Internet DRAFT - draft-ietf-tcpm-2140bis
draft-ietf-tcpm-2140bis
TCPM WG J. Touch
Internet Draft Independent
Intended status: Informational M. Welzl
Obsoletes: 2140 S. Islam
Expires: October 2021 University of Oslo
April 12, 2021
TCP Control Block Interdependence
draft-ietf-tcpm-2140bis-11.txt
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Abstract
This memo provides guidance to TCP implementers that is intended to
help improve connection convergence to steady-state operation
without affecting interoperability. It updates and replaces RFC
2140's description of sharing TCP state, as typically represented in
TCP Control Blocks, among similar concurrent or consecutive
connections.
Table of Contents
1. Introduction...................................................3
2. Conventions Used in This Document..............................4
3. Terminology....................................................4
4. The TCP Control Block (TCB)....................................5
5. TCB Interdependence............................................7
6. Temporal Sharing...............................................7
6.1. Initialization of a new TCB..................................7
6.2. Updates to the TCB cache.....................................8
6.3. Discussion..................................................10
7. Ensemble Sharing..............................................11
7.1. Initialization of a new TCB.................................11
7.2. Updates to the TCB cache....................................12
7.3. Discussion..................................................13
8. Issues with TCB information sharing...........................14
8.1. Traversing the same network path............................15
8.2. State dependence............................................15
8.3. Problems with sharing based on IP address...................16
9. Implications..................................................16
9.1. Layering....................................................17
9.2. Other possibilities.........................................17
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10. Implementation Observations..................................18
11. Changes Compared to RFC 2140.................................19
12. Security Considerations......................................19
13. IANA Considerations..........................................20
14. References...................................................20
14.1. Normative References....................................20
14.2. Informative References..................................21
15. Acknowledgments..............................................24
16. Change log...................................................24
Appendix A : TCB Sharing History.................................28
Appendix B : TCP Option Sharing and Caching......................29
Appendix C : Automating the Initial Window in TCP over Long
Timescales.......................................................31
C.1. Introduction.............................................31
C.2. Design Considerations....................................31
C.3. Proposed IW Algorithm....................................32
C.4. Discussion...............................................36
C.5. Observations.............................................37
1. Introduction
TCP is a connection-oriented reliable transport protocol layered
over IP [RFC793]. Each TCP connection maintains state, usually in a
data structure called the TCP Control Block (TCB). The TCB contains
information about the connection state, its associated local
process, and feedback parameters about the connection's transmission
properties. As originally specified and usually implemented, most
TCB information is maintained on a per-connection basis. Some
implementations share certain TCB information across connections to
the same host [RFC2140]. Such sharing is intended to lead to better
overall transient performance, especially for numerous short-lived
and simultaneous connections, as can be used in the World-Wide Web
and other applications [Be94][Br02]. This sharing of state is
intended to help TCP connections converge to long term behavior
(assuming stable application load, i.e., so-called "steady-state")
more quickly without affecting TCP interoperability.
This document updates RFC 2140's discussion of TCB state sharing and
provides a complete replacement for that document. This state
sharing affects only TCB initialization [RFC2140] and thus has no
effect on the long-term behavior of TCP after a connection has been
established nor on interoperability. Path information shared across
SYN destination port numbers assumes that TCP segments having the
same host-pair experience the same path properties, i.e., that
traffic is not routed differently based on port numbers or other
connection parameters (also addressed further in Section 8.1). The
observations about TCB sharing in this document apply similarly to
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any protocol with congestion state, including SCTP [RFC4960] and
DCCP [RFC4340], as well as for individual subflows in Multipath TCP
[RFC8684].
2. Conventions Used in This Document
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.
The core of this document describes behavior that is already
permitted by TCP standards. As a result, it provides informative
guidance but does not use normative language, except when quoting
other documents. Normative language is used in Appendix C as
examples of requirements for future consideration.
3. Terminology
The following terminology is used frequently in this document. Items
preceded with a "+" may be part of the state maintained as TCP
connection state in the associated connections TCB and are the focus
of sharing as described in this document. Note that terms are used
as originally introduced where possible; in some cases, direction is
indicated with a suffix (_S for send, _R for receive) and in other
cases spelled out (sendcwnd).
+cwnd - TCP congestion window size [RFC5681]
host - a source or sink of TCP segments associated with a single IP
address
host-pair - a pair of hosts and their corresponding IP addresses
+MMS_R - maximum message size that can be received, the largest
received transport payload of an IP datagram [RFC1122]
+MMS_S - maximum message size that can be sent, the largest
transmitted transport payload of an IP datagram [RFC1122]
path - an Internet path between the IP addresses of two hosts
PCB - protocol control block, the data associated with a protocol as
maintained by an endpoint; a TCP PCB is called a TCB
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PLPMTUD - packetization-layer path MTU discovery, a mechanism that
uses transport packets to discover the PMTU [RFC4821]
+PMTU - largest IP datagram that can traverse a path
[RFC1191][RFC8201]
PMTUD - path-layer MTU discovery, a mechanism that relies on ICMP
error messages to discover the PMTU [RFC1191][RFC8201]
+RTT - round-trip time of a TCP packet exchange [RFC793]
+RTTVAR - variation of round-trip times of a TCP packet exchange
[RFC6298]
+rwnd - TCP receive window size [RFC5681]
+sendcwnd - TCP send-side congestion window (cwnd) size [RFC5681]
+sendMSS - TCP maximum segment size, a value transmitted in a TCP
option that represents the largest TCP user data payload that can be
received [RFC6691]
+ssthresh - TCP slow-start threshold [RFC5681]
TCB - TCP Control Block, the data associated with a TCP connection
as maintained by an endpoint
TCP-AO - TCP Authentication Option [RFC5925]
TFO - TCP Fast Open option [RFC7413]
+TFO_cookie - TCP Fast Open cookie, state that is used as part of
the TFO mechanism, when TFO is supported [RFC7413]
+TFO_failure - an indication of when TFO option negotiation failed,
when TFO is supported
+TFOinfo - information cached when a TFO connection is established,
which includes the TFO_cookie [RFC7413]
4. The TCP Control Block (TCB)
A TCB describes the data associated with each connection, i.e., with
each association of a pair of applications across the network. The
TCB contains at least the following information [RFC793]:
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Local process state
pointers to send and receive buffers
pointers to retransmission queue and current segment
pointers to Internet Protocol (IP) PCB
Per-connection shared state
macro-state
connection state
timers
flags
local and remote host numbers and ports
TCP option state
micro-state
send and receive window state (size*, current number)
congestion window size (sendcwnd)*
congestion window size threshold (ssthresh)*
max window size seen*
sendMSS#
MMS_S#
MMS_R#
PMTU#
round-trip time and its variation#
The per-connection information is shown as split into macro-state
and micro-state, terminology borrowed from [Co91]. Macro-state
describes the protocol for establishing the initial shared state
about the connection; we include the endpoint numbers and components
(timers, flags) required upon commencement that are later used to
help maintain that state. Micro-state describes the protocol after a
connection has been established, to maintain the reliability and
congestion control of the data transferred in the connection.
