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


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

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

     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

RFC7232 requires maximum window size to be less than 2^30 as described below.

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.

                 S ≥ left edge && S ≤ right edge

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 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.

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 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

Yoshifumi Nishida GE Global Research 2623 Camino Ramon San Ramon, CA 94583 USA EMail:
Hirochika Asai The University of Tokyo 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8656 JP EMail: