TCP Maintenance and Minor Extensions R. Scheffenegger
(tcpm) NetApp, Inc.
Internet-Draft M. Kuehlewind
Updates: 1323 (if approved) University of Stuttgart
Intended status: Experimental October 31, 2011
Expires: May 3, 2012
Additional negotiation in the TCP Timestamp Option field
during the TCP handshake
draft-scheffenegger-tcpm-timestamp-negotiation-03
Abstract
A number of TCP enhancements in so diverse fields as congestion
control, loss recovery or side-band signaling could be improved by
allowing both ends of a TCP session to interpret the values carried
in the Timestamp option. Further enhancements are enabled by
changing the receiver side processing of timestamps in the presence
of Selective Acknowledgements.
This documents updates RFC1323 and specifies a backwards compatible
way of negotiating for Timestamp capabilities, and lists a number of
benefits and drawbacks of this approach.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Task Force (IETF). Note that other groups may also distribute
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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 May 3, 2012.
Copyright Notice
Copyright (c) 2011 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Overview of the TCP Timestamp Option . . . . . . . . . . . 6
3.2. Overview of the Timestamp Capabilities . . . . . . . . . . 7
4. Problem statement . . . . . . . . . . . . . . . . . . . . . . 8
5. Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Capability Flags . . . . . . . . . . . . . . . . . . . . . 10
5.2. Version 0 specific fields . . . . . . . . . . . . . . . . 11
5.3. Timestamp Capability Negotiation . . . . . . . . . . . . . 15
5.3.1. Implicit extended negotiation . . . . . . . . . . . . 16
5.3.2. Interaction with the Retransmission Timer . . . . . . 17
6. Possible use cases . . . . . . . . . . . . . . . . . . . . . . 18
6.1. One-way delay variation measurement . . . . . . . . . . . 18
6.2. Early spurious retransmit detection . . . . . . . . . . . 19
6.3. Early lost retransmission detection . . . . . . . . . . . 20
6.4. Integrity of the Timestamp value . . . . . . . . . . . . . 22
6.5. Disambiguation with slow Timestamp clock . . . . . . . . . 22
6.6. Masked timestamps as segment digest . . . . . . . . . . . 23
6.7. Timestamp value as covert channel . . . . . . . . . . . . 23
7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 25
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
9. Updates to Existing RFCs . . . . . . . . . . . . . . . . . . . 26
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11. Security Considerations . . . . . . . . . . . . . . . . . . . 28
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
12.1. Normative References . . . . . . . . . . . . . . . . . . . 28
12.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. Possible Extension . . . . . . . . . . . . . . . . . 30
A.1. Capability Flags . . . . . . . . . . . . . . . . . . . . . 31
A.2. Range Negotiation . . . . . . . . . . . . . . . . . . . . 32
Appendix B. Revision history . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
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1. Introduction
The timestamp option originally introduced in [RFC1323] was designed
solely for two-way delay measurement and to support a particular TCP
algorithm (Reno). It would be useful to be able to support one-way
delay measurement and to take advantage of developments since TCP
Reno, such as selective acknowledgements (SACK) [RFC2018].
This specification defines a protocol for the two ends of a TCP
session to negotiate alternative semantics for the timestamps they
will exchange during the rest of the session. It updates RFC1323 but
it is backwards compatible with implementations of RFC1323 timestamp
options.
The RFC1323 timestamp protocol presents the following problems when
trying to extend it for alternative uses:
a. Unclear meaning of the value in a timestamp.
* A timestamp value (TSval) as defined in [RFC1323] is
deliberately only meaningful to the end that sends it. The
other end is merely meant to echo the value without
understanding it. This is fine if one end is trying to
measure two-way delay (round trip time). However, to measure
one-way delay, timestamps from both ends need to be compared
by one end, which needs to relate the values in timestamps
from both ends to a notion of the passage of time that both
ends share.
b. No control over which timestamp to echo.
* A host implementing [RFC1323] is meant to echo the timestamp
value of the most recent in-order segment received. This was
fine for TCP Reno, but it is not the best choice for TCP
sessions using selective acknowledgement (SACK) [RFC2018].
* A [RFC1323] host is meant to echo the timestamp value of the
earliest unacknowledged segment, e.g. if a host delays ACKs
for one segment, it echoes the first timestamp not the second.
It is desirable to include delay due to ACK withholding when a
host is conservatively measuring RTT. However, is not useful
to include the delay due to ACK withholding when measuring
one-way delay.
c. Alternative protection against wrapped sequence numbers.
* [RFC1323] also points out that the timestamps it specifies
will always strictly monotonically increase in each window so
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they can be used to protect against wrapped sequence numbers
(PAWS). If the endpoints negotiate an alternative timestamp
scheme in which timestamps may not monotonically increase per
window, then it needs to be possible to negotiate alternative
protection against wrapped sequence numbers.
To solve these problems this specification changes the wire protocol
of the TCP timestamp option in two main ways:
1. It updates [RFC1323] to add the ability to negotiate the
semantics of timestamp options. The initiator of a TCP session
starts the negotiation in the TSecr field in the first <SYN>,
which is currently unused. This specification defines the
semantics of the TSecr field in a segment with the SYN flag set.
A version number is included to allow further extension of
capability negotiation in future.
2. A version independent ability to mask a specified number of the
lower significant bits of the timestamp values is present. These
masked bits are not considered for timestamp calculations, or in
an algorithm to protect against wrapped sequence numbers. Future
extensions can thereby change the timestamp signaling without
changing the modified treatment on the receiver side.
3. It updates [RFC1323] to define version 0 of timestamp
capabilities to include:
* the duration in seconds of a tick of the timestamp clock using
a floating point representation
* agreement that both ends will echo the timestamp on the most
recently received segment, rather than the one that would be
echoed by an [RFC1323] host. There is no specific option to
request this behavior, however it is implied by successful
negotiation of both SACK and timestamp capabilities.
With this new wire protocol, a number of new use-cases for the TCP
timestamp option become possible. Section 6 gives some examples.
Further extensions might be required in future. Appendix A gives an
example of a further version of timestamp capability negotiation that
could be defined in the future.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The reader is expected to be familiar with the definitions given in
[RFC1323].
Further terminology used within this document:
Timestamp clock interval
The Timestamp value is derived from a clock source running at a
reasonable constant frequency. The interval between two ticks of
that clock is signaled during the timestamp capability
negotiation. Note that the timestamp clock is not required to be
identical with the TCP clock, even though most implementations
use the same clock for practical purposes.
Timestamp option
This refers to the entire TCP timestamp option, including both
TSval and TSecr fields.
Timestamp capabilities
Refers only to the values and bits carried in the TSecr field of
<SYN> and <SYN,ACK> segments during a TCP handshake. For
signaling purposes, the timestamp capabilities are sent in clear
with the <SYN> segment, and in an encoded form (see Section 5 for
details) in the <SYN,ACK> segment.