We distinguish two other classes of shared micro-state that are
associated more with host-pairs than with application pairs. One
class is clearly host-pair dependent (shown above as "#", e.g.,
sendMSS, MMS_R, MMS_S, PMTU, RTT), because these parameters are
defined by the endpoint or endpoint pair (sendMSS, MMS_R, MMS_S,
RTT) or are already cached and shared on that basis (PMTU
[RFC1191][RFC4821]). The other is host-pair dependent in its
aggregate (shown above as "*", e.g., congestion window information,
current window sizes, etc.) because they depend on the total
capacity between the two endpoints.
Not all of the TCB state is necessarily sharable. In particular,
some TCP options are negotiated only upon request by the application
layer, so their use may not be correlated across connections. Other
options negotiate connection-specific parameters, which are
similarly not shareable. These are discussed further in Appendix B.
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Finally, we exclude rwnd from further discussion because its value
should depend on the send window size, so it is already addressed by
send window sharing and is not independently affected by sharing.
5. TCB Interdependence
There are two cases of TCB interdependence. Temporal sharing occurs
when the TCB of an earlier (now CLOSED) connection to a host is used
to initialize some parameters of a new connection to that same host,
i.e., in sequence. Ensemble sharing occurs when a currently active
connection to a host is used to initialize another (concurrent)
connection to that host.
6. Temporal Sharing
The TCB data cache is accessed in two ways: it is read to initialize
new TCBs and written when more current per-host state is available.
6.1. Initialization of a new TCB
TCBs for new connections can be initialized using cached context
from past connections as follows:
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TEMPORAL SHARING - TCB Initialization
Cached TCB New TCB
--------------------------------------
old_MMS_S old_MMS_S or not cached*
old_MMS_R old_MMS_R or not cached*
old_sendMSS old_sendMSS
old_PMTU old_PMTU+
old_RTT old_RTT
old_RTTVAR old_RTTVAR
old_option (option specific)
old_ssthresh old_ssthresh
old_sendcwnd old_sendcwnd
+Note that PMTU is cached at the IP layer [RFC1191][RFC4821].
*Note that some values are not cached when they are computed locally
(MMS_R) or indicated in the connection itself (MMS_S in the SYN).
The table below gives an overview of option-specific information
that can be shared. Additional information on some specific TCP
options and sharing is provided in Appendix B.
TEMPORAL SHARING - Option Info Initialization
Cached New
------------------------------------
old_TFO_cookie old_TFO_cookie
old_TFO_failure old_TFO_failure
6.2. Updates to the TCB cache
During a connection, the TCB cache can be updated based on events of
current connections and their TCBs as they progress over time, as
shown below:
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TEMPORAL SHARING - Cache Updates
Cached TCB Current TCB when? New Cached TCB
----------------------------------------------------------
old_MMS_S curr_MMS_S OPEN curr_MMS_S
old_MMS_R curr_MMS_R OPEN curr_MMS_R
old_sendMSS curr_sendMSS MSSopt curr_sendMSS
old_PMTU curr_PMTU PMTUD+ / curr_PMTU
PLPMTUD+
old_RTT curr_RTT CLOSE merge(curr,old)
old_RTTVAR curr_RTTVAR CLOSE merge(curr,old)
old_option curr_option ESTAB (depends on option)
old_ssthresh curr_ssthresh CLOSE merge(curr,old)
old_sendcwnd curr_sendcwnd CLOSE merge(curr,old)
+Note that PMTU is cached at the IP layer [RFC1191][RFC4821].
Merge() is the function that combines the current and previous (old)
values and may vary for each parameter of the TCB cache. The
particular function is not specified in this document; examples
include windowed averages (mean of the past N values, for some N)
and exponential decay (new = (1-alpha)*old + alpha *new, where alpha
is in the range [0..1]).
The table below gives an overview of option-specific information
that can be similarly shared. The TFO cookie is maintained until the
client explicitly requests it be updated as a separate event.
TEMPORAL SHARING - Option Info Updates
Cached Current when? New Cached
---------------------------------------------------------
old_TFO_cookie old_TFO_cookie ESTAB old_TFO_cookie
old_TFO_failure old_TFO_failure ESTAB old_TFO_failure
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6.3. Discussion
As noted, there is no particular benefit to caching MMS_S and MMS_R
as these are reported by the local IP stack. Caching sendMSS and
PMTU is trivial; reported values are cached (PMTU at the IP layer),
and the most recent values are used. The cache is updated when the
MSS option is received in a SYN or after PMTUD (i.e., when an ICMPv4
Fragmentation Needed [RFC1191] or ICMPv6 Packet Too Big message is
received [RFC8201] or the equivalent is inferred, e.g., as from
PLPMTUD [RFC4821]), respectively, so the cache always has the most
recent values from any connection. For sendMSS, the cache is
consulted only at connection establishment and not otherwise
updated, which means that MSS options do not affect current
connections. The default sendMSS is never saved; only reported MSS
values update the cache, so an explicit override is required to
reduce the sendMSS. Cached sendMSS affects only data sent in the SYN
segment, i.e., during client connection initiation or during
simultaneous open; all other segment MSS are based on the value
updated as included in the SYN.
RTT values are updated by formulae that merges the old and new
values, as noted in Section 6.2. Dynamic RTT estimation requires a
sequence of RTT measurements. As a result, the cached RTT (and its
variation) is an average of its previous value with the contents of
the currently active TCB for that host, when a TCB is closed. RTT
values are updated only when a connection is closed. The method for
merging old and current values needs to attempt to reduce the
transient effects of the new connections.
The updates for RTT, RTTVAR and ssthresh rely on existing
information, i.e., old values. Should no such values exist, the
current values are cached instead.
TCP options are copied or merged depending on the details of each
option. E.g., TFO state is updated when a connection is established
and read before establishing a new connection.
Sections 8 and 9 discuss compatibility issues and implications of
sharing the specific information listed above. Section 10 gives an
overview of known implementations.
