Internet DRAFT - draft-ietf-ipsecme-iptfs
draft-ietf-ipsecme-iptfs
Network Working Group C. Hopps
Internet-Draft LabN Consulting, L.L.C.
Intended status: Standards Track 4 September 2022
Expires: 8 March 2023
IP-TFS: Aggregation and Fragmentation Mode for ESP and its Use for IP
Traffic Flow Security
draft-ietf-ipsecme-iptfs-19
Abstract
This document describes a mechanism for aggregation and fragmentation
of IP packets when they are being encapsulated in ESP payloads. 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at 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 8 March 2023.
Copyright Notice
Copyright (c) 2022 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.
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology & Concepts . . . . . . . . . . . . . . . . . 4
2. The AGGFRAG Tunnel . . . . . . . . . . . . . . . . . . . . . 4
2.1. Tunnel Content . . . . . . . . . . . . . . . . . . . . . 5
2.2. Payload Content . . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Data Blocks . . . . . . . . . . . . . . . . . . . . . 6
2.2.2. End Padding . . . . . . . . . . . . . . . . . . . . . 7
2.2.3. Fragmentation, Sequence Numbers and All-Pad
Payloads . . . . . . . . . . . . . . . . . . . . . . 7
2.2.4. Empty Payload . . . . . . . . . . . . . . . . . . . . 9
2.2.5. IP Header Value Mapping . . . . . . . . . . . . . . . 9
2.2.6. IPv4 Time-To-Live (TTL), IPv6 Hop Limit, and ICMP
Messages . . . . . . . . . . . . . . . . . . . . . . 10
2.2.7. Effective MTU of the Tunnel . . . . . . . . . . . . . 11
2.3. Exclusive SA Use . . . . . . . . . . . . . . . . . . . . 11
2.4. Modes of Operation . . . . . . . . . . . . . . . . . . . 11
2.4.1. Non-Congestion-Controlled Mode . . . . . . . . . . . 11
2.4.2. Congestion-Controlled Mode . . . . . . . . . . . . . 12
2.5. Summary of Receiver Processing . . . . . . . . . . . . . 14
3. Congestion Information . . . . . . . . . . . . . . . . . . . 15
3.1. ECN Support . . . . . . . . . . . . . . . . . . . . . . . 16
4. Configuration of AGGFRAG Tunnels for IP-TFS . . . . . . . . . 16
4.1. Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2. Fixed Packet Size . . . . . . . . . . . . . . . . . . . . 17
4.3. Congestion Control . . . . . . . . . . . . . . . . . . . 17
5. IKEv2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1. USE_AGGFRAG Notification Message . . . . . . . . . . . . 17
6. Packet and Data Formats . . . . . . . . . . . . . . . . . . . 18
6.1. AGGFRAG_PAYLOAD Payload . . . . . . . . . . . . . . . . . 18
6.1.1. Non-Congestion Control AGGFRAG_PAYLOAD Payload
Format . . . . . . . . . . . . . . . . . . . . . . . 18
6.1.2. Congestion Control AGGFRAG_PAYLOAD Payload Format . . 19
6.1.3. Data Blocks . . . . . . . . . . . . . . . . . . . . . 21
6.1.4. IKEv2 USE_AGGFRAG Notification Message . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
7.1. ESP Next Header Value . . . . . . . . . . . . . . . . . . 24
7.2. AGGFRAG_PAYLOAD Sub-Type Registry . . . . . . . . . . . . 24
7.3. USE_AGGFRAG Notify Message Status Type . . . . . . . . . 25
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 25
8.1. Reference Implementation - VPP + Strongswan . . . . . . . 26
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8.2. In Progress Linux Kernel Implementation. . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 27
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
10.1. Normative References . . . . . . . . . . . . . . . . . . 28
10.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Example of An Encapsulated IP Packet Flow . . . . . 30
Appendix B. A Send and Loss Event Rate Calculation . . . . . . . 31
Appendix C. Comparisons of IP-TFS . . . . . . . . . . . . . . . 32
C.1. Comparing Overhead . . . . . . . . . . . . . . . . . . . 32
C.1.1. IP-TFS Overhead . . . . . . . . . . . . . . . . . . . 32
C.1.2. ESP with Padding Overhead . . . . . . . . . . . . . . 33
C.2. Overhead Comparison . . . . . . . . . . . . . . . . . . . 33
C.3. Comparing Available Bandwidth . . . . . . . . . . . . . . 34
C.3.1. Ethernet . . . . . . . . . . . . . . . . . . . . . . 34
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 36
Appendix E. Contributors . . . . . . . . . . . . . . . . . . . . 36
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 36
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 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.
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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, 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.
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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.
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 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 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.
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 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 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.
2.2.1. Data Blocks
+---------------------------------------------------------------+
| Type | rest of IPv4, IPv6 or pad.
+--------
Figure 2: Layout of a DataBlock
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
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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
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 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
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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 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 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.
A note on BlockOffset values, senders MUST encode the BlockOffset
consistent 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 native length encoding of 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 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
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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.
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 non-IKEv2 use cases.
2.2.5. IP Header Value Mapping
[RFC4301] provides some direction on when and how to map various
values from an inner IP header to the outer encapsulating header,
namely the Don't-Fragment (DF) bit ([RFC0791] and [RFC8200]), the
Differentiated Services (DS) field [RFC2474] and the Explicit
Congestion Notification (ECN) field [RFC3168]. Unlike [RFC4301],
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
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.
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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]
and [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
[RFC4301] specifies how to modify the inner packet IPv4 TTL [RFC0791]
or IPv6 Hop Limit [RFC8200].
[RFC4301] also 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 one
exception 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 encapsultation.
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 which cannot fit in it's own outer packet payload will
generate the appropriate ICMP "too-big" error as directed by
[RFC4301], and IPv4 packets without DF set will be IP fragmented as
directed by [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].
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2.2.7. Effective MTU of the Tunnel
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].
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-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 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
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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 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.
Non-congestion-controlled mode is also appropriate if ESP over TCP is
in use [RFC8229]. However, the use of TCP is considered a highly
non-preferred, and a fall-back only solution for IPsec. 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.
[RFC4342] provides an example of the [RFC5348] algorithm 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].
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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.
2.4.2.1. Circuit Breakers
In additional to congestion control, implementations that support
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 inellastic traffic.
One example of a simple slow-trip circuit breaker (CB) an
implementation may provide would utilize 2 values, the amount of
persistent loss rate required to trip the CB, and the required length
of time this persistent loss rate must be seen to trip the CB. These
2 value are required configuration from the user. When the CB is
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tripped the tunnel traffic is disabled, and an appropriate log
message or other management type alarm is triggered indicating
operate 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 transmits 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 re-order window, and the remaining outer packets in the re-
order 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 Section 2.4.2 and Section 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.
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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 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 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 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).
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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, 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.
[RFC3168] and [RFC4301], updated by [RFC6040] defines behaviors for
marking the outer ECN field value based on the ECN field of the inner
packet. As 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
[I-D.ietf-ipsecme-yang-iptfs] and [I-D.ietf-ipsecme-mib-iptfs].
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4.1. Bandwidth
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.
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 USE_TRANSPORT notification.
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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.
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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 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.
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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 CC 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.
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RTT:
A 22-bit value specifying the sender's current round-trip time
estimate in microseconds. The value MAY be zero prior to the
sender having calculated a round-trip time estimate. The value
SHOULD be set to zero on non-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 IPTFS 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
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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).
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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.
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D:
Don't Fragment bit. If set, indicates the sender of the notify
message does not support receiving packet fragments (i.e., inner
packets MUST be sent using a single Data Block). This value only
applies to what the sender is capable of receiving; the sender MAY
still send packet fragments unless similarly restricted by the
receiver in its USE_AGGFRAG notification.
7. IANA Considerations
7.1. ESP Next Header Value
Per the INT area directors direction, this document requests IANA
allocate an IP protocol number from "Protocol Numbers - Assigned
Internet Protocol Numbers" registry
Decimal:
144
Keyword:
AGGFRAG
Protocol:
AGGFRAG encapsulation payload for ESP (TEMPORARY - registered
2022-08-26, document sent to IESG Evaluation 2022-07-14)
Reference:
This document
7.2. AGGFRAG_PAYLOAD Sub-Type Registry
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]).