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3. Overview
3.1. Overview of the TCP Timestamp Option
The TCP Timestamp option (TSopt) provides timestamp echoing for
round-trip time (RTT) measurements. TSopt is widely deployed and
activated by default in many systems. [RFC1323] specifies TSopt the
following way:
Kind: 8
Length: 10 bytes
+-------+-------+---------------------+---------------------+
|Kind=8 | 10 | TS Value (TSval) |TS Echo Reply (TSecr)|
+-------+-------+---------------------+---------------------+
1 1 4 4
Figure 1: RFC1323 TSopt
"The Timestamps option carries two four-byte timestamp fields.
The Timestamp Value field (TSval) contains the current value of
the timestamp clock of the TCP sending the option.
The Timestamp Echo Reply field (TSecr) is only valid if the ACK
bit is set in the TCP header; if it is valid, it echos a times-
tamp value that was sent by the remote TCP in the TSval field of a
Timestamps option. When TSecr is not valid, its value must be
zero. The TSecr value will generally be from the most recent
Timestamp option that was received; however, there are exceptions
that are explained below.
A TCP may send the Timestamps option (TSopt) in an initial <SYN>
segment (i.e., segment containing a SYN bit and no ACK bit), and
may send a TSopt in other segments only if it received a TSopt in
the initial <SYN> segment for the connection."
The comparison of the timestamp in the TSecr field to the current
timestamp clock gives an estimation of the two-way delay (RTT). With
[RFC1323] the receiver is not supposed to interpret the TSVal field
for timing purposes, e.g. one-way delay measurments, but only to echo
the content in the TSecr field. [RFC1323] specifies various cases
when more than one timestamp is available to echo. The approach
taken by [RFC1323] is not always be the best choice, i.e. when the
TCP Selective Acknowledgment option (SACK) is used in conjunction.
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3.2. Overview of the Timestamp Capabilities
This document specifies a way of negotiating the timestamp
capabilities available between the end hosts. This is enabled by
using the TSecr field in the TCP <SYN> segment. In order to remain
backwards compatible, a receiver capable of timestamp capability
negotiation has to XOR the receivers (local) capabilities flags with
the received TSval, before echoing the result back in the TSecr
field. During the initial handshake, the sender has to store the
sent initial TSval, in order to determine if the receiver can support
this timestamp capability negotiation.
As there exist some benefit to change the receiver side treatment of
which timestamp value to echo, the negotiation protocol itself must
also provide some backwards compatibility. Therefore, even when a
sender tries to negotiate for a higher version than supported by the
receiver, the receiver MUST respond with at least version 0. Also, a
future protocol enhancement MUST make sure that any extension is
compatible with at least version 0.
As the importance of the timestamp option increases by using it in
more aspects of a TCP sender's operation e.g. congestion control, so
increases the importance of maintaining the integrity of the
reflected timestamps. At the same time this must not inhibit the
receiver to interpret a received timestamp in TSval.
This is achieved by indicating how many LSB bits of the timestamp
value MUST NOT be interpreted by the receiver. Apart from the
purpose of maintaining timestamp integrity for the use as input
signal into congestion control algorithms, this also allows the use
of timestamp based methods to discriminate at the earliest possible
moment (within 1 RTT after the retransmission) between spurious
retransmissions and genuine loss even when using slow running TCP
timestamp clocks.
In addition, by using synergistic signaling between timestamps
[RFC1323] and selective acknowledgments [RFC2018], enhancements in
loss recovery are possible by removing any remaining retransmission
and acknowledgment ambiguity. See Section 6 for a detailed
discussion.
As an optional extension, a timestamp clock interval range
negotiation is also briefly introduced in Appendix A. This is only
included as one potential example of further enhancements.
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4. Problem statement
Timestamp values are carried in each segment if negotiated for.
However, the content of this values is to be treated as an unmutable
and largely uninterpreted entity by the receiver. This document
describes an enhancement to the timestamp negotiation, and must meet
the following criteria:
o Indicate the (approximate) timestamp clock interval used by the
sender in a wide range. The longest interval should be around 10
seconds, while the shorted interval should allow unique timestamps
per segment, even at extremely high link speeds. At the time of
writing, the shortest meaningful duration was found to be a 64
byte packets (i.e. ACK segment) sent at a rate of 100 Gbit/s.
This corresponds to a maximum timestamp clock rate of around 200
MHz, or an interval between clock ticks of around 5 ns.
o Allow for timestamps that are not directly related to real time
(i.e. segment counting, or use of the timestamp value as a true
extension of sequence numbers).
o Provide means to prevent or at least detect tampering with the
echoed timestamp value, allowing for basic integrity and
consistency checks.
o Allow for future extensions that may use some of the timestamp
value bits for other signaling purposes during the remainder of
the session.
o Signaling must be backwards compatible with existing TCP stacks
implementing basic [RFC1323] timestamps. Current methods for
timestamp value generation must be supported.
o Allow to state timing information explicitly during the initial
handshake, to avoid a training phase extending beyond the initial
handshake.
o Provide a means to disambiguate between resent <SYN> segments.
o Cater for broken implementations, that either send a non-zero
TSecr value in the initial <SYN>, or a zero TSecr value in
<SYN,ACK>.
Some legacy implementations exist that violate [RFC1323] in that the
TSecr field in a <SYN> is not cleared (see
[I-D.ietf-tcpm-tcp-security]. The protocol should have some
resiliency in the presence of such misbehaving senders, and must not
lead to an unfair advantage for such wrongly negotiated sessions.
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As there exist some benefit to change the receiver side treatment of
which timestamp value to echo, the negotiation protocol itself must
also provide some backwards compatibility. Therefore, even when a
sender tries to negotiate for a higher version than supported by the
receiver, the receiver MUST respond with at least version 0. Also, a
future protocol enhancement MUST make sure that any extension is
compatible with at least version 0.
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5. Signaling
5.1. Capability Flags
In order to signal the supported capabilities, both the sender and
the receiver will idependently generate a timestamp capability
negotiation field, as indicated below. The TSecr value field of the
[RFC1323] TSopt is overloaded with the following flags and fields
during the initial <SYN> and <SYN,ACK> segments. The connection
initiator will send the timestamp capabilities in plain, as with
[RFC1323] the TSecr is not used in the inital <SYN>. The receiver
will XOR the local timestamp capabilities with the TSVal received
from the sender and send the result in the TSecr field. The
initiating host of a session with timestamp capability negotiation
has to keep minimal state to decode the returned capabilities XOR'ed
with the sent TSval.
Kind: 8
Length: 10 bytes
+-------+-------+---------------------+---------------------+
|Kind=8 | 10 | TS Value (TSval) |TS Echo Reply (TSecr)|
+-------+-------+---------------------+---------------------+
1 1 4 | 4 |
/ |
.-----------------------------------' |
/ \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| | # |
|X|VER| MSK # version specific contents |
|O| | # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Timestamp Capability flags
Common fields to all versions:
EXO - Extended Options (1 bit)
Indicates that the sender supports extended timestamp
capabilities as defined by this document, and MUST be set to one
by a compliant implementation. This flag also enables the
immediate echoing of the TSval with the next ACK, if both
timestamp capabilities and selective acknowledgement [RFC2018]
are successful negotiated during the initial handshake (see
Section 5.3.1). This change in semantics is independent of the
version in the signaled timestamp capabilities.
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VER - Version (2 bits)
Version of the capabilities fields definition. This document
specifies codepoint 0. With the exception of the immediate
mirroring - simplifying the receiver side processing - and the
masking of some LSB bits before performing the Protection Against
Wrapped Sequence Numbers (PAWS) test, hosts must not interpret
the received timestamps and not use a timestamp value as input
into advanced heuristics, if the version received is not
supported. This is an identical requirement as with current
[RFC1323] compliant implementations. The lower 3 octets of the
timestamp capability flags MUST be ignored if an unsupported
version is received. It is expected, that a host will implement
at least version 0. A receiver MUST respond with the appropriate
(equal or version 0) version when responding to a new session
request.
MSK - Mask Timestamps (5 bits)
The MaSK field indicates how many least significant bits should
be excluded by the receiver, before further processing the
timestamp (i.e. PAWS, or for timing purposes). The unmasked
portion of a TSval has to comply with the constraints imposed by
[RFC1323] on the generation of valid timestamps, e.g. must be
monotone increasing between segments, and strict monotone
increasing for each TCP window. Note that this does not impact
the reflected timestamp in any way - TSecr will always be equal
to an appropriate TSval. This field MUST be present in all
future version of timestamp capability fields. A value of 31
(all bits set) MUST be interpreted by a receiver that the full
TSval is to be ignored by any legacy heuristics, including PAWS.
For PAWS to be effective, at least 2 bits are required to
discriminate between an increase (and roll-over) versus outdated
segments.
5.2. Version 0 specific fields
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Kind: 8
Length: 10 bytes
+-------+-------+---------------------+---------------------+
|Kind=8 | 10 | TS Value (TSval) |TS Echo Reply (TSecr)|
+-------+-------+---------------------+---------------------+
1 1 4 | 4 |
/ |
.-----------------------------------' |
/ \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| | # | | |
|X|VER| MSK # RES | ADJ | INT |
|O| | # | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Timestamp Capability flags - version 0
RES - Reserved (8 bits)
Reserved for future use, and MUST be zero ("0") with version 0.
If timestamp capabilities are received with version set to 0, but
some of these bits set, the receiver MUST ignore the extended
options field and react as if the TSecr was zero (compatibility
mode).
ADJ - Adjustment factor (5 bits)
The scaling factor by which the signaled interval has to be left-
shifted. This is similar to the way the Window Scale option is
defined in [RFC1323]. All values between zero and 31 are valid.
This allows timestamp clock ticks of up to 15.99 s.
See Section 6.1 for details.
INT - Interval (11 bits)
The integer part of the timestamp clock interval can be signaled
with up to 11 bits of precision. This allows a range with the
highest resolution to cover clock intervals between 7.45 ns
(INT=0x400, ADJ=0) and 15.99 s (INT=0x7FF, ADJ=31). If a sender
is using a less precice clock source, fewer significant bits can
be used to implicitly signal this. For example, a timestamp
clock interval of approximately 1 ms (1/1024th sec) can be
represented by both (INT=0x001, ADJ=28) and (INT=0x400, ADJ=18).
A more accurate representation of 1 ms would be (INT=0x418,
ADJ=18). The latter representation carries more significant
bits, indicating a more stable clock source with low jitter.
Only non-zero values are valid when ADJ is non-zero. An invalid
combination of ADJ and INT MUST be treaded as if no timestamp
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capability negotiation is attempted. A compliant sender can
choose the value of the <SYN> TSval in such a way, that either
the EXO bit, or some of the RES bits are set, or all the INT bits
are cleared, in the encoded response from the receiver. A
receiver that does not reflect the initial TSval in it's
<SYN,ACK> and instead sends a zero value in TSecr, will not
erraneously negotiate for timestamp capabilities.
Conceptually, the timestamp clock interval can be represented as a
unsigned integer with 42 bits length. In this form, the least
significant bit represents an interval of 2^-38 sec (3.64 ps), while
still allowing a maximum interval of 16 sec. This value is then
shifted to the right, until it can be represented by only 11
significant bits, and the number of shift operations is stored as
scaling adjustment factor (ADJ).
A value of zero (both ADJ and INT are set to zero) is supported and
indicates, that the timestamp values are NOT correlated to wall-clock
time (i.e. the sender may perform some form of segment counting or
sequence number extension instead). A host receiving an interval of
zero from the other end host MUST NOT perform time-based heuristics
which take the received TSval into account, but SHOULD apply the
regular PAWS test.
Timestamp clock periods faster than 1 ms SHOULD be implemented by
inserting the timestamp "late" before transmitting a segment to avoid
unnecessary timing jitter. Shortest clock periods, with intervals of
only a few microseconds or less, are provided for hardware-assisted
implementations.
The range of possible values runs from 15.99 s to 7.45 ns with
highest precision, and down to 3.64 ps with reducing precision, which
is also the shortest difference in tick duration, that could be
resolved. This equates to clock frequencies of 0.06 Hz, 134 MHz and
275 GHz respectively.
Despite the provision of such a large dynamic range, a receiver
should consider, that a timestamp clock may deviate from the
indicated rate by a large fraction. Similarily, a sender SHOULD
refrain from signaling the clock interval with too much precision
(significan bits), if the clock can not be sampled with low variance
over time.
Example for an timestamp capability negotiation, to indicate that the
senders timestamp clock (tcp clock) is running with 1 ms per tick,
and using a clock source of typical quality (e.g. software timer
interrupt):
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SYN, TSval=<X>, TSecr=EXO|MSK|ADJ=22|INT=0x041
+-----------+------------+--------------------+---------------------+
| tick | tick | encoding at | encoding at lowest |
| interval | frequency | highest precision | precision |
+-----------+------------+--------------------+---------------------+
| 16 s | 0.06 Hz | ADJ=31, INT=0x7FF | ADJ=31, INT=0x7FF |
| 1 s | 1 Hz | ADJ=28, INT=0x400 | ADJ=31, INT=0x080 |
| 0.5 s | 2 Hz | ADJ=27, INT=0x400 | ADJ=31, INT=0x040 |
| 100 ms | 10 Hz | ADJ=24, INT=0x666 | ADJ=31, INT=0x00C |
| 10 ms | 100 Hz | ADJ=21, INT=0x51F | ADJ=31, INT=0x001 |
| 4 ms | 250 Hz | ADJ=20, INT=0x419 | ADJ=30, INT=0x001 |
| 1 ms | 1 kHz | ADJ=18, INT=0x418 | ADJ=28, INT=0x001 |
| 200 us | 5 kHz | ADJ=15, INT=0x68E | ADJ=25, INT=0x001 |
| 50 us | 20 kHz | ADJ=13, INT=0x68E | ADJ=23, INT=0x001 |
| 1 us | 1 MHz | ADJ=8, INT=0x432 | ADJ=18, INT=0x001 |
| 60 ns | 16.7 MHz | ADJ=4, INT=0x407 | ADJ=14, INT=0x001 |
+-----------+------------+--------------------+---------------------+
Table 1: Common used TCP Timestamp Clock intervals
The wide range of indicated timestamp clock intervals (spanning 9
orders of (decimal) magnitude, or 28 binary digits, and the
limitation to no more than 24 bits requires the use of a logarithmic
encoding. Since the precision of the timestamp clock value is most
valuable at low frequencies (long tick durations), the clock rate is
encoded as a time duration. This results in full precision for
common used timestamp clock tick durations, while allowing even
shorter intervals at reduced precision. A format was chosen that is
simple to implement and poses no risk of confusion with common
floating point representations.
