draft-ietf-tsvwg-initwin-03.txt   draft-ietf-tsvwg-initwin-04.txt 
Internet Engineering Task Force Mark Allman Internet Engineering Task Force Mark Allman
INTERNET DRAFT BBN/NASA GRC INTERNET DRAFT BBN/NASA GRC
File: draft-ietf-tsvwg-initwin-03.txt April, 2002 File: draft-ietf-tsvwg-initwin-04.txt Sally Floyd
Expires: October, 2002
Sally Floyd
ICIR ICIR
Craig Partridge Craig Partridge
BBN Technologies BBN Technologies
June, 2002
Expires: December, 2002
Increasing TCP's Initial Window Increasing TCP's Initial Window
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
skipping to change at page 1, line 38 skipping to change at page 1, line 37
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
Abstract Abstract
This document specifies an optional standard for TCP to increase the This document specifies an optional standard for TCP to increase the
permitted initial window from one segment to roughly 4K bytes, permitted initial window from one or two segment(s) to roughly 4K bytes,
replacing RFC 2414. This document discusses the advantages and replacing RFC 2414. This document discusses the advantages and
disadvantages of the higher initial window. The document includes disadvantages of the higher initial window. The document includes
discussion of experiments and simulations showing that the higher discussion of experiments and simulations showing that the higher
initial window does not lead to congestion collapse. Finally, the initial window does not lead to congestion collapse. Finally, the
document provides guidance on implementation issues. document provides guidance on implementation issues.
Terminology Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
1. TCP Modification 1. TCP Modification
This document updates [RFC2414] and specifies an increase in the This document updates [RFC2414] and specifies an increase in the
permitted upper bound for TCP's initial window from one segment to permitted upper bound for TCP's initial window from one or two
between two and four segments. In most cases, this change results segment(s) to between two and four segments. In most cases, this
in an upper bound on the initial window of roughly 4K bytes change results in an upper bound on the initial window of roughly 4K
(although given a large segment size, the permitted initial window bytes (although given a large segment size, the permitted initial
of two segments may be significantly larger than 4K bytes). The window of two segments may be significantly larger than 4K bytes).
upper bound for the initial window is given more precisely in (1): The upper bound for the initial window is given more precisely in
(1):
min (4*MSS, max (2*MSS, 4380 bytes)) (1) min (4*MSS, max (2*MSS, 4380 bytes)) (1)
Note: Sending a 1500 byte packet indicates an MSS of 1460 bytes
(assuming no IP or TCP options). Therefore, limiting the initial
window's MSS to 4380 bytes allows the sender to transmit three
segments initially in the common case when using 1500 byte packets.
Equivalently, the upper bound for the initial window size is based Equivalently, the upper bound for the initial window size is based
on the maximum segment size (MSS), as follows: on the maximum segment size (MSS), as follows:
If (MSS <= 1095 bytes) If (MSS <= 1095 bytes)
then win <= 4 * MSS; then win <= 4 * MSS;
If (1095 bytes < MSS < 2190 bytes) If (1095 bytes < MSS < 2190 bytes)
then win <= 4380; then win <= 4380;
If (2190 bytes <= MSS) If (2190 bytes <= MSS)
then win <= 2 * MSS; then win <= 2 * MSS;
This increased initial window is optional: that a TCP MAY start with This increased initial window is optional: a TCP MAY start with a
a larger initial window. However, we expect that most larger initial window. However, we expect that most general-purpose
general-purpose TCP implementations would choose to use the larger TCP implementations would choose to use the larger initial
initial congestion window given in equation (1) above. congestion window given in equation (1) above.
This upper bound for the initial window size represents a change This upper bound for the initial window size represents a change
from RFC 2581 [RFC2581], which specified that the congestion window from RFC 2581 [RFC2581], which specified that the congestion window
be initialized to one or two segments. be initialized to one or two segments.
This change applies to the initial window of the connection in the This change applies to the initial window of the connection in the
first round trip time (RTT) of data transmission following the TCP three- first round trip time (RTT) of data transmission following the TCP
way handshake. Neither the SYN/ACK nor its acknowledgment (ACK) in three- way handshake. Neither the SYN/ACK nor its acknowledgment
the three-way handshake should increase the initial window size (ACK) in the three-way handshake should increase the initial window
above that outlined in equation (1). If the SYN or SYN/ACK is lost, size above that outlined in equation (1). If the SYN or SYN/ACK is
the initial window used by a sender after a correctly transmitted lost, the initial window used by a sender after a correctly
SYN MUST be one segment consisting of MSS bytes. transmitted SYN MUST be one segment consisting of MSS bytes.
TCP implementations use slow start in as many as three different TCP implementations use slow start in as many as three different
ways: (1) to start a new connection (the initial window); (2) to ways: (1) to start a new connection (the initial window); (2) to
restart transmission after a long idle period (the restart window); restart transmission after a long idle period (the restart window);
and (3) to restart transmission after a retransmit timeout (the loss and (3) to restart transmission after a retransmit timeout (the loss
window). The change specified in this document affects the value of window). The change specified in this document affects the value of
the initial window. Optionally, a TCP MAY set the restart window to the initial window. Optionally, a TCP MAY set the restart window to
the minimum of the value used for the initial window and the current the minimum of the value used for the initial window and the current
value of cwnd (in other words, using a larger value for the restart value of cwnd (in other words, using a larger value for the restart
window should never increase the size of cwnd). These changes do window should never increase the size of cwnd). These changes do
skipping to change at page 2, line 51 skipping to change at page 3, line 4
window). The change specified in this document affects the value of window). The change specified in this document affects the value of
the initial window. Optionally, a TCP MAY set the restart window to the initial window. Optionally, a TCP MAY set the restart window to
the minimum of the value used for the initial window and the current the minimum of the value used for the initial window and the current
value of cwnd (in other words, using a larger value for the restart value of cwnd (in other words, using a larger value for the restart
window should never increase the size of cwnd). These changes do window should never increase the size of cwnd). These changes do
NOT change the loss window, which must remain 1 segment of MSS bytes NOT change the loss window, which must remain 1 segment of MSS bytes
(to permit the lowest possible window size in the case of severe (to permit the lowest possible window size in the case of severe
congestion). congestion).
2. Implementation Issues 2. Implementation Issues
When larger initial windows are implemented along with Path MTU When larger initial windows are implemented along with Path MTU
Discovery [RFC1191], and the MSS being used is found to be too large, Discovery [RFC1191], and the MSS being used is found to be too
the congestion window `cwnd' SHOULD be reduced to prevent large large, the congestion window `cwnd' SHOULD be reduced to prevent
bursts of smaller segments. Specifically, `cwnd' SHOULD be reduced large bursts of smaller segments. Specifically, `cwnd' SHOULD be
by the ratio of the old segment size to the new segment size. reduced by the ratio of the old segment size to the new segment
size.
When larger initial windows are implemented along with Path MTU When larger initial windows are implemented along with Path MTU
Discovery [RFC1191], alternatives are to set the "Don't Fragment" Discovery [RFC1191], alternatives are to set the "Don't Fragment"
(DF) bit in all segments in the initial window, or to set the "Don't (DF) bit in all segments in the initial window, or to set the "Don't
Fragment" (DF) bit in one of the segments. It is an open question Fragment" (DF) bit in one of the segments. It is an open question
which of these two alternatives is best; we would hope that which of these two alternatives is best; we would hope that
implementation experiences will shed light on this question. In the implementation experiences will shed light on this question. In the
first case of setting the DF bit in all segments, if the initial first case of setting the DF bit in all segments, if the initial
packets are too large, then all of the initial packets will be packets are too large, then all of the initial packets will be
dropped in the network. In the second case of setting the DF bit in dropped in the network. In the second case of setting the DF bit in
only one segment, if the initial packets are too large, then all but only one segment, if the initial packets are too large, then all but
one of the initial packets will be fragmented in the network. When one of the initial packets will be fragmented in the network. When
the second case is followed, setting the DF bit in the last segment the second case is followed, setting the DF bit in the last segment
in the initial window provides the least chance for needless in the initial window provides the least chance for needless
retransmissions when the initial segment size is found to be too retransmissions when the initial segment size is found to be too
large, because it minimizes the chances of duplicate ACKs triggering large, because it minimizes the chances of duplicate ACKs triggering
a Fast Retransmit. However, more attention needs to be paid to the a Fast Retransmit. However, more attention needs to be paid to the
interaction between larger initial windows and Path MTU Discovery. interaction between larger initial windows and Path MTU Discovery.
