| Internet Engineering Task Force | N. Kuhn |
| Internet-Draft | CNES |
| Intended status: Informational | E. Stephan |
| Expires: November 19, 2020 | Orange |
| G. Fairhurst | |
| T. Jones | |
| University of Aberdeen | |
| May 18, 2020 |
Transport parameters for 0-RTT connections
draft-kuhn-quic-0rtt-bdp-07
0-RTT mechanisms reduce the time it takes for the first bytes of application data to be processed in a transport connection and can greatly reduce connection latency during setup. The 0-RTT transport features described by quic-transport help clients establish secure connections with a minimal number of round-trips.
This document describes a generic method to exchange path parameters relating to transport. The additional transport parameters can help a connection that continues after an interruption or restarts by sharing connection properties. They can be used to increase the performance for a path with large RTT.
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Each transport connection typically starts without knowledge of the path between the endpoints. Transport protocols use implicit signals from the network to discover the properties of the path. This information is used to adapt the transport mechanisms to the network path. For example, an Internet transport endpoint is unable to determine a safe rate at which to start or continue their transmission, and uses slow-start to determine a safe rate. This applies to the 1-RTT mode of QUIC.
QUIC supports the sending of data in two different modes, after the transport handshake has completed, 1-RTT mode, and sending data along with handshake packets, 0-RTT mode. Using 0-RTT data an application is able to send transport parameters with the handshake packets, making it possible to reduce the latency of the connection setup.
In 0-RTT mode, a QUIC server must store a copy of a number of flow control related transport parameters, or receives an integrity-protected copy of these values in the ticket the client includes in the first message of the handshake, to enable the use of 0-RTT data. The setting or omission of one of these parameters can result in QUIC creating a connection, but flow control can still prevent any data being sent by the client.
For 0-RTT data to be sent, the QUIC server must record the values of:
These values set the flow control limits within which a connection must operate. The server has to store these parameters for a client to send data when resuming during 0-RTT. The stored values are used for any data that is transmitted before the handshake has completed and 1-RTT data is able to be sent on the connection. Once the handshake has completed, these values are discarded and the values established during the handshake are used.
This document proposes an extension to the transport parameters that are shared during the 0-RTT phase to allow resumption using additionnal transport and connection properties that were discovered in previous connections. These additional parameters aim to provide faster startup of flows and better buffer management. The BDP metadata extension is proposed and candidate parameters are discussed in Section 3.
Reducing the number of round-trips required to start a connection is an important way to reduce setup time and lower overall connection latency. 0-RTT mechanisms that allow a client to feed requests to a server in the first RTT do not alone improve the total time-to-service. The BDP extension described in this document aims to improve traffic delivery by allowing the connection to short-cut slow RTT-based processes that grow connection parameters.
Currently each side has a proprietary solution to measure and to store path characteristics. Recalling the parameters of a previous connection would allow:
There is an interest in sharing transport information across multiple connections. As one example, [I-D.ietf-tcpm-2140bis] considers the sharing of transport parameters between connections originating from the same host. The proposal in this document has the advantage of storing the information at the client and not requiring the server to retain additional state for each client.
Section 7.3.1 of [I-D.ietf-quic-transport] describes the parameters that must be remembered if a client wishes to send 0-RTT data. Both endpoints store the value of the server transport parameters from a connection and apply them to any 0-RTT packets that are sent in subsequent connections to the same peer. Of the six mandatory parameters, only initial_max_data improves the time-to-service of the 0-RTT connection. The BDP metadata extension augments the list of server transport parameters that are shared with the client to improve the time-to-service and save resources such as CPU, memory and power.
The BDP extension proposes three new parameters that help a connection startup in a minimal number of RTTs:
The BDP extension is protected by the same mechanism that protects the exchange of the 0-RTT transport parameters. A client that activates 0-RTT data sends back the transport parameters received from the server during the previous connection (see Section 7.3.1 of [I-D.ietf-quic-transport]).
The client reads the parameters in the BDP metadata extension, but can not change them.
Accept: A client MAY use the extension parameters. Then, it activates ingress optimization and sends back the transport parameters of the BDP metadata extension that it received from the server during the previous connection.
Refuse: A client could choose not to use there parameters. Then, it does not support ingress optimisation and drops the extension signal. A client that disagrees with the extension parameters received from the server refuses the optimization.
