TCP Maintenance Working Group K. Yang
Internet-Draft N. Cardwell
Intended status: Standards Track Y. Cheng
Expires: May 6, 2021 E. Dumazet
Google, Inc
November 2, 2020
TCP ETS: Extensible Timestamp Options
draft-yang-tcpm-ets-00
Abstract
This document presents ETS: an Extensible TimeStamps option for TCP.
It allows hosts to use microseconds as the unit for timestamps to
improve the precision of timestamps, and advertise the maximum ACK
delay for its own delayed ACK mechanism. Furthermore, it extends the
information provided in the [RFC7323] TCP Timestamps Option by
including the receiver delay in the TSecr echoing, so that the
receiver of the ACK is able to more accurately estimate the portion
of the RTT that resulted from time traveling through the network.
The ETS option format is extensible, so that future extensions can
add further information without the overhead of extra TCP option kind
and length fields.
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 May 6, 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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1. Terminology
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. In this document, these words will appear
with that interpretation only when in UPPER CASE. Lower case uses of
these words are not to be interpreted as carrying [RFC2119]
significance.
2. Introduction
Accurate round-trip time (RTT) estimation is necessary for TCP to
adapt to diverse and dynamic traffic conditions.
The TCP timestamp option specified in [RFC7323] is designed largely
for RTT samples intended for computing TCP's retransmission (RTO)
timer [RFC6298].
Some congestion control algorithms may wish to use a form of RTT
measurement as one of several congestion signals, since elevated RTT
measurements can reflect increases in network queueing delays. For
example, the Swift congestion control algorithm [KDJWWM20],
successfully deployed in data-center environments, requires precise
and accurate measurements of both network and host delays. However,
the existing TCP RTT sampling mechanisms that measure the delay
between data transmission and ACK receipt [RFC6298] do not separate
network and host delays, and cannot measure the RTT of retransmitted
data. Even the TCP timestamp option specified in [RFC7323] is not
well-suited to use as a congestion signal, for a number of reasons.
With the TCP Timestamps Option [RFC7323], data senders can measure an
RTT sample by computing the difference between the data sender's
current timestamp clock value and the received TSecr value. However,
there are some drawbacks in this [RFC7323] measurement method:
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1. The TCP endpoint can compute an [RFC7323] RTT measurement only if
the ACK advances the left edge of the send window, i.e. SND.UNA
is increased. (Without this rule, in one-way data flows the RTT
measured by the receiver would be inflated by gaps in the data
sending process; see [RFC7323] Section 4.1).
2. Even if an ACK advances the left edge of the send window, RTT
measurements can be greatly inflated by delayed ACKs: the host
receiving a data segment can delay an ACK segment in hopes of
piggybacking an ACK on the next outgoing data segment. This
delay can be as large as 500 milliseconds (as in [RFC1122]
Section 4.2.3.2).
3. There is delay included in the RTT measurement that is irrelevant
to network queuing, e.g. host-side transmit and receive delays,
including delays for waking CPUs from power management
"C-states".
4. [RFC7323] specifies a "timestamp clock frequency in the range 1
ms to 1 sec per tick" ([RFC732] Section 5.4). But today's
datacenter networks are capable of delivering network packets
within tens of microseconds [BMPR17]. In such networks the lower
bound of 1 ms limits the usefulness of [RFC7323] timestamps.
First, [RFC7323] is incapable of accurate RTT measurements in
such networks. Second, [RFC7323] timestamps are not suitable for
reverting spurious loss recovery and congestion control responses
[RFC4015] due to packet reordering in such networks.
In many of the cases above, an RTT sample computed using [RFC7323]
can be inflated for reasons other than network queuing. It is
difficult for the ACK receiver to infer how long the non-network
delay was, which makes it hard to use an [RFC7323] RTT measurement as
a clean signal for congestion control.
Delayed ACKs, as mentioned above, are particularly problematic. TCP
receivers typically implement a delayed ACK algorithm. To avoid
spurious timeouts due to these delayed ACKs, TCP senders can adapt to
this delayed ACK behavior by guessing the maximum delayed ACK value
of the remote receiver. Historically, many implementations tended to
delay ACKs by up to roughly 200ms [WS95], so some implementations
have correspondingly used a minimum RTO of 200ms. However, this
imposes a latency penalty that is very large compared to RTTs in some
of today's datacenter networks. If receivers had the ability to
signal their maximum delayed ACK timer delay, this could allow faster
timer-based loss recovery, while still avoiding spurious timeouts.
This document presents ETS: an Extensible TimeStamps option for TCP.
ETS extends the information provided in the [RFC7323] TCP Timestamps
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Option, adding several features. First, ETS allows connections to
use microseconds as the unit for timestamps, to improve the precision
of timestamps. Second, ETS allows endpoints to advertise the maximum
ACK delay for their own delayed ACK mechanism. Third, ETS allows
connections to include information about the delay between data
receipt and ACK generation, so that the receiver of the ACK is able
to more accurately estimate the portion of the RTT that resulted from
time that data and ACK segments spent traveling through the network.
Fourth, the ETS option format is extensible, so that future
extensions can add further information without the overhead of extra
TCP option kind and length fields.
2.1. High-level Design
The ETS protocol has two phases: an exchange of ETS options (ETSopt)
in the negotiation handshake in <SYN> and <SYN,ACK> segments, and
then ETS options included in all following segments.
In the negotiation handshake, the two senders exchange their
MaxACKDel: a hint characterizing the maximum amount of a delay that
the sender expects to schedule before acknowledging an incoming data
segment.
All segments include EcrDel, the delay in the TSecr echoing process
that was inserted by the data receiver, helping the receiver of an
ACK to estimate the portion of the RTT delay caused by the network.
An example of a handshake exchange is illustrated below:
TCP A (Client) TCP B (Server)
______________ ______________
CLOSED LISTEN
#1 SYN-SENT --- <SYN,TSval=X,TSecr=0,
EcrDel=0,MaxACKDel=M1> -----------> SYN-RCVD
<SYN,ACK,TSval=Y,TSecr=X, ---------- SYN-RCVD
#2 ESTABLISHED <-- EcrDel=E1,MaxACKDel=M2>
#3 ESTABLISHED -- <ACK,TSval=Z,TSecr=Y,EcrDel=E2> --> ESTABLISHED
Active connect: An actively connecting host that wishes to negotiate
ETSopt MUST include the ETSopt in the <SYN>. For backward
compatibility, the endpoint performing the active connect MAY also
include a [RFC7323] TSopt in the <SYN> segment, so that if the
passive side or middleboxes do not support and respond to the ETSopt,
the active and passive sides can proceed with the [RFC7323] TSopt
negotiation for the connection.
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Passive connect: For a passively connecting host that is willing to
proceed with ETSopt negotiation, if the <SYN> includes an ETSopt, the
host MUST include a TCP ETS option in the initial <SYN,ACK> segment.
A retransmission of the <SYN,ACK> segment may omit the ETSopt, to
increase robustness in the presence of middleboxes that block
segments containing ETSopt.
Processing of <SYN,ACK> for active connect: If the ETSopt is absent
from the <SYN,ACK> segment received by the actively connecting
endpoint, suggesting that the passive endpoint does not support
ETSopt, or some middlebox has stripped the option from the <SYN,ACK>
segment, then the actively connecting endpoint MUST disable ETSopt
for this connection. In such cases the actively connecting endpoint
MAY fall back to using [RFC7323] timestamps if both the <SYN> and
<SYN,ACK> segments include valid [RFC7323] timestamps.
3. Detailed Protocol
3.1. Definitions
The reader is expected to be familiar with the TCP Timestamps Option
(TSopt), including TSval, TSecr, and TS.Recent [RFC7323].
Variables introduced by this document are described below:
MaxACKDel: a hint characterizing the maximum amount of a delay that
the sender expects to schedule before acknowledging an incoming data
segment.
TSval: the value of the sender's timestamp clock when this segment is
scheduled for transmission, in microseconds.
TSecr: the echo of TS.recent
TS.Recent: the recently received TSval sent by the remote TCP
endpoint in the TSval field of an ETSopt, updated using the TS.Recent
rules specified in [RFC7323].
EcrDel: the field quantifying the delay between data receipt and ACK
generation, so that the receiver of the ACK is able to more
accurately estimate the NetworkRTT.