The larger initial window specified in this document is not intended The larger initial window specified in this document is not intended
as encouragement for web browsers to open multiple simultaneous as encouragement for web browsers to open multiple simultaneous TCP
TCP connections all with large initial windows. When web browsers connections all with large initial windows. When web browsers open
open simultaneous TCP connections to the same destination, this simultaneous TCP connections to the same destination, this works
works against TCP's congestion control mechanisms [FF98], regardless against TCP's congestion control mechanisms [FF98], regardless of
of the size of the initial window. Combining this behavior with the size of the initial window. Combining this behavior with larger
larger initial windows further increases the unfairness to other initial windows further increases the unfairness to other traffic in
traffic in the network. the network. We suggest the use of HTTP/1.1 [RFC2068] (persistent
TCP connections and pipelining) as a way to achieve better
performance of web transfers.
3. Advantages of Larger Initial Windows 3. Advantages of Larger Initial Windows
1. When the initial window is one segment, a receiver employing 1. When the initial window is one segment, a receiver employing
delayed ACKs [RFC1122] is forced to wait for a timeout before delayed ACKs [RFC1122] is forced to wait for a timeout before
generating an ACK. With an initial window of at least two generating an ACK. With an initial window of at least two
segments, the receiver will generate an ACK after the second segments, the receiver will generate an ACK after the second
data segment arrives. This eliminates the wait on the timeout data segment arrives. This eliminates the wait on the timeout
(often up to 200 msec, and possibly up to 500 msec [RFC1122]). (often up to 200 msec, and possibly up to 500 msec [RFC1122]).
skipping to change at page 5, line 38 skipping to change at page 5, line 47
For a network with a high segment drop rate, increasing the TCP For a network with a high segment drop rate, increasing the TCP
initial window could increase the segment drop rate even initial window could increase the segment drop rate even
further. This is in part because routers with Drop Tail queue further. This is in part because routers with Drop Tail queue
management have difficulties with bursty traffic in times of management have difficulties with bursty traffic in times of
congestion. However, given uncorrelated arrivals for TCP congestion. However, given uncorrelated arrivals for TCP
connections, the larger TCP initial window should not connections, the larger TCP initial window should not
significantly increase the segment drop rate. Simulation-based significantly increase the segment drop rate. Simulation-based
explorations of these issues are discussed in Section 7.2. explorations of these issues are discussed in Section 7.2.
These potential dangers for the network are explored in simulations These potential dangers for the network are explored in simulations
and experiments described in the section below. Our judgment is that and experiments described in the section below. Our judgment is
while there are dangers of congestion collapse in the current that while there are dangers of congestion collapse in the current
Internet (see [FF98] for a discussion of the dangers of congestion Internet (see [FF98] for a discussion of the dangers of congestion
collapse from an increased deployment of UDP connections without collapse from an increased deployment of UDP connections without
end-to-end congestion control), there is no such danger to the end-to-end congestion control), there is no such danger to the
network from increasing the TCP initial window to 4K bytes. network from increasing the TCP initial window to 4K bytes.
6. Interactions with the Retransmission Timer 6. Interactions with the Retransmission Timer
Using a larger initial burst of data can exacerbate existing Using a larger initial burst of data can exacerbate existing
problems with spurious retransmit timeouts on low-bandwidth paths, problems with spurious retransmit timeouts on low-bandwidth paths,
assuming the standard algorithm for determining the TCP assuming the standard algorithm for determining the TCP
retransmission timeout (RTO) [RFC2988]. The problem is that across retransmission timeout (RTO) [RFC2988]. The problem is that across
low-bandwidth network paths on which the transmission time of a low-bandwidth network paths on which the transmission time of a
packet is a large portion of the round-trip time, the small packets packet is a large portion of the round-trip time, the small packets
used to establish a TCP connection do not seed the RTO estimator appropriately. used to establish a TCP connection do not seed the RTO estimator
When the first window of data packets is transmitted, the sender's appropriately. When the first window of data packets is
retransmit timer could expire before the acknowledgments for those transmitted, the sender's retransmit timer could expire before the
packets are received. As each acknowledgment arrives, the acknowledgments for those packets are received. As each
retransmit timer is generally reset. Thus, the retransmit timer acknowledgment arrives, the retransmit timer is generally reset.
will not expire as long as an acknowledgment arrives at least once Thus, the retransmit timer will not expire as long as an
a second, given the one-second minimum on the RTO recommended in RFC acknowledgment arrives at least once a second, given the one-second
2988. minimum on the RTO recommended in RFC 2988.