Most cached TCB values are updated when a connection closes. The
exceptions are MMS_R and MMS_S, which are reported by IP [RFC1122],
PMTU which is updated after Path MTU Discovery and also reported by
IP [RFC1191][RFC4821][RFC8201], and sendMSS, which is updated if the
MSS option is received in the TCP SYN header.
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Sharing sendMSS information affects only data in the SYN of the next
connection, because sendMSS information is typically included in
most TCP SYN segments. Caching PMTU can accelerate the efficiency of
PMTUD but can also result in black-holing until corrected if in
error. Caching MMS_R and MMS_S may be of little direct value as they
are reported by the local IP stack anyway.
The way in which other TCP option state can be shared depends on the
details of that option. E.g., TFO state includes the TCP Fast Open
Cookie [RFC7413] or, in case TFO fails, a negative TCP Fast Open
response. RFC 7413 states, "The client MUST cache negative responses
from the server in order to avoid potential connection failures.
Negative responses include the server not acknowledging the data in
the SYN, ICMP error messages, and (most importantly) no response
(SYN-ACK) from the server at all, i.e., connection timeout." [RFC
7413]. TFOinfo is cached when a connection is established.
Other TCP option state might not be as readily cached. E.g., TCP-AO
[RFC5925] success or failure between a host pair for a single SYN
destination port might be usefully cached. TCP-AO success or failure
to other SYN destination ports on that host pair is never useful to
cache because TCP-AO security parameters can vary per service.
7. Ensemble Sharing
Sharing cached TCB data across concurrent connections requires
attention to the aggregate nature of some of the shared state. For
example, although MSS and RTT values can be shared by copying, it
may not be appropriate to simply copy congestion window or ssthresh
information; instead, the new values can be a function (f) of the
cumulative values and the number of connections (N).
7.1. Initialization of a new TCB
TCBs for new connections can be initialized using cached context
from concurrent connections as follows:
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ENSEMBLE SHARING - TCB Initialization
Cached TCB New TCB
------------------------------------------
old_MMS_S old_MMS_S
old_MMS_R old_MMS_R
old_sendMSS old_sendMSS
old_PMTU old_PMTU+
old_RTT old_RTT
old_RTTVAR old_RTTVAR
sum(old_ssthresh) f(sum(old_ssthresh), N)
sum(old_sendcwnd) f(sum(old_sendcwnd), N)
_
old_option (option specific)
+Note that PMTU is cached at the IP layer [RFC1191][RFC4821].
In the table, the cached sum() is a total across all active
connections because these parameters act in aggregate; similarly f()
is a function that updates that sum based on the new connection's
values, represented as "N".
The table below gives an overview of option-specific information
that can be similarly shared. Again, The TFO_cookie is updated upon
explicit client request, which is a separate event.
ENSEMBLE SHARING - Option Info Initialization
Cached New
------------------------------------
old_TFO_cookie old_TFO_cookie
old_TFO_failure old_TFO_failure
7.2. Updates to the TCB cache
During a connection, the TCB cache can be updated based on changes
to concurrent connections and their TCBs, as shown below:
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ENSEMBLE SHARING - Cache Updates
Cached TCB Current TCB when? New Cached TCB
---------------------------------------------------------------
old_MMS_S curr_MMS_S OPEN curr_MMS_S
old_MMS_R curr_MMS_R OPEN curr_MMS_R
old_sendMSS curr_sendMSS MSSopt curr_sendMSS
old_PMTU curr_PMTU PMTUD+ / curr_PMTU
PLPMTUD+
old_RTT curr_RTT update rtt_update(old, curr)
old_RTTVAR curr_RTTVAR update rtt_update(old, curr)
old_ssthresh curr_ssthresh update adjust sum as appropriate
old_sendcwnd curr_sendcwnd update adjust sum as appropriate
old_option curr_option (depends) (option specific)
+Note that the PMTU is cached at the IP layer [RFC1191][RFC4821].
In the table, rtt_update() is the function used to combine old and
current values, e.g., as a windowed average or exponentially decayed
average.
The table below gives an overview of option-specific information
that can be similarly shared.
ENSEMBLE SHARING - Option Info Updates
Cached Current when? New Cached
----------------------------------------------------------
old_TFO_cookie old_TFO_cookie ESTAB old_TFO_cookie
old_TFO_failure old_TFO_failure ESTAB old_TFO_failure
7.3. Discussion
For ensemble sharing, TCB information should be cached as early as
possible, sometimes before a connection is closed. Otherwise,
opening multiple concurrent connections may not result in TCB data
sharing if no connection closes before others open. The amount of
work involved in updating the aggregate average should be minimized,
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but the resulting value should be equivalent to having all values
measured within a single connection. The function "rtt_update" in
the ensemble sharing table indicates this operation, which occurs
whenever the RTT would have been updated in the individual TCP
connection. As a result, the cache contains the shared RTT
variables, which no longer need to reside in the TCB.
Congestion window size and ssthresh aggregation are more complicated
in the concurrent case. When there is an ensemble of connections, we
need to decide how that ensemble would have shared these variables,
in order to derive initial values for new TCBs.
Sections 8 and 9 discuss compatibility issues and implications of
sharing the specific information listed above.
There are several ways to initialize the congestion window in a new
TCB among an ensemble of current connections to a host. Current TCP
implementations initialize it to four segments as standard [RFC3390]
and 10 segments experimentally [RFC6928]. These approaches assume
that new connections should behave as conservatively as possible.
The algorithm described in [Ba12] adjusts the initial cwnd depending
on the cwnd values of ongoing connections. It is also possible to
use sharing mechanisms over long timescales to adapt TCP's initial
window automatically, as described further in Appendix C.
8. Issues with TCB information sharing
Here, we discuss various types of problems that may arise with TCB
information sharing.
For the congestion and current window information, the initial
values computed by TCB interdependence may not be consistent with
the long-term aggregate behavior of a set of concurrent connections
between the same endpoints. Under conventional TCP congestion
control, if the congestion window of a single existing connection
has converged to 40 segments, two newly joining concurrent
connections assume initial windows of 10 segments [RFC6928], and the
current connection's window doesn't decrease to accommodate this
additional load and connections can mutually interfere. One example
of this is seen on low-bandwidth, high-delay links, where concurrent
connections supporting Web traffic can collide because their initial
windows were too large, even when set at one segment.
The authors of [Hu12] recommend caching ssthresh for temporal
sharing only when flows are long. Some studies suggest that sharing
ssthresh between short flows can deteriorate the performance of
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individual connections [Hu12, Du16], although this may benefit
aggregate network performance.
