Distributed Denial-of-Service Open Threat Signaling (DOTS) Signal Channel

A distributed denial-of-service (DDoS) attack is an attempt to make machines or network resources unavailable to their intended users. In most cases, sufficient scale can be achieved by compromising enough end-hosts and using those infected hosts to perpetrate and amplify the attack. The victim in this attack can be an application server, a host, a router, a firewall, or an entire network. In many cases, it may not be possible for an enterprise network administrators to determine the causes of an attack, but instead just realize that certain resources seem to be under attack. This document, which adheres to the DOTS architecture , proposes that, in such cases, the DOTS client just inform its DOTS server(s) that the enterprise is under a potential attack and that the mitigator monitor traffic to the enterprise to mitigate any possible attacks. This cooperation between DOTS agents contributes to ensure a highly automated network that is also robust, reliable and secure. Protocol requirements for DOTS signal channel are obtained from DOTS requirements . This document satisfies all the use cases discussed in except the Third-party DOTS notifications use case in Section 3.2.3 of which is an optional feature and not a core use case. Third-party DOTS notifications are not part of the DOTS requirements document. Moreover, the DOTS architecture does not assess whether that use case may have an impact on the architecture itself and/or the DOTS trust model. This is a companion document to the DOTS data channel specification .

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 . (D)TLS: For brevity this term is used for statements that apply to both Transport Layer Security and Datagram Transport Layer Security . Specific terms will be used for any statement that applies to either protocol alone. The reader should be familiar with the terms defined in .

Network applications have finite resources like CPU cycles, number of processes or threads they can create and use, maximum number of simultaneous connections it can handle, limited resources of the control plane, etc. When processing network traffic, such applications are supposed to use these resources to offer the intended task in the most efficient fashion. However, an attacker may be able to prevent an application from performing its intended task by causing the application to exhaust the finite supply of a specific resource. TCP DDoS SYN-flood, for example, is a memory-exhaustion attack on the victim and ACK-flood is a CPU exhaustion attack on the victim (). Attacks on the link are carried out by sending enough traffic such that the link becomes excessively congested, and legitimate traffic suffers high packet loss. Stateful firewalls can also be attacked by sending traffic that causes the firewall to hold excessive state and the firewall runs out of memory, and can no longer instantiate the state required to pass legitimate flows. Other possible DDoS attacks are discussed in . In each of the cases described above, the possible arrangements between the DOTS client and DOTS server to mitigate the attack are discussed in . An example of network diagram showing a deployment of these elements is shown in . Architectural relationships between involved DOTS agents is explained in . In this example, the DOTS server is operating on the access network.

The DOTS server can also be running on the Internet, as depicted in .

In typical deployments, the DOTS client belongs to a different administrative domain than the DOTS server. For example, the DOTS client is a web server serving content owned and operated by an domain, while the DOTS server is owned and operated by a different domain providing DDoS mitigation services. That domain providing DDoS mitigation service might, or might not, also provide Internet access service to the website operator. The DOTS server may (not) be co-located with the DOTS mitigator. In typical deployments, the DOTS server belongs to the same administrative domain as the mitigator. The DOTS client can communicate directly with the DOTS server or indirectly via a DOTS gateway. This document focuses on the DOTS signal channel.

DOTS signaling can happen with DTLS over UDP and TLS over TCP. A DOTS client can use DNS to determine the IP address(es) of a DOTS server or a DOTS client may be provided with the list of DOTS server IP addresses. The DOTS client MUST know a DOTS server's domain name; hard-coding the domain name of the DOTS server into software is NOT RECOMMENDED in case the domain name is not valid or needs to change for legal or other reasons. The DOTS client performs A and/or AAAA record lookup of the domain name and the result will be a list of IP addresses, each of which can be used to contact the DOTS server using UDP and TCP. If an IPv4 path to reach a DOTS server is found, but the DOTS server's IPv6 path is not working, a dual-stack DOTS client can experience a significant connection delay compared to an IPv4-only DOTS client. The other problem is that if a middlebox between the DOTS client and DOTS server is configured to block UDP, the DOTS client will fail to establish a DTLS session with the DOTS server and will, then, have to fall back to TLS over TCP incurring significant connection delays. discusses that DOTS client and server will have to support both connectionless and connection-oriented protocols. To overcome these connection setup problems, the DOTS client can try connecting to the DOTS server using both IPv6 and IPv4, and try both DTLS over UDP and TLS over TCP in a fashion similar to the Happy Eyeballs mechanism . These connection attempts are performed by the DOTS client when its initializes, and the client uses that information for its subsequent alert to the DOTS server. In order of preference (most preferred first), it is UDP over IPv6, UDP over IPv4, TCP over IPv6, and finally TCP over IPv4, which adheres to address preference order and the DOTS preference that UDP be used over TCP (to avoid TCP's head of line blocking).