Name:
AGGFRAG_PAYLOAD Sub-Type Registry
Description:
AGGFRAG_PAYLOAD Payload Formats.
Reference:
This document
This initial content for this registry is as follows:
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Sub-Type Name Reference
--------------------------------------------------------
0 Non-Congestion Control Format This document
1 Congestion Control Format This document
3-255 Reserved
7.3. USE_AGGFRAG Notify Message Status Type
This document requests a status type USE_AGGFRAG be allocated from
the "IKEv2 Notify Message Types - Status Types" registry.
Decimal:
16442
Name:
USE_AGGFRAG
Reference:
This document
8. Implementation Status
[ RFC Ed.: please remove this entire section as well as the reference
to RFC7942 prior to publication. ]
[Section added during IESG review to help with evaluation]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. This is not intended
as, and must not be construed to be, a catalog of available
implementations or their features. Readers are advised to note that
other implementations may exist.
According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
Currently the author and contributors are aware of 1 full and
completed implementation and 1 underway implementation of IP-TFS as
defined in this document. These 2 are described below.
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8.1. Reference Implementation - VPP + Strongswan
The entire IP-TFS protocol including congestion control mode has been
implemented in VPP (Vector Packet Processor), and published to github
with an Open Source (Apache 2) License. VPP is a highly efficient
forwarding plane implemented in user-space utlizing direct control
and polling of physical devices to provide high speed low-latency
forwarding in Linux. By pinning packet processing threads directly
to CPU cores for their exclusive use a high degree of control is
given to the protocol designer.
The IKEv2 additions were implemented in Strongswan and are licensed
using the GNU public license used by the Strongswan project.
Finally, an extensive automation suite was also created and is
included with the open source implementation, which tests the
functionality as well as the performance of the implementation, and
most importantly verifies, through precise timing tracing and time-
stamping, the decoupling of the users offered load from the tunnel
packets (i.e., the Traffic Flow Security).
The verification process utilized the TREX (https://trex-
tgn.cisco.com/) packet generator for high bandwidth testing as well
as other tools such as iperf. The test hardware included large
servers with 10GE, 40GE and 100GE network interfaces, as well as
small SoC (system on a chip) network appliances, and also cloud
deployments.
Tested IP-TFS tunnel rates ranged from 10M all the way to 10GE on the
small network appliance, for the large servers multiple 10GE tunnel
rates were tested as well.
Offered loads included partial, full and oversubscribed bandwidths
from various flow types consisting of small packets, large packets,
random sized packets, sequential sized packets, and multiple IMIX
variations sized flows. Timing analysis was done with variable rate
traffic, impulse traffic and random bursty traffic.
The quality of the reference implementation should be considered
production level as it underwent extensive testing and verification.
The organization responsible for this implementation is LabN
Consulting, L.L.C.
URLs to the implementation follow.
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* VPP+IPTFS (https://github.com/LabNConsulting/vpp/tree/labn-
stable/2009-public), iptfs plugin
(https://github.com/LabNConsulting/vpp/tree/labn-stable/2009-
public/src/plugins/iptfs)
* Strongswan IKEv2
(https://github.com/LabNConsulting/strongswan/tree/labn-
5.8-public)
The implementation was last updated April, 2021.
8.2. In Progress Linux Kernel Implementation.
A second open source implementation has begun by LabN Consulting
L.L.C., within the Linux IPsec xfrm stack. Development has also been
coordinated with the Linux IPsec community, and was being worked by
the same during the most recent IETF 114 hackathon.
Currently the quality is alpha level with aggregation-only complete
and fragmentation support underway with congestion control to follow.
This implementation is licensed under the GNU public license and can
be found at the following URLs
* development environment: https://github.com/LabNConsulting/iptfs-
dev
* linux kernel source: https://github.com/LabNConsulting/iptfs-linux
* iproute2 source: https://github.com/LabNConsulting/iptfs-iproute2
9. 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.
