DOTS A. Mortensen
Internet-Draft Arbor Networks
Intended status: Informational F. Andreasen
Expires: December 7, 2017 Cisco
T. Reddy
McAfee, Inc.
C. Gray
Comcast
R. Compton
Charter
N. Teague
Verisign
June 05, 2017
Distributed-Denial-of-Service Open Threat Signaling (DOTS) Architecture
draft-ietf-dots-architecture-03
Abstract
This document describes an architecture for establishing and
maintaining Distributed Denial of Service (DDoS) Open Threat
Signaling (DOTS) within and between domains. The document does not
specify protocols or protocol extensions, instead focusing on
defining 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
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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 December 7, 2017.
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Copyright Notice
Copyright (c) 2017 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
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Table of Contents
1. Context and Motivation . . . . . . . . . . . . . . . . . . . 3
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. DOTS Architecture . . . . . . . . . . . . . . . . . . . . . . 5
2.1. DOTS Operations . . . . . . . . . . . . . . . . . . . . . 8
2.2. Components . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1. DOTS Client . . . . . . . . . . . . . . . . . . . . . 9
2.2.2. DOTS Server . . . . . . . . . . . . . . . . . . . . . 10
2.2.3. DOTS Gateway . . . . . . . . . . . . . . . . . . . . 11
2.3. DOTS Agent Relationships . . . . . . . . . . . . . . . . 12
2.3.1. Gatewayed Signaling . . . . . . . . . . . . . . . . . 14
3. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1. Signaling Sessions . . . . . . . . . . . . . . . . . . . 16
3.1.1. Preconditions . . . . . . . . . . . . . . . . . . . . 16
3.1.2. Establishing the Signaling Session . . . . . . . . . 16
3.1.3. Maintaining the Signaling Session . . . . . . . . . . 17
3.2. Modes of Signaling . . . . . . . . . . . . . . . . . . . 18
3.2.1. Direct Signaling . . . . . . . . . . . . . . . . . . 18
3.2.2. Redirected Signaling . . . . . . . . . . . . . . . . 18
3.2.3. Recursive Signaling . . . . . . . . . . . . . . . . . 19
3.2.4. Anycast Signaling . . . . . . . . . . . . . . . . . . 22
3.3. Triggering Requests for Mitigation . . . . . . . . . . . 23
3.3.1. Manual Mitigation Request . . . . . . . . . . . . . . 24
3.3.2. Automated Conditional Mitigation Request . . . . . . 25
3.3.3. Automated Mitigation on Loss of Signal . . . . . . . 26
4. Security Considerations . . . . . . . . . . . . . . . . . . . 26
5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
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7. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 27
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1. Normative References . . . . . . . . . . . . . . . . . . 27
8.2. Informative References . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
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 defensive
response. The signaling layer and supplementary messaging is the
focus of DDoS Open Threat Signaling (DOTS). DOTS defines a method of
coordinating defensive measures among willing peers to mitigate
attacks quickly and efficiently, enabling hybrid attack responses
coordinated locally at or near the target of an active attack, or
anywhere in-path between attack sources and target. Sample DOTS use
cases are elaborated in [I-D.ietf-dots-use-cases].
This document describes an architecture used in establishing,
maintaining or terminating a DOTS relationship within a domain or
between domains.
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
In this architecture, DOTS clients and servers communicate using DOTS
signaling. As a result of signals from a DOTS client, the DOTS
server may modify the forwarding path of traffic destined for the
attack target(s), for example by diverting traffic to a mitigator or
pool of mitigators, where policy may be applied to distinguish and
discard attack traffic. Any such policy is deployment-specific.
The DOTS architecture presented here is applicable across network
administrative domains - for example, between an enterprise domain
and the domain of a third-party attack mitigation service - as well
as to a single administrative domain. DOTS is generally assumed to
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be most effective when aiding coordination of attack response between
two or more participating networks, but single domain scenarios are
valuable in their own right, as when aggregating intra-domain DOTS
client signals for inter-domain coordinated attack response.
This document does not address any administrative or business
agreements that may be established between involved DOTS parties.
Those considerations are out of scope. Regardless, this document
assumes necessary authentication and authorization mechanisms are put
in place so that only authorized clients can invoke the DOTS service.
A detailed set of DOTS requirements are discussed in
[I-D.ietf-dots-requirements], and the DOTS architecture is assumed to
follow those requirements. Only new behavioral requirements are
described in this document.
1.3. Assumptions
This document makes the following assumptions:
o All domains 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, but in general conditions should be assumed
to 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 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 Mitigation requests may be sent to one or more upstream DOTS
servers based on criteria determined by DOTS client administrators
and the underlying network configuration. The number of DOTS
servers with which a given DOTS client has established signaling
sessions is determined by local policy and is deployment-specific.
For example, a DOTS client of a multi-homed network may support
built-in policies to establish DOTS relationships with DOTS
servers located upstream of each interconnection link.
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o The mitigation capacity and/or capability of domains receiving
requests for coordinated attack response is opaque to the domains
sending the request. The domain 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. In either
case, the upstream DOTS server may redirect a request to another
DOTS server. Redirection may be local to the redirecting DOTS
server's domain, or may involve a third-party domain.
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 domain and the DOTS server domain. 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. Such preferential
treatment may be available in some deployments (e.g., intra-domain
scenarios), and the DOTS architecture does not preclude its use
when available. However, DOTS itself does not address how that
may be done.
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 the DOTS
architecture.
o The signal and data channels may be loosely coupled, and may not
terminate on the same DOTS server.
2. DOTS Architecture
The basic high-level DOTS architecture is illustrated in Figure 1:
+-----------+ +-------------+
| Mitigator | ~~~~~~~~~~ | DOTS Server |
+-----------+ +-------------+
|
|
|
+---------------+ +-------------+
| Attack Target | ~~~~~~ | DOTS Client |
+---------------+ +-------------+
Figure 1: Basic DOTS Architecture
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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 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 and DOTS server. 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 is 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. 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 domain's capacity and willingness to
mitigate on behalf of the DOTS client's domain.
As illustrated in Figure 2, there are two interfaces between the DOTS
server and the DOTS client.
+---------------+ +---------------+
| | <------- Signal Channel ------> | |
| DOTS Client | | DOTS Server |
| | <======= Data Channel ======> | |
+---------------+ +---------------+
Figure 2: DOTS Interfaces
The DOTS client may be provided with a list of DOTS servers, each
associated with one or more IP addresses. These addresses may or may
not be of the same address family. The DOTS client establishes one
or more signaling sessions by connecting to the provided DOTS server
addresses.
The primary purpose of the signal channel is for a DOTS client to ask
a 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(s). 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
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mitigation request. Assuming it will, the DOTS server initiates
attack mitigation, 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 lifetime is exceeded. 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, but are not limited to:
o Creating identifiers, such as names or aliases, for resources for
which mitigation may be requested. Such identifiers may then be
used in subsequent signal channel exchanges to refer more
efficiently to the resources under attack, as seen in Figure 3,
using JSON to serialize the data:
{
"https1": [
"192.0.2.1:443",
"198.51.100.2:443",
],
"proxies": [
"203.0.113.3:3128",
"[2001:db8:ac10::1]:3128"
],
"api_urls": "https://apiserver.example.com/api/v1",
}
Figure 3: Protected resource identifiers
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 is always accepted.
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o Filter management, which enables a DOTS client to install or
remove traffic filters dropping or rate-limiting unwanted traffic.
o DOTS client provisioning.
Note that while it is possible to exchange the above information
before, during or after a DDoS attack, DOTS requires reliable
delivery of this information and does not provide any special means
for ensuring timely delivery of it during an attack. In practice,
this means that DOTS deployments should not rely on such information
being exchanged during a DDoS attack.
2.1. DOTS Operations
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 a domain that has an identity
which can be authenticated and authorized. DOTS agents communicate
with each other over a mutually authenticated signal channel and 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.
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 is 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 active. 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.
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DDoS mitigation with the help of an upstream mitigator may involve
some form of traffic redirection whereby traffic destined for the
attack target is steered towards the mitigator. Common mechanisms to
achieve this redirection depend on BGP [RFC4271] and DNS [RFC1035].
The mitigator in turn inspects and scrubs the traffic, and forwards
the resulting (hopefully non-attack) traffic to the attack target.
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.
DOTS relies on mutual authentication and the pre-established service
relationship between the DOTS client's domain and the DOTS server's
domain to provide basic authorization. The DOTS server should
enforce additional authorization mechanisms to restrict the
mitigation scope a DOTS client can request, but such authorization
mechanisms are deployment-specific.
Although co-location of DOTS server and mitigator within the same
domain is expected to be a common deployment model, it is assumed
that operators may require alternative models. Nothing in this
document precludes such alternatives.
2.2. Components
2.2.1. 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. Operators may wish to have upstream mitigators 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 administrator. Such
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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.
2.2.2. DOTS Server
A DOTS server is a DOTS agent capable of receiving, processing and
possibly acting on requests for help coordinating attack response
DOTS clients. The DOTS server authenticates and authorizes DOTS
clients as described in Section 3.1, 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 domain, such that a DOTS client from one domain is not
able to influence the network path to another domain. A DOTS server
MUST reject requests for mitigation of resources not owned by the
requesting DOTS client's administrative domain. 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(s)
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.
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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
server MUST send 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.
2.2.3. DOTS Gateway
Traditional client/server relationships may be expanded by chaining
DOTS sessions. This chaining is enabled through "logical
concatenation" of a DOTS server and a DOTS client, resulting in an
application analogous to the SIP logical entity of a Back-to-Back
User Agent (B2BUA) [RFC7092]. The term DOTS gateway is used here in
the descriptions of selected scenarios involving this application.
A DOTS gateway may be deployed client-side, server-side or both. The
gateway may terminate multiple discrete client connections and may
aggregate these into a single or multiple DOTS signaling sessions.
The DOTS gateway will appear as a server to its downstream agents and
as a client to its upstream agents, a functional concatenation of the
DOTS client and server roles, as depicted in Figure 4:
+-------------+
| | D | |
+----+ | | O | | +----+
| c1 |----------| s1 | T | c2 |---------| s2 |
+----+ | | S | | +----+
| | G | |
+-------------+
Figure 4: DOTS gateway
The DOTS gateway MUST perform full stack DOTS session termination and
reorigination between its client and server side. The details of how
this is achieved are implementation specific. The DOTS protocol does
not include any special features related to DOTS gateways, and hence
from a DOTS perspective, whenever a DOTS gateway is present, the DOTS
session simply terminates/originates there.
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2.3. 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 domains. An example
scenario is a mitigation provider serving multiple attack targets
(Figure 5).
DOTS clients DOTS server
+---+
| c |-----------
+---+ \
c1.example.org \
\
+---+ \ +---+
| c |----------------| S |
+---+ / +---+
c1.example.com / dots1.example.net
/
+---+ /
| c |-----------
+---+
c2.example.com
Figure 5: DOTS server with multiple clients
A DOTS client may be associated with one or more DOTS servers, and
those DOTS servers may belong to different domains. 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 6.
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DOTS client DOTS servers
+---+
-----------| S |
/ +---+
/ dots1.example.net
/
+---+/ +---+
| c |---------------| S |
+---+\ +---+
\ dots.example.org
\
\ +---+
-----------| S |
+---+
c.example.com dots2.example.net
Figure 6: Multi-Homed DOTS Client
Deploying a multi-homed client requires extra care and planning, as
the DOTS servers with which the multi-homed client communicates may
not be affiliated. Should the multi-homed client simultaneously
request for mitigation from all servers with which it has established
signal channels, the client may unintentionally inflict additional
network disruption on the resources it intends to protect. In one of
the worst cases, a multi-homed DOTS client could cause a permanent
routing loop of traffic destined for the client's protected protected
services, as the uncoordinated DOTS servers' mitigators all try to
divert that traffic to their own scrubbing centers.
The DOTS protocol itself provides no fool-proof method to prevent
such self-inflicted harms as a result deploying multi-homed DOTS
clients. If DOTS client implementations nevertheless include support
for multi-homing, they are expected to be aware of the risks, and
consequently to include measures aimed at reducing the likelihood of
negative outcomes. Simple measures might include:
o Requesting mitigation serially, ensuring only one mitigation
request for a given address space is active at any given time;
o Dividing the protected resources among the DOTS servers, such that
no two mitigators will be attempting to divert and scrub the same
traffic;
o Using multi-homing for redundancy only. In this case, the DOTS
servers would be affiliated, and through shared communications
ensure that the redundant mitigation requests do not result in
overlapping traffic diversion announcements.
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2.3.1. Gatewayed Signaling
As discussed in Section 2.2.3, a DOTS gateway is a logical function
chaining signaling sessions through concatenation of a DOTS server
and DOTS client.
An example scenario, as shown in Figure 7 and Figure 8, 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 DOTS gateway 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 gateway and its upstream agents is opaque to the initial clients.
+---+
| c |\
+---+ \ +---+
\-----TCP-----| D | +---+
+---+ | O | | |
| c |--------TCP-----| T |------UDP------| S |
+---+ | S | | |
/-----TCP-----| G | +---+
+---+ / +---+
| c |/
+---+
example.com example.com example.net
DOTS clients DOTS gateway (DOTSG) DOTS server
Figure 7: Client-Side Gateway with Aggregation
+---+
| c |\
+---+ \ +---+
\-----TCP-----| D |------UDP------+---+
+---+ | O | | |
| c |--------TCP-----| T |------UDP------| S |
+---+ | S | | |
/-----TCP-----| G |------UDP------+---+
+---+ / +---+
| c |/
+---+
example.com example.com example.net
DOTS clients DOTS gateway (DOTSG) DOTS server
Figure 8: Client-Side Gateway without Aggregation
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This may similarly be deployed in the inverse scenario where the
gateway resides in the server-side domain and may be used to
terminate and/or aggregate multiple clients to single transport as
shown in figures Figure 9 and Figure 10.
+---+
| c |\
+---+ \ +---+
\-----UDP-----| D | +---+
+---+ | O | | |
| c |--------TCP-----| T |------TCP------| S |
+---+ | S | | |
/-----TCP-----| G | +---+
+---+ / +---+
| c |/
+---+
example.com example.net example.net
DOTS clients DOTS gateway (DOTSG) DOTS server
Figure 9: Server-Side Gateway with Aggregation
+---+
| c |\
+---+ \ +---+
\-----UDP-----| D |------TCP------+---+
+---+ | O | | |
| c |--------TCP-----| T |------TCP------| S |
+---+ | S | | |
/-----UDP-----| G |------TCP------+---+
+---+ / +---+
| c |/
+---+
example.com example.net example.net
DOTS clients DOTS gateway (DOTSG) DOTS server
Figure 10: Server-Side Gateway without Aggregation
This document anticipates scenarios involving multiple DOTS gateways.
An example is a DOTS gateway at the network client's side, and
another one at the server side. The first gateway can be located at
a CPE to aggregate requests from multiple DOTS clients enabled in an
enterprise network. The second DOTS gateway is deployed on the
provider side. This scenario can be seen as a combination of the
client-side and server-side scenarios.
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3. Concepts
3.1. 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.
3.1.1. 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.
3.1.2. Establishing the Signaling Session
With the required business agreements in place, the DOTS client
initiates a signal session by contacting its DOTS server(s) over the
signal channel and the data channel. To allow for DOTS service
flexibility, neither the order of contact nor the time interval
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
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to use a specific DOTS server IP 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 domain 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 domain
operating the DOTS server or servers might provide the client domain
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.
As described in [I-D.ietf-dots-requirements], the DOTS client can
configure preferred values for acceptable signal loss, mitigation
lifetime, and heartbeat intervals when establishing the signaling
session. A signaling session is not active until DOTS agents have
agreed on the values for these signaling session parameters, a
process defined by the protocol.
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]).
3.1.3. Maintaining the Signaling Session
DOTS clients and servers 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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3.2. Modes of Signaling
This section examines the modes of signaling between agents in a DOTS
architecture.
3.2.1. Direct Signaling
A signaling session may take the form of direct signaling between the
DOTS clients and servers, as shown in Figure 11.
+-------------+ +-------------+
| DOTS client |<------signal session------>| DOTS server |
+-------------+ +-------------+
Figure 11: Direct Signaling
In a direct signaling session, DOTS client and server are
communicating directly. 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
adjacent.
3.2.2. 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 12.
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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 12: Redirected Signaling
1. Previously established signaling session 1 exists between a DOTS
client and DOTS server 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.
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.
3.2.3. 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 domains 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
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 domain's
capabilities.
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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 domain 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 domain is the
upstream service provider for the attack target, this may mean the
mitigating domain and its other services and users continue to suffer
the incidental effects of the attack.
A recursive signaling model as shown in Figure 13 offers an
alternative. In a variation of the use case "Enterprise with an
upstream transit provider DDoS mitigation service" described in
[I-D.ietf-dots-use-cases], a domain operating a DOTS server and
mitigator also operates a DOTS client. This DOTS client has an
established signaling session with a DOTS server belonging to a
separate administrative domain.
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
domain. 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 domain
. . . . . . . . . . . . . . . . .
. Gn .
+----+ A . +----+ +-----------+ .
| Cc |<--------->| Sn |~~~~~~~| Mitigator | .
+----+ . +====+ | Mn | .
. | Cn | +-----------+ .
example.com . +----+ .
client . ^ .
. . .|. . . . . . . . . . . . . .
|
B |
|
. . .|. . . . . . . . . . . . . .
. v .
. +----+ +-----------+ .
. | So |~~~~~~~| Mitigator | .
. +----+ | Mo | .
. +-----------+ .
. .
. . . . . . . . . . . . . . . . .
example.org domain
Figure 13: Recursive Signaling
In Figure 13, client Cc signals a request for mitigation across
inter-domain signaling session A to the DOTS server Sn belonging to
the example.net domain. DOTS server Sn enables mitigation on
mitigator Mn. DOTS server Sn is half of DOTS gateway Gn, being
deployed logically back-to-back with DOTS client Cn, which has pre-
existing inter-domain signaling session B with the DOTS server So
belonging to the example.org domain. At any point, DOTS server Sn
MAY recurse an on-going mitigation request through DOTS client Cn 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
domain SHOULD provide recursed mitigation feedback in signals
reporting on mitigation status to the DOTS client. For example, the
recursing domain'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 recursing mitigation requests 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]).
[[EDITOR'S NOTE: Recursive signaling raises questions about
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.]]
3.2.4. Anycast Signaling
The DOTS architecture does not assume the availability of anycast
within a DOTS deployment, but neither does the architecture exclude
it. Domains operating DOTS servers MAY deploy DOTS servers with an
anycast Service Address as described in BCP 126 [RFC4786]. In such a
deployment, DOTS clients connecting to the DOTS Service Address may
be communicating with distinct DOTS servers, depending on the network
configuration at the time the DOTS clients connect. Among other
benefits, anycasted signaling potentially offers the following:
o Simplified DOTS client configuration, including service discovery
through the methods described in [RFC7094]. In this scenario, the
"instance discovery" message would be a DOTS client initiating a
signaling session to the DOTS server anycast Service Address, to
which the DOTS server would reply with a redirection to the DOTS
server unicast address the client should use for DOTS.
o Region- or customer-specific deployments, in which the DOTS
Service Addresses route to distinct DOTS servers depending on the
client region or the customer network in which a DOTS client
resides.
o Operational resiliency, spreading DOTS signaling traffic across
the DOTS server domain's networks, and thereby also reducing the
potential attack surface, as described in BCP 126 [RFC4786].
3.2.4.1. Anycast Signaling Considerations
As long as network configuration remains stable, anycast DOTS
signaling is to the individual DOTS client indistinct from direct
signaling. However, the operational challenges inherent in anycast
signaling are anything but negligible, and DOTS server operators must
carefully weigh the risks against the benefits before deploying.
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While the DOTS signal channel primarily operates over UDP per
[I-D.ietf-dots-requirements], the signal channel also requires mutual
authentication between DOTS agents, with associated security state on
both ends. The resulting considerations therefore superficially
resemble the deployment of anycast DNS over DTLS, as described in
[RFC8094], but the similarities only go so far.
Network instability is of particular concern with anycast signaling,
as DOTS signaling sessions are expected to be long-lived, and
potentially operating under congested network conditions caused by a
volumetric DDoS attack.
For example, a network configuration altering the route to the DOTS
server during active anycast signaling may cause the DOTS client to
send messages to a DOTS server other than the one with which it
initially established a signaling session. That second DOTS server
may not have the security state of the existing session, forcing the
DOTS client to initialize a new signaling session. This challenge
may in part be mitigated by use of pre-shared keys, as described in
[I-D.ietf-tls-tls13], but keying material must be available to all
DOTS servers sharing the anycast Service Address in that case.
While the DOTS client will try to establish a new signaling session
with the DOTS server now acting as the anycast DOTS Service Address,
the link between DOTS client and server may be congested with attack
traffic, making signal session establishment difficult. In such a
scenario, anycast Service Address instability becomes a sort of
signal session flapping, with obvious negative consequences for the
DOTS deployment.
Anycast signaling deployments similarly must also take into account
active mitigations. Active mitigations initiated through a signaling
session may involve diverting traffic to a scrubbing center. If the
signaling session flaps due to anycast changes as described above,
mitigation may also flap as the DOTS servers sharing the anycast DOTS
service address toggles mitigation on detecting signaling session
loss, depending on whether the client has configured mitigation on
loss of signal.
3.3. Triggering Requests for Mitigation
[I-D.ietf-dots-requirements] places no limitation on the
circumstances in which a DOTS client operator may request mitigation,
nor does it demand justification for any mitigation request, thereby
reserving operational control over DDoS defense for the domain
requesting mitigation. This architecture likewise does not prescribe
the network conditions and mechanisms triggering a mitigation request
from a DOTS client.
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However, considering selected possible mitigation triggers from an
architectural perspective offers a model for alternative or
unanticipated triggers for DOTS deployments. In all cases, what
network conditions merit a mitigation request are at the discretion
of the DOTS client operator.
The interfaces required to trigger the mitigation request in the
following scenarios are implementation-specific.
3.3.1. Manual Mitigation Request
A DOTS client operator may manually prepare a request for mitigation,
including scope and duration, and manually instruct the DOTS client
to send the mitigation request to the DOTS server. In context, a
manual request is a request directly issued by the operator without
automated decision-making performed by a device interacting with the
DOTS client. Modes of manual mitigation requests include an operator
entering a command into a text interface, or directly interacting
with a graphical interface to send the request.
An operator might do this, for example, in response to notice of an
attack delivered by attack detection equipment or software, and the
alerting detector lacks interfaces or is not configured to use
available interfaces to translate the alert to a mitigation request
automatically.
In a variation of the above scenario, the operator may have
preconfigured on the DOTS client mitigation request for various
resources in the operator's domain. When notified of an attack, the
DOTS client operator manually instructs the DOTS client to send the
preconfigured mitigation request for the resources under attack.
A further variant involves recursive signaling, as described in
Section 3.2.3. The DOTS client in this case is the second half of a
DOTS gateway (back-to-back DOTS server and client). As in the
previous scenario, the scope and duration of the mitigation request
are pre-existing, but in this case are derived from the mitigation
request received from a downstream DOTS client by the DOTS server.
Assuming the preconditions required by Section 3.2.3 are in place,
the DOTS gateway operator may at any time manually request mitigation
from an upstream DOTS server, sending a mitigation request derived
from the downstream DOTS client's request.
The motivations for a DOTS client operator to request mitigation
manually are not prescribed by this architecture, but are expected to
include some of the following:
o Notice of an attack delivered via e-mail or alternative messaging
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o Notice of an attack delivered via phone call
o Notice of an attack delivered through the interface(s) of
networking monitoring software deployed in the operator's domain
o Manual monitoring of network behavior through network monitoring
software
3.3.2. Automated Conditional Mitigation Request
Unlike manual mitigation requests, which depend entirely on the DOTS
client operator's capacity to react with speed and accuracy to every
detected or detectable attack, mitigation requests triggered by
detected attack conditions reduce the operational burden on the DOTS
client operator, and minimize the latency between attack detection
and the start of mitigation.
Mitigation requests are triggered in this scenario by operator-
specified network conditions. Attack detection is deployment-
specific, and not constrained by this architecture. Similarly the
specifics of a condition are left to the discretion of the operator,
though common conditions meriting mitigation include the following:
o Detected attack exceeding a rate in packets per second (pps).
o Detected attack exceeding a rate in bytes per second (bps).
o Detected resource exhaustion in an attack target.
o Detected resource exhaustion in the local domain's mitigator.
o Number of open connections to an attack target.
o Number of attack sources in a given attack.
o Number of active attacks against targets in the operator's domain.
o Conditional detection developed through arbitrary statistical
analysis or deep learning techniques.
o Any combination of the above.
When automated conditional mitigation requests are enabled,
violations of any of the above conditions, or any additional
operator-defined conditions, will trigger a mitigation request from
the DOTS client to the DOTS server. The interfaces between the
application detecting the condition violation and the DOTS client are
implementation-specific.
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3.3.3. Automated Mitigation on Loss of Signal
To maintain a signaling session, the DOTS client and the DOTS server
exchange regular but infrequent messages across the signaling
channel. In the absence of an attack, the probability of message
loss in the signaling channel should be extremely low. Under attack
conditions, however, some signal loss may be anticipated as attack
traffic congests the link, depending on the attack type.
While [I-D.ietf-dots-requirements] specifies the DOTS protocol be
robust when signaling under attack conditions, there are nevertheless
scenarios in which the DOTS signal is lost in spite of protocol best
efforts. To handle such scenarios, a DOTS client operator may
configure the signaling session to trigger mitigation when the DOTS
server ceases receiving DOTS client signals (or vice versa) beyond
the miss count or period permitted by the protocol.
The impact of mitigating due to loss of signal in either direction
must be considered carefully before enabling it. Signal loss is not
caused by links congested with attack traffic alone, and as such
mitigation requests triggered by signal channel degradation in either
direction may incur unnecessary costs, in network performance and
operational expense alike.
4. 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 DOTS
agents.
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.
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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 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
in failure to fulfill a mitigation request. DOTS agents SHOULD be
given adequate protections, again in accordance with best current
practices for network and host security.
5. Contributors
Mohamed Boucadair
Orange
mohamed.boucadair@orange.com
6. Acknowledgments
Thanks to Matt Richardson and Med Boucadair for their 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, "Distributed
Denial of Service (DDoS) Open Threat Signaling
Requirements", draft-ietf-dots-requirements-04 (work in
progress), March 2017.
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[I-D.ietf-dots-use-cases]
Dobbins, R., Fouant, S., Migault, D., Moskowitz, R.,
Teague, N., Xia, L., and K. Nishizuka, "Use cases for DDoS
Open Threat Signaling (DDoS) Open Threat Signaling",
draft-ietf-dots-use-cases-05 (work in progress), May 2017.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-20 (work in progress),
April 2017.
[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>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[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>.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <http://www.rfc-editor.org/info/rfc4786>.
[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>.
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[RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents",
RFC 7092, DOI 10.17487/RFC7092, December 2013,
<http://www.rfc-editor.org/info/rfc7092>.
[RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014,
<http://www.rfc-editor.org/info/rfc7094>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<http://www.rfc-editor.org/info/rfc8094>.
Authors' Addresses
Andrew Mortensen
Arbor Networks
2727 S. State St
Ann Arbor, MI 48104
United States
EMail: amortensen@arbor.net
Flemming Andreasen
Cisco
United States
EMail: fandreas@cisco.com
Tirumaleswar Reddy
McAfee, Inc.
Embassy Golf Link Business Park
Bangalore, Karnataka 560071
India
EMail: tireddy@cisco.com
Christopher Gray
Comcast
United States
EMail: Christopher_Gray3@cable.comcast.com
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Rich Compton
Charter
EMail: Rich.Compton@charter.com
Nik Teague
Verisign
United States
EMail: nteague@verisign.com
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