DOTS A. Mortensen
Internet-Draft Arbor Networks, Inc.
Intended status: Informational F. Andreasen
Expires: September 20, 2016 T. Reddy
Cisco Systems, Inc.
C. Gray
Comcast, Inc.
R. Compton
Charter Communications, Inc.
N. Teague
Verisign, Inc.
March 19, 2016
Distributed-Denial-of-Service (DDoS) Open Threat Signaling Architecture
draft-mortensen-dots-architecture-00
Abstract
This document describes an architecture for establishing and
maintaining Distributed Denial of Service (DDoS) Open Threat
Signaling (DOTS) within and between networks. The document makes no
attempt to suggest protocols or protocol extensions, instead focusing
on architectural relationships, components and concepts used in a
DOTS deployment.
Status of This Memo
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-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 20, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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Table of Contents
1. Context and Motivation . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Definition of Terms . . . . . . . . . . . . . . . . . 3
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 4
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. DOTS Operations . . . . . . . . . . . . . . . . . . . . . 7
2.2. DOTS Agent Relationships . . . . . . . . . . . . . . . . 9
3. Components . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1. DOTS client . . . . . . . . . . . . . . . . . . . . . . . 13
3.2. DOTS server . . . . . . . . . . . . . . . . . . . . . . . 14
4. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Signaling Sessions . . . . . . . . . . . . . . . . . . . 15
4.1.1. Preconditions . . . . . . . . . . . . . . . . . . . . 15
4.1.2. Establishing the Signaling Session . . . . . . . . . 15
4.1.3. Maintaining the Signaling Session . . . . . . . . . . 16
4.2. Modes of Signaling . . . . . . . . . . . . . . . . . . . 17
4.2.1. Direct Signaling . . . . . . . . . . . . . . . . . . 17
4.2.2. Relayed Signaling . . . . . . . . . . . . . . . . . . 17
4.2.3. Redirected Signaling . . . . . . . . . . . . . . . . 18
4.2.4. Recursive Signaling . . . . . . . . . . . . . . . . . 19
5. Security Considerations . . . . . . . . . . . . . . . . . . . 22
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
7. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 23
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. Normative References . . . . . . . . . . . . . . . . . . 23
8.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Context and Motivation
Signaling the need for help defending against an active distributed
denial of service (DDoS) attack requires a common understanding of
mechanisms and roles among the parties coordinating attack response.
The proposed signaling layer and supplementary messaging is the focus
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of DDoS Open Threat Signaling (DOTS). DOTS proposes to standardize a
method of coordinating defensive measures among willing peers to
mitigate attacks quickly and efficiently.
This document describes an architecture used in establishing,
maintaining or terminating a DOTS relationship in a network or
between networks. DOTS enables hybrid attack responses, coordinated
locally at or near the target of an active attack, as well as closer
to attack sources in the network path.
1.1. Terminology
1.1.1. Key Words
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].
1.1.2. Definition of Terms
This document uses the terms defined in [I-D.ietf-dots-requirements].
1.2. Scope
This document defines an architecture for the proposed DOTS standard
in the IETF.
In this architecture, DOTS clients and servers communicate using the
signaling mechanism established in the proposed DOTS standard. As a
result of signals from a DOTS client, the DOTS server may modify the
network path of traffic destined for the attack target or targets,
for example by diverting traffic to a scrubbing center. Packets
deemed part of an active attack may be dropped.
The architecture presented here is assumed to be applicable across
network administrative domains - for example, between an enterprise
domain and the domain of a third-party attack scrubbing service - as
well as to a single administrative domain. DOTS is generally assumed
to be most effective when aiding coordination of attack response
between two or more participating network domains, but single domain
scenarios are valuable in their own right, as when aggregating intra-
domain DOTS client signals for inter-domain coordinated attack
response.
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1.3. Assumptions
This document makes the following assumptions:
o The network or networks in which DOTS is deployed are assumed to
offer the required connectivity between DOTS agents and any
intermediary network elements, but the architecture imposes no
additional limitations on the form of connectivity.
o Congestion and resource exhaustion are intended outcomes of a DDoS
attack [RFC4732]. Some operators may utilize non-impacted paths
or networks for DOTS, however, it should be assumed that, in
general, conditions will be hostile and that DOTS must be able to
function in all circumstances, including when the signaling path
is significantly impaired.
o There is no universal DDoS attack scale threshold triggering a
coordinated response across network administrative domains. A
network domain administrator, or service or application owner may
arbitrarily set attack scale threshold triggers, or manually send
requests for mitigation.
o The mitigation capacity and/or capability of networks receiving
requests for coordinated attack response is opaque to the network
sending the request. The entity receiving the DOTS client signal
may or may not have sufficient capacity or capability to filter
any or all DDoS attack traffic directed at a target.
o DOTS client and server signals, as well as messages sent through
the data channel, are sent across any transit networks with the
same probability of delivery as any other traffic between the DOTS
client network and the DOTS server network. Any encapsulation
required for successful delivery is left untouched by transit
network elements. DOTS server and DOTS client cannot assume any
preferential treatment of DOTS signals.
o The architecture allows for, but does not assume, the presence of
Quality of Service (QoS) policy agreements between DOTS-enabled
peer networks or local QoS prioritization aimed at ensuring
delivery of DOTS messages between DOTS agents. QoS is an
operational consideration only, not a functional part of a DOTS
architecture.
o There is no assumption that the signal channel and the data
channel should terminate on the same DOTS server: they may be
loosely coupled.
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2. Architecture
DOTS enables a target that is under a Distributed Denial-of-Service
(DDoS) attack to signal another entity for help in mitigating the
DDoS attack. The basic high-level DOTS architecture is illustrated
in Figure 1:
+-----------+ +-------------+
| Mitigator | ~~~~~~~~~~ | DOTS Server |
+-----------+ +-------------+
| |
| |
| |
| +------------+
| | DOTS Relay |
| +------------+
| |
| |
| |
+---------------+ +-------------+
| Attack Target | ~~~~~~~ | DOTS Client |
+---------------+ +-------------+
Figure 1: Basic DOTS Architecture
A simple example instantiation of the DOTS architecture could be an
enterprise as the attack target for a volumetric DDoS attack, and an
upstream DDoS mitigation service as the Mitigator. The enterprise
(attack target) is connected to the Internet via a link that is
getting saturated, and the enterprise suspects it is under DDoS
attack. The enterprise has a DOTS client, which obtains information
about the DDoS attack, and signals the DOTS server for help in
mitigating the attack. The communication may be direct from the DOTS
client to the DOTS Server, or it may traverse one or more DOTS
Relays, which act as intermediaries. The DOTS Server in turn invokes
one or more mitigators, which are tasked with mitigating the actual
DDoS attack, and hence aim to suppress the attack traffic while
allowing valid traffic to reach the attack target.
The scope of the DOTS specifications is the interfaces between the
DOTS client, DOTS server, and DOTS relay. The interfaces to the
attack target and the mitigator are out of scope of DOTS. Similarly,
the operation of both the attack target and the mitigator are out of
scope of DOTS. Thus, DOTS neither specifies how an attack target
decides it is under DDoS attack, nor does DOTS specify how a
mitigator may actually mitigate such an attack. Indeed, a DOTS
client's request for mitigation is advisory in nature, and may not
lead to any mitigation at all, depending on the DOTS server entity's
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capacity and willingness to mitigate on behalf of the DOTS client's
entity.
As illustrated in Figure 2, there are two interfaces between the DOTS
Server and the DOTS Client (and possibly the DOTS Relay):
+---------------+ +---------------+
| | <------- Signal Channel ------> | |
| DOTS Client | | DOTS Server |
| | <======= Data Channel ======> | |
+---------------+ +---------------+
Figure 2: DOTS Interfaces
The primary purpose of the signal channel is for the DOTS client to
ask the DOTS server for help in mitigating an attack, and for the
DOTS server to inform the DOTS client about the status of such
mitigation. The DOTS client does this by sending a client signal,
which contains information about the attack target or targets. The
client signal may also include telemetry information about the
attack, if the DOTS client has such information available. The DOTS
Server in turn sends a server signal to inform the DOTS client of
whether it will honor the mitigation request. Assuming it will, the
DOTS Server initiates attack mitigation (by means outside of DOTS),
and periodically informs the DOTS client about the status of the
mitigation. Similarly, the DOTS client periodically informs the DOTS
server about the client's status, which at a minimum provides client
(attack target) health information, but it may also include telemetry
information about the attack as it is now seen by the client. At
some point, the DOTS client may decide to terminate the server-side
attack mitigation, which it indicates to the DOTS server over the
signal channel. A mitigation may also be terminated if a DOTS
client-specified mitigation time limit is exceeded; additional
considerations around mitigation time limits may be found below.
Note that the signal channel may need to operate over a link that is
experiencing a DDoS attack and hence is subject to severe packet loss
and high latency.
While DOTS is able to request mitigation with just the signal
channel, the addition of the DOTS data channel provides for
additional and more efficient capabilities; both channels are
required in the DOTS architecture. The primary purpose of the data
channel is to support DOTS related configuration and policy
information exchange between the DOTS client and the DOTS server.
Examples of such information include
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o Defining names or aliases for attack targets (resources). Those
names can be used in subsequent signal channel exchanges to more
efficiently refer to the resources (attack targets) in question.
o Black-list management, which enables a DOTS client to inform the
DOTS server about sources to suppress.
o White-list management, which enables a DOTS client to inform the
DOTS server about sources from which traffic should always be
accepted.
o DOTS client provisioning.
o Vendor-specific extensions, supplementing or in some other way
facilitating mitigation when the mitigator relies on particular
proprietary interfaces.
Note that while it is possible to exchange the above information
before, during or after a DDoS attack, DOTS requires reliable
delivery of the above information and does not provide any special
means for ensuring timely delivery of it during an attack. In
practice, this means that DOTS entities SHOULD NOT rely on such
information being exchanged during a DDoS attack.
2.1. DOTS Operations
The scope of DOTS is focused on the signaling and data exchange
between the DOTS client, DOTS server and (possibly) the DOTS relay.
DOTS does not prescribe any specific deployment models, however DOTS
is designed with some specific requirements around the different DOTS
agents and their relationships.
First of all, a DOTS agent belongs to an entity, and that entity has
an identity which can be authenticated. DOTS agents communicate with
each other over a mutually authenticated signal channel and bulk data
channel. However, before they can do so, a service relationship
needs to be established between them. The details and means by which
this is done is outside the scope of DOTS, however an example would
be for an enterprise A (DOTS client) to sign up for DDoS service from
provider B (DOTS server). This would establish a (service)
relationship between the two that enables enterprise A's DOTS client
to establish a signal channel with provider B's DOTS server. A and B
will authenticate each other, and B can verify that A is authorized
for its service. A and B may each have one or more DOTS relays in
front of their DOTS client and DOTS server.
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[[EDITOR'S NOTE: we request working group feedback and discussion of
considerations of end-to-end signaling and agent authentication/
authorization with relays in the signaling path.]]
From an operational and design point of view, DOTS assumes that the
above relationship is established prior to a request for DDoS attack
mitigation. In particular, it is assumed that bi-directional
communication is possible at this time between the DOTS client and
DOTS server. Furthermore, it as assumed that additional service
provisioning, configuration and information exchange can be performed
by use of the data channel, if operationally required. It is not
until this point that the mitigation service is available for use.
Once the mutually authenticated signal channel has been established,
it will remain in place. This is done to increase the likelihood
that the DOTS client can signal the DOTS server for help when the
attack target is being flooded, and similarly raise the probability
that DOTS server signals reach the client regardless of inbound link
congestion. This does not necessarily imply that the attack target
and the DOTS client have to be co-located in the same administrative
domain, but it is expected to be a common scenario.
DDoS mitigation service with the help of an upstream mitigator will
often involve some form of traffic redirection whereby traffic
destined for the attack target is diverted towards the mitigator,
e.g. by use of BGP [RFC4271] or DNS [RFC1034]. The mitigator in turn
inspects and scrubs the traffic, and forwards the resulting
(hopefully non-attack) traffic to the attack target, e.g. via a GRE
tunnel. Thus, when a DOTS server receives an attack mitigation
request from a DOTS client, it can be viewed as a way of causing
traffic redirection for the attack target indicated. Note that DOTS
does not consider any authorization aspects around who should be
allowed to issue such requests for what attack targets. Instead,
DOTS merely relies on the mutual authentication and the pre-
established (service) relationship between the entity owning the DOTS
client and the entity owning the DOTS server. The entity owning the
DOTS server SHOULD limit the attack targets that a particular DOTS
client can request mitigation for as part of establishing this
relationship. The method of such limitation is not in scope for this
document.
Although co-location of DOTS server and mitigator within the same
entity is expected to be a common deployment model, it is assumed
that operators may require alternative models. Nothing in this
document precludes such alternatives.
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2.2. DOTS Agent Relationships
So far, we have only considered a relatively simple scenario of a
single DOTS client associated with a single DOTS server, however DOTS
supports more advanced relationships.
A DOTS server may be associated with one or more DOTS clients, and
those DOTS clients may belong to different entities. An example
scenario is a mitigation provider serving multiple attack targets
(Figure 3):
+---+
| c |-----------
+---+ \
\
+---+ \ +---+
| c |---------------| S |
+---+ / +---+
/
+---+ /
| c |-----------
+---+
example.com/.org example.net
DOTS Clients DOTS Server
Figure 3: Multiple DOTS clients for a DOTS server
A DOTS client may be associated with one or more DOTS servers, and
those DOTS servers may belong to different entities. This may be to
ensure high availability or co-ordinate mitigation with more than one
directly connected ISP. An example scenario is for an enterprise to
have DDoS mitigation service from multiple providers, as shown in
Figure 4 below. Operational considerations relating to co-ordinating
multiple provider responses are beyond the scope of DOTS.
[[EDITOR'S NOTE: we request working group feedback and discussion of
operational considerations relating to coordinating multiple provider
responses to a mitigation request.]]
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+---+
------------| S |
/ +---+
/
+---+/ +---+
| c |---------------| S |
+---+\ +---+
\
\ +---+
------------| S |
+---+
example.com example.net/.org
DOTS Client DOTS Servers
Figure 4: Multi-Homed DOTS Client
DOTS Relays may be either server-side or client-side, or both. A
DOTS server-side relay belongs to the entity owning the DOTS server.
A relay will terminate multiple discrete client connections as if it
were a server and may aggregate these into a single (Figure 5) or
multiple DOTS signaling sessions (Figure 6) depending upon locally
applied policy. A relay will function as a server to its downstream
agents and as a client to its upstream agents. Aside from the
exceptions discussed in Section 4.2.2 below, the relationship between
the relay and its upstream agents is opaque to the relayed clients.
An example scenario is for an enterprise to have deployed multiple
DOTS capable devices which are able to signal intra-domain using TCP
[RFC0793] on un-congested links to a relay which may then transform
these to a UDP [RFC0768] transport inter-domain where connection
oriented transports may degrade; this applies to the signal channel
only, as the data channel requires a connection-oriented transport.
The relationship between the relay and its upstream agents is opaque
to the relayed clients.
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+---+
| c |\
+---+ \ +---+
\-----TCP-----| r | +---+
+---+ | e | | |
| c |--------TCP-----| l |------UDP-----| S |
+---+ | a | | |
/-----TCP-----| y | +---+
+---+ / +---+
| c |/
+---+
example.com example.com example.net
DOTS Clients DOTS Relay DOTS Server
Figure 5: Client-Side Relay with Aggregation
+---+
| c |\
+---+ \ +---+
\-----TCP-----| r |------UDP-----+---+
+---+ | e | | |
| c |--------TCP-----| l |------UDP-----| S |
+---+ | a | | |
/-----TCP-----| y |------UDP-----+---+
+---+ / +---+
| c |/
+---+
example.com example.com example.net
DOTS Clients DOTS Relay DOTS Server
Figure 6: Client-Side Relay without Aggregation
A variation of this scenario would be a DDoS mitigation provider
deploying relays at their perimeter to consume signals across
multiple transports and to consolidate these into a single transport
suitable for the providers deployment, as shown in Figure 7 and
Figure 8 below. The relationship between the relay and its upstream
agents is opaque to the relayed clients.
[[EDITOR'S NOTE: we request working group feedback and discussion of
DOTS client visibility into relayed signaling.]]
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+---+
| c |\
+---+ \ +---+
\-----UDP-----| r | +---+
+---+ | e | | |
| c |--------TCP-----| l |------TCP-----| S |
+---+ | a | | |
/-----TCP-----| y | +---+
+---+ / +---+
| c |/
+---+
example.com example.net example.net
DOTS Clients DOTS Relay DOTS Server
Figure 7: Server-Side Relay with Aggregation
+---+
| c |\
+---+ \ +---+
\-----UDP-----| r |------TCP-----+---+
+---+ | e | | |
| c |--------TCP-----| l |------TCP-----| S |
+---+ | a | | |
/-----UDP-----| y |------TCP-----+---+
+---+ / +---+
| c |/
+---+
example.com example.net example.net
DOTS Clients DOTS Relay DOTS Server
Figure 8: Server-Side Relay without Aggregation
In the context of relays, sessions are established directly between
peer DOTS agents and may not be end-to-end. In spite of this
distinction a method must exist to uniquely identify the originating
DOTS client. The relay should identify itself as such to any clients
or servers it interacts with. Greater abstraction by way of
additional layers of relays may introduce undesired complexity in
regard to authentication and authorization and should be avoided.
[[EDITOR'S NOTE: we request working group feedback and discussion of
the many-to-one and one-to-many client/server, client/relay, and
relay/server relationships described above. We additionally request
working group feedback and discussion of end-to-end signaling
considerations in the context of relayed signaling.]]
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3. Components
The architecture in this document is comprised of a few basic
components on top of the assumed underlay network or networks
described above. When connected to one another, the components
represent an operational DOTS architecture.
This section describes the components themselves. Section 4 below
describes the architectural concepts involved.
3.1. DOTS client
A DOTS client is a DOTS agent from which requests for help
coordinating attack response originate. The requests may be in
response to an active, ongoing attack against a target in the DOTS
client's domain, but no active attack is required for a DOTS client
to request help. Local operators may wish to have upstream traffic
scrubbers in the network path for an indefinite period, and are
restricted only by business relationships when it comes to duration
and scope of requested mitigation.
The DOTS client requests attack response coordination from a DOTS
server over the signal channel, including in the request the DOTS
client's desired mitigation scoping, as described in
[I-D.ietf-dots-requirements]. The actual mitigation scope and
countermeasures used in response to the attack are up to the DOTS
server and Mitigator operators, as the DOTS client may have a narrow
perspective on the ongoing attack. As such, the DOTS client's
request for mitigation should be considered advisory: guarantees of
DOTS server availability or mitigation capacity constitute service
level agreements and are out of scope for this document.
The DOTS client adjusts mitigation scope and provides available
attack details at the direction of its local operator. Such
direction may involve manual or automated adjustments in response to
feedback from the DOTS server.
To provide a metric of signal health and distinguish an idle
signaling session from a disconnected or defunct session, the DOTS
client sends a heartbeat over the signal channel to maintain its half
of the signaling session. The DOTS client similarly expects a
heartbeat from the DOTS server, and MAY consider a signaling session
terminated in the extended absence of a DOTS server heartbeat.
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3.2. DOTS server
A DOTS server is a DOTS agent capable of receiving, processing and
possibly acting on requests for help coordinating attack response
from one or more DOTS clients. The DOTS server authenticates and
authorizes DOTS clients as described in Signaling Sessions below, and
maintains signaling session state, tracking requests for mitigation,
reporting on the status of active mitigations, and terminating
signaling sessions in the extended absence of a client heartbeat or
when a session times out.
Assuming the preconditions discussed below exist, a DOTS client
maintaining an active signaling session with a DOTS server may
reasonably expect some level of mitigation in response to a request
for coordinated attack response.
The DOTS server enforces authorization of DOTS clients' signals for
mitigation. The mechanism of enforcement is not in scope for this
document, but is expected to restrict requested mitigation scope to
addresses, prefixes, and/or services owned by the DOTS client's
administrative entity, such that a DOTS client from one entity is not
able to influence the network path to another entity. A DOTS server
MUST reject requests for mitigation of resources not owned by the
requesting DOTS client's administrative entity. A DOTS server MAY
also refuse a DOTS client's mitigation request for arbitrary reasons,
within any limits imposed by business or service level agreements
between client and server domains. If a DOTS server refuses a DOTS
client's request for mitigation, the DOTS server SHOULD include the
refusal reason in the server signal sent to the client.
A DOTS server is in regular contact with one or more mitigators. If
a DOTS server accepts a DOTS client's request for help, the DOTS
server forwards a translated form of that request to the mitigator or
mitigators responsible for scrubbing attack traffic. Note that the
form of the translated request passed from the DOTS server to the
mitigator is not in scope: it may be as simple as an alert to
mitigator operators, or highly automated using vendor or open
application programming interfaces supported by the mitigator. The
DOTS server MUST report the actual scope of any mitigation enabled on
behalf of a client.
The DOTS server SHOULD retrieve available metrics for any mitigations
activated on behalf of a DOTS client, and SHOULD include them in
server signals sent to the DOTS client originating the request for
mitigation.
To provide a metric of signal health and distinguish an idle
signaling session from a disconnected or defunct session, the DOTS
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server sends a heartbeat over the signal channel to maintain its half
of the signaling session. The DOTS server similarly expects a
heartbeat from the DOTS client, and MAY consider a signaling session
terminated in the extended absence of a DOTS client heartbeat.
4. Concepts
4.1. Signaling Sessions
In order for DOTS to be effective as a vehicle for DDoS mitigation
requests, one or more DOTS clients must establish ongoing
communication with one or more DOTS servers. While the preconditions
for enabling DOTS in or among network domains may also involve
business relationships, service level agreements, or other formal or
informal understandings between network operators, such
considerations are out of scope for this document.
An established communication layer between DOTS agents is a Signaling
Session. At its most basic, for a DOTS signaling session to exist
both signal channel and data channel must be functioning between DOTS
agents. That is, under nominal network conditions, signals actively
sent from a DOTS client are received by the specific DOTS server
intended by the client, and vice versa.
4.1.1. Preconditions
Prior to establishing a signaling session between agents, the owners
of the networks, domains, services or applications involved are
assumed to have agreed upon the terms of the relationship involved.
Such agreements are out of scope for this document, but must be in
place for a functional DOTS architecture.
It is assumed that as part of any DOTS service agreement, the DOTS
client is provided with all data and metadata required to establish
communication with the DOTS server. Such data and metadata would
include any cryptographic information necessary to meet the message
confidentiality, integrity and authenticity requirement in
[I-D.ietf-dots-requirements], and might also include the pool of DOTS
server addresses and ports the DOTS client should use for signal and
data channel messaging.
4.1.2. Establishing the Signaling Session
With the required business or service agreements in place, the DOTS
client initiates a signal session by contacting the DOTS server over
the signal channel and the data channel. To allow for DOTS service
flexibility, neither the order of contact nor the time interval
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between channel creations is specified. A DOTS client MAY establish
signal channel first, and then data channel, or vice versa.
The methods by which a DOTS client receives the address and
associated service details of the DOTS server are not prescribed by
this document. For example, a DOTS client may be directly configured
to use a specific DOTS server address and port, and directly provided
with any data necessary to satisfy the Peer Mutual Authentication
requirement in [I-D.ietf-dots-requirements], such as symmetric or
asymmetric keys, usernames and passwords, etc. All configuration and
authentication information in this scenario is provided out-of-band
by the entity operating the DOTS server.
At the other extreme, the architecture in this document allows for a
form of DOTS client auto-provisioning. For example, the entity
operating the DOTS server or servers might provide the client entity
only with symmetric or asymmetric keys to authenticate the
provisioned DOTS clients. Only the keys would then be directly
configured on DOTS clients, but the remaining configuration required
to provision the DOTS clients could be learned through mechanisms
similar to DNS SRV [RFC2782] or DNS Service Discovery [RFC6763].
The DOTS client SHOULD successfully authenticate and exchange
messages with the DOTS server over both signal and data channel as
soon as possible to confirm that both channels are operational.
Once the DOTS client begins receiving DOTS server signals, the
signaling session is active. At any time during the signaling
session, the DOTS client MAY use the data channel to adjust initial
configuration, manage black- and white-listed prefixes or addresses,
leverage vendor-specific extensions, and so on. Note that unlike the
signal channel, there is no requirement that the data channel remain
operational in attack conditions (See Data Channel Requirements,
[I-D.ietf-dots-requirements]).
4.1.3. Maintaining the Signaling Session
DOTS clients, servers and relays periodically send heartbeats to each
other over the signal channel, per Operational Requirements discussed
in [I-D.ietf-dots-requirements]. DOTS agent operators SHOULD
configure the heartbeat interval such that the frequency does not
lead to accidental denials of service due to the overwhelming number
of heartbeats a DOTS agent must field.
Either DOTS agent may consider a signaling session terminated in the
extended absence of a heartbeat from its peer agent. The period of
that absence will be established in the protocol definition.
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4.2. Modes of Signaling
This section examines the modes of signaling between agents in a DOTS
architecture.
4.2.1. Direct Signaling
A signaling session may take the form of direct signaling between the
DOTS clients and servers, as shown in Figure 9 below:
+-------------+ +-------------+
| DOTS client |<------signal session------>| DOTS server |
+-------------+ +-------------+
Figure 9: Direct Signaling
In a direct signaling session, DOTS client and server are
communicating directly, with no relays in the signaling path. A
direct signaling session MAY exist inter- or intra-domain. The
signaling session is abstracted from the underlying networks or
network elements the signals traverse: in a direct signaling session,
the DOTS client and server are logically peer DOTS agents.
4.2.2. Relayed Signaling
A signaling session may also include one or more DOTS relays in the
signaling path between the clients and servers, as shown in
Figure 10:
+-------------+ +-------------+
| DOTS client | | DOTS server |
+-------------+ +-------------+
^ ^
| +------------+ +------------+ |
+--->| DOTS relay |<----->| DOTS relay |<---+
+------------+ +------------+
Figure 10: Relayed Signaling
To allow for maximum architectural flexibility, no restriction is
placed on the number of relays in the signaling path. Operators of
DOTS agents should consider the impact on signal latency incurred by
each additional DOTS relay in the signaling path, as well as the
increased operational complexity, when deploying DOTS relays.
[[EDITOR'S NOTE: we request working group feedback and discussion of
operational considerations related to DOTS relays, particularly with
respect to the implications of multiple relays in the signal path.]]
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As discussed above in Section 2.2, relays may be client-side or
server-side. In either case, the relay appears to the peer agent as
its logical opposite. That is, a DOTS relay appears to a DOTS client
or downstream relay as a DOTS server. Conversely, a DOTS relay
appears to a DOTS server or upstream DOTS relay as a DOTS client.
Thus relayed signaling may be thought of as chained direct signaling
sessions.
4.2.3. Redirected Signaling
In certain circumstances, a DOTS server may want to redirect a DOTS
client to an alternative DOTS server for a signaling session. Such
circumstances include but are not limited to:
o Maximum number of signaling sessions with clients has been
reached;
o Mitigation capacity exhaustion in the Mitigator with which the
specific DOTS server is communicating;
o Mitigator outage or other downtime, such as scheduled maintenance;
o Scheduled DOTS server maintenance;
o Scheduled modifications to the network path between DOTS server
and DOTS client.
A basic redirected signaling session resembles the following, as
shown in Figure 11:
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+-------------+ +---------------+
| |<-(1)-- signal session 1 -->| |
| | | |
| |<=(2)== redirect to B ======| |
| DOTS client | | DOTS server A |
| |X-(4)-- signal session 1 --X| |
| | | |
| | | |
+-------------+ +---------------+
^
|
(3) signal session 2
|
v
+---------------+
| DOTS server B |
+---------------+
Figure 11: Redirected Signaling
1. Previously established signaling session 1 exists between a DOTS
client and DOTS server with address A.
2. DOTS server A sends a server signal redirecting the client to
DOTS server B.
3. If the DOTS client does not already have a separate signaling
session with the redirection target, the DOTS client initiates
and establishes a signaling session with DOTS server B as
described above.
4. Having redirected the DOTS client, DOTS server A ceases sending
server signals. The DOTS client likewise stops sending client
signals to DOTS server A. Signal session 1 is terminated.
[[EDITOR'S NOTE: we request working group feedback and discussion of
the need for redirected signaling.]]
4.2.4. Recursive Signaling
DOTS is centered around improving the speed and efficiency of
coordinated response to DDoS attacks. One scenario not yet discussed
involves coordination among federated entities operating DOTS servers
and mitigators.
In the course of normal DOTS operations, a DOTS client communicates
the need for mitigation to a DOTS server, and that server initiates
mitigation on a mitigator with which the server has an established
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service relationship. The operator of the mitigator may in turn
monitor mitigation performance and capacity, as the attack being
mitigated may grow in severity beyond the mitigating entity's
capabilities.
The operator of the mitigator has limited options in the event a DOTS
client-requested mitigation is being overwhelmed by the severity of
the attack. Out-of-scope business or service level agreements may
permit the mitigating entity to drop the mitigation and let attack
traffic flow unchecked to the target, but this is only encourages
attack escalation. In the case where the mitigating entity is the
upstream service provider for the attack target, this may mean the
mitigating entity and its other services and users continue to suffer
the incidental effects of the attack.
A recursive signaling model as shown in Figure 12 below offers an
alternative. In a variation of the primary use case "Successful
Automatic or Operator-Assisted CPE or PE Mitigators Request Upstream
DDoS Mitigation Services" described in [I-D.ietf-dots-use-cases], an
entity operating a DOTS server and mitigation has a mitigator that is
itself capable of acting as a DOTS client. The mitigator with DOTS
client capabilities has an established signaling session with a DOTS
server belonging to a separate administrative entity.
With these preconditions in place, the operator of the mitigator
being overwhelmed or otherwise performing inadequately may request
mitigation for the attack target from this separate DOTS-aware
entity. Such a request recurses the originating mitigation request
to the secondary DOTS server, in the hope of building a cumulative
mitigation against the attack:
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example.net entity
. . . . . . . . . . . . . . . . .
. .
+----+ A . +----+ +-----------+ .
| Cc |<--------->| Sn |~~~~~~~| Mitigator | .
+----+ . +----+ | Mn | .
. | +----+ | .
example.com . +---| Cn |--+ .
client . +----+ .
. ^ .
. . . . . . . . . . . | . . . . .
|
| B
|
. . . . . . . . . . . | . . . . .
. v .
. +-----------+ +----+ .
. | Mitigator |~~~~| So | .
. | Mo | +----+ .
. +-----------+ .
. .
. . . . . . . . . . . . . . . . .
example.org entity
Figure 12: Recursive Signaling
In Figure 12 above, client Cc signals a request for mitigation across
inter-domain signaling session A to the DOTS server Sn belonging to
the example.net entity. DOTS server Sn enables mitigation on
mitigator Mn, which, acting as DOTS client Cn, has pre-existing
inter-domain signaling session B with the DOTS server So belonging to
the example.org entity. At any point, DOTS client Cn MAY recurse an
on-going mitigation request to DOTS server So, in the expectation
that mitigator Mo will be activated to aid in the defense of the
attack target.
Recursive signaling is opaque to the DOTS client. To maximize
mitigation visibility to the DOTS client, however, the recursing
entity SHOULD provide recursed mitigation feedback in signals
reporting on mitigation status to the DOTS client. For example, the
recursing entity's mitigator should incorporate into mitigation
status messages available metrics such as dropped packet or byte
counts from the recursed mitigation.
DOTS clients involved in recursive signaling MUST be able to withdraw
requests for mitigation without warning or justification, per
[I-D.ietf-dots-requirements].
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Operators of recursing mitigators MAY maintain the recursed
mitigation for a brief, protocol-defined period in the event the DOTS
client originating the mitigation withdraws its request for help, as
per the discussion of managing mitigation toggling in the operational
requirements ([I-D.ietf-dots-requirements]). Service or business
agreements between recursing entities are not in scope for this
document.
[[EDITOR'S NOTE: Recursive signaling raises questions about how to
authenticate and authorize the recursed request, how end-to-end
signaling functions in such a scenario, and implications for
operational and data privacy, as well as what level of visibility a
client has into the recursed mitigation. We ask the working group
for feedback and additional discussion of these issues to help settle
the way forward.]]
5. Security Considerations
This section describes identified security considerations for the
DOTS architecture.
DOTS is at risk from three primary attack vectors: agent
impersonation, traffic injection and signal blocking. These vectors
may be exploited individually or in concert by an attacker to
confuse, disable, take information from, or otherwise inhibit the
DOTS system.
Any attacker with the ability to impersonate a legitimate client or
server or, indeed, inject false messages into the stream may
potentially trigger/withdraw traffic redirection, trigger/cancel
mitigation activities or subvert black/whitelists. From an
architectural standpoint, operators SHOULD ensure best current
practices for secure communication are observed for data and signal
channel confidentiality, integrity and authenticity. Care must be
taken to ensure transmission is protected by appropriately secure
means, reducing attack surface by exposing only the minimal required
services or interfaces. Similarly, received data at rest SHOULD be
stored with a satisfactory degree of security.
As many mitigation systems employ diversion to scrub attack traffic,
operators of DOTS agents SHOULD ensure signaling sessions are
resistant to Man-in-the-Middle (MitM) attacks. An attacker with
control of a DOTS client or relay may negatively influence network
traffic by requesting and withdrawing requests for mitigation for
particular prefixes, leading to route or DNS flapping.
Any attack targeting the availability of DOTS servers may disrupt the
ability of the system to receive and process DOTS signals resulting
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in failure to fulfill a mitigation request. Similarly, DOTS relays
represent high-value targets in a DOTS architecture. Disrupting any
DOTS relay in a signaling path represents a denial-of-service against
DOTS in general. DOTS systems SHOULD be given adequate protections,
again, in accordance with best current practices for network and host
security.
6. Acknowledgments
Thanks to Matt Richardson for last minute comments and suggestions.
7. Change Log
2016-03-18 Initial revision
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
8.2. Informative References
[I-D.ietf-dots-requirements]
Mortensen, A., Moskowitz, R., and T. Reddy, "DDoS Open
Threat Signaling Requirements", draft-ietf-dots-
requirements-00 (work in progress), October 2015.
[I-D.ietf-dots-use-cases]
Dobbins, R., Fouant, S., Migault, D., Moskowitz, R.,
Teague, N., and L. Xia, "Use cases for DDoS Open Threat
Signaling", draft-ietf-dots-use-cases-00 (work in
progress), October 2015.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI
10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
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[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<http://www.rfc-editor.org/info/rfc2782>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI
10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.
[RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732, DOI 10.17487/
RFC4732, December 2006,
<http://www.rfc-editor.org/info/rfc4732>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
Authors' Addresses
Andrew Mortensen
Arbor Networks, Inc.
2727 S. State St
Ann Arbor, MI 48104
United States
EMail: amortensen@arbor.net
Flemming Andreasen
Cisco Systems, Inc.
United States
EMail: fandreas@cisco.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
EMail: tireddy@cisco.com
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Christopher Gray
Comcast, Inc.
United States
EMail: Christopher_Gray3@cable.comcast.com
Rich Compton
Charter Communications, Inc.
EMail: Rich.Compton@charter.com
Nik Teague
Verisign, Inc.
United States
EMail: nteague@verisign.com
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