| Network Working Group | M. Kuehlewind |
| Internet-Draft | B. Trammell |
| Intended status: Informational | ETH Zurich |
| Expires: January 3, 2019 | July 02, 2018 |
Manageability of the QUIC Transport Protocol
draft-ietf-quic-manageability-02
This document discusses manageability of the QUIC transport protocol, focusing on caveats impacting network operations involving QUIC traffic. Its intended audience is network operators, as well as content providers that rely on the use of QUIC-aware middleboxes, e.g. for load balancing.
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QUIC [QUIC-TRANSPORT] is a new transport protocol currently under development in the IETF quic working group, focusing on support of semantics as needed for HTTP/2 [QUIC-HTTP]. Based on current deployment practices, QUIC is encapsulated in UDP and encrypted by default. The current version of QUIC integrates TLS [QUIC-TLS] to encrypt all payload data and most control information. Given QUIC is an end-to-end transport protocol, all information in the protocol header, even that which can be inspected, is is not meant to be mutable by the network, and is therefore integrity-protected to the extent possible.
This document provides guidance for network operation on the management of QUIC traffic. This includes guidance on how to interpret and utilize information that is exposed by QUIC to the network as well as explaining requirement and assumptions that the QUIC protocol design takes toward the expected network treatment. It also discusses how common network management practices will be impacted by QUIC.
Of course, network management is not a one-size-fits-all endeavour: practices considered necessary or even mandatory within enterprise networks with certain compliance requirements, for example, would be impermissible on other networks without those requirements. This document therefore does not make any specific recommendations as to which practices should or should not be applied; for each practice, it describes what is and is not possible with the QUIC transport protocol as defined.
QUIC is at the moment very much a moving target. This document refers the state of the QUIC working group drafts as well as to changes under discussion, via issues and pull requests in GitHub current as of the time of writing.
The words “MUST”, “MUST NOT”, “SHOULD”, and “MAY” are used in this document. It’s not shouting; when these words are capitalized, they have a special meaning as defined in [RFC2119].
In this section, we discusses those aspects of the QUIC transport protocol that have an impact on the design and operation of devices that forward QUIC packets. Here, we are concerned primarily with QUIC’s unencrypted wire image [WIRE-IMAGE], which we define as the information available in the packet header in each QUIC packet, and the dynamics of that information. Since QUIC is a versioned protocol, the wire image of the header format can also change from version to version. However, at least the mechanism by which a receiver can determine which version is used and the meaning and location of fields used in the version negotiation process is invariant [QUIC-INVARIANTS].
This document is focused on the protocol as presently defined in [QUIC-TRANSPORT] and [QUIC-TLS], and will change to track those documents.
QUIC packets may have either a long header, or a short header. The first bit of the QUIC header indicates which type of header is present.
The long header exposes more information. It is used during connection establishment, including version negotiation, server retry, and 0-RTT data. It contains a version number, as well as source and destination connection IDs for grouping packets belonging to the same flow. The definition and location of these fields in the QUIC long header are invariant for future versions of QUIC, although future versions of QUIC may provide additional fields in the long header [QUIC-INVARIANTS].
Short headers are used after connection establishment. The only information they contain for inspection on the path is an optional, variable-length destination connection ID.
As of draft version 13 of the QUIC transport document, the following information may be exposed in QUIC packet headers:
Multiple QUIC packets may be coalesced into a UDP datagram, with a datagram carrying one or more long header packets followed by zero or one short header packets. When packets are coalesced, the Length fields in the long headers are used to separate QUIC packets. The length header field is variable length and its position in the header is also variable depending on the length of the source and destionation connection ID. See Section 4.6 of [QUIC-TRANSPORT].
As soon as the cryptographic context is established, all information in the QUIC header, including those exposed in the packet header, is integrity protected. Further, information that were sent and exposed in previous packets when the cryptographic context was established yet, e.g. for the cryptographic initial handshake itself, will be validated later during the cryptographic handshake. Therefore, devices on path MUST NOT change any information or bits in QUIC packet headers, since alteration of header information will lead to a failed integrity check at the receiver, and can even lead to connection termination.