X | |--TCP SYN, IPv6-------------->X | |--DTLS ClientHello, IPv4 ---->X | |--TCP SYN, IPv4----------------------------------------->| |--DTLS ClientHello, IPv6 ---->X | |--TCP SYN, IPv6-------------->X | |<-TCP SYNACK---------------------------------------------| |--DTLS ClientHello, IPv4 ---->X | |--TCP ACK----------------------------------------------->| |<------------Establish TLS Session---------------------->| |----------------DOTS signal----------------------------->| | | ]]> In reference to , the DOTS client sends two TCP SYNs and two DTLS ClientHello messages at the same time over IPv6 and IPv4. In this example, it is assumed that the IPv6 path is broken and UDP is dropped by a middle box but has little impact to the DOTS client because there is no long delay before using IPv4 and TCP. The IPv6 path and UDP over IPv6 and IPv4 is retried until the DOTS client gives up.

Constrained Application Protocol (CoAP) is used for DOTS signal channel (). COAP was designed according to the REST architecture, and thus exhibits functionality similar to that of HTTP, it is quite straightforward to map from CoAP to HTTP and from HTTP to CoAP. CoAP has been defined to make use of both DTLS over UDP and TLS over TCP. The advantages of COAP are: (1) Like HTTP, CoAP is based on the successful REST model, (2) CoAP is designed to use minimal resources, (3) CoAP integrates with JSON, CBOR or any other data format, (4) asynchronous message exchanges, (5) includes a congestion control mechanism (6) allows configuration of message transmission parameters specific to the application environment (including dynamically adjusted values, see Section 4.8.1 in ) etc.

A single DOTS signal channel between DOTS agents can be used to exchange multiple DOTS signal messages. To reduce DOTS client and DOTS server workload, DOTS client SHOULD re-use the (D)TLS session. JSON payloads are used to convey signal channel specific payload messages that convey request parameters and response information such as errors. TBD: Do we want to use CBOR [RFC7049] instead of JSON?

The following APIs define the means to convey a DOTS signal from a DOTS client to a DOTS server: are used to convey the DOTS signal from a DOTS client to a DOTS server over the signal channel, possibly traversing a DOTS gateway, indicating the DOTS client's need for mitigation, as well as the scope of any requested mitigation (). DOTS gateway act as a CoAP-to-CoAP Proxy (explained in ). are used by the DOTS client to withdraw the request for mitigation from the DOTS server (). are used by the DOTS client to retrieve the DOTS signal(s) it had conveyed to the DOTS server (). are used by the DOTS client to convey mitigation efficacy updates to the DOTS server (). Reliability is provided to the POST, DELETE, GET, and PUT requests by marking them as Confirmable (CON) messages. As explained in Section 2.1 of , a Confirmable message is retransmitted using a default timeout and exponential back-off between retransmissions, until the DOTS server sends an Acknowledgement message (ACK) with the same Message ID conveyed from the DOTS client. Message transmission parameters are defined in Section 4.8 of . Reliability is provided to the responses by marking them as Confirmable (CON) messages. The DOTS server can either piggback the response in the acknowledgement message or if the DOTS server is not able to respond immediately to a request carried in a Confirmable message, it simply responds with an Empty Acknowledgement message so that the DOTS client can stop retransmitting the request. Empty Acknowledgement message is explained in Section 2.2 of . When the response is ready, the server sends it in a new Confirmable message which then in turn needs to be acknowledged by the DOTS client (see Sections 5.2.1 and Sections 5.2.2 in ). Implementation Note: A DOTS client that receives a response in a CON message may want to clean up the message state right after sending the ACK. If that ACK is lost and the DOTS server retransmits the CON, the DOTS client may no longer have any state to which to correlate this response, making the retransmission an unexpected message; the DOTS client will send a Reset message so it does not receive any more retransmissions. This behavior is normal and not an indication of an error (see Section 5.3.2 in for more details).

When suffering an attack and desiring DoS/DDoS mitigation, a DOTS signal is sent by the DOTS client to the DOTS server. A POST request is used to convey a DOTS signal to the DOTS server (). The DOTS server can enable mitigation on behalf of the DOTS client by communicating the DOTS client's request to the mitigator and relaying any mitigator feedback to the requesting DOTS client.

The header fields are described below. Identifier of the policy represented using an integer. This identifier MUST be unique for each policy bound to the DOTS client, i.e. ,the policy-id needs to be unique relative to the active policies with the DOTS server. This identifier MUST be generated by the DOTS client. This document does not make any assumption about how this identifier is generated. This is a mandatory attribute. A list of IP addresses or prefixes under attack. IP addresses and prefixes are separated by commas. Prefixes are represented using CIDR notation . This is an optional attribute. A list of ports under attack. Ports are separated by commas and port number range (using "-"). For TCP, UDP, SCTP, or DCCP: the range of ports (e.g., 1024-65535). This is an optional attribute. A list of protocols under attack. Valid protocol values include tcp, udp, sctp, and dccp. Protocol values are separated by commas. This is an optional attribute. Fully Qualified Domain Name, is the full name of a system, rather than just its hostname. For example, "venera" is a hostname, and "venera.isi.edu" is an FQDN. This is an optional attribute. Uniform Resource Identifier (URI). This is an optional attribute. E.164 number. This is an optional attribute. Name of the alias (see Section 3.1.1 in ). This is an optional attribute. Lifetime of the mitigation request policy in seconds. Upon the expiry of this lifetime, and if the request is not refreshed, the mitigation request is removed. The request can be refreshed by sending the same request again. The default lifetime of the policy is 60 minutes -- this value was chosen to be long enough so that refreshing is not typically a burden on the DOTS client, while expiring the policy where the client has unexpectedly quit in a timely manner. A lifetime of zero indicates indefinite lifetime for the mitigation request. The server MUST always indicate the actual lifetime in the response. This is an optional attribute in the request. In the POST request at least one of the attributes target-ip or target-port or target-protocol or FQDN or URI or E.164 or alias MUST be present. The relative order of two rules is determined by comparing their respective policy identifiers. The rule with lower numeric policy identifier value has higher precedence (and thus will match before) than the rule with higher numeric policy identifier value. To avoid DOTS signal message fragmentation and the consequently decreased probability of message delivery, DOTS agents MUST ensure that the DTLS record MUST fit within a single datagram. If the Path MTU is not known to the DOTS server, an IP MTU of 1280 bytes SHOULD be assumed. The length of the URL MUST NOT exceed 256 bytes. If UDP is used to convey the DOTS signal messages then the DOTS client must consider the amount of record expansion expected by the DTLS processing when calculating the size of CoAP message that fits within the path MTU. Path MTU MUST be greater than or equal to [CoAP message size + DTLS overhead of 13 octets + authentication overhead of the negotiated DTLS cipher suite + block padding (Section 4.1.1.1 of ]. If the request size exceeds the Path MTU then the DOTS client MUST split the DOTS signal into separate messages, for example the list of addresses in the 'target-ip' field could be split into multiple lists and each list conveyed in a new POST request. Implementation Note: DOTS choice of message size parameters works well with IPv6 and with most of today's IPv4 paths. However, with IPv4, it is harder to absolutely ensure that there is no IP fragmentation. If IPv4 support on unusual networks is a consideration and path MTU is unknown, implementations may want to limit themselves to more conservative IPv4 datagram sizes such as 576 bytes, as per IP packets up to 576 bytes should never need to be fragmented, thus sending a maximum of 500 bytes of DOTS signal over a UDP datagram will generally avoid IP fragmentation. shows a POST request to signal that ports 80, 8080, and 443 on the servers 2002:db8:6401::1 and 2002:db8:6401::2 are being attacked.

The DOTS server indicates the result of processing the POST request using CoAP response codes. CoAP 2xx codes are success, CoAP 4xx codes are some sort of invalid requests and 5xx codes are returned if the DOTS server has erred or is incapable of performing the mitigation. Response code 2.01 (Created) will be returned in the response if the DOTS server has accepted the mitigation request and will try to mitigate the attack. If the request is missing one or more mandatory attributes, then 4.00 (Bad Request) will be returned in the response or if the request contains invalid or unknown parameters then 4.02 (Invalid query) will be returned in the response. The CoAP response will include the JSON body received in the request.