8.1. Traversing the same network path
TCP is sometimes used in situations where packets of the same host-
pair do not always take the same path, such as when connection-
specific parameters are used for routing (e.g., for load balancing).
Multipath routing that relies on examining transport headers, such
as ECMP and LAG [RFC7424], may not result in repeatable path
selection when TCP segments are encapsulated, encrypted, or altered
- for example, in some Virtual Private Network (VPN) tunnels that
rely on proprietary encapsulation. Similarly, such approaches cannot
operate deterministically when the TCP header is encrypted, e.g.,
when using IPsec ESP (although TCB interdependence among the entire
set sharing the same endpoint IP addresses should work without
problems when the TCP header is encrypted). Measures to increase the
probability that connections use the same path could be applied:
e.g., the connections could be given the same IPv6 flow label
[RFC6437]. TCB interdependence can also be extended to sets of host
IP address pairs that share the same network path conditions, such
as when a group of addresses is on the same LAN (see Section 9).
Traversing the same path is not important for host-specific
information such as rwnd and TCP option state, such as TFOinfo, or
for information that is already cached per-host, such as path MTU.
When TCB information is shared across different SYN destination
ports, path-related information can be incorrect; however, the
impact of this error is potentially diminished if (as discussed
here) TCB sharing affects only the transient event of a connection
start or if TCB information is shared only within connections to the
same SYN destination port.
In case of Temporal Sharing, TCB information could also become
invalid over time, i.e., indicating that although the path remains
the same, path properties have changed. Because this is similar to
the case when a connection becomes idle, mechanisms that address
idle TCP connections (e.g., [RFC7661]) could also be applied to TCB
cache management, especially when TCP Fast Open is used [RFC7413].
8.2. State dependence
There may be additional considerations to the way in which TCB
interdependence rebalances congestion feedback among the current
connections, e.g., it may be appropriate to consider the impact of a
connection being in Fast Recovery [RFC5681] or some other similar
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unusual feedback state, e.g., as inhibiting or affecting the
calculations described herein.
8.3. Problems with sharing based on IP address
It can be wrong to share TCB information between TCP connections on
the same host as identified by the IP address if an IP address is
assigned to a new host (e.g., IP address spinning, as is used by
ISPs to inhibit running servers). It can be wrong if Network Address
(and Port) Translation (NA(P)T) [RFC2663] or any other IP sharing
mechanism is used. Such mechanisms are less likely to be used with
IPv6. Other methods to identify a host could also be considered to
make correct TCB sharing more likely. Moreover, some TCB information
is about dominant path properties rather than the specific host. IP
addresses may differ, yet the relevant part of the path may be the
same.
9. Implications
There are several implications to incorporating TCB interdependence
in TCP implementations. First, it may reduce the need for
application-layer multiplexing for performance enhancement
[RFC7231]. Protocols like HTTP/2 [RFC7540] avoid connection
reestablishment costs by serializing or multiplexing a set of per-
host connections across a single TCP connection. This avoids TCP's
per-connection OPEN handshake and also avoids recomputing the MSS,
RTT, and congestion window values. By avoiding the so-called "slow-
start restart", performance can be optimized [Hu01]. TCB
interdependence can provide the "slow-start restart avoidance" of
multiplexing, without requiring a multiplexing mechanism at the
application layer.
Like the initial version of this document [RFC2140], this update's
approach to TCB interdependence focuses on sharing a set of TCBs by
updating the TCB state to reduce the impact of transients when
connections begin, end, or otherwise significantly change state.
Other mechanisms have since been proposed to continuously share
information between all ongoing communication (including
connectionless protocols), updating the congestion state during any
congestion-related event (e.g., timeout, loss confirmation, etc.)
[RFC3124]. By dealing exclusively with transients, the approach in
this document is more likely to exhibit the "steady-state" behavior
as unmodified, independent TCP connections.
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9.1. Layering
TCB interdependence pushes some of the TCP implementation from the
traditional transport layer (in the ISO model), to the network
layer. This acknowledges that some state is in fact per-host-pair or
can be per-path as indicated solely by that host-pair. Transport
protocols typically manage per-application-pair associations (per
stream), and network protocols manage per-host-pair and path
associations (routing). Round-trip time, MSS, and congestion
information could be more appropriately handled at the network
layer, aggregated among concurrent connections, and shared across
connection instances [RFC3124].
An earlier version of RTT sharing suggested implementing RTT state
at the IP layer, rather than at the TCP layer. Our observations
describe sharing state among TCP connections, which avoids some of
the difficulties in an IP-layer solution. One such problem of an IP
layer solution is determining the correspondence between packet
exchanges using IP header information alone, where such
correspondence is needed to compute RTT. Because TCB sharing
computes RTTs inside the TCP layer using TCP header information, it
can be implemented more directly and simply than at the IP layer.
This is a case where information should be computed at the transport
layer but could be shared at the network layer.
9.2. Other possibilities
Per-host-pair associations are not the limit of these techniques. It
is possible that TCBs could be similarly shared between hosts on a
subnet or within a cluster, because the predominant path can be
subnet-subnet, rather than host-host. Additionally, TCB
interdependence can be applied to any protocol with congestion
state, including SCTP [RFC4960] and DCCP [RFC4340], as well as for
individual subflows in Multipath TCP [RFC8684].
There may be other information that can be shared between concurrent
connections. For example, knowing that another connection has just
tried to expand its window size and failed, a connection may not
attempt to do the same for some period. The idea is that existing
TCP implementations infer the behavior of all competing connections,
including those within the same host or subnet. One possible
optimization is to make that implicit feedback explicit, via
extended information associated with the endpoint IP address and its
TCP implementation, rather than per-connection state in the TCB.
This document focuses on sharing TCB information at connection
initialization. Subsequent to RFC 2140, there have been numerous
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approaches that attempt to coordinate ongoing state across
concurrent connections, both within TCP and other congestion-
reactive protocols, which are summarized in [Is18]. These approaches
are more complex to implement and their comparison to steady-state
TCP equivalence can be more difficult to establish, sometimes
intentionally (i.e., they sometimes intend to provide a different
kind of "fairness" than emerges from TCP operation).
10. Implementation Observations
The observation that some TCB state is host-pair specific rather
than application-pair dependent is not new and is a common
engineering decision in layered protocol implementations. Although
now deprecated, T/TCP [RFC1644] was the first to propose using
caches in order to maintain TCB states (see Appendix A).
The table below describes the current implementation status for TCB
temporal sharing in Windows as of December 2020, Apple variants
(macOS, iOS, iPadOS, tvOS, watchOS) as of January 2021, Linux kernel
version 5.10.3, and FreeBSD 12. Ensemble sharing is not yet
implemented.
KNOWN IMPLEMENTATION STATUS
TCB data Status
------------------------------------------------------------
old_MMS_S Not shared
old_MMS_R Not shared
old_sendMSS Cached and shared in Apple, Linux (MSS)
old_PMTU Cached and shared in Apple, FreeBSD, Windows (PMTU)
old_RTT Cached and shared in Apple, FreeBSD, Linux, Windows
old_RTTVAR Cached and shared in Apple, FreeBSD, Windows
old_TFOinfo Cached and shared in Apple, Linux, Windows
old_sendcwnd Not shared
old_ssthresh Cached and shared in Apple, FreeBSD*, Linux*
TFO failure Cached and shared in Apple
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In the table above, "Apple" refers to all Apple OSes, i.e.,
desktop/laptop macOS, phone iOS, pad iPadOS, video player tvOS, and
watch watchOS, which all share the same Internet protocol stack.
*Note: In FreeBSD, new ssthresh is the mean of curr_ssthresh and
previous value if a previous value exists; in Linux, the calculation
depends on state and is max(curr_cwnd/2, old_ssthresh) in most
cases.
11. Changes Compared to RFC 2140
This document updates the description of TCB sharing in RFC 2140 and
its associated impact on existing and new connection state,
providing a complete replacement for that document [RFC2140]. It
clarifies the previous description and terminology and extends the
mechanism to its impact on new protocols and mechanisms, including
multipath TCP, fast open, PLPMTUD, NAT, and the TCP Authentication
Option.
The detailed impact on TCB state addresses TCB parameters in greater
detail, addressing MSS in both the send and receive direction, MSS
and sendMSS separately, adds path MTU and ssthresh, and addresses
the impact on TCP option state.
New sections have been added to address compatibility issues and
implementation observations. The relation of this work to T/TCP has
been moved to 0 on history, partly to reflect the deprecation of
that protocol.
Appendix C has been added to discuss the potential to use temporal
sharing over long timescales to adapt TCP's initial window
automatically, avoiding the need to periodically revise a single
global constant value.
Finally, this document updates and significantly expands the
referenced literature.
12. Security Considerations
These presented implementation methods do not have additional
ramifications for direct (connection-aborting or information
injecting) attacks on individual connections. Individual
connections, whether using sharing or not, also may be susceptible
to denial-of-service attacks that reduce performance or completely
deny connections and transfers if not otherwise secured.
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TCB sharing may create additional denial-of-service attacks that
affect the performance of other connections by polluting the cached
information. This can occur across whatever set of connections where
the TCB is shared, between connections in a single host, or between
hosts if TCB sharing is implemented within a subnet (see
Implications section). Some shared TCB parameters are used only to
create new TCBs, others are shared among the TCBs of ongoing
connections. New connections can join the ongoing set, e.g., to
optimize send window size among a set of connections to the same
host. PMTU is defined as shared at the IP layer, and is already
susceptible in this way.
Options in client SYNs can be easier to forge than complete, two-way
connections. As a result, their values may not be safely
incorporated in shared values until after the three-way handshake
completes.
Attacks on parameters used only for initialization affect only the
transient performance of a TCP connection. For short connections,
the performance ramification can approach that of a denial-of-
service attack. E.g., if an application changes its TCB to have a
false and small window size, subsequent connections will experience
performance degradation until their window grew appropriately.
TCB sharing reuses and mixes information from past and current
connections. Although reusing information could create a potential
for fingerprinting to identify hosts, the mixing reduces that
potential. There has been no evidence of fingerprinting based on
this technique and it is currently considered safe in that regard.
Further, information about the performance of a TCP connection has
not been considered as private.
13. IANA Considerations
There are no IANA implications or requests in this document.
This section should be removed upon final publication as an RFC.
14. References
14.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol," Network
Working Group RFC-793/STD-7, ISI, Sept. 1981.
[RFC1122] Braden, R. (ed), "Requirements for Internet Hosts --
Communication Layers", RFC-1122, Oct. 1989.
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[RFC1191] Mogul, J., Deering, S., "Path MTU Discovery," RFC 1191,
Nov. 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4821] Mathis, M., Heffner, J., "Packetization Layer Path MTU
Discovery," RFC 4821, Mar. 2007.
[RFC5681] Allman, M., Paxson, V., Blanton, E., "TCP Congestion
Control," RFC 5681 (Standards Track), Sep. 2009.
[RFC6298] Paxson, V., Allman, M., Chu, J., Sargent, M., "Computing
TCP's Retransmission Timer," RFC 6298, June 2011.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., Jain, A., "TCP Fast
Open", RFC 7413, Dec. 2014.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", RFC 8174, May 2017.
[RFC8201] McCann, J., Deering. S., Mogul, J., Hinden, R. (Ed.),
"Path MTU Discovery for IP version 6," RFC 8201, Jul.
2017.
14.2. Informative References
[Al10] Allman, M., "Initial Congestion Window Specification",
(work in progress), draft-allman-tcpm-bump-initcwnd-00,
Nov. 2010.
[Ba12] Barik, R., Welzl, M., Ferlin, S., Alay, O., " LISA: A
Linked Slow-Start Algorithm for MPTCP", IEEE ICC, Kuala
Lumpur, Malaysia, May 23-27 2016.
[Ba20] Bagnulo, M., Briscoe, B., "ECN++: Adding Explicit
Congestion Notification (ECN) to TCP Control Packets",
draft-ietf-tcpm-generalized-ecn-07, Feb. 2021.
[Be94] Berners-Lee, T., et al., "The World-Wide Web,"
Communications of the ACM, V37, Aug. 1994, pp. 76-82.
[Br94] Braden, B., "T/TCP -- Transaction TCP: Source Changes for
Sun OS 4.1.3,", Release 1.0, USC/ISI, September 14, 1994.
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[Br02] Brownlee, N., Claffy, K., "Understanding Internet Traffic
Streams: Dragonflies and Tortoises", IEEE Communications
Magazine p110-117, 2002.
[Co91] Comer, D., Stevens, D., Internetworking with TCP/IP, V2,
Prentice-Hall, NJ, 1991.
[Du16] Dukkipati, N., Yuchung C., Amin V., "Research Impacting
the Practice of Congestion Control." ACM SIGCOMM CCR
(editorial), on-line post, July 2016.
[FreeBSD] FreeBSD source code, Release 2.10, http://www.freebsd.org/
[Hu01] Hughes, A., Touch, J., Heidemann, J., "Issues in Slow-
Start Restart After Idle", draft-hughes-restart-00
(expired), Dec. 2001.
[Hu12] Hurtig, P., Brunstrom, A., "Enhanced metric caching for
short TCP flows," 2012 IEEE International Conference on
Communications (ICC), Ottawa, ON, 2012, pp. 1209-1213.
[IANA] IANA TCP Parameters (options) registry,
https://www.iana.org/assignments/tcp-parameters
[Is18] Islam, S., Welzl, M., Hiorth, K., Hayes, D., Armitage, G.,
Gjessing, S., "ctrlTCP: Reducing Latency through Coupled,
Heterogeneous Multi-Flow TCP Congestion Control," Proc.
IEEE INFOCOM Global Internet Symposium (GI) workshop (GI
2018), Honolulu, HI, April 2018.
[Ja88] Jacobson, V., Karels, M., "Congestion Avoidance and
Control", Proc. Sigcomm 1988.
[RFC1644] Braden, R., "T/TCP -- TCP Extensions for Transactions
Functional Specification," RFC-1644, July 1994.
[RFC1379] Braden, R., "Transaction TCP -- Concepts," RFC-1379,
September 1992.
[RFC2001] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC2001
(Standards Track), Jan. 1997.
[RFC2140] Touch, J., "TCP Control Block Interdependence", RFC 2140,
April 1997.
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[RFC2414] Allman, M., Floyd, S., Partridge, C., "Increasing TCP's
Initial Window", RFC 2414 (Experimental), Sept. 1998.
[RFC2663] Srisuresh, P., Holdrege, M., "IP Network Address
Translator (NAT) Terminology and Considerations", RFC-
2663, August 1999.
[RFC3390] Allman, M., Floyd, S., Partridge, C., "Increasing TCP's
Initial Window," RFC 3390, Oct. 2002.
[RFC3124] Balakrishnan, H., Seshan, S., "The Congestion Manager,"
RFC 3124, June 2001.
[RFC4340] Kohler, E., Handley, M., Floyd, S., "Datagram Congestion
Control Protocol (DCCP)," RFC 4340, Mar. 2006.
[RFC4960] Stewart, R., (Ed.), "Stream Control Transmission
Protocol," RFC4960, Sept. 2007.
[RFC5925] Touch, J., Mankin, A., Bonica, R., "The TCP Authentication
Option," RFC 5925, June 2010.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., Rajajalme, J., "IPv6
Flow Label Specification," RFC 6437, Nov. 2011.
[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS),"
RFC 6691, July 2012.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., Mathis, M., "Increasing
TCP's Initial Window," RFC 6928, Apr. 2013.
[RFC7231] Fielding, R., Reshke, J., Eds., "HTTP/1.1 Semantics and
Content," RFC-7231, June 2014.
[RFC7323] Borman, D., Braden, B., Jacobson, V., Scheffenegger, R.,
(Ed.), "TCP Extensions for High Performance," RFC 7323,
Sept. 2014.
[RFC7424] Krishnan, R., Yong, L., Ghanwani, A., So, N., Khasnabish,
B., "Mechanisms for Optimizing Link Aggregation Group
(LAG) and Equal-Cost Multipath (ECMP) Component Link
Utilization in Networks", RFC 7424, Jan. 2015
[RFC7540] Belshe, M., Peon, R., Thomson, M., "Hypertext Transfer
Protocol Version 2 (HTTP/2)", RFC 7540, May 2015.
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[RFC7661] Fairhurst, G., Sathiaseelan, A., Secchi, R., "Updating TCP
to Support Rate-Limited Traffic", RFC 7661, Oct. 2015.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O.,
Paasch, C., "TCP Extensions for Multipath Operation with
Multiple Addresses," RFC 8684, Mar. 2020.
15. Acknowledgments
The authors would like to thank for Praveen Balasubramanian for
information regarding TCB sharing in Windows, Christoph Paasch for
information regarding TCB sharing in Apple OSes, and Yuchung Cheng,
Lars Eggert, Ilpo Jarvinen and Michael Scharf for comments on
earlier versions of the draft, as well as members of the TCPM WG.
Earlier revisions of this work received funding from a collaborative
research project between the University of Oslo and Huawei
Technologies Co., Ltd. and were partly supported by USC/ISI's Postel
Center.
This document was prepared using 2-Word-v2.0.template.dot.
16. Change log
This section should be removed upon final publication as an RFC.
ietf-11:
- Addressed gen-art review and IESG feedback
ietf-10:
- Addressed IETF last call feedback
ietf-09:
- Correction of typographic errors
ietf-08:
- Address TSV AD comments, add Apple OS implementation status
ietf-07:
- Update per id-nits and normative language for consistency
ietf-06:
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- Address WGLC comments
ietf-05:
- Correction of typographic errors, expansion of terminology
ietf-04:
- Fix internal cross-reference errors that appeared in ietf-02
- Updated tables to re-center; clarified text
ietf-03:
- Correction of typographic errors, minor rewording in appendices
ietf-02:
- Minor reorganization and correction of typographic errors
- Added text to address fingerprinting in Security section
- Now retains Appendix B and body option tables upon publication
ietf-01:
- Added Appendix C to address long-timescale temporal adaptation
ietf-00:
- Re-issued as draft-ietf-tcpm-2140bis due to WG adoption.
- Cleaned orphan references to T/TCP, removed incomplete refs
- Moved references to informative section and updated Sec 2
- Updated to clarify no impact to interoperability
- Updated appendix B to avoid 2119 language
06:
- Changed to update 2140, cite it normatively, and summarize the
updates in a separate section
05:
- Fixed some TBDs
04:
- Removed BCP-style recommendations and fixed some TBDs
03:
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- Updated Touch's affiliation and address information
02:
- Stated that our OS implementation overview table only covers
temporal sharing.
- Correctly reflected sharing of old_RTT in Linux in the
implementation overview table.
- Marked entries that are considered safe to share with an
asterisk (suggestion was to split the table)
- Discussed correct host identification: NATs may make IP
addresses the wrong input, could e.g., use HTTP cookie.
- Included MMS_S and MMS_R from RFC1122; fixed the use of MSS and
MTU
- Added information about option sharing, listed options in
Appendix B
Authors' Addresses
Joe Touch
Manhattan Beach, CA 90266
USA
Phone: +1 (310) 560-0334
Email: touch@strayalpha.com
Michael Welzl
University of Oslo
PO Box 1080 Blindern
Oslo N-0316
Norway
Phone: +47 22 85 24 20
Email: michawe@ifi.uio.no
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Safiqul Islam
University of Oslo
PO Box 1080 Blindern
Oslo N-0316
Norway
Phone: +47 22 84 08 37
Email: safiquli@ifi.uio.no
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Appendix A: TCB Sharing History
T/TCP proposed using caches to maintain TCB information across
instances (temporal sharing), e.g., smoothed RTT, RTT variation,
congestion avoidance threshold, and MSS [RFC1644]. These values were
in addition to connection counts used by T/TCP to accelerate data
delivery prior to the full three-way handshake during an OPEN. The
goal was to aggregate TCB components where they reflect one
association - that of the host-pair, rather than artificially
separating those components by connection.
At least one T/TCP implementation saved the MSS and aggregated the
RTT parameters across multiple connections but omitted caching the
congestion window information [Br94], as originally specified in
[RFC1379]. Some T/TCP implementations immediately updated MSS when
the TCP MSS header option was received [Br94], although this was not
addressed specifically in the concepts or functional specification
[RFC1379][RFC1644]. In later T/TCP implementations, RTT values were
updated only after a CLOSE, which does not benefit concurrent
sessions.
Temporal sharing of cached TCB data was originally implemented in
the SunOS 4.1.3 T/TCP extensions [Br94] and the FreeBSD port of same
[FreeBSD]. As mentioned before, only the MSS and RTT parameters were
cached, as originally specified in [RFC1379]. Later discussion of
T/TCP suggested including congestion control parameters in this
cache; for example, [RFC1644] (Section 3.1) hints at initializing
the congestion window to the old window size.
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Appendix B: TCP Option Sharing and Caching
In addition to the options that can be cached and shared, this memo
also lists known TCP options [IANA] for which state is unsafe to be
kept. This list is not intended to be authoritative or exhaustive.
Obsolete (unsafe to keep state):
ECHO
ECHO REPLY
PO Conn permitted
PO service profile
CC
CC.NEW
CC.ECHO
Alt CS req
Alt CS data
No state to keep:
EOL
NOP
WS
SACK
TS
MD5
TCP-AO
EXP1
EXP2
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Unsafe to keep state:
Skeeter (DH exchange, known to be vulnerable)
Bubba (DH exchange, known to be vulnerable)
Trailer CS
SCPS capabilities
S-NACK
Records boundaries
Corruption experienced
SNAP
TCP Compression
Quickstart response
UTO
MPTCP negotiation success (see below for negotiation failure)
TFO negotiation success (see below for negotiation failure)
Safe but optional to keep state:
MPTCP negotiation failure (to avoid negotiation retries)
MSS
TFO negotiation failure (to avoid negotiation retries)
Safe and necessary to keep state:
TFO cookie (if TFO succeeded in the past)
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Appendix C: Automating the Initial Window in TCP over Long Timescales
C.1. Introduction
Temporal sharing, as described earlier in this document, builds on
the assumption that multiple consecutive connections between the
same host pair are somewhat likely to be exposed to similar
environment characteristics. The stored information can become less
accurate over time and suitable precautions should take this ageing
into consideration (this is discussed further in section 8.1).
However, there are also cases where it can make sense to track these
values over longer periods, observing properties of TCP connections
to gradually influence evolving trends in TCP parameters. This
appendix describes an example of such a case.
TCP's congestion control algorithm uses an initial window value
(IW), both as a starting point for new connections and as an upper
limit for restarting after an idle period [RFC5681][RFC7661]. This
value has evolved over time, originally one maximum segment size
(MSS), and increased to the lesser of four MSS or 4,380 bytes
[RFC3390][RFC5681]. For a typical Internet connection with a maximum
transmission unit (MTU) of 1500 bytes, this permits three segments
of 1,460 bytes each.
The IW value was originally implied in the original TCP congestion
control description and documented as a standard in 1997
[RFC2001][Ja88]. The value was updated in 1998 experimentally and
moved to the standards track in 2002 [RFC2414][RFC3390]. In 2013, it
was experimentally increased to 10 [RFC6928].
This appendix discusses how TCP can objectively measure when an IW
is too large, and that such feedback should be used over long
timescales to adjust the IW automatically. The result should be
safer to deploy and might avoid the need to repeatedly revisit IW
over time.
Note that this mechanism attempts to make the IW more adaptive over
time. It can increase the IW beyond that which is currently
recommended for widescale deployment, and so its use should be
carefully monitored.
C.2. Design Considerations
TCP's IW value has existed statically for over two decades, so any
solution to adjusting the IW dynamically should have similarly
stable, non-invasive effects on the performance and complexity of
TCP. In order to be fair, the IW should be similar for most machines
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on the public Internet. Finally, a desirable goal is to develop a
self-correcting algorithm, so that IW values that cause network
problems can be avoided. To that end, we propose the following
design goals:
o Impart little to no impact to TCP in the absence of loss, i.e.,
it should not increase the complexity of default packet
processing in the normal case.
o Adapt to network feedback over long timescales, avoiding values
that persistently cause network problems.
o Decrease the IW in the presence of sustained loss of IW segments,
as determined over a number of different connections.
o Increase the IW in the absence of sustained loss of IW segments,
as determined over a number of different connections.
o Operate conservatively, i.e., tend towards leaving the IW the
same in the absence of sufficient information, and give greater
consideration to IW segment loss than IW segment success.
We expect that, without other context, a good IW algorithm will
converge to a single value, but this is not required. An endpoint
with additional context or information, or deployed in a constrained
environment, can always use a different value. In particular,
information from previous connections, or sets of connections with a
similar path, can already be used as context for such decisions (as
noted in the core of this document).
However, if a given IW value persistently causes packet loss during
the initial burst of packets, it is clearly inappropriate and could
be inducing unnecessary loss in other competing connections. This
might happen for sites behind very slow boxes with small buffers,
which may or may not be the first hop.
C.3. Proposed IW Algorithm
Below is a simple description of the proposed IW algorithm. It
relies on the following parameters:
o MinIW = 3 MSS or 4,380 bytes (as per [RFC3390])
o MaxIW = 10 MSS (as per [RFC6928])
o MulDecr = 0.5
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o AddIncr = 2 MSS
o Threshold = 0.05
We assume that the minimum IW (MinIW) should be as currently
specified as standard [RFC3390]. The maximum IW can be set to a
fixed value (we suggest using the experimental and now somewhat de-
facto standard in [RFC6928]) or set based on a schedule if trusted
time references are available [Al10]; here we prefer a fixed value.
We also propose to use an AIMD algorithm, with increase and
decreases as noted.
Although these parameters are somewhat arbitrary, their initial
values are not important except that the algorithm is AIMD and the
MaxIW should not exceed that recommended for other systems on the
Internet (here we selected the current de-facto standard rather than
the actual standard). Current proposals, including default current
operation, are degenerate cases of the algorithm below for given
parameters - notably MulDec = 1.0 and AddIncr = 0 MSS, thus
disabling the automatic part of the algorithm.
The proposed algorithm is as follows:
1. On boot:
IW = MaxIW; # assume this is in bytes, and indicates an integer
multiple of 2 MSS (an even number to support ACK compression)
2. Upon starting a new connection:
CWND = IW;
conncount++;
IWnotchecked = 1; # true
3. During a connection's SYN-ACK processing, if SYN-ACK includes ECN
(as similarly addressed in Sec 5 of ECN++ for TCP [Ba20]), treat
as if the IW is too large:
if (IWnotchecked && (synackecn == 1)) {
losscount++;
IWnotchecked = 0; # never check again
}
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4. During a connection, if retransmission occurs, check the seqno of
the outgoing packet (in bytes) to see if the resent segment fixes
an IW loss:
if (Retransmitting && IWnotchecked && ((seqno - ISN) < IW))) {
losscount++;
IWnotchecked = 0; # never do this entire "if" again
} else {
IWnotchecked = 0; # you're beyond the IW so stop checking
}
5. Once every 1000 connections, as a separate process (i.e., not as
part of processing a given connection):
if (conncount > 1000) {
if (losscount/conncount > threshold) {
# the number of connections with errors is too high
IW = IW * MulDecr;
} else {
IW = IW + AddIncr;
}
}
As presented, this algorithm can yield a false positive when the
sequence number wraps around, e.g., the code might increment
losscount in step 4 when no loss occurred or fail to increment
losscount when a loss did occur. This can be avoided using either
PAWS [RFC7323] context or internal extended sequence number
representations (as in TCP-AO [RFC5925]). Alternately, false
positives can be tolerated because they are expected to be
infrequent and thus will not significantly impact the algorithm.
A number of additional constraints need to be imposed if this
mechanism is implemented to ensure that it defaults to values that
comply with current Internet standards, is conservative in how it
extends those values, and returns to those values in the absence of
positive feedback (i.e., success). To that end, we recommend the
following list of example constraints:
>> The automatic IW algorithm MUST initialize MaxIW a value no
larger than the currently recommended Internet default, in the
absence of other context information.
Thus, if there are too few connections to make a decision or if
there is otherwise insufficient information to increase the IW, then
the MaxIW defaults to the current recommended value.
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>> An implementation MAY allow the MaxIW to grow beyond the
currently recommended Internet default, but not more than 2 segments
per calendar year.
Thus, if an endpoint has a persistent history of successfully
transmitting IW segments without loss, then it is allowed to probe
the Internet to determine if larger IW values have similar success.
This probing is limited and requires a trusted time source,
otherwise the MaxIW remains constant.
>> An implementation MUST adjust the IW based on loss statistics at
least once every 1000 connections.
An endpoint needs to be sufficiently reactive to IW loss.
>> An implementation MUST decrease the IW by at least one MSS when
indicated during an evaluation interval.
An endpoint that detects loss needs to decrease its IW by at least
one MSS, otherwise it is not participating in an automatic reactive
algorithm.
>> An implementation MUST increase by no more than 2 MSS per
evaluation interval.
An endpoint that does not experience IW loss needs to probe the
network incrementally.
>> An implementation SHOULD use an IW that is an integer multiple of
2 MSS.
The IW should remain a multiple of 2 MSS segments, to enable
efficient ACK compression without incurring unnecessary timeouts.
>> An implementation MUST decrease the IW if more than 95% of
connections have IW losses.
Again, this is to ensure an implementation is sufficiently reactive.
>> An implementation MAY group IW values and statistics within
subsets of connections. Such grouping MAY use any information about
connections to form groups except loss statistics.
There are some TCP connections which might not be counted at all,
such as those to/from loopback addresses, or those within the same
subnet as that of a local interface (for which congestion control is
sometimes disabled anyway). This may also include connections that
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terminate before the IW is full, i.e., as a separate check at the
time of the connection closing.
The period over which the IW is updated is intended to be a long
timescale, e.g., a month or so, or 1,000 connections, whichever is
longer. An implementation might check the IW once a month, and
simply not update the IW or clear the connection counts in months
where the number of connections is too small.
C.4. Discussion
There are numerous parameters to the above algorithm that are
compliant with the given requirements; this is intended to allow
variation in configuration and implementation while ensuring that
all such algorithms are reactive and safe.
This algorithm continues to assume segments because that is the
basis of most TCP implementations. It might be useful to consider
revising the specifications to allow byte-based congestion given
sufficient experience.
The algorithm checks for IW losses only during the first IW after a
connection start; it does not check for IW losses elsewhere the IW
is used, e.g., during slow-start restarts.
>> An implementation MAY detect IW losses during slow-start restarts
in addition to losses during the first IW of a connection. In this
case, the implementation MUST count each restart as a "connection"
for the purposes of connection counts and periodic rechecking of the
IW value.
False positives can occur during some kinds of segment reordering,
e.g., that might trigger spurious retransmissions even without a
true segment loss. These are not expected to be sufficiently common
to dominate the algorithm and its conclusions.
This mechanism does require additional per-connection state, which
is currently common in some implementations, and is useful for other
reasons (e.g., the ISN is used in TCP-AO [RFC5925]). The mechanism
also benefits from persistent state kept across reboots, as would be
other state sharing mechanisms (e.g., TCP Control Block Sharing per
the main body of this document).
The receive window (rwnd) is not involved in this calculation. The
size of rwnd is determined by receiver resources and provides space
to accommodate segment reordering. It is not involved with
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congestion control, which is the focus of this document and its
management of the IW.
C.5. Observations
The IW may not converge to a single, global value. It also may not
converge at all, but rather may oscillate by a few MSS as it
repeatedly probes the Internet for larger IWs and fails. Both
properties are consistent with TCP behavior during each individual
connection.
This mechanism assumes that losses during the IW are due to IW size.
Persistent errors that drop packets for other reasons - e.g., OS
bugs, can cause false positives. Again, this is consistent with
TCP's basic assumption that loss is caused by congestion and
requires backoff. This algorithm treats the IW of new connections as
a long-timescale backoff system.
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