Increasing Maximum Window Size of TCP

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 achive better performance. Original TCP's maximum window size defined in RFC793 is 2^16 -1 (65,535), however, 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 . 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.

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 .

RFC7232 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 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 RFC7232 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.

In this proposal, an endpoint that supports new maximum window size simply sets 15 to the shift count in Window Scale option. A potential problem of this approach is that an endpoint cannot verify if the peer supports shift count 15 unless the peer also uses shift count 15. However, we believe this is not a significant problem. As specified in , if an endpoint does not support shift count 15, it simply interprets it as shift count 14. while the window size used by the sender will be smaller than the actual available window size at the receiver, this will not cause any fatal issues In order to avoid the performance degradation caused by misinterpretation of shift count, an endpoint MAY allocate more than 65535 * 2^15 + 65535 * 2^14 bytes memory for connections that use shift count 15. In this case, the endpoint that supports shiftcount 15 can always offer maximum window value and shift count 15 when the peer does not support new shift count. Because the maximum window value conventional endpoint can consume is 65535 * 2^14 bytes. As long as an endpoint can offer shift count 15 with maximum window value, conventional endpoints won't have performance issue.

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 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, and 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.

The authors gratefully acknowledge significant inputs for this document from Richard Scheffenegger and Ilpo Jarvinen.

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 to check its implications.

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