TCP Maintenance and Minor R. Scheffenegger, Ed.
Extensions (tcpm) NetApp, Inc.
Internet-Draft M. Kuehlewind
Intended status: Experimental University of Stuttgart
Expires: September 15, 2011 March 14, 2011
Additional negotiation in the TCP Timestamp Option field
during the TCP handshake
draft-scheffenegger-tcpm-timestamp-negotiation-01
Abstract
RFC 1323 defines the TSecr field of a SYN packet to be not valid and
thus this field will always be zero. This documents specifies the
use of this field to signal and negotiate additional information
about the content of the TSopt field as well as the behavior of the
receiver. If the receiver understands this extension, it will use
the TSecr field of the SYN/ACK to reply. Otherwise the receiver will
ignore the TSecr field and set a timestamp in the TSecr field as
specified in RFC 1323.
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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This Internet-Draft will expire on September 15, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Capability Flags . . . . . . . . . . . . . . . . . . . . . 5
4.2. Implicit extended negotiation . . . . . . . . . . . . . . 7
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Optional Capability Flags . . . . . . . . . . . . . . 10
A.1. Range Negotiation . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Revision history . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
The TCP Timestamps Option (TSopt) provides timestamp echoing for
Round-trip Time (RTT) measurments. TSopt is widely deployed and
activated by default in many systems. RFC 1323 [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
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 timestamp
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
time gives an estimation of the RTT. RFC 1323 [RFC1323] specifies
various cases when more than one timestamp is available to echo. The
proposed solution might not always be the best choice, e.g. when the
TCP Selective Acknowledgment Option (SACK) is used. Moreover, more
and more use cases arise where one-way delay (OWD) measurements are
needed. These mechanism misuse usually the TSopt to estimated the
variation in OWD. To enable such mechanisms the TSecr field in the
TCP SYN packet could be used for additional negotiation.
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1.1. Requirements Language
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].
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2. Overview
Enhancements in the area of TCP congestion control can use the
measurement of the one-way delay variance as one input. However,
without explicit knowledge of the partner's timestamp clock, arriving
at a good estimate requires multiple segent exchanges over a few
round trip times. Nevertheless, any such calculation has to make
assumptions about the network state at the time of the measurement.
In order to assist such algorithms, explicit knowledge at a early
phase of the session can be negotiated.
In addition, by using synergistic signalling between Timestamps and
other options such as selective acknowledgment, enhancments in loss
recovery are possible by removing any retransmission ambiguity .
However, currently receivers are required to only reflect the
timestamp of the last segment that was received in order. Therefore,
a backwards compatible way of changing this behavior is required.
Furthermore, as the importance of the timestamp option increases by
using it in more aspects of a TCP senders algorithm, so increases the
importance of maintaining the integrity of the reflected timestamps,
while allowing the receiver to make use of a senders timestamp.
As an optional extension, a timestamp clock rate range negotiation is
also introduced. However, this is only included as example of
further possible enhancements.
3. Definitions
The reader is expected to be familiar with the definitions given in
[RFC1323].
4. Signaling
During the initial TCP three-way handshake, timestamp options are
negotiated using the TSecr field. A compliant TCP receiver will XOR
the flags with the received TSval, when responding with the SYN+ACK.
Timestamp Options MAY only be present when the SYN bit is set.
4.1. Capability Flags
In order to signal the supported capabilities, the TSecr is
overloaded with the following flags and fields during the three-way
handshake. If optional capabilities such as tcp clock range are
presented, minimal state will be required in the host to decode the
returned Flags xor'ed with the TSval.
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Kind: 8
Length: 10 bytes
+-------+-------+---------------------+---------------------+
|Kind=8 | 10 | TS Value (TSval) |TS Echo Reply (TSecr)|
+-------+-------+---------------------+---------------------+
1 1 4 | 4 |
/ |
.-----------------------------------' |
/ \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|R|R| | |S| | |
|X|E|E| MASK | RES |G| EXP16 | FRAC16 |
|O|S|S| | |N| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
timestamp option flags
EXO - Extended Options
Indicated that the sender supports extended timestamp options as
defined by this document, and MUST be set ("1") by compliant
implementations.