The timestamp clock values a host is using must not necessarily run
synchronous with the internal TCP clock. Different clock sources,
such as a NTP stratum, RTC, CPU cycle counters, or other independent
clocks can be used to derive the TSval. This allows the de-coupling
of the coarse-grained TCP clock used for retransmission and delayed
ACK timeouts, from the clock frequency indicated in the TSval itself.
Since [RFC1323] timestamp clocks used to be only useful for RTT
measurement, and calculation of the RTO, the straight forward use of
the TCP timer directly seemed natural to minimize subsequent RTT
calculations.
Most stacks will at first not be able to dynamically adjust their
timestamp clock interval. Therefore, the indicated clock duration
can be a static, compile time value. To use the indicated clock
interval, for example to perform one-way delay variation
calculations, simple integer operations can be used after an initial
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conversion of the wire presentation to longer (i.e. 32 or 64 bit)
integer values.
5.3. Timestamp Capability Negotiation
During the initial TCP three-way handshake, timestamp capabilities
are negotiated using the TSecr field. Timestamp capabilities MAY
only be negotiated in TSecr when the SYN bit is set. A host detects
the presence of timestamp capability flags when the EXO bit is set in
the TSecr field of the received <SYN> segment. When receiving a
session request (<SYN> segment with timestamp capabilities), a
compliant TCP receiver is required to XOR the received TSval with the
receivers timestamp capabilities. The resulting value is then sent
in the <SYN,ACK> response.
To support these design goals stated in Section 4, only the TSecr
field in the initial <SYN> can be used directly. The response from
the receiver has to be encoded, since no unused field is available in
the <SYN,ACK>. The most straightforward encoding is a XOR with a
value that is known to the sender. Therefore, the receiver also uses
TSecr to indicate it's capabilities, but calculates the XOR sum with
the received TSval. This allows the receiver to remain stateless and
functionalities like syncache (see [RFC4987]) can be maintained with
no change.
If a sender has to retransmit the <SYN>, this encoding also allows to
detect which segment was received and responeded to. This is
possible by changing the timestamp clock offset between
retransmissions in such a way, that the decoding on the sender side
would result in an invalid timestamp capability negotiation field
(e.g. some RES bits are set). If the sender does not require the
capability to differentiate which <SYN> was received, the timestamp
clock offset for each new <SYN> can be set in such a way, that the
TSopt of the <SYN> is identical for each retransmission.
As a receiver MAY report back a zero value at any time, in particular
during the <SYN,ACK>, the sender is slightly constrained in it's
selection of an initial Timestamp value. The Timestamp value sent in
the <SYN> should be selected in such a way, that it does not resemble
a valid Timestamp capabilities field. This prevents a compliant
sender to erraneously detect a compliant receiver, if the returned
TSecr value is zero.
A host initiating a TCP session must verify if the partner also
supports timestamp capability negotiation and a supported version,
before using enhanced algorithms. Note that this change in semantics
does not necessarily change the signaling of timestamps on the wire
after initial negotiation.
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To mitigate the effect from misbehaving TCP senders appearing to
negotiate for timestamp capabilities, a receiver MUST verify that one
specific bit (EXO) is set, and any reserved bits (currently 8, RES
field) are cleared. This limits the chance for a receiver to
mistakenly negotiate for version 0 capabilities to around 0.05%.
However, as a receiver has to use changed semantics when reflecting
TSval also for higher values in the version field, a misbehaving
sender negotiating for SACK, but not properly clearing TSecr, may
have a 37.5% chance of receiving timestamp values with modified
receiver behavior. This may lead to an increased number of spurious
retransmission timeouts, putting such a session to a disadvantage.
Once timestamp capabilities are successfully negotiated, the receiver
MUST ignore an indicated number of masked, low-order bits, before
applying the heuristics defined in [RFC1323]. The monotonic increase
of the timestamp value for each new segment could be violated if the
full 32 bit field, including the masked bits, are used. This
conflicts with the constraints imposed by PAWS. The use of generic
(secure) hash algorithms makes it possible to protect the integrity
of the timestamp value, without any compromise to fulfill the PAWS
requirement of monotonic increasing values.
The presented distribution of the common three fields, EXO, VER and
MASK, that MUST be present regardless of which version is implemented
in a compliant TCP stack, is a result of the previously mentioned
design goals. The lower three octets MAY be redefined freely with
subsequent versions of the timestamp capability negotiation protocol.
This allows a future version to be implemented in such a way, that a
receiver can still operate with the modified behavior, and a minimum
amount of processing (PAWS)
5.3.1. Implicit extended negotiation
If both Timestamp capabilities and Selective Acknowledgement options
[RFC2018] are negotiated (both hosts send these options in their
respective segments), both hosts MUST echo the timestamp value of the
last received segment, irrespective of the order of delivery. Note
that this is in conflict with [RFC1323], where only the timestamp of
the last segment received in sequence is mirrored. As SACK allows
discrimination of reordered or lost segments, the reflected
timestamps are not required to convey the most conservative
information. If SACK indicates lost or reordered packets at the
receiver, the sender MUST take appropriate action such as ignoring
the received timestamps for calculating the round trip time, or
assuming a delayed packet (with appropriate handling). An updated
algorithm to calculate the retransmission timeout timer (RTO) is not
discribed in this document.
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The immediate echoing of the last received timestamp value allowed by
the synergistic use of the timestamp option with the SACK option
enables enhancements to improve loss recovery, round trip time (RTT)
and one-way delay (OWD) variation measurements (see Section 6) even
during loss or reordering episodes. This is enabled by removing any
retransmission ambiguity using unique timestamps for every
retransmission, while simultaneously the SACK option indicates the
ordering of received segments even in the presence of ACK loss or
reordering.
