Internet DRAFT - draft-nishida-tcpm-maxwin
draft-nishida-tcpm-maxwin
Network Working Group Y. Nishida
Internet-Draft GE Global Research
Intended status: Experimental H. Asai
Expires: August 3, 2017 The University of Tokyo
M. Bagnulo
UC3M
January 30, 2017
Increasing Maximum Window Size of TCP
draft-nishida-tcpm-maxwin-03.txt
Abstract
This document proposes to increase the current max window size
allowed in TCP. It describes the current logic that limits the
maximum window size and provides a rationale to relax the limitation
as well as the negotiation mechanism to enable this feature safely.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 2
3. Increasing Maximum Window Size . . . . . . . . . . . . . . . 3
4. Updating the Window Scale Option . . . . . . . . . . . . . . 4
5. Use Cases, Benefits to Explore Maximum Window Size . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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 achieve
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. 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].
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3. Increasing Maximum Window Size
RFC7323 requires maximum window size to be less than 2^30 as
described below.
"
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
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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
the problem that the referred paragraphs from RFC7323 [RFC7323]
describe. 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.
Using the larger window size implies that the sequence number space
can wrap around in less than 3 RTTs. This can pose problems to
distinguish old retransmitted packets from new packets solely using
the same sequence number. Because of this, a sender using the larger
window size defined in this specification is recommended to use
Protection Against Wrapped Sequences (PAWS) as defined in RFC7323
[RFC7323].
4. Updating the Window Scale Option
As shown in Figure 1, the Window Scale Option (WSO) defined in
[RFC7323] has three 1-byte fields, the Kind field (which specifies
the option type), the Length field (set to 3 because the WSO is 3
bytes long) and the shift.cnt field (which specifies the shift count
applied to the window to scale it).
+-----------+-----------+-----------+
| Kind=3 | Length=3 | shift.cnt |
+-----------+-----------+-----------+
1 1 1
Figure 1: Window Scale Option (WSO) format
RFC7323 [RFC7323] defines that the shift.cnt field can have a maximum
value of 14 and upon reception of a larger value in this field, the
receiver must proceed as if it had received a shift.cnt of 14.
This specification updates the shift.cnt field definition. Figure 2
represents the new format of the shift.cnt field. The eight bits
contained in the shift.cnt field are formatted as "SSSSLRRR".
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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S S S S L R R R|
+-+-+-+-+-+-+-+-+
Figure 2: New shift.cnt field format
These bits are parsed as follows:
o The four leftmost bits "SSSS" express the shift-count, as in
RFC7323 [RFC7323], only that now the maximum shift count value
allowed is 15.
o The "L" bit expresses if the sender supports the large window
defined in this specification i.e. the bit is set if the sender
supports this specification.
o The three rightmost bits "RRR" are reserved for future use and
MUST be set to zero.
This new format for the shift.count field allows an updated client to
initiate a TCP connection and express that it supports the larger
window by setting the "L" bit, while still conveying information
about the shift count that it wants to use for its own RCV.WND in the
four leftmost bits "SSSS" (which do not necessarily have to be set to
15). A server that supports this specification that receives a SYN
with the WSO with the "L" bit set knows that it can reply using a
shift count of 15. A legacy server that receives the WSO with the
"L"" bit set will interpret it using the RFC7323 format and will then
read it as a shift count value larger than 14. As per RFC7323 the
server MUST then assume a shift count of 14. The legacy server will
then reply with a WSO with the "L" bit set to zero, so the client
knows that the server does not support this specification and that
the server will assume a shift count of 14 for the client's receive
window.
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 80 Gbps 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 maximum 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. Acknowledgments
The authors gratefully acknowledge significant inputs for this
document from Richard Scheffenegger and Ilpo Jarvinen.
7. 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.
8. IANA Considerations
If approved, this document overrides the definition of the WSO option
defined in RFC7323 and so the IANA registry should be update
accordingly (at least to add a pointer to this specification as
reference for the WSO in the IANA registry).
9. References
9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<http://www.rfc-editor.org/info/rfc7323>.
9.2. Informative References
[RFC1263] O'Malley, S. and L. Peterson, "TCP Extensions Considered
Harmful", RFC 1263, DOI 10.17487/RFC1263, October 1991,
<http://www.rfc-editor.org/info/rfc1263>.
Authors' Addresses
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
Marcelo Bagnulo
UC3M
Email: marcelo@it.uc3m.es
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