The connection ID in the QUIC packet headers allows routing of QUIC packets at load balancers on other than five-tuple information, ensuring that related flows are appropriately balanced together; and to allow rebinding of a connection after one of the endpoint’s addresses changes - usually the client’s, in the case of the HTTP binding. Client and server negotiate connection IDs during the handshake; typically, however, only the server will request a connection ID for the lifetime of the connection. Connection IDs for either endpoint may change during the lifetime of a connection, with the new connection ID being negotiated via encrypted frames. See Section 6.1 of [QUIC-TRANSPORT].
The packet number field is always present in the QUIC packet header; however, it is always encrypted. The encryption key for packet number protection on handshake packets sent before cryptographic context establishment is specific to the QUIC version, while packet number protection on subsequent packets uses secrets derived from the end-to-end cryptographic context. Packet numbers are therefore not part of the wire image that is useful to on-path observers.
Version negotiation is not protected, given the used protection mechanism can change with the version. However, the choices provided in the list of version in the Version Negotiation packet will be validated as soon as the cryptographic context has been established. Therefore any manipulation of this list will be detected and will cause the endpoints to terminate the connection.
Also note that the list of versions in the Version Negotiation packet may contain reserved versions. This mechanism is used to avoid ossification in the implementation on the selection mechanism. Further, a client may send a Initial Client packet with a reserved version number to trigger version negotiation. In the Version Negotiation packet the connection ID and packet number of the Client Initial packet are reflected to provide a proof of return-routability. Therefore changing these information will also cause the connection to fail.
This section addresses the different kinds of observations and inferences that can be made about QUIC flows by a passive observer in the network based on the wire image in Section 2. Here we assume a bidirectional observer (one that can see packets in both directions in the sequence in which they are carried on the wire) unless noted.
The QUIC wire image is not specifically designed to be distinguishable from other UDP traffic.
The only application binding currently defined for QUIC is HTTP [QUIC-HTTP]. HTTP over QUIC uses UDP port 443 by default, although URLs referring to resources available over HTTP over QUIC may specify alternate port numbers. Simple assumptions about whether a given flow is using QUIC based upon a UDP port number may therefore not hold; see also [RFC7605] section 5.
An in-network observer assuming that a set of packets belongs to a QUIC flow can infer the version number in use by observing the handshake: an Initial packet with a given version from a client to which a server responds with an Initial packet with the same version implies acceptance of that version.
Negotiated version cannot be identified for flows for which a handshake is not observed, such as in the case of NAT rebinding; however, these flows can be associated with flows for which a version has been identified; see Section 3.4.
In the rest of this section, we discuss only packets belonging to Version 1 QUIC flows, and assume that these packets have been identified as such through the observation of a version negotiation.
A related question is whether a first packet of a given flow on known QUIC-associated port is a valid QUIC packet, in order to support in-network filtering of garbage UDP packets (reflection attacks, random backscatter). While heuristics based on the first byte of the packet (packet type) could be used to separate valid from invalid first packet types, the deployment of such heuristics is not recommended, as packet types may have different meanings in future versions of the protocol.
Connection establishment uses Initial, Handshake, and Retry packets containing a TLS handshake on Stream 0. Connection establishment can therefore be detected using heuristics similar to those used to detect TLS over TCP. A client using 0-RTT connection may also send data packets in 0-RTT Protected packets directly after the Initial packet containing the TLS Client Hello. Since these packets may be reordered in the network, note that 0-RTT Protected data packets may be seen before the Initial packet. Note that only clients send Initial packets, so the sides of a connection can be distinguished by QUIC packet type in the handshake.
The cleartext TLS handshake may contain Server Name Indication (SNI) [RFC6066], by which the client reveals the name of the server it intends to connect to, in order to allow the server to present a certificate based on that name. It may also contain information from Application-Layer Protocol Negotiation (ALPN) [RFC7301], by which the client exposes the names of application-layer protocols it supports; an observer can deduce that one of those protocols will be used if the connection continues.
Work is currently underway in the TLS working group to encrypt the SNI in TLS 1.3 [TLS-ENCRYPT-SNI], reducing the information available in the SNI to the name of a fronting service, which can generally be identified by the IP address of the server anyway. If used with QUIC, this would make SNI-based application identification impossible through passive measurement.
The QUIC Connection ID (see Section 2.4) is designed to allow an on-path device such as a load-balancer to associate two flows as identified by five-tuple when the address and port of one of the endpoints changes; e.g. due to NAT rebinding or server IP address migration. An observer keeping flow state can associate a connection ID with a given flow, and can associate a known flow with a new flow when when observing a packet sharing a connection ID and one endpoint address (IP address and port) with the known flow.
The connection ID to be used for a long-running flow is chosen by the server (see [QUIC-TRANSPORT] section 5.6) during the handshake. This value should be treated as opaque; see Section 4.3 for caveats regarding connection ID selection at servers.
The QUIC does not expose the end of a connection; the only indication to on-path devices that a flow has ended is that packets are no longer observed. Stateful devices on path such as NATs and firewalls must therefore use idle timeouts to determine when to drop state for QUIC flows.
Changes to this behavior have been discussed in the working group, but there is no current proposal to implement these changes: see https://github.com/quicwg/base-drafts/issues/602.
Round-trip time of QUIC flows can be inferred by observation once per flow, during the handshake, as in passive TCP measurement; this requires parsing of the QUIC packet header and the cleartext TLS handshake on stream 0.
In the common case, the delay between the Initial packet containing the TLS Client Hello and the Handshake packet containing the TLS Server Hello represents the RTT component on the path between the observer and the server. The delay between the TLS Server Hello and the Handshake packet containing the TLS Finished message sent by the client represents the RTT component on the path between the observer and the client. While the client may send 0-RTT Protected packets after the Initial packet during 0-RTT connection re-establishment, these can be ignored for RTT measurement purposes.
Handshake RTT can be measured by adding the client-to-observer and observer-to-server RTT components together. This measurement necessarily includes any transport and application layer delay at both sides.
The spin bit experiment, detailed in [QUIC-SPIN], provides an additional method to measure intraflow per-flow RTT. When a QUIC flow is sending at full rate (i.e., neither application nor flow control limited), the latency spin bit described in that document changes value once per round-trip time (RTT). An on-path observer can observe the time difference between edges in the spin bit signal in a single direction to measure one sample of end-to-end RTT. Note that this measurement, as with passive RTT measurement for TCP, includes any transport protocol delay (e.g., delayed sending of acknowledgements) and/or application layer delay (e.g., waiting for a request to complete). It therefore provides devices on path a good instantaneous estimate of the RTT as experienced by the application. A simple linear smoothing or moving minimum filter can be applied to the stream of RTT information to get a more stable estimate.
An on-path observer that can see traffic in both directions (from client to server and from server to client) can also use the spin bit to measure “upstream” and “downstream” component RTT; i.e, the component of the end-to-end RTT attributable to the paths between the observer and the server and the observer and the client, respectively. It does this by measuring the delay between a spin edge observed in the upstream direction and that observed in the downstream direction, and vice versa.
Application-limited and flow-control-limited senders can have application and transport layer delay, respectively, that are much greater than network RTT. Therefore, the spin bit provides network latency information only when the sender is neither application nor flow control limited. When the sender is application-limited by periodic application traffic, where that period is longer than the RTT, measuring the spin bit provides information about the application period, not the RTT. Simple heuristics based on the observed data rate per flow or changes in the RTT series can be used to reject bad RTT samples due to application or flow control limitation.
Since the spin bit logic at each endpoint considers only samples on packets that advance the largest packet number seen, signal generation itself is resistant to reordering. However, reordering can cause problems at an observer by causing spurious edge detection and therefore low RTT estimates, if reordering occurs across a spin bit flip in the stream. This can be probabilistically mitigated by the observer also tracking the low-order bits of the packet number, and rejecting edges that appear out-of-order [RFC4737].
QUIC explicitly exposes which side of a connection is a client and which side is a server during the handshake. In addition, the symmerty of a flow (whether primarily client-to-server, primarily server-to-client, or roughly bidirectional, as input to basic traffic classification techniques) can be inferred through the measurement of data rate in each direction. While QUIC traffic is protected and ACKS may be padded, padding is not required.
In this section, we address specific network management and measurement techniques and how QUIC’s design impacts them.