For instance, consider a 9.6 Kbps link. The initial RTT measurement For instance, consider a 9.6 Kbps link. The initial RTT measurement
will be on the order of 67 msec, if we simply consider the will be on the order of 67 msec, if we simply consider the
transmission time of 2 packets (the SYN and SYN-ACK) each consisting transmission time of 2 packets (the SYN and SYN-ACK) each consisting
of 40 bytes. Using the RTO estimator given in [RFC2988], this of 40 bytes. Using the RTO estimator given in [RFC2988], this
yields an initial RTO of 201 msec (67 + 4*(67/2)). However, we yields an initial RTO of 201 msec (67 + 4*(67/2)). However, we
round the RTO to 1 second as specified in RFC 2988. Then assume we round the RTO to 1 second as specified in RFC 2988. Then assume we
send an initial window of one or more 1500-byte packets (1460 data send an initial window of one or more 1500-byte packets (1460 data
bytes plus overhead). Each packet will take on the order of 1.25 bytes plus overhead). Each packet will take on the order of 1.25
seconds to transmit. Clearly the RTO will fire before the ACK for seconds to transmit. Therefore, the RTO will fire before the ACK
the first packet returns, causing a spurious timeout. In this case, for the first packet returns, causing a spurious timeout. In this
a larger initial window of three or four packets exacerbates the case, a larger initial window of three or four packets exacerbates
problems caused by this spurious timeout. the problems caused by this spurious timeout.
One way to deal with this problem is to make the RTO algorithm more One way to deal with this problem is to make the RTO algorithm more
conservative. During the initial window of data, for instance, we conservative. During the initial window of data, for instance, the
could update the RTO for each acknowledgment received. In RTO could be updated for each acknowledgment received. In addition,
addition, if the retransmit timer expires for some packet lost in if the retransmit timer expires for some packet lost in the first
the first window of data, we could leave the exponential-backoff of window of data, we could leave the exponential-backoff of the
the retransmit timer engaged until at least one valid RTT measurement is retransmit timer engaged until at least one valid RTT measurement is
received that involves a data packet. received that involves a data packet.
Another method would be to refrain from taking a RTT sample during Another method would be to refrain from taking a RTT sample during
connection establishment, leaving the default RTO in place until TCP connection establishment, leaving the default RTO in place until TCP
takes a sample from a data segment and the corresponding ACK. While takes a sample from a data segment and the corresponding ACK. While
this method likely helps prevent spurious retransmits it also slows this method likely helps prevent spurious retransmits it also may
the data transfer down if loss occurs before the RTO is seeded. slow the data transfer down if loss occurs before the RTO is
seeded. The use of limited transmit [RFC3042] to aid a TCP
connection in recovering from loss using fast retransmit rather than
the RTO timer mitigates the performance degradation caused by using
the high default RTO during the initial window of data
transmission.
This specification leaves the decision about what to do (if This specification leaves the decision about what to do (if
anything) with regards to the RTO when using a larger initial window anything) with regards to the RTO when using a larger initial window
to the implementer. to the implementer. However, the RECOMMENDED approach is to refrain
from sampling the RTT during the three-way handshake, keeping the
default RTO in place until a RTT sample involving a data packet is
taken. In addition, it is RECOMMENDED that TCPs use limited
transmit [RFC3042].
7. Typical Levels of Burstiness for TCP Traffic. 7. Typical Levels of Burstiness for TCP Traffic.
Larger TCP initial windows would not dramatically increase the Larger TCP initial windows would not dramatically increase the
burstiness of TCP traffic in the Internet today, because such burstiness of TCP traffic in the Internet today, because such
traffic is already fairly bursty. Bursts of two and three segments traffic is already fairly bursty. Bursts of two and three segments
are already typical of TCP [Flo97]; A delayed ACK (covering two are already typical of TCP [Flo97]; A delayed ACK (covering two
previously unacknowledged segments) received during congestion previously unacknowledged segments) received during congestion
avoidance causes the congestion window to slide and two segments to avoidance causes the congestion window to slide and two segments to
be sent. The same delayed ACK received during slow start causes the be sent. The same delayed ACK received during slow start causes the
skipping to change at page 7, line 14 skipping to change at page 7, line 32
deployment of higher-speed links in some parts of the network, where deployment of higher-speed links in some parts of the network, where
a burst of 4K bytes can represent a small quantity of data. A a burst of 4K bytes can represent a small quantity of data. A
second change, for routers with sufficient buffering, is the second change, for routers with sufficient buffering, is the
deployment of queue management mechanisms such as RED, which is deployment of queue management mechanisms such as RED, which is
designed to be tolerant of transient traffic bursts. designed to be tolerant of transient traffic bursts.
