Network Working Group Y. Nishida
Internet-Draft GE Global Research
Intended status: Experimental H. Asai
Expires: September 10, 2015 The University of Tokyo
March 9, 2015
Increasing Maximum Window Size of TCP
draft-nishida-tcpm-maxwin-00.txt
Abstract
This document proposes to increase the current max window size
allowed in TCP. It describes the current logic that limits the max
window size and provides a rationale to relax the limitation as well
as the negotiation mechanism to enable this feature safely.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 2
3. Increasing Maximum Window Size . . . . . . . . . . . . . . . 2
4. Signaling Method . . . . . . . . . . . . . . . . . . . . . . 4
5. Use Cases, Benefits to Explore Maximum Window Size . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
TCP throughput is determined by two factors: Round Trip Time and
Receive Window size. It can never exceed Receive Window size divided
by RTT. This implies larger window size is important to archive
better performance. Original TCP's maximum window size defined in
RFC793 [RFC0793] is 2^16 -1 (65,535), however, RFC7323 [RFC7323]
defines TCP Window Scale option which allows TCP to use larger window
size. Window Scale uses a shift count stored in 1-byte field in the
option. The receiver of the option uses left-shifted window size
value by the shift count as actual window size. When Window Scale is
used, TCP can extend maximum window size to 2^30 - 2^14
(1,073,725,440). This is because the maximum shift count is 14 as
described in the Section 2.3 of RFC7323 [RFC7323]. However, since
TCP's sequence number space is 2^32, we believe it is still possible
to use larger window size than this while careful design of the logic
that can identify segments inside the window is required. In this
document, we propose to increase the maximum shift count to 15, which
extend window size to 2^31 - 2^15 (2,147,450,880).
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Increasing Maximum Window Size
RFC7232 requires maximum window size to be less than 2^30 as
described below.
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"
TCP determines if a data segment is "old" or "new" by testing whether
its sequence number is within 2^31 bytes of the left edge of the
window, and if it is not, discarding the data as "old". To insure
that new data is never mistakenly considered old and vice versa, the
left edge of the sender's window has to be at most 2^31 away from the
right edge of the receiver's window. The same is true of the
sender's right edge and receiver's left edge. Since the right and
left edges of either the sender's or receiver's window differ by the
window size, and since the sender and receiver windows can be out of
phase by at most the window size, the above constraints imply that
two times the maximum window size must be less than 2^31, or
max window < 2^30
"
However, TCP does not necessarily need to determine if a segment is
old or new. Because important point is to determine if a receive
segment is inside of the window or not. It basically does not matter
if a segment is too old (left side of the window) or too new (right
side of the window) as long as it is outside of the window. Based on
this viewpoint, we propose to extend maximum window to 2^31 - 2^15,
which can be attained by increasing maximum shift count to 15.
To demonstrate the feasibility of the proposal, we would like to use
the following worst case example where the sender and the receiver
windows are completely out of phase. In this example, we define S as
the sender's left edge of the window and W as the sender's window
size. Hence, the sender's right edge of the window is S+W. Also,
the receiver's left edge of the window is S+W+1 and the right edge of
the window is S+2W+1, as they are out of phase. This situation can
happen when the sender sent all segments in the window and the
receiver received all segments while no ACK has been received by the
sender yet. Now, we presume a segment that contains sequence number
S has arrived at the receiver. This segment should be excluded by
the receiver, although it can easily happen when the sender
retransmits segments.
In case of W=2^31, the receiver cannot exclude this segment as S+2W =
S. It is considered inside of the window. (S+W+1 < S < S+2W+1)
However, our proposed window size is W=2^31-X, where X is 2^15. In
this case, when segment S has arrived, the following checks will be
performed. First, TCP checks it with the left edge of the window and
it considers the segment is left side of the left edge. (S < S+W+1
Note: W=2^31-X) Second, TCP checks it with the right edge of the
window and it considers the segment is right of the right edge. (S >
S+2W+1) You might notice that the result of the second check is not
expected one as the segment S is actually an old segment. This is
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the problem depicted by the referred paragraphs from RFC7232
[RFC7323] . However, the segment is properly excluded by the receiver
as both checks indicate it is outside of the window. It should be
noted that the principle of TCP requires to accept the segment S only
when it has passed both checks successfully, which means S must
satisfy the following condition.
S >= left edge && S <= right edge
As we have shown in the example, our proposed maximum window size:
W=2^31-2^15 does not affect this principle.
4. Signaling Method
This proposal requires both endnodes to agree to use new maximum
window size. This can be archived by setting 15 to the shift count
in Window Scale option, which requires updating RFC7323 [RFC7323]. A
potential problem of this approach is that the sender of shift count
15 cannot confirm if the receiver supports it. This is because
Window Scale does not have a way to check the response from its peer.
Both endpoints just exchange their preferred shift count, but do not
negotiate the value. One possible method for the feature is to
enable it when both endpoints use shift count 15 in the Window Scale
option. As using shift count 15 is prohibited by RFC7323 [RFC7323],
we can presume that an endpoint supports the new maximum value when
it is seen in the option.
More explicit negotiation can be archived by introducing a new TCP
option. In this case, when a sender sends Window Scale option with
shift count 15 in the SYN segment, it MUST include a new TCP option
to indicate it supports new maximum value. The receiver of the
segment MUST include this new option in the SYN/ACK segment if it
receives the option and it supports new maximum value. When the
sender receives this option in the SYN/ACK, it can use shift count
15, otherwise it should use shift count 14. This method guarantees
that both endpoints agree to use new maximum value.
5. Use Cases, Benefits to Explore Maximum Window Size
One of the use cases of the extended maximum window size is high
volume data transfer over paths with long RTT delays and high
bandwidth, called long fat pipes. The proposed extension improves
and doubles at most the maximum throughput when bandwidth-latency
product is greater than 1 GB. As propagation delay in an optical
fiber is around 20 cm/ns, RTT will be over 100 milliseconds when the
distance of the transmission is more than 10000km. This distance is
not extraordinary for trans-pacific communications. In this case,
the maximum throughput will be limited to 80Gbps with the current
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maximum window size, although network technologies for more than 100
GBps are becoming common these days.
As the current TCP sequence number space is limited to 32 bits, it
will not be possible to increase maxmimum window size any further.
However, TCP may eventually have other extensions to increase
sequence number space, for example, [RFC7323] and [RFC1263] mention
about increasing sequence number space to 64 bits. We believe the
information in this document will be useful when such extensions are
proposed as they need to define new maximum window size.
6. Security Considerations
It is known that an attacker can have more chances to insert forged
packets into a TCP connection when large window size is used. This
is not a specific problem of this proposal, but a generic problem to
use larger window. Using PAWS can mitigate this problem, however, it
is recommended to consult the Security Considerations section of
RFC7323 [RFC7323] to check its implications.
7. IANA Considerations
This document may request new TCP option codepoint.
8. References
8.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, "TCP Extensions for High Performance", RFC
7323, September 2014.
8.2. Informative References
[RFC1263] O'Malley, S. and L. Peterson, "TCP Extensions Considered
Harmful", RFC 1263, October 1991.
Authors' Addresses
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Yoshifumi Nishida
GE Global Research
2623 Camino Ramon
San Ramon, CA 94583
USA
Email: nishida@wide.ad.jp
Hirochika Asai
The University of Tokyo
7-3-1 Hongo
Bunkyo-ku, Tokyo 113-8656
JP
Email: panda@wide.ad.jp
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