RES - Reserved
Reserved for future use. MUST not be set ("0"). If a timestamp
option is received with this bit set, the receiver MUST ignore
the extended options field and react as if the Flags were not set
(compatibility mode).
MASK - Mask Timestamps
If the timestamp is used for congestion control purposes, an
incentive exists for malicious receivers to reflect tampered
timestamps. A 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
would prevent a receiver from using the timestamp for other
purposes. The MASK field indicates how many least significant
bits should be excluded by the receiver, when processing a
timestamp for timing purposes. Note that this does not impact
the reflected timestamp in any way - TSecr will always be equal
to an appropriate TSval. Another use case would be when the
sender does not support a timestamp clock which can guarantee
unique timestamps for retransmitted segments. For unambigously
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
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running timestamp clocks to make use of this feature. Note that
the use of this option as implications in the protection against
wraped sequence numbers (PAWS - [RFC1323]), as the more bits are
set aside for tamper prevention, the faster the timestamp number
space cycles itself.
SGN - binary16 Sign
This is the sign bit of the IEEE 754-2008 binary16 floating point
representation of the timestamp clock. Timestamp clocks MUST be
positive, thus this bit MUST be zero.
EXP16 - binary16 Exponent
The exponent component of a binary16 floating point number
indicating the timestamp clock. The exponent bias is 28, which
is not identical to the binary16 definition in IEEE 754-2008.
Subnormal numbers (lower precision), where the exponent is set to
zero, extend the lowest possible value representation to 2^-38
(or 7.276 ps) at reduced precision. Infinity and NaN (all
exponent bits set) are not suppored, an exponent value of 31 is
to be treated as normal exponent. This allows timestamp clock
rates of up to 15.999 sec.
FRAC16 - binary16 Fraction
The fraction component of a binary16 floating point number
indicating the timestamp clock. The clock rate is measured in
seconds between ticks. The range with the highest resolution,
excluding subnormal numbers, covers clock ranges between 7.45 ns
and 15.99 sec. It is expected, that timestamp clock rates in
excess of 0.1 ms are implemented by inserting the timestamp
"late" before transmitting a segment.
Example for an extended timestamp option, to indicate that the
senders timestamp clock (tcp clock) is running with 1 ms per tick:
SYN, TSopt=<X>, TSecr=EXO|MASK|EXP16=18|FRAC16=0x018
The clock rate calculates as 2^(18-28)*1.0000011b, thus indicates an
actual clock rate of 999.45 us
4.2. Implicit extended negotiation
If both timestamp extended options and selective acknowledgement
options ([RFC2018]) are negotiated, both hosts MUST mirror the
timestamp option immediately after receiving it. 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
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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). The exact
implications are beyond the scope of this note.
This allows the synergistic use of the timestamp option with the SACK
option to improve loss recovery, round trip time and one way delay
variance measurements 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.
5. Discussion
One-way delay (variation) based congestion controls would benefit
from knowing the clock resolution on both sides.
RTT variance 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 - may allow the refinement of
these algorithms further, as well as enabling new research to
distinguish between corruption loss (without RTT / one-way delay
impact) and congestion loss (with RTT / one-way delay impact).
Research into this field appears to be a 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. 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
signaling (receiver behavior) is a necessity.
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6. Acknowledgements
The authors would like to thank Dragana Damjanovic for some initial
thoughts around Timestamps and their extended potential use.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
The algorithm presented in this paper shares security considerations
with [RFC1323].
Some implementations address the vulerabilities of [RFC1323], by
dedicating a few low-order bits of the timestamp fields for use with
a (secure) hash, that protects against malicious tweaking of TSecr
values. A Flag-field has been provided to transparently notify the
receiver about that use of low-order bits, so that they can be
excluded in one-way delay calculations.
9. References
9.1. 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.
9.2. Informative References
[Chirp] Kuehlewind, M. and B. Briscoe, "Chirping for Congestion
Control - Implementation Feasibility", Nov 2010, <http://
bobbriscoe.net/projects/netsvc_i-f/chirp_pfldnet10.pdf>.
[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.
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Appendix A. Optional Capability Flags
Certain hosts may want to negotiate a certain optimal Timestamp Clock
Rate 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 certain algorithms want to have identical
timestamp clock rates both at the sender and receiver, negotiating
the clock rate would be preferrable. However, without a full three
way handshake, full negotiation of the timestamp clock rate is not
possible.
For this purpose, the following extension of this proposal is
suggested.
Kind: 8
Length: 10 bytes
+-------+-------+---------------------+---------------------+
|Kind=8 | 10 | TS Value (TSval) |TS Echo Reply (TSecr)|
+-------+-------+---------------------+---------------------+
1 1 4 | 4 |
/ |
.-----------------------------------' |
/ \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|R|R| | | | | |
|X|E|N| MASK | EXP12lo | FRAC12lo | EXP12hi | FRAC12hi |
|O|S|G| | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
timestamp option flags
The following additonal fields are defined:
RNG - Range negotiation
Indicated that the sender is capable of adjusting the timestamp
clock rate within the bounds of the two 12 bit fields (see
Appendix A.1). This flag is only valid in the initial SYN
segment, and invalid when the ACK bit is set.
EXP12lo and
EXP12hi - binary12 Exponent
The exponent component of a truncated, 12 bit floating point
number indicating the possible timestamp clock ranges. The
exponent bias is also 28, and no special numbers (infinity, NaN)
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are allowed. The exponent value 31 is treated like any other
exponent value. Only the host initiating a TCP session MAY offer
a timestamp clock range, while the receiver SHOULD select a
timestamp clock 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 rate within the indicated bounds, the
sender MUST set it's timestamp clock rate to the negotiated rate.
If the receiver uses a timestamp clock rate outside the indicated
bounds, the sender MUST set the local timestamp clock rate to the
value indicated at the closer bound.
FRAC12lo and
FRAC12hi - binary12 Fraction
The fraction component of a 12 bit floating point number.
Subnormal numbers are allowed (Exponent value 0). This allows a
range between 7.45 ns and 15.99 s with full resolution (lower
bound is 0.06 ns using subnormal values).
A.1. Range Negotiation
The following sequence would negotiate the timestamp clock rate for
both sender and receiver, where both finally know the clock rate of
the respective partner.
SYN, TSopt=<X>, TSecr=EXO|RNG|MASK|12bit-lo=1ms|12bit-hi=100ms
SYN,ACK, TSopt=<Y>, TSecr=<X>^EXO|MASK|16bit=10ms
In this example, both hosts would run their respective timestamp
clocks with a resolution of 10 ms.
SYN, TSopt=<X>, TSecr=EXO|RNG|MASK|12bit-lo=1ms|12bit-hi=100ms
SYN,ACK, TSopt=<Y>, TSecr=<X>^EXO|MASK|16bit=1000ms
In this example, the sender would run the timestamp clocks with a
resolution of 100 ms (closer to the receivers clock rate of 1 sec),
while the receiver will have a timestamp clock rate running at 1 sec.
SYN, TSopt=<X>, TSecr=EXO|RNG|MASK|12bit-lo=1ms|12bit-hi=100ms
SYN,ACK, TSopt=<Y>, TSecr=<X>^EXO|MASK|16bit=100us
In this example, the sender would run the timestamp clocks with a
resolution of 10 ms (closer to the receivers clock rate of 0.1 ms),
while the receiver will have a timestamp clock rate running at 0.1ms.
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Appendix B. Revision history
00 ... initial draft, early submission to meet deadline
01 ... refined draft, focusing only on those options that have an
immediate use case. Also excluding flags that can be substituted by
other means (MIR - synergistic with SACK options only, RNG moved to
appendix, BIA removed while the exponent bias is at a fixed value.
Also extended other paragraphs.
Authors' Addresses
Richard Scheffenegger (editor)
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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