The recon_bytes_in_flight parameter is higher than the number of bytes in the actual BDP since it may include bytes in buffers along the path.
The recon_bytes_in_flight parameter is an indication of the end-to-end BDP that is experienced by the congestion and flow control. If the recon_bytes_in_flight is high, the server may decide to increase the maximum amount of packets it will send when reconnecting using the recon_max_pkt_number parameter.
The maximum number of initial data packets that can be sent without acknowledgment needs to be chosen to avoid congestion collapse. An example of a server solution is proposed in Appendix A. In short:
Other parameters can contribute to the optimization of 0-RTT connection. There are good candidates, like max_ack_delay, in the Appendix of [I-D.ietf-quic-recovery].
The authors would like to thank Gabriel Montenegro, Patrick McManus, Ian Swett, Igor Lubashev, Christian Huitema and Tom Jones for their fruitful comments on earlier versions of this document.
TBD: Text is required to register the extension BDP_metadata field. Parameters are registered using the procedure defined in [I-D.ietf-quic-transport].
The BDP metadata parameters are measured by the server during a previous connection.
The BDP extension is protected by the mechanism that protects the exchange of the 0-RTT transport parameters. For the version 1 of QUIC, the BDP extension is protected using the mechanism that already protects the "initial_max_data" parameter. This is defined in sections 4.5 to 4.7 of [I-D.ietf-quic-tls]. It provides the server with a way to check the parameters proposed by the client are those that the server sent to the client during the previous connexion.
| [I-D.cardwell-iccrg-bbr-congestion-control] | Cardwell, N., Cheng, Y., Yeganeh, S. and V. Jacobson, "BBR Congestion Control", Internet-Draft draft-cardwell-iccrg-bbr-congestion-control-00, July 2017. |
| [I-D.ietf-quic-recovery] | Iyengar, J. and I. Swett, "QUIC Loss Detection and Congestion Control", Internet-Draft draft-ietf-quic-recovery-27, March 2020. |
| [I-D.ietf-quic-tls] | Thomson, M. and S. Turner, "Using TLS to Secure QUIC", Internet-Draft draft-ietf-quic-tls-27, February 2020. |
| [I-D.ietf-quic-transport] | Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", Internet-Draft draft-ietf-quic-transport-27, February 2020. |
| [I-D.ietf-tcpm-2140bis] | Touch, J., Welzl, M. and S. Islam, "TCP Control Block Interdependence", Internet-Draft draft-ietf-tcpm-2140bis-05, April 2020. |
| [RFC2914] | Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, DOI 10.17487/RFC2914, September 2000. |
| [RFC4782] | Floyd, S., Allman, M., Jain, A. and P. Sarolahti, "Quick-Start for TCP and IP", RFC 4782, DOI 10.17487/RFC4782, January 2007. |
This section details a solution at the server to safely increase the maximum amount of packets that the server sends when receiving a 0-RTT packet from a client.
The initial window is considered a safe starting point for an unknown path to avoid adding congestion to a congested network. The general assumption is that a path does not currently suffer persistent congestion, and therefore the initial window is applicable until feedback about the path is received. The resulting initial sending rate is only tentative until the capacity is confirmed to be available. If there is loss within this initial transmission, then this could be evidence that the path is congested, and the sender needs to adjust to this congestion.
Significant loss could be an indication of congestion collapse - i.e. persistent loss, requiring back-off of the sending rate.
If however, the reception of IW is confirmed for the first RTT of data, and also the path is determined to be similar to that of a recent previous session (e.g., similar RTT), the method permits the sender to use the previous path information as an input to help determine a new safe rate. One possibility is to immediately jump to a new sending rate that is derived from the previously sustained rate. This follows the ideas of [I-D.cardwell-iccrg-bbr-congestion-control] and [RFC4782].
The QoS mechanisms that are deployed in the networks can help in prevent the congestion collapse from occurring. However, the sender must provide a significant reduction if there is evidence of potential congestion collapse [RFC2914] from his point of view. Precautions can be taken to guarantee that it is reasonably safe to jump to a high sending rate : measuring that network conditions did not change, allowing some space for other flows that have started and pace transmission of packets. The sender might need to rapidly reduce its rate, if the higher sending rate does not prove to be supported. It if is supported, the sender can resume standard congestion control.