| DOTS | T. Reddy |
| Internet-Draft | Cisco |
| Intended status: Standards Track | M. Boucadair |
| Expires: February 19, 2017 | Orange |
| D. Wing | |
| P. Patil | |
| Cisco | |
| August 18, 2016 |
Distributed Denial-of-Service Open Threat Signaling (DOTS) Signal Channel
draft-reddy-dots-signal-channel-00
This document specifies a mechanism that a DOTS client can use to signal that a network is under a Distributed Denial-of-Service (DDoS) attack to an upstream DOTS server so that appropriate mitigation actions are undertaken (including, blackhole, drop, rate-limit, or add to watch list) on the suspect traffic. The document specifies the DOTS signal channel including Happy Eyeballs considerations. The specification of the DOTS data channel is elaborated in a companion document.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on February 19, 2017.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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 [I-D.ietf-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 [I-D.ietf-dots-requirements].
This document satisfies all the use cases discussed in [I-D.ietf-dots-use-cases] except the Third-party DOTS notifications use case in Section 3.2.3 of [I-D.ietf-dots-use-cases] 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 [I-D.reddy-dots-data-channel].
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 [RFC2119].
(D)TLS: For brevity this term is used for statements that apply to both Transport Layer Security [RFC5246] and Datagram Transport Layer Security [RFC6347]. Specific terms will be used for any statement that applies to either protocol alone.
The reader should be familiar with the terms defined in [I-D.ietf-dots-architecture].
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 ([RFC4987]). 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 [RFC4732].
In each of the cases described above, the possible arrangements between the DOTS client and DOTS server to mitigate the attack are discussed in [I-D.ietf-dots-use-cases]. An example of network diagram showing a deployment of these elements is shown in Figure 1. Architectural relationships between involved DOTS agents is explained in [I-D.ietf-dots-architecture]. In this example, the DOTS server is operating on the access network.
Network Resource CPE router Access network __________ +-----------+ +--------------+ +-------------+ / \ | |____| |_______| |___ | Internet | |DOTS client| | DOTS gateway | | DOTS server | | | | | | | | | | | +-----------+ +--------------+ +-------------+ \__________/
Figure 1
The DOTS server can also be running on the Internet, as depicted in Figure 2.
Network DDoS mitigation Resource CPE router __________ service +-----------+ +-------------+ / \ +-------------+ | |____| |_______| |___ | | |DOTS client| |DOTS gateway | | Internet | | DOTS server | | | | | | | | | | +-----------+ +-------------+ \__________/ +-------------+
Figure 2
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 [RFC6347] over UDP and TLS [RFC5246] 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. [I-D.ietf-dots-requirements] 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 [RFC6555]. 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 [RFC6724] and the DOTS preference that UDP be used over TCP (to avoid TCP's head of line blocking).
DOTS client DOTS server | | |--DTLS ClientHello, IPv6 ---->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----------------------------->| | |
Figure 3: Happy Eyeballs
In reference to Figure 3, 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) [RFC7252] is used for DOTS signal channel (Figure 4). 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, etc.
+--------------+
| DOTS |
+--------------+
| CoAP |
+--------------+
| TLS | DTLS |
+--------------+
| TCP | UDP |
+--------------+
| IP |
+--------------+
Figure 4: Abstract Layering of DOTS signal channel over CoAP over (D)TLS
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 [RFC7159] 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:
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 [RFC7252], 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 [RFC7252]. 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 [RFC7252]. 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 [RFC7252]).
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 [RFC7252] 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 (Figure 5). 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.
Header: POST (Code=0.02)
Uri-Host: "host"
Uri-Path: ".well-known"
Uri-Path: "DOTS-signal"
Uri-Path: "version"
Content-Type: "application/json"
{
"policy-id": "integer",
"target-ip": "string",
"target-port": "string",
"target-protocol": "string",
"alias": "string"
"lifetime": "number"
}
Figure 5: POST to convey DOTS signals
The header fields are described below.
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 and 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 [RFC0791] 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.
Figure 6 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.
