Distributed Denial of Service (DDoS) Open Threat Signaling Requirements

Distributed Denial of Service (DDoS) attacks afflict networks connected to the Internet, plaguing network operators at service providers and enterprises around the world. High-volume attacks saturating inbound links are now common, as attack scale and frequency continue to increase. The prevalence and impact of these DDoS attacks has led to an increased focus on coordinated attack response. However, many enterprises lack the resources or expertise to operate on-premises attack mitigation solutions themselves, or are constrained by local bandwidth limitations. To address such gaps, service providers have begun to offer on-demand traffic scrubbing services, which are designed to separate the DDoS attack traffic from legitimate traffic and forward only the latter. Today, these services offer proprietary interfaces for subscribers to request attack mitigation. Such proprietary interfaces tie a subscriber to a service and limit the abilities of network elements that would otherwise be capable of participating in attack mitigation. As a result of signaling interface incompatibility, attack responses may be fragmented or otherwise incomplete, leaving operators in the attack path unable to assist in the defense. A standardized method to coordinate a real-time response among involved operators will increase the speed and effectiveness of DDoS attack mitigation, and reduce the impact of these attacks. This document describes the required characteristics of protocols that enable attack response coordination and mitigation of DDoS attacks. DDoS Open Threat Signaling (DOTS) communicates the need for defensive action in anticipation of or in response to an attack, but does not dictate the implementation of these actions. The DOTS use cases are discussed in and the DOTS architecture is discussed in .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP14 , when, and only when, they appear in all capitals. These capitalized words are used to signify the requirements for DOTS protocols design. This document adopts the following terms: A distributed denial-of-service attack, in which traffic originating from multiple sources is directed at a target on a network. DDoS attacks are intended to cause a negative impact on the availability and/or other functionality of an attack target. Denial-of-service considerations are discussed in detail in . A network connected entity that is the target of a DDoS attack. Potential targets include (but are not limited to) network elements, network links, servers, and services. Collected measurements and behavioral characteristics defining the nature of a DDoS attack. An action or set of actions focused on recognizing and filtering out specific types of DDoS attack traffic while passing legitimate traffic to the attack target. Distinct countermeasures can be layered to defend against attacks combining multiple DDoS attack types. A set of countermeasures enforced against traffic destined for the target or targets of a detected or reported DDoS attack, where countermeasure enforcement is managed by an entity in the network path between attack sources and the attack target. Mitigation methodology is out of scope for this document. An entity, typically a network element, capable of performing mitigation of a detected or reported DDoS attack. The means by which this entity performs these mitigations and how they are requested of it are out of scope for this document. The mitigator and DOTS server receiving a mitigation request are assumed to belong to the same administrative entity. A DOTS-aware software module responsible for requesting attack response coordination with other DOTS-aware elements. A DOTS-aware software module handling and responding to messages from DOTS clients. The DOTS server enables mitigation on behalf of the DOTS client, if requested, by communicating the DOTS client's request to the mitigator and returning selected mitigator feedback to the requesting DOTS client. Any DOTS-aware software module capable of participating in a DOTS signal or data channel. It can be a DOTS client, DOTS server, or, as a logical agent, a DOTS gateway. A DOTS-aware software module resulting from the logical concatenation of the functionality of a DOTS server and a DOTS client into a single DOTS agent. This functionality is analogous to a Session Initiation Protocol (SIP) Back-to-Back User Agent (B2BUA) . A DOTS gateway has a client-facing side, which behaves as a DOTS server for downstream clients, and a server-facing side, which performs the role of DOTS client for upstream DOTS servers. Client-domain DOTS gateways are DOTS gateways that are in the DOTS client's domain, while server-domain DOTS gateways denote DOTS gateways that are in the DOTS server's domain. A DOTS gateway may terminate multiple discrete DOTS client connections and may aggregate these into a single or multiple connections. DOTS gateways are described further in . A bidirectional, mutually authenticated communication channel between DOTS agents that is resilient even in conditions leading to severe packet loss, such as a volumetric DDoS attack causing network congestion. A status/control message transmitted over the authenticated signal channel between DOTS agents, used to indicate the client's need for mitigation, or to convey the status of any requested mitigation. A message transmitted between DOTS agents over the signal channel, used as a keep-alive and to measure peer health. A bidirectional, mutually authenticated communication channel between two DOTS agents used for infrequent but reliable bulk exchange of data not easily or appropriately communicated through the signal channel. Reliable bulk data exchange may not function well or at all during attacks causing network congestion. The data channel is not expected to operate in such conditions. A specification of a matching network traffic flow or set of flows. The filter will typically have a policy associated with it, e.g., rate-limiting or discarding matching traffic . A list of filters indicating sources from which traffic should be blocked, regardless of traffic content. A list of filters indicating sources from which traffic should always be allowed, regardless of contradictory data gleaned in a detected attack. A DOTS client exchanging messages with multiple DOTS servers, each in a separate administrative domain.

