6. Performance Considerations
- Performance Considerations
Bursty traffic can often lead to temporally correlated packet losses; in turn, this can lead to suboptimal responses from congestion controllers in protocols running over UDP. To avoid this, UDP proxies SHOULD strive to avoid increasing burstiness of UDP traffic; they SHOULD NOT queue packets in order to increase batching.
When the protocol running over UDP that is being proxied uses congestion control (e.g., [QUIC]), the proxied traffic will incur at least two nested congestion controllers. The underlying HTTP connection MUST NOT disable congestion control unless it has an out- of-band way of knowing with absolute certainty that the inner traffic is congestion-controlled.
If a client or UDP proxy with a connection containing a UDP Proxying request stream disables congestion control, it MUST NOT signal Explicit Congestion Notification (ECN) [ECN] support on that connection. That is, it MUST mark all IP headers with the Not-ECT codepoint. It MAY continue to report ECN feedback via QUIC ACK_ECN frames or the TCP ECE bit, as the peer may not have disabled congestion control.
When the protocol running over UDP that is being proxied uses loss recovery (e.g., [QUIC]), and the underlying HTTP connection runs over TCP, the proxied traffic will incur at least two nested loss recovery mechanisms. This can reduce performance as both can sometimes independently retransmit the same data. To avoid this, UDP proxying SHOULD be performed over HTTP/3 to allow leveraging the QUIC DATAGRAM frame.
6.1. MTU Considerations
When using HTTP/3 with the QUIC Datagram extension [QUIC-DGRAM], UDP payloads are transmitted in QUIC DATAGRAM frames. Since those cannot be fragmented, they can only carry payloads up to a given length determined by the QUIC connection configuration and the Path MTU (PMTU). If a UDP proxy is using QUIC DATAGRAM frames and it receives a UDP payload from the target that will not fit inside a QUIC DATAGRAM frame, the UDP proxy SHOULD NOT send the UDP payload in a DATAGRAM capsule, as that defeats the end-to-end unreliability characteristic that methods such as Datagram Packetization Layer PMTU Discovery (DPLPMTUD) depend on [DPLPMTUD]. In this scenario, the UDP proxy SHOULD drop the UDP payload and send an ICMP Packet Too Big message to the target; see Section 3.2 of [ICMP6].
6.2. Tunneling of ECN Marks
UDP proxying does not create an IP-in-IP tunnel, so the guidance in [ECN-TUNNEL] about transferring ECN marks between inner and outer IP headers does not apply. There is no inner IP header in UDP proxying tunnels.
In this specification, note that UDP proxying clients do not have the ability to control the ECN codepoints on UDP packets the UDP proxy sends to the target, nor can UDP proxies communicate the markings of each UDP packet from target to UDP proxy.
A UDP proxy MUST ignore ECN bits in the IP header of UDP packets received from the target, and it MUST set the ECN bits to Not-ECT on UDP packets it sends to the target. These do not relate to the ECN markings of packets sent between client and UDP proxy in any way.