8. Security Considerations
8.1. Loss and Congestion Signals
Loss detection and congestion control fundamentally involve the consumption of signals, such as delay, loss, and ECN markings, from unauthenticated entities. An attacker can cause endpoints to reduce their sending rate by manipulating these signals: by dropping packets, by altering path delay strategically, or by changing ECN codepoints.
8.2. Traffic Analysis
Packets that carry only ACK frames can be heuristically identified by observing packet size. Acknowledgment patterns may expose information about link characteristics or application behavior. To reduce leaked information, endpoints can bundle acknowledgments with other frames, or they can use PADDING frames at a potential cost to performance.
8.3. Misreporting ECN Markings
A receiver can misreport ECN markings to alter the congestion response of a sender. Suppressing reports of ECN-CE markings could cause a sender to increase their send rate. This increase could result in congestion and loss.
A sender can detect suppression of reports by marking occasional packets that it sends with an ECN-CE marking. If a packet sent with an ECN-CE marking is not reported as having been CE marked when the packet is acknowledged, then the sender can disable ECN for that path by not setting ECN-Capable Transport (ECT) codepoints in subsequent packets sent on that path [RFC3168].
Reporting additional ECN-CE markings will cause a sender to reduce their sending rate, which is similar in effect to advertising reduced connection flow control limits and so no advantage is gained by doing so.
Endpoints choose the congestion controller that they use. Congestion controllers respond to reports of ECN-CE by reducing their rate, but the response may vary. Markings can be treated as equivalent to loss [RFC3168], but other responses can be specified, such as [RFC8511] or [RFC8311].