Endpoint support
We employ PathSpider to determine whether endpoints support ECN.
Results on endpoint support for ECN
We examine ECN-awareness of web servers and IPv6 Hitlist targets and characterize ECN support at both the endpoint and network levels.
On ECN Awareness
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ECN-awareness was consistently high across all datasets, with most servers successfully negotiating ECN from all vantage points, suggesting limited interference from routers and middleboxes along the path. Tranco and university websites exhibited particularly high levels of ECN support, especially over IPv6, where nearly all reachable servers were ECN-aware. While IPv4 also showed widespread support, it experienced more connection failures and a slightly larger fraction of ECN-unaware servers. The IPv6 Hitlist displayed somewhat lower ECN-awareness, likely due to its broader coverage of Internet services beyond web infrastructure. These trends are reflected in Fig. 1, which shows ECN negotiation outcomes across vantage points, and in Table 1 (See the paper for more details), which summarizes ECN-awareness across the different datasets.
a. IPv4.
b. IPv6.
Figure 1. Counts of IP addresses that successfully negotiate ECN, as a function of the number of vantage points from which we observe that outcome. Most web servers negotiate ECN regardless of vantage point location; a small but non-trivial fraction consistently fail to do so.
ECN-awareness of Networks
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We further examined ECN-awareness at the AS level by analyzing the proportion of ECN-unaware endpoints within each network. While most ASes exhibited high levels of ECN support, several large providers showed substantial fractions of ECN-unaware hosts. Notably, Microsoft (AS8075) exhibited a high proportion of ECN-unaware IPv4 endpoints, while Google (AS15169) consistently appeared ECN-unaware over IPv6 in our measurements. These observations suggest that ECN deployment remains uneven across some major operators, potentially reflecting differences in deployment practices or path interference. Additional results and AS-specific analyses are provided in the paper.