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10. References
10.1. 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>.
10.2. Informative References
[AppCrypt] Schneier, B., "Applied Cryptography: Protocols,
Algorithms, and Source Code in C", 1 November 2017.
[I-D.ietf-ipsecme-mib-iptfs]
Fedyk, D. and E. Kinzie, "Definitions of Managed Objects
for IP Traffic Flow Security", Work in Progress, Internet-
Draft, draft-ietf-ipsecme-mib-iptfs-03, 18 November 2021,
<https://www.ietf.org/archive/id/draft-ietf-ipsecme-mib-
iptfs-03.txt>.
[I-D.ietf-ipsecme-yang-iptfs]
Fedyk, D. and C. Hopps, "A YANG Data Model for IP Traffic
Flow Security", Work in Progress, Internet-Draft, draft-
ietf-ipsecme-yang-iptfs-10, 31 August 2022,
<https://www.ietf.org/archive/id/draft-ietf-ipsecme-yang-
iptfs-10.txt>.
[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>.
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[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>.
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[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>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[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>.
[RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
August 2017, <https://www.rfc-editor.org/info/rfc8229>.
[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>.
Appendix A. Example of An Encapsulated IP Packet Flow
Below, an example inner IP packet flow within the encapsulating
tunnel packet stream is shown. Notice how encapsulated IP packets
can start and end anywhere, and more than one or less than 1 may
occur in a single encapsulating packet.
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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 ESPupayload space is a fixed-size of 1404
octets the first 4 octets of which contains the AGGFRAG header. The
encapsulated IP packet flow (lengths include IP header and payload)
is as follows: a 750-octet packet, a 750-octet packet, a 60-octet
packet, a 240-octet packet, a 3000-octet packet.
The BlockOffset values in the 4 AGGFRAG payload headers for this
packet flow would thus be: 0, 100, 2000, 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 which 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))
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
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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%
Figure 12: IP-TFS Overhead as Percentage of Inner Packet Size
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C.1.2. ESP with Padding Overhead
The overhead per inner packet for constant-send-rate padded ESP
(i.e., traditional IPsec TFC) is 36 octets plus any padding, unless
fragmentation is required.
When fragmentation of the inner packet is required to fit in the
outer IPsec packet, overhead is the number of outer packets required
to carry the fragmented inner packet times both the inner IP overhead
(20) and the outer packet overhead (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.
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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
Figure 13: 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%
Figure 14: Overhead as Percentage of Inner Packet Size
C.3. Comparing Available Bandwidth
Another way to compare the two solutions is to look at the amount of
available bandwidth each solution provides. The following sections
consider and compare the percentage of available bandwidth. For the
sake of providing a well-understood baseline normal (unencrypted)
Ethernet as well as normal ESP values are included.
C.3.1. Ethernet
In order to calculate the available bandwidth the per packet overhead
is calculated first. The total overhead of Ethernet is 14+4 octets
of header and CRC plus 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.
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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
Figure 15: L2 Octets Per Packet
Size E + P E + P E + P IPTFS IPTFS IPTFS Enet ESP
MTU 590 1514 9014 590 1514 9014 any any
OH 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
Figure 16: Packets Per Second on 10G Ethernet
Size E + P E + P E + P IPTFS IPTFS IPTFS Enet ESP
590 1514 9014 590 1514 9014 any any
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%
Figure 17: Percentage of Bandwidth on 10G Ethernet
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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
Figure 18: Added Latency
Notice that the latency values are very similar between the two
solutions; however, whereas IP-TFS provides for constant high
bandwidth, in some cases even exceeding native Ethernet, ESP with
padding often greatly reduces available bandwidth.
Appendix D. Acknowledgements
We would like to thank Don Fedyk for help in reviewing and editing
this work. 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.
Appendix E. Contributors
The following people made significant contributions to this document.
Lou Berger
LabN Consulting, L.L.C.
Email: lberger@labn.net
Author's Address
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Christian Hopps
LabN Consulting, L.L.C.
Email: chopps@chopps.org
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