A DELETE request is used to withdraw a DOTS signal from a DOTS server ().

If the DOTS server does not find the policy number conveyed in the DELETE request in its policy state data, then it responds with a 4.04 (Not Found) error response code. The DOTS server successfully acknowledges a DOTS client's request to withdraw the DOTS signal using 2.02 (Deleted) response code, and ceases mitigation activity as quickly as possible.

A GET request is used to retrieve information and status of a DOTS signal from a DOTS server (). If the DOTS server does not find the policy number conveyed in the GET request in its policy state data, then it responds with a 4.04 (Not Found) error response code.

shows the response of all the active policies on the DOTS server.

The various possible values of status field are explained below: Attack mitigation is in progress (e.g., changing the network path to re-route the inbound traffic to DOTS mitigator). Attack is successfully mitigated (e.g., attack traffic is dropped). Attack has stopped and the DOTS client can withdraw the mitigation request. Attack has exceeded the mitigation provider capability. The observe option defined in extends the CoAP core protocol with a mechanism for a CoAP client to "observe" a resource on a CoAP server: the client retrieves a representation of the resource and requests this representation be updated by the server as long as the client is interested in the resource. A DOTS client conveys the observe option set to 0 in the GET request to receive unsolicited notifications of attack mitigation status from the DOTS server. Unidirectional notifications within the bidirectional signal channel allows unsolicited message delivery, enabling asynchronous notifications between the agents. A DOTS client that is no longer interested in receiving notifications from the DOTS server can simply "forget" the observation. When the DOTS server then sends the next notification, the DOTS client will not recognize the token in the message and thus will return a Reset message. This causes the DOTS server to remove the associated entry.

A DOTS client retrieves the information about a DOTS signal at frequent intervals to determine the status of an attack. If the DOTS server has been able to mitigate the attack and the attack has stopped, the DOTS server indicates as such in the status, and the DOTS client recalls the mitigation request. A DOTS client should react to the status of the attack from the DOTS server and not the fact that it has recognized, using its own means, that the attack has been mitigated. This ensures that the DOTS client does not recall a mitigation request in a premature fashion because it is possible that the DOTS client does not sense the DDOS attack on its resources but the DOTS server could be actively mitigating the attack and the attack is not completely averted.

While DDoS mitigation is active, a DOTS client MAY frequently transmit DOTS mitigation efficacy updates to the relevant DOTS server. An PUT request () is used to convey the mitigation efficacy update to the DOTS server. The PUT request MUST include all the header fields used in the POST request to convey the DOTS signal (). If the DOTS server does not find the policy number conveyed in the PUT request in its policy state data, it responds with a 4.04 (Not Found) error response code.

The 'attack-status' field is a mandatory attribute. The various possible values contained in the 'attack-status' field are explained below: DOTS client determines that it is still under attack. Attack is successfully mitigated (e.g., attack traffic is dropped).

This section defines the (D)TLS protocol profile of DOTS signal channel over (D)TLS and DOTS data channel over TLS. There are known attacks on (D)TLS, such as machine-in-the-middle and protocol downgrade. These are general attacks on (D)TLS and not specific to DOTS over (D)TLS; please refer to the (D)TLS RFCs for discussion of these security issues. DOTS agents MUST adhere to the (D)TLS implementation recommendations and security considerations of except with respect to (D)TLS version. Since encryption of DOTS using (D)TLS is virtually a green-field deployment DOTS agents MUST implement only (D)TLS 1.2 or later. Implementations compliant with this profile MUST implement all of the following items: DOTS client can use (D)TLS session resumption without server-side state to resume session and convey the DOTS signal. While the communication to the DOTS server is quiescent, the DOTS client MAY probe the server to ensure it has maintained cryptographic state. Such probes can also keep alive firewall or NAT bindings. This probing reduces the frequency of needing a new handshake when a DOTS signal needs to be conveyed to the DOTS server. A (D)TLS heartbeat verifies the DOTS server still has DTLS state by returning a DTLS message. If the server has lost state, it returns a DTLS Alert. Upon receipt of an unauthenticated DTLS Alert, the DTLS client validates the Alert is within the replay window (Section 4.1.2.6 of ). It is difficult for the DTLS client to validate the DTLS Alert was generated by the DTLS server in response to a request or was generated by an on- or off-path attacker. Thus, upon receipt of an in-window DTLS Alert, the client SHOULD continue re-transmitting the DTLS packet (in the event the Alert was spoofed), and at the same time it SHOULD initiate DTLS session resumption. TLS runs over TCP, so a simple probe is a 0-length TCP packet (a "window probe"). This verifies the TCP connection is still working, which is also sufficient to prove the server has retained TLS state, because if the server loses TLS state it abandons the TCP connection. If the server has lost state, a TCP RST is returned immediately. Raw public keys which reduce the size of the ServerHello, and can be used by servers that cannot obtain certificates (e.g., DOTS gateways on private networks). Implementations compliant with this profile SHOULD implement all of the following items to reduce the delay required to deliver a DOTS signal: TLS False Start which reduces round-trips by allowing the TLS second flight of messages (ChangeCipherSpec) to also contain the DOTS signal. Cached Information Extension which avoids transmitting the server's certificate and certificate chain if the client has cached that information from a previous TLS handshake. TCP Fast Open can reduce the number of round-trips to convey DOTS signal.