8. Simulations and Experimental Results 8. Simulations and Experimental Results
8.1 Studies of TCP Connections using that Larger Initial Window 8.1 Studies of TCP Connections using that Larger Initial Window
This section surveys simulations and experiments that have been used This section surveys simulations and experiments that explore the
to explore the effect of larger initial windows on TCP effect of larger initial windows on TCP connections. The first set
connections. The first set of experiments of experiments explores performance over satellite links. Larger
explores performance over satellite links. Larger initial windows initial windows have been shown to improve performance of TCP
have been shown to improve performance of TCP connections over connections over satellite channels [All97b]. In this study, an
satellite channels [All97b]. In this study, an initial window of initial window of four segments (512 byte MSS) resulted in
four segments (512 byte MSS) resulted in throughput improvements of throughput improvements of up to 30% (depending upon transfer size).
up to 30% (depending upon transfer size). [KAGT98] shows that the [KAGT98] shows that the use of larger initial windows results in a
use of larger initial windows results in a decrease in transfer time decrease in transfer time in HTTP tests over the ACTS satellite
in HTTP tests over the ACTS satellite system. A study involving system. A study involving simulations of a large number of HTTP
simulations of a large number of HTTP transactions over hybrid fiber transactions over hybrid fiber coax (HFC) indicates that the use of
coax (HFC) indicates that the use of larger initial windows larger initial windows decreases the time required to load WWW pages
decreases the time required to load WWW pages [Nic97]. [Nic97].
A second set of experiments has explored TCP performance over dialup A second set of experiments explored TCP performance over dialup
modem links. In experiments over a 28.8 bps dialup channel [All97a, modem links. In experiments over a 28.8 bps dialup channel [All97a,
AHO98], a four-segment initial window decreased the transfer time of AHO98], a four-segment initial window decreased the transfer time of
a 16KB file by roughly 10%, with no accompanying increase in the a 16KB file by roughly 10%, with no accompanying increase in the
drop rate. A particular area of concern has been TCP performance drop rate. A simulation study [RFC2416] investigated the effects of
over low speed tail circuits (e.g., dialup modem links) with routers using a larger initial window on a host connected by a slow modem
with small buffers. A simulation study [RFC2416] investigated the link and a router with a 3 packet buffer. The study concluded that
effects of using a larger initial window on a host connected by a for the scenario investigated, the use of larger initial windows was
slow modem link and a router with a 3 packet buffer. The study not harmful to TCP performance.
concluded that for the scenario investigated, the use of larger
initial windows was not harmful to TCP performance. Questions have
been raised concerning the effects of larger initial windows on the
transfer time for short transfers in this environment, but these
effects have not been quantified. A question has also been raised
concerning the possible effect on existing TCP connections sharing
the link.
Finally, [All00] illustrates that the percentage of connections at a Finally, [All00] illustrates that the percentage of connections at a
particular web server that experience loss in the initial window of particular web server that experience loss in the initial window of
data transmission increases with the size of the initial congestion data transmission increases with the size of the initial congestion
window. However, the increase is in line with what would be window. However, the increase is in line with what would be
expected from sending a larger burst into the network. expected from sending a larger burst into the network.
8.2 Studies of Networks using Larger Initial Windows 8.2 Studies of Networks using Larger Initial Windows
This section surveys simulations and experiments investigating the This section surveys simulations and experiments investigating the
impact of the larger window on other TCP connections sharing the impact of the larger window on other TCP connections sharing the
path. Experiments in [All97a, AHO98] show that for 16 KB transfers path. Experiments in [All97a, AHO98] show that for 16 KB transfers
to 100 Internet hosts, four-segment initial windows resulted in a to 100 Internet hosts, four-segment initial windows resulted in a
small increase in the drop rate of 0.04 segments/transfer. While small increase in the drop rate of 0.04 segments/transfer. While
the drop rate increased slightly, the transfer time was reduced by the drop rate increased slightly, the transfer time was reduced by
roughly 25% for transfers using the four-segment (512 byte MSS) roughly 25% for transfers using the four-segment (512 byte MSS)
initial window when compared to an initial window of one segment. initial window when compared to an initial window of one segment.
