Percentage of Inner Packet Size
C.3. Comparing Available Bandwidth
Another way to compare the two solutions is to look at the amount of
available bandwidth each solution provides. The following sections
consider and compare the percentage of available bandwidth. For the
sake of providing a well-understood baseline, normal (unencrypted)
Ethernet and normal ESP values are included.
C.3.1. Ethernet
In order to calculate the available bandwidth, the per-packet
overhead is calculated first. The total overhead of Ethernet is 14+4
octets of header and Cyclic Redundancy Check (CRC) plus an 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 |
+----+-------+-------+-------+-------+-------+-------+------+------+
Table 5: L2 Octets Per Packet
+====+=======+=======+======+=======+=======+=======+=======+=======+
|Size| E + P | E + | E + | IPTFS | IPTFS | IPTFS | Enet | ESP |
| | | P | P | | | | | |
+====+=======+=======+======+=======+=======+=======+=======+=======+
|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 |
+----+-------+-------+------+-------+-------+-------+-------+-------+
Table 6: Packets Per Second on 10G Ethernet
+====+======+======+======+======+======+========+========+========+
|Size|E + P |E + P |E + P |IP-TFS|IP-TFS| IP-TFS | Enet | ESP |
+====+======+======+======+======+======+========+========+========+
|MTU |590 |1514 |9014 |590 |1514 | 9014 | any | any |
+====+======+======+======+======+======+========+========+========+
|OH |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% |
+----+------+------+------+------+------+--------+--------+--------+
Table 7: Percentage of Bandwidth on 10G Ethernet
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 plain
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.
+======+=========+=========+=========+=========+
| Size | ESP+Pad | ESP+Pad | IP-TFS | IP-TFS |
+======+=========+=========+=========+=========+
| MTU | 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 |
+------+---------+---------+---------+---------+
Table 8: 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 plain Ethernet, ESP with
padding often greatly reduces available bandwidth.
Acknowledgements
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.
Contributors
The following person made significant contributions to this document.
Lou Berger
LabN Consulting, L.L.C.
Email: lberger@labn.net
Author's Address
Christian Hopps
LabN Consulting, L.L.C.
Email: chopps@chopps.org