The expected layout and interactions amongst DOTS entities is described in the DOTS Architecture . The goal of the DOTS requirements specification is to specify the requirements for DOTS signal channel and data channel protocols that have different application and transport layer requirements. This section describes the required features and characteristics of the DOTS protocols. The goal of DOTS protocols is to enable and manage mitigation on behalf of a network domain or resource which is or may become the focus of a DDoS attack. An active DDoS attack against the entity controlling the DOTS client need not be present before establishing a communication channel between DOTS agents. Indeed, establishing a relationship with peer DOTS agents during normal network conditions provides the foundation for more rapid attack response against future attacks, as all interactions setting up DOTS, including any business or service level agreements, are already complete. Reachability information of peer DOTS agents is provisioned to a DOTS client using a variety of manual or dynamic methods. Once a relationship between DOTS agents is established, regular communication between DOTS clients and servers enables a common understanding of the DOTS agents' health and activity. The DOTS protocol must at a minimum make it possible for a DOTS client to request aid mounting a defense against a suspected attack. This defense could be coordinated by a DOTS server and include signaling within or between domains as requested by local operators. DOTS clients should similarly be able to withdraw aid requests. DOTS requires no justification from DOTS clients for requests for help, nor do DOTS clients need to justify withdrawing help requests: the decision is local to the DOTS clients' domain. Multi-homed DOTS clients must be able to select the appropriate DOTS server(s) to which a mitigation request is to be sent. The method for selecting the appropriate DOTS server in a multi-homed environment is out of scope for this document. DOTS protocol implementations face competing operational goals when maintaining this bidirectional communication stream. On the one hand, DOTS must include measures to ensure message confidentiality, integrity, authenticity, and replay protection to keep the protocols from becoming additional vectors for the very attacks it is meant to help fight off. On the other hand, the protocol must be resilient under extremely hostile network conditions, providing continued contact between DOTS agents even as attack traffic saturates the link. Such resiliency may be developed several ways, but characteristics such as small message size, asynchronous, redundant message delivery and minimal connection overhead (when possible given local network policy) will tend to contribute to the robustness demanded by a viable DOTS protocol. Operators of peer DOTS-enabled domains may enable quality- or class-of-service traffic tagging to increase the probability of successful DOTS signal delivery, but DOTS does not require such policies be in place, and should be viable in their absence. The DOTS server and client must also have some standardized method of defining the scope of any mitigation, as well as managing other mitigation-related configuration. Finally, DOTS should be sufficiently extensible to meet future needs in coordinated attack defense, although this consideration is necessarily superseded by the other operational requirements.

Extensibility: Protocols and data models developed as part of DOTS MUST be extensible in order to keep DOTS adaptable to proprietary DDoS defenses. Future extensions MUST be backward compatible. Implementations of older protocol versions MUST ignore optional information added to DOTS messages as part of newer protocol versions. Implementations of older protocol versions MUST reject DOTS messages carrying mandatory information as part of newer protocol versions. Resilience and Robustness: The signaling protocol MUST be designed to maximize the probability of signal delivery even under the severely constrained network conditions caused by attack traffic. Additional means to enhance the resilience of DOTS protocols, including when multiple DOTS servers are provisioned to the DOTS clients, SHOULD be considered. The protocol MUST be resilient, that is, continue operating despite message loss and out-of-order or redundant message delivery. In support of signaling protocol robustness, DOTS signals SHOULD be conveyed over transport and application protocols not susceptible to Head of Line Blocking. These requirements are at SHOULD strength to handle middle-boxes and firewall traversal. Bulk Data Exchange: Infrequent bulk data exchange between DOTS agents can also significantly augment attack response coordination, permitting such tasks as population of drop- or accept-listed source addresses; address or prefix group aliasing; exchange of incident reports; and other hinting or configuration supplementing attack response. As the resilience requirements for the DOTS signal channel mandate small signal message size, a separate, secure data channel utilizing a reliable transport protocol MUST be used for bulk data exchange. However, reliable bulk data exchange may not be possible during attacks causing network congestion. Mitigation Hinting: DOTS clients may have access to attack details which can be used to inform mitigation techniques. Example attack details might include locally collected fingerprints for an on-going attack, or anticipated or active attack focal points based on other threat intelligence. DOTS clients MAY send mitigation hints derived from attack details to DOTS servers, in the full understanding that the DOTS server MAY ignore mitigation hints. Mitigation hints MUST be transmitted across the signal channel, as the data channel may not be functional during an attack. DOTS server handling of mitigation hints is implementation-specific. Loop Handling: In certain scenarios, typically involving misconfiguration of DNS or routing policy, it may be possible for communication between DOTS agents to loop. Signal and data channel implementations should be prepared to detect and terminate such loops to prevent service disruption.

Use of Common Transport Protocols: DOTS MUST operate over common widely deployed and standardized transport protocols. While connectionless transport such as the User Datagram Protocol (UDP) SHOULD be used for the signal channel, the Transmission Control Protocol (TCP) MAY be used if necessary due to network policy or middlebox capabilities or configurations. Sub-MTU Message Size: To avoid message fragmentation and the consequently decreased probability of message delivery over a congested link, signaling protocol message size MUST be kept under signaling Path Maximum Transmission Unit (PMTU), including the byte overhead of any encapsulation, transport headers, and transport- or message-level security. If the total message size exceeds the path MTU, the DOTS agent MUST split the message into separate messages; for example, the list of mitigation scope types could be split into multiple lists and each list conveyed in a new message. DOTS agents can attempt to learn PMTU using the procedures discussed in . If the PMTU cannot be discovered, DOTS agents MUST assume a PMTU of 1280 bytes, as IPv6 requires that every link in the Internet have an MTU of 1280 octets or greater as specified in . If IPv4 support on legacy or otherwise unusual networks is a consideration and the PMTU is unknown, DOTS implementations MAY assume on a PMTU of 576 bytes for IPv4 datagrams, as every IPv4 host must be capable of receiving a packet whose length is equal to 576 bytes as discussed in and . Bidirectionality: To support peer health detection, to maintain an active signal channel, and to increase the probability of signal delivery during an attack, the signal channel MUST be bidirectional, with client and server transmitting signals to each other at regular intervals, regardless of any client request for mitigation. The bidirectional signal channel MUST support unidirectional messaging to enable notifications between DOTS agents. Channel Health Monitoring: DOTS agents MUST support exchange of heartbeat messages over the signal channel to monitor channel health. These keepalives serve the purpose to maintain any on-path NAT or Firewall bindings to avoid cryptographic handshake for new mitigation requests. The heartbeat interval during active mitigation could be negotiable based on NAT/Firewall characteristics. Absent information about the NAT/Firewall characteristics, DOTS agent needs to ensure its on-path NAT or Firewall bindings do not expire, by using the keep-alive frequency discussed in Section 3.5 of . To support scenarios in which loss of heartbeat is used to trigger mitigation, and to keep the channel active, DOTS servers MUST solicit heartbeat exchanges after successful mutual authentication. When DOTS agents are exchanging heartbeats and no mitigation request is active, either agent MAY request changes to the heartbeat rate. For example, a DOTS server might want to reduce heartbeat frequency or cease heartbeat exchanges when an active DOTS client has not requested mitigation, in order to control load. Following mutual authentication, a signal channel MUST be considered active until a DOTS agent explicitly ends the session. When no attack traffic is present, the signal channel MUST be considered active until either DOTS agent fails to receive heartbeats from the other peer after a mutually agreed upon retransmission procedure has been exhausted. Peer DOTS agents MUST regularly send heartbeats to each other while a mitigation request is active. Because heartbeat loss is much more likely during volumetric attack, DOTS agents SHOULD avoid signal channel termination when mitigation is active and heartbeats are not received by either DOTS agent for an extended period. The exception circumstances to terminate the signal channel session during active mitigation are discussed below: To handle possible DOTS server restart or crash, the DOTS clients MAY attempt to establish a new signal channel session, but MUST continue to send heartbeats on the current session so that the DOTS server knows the session is still alive. If the new session is successfully established, the DOTS client can terminate the current session. DOTS servers are assumed to have the ability to monitor the attack, using feedback from the mitigator and other available sources, and MAY use the absence of attack traffic and lack of client heartbeats as an indication the signal channel is defunct. Channel Redirection: In order to increase DOTS operational flexibility and scalability, DOTS servers SHOULD be able to redirect DOTS clients to another DOTS server at any time. DOTS clients MUST NOT assume the redirection target DOTS server shares security state with the redirecting DOTS server. DOTS clients are free to attempt abbreviated security negotiation methods supported by the protocol, such as DTLS session resumption, but MUST be prepared to negotiate new security state with the redirection target DOTS server. The redirection DOTS server and redirecting DOTS server MUST belong to the same administrative domain. Due to the increased likelihood of packet loss caused by link congestion during an attack, DOTS servers SHOULD NOT redirect while mitigation is enabled during an active attack against a target in the DOTS client's domain. Mitigation Requests and Status: Authorized DOTS clients MUST be able to request scoped mitigation from DOTS servers. DOTS servers MUST send status to the DOTS clients about mitigation requests. If a DOTS server rejects an authorized request for mitigation, the DOTS server MUST include a reason for the rejection in the status message sent to the client. DOTS servers MUST regularly send mitigation status updates to authorized DOTS clients which have requested and been granted mitigation. If unreliable transport is used for the signal channel protocol, due to the higher likelihood of packet loss during a DDoS attack, DOTS servers needs to send mitigation status multiple times at regular intervals following the the data transmission guidelines discussed in Section 3.1.3 of . When DOTS client-requested mitigation is active, DOTS server status messages MUST include the following mitigation metrics: Total number of packets blocked by the mitigation Current number of packets per second blocked Total number of bytes blocked Current number of bytes per second blocked DOTS clients MAY take these metrics into account when determining whether to ask the DOTS server to cease mitigation. A DOTS client MAY withdraw a mitigation request at any time, regardless of whether mitigation is currently active. The DOTS server MUST immediately acknowledge a DOTS client's request to stop mitigation. To protect against route or DNS flapping caused by a client rapidly toggling mitigation, and to dampen the effect of oscillating attacks, DOTS servers MAY allow mitigation to continue for a limited period after acknowledging a DOTS client's withdrawal of a mitigation request. During this period, DOTS server status messages SHOULD indicate that mitigation is active but terminating. DOTS clients MAY reverse the mitigation termination during this active-but-terminating period with a new mitigation request for the same scope. The DOTS server MUST treat this request as a mitigation lifetime extension (see SIG-007 below). The initial active-but-terminating period is implementation- and deployment- specific, but SHOULD be sufficiently long to absorb latency incurred by route propagation. If a DOTS client refreshes the mitigation before the active-but-terminating period elapses, the DOTS server MAY increase the active-but-terminating period up to a maximum of 300 seconds (5 minutes). After the active-but-terminating period elapses, the DOTS server MUST treat the mitigation as terminated, as the DOTS client is no longer responsible for the mitigation. Mitigation Lifetime: DOTS servers MUST support mitigations for a negotiated time interval, and MUST terminate a mitigation when the lifetime elapses. DOTS servers also MUST support renewal of mitigation lifetimes in mitigation requests from DOTS clients, allowing clients to extend mitigation as necessary for the duration of an attack. DOTS servers MUST treat a mitigation terminated due to lifetime expiration exactly as if the DOTS client originating the mitigation had asked to end the mitigation, including the active-but-terminating period, as described above in SIG-005. DOTS clients MUST include a mitigation lifetime in all mitigation requests. DOTS servers SHOULD support indefinite mitigation lifetimes, enabling architectures in which the mitigator is always in the traffic path to the resources for which the DOTS client is requesting protection. DOTS clients MUST be prepared to not be granted mitigations with indefinite lifetimes. DOTS servers MAY refuse mitigations with indefinite lifetimes, for policy reasons. The reasons themselves are out of scope. If the DOTS server does not grant a mitigation request with an indefinite mitigation lifetime, it MUST set the lifetime to a value that is configured locally. That value MUST be returned in a reply to the requesting DOTS client. Mitigation Scope: DOTS clients MUST indicate desired mitigation scope. The scope type will vary depending on the resources requiring mitigation. All DOTS agent implementations MUST support the following required scope types: IPv4 prefixes IPv6 prefixes Domain names The following mitigation scope types are OPTIONAL: Uniform Resource Identifiers DOTS servers MUST be able to resolve domain names and (when supported) URIs. How name resolution is managed on the DOTS server is implementation-specific. DOTS agents MUST support mitigation scope aliases, allowing DOTS clients and servers to refer to collections of protected resources by an opaque identifier created through the data channel, direct configuration, or other means. Domain name and URI mitigation scopes may be thought of as a form of scope alias, in which the addresses to which the domain name or URI resolve represent the full scope of the mitigation. If there is additional information available narrowing the scope of any requested attack response, such as targeted port range, protocol, or service, DOTS clients SHOULD include that information in client mitigation requests. DOTS clients MAY also include additional attack details. DOTS servers MAY ignore such supplemental information when enabling countermeasures on the mitigator. As an active attack evolves, DOTS clients MUST be able to adjust as necessary the scope of requested mitigation by refining the scope of resources requiring mitigation. A DOTS client may obtain the mitigation scope through direct provisioning or through implementation-specific methods of discovery. DOTS clients MUST support at least one mechanism to obtain mitigation scope. Mitigation Efficacy: When a mitigation request is active, DOTS clients MUST be able to transmit a metric of perceived mitigation efficacy to the DOTS server. DOTS servers MAY use the efficacy metric to adjust countermeasures activated on a mitigator on behalf of a DOTS client. Conflict Detection and Notification: Multiple DOTS clients controlled by a single administrative entity may send conflicting mitigation requests as a result of misconfiguration, operator error, or compromised DOTS clients. DOTS servers in the same administrative domain attempting to honor conflicting requests may flap network route or DNS information, degrading the networks attempting to participate in attack response with the DOTS clients. DOTS servers in a single administrative domain SHALL detect such conflicting requests, and SHALL notify the DOTS clients in conflict. The notification MUST indicate the nature and scope of the conflict, for example, the overlapping prefix range in a conflicting mitigation request. Network Address Translator Traversal: DOTS clients may be deployed behind a Network Address Translator (NAT), and need to communicate with DOTS servers through the NAT. DOTS protocols MUST therefore be capable of traversing NATs. If UDP is used as the transport for the DOTS signal channel, all considerations in "Middlebox Traversal Guidelines" in apply to DOTS. Regardless of transport, DOTS protocols MUST follow established best common practices established in BCP 127 for NAT traversal .

The data channel is intended to be used for bulk data exchanges between DOTS agents. Unlike the signal channel, the data channel is not expected to be constructed to deal with attack conditions. As the primary function of the data channel is data exchange, a reliable transport is required in order for DOTS agents to detect data delivery success or failure. The data channel provides a protocol for DOTS configuration, management. For example, a DOTS client may submit to a DOTS server a collection of prefixes it wants to refer to by alias when requesting mitigation, to which the server would respond with a success status and the new prefix group alias, or an error status and message in the event the DOTS client's data channel request failed. Reliable transport: Messages sent over the data channel MUST be delivered reliably, in order sent. Resource Configuration: To help meet the general and signal channel requirements in and , DOTS server implementations MUST provide an interface to configure resource identifiers, as described in SIG-008. DOTS server implementations MAY expose additional configurability. Additional configurability is implementation-specific. Policy management: DOTS servers MUST provide methods for DOTS clients to manage drop- and accept-lists of traffic destined for resources belonging to a client. For example, a DOTS client should be able to create a drop- or accept-list entry, retrieve a list of current entries from either list, update the content of either list, and delete entries as necessary. How a DOTS server authorizes DOTS client management of drop- and accept-list entries is implementation-specific.

DOTS must operate within a particularly strict security context, as an insufficiently protected signal or data channel may be subject to abuse, enabling or supplementing the very attacks DOTS purports to mitigate. Peer Mutual Authentication: DOTS agents MUST authenticate each other before a DOTS signal or data channel is considered valid. The method of authentication is not specified in this document, but should follow current IETF best practices with respect to any cryptographic mechanisms to authenticate the remote peer. Message Confidentiality, Integrity and Authenticity: DOTS protocols MUST take steps to protect the confidentiality, integrity and authenticity of messages sent between client and server. While specific transport- and message-level security options are not specified, the protocols MUST follow current IETF best practices for encryption and message authentication. Client-domain DOTS gateways are more trusted than DOTS clients, while server-domain DOTS gateways and DOTS servers share the same level of trust. A security mechanism at the transport layer TLS/DTLS is thus adequate to provide security between peer DOTS agents. In order for DOTS protocols to remain secure despite advancements in cryptanalysis and traffic analysis, DOTS agents MUST support secure negotiation of the terms and mechanisms of protocol security, subject to the interoperability and signal message size requirements in . While the interfaces between downstream DOTS server and upstream DOTS client within a DOTS gateway are implementation-specific, those interfaces nevertheless MUST provide security equivalent to that of the signal channels bridged by gateways in the signaling path. For example, when a DOTS gateway consisting of a DOTS server and DOTS client is running on the same logical device, the two DOTS agents could be implemented within the same process security boundary. Data privacy and integrity: Transmissions over the DOTS protocols are likely to contain operationally or privacy-sensitive information or instructions from the remote DOTS agent. Theft, modification, or replay of message transmissions could lead to information leaks or malicious transactions on behalf of the sending agent (see below). Consequently data sent over the DOTS protocols MUST be encrypted using secure transports TLS/DTLS. DOTS servers MUST enable means to prevent leaking operationally or privacy-sensitive data. Although administrative entities participating in DOTS may detail what data may be revealed to third-party DOTS agents, such considerations are not in scope for this document. Message Replay Protection: To prevent a passive attacker from capturing and replaying old messages, and thereby potentially disrupting or influencing the network policy of the receiving DOTS agent's domain, DOTS protocols MUST provide a method for replay detection and prevention. Within the signal channel, messages MUST be uniquely identified such that replayed or duplicated messages can be detected and discarded. Unique mitigation requests MUST be processed at most once. Authorization: DOTS servers MUST authorize all messages from DOTS clients which pertain to mitigation, configuration, filtering, or status. DOTS servers MUST reject mitigation requests with scopes which the DOTS client is not authorized to manage. Likewise, DOTS servers MUST refuse to allow creation, modification or deletion of scope aliases and drop-/accept-lists when the DOTS client is unauthorized. The modes of authorization are implementation-specific.

A well-structured DOTS data model is critical to the development of successful DOTS protocols. Structure: The data model structure for the DOTS protocol MAY be described by a single module, or be divided into related collections of hierarchical modules and sub-modules. If the data model structure is split across modules, those distinct modules MUST allow references to describe the overall data model's structural dependencies. Versioning: To ensure interoperability between DOTS protocol implementations, data models MUST be versioned. How the protocols represent data model versions is not defined in this document. Mitigation Status Representation: The data model MUST provide the ability to represent a request for mitigation and the withdrawal of such a request. The data model MUST also support a representation of currently requested mitigation status, including failures and their causes. Mitigation Scope Representation: The data model MUST support representation of a requested mitigation's scope. As mitigation scope may be represented in several different ways, per SIG-008 above, the data model MUST include extensible representation of mitigation scope. Mitigation Lifetime Representation: The data model MUST support representation of a mitigation request's lifetime, including mitigations with no specified end time. Mitigation Efficacy Representation: The data model MUST support representation of a DOTS client's understanding of the efficacy of a mitigation enabled through a mitigation request. Acceptable Signal Loss Representation: The data model MUST be able to represent the DOTS agent's preference for acceptable signal loss when establishing a signal channel. Measurements of loss might include, but are not restricted to, number of consecutive missed heartbeat messages, retransmission count, or request timeouts. Heartbeat Interval Representation: The data model MUST be able to represent the DOTS agent's preferred heartbeat interval, which the client may include when establishing the signal channel, as described in SIG-003. Relationship to Transport: The DOTS data model MUST NOT make any assumptions about specific characteristics of any given transport into the data model, but instead, represent the fields in the model explicitly.