One scenario of concern is heavily loaded links. For instance, A simulation study in [RFC2415] explores the impact of a larger
several years ago one of the trans-Atlantic links was so heavily initial window on competing network traffic. In this investigation,
loaded that the correct congestion window size for each connection was HTTP and FTP flows share a single congested gateway (where the
about one segment. In this environment, new connections using number of HTTP and FTP flows varies from one simulation set to
larger initial windows would be starting with windows that were four another). For each simulation set, the paper examines aggregate
times too big. What would the effects be? Do connections thrash? link utilization and packet drop rates, median web page delay, and
network power for the FTP transfers. The larger initial window
A simulation study in [RFC2415] explores the impact of a larger initial generally resulted in increased throughput, slightly-increased
window on competing network traffic. In this investigation, HTTP packet drop rates, and an increase in overall network power. With
and FTP flows share a single congested gateway (where the number of the exception of one scenario, the larger initial window resulted in
HTTP and FTP flows varies from one simulation set to another). For an increase in the drop rate of less than 1% above the loss rate
each simulation set, the paper examines aggregate link utilization experienced when using a one-segment initial window; in this
and packet drop rates, median web page delay, and network power for scenario, the drop rate increased from 3.5% with one-segment initial
the FTP transfers. The larger initial window generally resulted in windows, to 4.5% with four-segment initial windows. The overall
increased throughput, slightly-increased packet drop rates, and an conclusions were that increasing the TCP initial window to three
increase in overall network power. With the exception of one packets (or 4380 bytes) helps to improve perceived performance.
scenario, the larger initial window resulted in an increase in the
drop rate of less than 1% above the loss rate experienced when using
a one-segment initial window; in this scenario, the drop rate
increased from 3.5% with one-segment initial windows, to 4.5% with
four-segment initial windows. The overall conclusions were that
increasing the TCP initial window to three packets (or 4380 bytes)
helps to improve perceived performance.
Morris [Mor97] investigated larger initial windows in a very Morris [Mor97] investigated larger initial windows in a highly
congested network with transfers of size 20K. The loss rate in congested network with transfers of size 20K. The loss rate in
networks where all TCP connections use an initial window of four networks where all TCP connections use an initial window of four
segments is shown to be 1-2% greater than in a network where all segments is shown to be 1-2% greater than in a network where all
connections use an initial window of one segment. This relationship connections use an initial window of one segment. This relationship
held in scenarios where the loss rates with one-segment initial held in scenarios where the loss rates with one-segment initial
windows ranged from 1% to 11%. In addition, in networks where windows ranged from 1% to 11%. In addition, in networks where
connections used an initial window of four segments, TCP connections connections used an initial window of four segments, TCP connections
spent more time waiting for the retransmit timer (RTO) to expire to spent more time waiting for the retransmit timer (RTO) to expire to
resend a segment than was spent when using an initial window of one resend a segment than was spent when using an initial window of one
segment. The time spent waiting for the RTO timer to expire segment. The time spent waiting for the RTO timer to expire
skipping to change at page 9, line 4 skipping to change at page 9, line 7
cause a perceptible increase in both loss rates and retransmit cause a perceptible increase in both loss rates and retransmit
timeouts. timeouts.
9. Security Considerations 9. Security Considerations
This document discusses the initial congestion window permitted for This document discusses the initial congestion window permitted for
TCP connections. Changing this value does not raise any known new TCP connections. Changing this value does not raise any known new
security issues with TCP. security issues with TCP.
10. Conclusion 10. Conclusion
This document specifies a small change to TCP that will likely be beneficial
to short-lived TCP connections and those over links with long RTTs This document specifies a small change to TCP that will likely be
(saving several RTTs during the initial slow-start phase). beneficial to short-lived TCP connections and those over links with
long RTTs (saving several RTTs during the initial slow-start phase).
11. Acknowledgments 11. Acknowledgments
We would like to acknowledge Vern Paxson, Tim Shepard, members of We would like to acknowledge Vern Paxson, Tim Shepard, members of
the End-to-End-Interest Mailing List, and members of the IETF TCP the End-to-End-Interest Mailing List, and members of the IETF TCP
Implementation Working Group for continuing discussions of these Implementation Working Group for continuing discussions of these
issues for discussions and feedback on this document. issues for discussions and feedback on this document.
12. References 12. References
 End of changes. 

This html diff was produced by rfcdiff 1.25, available from http://www.levkowetz.com/ietf/tools/rfcdiff/