NetworkRTT: the time from when the data segment leaves the sender
until when it arrives at the receiver, plus the time from when the
corresponding ACK leaves the (data) receiver until the ACK arrives at
the data sender (here sender and receiver refer to the TCP layer
only).
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3.2. TCP ETS option header format
The header format for TCP ETS options (ETSopt) is as follows:
01234567 89012345 67890123 45678901
+--------+--------+--------+--------+
| Kind | Length | ExID |
+--------+--------+--------+--------+
| TSval |
+--------+--------+--------+--------+
| TSecr |
+--------+--------+--------+--------+
|Un| EcrDel |R| MaxACKDel |
+-----------------+-----------------+
| |
/ \
.---' `---------------------.
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unit | EcrDel |Reserved|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2 bits | 13 bits | 1 bit
Kind: 1 byte, has value 254,
TCP experimental option codepoint [RFC6994]
Length: 1 byte option length, value is 16 if SYN bit is set,
otherwise 14 if SYN bit is not set
(value MAY be higher in later ETSOpt protocol versions).
ExID: 2 byte [RFC6994] experiment ID; MUST be 0x4554.
TSval and TSecr: 32 bits each, have the same definition as
[RFC7323] except that both are in microseconds.
EcrDelUnit: 2 bits, has value:
0: indicates EcrDel is in microsecond units
1: indicates EcrDel is in millisecond units
2: indicates EcrDel is invalid
3: reserved
EcrDel: 13 bits, the value of EcrDel.
Reserved: 1 bit, in this protocol version; sender MUST set to 0;
receiver MUST ignore field and tolerate 0 or 1
MaxACKDel: 16 bits, max expected ACK delay in microseconds,
MUST only be present when SYN bit is set.
The semantics of the option fields are as follows:
TSval: The TSval field contains the value of the sender host's
timestamp clock, in microseconds, at the time the sender schedules
transmission of the segment.
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TSecr: The TSecr field contains the current value of TS.Recent, the
recently received TSval sent by the remote TCP that is recorded using
the TS.recent update algorithm described in [RFC7323], section 4.3.
TSecr is only valid when the ACK bit is set. When the ACK bit is not
set, senders MUST set this field to 0, and receivers MUST ignore the
value in this field (and MUST tolerate any value in this field).
MaxACKDel: When the SYN bit is set, the sender advertises MaxACKDel
as a hint characterizing the maximum amount of a delay that the
sender expects to apply before ACKing an incoming data segment. The
unit of MaxACKDel is microseconds. Field MaxACKDel MUST be set to
0xFFFE if the value is equal to or greater than 0xFFFE. If the
MaxACKDel value is invalid, or the sender does not want to advertise
this value, field MaxACKDel MUST be set to the max value allowed by
the field, i.e. 0xFFFF. When a host performing an active TCP
connection attempt is willing to enable ETS, in a <SYN> segment it
MUST include a TCP ETS option (ETSopt) with the MaxACKDel field. If
the SYN bit is not set, the field MaxACKDel MUST NOT be present in
the option.
EcrDel: Field EcrDel contains the delay inserted by the receiver in
this TSecr echoing process. Field EcrDel is only valid when the ACK
bit is set, otherwise MUST be set to 0 by the sender and ignored by
the receiver. When the ACK bit is set, the sender computes the
EcrDel using the following algorithm:
(1) When a <SYN> or non-empty data segment SEG is received:
If SYN is set, or SEG.TSval is after TS.Latest:
TS.Latest = SEG.TSval
TS.LatestClock = ArrivalTime of SEG
(2) When an ETSopt is sent:
TSecr = TS.Recent (as in [RFC7323])
LatestACKDel = ACKSendTime - TS.LatestClock
TSecrAge = TS.Latest - TSecr
EcrDel = LatestACKDel + TSecrAge
In (1), a non-empty data segment is defined as a segment containing a
non-empty data payload. The timestamps are in a modular 32-bit
space, and a timestamp s is "after" timestamp t if and only if 0 < (s
- t) < 2^31.
In (2), the semantics of the two temporary variables LatestACKDel and
TSecrAge are as follows:
LatestACKDel, the time from the arrival of the segment with the
latest TSval until the time when an ACK segment is sent.
TSecrAge, the age of TSecr compared to TS.Latest.
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We discuss how the ACK receiver can estimate the latest NetworkRTT
using EcrDel in the next section.
3.3. Estimating the Network RTT
NetworkRTT is the time from when the data segment leaves the sender
until when it arrives at the receiver, plus the time from when the
corresponding ACK leaves the (data) receiver until the ACK arrives at
the data sender (here sender and receiver refer to the TCP layer
only).
With EcrDel in ETSopt when a data sender (TCP A) receives an ACK
segment with ETSopt from the remote endpoint (TCP B), NetworkRTT can
be eatimated by:
NetworkRTT = ACKArrivalTime - SEG.TSecr - SEG.EcrDel
where ACKArrivalTime is the TCP A's timestamp clock when the ACK
segment arrives, and SEG.TSecr and SEG.EcrDel are the values from the
incoming ETSopt.
This gives us the latest network RTT measurement by:
NetworkRTT = ACKArrivalTime - SEG.TSecr - SEG.EcrDel
= ACKArrivalTime - SEG.TSecr - B.TSecrAge - B.LatestACKDel
= ACKArrivalTime - (SEG.TSecr + B.TSecrAge)
- B.LatestACKDel
= ACKArrivalTime - (SEG.TSecr + B.TS.Latest - SEG.TSecr)
- B.LatestACKDel
= ACKArrivalTime - B.TS.Latest - B.LatestACKDel
In the calculation above, B.TSecrAge, B.TS.Latest and B.LatestACKDel
are TSecrAge, TS.Latest and LatestACKDel on TCP B respectively at the
time this ACK segment was sent.
At the time TCP B sends an ETSopt, TS.Latest can be different from
TS.Recent, e.g. if there is a sequence hole in B's receiving window.
If this is the case, the NetworkRTT measures the network RTT of the
latest segment received by TCP B.
The following example shows how NetworkRTT is computed when TS.Latest
is different from TS.Recent:
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TCP A TCP B
______________ ______________
-- <TSval=1> --> arrives at t=2
TS.Latest=TS.Recent=1
TS.LatestClock=2
-- <TSval=2> --> lost
-- <TSval=3> --> arrives at t=10
TS.Recent=1, TS.Latest=3
TS.LatestClock=10
arrive at t=11 <-- <ACK, TSecr=1, EcrDel=2> -- Send ACK at t=10
In this example, EcrDel is LatestACKDel + TSecrAge = (10 - 10) + (3 -
1) = 2, and the NetworkRTT from the last <ACK> segment is estimated
as 11 - 1 - 2 = 8, which is the network RTT of the segment sent with
TSval = 3.
In order to make use of this NetworkRTT estimate as a clean signal
that more precisely reflects network queuing, it is RECOMMENDED that
timestamps ACKArrivalTime and TS.LatestClock use the time at which
the segment arrives at the host, e.g. the time the Network Interface
Controller (NIC) receives the segment, if the NIC supports hardware
receive timestamping. Within this context, NetworkRTT is then
defined as the time from when the data segment leaves the sender's
TCP until when it is received at the receiver's NIC, plus the time
from when the corresponding ACK leaves the (data) receiver's TCP
until the ACK is received at the data sender's NIC.
4. Interaction with other TCP Mechanisms
4.1. Interaction with PAWS
Protection Against Wrapped Sequences (PAWS), introduced by [RFC7323]
Section 5, is a mechanism to reject old duplicate segments that might
corrupt an open TCP connection. In the PAWS mechanism, a segment can
be discarded as an old duplicate if it is received with a timestamp
SEG.TSval that is "before" some timestamps recently received on this
connection.
As in [RFC7323], ETS receivers need to exercise care to avoid
spurious PAWS discards due to wrapping 32-bit timestamp values during
periods in which the connection is idle. When microsecond units are
used, as in ETS, the 32-bit timestamp could trigger wrapping issues
and spurious PAWS discards after 2^31 ticks of idleness, which is
around 2147 seconds (or around 35.7 minutes). To prevent a false
positive PAWS rejection of a valid segment, an ETS receiver MUST skip
the PAWS check for the first arriving segment after the timestamp
used by PAWS, e.g. TS.Recent, has not been updated for 2147 seconds
or more.
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4.2. Eifel Considerations
The Eifel Detection Algorithm [RFC3522] detects a spurious recovery
by comparing a received TSecr to RetransmitTS, the value of the TSval
in the retransmit sent when loss recovery is initiated. ETS allows
Eifel to work as-is because the fields TSval and TSecr in the ETSopt
have the same semantics as in TSopt [RFC7323]. Further in sub-
millisecond environment, ETS microsecond precision is more effective
detecting spurious retransmission compared to TSopt's more coarse
unit.
4.3. RTO Calculation Considerations
The RTT measurement used in the calculation of RTO (retransmission
timeout) [RFC6298] stays the same as described in [RFC7323]. It is
NOT RECOMMENDED to use only NetworkRTT measurements for RTO
calculation because RTO needs to include the host side delays to
avoid spurious RTO events due to host delays.
With MaxACKDel information exchanged in the ETSopt of <SYN> and
<SYN,ACK>, senders MAY impose a minimum RTO that is greater than or
equal to the MaxACKDel.
4.4. SACK Considerations
The ETSopt has a length of 14 bytes on non-<SYN> segments. This
leaves a remaining space of 26 bytes for other TCP options. A SACK
option [RFC2018] with at most 3 SACK blocks is able to coexist with
ETSopt in a single TCP segment as with TSopt.
4.5. Interaction with Middleboxes
[HNRGHT11] shows that middleboxes could drop an unrecognized TCP
option or even drop the whole segment.
In order to fall back on [RFC7323] TSopt, the sender MAY include a
[RFC7323] TSopt in the <SYN> and <SYN,ACK> segments, so that the
[RFC7323] TSopt can be adopted when the ETSopt is stripped by a
middlebox.
Once an expected ETSopt is missing from an incoming segment, the
sender MUST NOT include an ETSopt for all future segments of this TCP
connection. An implementation could negatively cache such incidents
to avoid using ETS on these hosts or routes on future connections.
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5. IANA Considerations
This document specifies a new TCP option that uses the shared
experimental options format [RFC6994], with ExID in network-standard
byte order.
The authors plan to request the allocation of ExID value 0x4554 for
the TCP option specified in this document.
6. Security Considerations
A malicious receiver can manipulate the sender's network RTT
estimated by forging an EcrDel value. However this does not
introduce a new vulnerability relative to the Timestamp option
[RFC7323], because a malicious receiver could already forge the TSecr
field to manipulate the RTT measured by the other side.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options",
August 2013.
7.2. Informative References
[BMPR17] Barroso, L., Marty, M., Patterson, D., and P. Ranganathan,
"Attack of the Killer Microseconds", Communications of the
ACM , April 2017.
[HNRGHT11]
Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M., and H. Tokuda, "Is it still possible to
extend TCP?", InProceedings of the 2011 ACM SIGCOMM
conference on Internet measurement conference 2011 Nov 2
(pp.181-194) , 2011.
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[KDJWWM20]
Kumar, G., Dukkipati, N., Jang, K., Wassel, HM., Wu, X.,
Montazeri, B., Wang, Y., Springborn, K., Alfeld, C., Ryan,
M., and D. Wetherall, "Swift: Delay is Simple and
Effective for Congestion Control in the Datacenter",
InProceedings of the Annual conference of the ACM Special
Interest Group on Data Communication on the applications,
technologies, architectures, and protocols for computer
communication 2020 Jul 30 (pp514-528) , 2020.
[RFC1122] Braden, R., "Requirements for Internet hosts-communication
layers", October 1989.
[RFC3522] Ludwig, R. and M. Meyer, "The Eifel detection algorithm
for TCP", April 2003.
[RFC4015] Ludwig, R. and A. Gurtov, "The Eifel response algorithm
for TCP", February 2005.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", June 2011.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, "TCP Extensions for High Performance",
September 2014.
[WS95] Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:
The Implementation", 1995.
Authors' Addresses
Kevin (Yudong) Yang
Google, Inc
Email: yyd@google.com
Neal Cardwell
Google, Inc
Email: ncardwell@google.com
Yuchung Cheng
Google, Inc
Email: ycheng@google.com
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Eric Dumazet
Google, Inc
Email: edumazet@google.com
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