The HTTP Alternative Services (Alt-Svc) method is defined as an application to check connectivity in HTTP/3. This method is designed based on the fact that communication with old HTTP is guaranteed and the HTTP/3 adoption rate is not necessarily dominant, and it is considered effective in the early stages of transition. However, once HTTP/3 has reached its peak and the transitional period has passed, the uncertainty and redundancy of the Alt-Svc procedure become detrimental. In Alt-Svc, the procedure involves first completing the old HTTP connection to use HTTP/3, and then migrating to HTTP/3 if possible; however, because HTTP/3 is a protocol that eliminates the waste of the old HTTP handshake (TCP handshake followed by TLS handshake), HTTP/3 does not fully benefit from the rapid connection establishment of HTTP/3. Therefore, we propose a method to apply the Happy Eyeballs algorithm, which is used for IPv4 and IPv6 connectivity checks, to the old HTTP and HTTP/3 connectivity checks. The Happy Eyeballs algorithm performs the two selections in parallel to eliminate the delay that occurs in sequential processing, but the proposed method differs from the conventional Happy Eyeballs algorithm in that, even if the old HTTP is adopted once, it switches to the HTTP/3 connection if it is possible to connect using HTTP/3. The proposed method differs from the conventional Happy Eyeballs algorithm by introducing a mechanism to switch to HTTP/3 connections when HTTP/3 connections are available, even when the old HTTP is adopted. Results of the evaluation experiments demonstrated that the adoption rate of HTTP/3 increases in environments with high communication latency because the old HTTP performs the TLS handshake after the TCP handshake, but with this improvement, HTTP/3 is preferentially selected even in low latency environments when it is selectable.
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