The use of RTT and OWD measurements during loss episodes is an open
research topic. A sender has to accomodate for the changed timestamp
semantics in order to maintain a conservative estimate of the
Retransmission Timer as defined in [RFC6298], when a TCP sender has
negotiated for an immideate reflection of the timestamp triggering an
ACK (i.e. both timestamp capability negotiation and Selective
Acknowledgements are enabled for the session). As the presence of a
SACK option in an ACK signals an ongoing reordering or loss episode,
timestamps conveyed in such segments MUST NOT be used to update the
retransmission timeout. Also note that the presence of a SACK option
alleviates the need of the receiver to reflect the last in-order
timestamp, as lost ACKs can no longer cause erraneous updates of the
retransmission timeout.
5.3.2. Interaction with the Retransmission Timer
The above stated rule, to ignore timestamps as soon as a SACK option
is present, is fully consistent with the guidance given in [RFC1323],
even though most implementations skip over such an additional
verification step in the precense of the SACK option.
To address the additional delay imposed by delayed ACKs, a compliant
sender SHOULD modify the update procedure when receiving normal, in-
sequence ACKs that acknowledge more than SMSS bytes, so that the
input (denoted R in [RFC6298]) is calculated as
R = RTT * ( 1 + 1/(cwnd/smss) )
If RTT (as measured in units of the timestamp clock) is smaller than
the congestion window measured in full sized segments, the above
heuristic MAY be bypassed before updating the retransmission timeout
value.
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6. Possible use cases
6.1. One-way delay variation measurement
New congestion control algorithms are currently proposed, that react
on the measured one-way delay variation (i.e.
[I-D.ietf-ledbat-congestion], [Chirp]). This control variable is
updated after each received ACK:
C(t) = TSval(t) - TSecr(t)
V(t) = C(t) - C(t-1)
provided that the timestamp clocks at both ends are running at
roughly the same rate. Without prior knowledge of the timestamp
clock interval used by the partner, a sender can try to learn this
interval by observing the exchanged segments for a duration of a few
RTTs. However, such a scheme fails if the partner uses some form of
implicit integrity check of the timestamp values, which would appear
as either random scrambling of LSB bits in the timestamp, or give the
impression of much shorter clock intervals than what is actually
used. If the partner uses some form of segment counting as timestamp
value, without any direct relationship to the wall-clock time, the
above formula will fail to yield meaningful results. Finally the
network conditions need to remain stable during any such training
phase, so that the sender can arrive at reasonable estimates of the
partners timestamp clock tick duration.
This note addresses these concerns by providing a means by which both
host are required to use a timestamp clock that is closely related to
the wall-clock time, with known clock rate, and also provides means
by which a host can signal the use of a few LSB bits for timestamp
value integrity checks. To arrive at a valid one-way delay (OWD)
variation, first the timestamp received from the partner has to be
right-shifted by a known amount of bits as defined by the mask field.
Next the local and remote timestamp values need to be normalized to a
common base clock interval (typically, the local clock interval):
remote interval
C = (TSecr >> local mask) - (TSval >> remote mask) * ---------------
t local interval
V(t) = C(t) - C(t-1)
The adjustment factor can be calculated once during the timestamp
capability negotiation phase, and pure integer arithmetic can be used
during per-segment processing:
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EXP.min = min(EXP.loc, EXP.rem)
EXP.rem -= EXP.min
EXP.loc -= EXP.min
FRAC.rem = (0x800 | FRAC.rem) << EXP.rem
FRAC.loc = (0x800 | FRAC.loc) << EXP.loc
and assuming that the local clock tick duration is lower
ADJ = FRAC.rem / FRAC.loc
with ADJ being a integer variable. For higher precision, two
appropriately calculated integers can be used.
Any previously required training on the remote clock interval can be
removed, resulting in a simpler and more dependable algorithm.
Furthermore, transient network effects during the training phase
which may result in a wrong inference of the remote clock interval
are eliminated completely.
6.2. Early spurious retransmit detection
Using the provided timestamp negotiation scheme, clients utilizing
slow running timestamp clocks can set aside a small number of least
significant bits in the timestamps. These bits can be used to
differentiate between original and retransmitted segments, even
within the same timestamp clock tick (i.e. when RTT is shorter than
the TCP timestamp clock interval). It is recommended to use only a
single bit (mask = 1), unless the sender can also perform lost
retransmission detection. Using more than 2 bits for this purpose is
discouraged due to the diminishing probability of loosing
retransmitted packets more than one time. A simple scheme could send
out normal data segments with the so masked bits all cleared. Each
advance of the timestamp clock also clears those bits again. When a
segment is retransmitted without the timestamp clock increasing,
these bits increased by one for each consecutive retry of the same
segment, until the maximum value is reached. Newly sent segments
(during the same clock interval) should maintain these bits, in order
to maintain monotonically increasing values, even though compliant
end hosts do not require this property. This scheme maintains
monotonically increasing timestamp values - including the masked
bits. Even without negotiating the immediate mirroring of timestamps
(done by simultaneously doing timestamp capabilities negotiation, and
selective acknowledgments), this extends the use of the Eifel
Detection [RFC3522] and Eifel Response [RFC4015] algorithm to detect
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and react to spurious retransmissions under all circumstances. Also,
currently experimental schemes such as ER-SRTO [Cho08] could be
deployed without requiring the receiver to explicitly support that
capability.
Seg0 Seg1 Seg2 Seg3 Seg4
TS00 TS00 TS00 TS00 TS00
X
Seg1 Seg5
TS01 TS01
Seg6 Seg7
TS01 TS10
Figure 4: timestamp for spurious retranmit detection
Masked bits are the 2nd digit, the timestamp value is represented by
the first digit. The timestamp clock "ticks" between segment 6 and
7.
6.3. Early lost retransmission detection
During phases where multiple segments in short succession (but not
necessarily successive segments) are lost, there is a high likelihood
that at least one segment is retransmitted, while the cause of loss
(i.e. congestion, fading) is still persisting. The best current
algorithms can recover such a lost retransmission with a few
constraints, for example, that the session has to have at least
DupThresh more segments to send beyond the current recovery phase.
During loss recovery, when a retransmission is lost again, currently
the timestamp can also not be used as means of conveying additional
information, to allow more rapid loss recovery while maintaining
packet conservation principles. Only the timestamp of the last
segment preceding the continuous loss will be reflected. Using the
extended timestamp option negotiation together with selective
acknowledgements, the receiver will immediately reflect the timestamp
of the last seen segment. Using both SACK and TS information
synergistically, a sender can infer the exact order in which original
and retransmitted segments are received. This allows a slightly less
conservative and faster approach to retransmit lost retransmitted
segments.
This can be implemented in combination with the masked bit approach
described in the previous paragraph, or without. However, if the
timestamp clock interval is lower than 1/2 RTT, both the original and
the retransmitted segment may carry an identical timestamp. If the
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sender cannot discriminate between the original and the retransmitted
segments, is MUST refrain from taking any action before such a
determination can be made.
In this example, masked bits are used, with a simple marking method.
As the timestamp value of the retransmission itself is already
different from the original segments, such an additional
discrimination would not strictly be required here. The timestamp
clock ticks in the first digit and the dupthresh value is 3.
Seg0 Seg1 Seg2 Seg3 Seg4 Seg5 Seg6 Seg7
TS00 TS10 TS10 TS10 TS10 TS10 TS10 TS20
X X X *
Seg1 Seg2 Seg3 Seg4
TS21 TS30 TS30 TS30
X
Seg1 Seg8 Seg9
TS31 TS31 TS40
Figure 5: timestamp under loss
If Seg1,TS00 is lost twice, and Seg4,TS10 is also lost, the sender
could resend Seg1 once more after seeing dupthresh number of segments
sent after the first retransmission of Seg1 being received (ie, when
Seg4 is SACKed). However, there is a ambiguity between retransmitted
segments and original segments, as the sender cannot know, if a SACK
for one particular segment was due to the retransmitted segment, or a
delayed original segment. The timestamp value will not help in this
case, as per RFC1323 it will be held at TS00 for the entire loss
recovery episode. Therefore, currently a sender has to assume that
any SACKed segments may be due to delayed original sent segments, and
can only resolve this conflict by injecting additional, previously
unsent segments. Once dupthresh newly injected segments are SACKed,
continuous loss (and not further delay) of Seg1 can safely be
assumed, and that segment be resent. This approach is conservative
but constrained by the requirement that additional segments can be
sent, and thereby delayed in the response.
With the synergistic use of timestamp extended options together with
selective acknowledgments, the receiver would immediately reflect
back the timestamp of the last received segment. This allows the
sender to discriminate between a SACK due to a delayed Seg4,TS10, or
a SACK because of Seg4,TS30. Therefore, the appropriate decision
(retransmission of Seg1 once more, or addressing the observed
reordering/delay accordingly [I-D.blanton-tcp-reordering] can be
taken with high confidence.
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6.4. Integrity of the Timestamp value
If the timestamp is used for congestion control purposes, an
incentive exists for malicious receivers to reflect tampered
timestamps, as demonstrated with some exploits [CUBIC].
One way to address this is to not use timestamp information directly,
but to keep state in the sender for each sent segment, and track the
round trip time independent of sent timestamps. Such an approach has
the drawback, that it is not straightforward to make it work during
loss recovery phases for those segments possibly lost (or reordered).
In addition there is processing and memory overhead to maintain
possibly extensive lists in the sender that need to be consulted with
each ACK. Despite these drawbacks, this approach is currently
implemented due to lack of alternatives (see [Linux], and [BSD10]).
The preferred approach is that the sender MAY choose to protect
timestamps from such modifications by including a fingerprint (secure
hash of some kind) in some of the least significant bits. However,
doing so prevents a receiver from using the timestamp for other
purposes, unless the receiver has prior knowledge about this use of
some bits in the timestamp value. Furthermore, strict monotonic
increasing values are still to be maintained. That constraint
restricts this approach somewhat and limits or inhibits the use of
timestamp values for direct use by the receiver (i.e. for one-way
delay variation measurement, as the hash bits would look like random
noise in the delay measurement).
6.5. Disambiguation with slow Timestamp clock
In addition, but somewhat orthogonal to maintaining timestamp value
integrity, there is a use case when the sender does not support a
timestamp clock interval that can guarantee unique timestamps for
retransmitted segments. This may happen whenever the TCP timestamp
clock interval is higher than the round-trip time of the path. For
unambiguously identifying regular from retransmitted segments, the
timestamp must be unique for otherwise identical segments. Reserving
the least significant bits for this purpose allows senders with slow
running timestamp clocks to make use of this feature. However,
without modifying the receiver behavior, only limited benefits can be
extracted from such an approach. Furthermore the use of this option
has implications in the protection against wrapped sequence numbers
(PAWS - [RFC1323]), as the more bits are set aside for tamper
prevention, the faster the timestamp number space cycles.
Using Timestamp capabilities to explicitly negotiate mask bits, and
set aside a (low) number of least significant bits for the above
listed purposes, allows a sender to use more reliable integrity
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checks. These masked bits are not to be considered part of the
timestamp value, for the purposes described in [RFC1323] (i.e. PAWS)
and subsequent heuristics using timestamp values (i.e. Eifel
Detection), thereby lifting the strict requirement of always
monotonically increasing timestamp values. However, care should be
taken to not mask too many bits, for the reasons outlined in
[RFC1323]. Using a mask value higher than 8 is therefore
discouraged.
The reason for having 5 bits for the mask field nevertheless is to
allow the implementation of this protocol in conjunction with TCP
cookie transaction (TCPCT) extended timestamps [RFC6013]. That
allows for nearly a quarter of a 128 bit timestamp to be set aside.
6.6. Masked timestamps as segment digest
After making TCP alternate checksums historic (see [RFC6247]), there
still remains a need to address increased corruption probabilities
when segment sizes are increased (see
[I-D.ietf-tcpm-anumita-tcp-stronger-checksum]).
Utilizing a completely masked TSval field allows the sender to
include a stronger CRC32, with semantics independent of the fixed TCP
header fields. However, such a use would again exclude the use of
PAWS on the receiver side, and a receiver would need to know the
specifics of the digest for processing. It is assumed, that such a
digest would only cover the data payload of a TCP segment. In order
to allow disambiguation of retransmissions, a special TSval can be
defined (e.g. TSval=0) which bypasses regular CRC processing but
allows the identification of retransmitted segments.
The full semantics of such a data-only CRC scheme are beyond the
scope of this document, but would require a different version of the
timestamp capability. Nevertheless, allowing the full TSval to
remain unprocessed by the receiver for the purpose of PAWS even in
version 0 could still allow the successful negotiation of sender-side
enhancements such as loss recovery improvements (see Section 6.2, and
Section 6.3).
In effect, the masked portion of the timestamp value represent an
unreliable out of band signal channel, that could also be used for
other purposes than solely performing timestamp integrity checks (for
example, this would allow ER-SRTO algorithms [Cho08]).
6.7. Timestamp value as covert channel
Covert channels SHOULD NOT be implemented by using the mask field, as
the explicit masking clearly points to such a channel. As the
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regular operation of the timestamp clock is still maintained, covert
channels working by artificially delaying data segments in an
application (and thereby influencing the timestamp inserted into the
segment) work unaffected. The received TSval would need to be
shifted by the appropriate number of bits, before extracting the data
from the covert channel by the receiver.
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7. Discussion
RTT and OWD variation during loss episodes is not deeply researched.
Current heuristics ([RFC1122], [RFC1323], Karn's algorithm [RFC2988])
explicitly exclude (and prevent) the use of RTT samples when loss
occurs. However, solving the retransmission ambiguity problem - and
the related reliable ACK delivery problem - would enable new
functionality to improve TCP processing. Also, having an immediate
echo of the last received timestamp value would enable new research
to distinguish between corruption loss (assumed to have no RTT / OWD
impact) and congestion loss (assumed to have RTT / OWD impact).
Research into this field appears to be rather neglected, especially
when it comes to large scale, public internet investigations. Due to
the very nature of this, passive investigations without signals
contained within the headers are only of limited use in empirical
research.
Retransmission ambiguity detection during loss recovery would allow
an additional level of loss recovery control without reverting to
timer-based methods. As with the deployment of SACK, separating
"what" to send from "when" to send it could be driven one step
further. In particular, less conservative loss recovery schemes
which do not trade principles of packet conservation against
timeliness, require a reliable way of prompt and best possible
feedback from the receiver about any delivered segment and their
ordering. [RFC2018] SACK alone goes quite a long way, but using
timestamp information in addition could remove any ambiguity.
However, the current specs in [RFC1323] make that use impossible,
thus a modified semantic (receiver behavior) is a necessity.
A synergistic signaling with immediate timestamp value echoes would
however break legacy, per-packet RTT measurements. The reason is,
that delayed ACKs would not be covered. Research has shown, that
per-packet updates of the RTT estimation (for the purpose of
calculating a reasonable RTO value) are only of limited benefit (see
[Path99], and [PH04]). This is the most serious implication of the
proposed synergistic signaling scheme with directly echoing the
timestamp value of the segment triggering the ACK. Even when using
the directly reflected timestamp values in an unmodified RTT
estimator, the immediate impact would be limited to causing premature
RTOs when the sending rate suddenly drops below two segments per RTT.
That is, assuming the receiver implements delayed ACK and sending one
ACK for every other data segment received. If the receiver has
D-SACK [RFC2883] enabled, such premature RTOs can be detected and
mitigated by the sender (for example, by increasing minRTO for low
bandwidth flows).
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8. Acknowledgements
The authors would like to thank Dragana Damjanovic for some initial
thoughts around Timestamps and their extended potential use.
The editor would like to thank Bob Briscoe for his insightful
comments, and the gratuitous donation of text, that have resulted in
a substantially improved document.
9. Updates to Existing RFCs
Care has been taken to make sure the updates in this specification
can be deployed incrementally.
Updates to existing [RFC1323] implementations are only REQUIRED if
they do not clear the TSecr value in the initial <SYN> segment. This
is a misinterpretation of [RFC1323] and may leak data anyway (see
[I-D.ietf-tcpm-tcp-security]). Otherwise, there will be no need to
update an RFC1323-compliant TCP stack unless the timestamp
capabilities negotiation is to be used.
Implementations compliant with the definitions in this document shall
be prepared to encounter misbehaving senders, that don't clear TSecr
in their initial <SYN>. It is believed, that checking the reserved
bits to be all zero provides enough protection against misbehaving
senders.
In the unlikely case of an erraneous negotiation of timestamp
capabilities between a compliant receiver, and a misbehaving sender,
the proposed semantic changes will trigger a higher rate of spurious
retransmissions, while time-based heuristics on the receiver side may
further negatively impact congestion control decisions. Overall,
misbehaving receivers will suffer from self-inflicted reductions in
TCP performance.
10. IANA Considerations
With this document, the IANA is requested to establish a new registry
to record the timestamp capability flags defined with future versions
(codepoints 1, 2 and 3).
The lower 24 bits (3 octets) of the timestamp capabilities field may
be freely assigned in future versions. The first octet must always
contain the EXO, VER and MASK fields for compatibility, and the MASK
field MUST be set to allow interoperation with a version 0 receiver.
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This document specifies version 0 and the use of the last three
octets to signal the senders timestamp clock interval to the
receiver.
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11. Security Considerations
The algorithm presented in this paper shares security considerations
with [RFC1323] (see [I-D.ietf-tcpm-tcp-security]).
An implementation can address the vulnerabilities of [RFC1323], by
dedicating a few low-order bits of the timestamp fields for use with
a (secure) hash, that protects against malicious modification of
returned timestamp value by the receiver. A MASK field has been
provided to explicitly notify the receiver about that alternate use
of low-order bits. This allows the use of timestamps for purposes
requiring higher integrity and security while allowing the receiver
to extract useful information nevertheless.
12. References
12.1. Normative References
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[BSD10] Hayes, D., "Timing enhancements to the FreeBSD kernel to
support delay and rate based TCP mechanisms", Feb 2010, <h
ttp://caia.swin.edu.au/reports/100219A/
CAIA-TR-100219A.pdf>.
[CUBIC] Rhee, I., Ha, S., and L. Xu, "CUBIC: A New TCP-Friendly
High-Speed TCP Variant", Feb 2005, <http://
citeseerx.ist.psu.edu/viewdoc/
download?doi=10.1.1.153.3152&rep=rep1&type=pdf>.
[Chirp] Kuehlewind, M. and B. Briscoe, "Chirping for Congestion
Control - Implementation Feasibility", Nov 2010, <http://
bobbriscoe.net/projects/netsvc_i-f/chirp_pfldnet10.pdf>.
[Cho08] Cho, I., Han, J., and J. Lee, "Enhanced Response Algorithm
for Spurious TCP Timeout (ER-SRTO)", Jan 2008, <http://
ubinet.yonsei.ac.kr/v2/publication/hpmn_papaers/ic/
2008_Enhanced%20Response%20Algorithm%20for%20Spurious%
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20TCP.pdf>.
[I-D.blanton-tcp-reordering]
Blanton, E., Dimond, R., and M. Allman, "Practices for TCP
Senders in the Face of Segment Reordering",
draft-blanton-tcp-reordering-00 (work in progress),
February 2003.
[I-D.ietf-ledbat-congestion]
Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)",
draft-ietf-ledbat-congestion-08 (work in progress),
October 2011.
[I-D.ietf-tcpm-anumita-tcp-stronger-checksum]
Biswas, A., "Support for Stronger Error Detection Codes in
TCP for Jumbo Frames",
draft-ietf-tcpm-anumita-tcp-stronger-checksum-00 (work in
progress), May 2010.
[I-D.ietf-tcpm-tcp-security]
Gont, F., "Security Assessment of the Transmission Control
Protocol (TCP)", draft-ietf-tcpm-tcp-security-02 (work in
progress), January 2011.
[Linux] Sarolahti, P., "Linux TCP", Apr 2007,
<http://www.cs.clemson.edu/~westall/853/linuxtcp.pdf>.
[PH04] Eckstroem, H. and R. Ludwig, "The Peak-Hopper: A New End-
to-End Retransmission Timer for Reliable Unicast
Transport", Apr 2004, <citeseerx.ist.psu.edu/viewdoc/
download?doi=10.1.1.76.2748&rep=rep1&type=pdf>.
[Path99] Allman, M. and V. Paxson, "On Estimating End-to-End
Network Path Properties", Sep 1999,
<http://www.icir.org/mallman/papers/estimation.ps>.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option
for TCP", RFC 2883, July 2000.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
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for TCP", RFC 3522, April 2003.
[RFC4015] Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
for TCP", RFC 4015, February 2005.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013,
January 2011.
[RFC6247] Eggert, L., "Moving the Undeployed TCP Extensions RFC
1072, RFC 1106, RFC 1110, RFC 1145, RFC 1146, RFC 1379,
RFC 1644, and RFC 1693 to Historic Status", RFC 6247,
May 2011.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
June 2011.
Appendix A. Possible Extension
This section is not intended as normative description of an
extension, but merely as an example of a possible extension. Future
extensions MUST set the common fields in such a way that a receiver
capable of version 0 only can react appropriately.
Certain hosts may want to negotiate a common optimal timestamp clock
interval between each other for various purposes. For example, the
balance between PAWS ([RFC1323]) and the timestamp clock resolution
should be more towards one or the other. Also, if a hosts wants to
have identical timestamp clock intervals both at the sender and
receiver to simplify one-way delay variation calculation, negotiating
the clock interval could be useful. With identical timestamp clock
intervals, instead of multiplications and divisions, only additions
and subtractions are required for OWD variation calculation.
Without a full three way handshake, full negotiation of the timestamp
clock intervals is not possible. For this reason, a special semantic
is required during negotiation. This allows both ends to know the
exact timestamp clock interval with only two exchanged segments,
while at the same time remaining compatible with version 0.
For this purpose, the following extension (version 1) of this
protocol is one suggestion. Depending on the exact requirements, a
different signaling may be more appropriate. For example, only the
two different EXP fields could be required, while a single, but
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higher precision FRAC field for both low and high boundaries could
suffice, and some additional signaling bits could be made available.
A.1. Capability Flags
Kind: 8
Length: 10 bytes
+-------+-------+---------------------+---------------------+
|Kind=8 | 10 | TS Value (TSval) |TS Echo Reply (TSecr)|
+-------+-------+---------------------+---------------------+
1 1 4 | 4 |
/ |
.-----------------------------------' |
/ \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| | # DUR12lo | DUR12hi |
|X|VER| MASK #-----------------------|-----------------------|
|O| | # ADJ12lo | INT12lo | ADJ12hi | INT12hi |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Timestamp Capability enhanced flags
The following additional fields are defined:
VER - version (2 bits)
Version 1 could indicated that the sender is capable of adjusting
the timestamp clock interval within the bounds of the two 12 bit
fields (see Appendix A.2). A receiver that only implements
version 0 SHOULD NOT ignore the timestamp capability negotiation
entirely when encountering an unsupported version, any SHOULD
respond with a version 0 response nevertheless (see below) -
thereby enabling enhanced uses of the timestamp value and the
modification of the receiver side timestamp processing.
DUR12lo and
DUR12hi - Duration (12 bits each)
The sender provides a range of two timestamp clock intervals in
the initial <SYN> to ask the receiver to operate preferred in
this range.
ADJ12lo and
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ADJ12hi - Adjustment factor (5 bits each)
The scale adjustment factor indicating the possible timestamp
clock ranges. All values between zero and 31 are allowed, with
the only limitation that ADJ12hi must be equal or greater than
ADJ12lo. As the base value representation is shorter by 4 bits
than the single interval representation, the values need to be
left shifted always by 4. left-
INT12lo and
INT12hi - Base Interval (7 bits each)
The integer part of the timestamp clock interval before being
left-shifted. A a value of zero would have a special meaning,
and is not a valid number for range negotiation. The properly
scaled intervals MUST be given in the correct order (lower
interval in DUR12lo and higher interval in DUR12hi).
A.2. Range Negotiation
Only the host initiating a TCP session MAY offer a timestamp clock
interval, while the receiver SHOULD select a timestamp clock interval
within these bounds. If the receiver can not adjust it's timestamp
clock to match the range, it MAY use a timestamp clock rate outside
these bounds. If the receiver indicated a timestamp clock interval
within the indicated bounds, the sender MUST set it's timestamp clock
interval to the negotiated rate. If the receiver uses a timestamp
clock interval outside the indicated bounds, the sender MUST set the
local timestamp clock interval to the value indicated by the closer
boundary.
The following example sequence is provided to demonstrate how
timestamp clock range negotiation works. Both sender and receiver
finally know the clock interval of their respective partner.
SYN, TSopt=<X>, TSecr=EXO|VER=1|MSK|DUR12lo=1ms|DUR12hi=100ms
SYN,ACK, TSopt=<Y>, TSecr=<X>^EXO|VER=0|MSK|DUR=10ms
In this example, both hosts would run their respective timestamp
clocks with one tick every 10 ms.
SYN, TSopt=<X>, TSecr=EXO|VER=1|MSK|DUR12lo=1ms|DUR12hi=100ms
SYN,ACK, TSopt=<Y>, TSecr=<X>^EXO|VER=0|MSK|DUR=1000ms
In this example, the sender would set the timestamp clock interval to
100 ms (closer to the receivers clock interval of 1 sec), while the
receiver will have a timestamp clock interval running at 1 sec.
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SYN, TSopt=<X>, TSecr=EXO|VER=1|MSK|DUR12lo=1ms|DUR12hi=100ms
SYN,ACK, TSopt=<Y>, TSecr=<X>^EXO|VER=0|MSK|DUR=100us
In this example, the sender would set the timestamp clock rate to one
tick every 10 ms (closest to the receiver's clock interval of 100
us), while the receiver will have the timestamp clock running at 100
us per tick.
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Appendix B. Revision history
00 ... initial draft, early submission to meet deadline.
01 ... refined draft, focusing only on those capabilities that have
an immediate use case. Also excluding flags that can be substituted
by other means (MIR - synergistic with SACK option only, RNG moved to
appendix A, BIA removed and the exponent bias set to a fixed value.
Also extended other paragraphs.
02 ... updated document after IETF80 - referrals to "timestamp
options" were seen to be ambiguous with "timestamp option", and
therefore replaced by "timestamp capabilities". Also, the document
was reworked to better align with RFC4101. Removed SGN and increased
FRAC to allow higher precision.
03 ... removed references to "opaque" and "transparent". substituted
"timestamp clock interval" for all instances of rate. Changed signal
encoding to resemble a scale/value approach like what is done with
Window Scaling. As added benefit, clock quality can be implicitly
signaled, since multiple representations can map to idential time
intervals. Added discussion around resilience against broken RFC1323
implementations (Win95, Linux 2.3.41+), which deviate from expected
Timestamp signaling behavior.
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Authors' Addresses
Richard Scheffenegger
NetApp, Inc.
Am Euro Platz 2
Vienna, 1120
Austria
Phone: +43 1 3676811 3146
Email: rs@netapp.com
Mirja Kuehlewind
University of Stuttgart
Pfaffenwaldring 47
Stuttgart 70569
Germany
Email: mirja.kuehlewind@ikr.uni-stuttgart.de
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