Header: POST (Code=0.02)
Uri-Host: "www.example.com"
Uri-Path: ".well-known"
Uri-Path: "v1"
Uri-Path: "DOTS-signal"
Content-Type: "application/json"
{
"policy-id":123321333242,
"target-ip":[
"2002:db8:6401::1",
"2002:db8:6401::2"
],
"target-port":[
"80",
"8080",
"443"
],
"target-protocol":"tcp"
}
Figure 6: POST for DOTS signal
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 (Figure 7).
Header: DELETE (Code=0.04)
Uri-Host: "host"
Uri-Path: ".well-known"
Uri-Path: "version"
Uri-Path: "DOTS-signal"
Content-Type: "application/json"
{
"policy-id": "number"
}
Figure 7: Withdraw DOTS signal
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 (Figure 8). 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.
1) To retrieve all DOTS signals signaled by the DOTS client. Header: GET (Code=0.01) Uri-Host: "host" Uri-Path: ".well-known" Uri-Path: "version" Uri-Path: "DOTS-signal" Uri-Path: "list" Observe : 0 2) To retrieve a specific DOTS signal signaled by the DOTS client. The policy information in the response will be formatted in the same order it was processed at the DOTS server. Header: GET (Code=0.01) Uri-Host: "host" Uri-Path: ".well-known" Uri-Path: "version" Uri-Path: "DOTS-signal" Uri-Path: "policy-id value" Observe : 0
Figure 8: GET to retrieve the rules
Figure 9 shows the response of all the active policies on the DOTS server.
{
"policy-data":[
{
"policy-id":123321333242,
"target-protocol":"tcp",
"lifetime":3600,
"status":"mitigation in progress"
},
{
"policy-id":123321333244,
"target-protocol":"udp",
"lifetime":1800,
"status":"mitigation complete"
},
{
"policy-id":123321333245,
"target-protocol":"tcp",
"lifetime":1800,
"status":"attack stopped"
}
]
}
Figure 9: Response body
The various possible values of status field are explained below:
The observe option defined in [RFC7641] 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.
DOTS Client DOTS Server
| |
| GET /<policy-id number> |
| Token: 0x4a | Registration
| Observe: 0 |
+-------------------------->|
| |
| 2.05 Content |
| Token: 0x4a | Notification of
| Observe: 12 | the current state
| status: "mitigation |
| in progress" |
|<--------------------------+
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 44 | a state change
| status: "mitigation |
| complete" |
|<--------------------------+
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 60 | a state change
| status: "attack stopped" |
|<--------------------------+
| |
Figure 10: Notifications of attack mitigation status
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 (Figure 11) 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 (Section 5.2.1). 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.
Header: PUT (Code=0.03)
Uri-Host: "host"
Uri-Path: ".well-known"
Uri-Path: "version"
Uri-Path: "DOTS-signal"
Uri-Path: "policy-id value"
Content-Type: "application/json"
{
"target-ip": "string",
"target-port": "string",
"target-protocol": "string",
"lifetime": "number",
"attack-status": "string"
}
Figure 11: Efficacy Update
The 'attack-status' field is a mandatory attribute. The various possible values contained in the 'attack-status' field are explained below:
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 [RFC7525] 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:
Implementations compliant with this profile SHOULD implement all of the following items to reduce the delay required to deliver a 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.
+-------------------------------------------------+ | example.com domain +---------+ | | | AAA | | | +---------------+ | Server | | | | Application | +------+--+ | | | server + ^ | | (DOTS client) |<-----------------+ | | | +---------------+ + | | example.net domain | V V | | +-------------+ | +---------------+ | +--------------+ | | | | | | | Guest +<-----x----->+ +<---------------->+ DOTS | | | (DOTS client)| | DOTS | | | Server | | +--------------+ | Gateway | | | | | +----+--------+ | +---------------+ | ^ | | | | | +----------------+ | | | | DDOS detector | | | | | (DOTS client) +<--------------+ | | +----------------+ | | | +-------------------------------------------------+
Figure 12: Example of Authentication and Authorization of DOTS Agents
Figure 12, 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 Figure 12, 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 [RFC7525] 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 [RFC5925]. 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.