(D)TLS based on client certificate can be used for mutual authentication between DOTS agents. If a DOTS gateway is involved, DOTS clients and DOTS gateway MUST perform mutual authentication; only authorized DOTS clients are allowed to send DOTS signals to a DOTS gateway. DOTS gateway and DOTS server MUST perform mutual authentication; DOTS server only allows DOTS signals from authorized DOTS gateway, creating a two-link chain of transitive authentication between the DOTS client and the DOTS server.

+ +<---------------->+ DOTS | | | (DOTS client)| | DOTS | | | Server | | +--------------+ | Gateway | | | | | +----+--------+ | +---------------+ | ^ | | | | | +----------------+ | | | | DDOS detector | | | | | (DOTS client) +<--------------+ | | +----------------+ | | | +-------------------------------------------------+ ]]> In the example depicted in , the DOTS gateway and DOTS clients within the 'example.com' domain mutually authenticate with each other. After the DOTS gateway validates the identity of a DOTS client, it communicates with the AAA server in the 'example.com' domain to determine if the DOTS client is authorized to request DDOS mitigation. If the DOTS client is not authorized, a 4.01 (Unauthorized) is returned in the response to the DOTS client. In this example, the DOTS gateway only allows the application server and DDOS detector to request DDOS mitigation, but does not permit the user of type 'guest' to request DDOS mitigation. Also, DOTS gateway and DOTS server MUST perform mutual authentication using certificates. A DOTS server will only allow a DOTS gateway with a certificate for a particular domain to request mitigation for that domain. In reference to , the DOTS server only allows the DOTS gateway to request mitigation for 'example.com' domain and not for other domains.

TODO [TBD: DOTS WG will probably have to do something similar to https://tools.ietf.org/html/rfc7519#section-10, create JSON DOTS claim registry and register the JSON attributes defined in this specification].

Authenticated encryption MUST be used for data confidentiality and message integrity. (D)TLS based on client certificate MUST be used for mutual authentication. The interaction between the DOTS agents requires Datagram Transport Layer Security (DTLS) and Transport Layer Security (TLS) with a cipher suite offering confidentiality protection and the guidance given in MUST be followed to avoid attacks on (D)TLS. If TCP is used between DOTS agents, an attacker may be able to inject RST packets, bogus application segments, etc., regardless of whether TLS authentication is used. Because the application data is TLS protected, this will not result in the application receiving bogus data, but it will constitute a DoS on the connection. This attack can be countered by using TCP-AO . If TCP-AO is used, then any bogus packets injected by an attacker will be rejected by the TCP-AO integrity check and therefore will never reach the TLS layer. Special care should be taken in order to ensure that the activation of the proposed mechanism won't have an impact on the stability of the network (including connectivity and services delivered over that network). Involved functional elements in the cooperation system must establish exchange instructions and notification over a secure and authenticated channel. Adequate filters can be enforced to avoid that nodes outside a trusted domain can inject request such as deleting filtering rules. Nevertheless, attacks can be initiated from within the trusted domain if an entity has been corrupted. Adequate means to monitor trusted nodes should also be enabled.

The following individuals have contributed to this document: Mike Geller Cisco Systems, Inc. 3250 Florida 33309 USA Email: mgeller@cisco.com Robert Moskowitz HTT Consulting Oak Park, MI 42837 United States Email: rgm@htt-consult.com

Thanks to Christian Jacquenet, Roland Dobbins, Andrew Mortensen, Roman D. Danyliw, and Gilbert Clark for the discussion and comments.