Stateful treatment of QUIC traffic is possible through QUIC traffic and version identification (Section 3.1) and observation of the handshake for connection confirmation (Section 3.2). The lack of any visible end-of-flow signal (Section 3.5) means that this state must be purged either through timers or through least-recently-used eviction, depending on application requirements.
Limited RTT measurement is possible by passive observation of QUIC traffic; see Section 3.6. No passive measurement of loss is possible with the present wire image. Extremely limited observation of upstream congestion may be possible via the observation of CE markings on ECN-enabled QUIC traffic.
In the case of content distribution networking architectures including load balancers, the connection ID provides a way for the server to signal information about the desired treatment of a flow to the load balancers.
Server-generated Connection IDs must not encode any information other that that needed to route packets to the appropriate backend server(s): typically the identity of the backend server or pool of servers, if the data-center’s load balancing system keeps “local” state of all flows itself. Care must be exercised to ensure that the information encoded in the Connection ID is not sufficient to identify unique end users. Note that by encoding routing information in the Connection ID, load balancers open up a new attack vector that allows bad actors to direct traffic at a specific backend server or pool. It is therefore recommended that Server-Generated Connection ID includes a cryptographic MAC that the load balancer pool server are able to identify and discard packets featuring an invalid MAC.
Current practices in detection and mitigation of Distributed Denial of Service (DDoS) attacks generally involve passive measurement using network flow data [RFC7011], classification of traffic into “good” (productive) and “bad” (DoS) flows, and filtering of these bad flows in a “scrubbing” environment. Key to successful DDoS mitigation is efficient classification of this traffic.
Limited first-packet garbage detection as in Section 3.1.2 and stateful tracking of QUIC traffic as in Section 4.1 above can be used in this classification step.
Note that the use of a connection ID to support connection migration renders 5-tuple based filtering insufficient, and requires more state to be maintained by DDoS defense systems, and linkability resistance in connection ID update mechanisms means that a connection ID aware DDoS defense system must have the same information about flows as the load balancer.
However, it is questionable if connection migrations needs to be supported in a DDOS attack. If the connection migration is not visible to the network that performs the DDoS detection, an active, migrated QUIC connection may be blocked by such a system under attack. However, a defense system might simply rely on the fast resumption mechanism provided by QUIC.
[EDITOR’S NOTE: this is a bit speculative; keep?]
QUIC does not provide any additional information on requirements on Quality of Service (QoS) provided from the network. QUIC assumes that all packets with the same 5-tuple {dest addr, source addr, protocol, dest port, source port} will receive similar network treatment. That means all stream that are multiplexed over the same QUIC connection require the same network treatment and are handled by the same congestion controller. If differential network treatment is desired, multiple QUIC connections to the same server might be used, given that establishing a new connection using 0-RTT support is cheap and fast.
QoS mechanisms in the network MAY also use the connection ID for service differentiation, as a change of connection ID is bound to a change of address which anyway is likely to lead to a re-route on a different path with different network characteristics.
Given that QUIC is more tolerant of packet re-ordering than TCP (see Section 2.5), Equal-cost multi-path routing (ECMP) does not necessarily need to be flow based. However, 5-tuple (plus eventually connection ID if present) matching is still beneficial for QoS given all packets are handled by the same congestion controller.
This document has no actions for IANA.
Supporting manageability of QUIC traffic inherently involves tradeoffs with the confidentiality of QUIC’s control information; this entire document is therefore security-relevant.
Some of the properties of the QUIC header used in network management are irrelevant to application-layer protocol operation and/or user privacy. For example, packet number exposure (and echo, as proposed in this document), as well as connection establishment exposure for 1-RTT establishment, make no additional information about user traffic available to devices on path.
At the other extreme, supporting current traffic classification methods that operate through the deep packet inspection (DPI) of application-layer headers are directly antithetical to QUIC’s goal to provide confidentiality to its application-layer protocol(s); in these cases, alternatives must be found.
Dan Druta contributed text to Section 4.4. Igor Lubashev contributed text to Section 4.3 on the use of the connection ID for load balancing. Marcus Ilhar contributed text to Section 3.6 on the use of the spin bit.
This work is partially supported by the European Commission under Horizon 2020 grant agreement no. 688421 Measurement and Architecture for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat for Education, Research, and Innovation under contract no. 15.0268. This support does not imply endorsement.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |