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RFC 6940

 
 
 

REsource LOcation And Discovery (RELOAD) Base Protocol

Part 6 of 7, p. 124 to 152
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11.  Enrollment and Bootstrap

   The section defines the format of the configuration data as well the
   process to join a new overlay.

11.1.  Overlay Configuration

   This specification defines a new content type
   "application/p2p-overlay+xml" for a MIME entity that contains overlay
   information.  An example document is shown below:

   <?xml version="1.0" encoding="UTF-8"?>
   <overlay xmlns="urn:ietf:params:xml:ns:p2p:config-base"
      xmlns:ext="urn:ietf:params:xml:ns:p2p:config-ext1"
      xmlns:chord="urn:ietf:params:xml:ns:p2p:config-chord">
      <configuration instance-name="overlay.example.org" sequence="22"
          expiration="2002-10-10T07:00:00Z" ext:ext-example="stuff" >
          <topology-plugin> CHORD-RELOAD </topology-plugin>
          <node-id-length>16</node-id-length>
          <root-cert>
   MIIDJDCCAo2gAwIBAgIBADANBgkqhkiG9w0BAQUFADBwMQswCQYDVQQGEwJVUzET
   MBEGA1UECBMKQ2FsaWZvcm5pYTERMA8GA1UEBxMIU2FuIEpvc2UxDjAMBgNVBAoT
   BXNpcGl0MSkwJwYDVQQLEyBTaXBpdCBUZXN0IENlcnRpZmljYXRlIEF1dGhvcml0
   eTAeFw0wMzA3MTgxMjIxNTJaFw0xMzA3MTUxMjIxNTJaMHAxCzAJBgNVBAYTAlVT
   MRMwEQYDVQQIEwpDYWxpZm9ybmlhMREwDwYDVQQHEwhTYW4gSm9zZTEOMAwGA1UE

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   ChMFc2lwaXQxKTAnBgNVBAsTIFNpcGl0IFRlc3QgQ2VydGlmaWNhdGUgQXV0aG9y
   aXR5MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDDIh6DkcUDLDyK9BEUxkud
   +nJ4xrCVGKfgjHm6XaSuHiEtnfELHM+9WymzkBNzZpJu30yzsxwfKoIKugdNUrD4
   N3viCicwcN35LgP/KnbN34cavXHr4ZlqxH+OdKB3hQTpQa38A7YXdaoz6goW2ft5
   Mi74z03GNKP/G9BoKOGd5QIDAQABo4HNMIHKMB0GA1UdDgQWBBRrRhcU6pR2JYBU
   bhNU2qHjVBShtjCBmgYDVR0jBIGSMIGPgBRrRhcU6pR2JYBUbhNU2qHjVBShtqF0
   pHIwcDELMAkGA1UEBhMCVVMxEzARBgNVBAgTCkNhbGlmb3JuaWExETAPBgNVBAcT
   CFNhbiBKb3NlMQ4wDAYDVQQKEwVzaXBpdDEpMCcGA1UECxMgU2lwaXQgVGVzdCBD
   ZXJ0aWZpY2F0ZSBBdXRob3JpdHmCAQAwDAYDVR0TBAUwAwEB/zANBgkqhkiG9w0B
   AQUFAAOBgQCWbRvv1ZGTRXxbH8/EqkdSCzSoUPrs+rQqR0xdQac9wNY/nlZbkR3O
   qAezG6Sfmklvf+DOg5RxQq/+Y6I03LRepc7KeVDpaplMFGnpfKsibETMipwzayNQ
   QgUf4cKBiF+65Ue7hZuDJa2EMv8qW4twEhGDYclpFU9YozyS1OhvUg==
          </root-cert>
          <root-cert> YmFkIGNlcnQK </root-cert>
          <enrollment-server>https://example.org</enrollment-server>
          <enrollment-server>https://example.net</enrollment-server>
          <self-signed-permitted
                    digest="sha1">false</self-signed-permitted>
          <bootstrap-node address="192.0.0.1" port="6084" />
          <bootstrap-node address="192.0.2.2" port="6084" />
          <bootstrap-node address="2001:DB8::1" port="6084" />
          <turn-density> 20 </turn-density>
          <clients-permitted> false </clients-permitted>
          <no-ice> false </no-ice>
          <chord:chord-update-interval>
              400</chord:chord-update-interval>
          <chord:chord-ping-interval>30</chord:chord-ping-interval>
          <chord:chord-reactive> true </chord:chord-reactive>
          <shared-secret> password </shared-secret>
          <max-message-size>4000</max-message-size>
          <initial-ttl> 30 </initial-ttl>
          <overlay-reliability-timer> 3000 </overlay-reliability-timer>
          <overlay-link-protocol>TLS</overlay-link-protocol>
          <configuration-signer>47112162e84c69ba</configuration-signer>
          <kind-signer> 47112162e84c69ba </kind-signer>
          <kind-signer> 6eba45d31a900c06 </kind-signer>
          <bad-node> 6ebc45d31a900c06 </bad-node>
          <bad-node> 6ebc45d31a900ca6 </bad-node>

          <ext:example-extension> foo </ext:example-extension>

          <mandatory-extension>
              urn:ietf:params:xml:ns:p2p:config-ext1
          </mandatory-extension>

          <required-kinds>
            <kind-block>
              <kind name="SIP-REGISTRATION">

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                  <data-model>SINGLE</data-model>
                  <access-control>USER-MATCH</access-control>
                  <max-count>1</max-count>
                  <max-size>100</max-size>
              </kind>
              <kind-signature>
                   VGhpcyBpcyBub3QgcmlnaHQhCg==
              </kind-signature>
            </kind-block>
            <kind-block>
              <kind id="2000">
                  <data-model>ARRAY</data-model>
                  <access-control>NODE-MULTIPLE</access-control>
                  <max-node-multiple>3</max-node-multiple>
                  <max-count>22</max-count>
                  <max-size>4</max-size>
                  <ext:example-kind-extension> 1
                          </ext:example-kind-extension>
              </kind>
              <kind-signature>
                 VGhpcyBpcyBub3QgcmlnaHQhCg==
              </kind-signature>
            </kind-block>
          </required-kinds>
      </configuration>
      <signature> VGhpcyBpcyBub3QgcmlnaHQhCg== </signature>

      <configuration instance-name="other.example.net">
      </configuration>
      <signature> VGhpcyBpcyBub3QgcmlnaHQhCg== </signature>

    </overlay>

   The file MUST be a well-formed XML document, and it SHOULD contain an
   encoding declaration in the XML declaration.  The file MUST use the
   UTF-8 character encoding.  The namespaces for the elements defined in
   this specification are urn:ietf:params:xml:ns:p2p:config-base and
   urn:ietf:params:xml:ns:p2p:config-chord.

   Note that elements or attributes that are defined as type xsd:boolean
   in the RELAX NG schema (Section 11.1.1) have two lexical
   representations, "1" or "true" for the concept true, and "0" or
   "false" for the concept false.  Whitespace and case processing
   follows the rules of [OASIS.relax_ng] and XML Schema Datatypes
   [W3C.REC-xmlschema-2-20041028].

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   The file MAY contain multiple "configuration" elements, where each
   one contains the configuration information for a different overlay.
   Each configuration element MAY be followed by signature elements that
   provide a signature over the preceding configuration element.  Each
   configuration element has the following attributes:

   instance-name
      The name of the overlay (referred to as "overlay name" in this
      specification)

   expiration
      Time in the future at which this overlay configuration is no
      longer valid.  The node SHOULD retrieve a new copy of the
      configuration at a randomly selected time that is before the
      expiration time.  Note that if the certificates expire before a
      new configuration is retried, the node will not be able to
      validate the configuration file.  All times MUST conform to the
      Internet date/time format defined in [RFC3339] and be specified
      using UTC.

   sequence
      A monotonically increasing sequence number between 0 and 2^16-2.

   Inside each overlay element, the following elements can occur:

   topology-plug-in
      This element defines the overlay algorithm being used.  If
      missing, the default is "CHORD-RELOAD".

   node-id-length
      This element contains the length of a NodeId (NodeIdLength), in
      bytes.  This value MUST be between 16 (128 bits) and 20 (160
      bits).  If this element is not present, the default of 16 is used.

   root-cert
      This element contains a base-64-encoded X.509v3 certificate that
      is a root trust anchor used to sign all certificates in this
      overlay.  There can be more than one root-cert element.

   enrollment-server
      This element contains the URL at which the enrollment server can
      be reached in a "url" element.  This URL MUST be of type "https:".
      More than one enrollment-server element MAY be present.  Note that
      there is no necessary relationship between the overlay name/
      configuration server name and the enrollment server name.

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   self-signed-permitted
      This element indicates whether self-signed certificates are
      permitted.  If it is set to "true", then self-signed certificates
      are allowed, in which case the enrollment-server and root-cert
      elements MAY be absent.  Otherwise, it SHOULD be absent, but MAY
      be set to "false".  This element also contains an attribute
      "digest", which indicates the digest to be used to compute the
      Node-ID.  Valid values for this parameter are "sha1" and "sha256",
      representing SHA-1 [RFC3174] and SHA-256 [RFC6234], respectively.
      Implementations MUST support both of these algorithms.

   bootstrap-node
      This element represents the address of one of the bootstrap nodes.
      It has an attribute called "address" that represents the IP
      address (either IPv4 or IPv6, since they can be distinguished) and
      an optional attribute called "port" that represents the port and
      defaults to 6084.  The IPv6 address is in typical hexadecimal form
      using standard period and colon separators as specified in
      [RFC5952].  More than one bootstrap-node element MAY be present.

   turn-density
      This element is a positive integer that represents the approximate
      reciprocal of density of nodes that can act as TURN servers.  For
      example, if 5% of the nodes can act as TURN servers, this element
      would be set to 20.  If it is not present, the default value is 1.
      If there are no TURN servers in the overlay, it is set to zero.

   clients-permitted
      This element represents whether clients are permitted or whether
      all nodes must be peers.  If clients are permitted, the element
      MUST be set to "true" or be absent.  If the nodes are not allowed
      to remain clients after the initial join, the element MUST be set
      to "false".  There is currently no way for the overlay to enforce
      this.

   no-ice
      This element represents whether nodes are REQUIRED to use the
      "No-ICE" Overlay Link protocols in this overlay.  If it is absent,
      it is treated as if it were set to "false".

   chord-update-interval
      The update frequency for the CHORD-RELOAD Topology Plug-in (see
      Section 10).

   chord-ping-interval
      The Ping frequency for the CHORD-RELOAD Topology Plug-in (see
      Section 10).

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   chord-reactive
      Whether reactive recovery SHOULD be used for this overlay.  It is
      set to "true" or "false".  If missing, the default is "true" (see
      Section 10).

   shared-secret
      If shared secret mode is used, this element contains the shared
      secret.  The security guarantee here is that any agent which is
      able to access the Configuration Document (presumably protected by
      some sort of HTTP access control or network topology) is able to
      recover the shared secret and hence join the overlay.

   max-message-size
      Maximum size, in bytes, of any message in the overlay.  If this
      value is not present, the default is 5000.

   initial-ttl
      Initial default TTL for messages (see Section 6.3.2).  If this
      value is not present, the default is 100.

   overlay-reliability-timer
      Default value for the end-to-end retransmission timer for
      messages, in milliseconds.  If not present, the default value is
      3000.  The value MUST be at least 200 milliseconds, which means
      the minimum time delay before dropping a link is 1000
      milliseconds.

   overlay-link-protocol
      Indicates a permissible overlay link protocol (see Section 6.6.1
      for requirements for such protocols).  An arbitrary number of
      these elements may appear.  If none appear, then this implies the
      default value, "TLS", which refers to the use of TLS and DTLS.  If
      one or more elements appear, then no default value applies.

   kind-signer
      This contains a single Node-ID in hexadecimal and indicates that
      the certificate with this Node-ID is allowed to sign Kinds.
      Identifying kind-signer by Node-ID instead of certificate allows
      the use of short-lived certificates without constantly having to
      provide an updated configuration file.

   configuration-signer
      This contains a single Node-ID in hexadecimal and indicates that
      the certificate with this Node-ID is allowed to sign
      configurations for this instance-name.  Identifying the signer by
      Node-ID instead of certificate allows the use of short-lived
      certificates without constantly having to provide an updated
      configuration file.

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   bad-node
      This contains a single Node-ID in hexadecimal and indicates that
      the certificate with this Node-ID MUST NOT be considered valid.
      This allows certificate revocation.  An arbitrary number of these
      elements can be provided.  Note that because certificates may
      expire, bad-node entries need be present only for the lifetime of
      the certificate.  Technically speaking, bad Node-IDs may be reused
      after their certificates have expired.  The requirement for
      Node-IDs to be pseudorandomly generated gives this event a
      vanishing probability.

   mandatory-extension
      This element contains the name of an XML namespace that a node
      joining the overlay MUST support.  The presence of a mandatory-
      extension element does not require the extension to be used in the
      current configuration file, but can indicate that it may be used
      in the future.  Note that the namespace is case-sensitive, as
      specified in Section 2.3 of [w3c-xml-namespaces].  More than one
      mandatory-extension element MAY be present.

   Inside each configuration element, the required-kinds element MAY
   also occur.  This element indicates the Kinds that members MUST
   support and contains multiple kind-block elements that each define a
   single Kind that MUST be supported by nodes in the overlay.  Each
   kind-block consists of a single kind element and a kind-signature.
   The kind element defines the Kind.  The kind-signature is the
   signature computed over the kind element.

   Each kind element has either an id attribute or a name attribute.
   The name attribute is a string representing the Kind (the name
   registered to IANA), while the id is an integer Kind-ID allocated out
   of private space.

   In addition, the kind element MUST contain the following elements:

   max-count
      The maximum number of values which members of the overlay must
      support.

   data-model
      The data model to be used.

   max-size
      The maximum size of individual values.

   access-control
      The access control model to be used.

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   The kind element MAY also contain the following element:

   max-node-multiple
      If the access control is NODE-MULTIPLE, this element MUST be
      included.  This indicates the maximum value for the i counter.  It
      MUST be an integer greater than 0.

   All of the non-optional values MUST be provided.  If the Kind is
   registered with IANA, the data-model and access-control elements MUST
   match those in the Kind registration, and clients MUST ignore them in
   favor of the IANA versions.  Multiple kind-block elements MAY be
   present.

   The kind-block element also MUST contain a "kind-signature" element.
   This signature is computed across the kind element from the beginning
   of the first < of the kind element to the end of the last > of the
   kind element in the same way as the signature element described later
   in this section. kind-block elements MUST be signed by a node listed
   in the kind-signers block of the current configuration.  Receivers
   MUST verify the signature prior to accepting a kind-block.

   The configuration element MUST be treated as a binary blob that
   cannot be changed -- including any whitespace changes -- or the
   signature will break.  The signature MUST be computed by taking each
   configuration element and starting from, and including, the first <
   at the start of <configuration> up to and including the > in </
   configuration> and treating this as a binary blob that MUST be signed
   using the standard SecurityBlock defined in Section 6.3.4.  The
   SecurityBlock MUST be base-64 encoded using the base64 alphabet from
   [RFC4648] and MUST be put in the signature element following the
   configuration object in the configuration file.  Any configuration
   file MUST be signed by one of the configuration-signer elements from
   the previous extant configuration.  Recipients MUST verify the
   signature prior to accepting the configuration file.

   When a node receives a new configuration file, it MUST change its
   configuration to meet the new requirements.  This may require the
   node to exit the DHT and rejoin.  If a node is not capable of
   supporting the new requirements, it MUST exit the overlay.  If some
   information about a particular Kind changes from what the node
   previously knew about the Kind (for example, the max size), the new
   information in the configuration files overrides any previously
   learned information.  If any Kind data was signed by a node that is
   no longer allowed to sign Kinds, that Kind MUST be discarded along
   with any stored information of that Kind.  Note that forcing an
   avalanche restart of the overlay with a configuration change that
   requires rejoining the overlay may result in serious performance
   problems, including total collapse of the network if configuration

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   parameters are not properly considered.  Such an event may be
   necessary in case of a compromised CA or similar problem, but for
   large overlays, it should be avoided in almost all circumstances.

11.1.1.  RELAX NG Grammar

   The grammar for the configuration data is:

   namespace chord = "urn:ietf:params:xml:ns:p2p:config-chord"
   namespace local = ""
   default namespace p2pcf = "urn:ietf:params:xml:ns:p2p:config-base"
   namespace rng = "http://relaxng.org/ns/structure/1.0"

   anything =
       (element * { anything }
        | attribute * { text }
        | text)*

   foreign-elements = element * - (p2pcf:* | local:* | chord:*)
                      { anything }*
   foreign-attributes = attribute * - (p2pcf:*|local:*|chord:*)
                        { text }*
   foreign-nodes = (foreign-attributes | foreign-elements)*

   start =  element p2pcf:overlay {
         overlay-element
   }

   overlay-element &=  element configuration {
               attribute instance-name { xsd:string },
               attribute expiration { xsd:dateTime }?,
               attribute sequence { xsd:long }?,
               foreign-attributes*,
               parameter
           }+
   overlay-element &= element signature {
               attribute algorithm { signature-algorithm-type }?,
               xsd:base64Binary
           }*

   signature-algorithm-type |= "rsa-sha1"
   signature-algorithm-type |=  xsd:string # signature alg extensions

   parameter &= element topology-plugin { topology-plugin-type }?
   topology-plugin-type |= xsd:string # topo plugin extensions
   parameter &= element max-message-size { xsd:unsignedInt }?
   parameter &= element initial-ttl { xsd:int }?
   parameter &= element root-cert { xsd:base64Binary }*

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   parameter &= element required-kinds { kind-block* }?
   parameter &= element enrollment-server { xsd:anyURI }*
   parameter &= element kind-signer {  xsd:string }*
   parameter &= element configuration-signer {  xsd:string }*
   parameter &= element bad-node {  xsd:string }*
   parameter &= element no-ice { xsd:boolean }?
   parameter &= element shared-secret { xsd:string }?
   parameter &= element overlay-link-protocol { xsd:string }*
   parameter &= element clients-permitted { xsd:boolean }?
   parameter &= element turn-density { xsd:unsignedByte }?
   parameter &= element node-id-length { xsd:int }?
   parameter &= element mandatory-extension { xsd:string }*
   parameter &= foreign-elements*

   parameter &=
       element self-signed-permitted {
           attribute digest { self-signed-digest-type },
           xsd:boolean
       }?
   self-signed-digest-type |= "sha1"
   self-signed-digest-type |=  xsd:string # signature digest extensions

   parameter &= element bootstrap-node {
                   attribute address { xsd:string },
                   attribute port { xsd:int }?
                }*

   kind-block = element kind-block {
       element kind {
           (  attribute name { kind-names }
              | attribute id { xsd:unsignedInt } ),
           kind-parameter
       } &
       element kind-signature  {
           attribute algorithm { signature-algorithm-type }?,
           xsd:base64Binary
       }?
   }

   kind-parameter &= element max-count { xsd:int }
   kind-parameter &= element max-size { xsd:int }
   kind-parameter &= element max-node-multiple { xsd:int }?

   kind-parameter &= element data-model { data-model-type }
   data-model-type |= "SINGLE"
   data-model-type |= "ARRAY"
   data-model-type |= "DICTIONARY"
   data-model-type |=  xsd:string # data model extensions

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   kind-parameter &= element access-control { access-control-type }
   access-control-type |= "USER-MATCH"
   access-control-type |= "NODE-MATCH"
   access-control-type |= "USER-NODE-MATCH"
   access-control-type |= "NODE-MULTIPLE"
   access-control-type |= xsd:string # access control extensions

   kind-parameter &= foreign-elements*

   kind-names |= "TURN-SERVICE"
   kind-names |= "CERTIFICATE_BY_NODE"
   kind-names |= "CERTIFICATE_BY_USER"
   kind-names |= xsd:string # kind extensions

   # Chord specific parameters
   topology-plugin-type |= "CHORD-RELOAD"
   parameter &= element chord:chord-ping-interval { xsd:int }?
   parameter &= element chord:chord-update-interval { xsd:int }?
   parameter &= element chord:chord-reactive { xsd:boolean }?

11.2.  Discovery through Configuration Server

   When a node first enrolls in a new overlay, it starts with a
   discovery process to find a configuration server.

   The node MAY start by determining the overlay name.  This value MUST
   be provided by the user or some other out-of-band provisioning
   mechanism.  The out-of-band mechanism MAY also provide an optional
   URL for the configuration server.  If a URL for the configuration
   server is not provided, the node MUST do a DNS SRV query using a
   Service name of "reload-config" and a protocol of TCP to find a
   configuration server and form the URL by appending a path of
   "/.well-known/reload-config" to the overlay name.  This uses the
   "well-known URI" framework defined in [RFC5785].  For example, if the
   overlay name was example.com, the URL would be
   "https://example.com/.well-known/reload-config".

   Once an address and URL for the configuration server are determined,
   the peer MUST form an HTTPS connection to that IP address.  If an
   optional URL for the configuration server was provided, the
   certificate MUST match the domain name from the URL as described in
   [RFC2818]; otherwise, the certificate MUST match the overlay name as
   described in [RFC2818].  If the HTTPS certificates pass the name
   matching, the node MUST fetch a new copy of the configuration file.
   To do this, the peer performs a GET to the URL.  The result of the
   HTTP GET is an XML configuration file described above.  If the XML is
   not valid or the instance-name attribute of the overlay-element in
   the XML does not match the overlay name, this configurations file

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   SHOULD be discarded.  Otherwise, the new configuration MUST replace
   any previously learned configuration file for this overlay.

   For overlays that do not use a configuration server, nodes MUST
   obtain the configuration information needed to join the overlay
   through some out-of-band approach, such as an XML configuration file
   sent over email.

11.3.  Credentials

   If the Configuration Document contains an enrollment-server element,
   credentials are REQUIRED to join the Overlay Instance.  A peer which
   does not yet have credentials MUST contact the enrollment server to
   acquire them.

   RELOAD defines its own trivial certificate request protocol.  We
   would have liked to have used an existing protocol, but were
   concerned about the implementation burden of even the simplest of
   those protocols, such as [RFC5272] and [RFC5273].  The objective was
   to have a protocol which could be easily implemented in a Web server
   which the operator did not control (e.g., in a hosted service) and
   which was compatible with the existing certificate-handling tooling
   as used with the Web certificate infrastructure.  This means
   accepting bare PKCS#10 requests and returning a single bare X.509
   certificate.  Although the MIME types for these objects are defined,
   none of the existing protocols support exactly this model.

   The certificate request protocol MUST be performed over HTTPS.  The
   server certificate MUST match the overlay name as described in
   [RFC2818].  The request MUST be an HTTP POST with the parameters
   encoded as described in [RFC2388] and with the following properties:

   o  If authentication is required, there MUST be form parameters of
      "password" and "username" containing the user's account name and
      password in the clear (hence the need for HTTPS).  The username
      and password strings MUST be UTF-8 strings compared as binary
      objects.  Applications using RELOAD SHOULD define any needed
      string preparation as per [RFC4013] or its successor documents.

   o  If more than one Node-ID is required, there MUST be a form
      parameter of "nodeids" containing the number of Node-IDs required.

   o  There MUST be a form parameter of "csr" with a content type of
      "application/pkcs10", as defined in [RFC2311], that contains the
      certificate signing request (CSR).

   o  The Accept header MUST contain the type "application/pkix-cert",
      indicating the type that is expected in the response.

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   The enrollment server MUST authenticate the request using the
   provided account name and password.  The reason for using the RFC
   2388 "multipart/form-data" encoding is so that the password parameter
   will not be encoded in the URL, to reduce the chance of accidental
   leakage of the password.  If the authentication succeeds and the
   requested user name in the CSR is acceptable, the server MUST
   generate and return a certificate for the CSR in the "csr" parameter
   of the request.  The SubjectAltName field in the certificate MUST
   contain the following values:

   o  One or more Node-IDs which MUST be cryptographically random
      [RFC4086].  Each MUST be chosen by the enrollment server in such a
      way that it is unpredictable to the requesting user.  For example,
      the user MUST NOT be informed of potential (random) Node-IDs prior
      to authenticating.  Each is placed in the subjectAltName using the
      uniformResourceIdentifier type, each MUST contain RELOAD URI, as
      described in Section 14.15, and each MUST contain a Destination
      List with a single entry of type "node_id".  The enrollment server
      SHOULD maintain a mapping of users to Node-IDs and if the same
      user returns (e.g., to have their certificate re-issued), the
      enrollment server should return the same Node-IDs, thus avoiding
      the need for implementations to re-store all their data when their
      certificates expire.

   o  A single name (the "user name") that this user is allowed to use
      in the overlay, using type rfc822Name.  Enrollment servers SHOULD
      take care to allow only legal characters in the name (e.g., no
      embedded NULs), rather than simply accepting any name provided by

      the user.  In some usages, the right side of the user name will
      match the overlay name, but there is no requirement for this match
      in this specification.  Applications using this specification MAY
      define such a requirement or MAY otherwise limit the allowed range
      of allowed user names.

   The SubjectAltName field in the certificate MUST NOT contain any
   identities other than those listed above.  The subject distinguished
   name in the certificate MUST be empty.

   The certificate MUST be returned as type "application/pkix-cert", as
   defined in [RFC2585], with an HTTP status code of 200 OK.

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   Certificate processing errors SHOULD result in an HTTP return code of
   403 Forbidden, along with a body of type "text/plain" and body that
   consists of one of the tokens defined in the following list:

   failed_authentication
      The account name and password combination used in the HTTPS
      request was not valid.

   username_not_available
      The requested user name in the CSR was not acceptable.

   Node-IDs_not_available
      The number of Node-IDs requested was not acceptable.

   bad_CSR
      There was some other problem with the CSR.

   If the client receives an unknown token in the body, it SHOULD treat
   it as a failure for an unknown reason.

   The client MUST check that the returned certificate chains back to
   one of the certificates received in the "root-cert" list of the
   overlay configuration data (including PKIX BasicConstraints checks).
   The node then reads the certificate to find the Node-ID it can use.

11.3.1.  Self-Generated Credentials

   If the "self-signed-permitted" element is present in the
   configuration and is set to "true", then a node MUST generate its own
   self-signed certificate to join the overlay.  The self-signed
   certificate MAY contain any user name of the user's choice.

   For self-signed certificates containing only one Node-ID, the Node-ID
   MUST be computed by applying the digest specified in the self-signed-
   permitted element to the DER representation of the user's public key
   (more specifically, the subjectPublicKeyInfo) and taking the high-
   order bits.  For self-signed certificates containing multiple
   Node-IDs, the index of the Node-ID (from 1 to the number of Node-IDs
   needed) must be prepended as a 4-byte big-endian integer to the DER
   representation of the user's public key and taking the high-order
   bits.  When accepting a self-signed certificate, nodes MUST check
   that the Node-ID and public keys match.  This prevents Node-ID theft.

   Once the node has constructed a self-signed certificate, it MAY join
   the overlay.  It MUST store its certificate in the overlay
   (Section 8), but SHOULD look to see if the user name is already taken
   and, if so, choose another user name.  Note that this provides
   protection only against accidental name collisions.  Name theft is

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   still possible.  If protection against name theft is desired, then
   the enrollment service MUST be used.

11.4.  Contacting a Bootstrap Node

   In order to join the overlay, the Joining Node MUST contact a node in
   the overlay.  Typically this means contacting the bootstrap nodes,
   since they are reachable by the local peer or have public IP
   addresses.  If the Joining Node has cached a list of peers that it
   has previously been connected with in this overlay, as an
   optimization it MAY attempt to use one or more of them as bootstrap
   nodes before falling back to the bootstrap nodes listed in the
   configuration file.

   When contacting a bootstrap node, the Joining Node MUST first form
   the DTLS or TLS connection to the bootstrap node and then send an
   Attach request over this connection with the destination Resource-ID
   set to the Joining Node's Node-ID plus 1.

   When the requester node finally does receive a response from some
   responding node, it MUST use the Node-ID in the response to start
   sending requests to join the Overlay Instance as described in
   Section 6.4.

   After a node has successfully joined the overlay network, it will
   have direct connections to several peers.  Some MAY be added to the
   cached bootstrap nodes list and used in future boots.  Peers that are
   not directly connected MUST NOT be cached.  The suggested number of
   peers to cache is 10.  Algorithms for determining which peers to
   cache are beyond the scope of this specification.

12.  Message Flow Example

   The following abbreviations are used in the message flow diagrams:
   JN = Joining Node, AP = Admitting Peer, NP = next peer after the AP,
   NNP = next next peer which is the peer after NP, PP = previous peer
   before the AP, PPP = previous previous peer which is the peer before
   the PP, BP = bootstrap node.

   In the following example, we assume that JN has formed a connection
   to one of the bootstrap nodes.  JN then sends an Attach through that
   peer to a Resource-ID of itself plus 1 (JN+1).  It gets routed to the
   AP, because JN is not yet part of the overlay.  When AP responds, JN
   and the AP use ICE to set up a connection and then set up DTLS.  Once
   AP has connected to JN, AP sends to JN an Update to populate its
   Routing Table.  The following example shows the Update happening
   after the DTLS connection is formed, but it could also happen before,
   in which case the Update would often be routed through other nodes.

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       JN        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachReq Dest=JN+1|         |         |         |         |
        |---------------------------------------------------------->|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |AttachReq Dest=JN+1|         |
        |         |         |         |<----------------------------|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |AttachAns          |         |
        |         |         |         |---------------------------->|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachAns          |         |         |         |         |
        |<----------------------------------------------------------|
        |         |         |         |         |         |         |
        |ICE      |         |         |         |         |         |
        |<===========================>|         |         |         |
        |         |         |         |         |         |         |
        |TLS      |         |         |         |         |         |
        |<...........................>|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |

                                 Figure 1

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   The JN then forms connections to the appropriate neighbors, such as
   NP, by sending an Attach which gets routed via other nodes.  When NP
   responds, JN and NP use ICE and DTLS to set up a connection.

       JN        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachReq NP       |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |AttachReq NP       |         |
        |         |         |         |-------->|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |AttachAns|         |         |
        |         |         |         |<--------|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachAns|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |ICE      |         |         |         |         |         |
        |<=====================================>|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |TLS      |         |         |         |         |         |
        |<.....................................>|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |


                                 Figure 2

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   The JN also needs to populate its Finger Table (for the Chord-based
   DHT).  It issues an Attach to a variety of locations around the
   overlay.  The diagram below shows JN sending an Attach halfway around
   the Chord ring to the JN + 2^127.

       JN        NP        XX        TP
        |         |         |         |
        |         |         |         |
        |         |         |         |
        |AttachReq JN+2<<126|         |
        |-------->|         |         |
        |         |         |         |
        |         |         |         |
        |         |AttachReq JN+2<<126|
        |         |-------->|         |
        |         |         |         |
        |         |         |         |
        |         |         |AttachReq JN+2<<126
        |         |         |-------->|
        |         |         |         |
        |         |         |         |
        |         |         |AttachAns|
        |         |         |<--------|
        |         |         |         |
        |         |         |         |
        |         |AttachAns|         |
        |         |<--------|         |
        |         |         |         |
        |         |         |         |
        |AttachAns|         |         |
        |<--------|         |         |
        |         |         |         |
        |ICE      |         |         |
        |<===========================>|
        |         |         |         |
        |TLS      |         |         |
        |<...........................>|
        |         |         |         |
        |         |         |         |

                                 Figure 3

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   Once JN has a reasonable set of connections, it is ready to take its
   place in the DHT.  It does this by sending a Join to AP.  AP sends a
   series of Store requests to JN to store the data that JN will be
   responsible for.  AP then sends JN an Update that explicitly labels
   JN as its predecessor.  At this point, JN is part of the ring and is
   responsible for a section of the overlay.  AP can now forget any data
   which is assigned to JN and not to AP.

       JN        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |JoinReq  |         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |JoinAns  |         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreReq Data A    |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreAns |         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreReq Data B    |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreAns |         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |

                                 Figure 4

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   In Chord, JN's Neighbor Table needs to contain its own predecessors.
   It couldn't connect to them previously, because it did not yet know
   their addresses.  However, now that it has received an Update from
   AP, as in the previous diagram, it has AP's predecessors, which are
   also its own, so it sends Attaches to them.  Below, it is shown
   connecting only to AP's closest predecessor, PP.

       JN        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachReq Dest=PP  |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |AttachReq Dest=PP  |         |         |
        |         |         |<--------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |AttachAns|         |         |         |
        |         |         |-------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachAns|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |TLS      |         |         |         |         |         |
        |...................|         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |------------------>|         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |<------------------|         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |

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        |-------------------------------------->|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |<--------------------------------------|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |

                                 Figure 5

   Finally, now that JN has a copy of all the data and is ready to route
   messages and receive requests, it sends Updates to everyone in its
   Routing Table to tell them it is ready to go.  Below, it is shown
   sending such an update to TP.


           JN        NP        XX        TP
            |         |         |         |
            |         |         |         |
            |         |         |         |
            |UpdateReq|         |         |
            |---------------------------->|
            |         |         |         |
            |         |         |         |
            |UpdateAns|         |         |
            |<----------------------------|
            |         |         |         |
            |         |         |         |
            |         |         |         |
            |         |         |         |

                                 Figure 6

13.  Security Considerations

13.1.  Overview

   RELOAD provides a generic storage service, albeit one designed to be
   useful for P2PSIP.  In this section, we discuss security issues that
   are likely to be relevant to any usage of RELOAD.  More background
   information can be found in [RFC5765].

   In any Overlay Instance, any given user depends on a number of peers
   with which they have no well-defined relationship except that they
   are fellow members of the Overlay Instance.  In practice, these other
   nodes may be friendly, lazy, curious, or outright malicious.  No
   security system can provide complete protection in an environment
   where most nodes are malicious.  The goal of security in RELOAD is to

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   provide strong security guarantees of some properties even in the
   face of a large number of malicious nodes and to allow the overlay to
   function correctly in the face of a modest number of malicious nodes.

   P2PSIP deployments require the ability to authenticate both peers and
   resources (users) without the active presence of a trusted entity in
   the system.  We describe two mechanisms.  The first mechanism is
   based on public key certificates and is suitable for general
   deployments.  The second is an admission control mechanism based on
   an overlay-wide shared symmetric key.

13.2.  Attacks on P2P Overlays

   The two basic functions provided by overlay nodes are storage and
   routing: some peer is responsible for storing a node's data and for
   allowing a third node to fetch this stored data, while other peers
   are responsible for routing messages to and from the storing nodes.
   Each of these issues is covered in the following sections.

   P2P overlays are subject to attacks by subversive nodes that may
   attempt to disrupt routing, corrupt or remove user registrations, or
   eavesdrop on signaling.  The certificate-based security algorithms we
   describe in this specification are intended to protect overlay
   routing and user registration information in RELOAD messages.

   To protect the signaling from attackers pretending to be valid nodes
   (or nodes other than themselves), the first requirement is to ensure
   that all messages are received from authorized members of the
   overlay.  For this reason, RELOAD MUST transport all messages over a
   secure channel (TLS and DTLS are defined in this document) which
   provides message integrity and authentication of the directly
   communicating peer.  In addition, messages and data MUST be digitally
   signed with the sender's private key, providing end-to-end security
   for communications.

13.3.  Certificate-Based Security

   This specification stores users' registrations and possibly other
   data in an overlay network.  This requires a solution both to
   securing this data and to securing, as well as possible, the routing
   in the overlay.  Both types of security are based on requiring that
   every entity in the system (whether user or peer) authenticate
   cryptographically using an asymmetric key pair tied to a certificate.

   When a user enrolls in the Overlay Instance, they request or are
   assigned a unique name, such as "alice@dht.example.net".  These names
   MUST be unique and are meant to be chosen and used by humans much
   like a SIP address-of-record (AOR) or an email address.  The user

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   MUST also be assigned one or more Node-IDs by the central enrollment
   authority.  Both the name and the Node-IDs are placed in the
   certificate, along with the user's public key.

   Each certificate enables an entity to act in two sorts of roles:

   o  As a user, storing data at specific Resource-IDs in the Overlay
      Instance corresponding to the user name.

   o  As a overlay peer with the Node-IDs listed in the certificate.

   Note that since only users of this Overlay Instance need to validate
   a certificate, this usage does not require a global Public Key
   Infrastructure (PKI).  Instead, certificates MUST be signed by a
   central enrollment authority which acts as the certificate authority
   for the Overlay Instance.  This authority signs each node's
   certificate.  Because each node possesses the CA's certificate (which
   they receive upon enrollment), they can verify the certificates of
   the other entities in the overlay without further communication.
   Because the certificates contain the user's/node's public key,
   communications from the user/node can, in turn, be verified.

   If self-signed certificates are used, then the security provided is
   significantly decreased, since attackers can mount Sybil attacks.  In
   addition, attackers cannot trust the user names in certificates
   (although they can trust the Node-IDs, because they are
   cryptographically verifiable).  This scheme may be appropriate for
   some small deployments, such as a small office or an ad hoc overlay
   set up among participants in a meeting where all hosts on the network
   are trusted.  Some additional security can be provided by using the
   shared secret admission control scheme as well.

   Because all stored data is signed by the owner of the data, the
   storing node can verify that the storer is authorized to perform a
   store at that Resource-ID and also can allow any consumer of the data
   to verify the provenance and integrity of the data when it retrieves
   it.

   Note that RELOAD does not itself provide a revocation/status
   mechanism (although certificates may, of course, include Online
   Certificate Status Protocol [OCSP] responder information).  Thus,
   certificate lifetimes SHOULD be chosen to balance the compromise
   window versus the cost of certificate renewal.  Because RELOAD is
   already designed to operate in the face of some fraction of malicious
   nodes, this form of compromise is not fatal.

   All implementations MUST implement certificate-based security.

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13.4.  Shared-Secret Security

   RELOAD also supports a shared secret admission control scheme that
   relies on a single key that is shared among all members of the
   overlay.  It is appropriate for small groups that wish to form a
   private network without complexity.  In shared secret mode, all the
   peers MUST share a single symmetric key which is used to key TLS-PSK
   or TLS-SRP mode.  A peer which does not know the key cannot form TLS
   connections with any other peer and therefore cannot join the
   overlay.

   One natural approach to a shared-secret scheme is to use a user-
   entered password as the key.  The difficulty with this is that in
   TLS-PSK mode, such keys are very susceptible to dictionary attacks.
   If passwords are used as the source of shared keys, then TLS-SRP is a
   superior choice, because it is not subject to dictionary attacks.

13.5.  Storage Security

   When certificate-based security is used in RELOAD, any given
   Resource-ID/Kind-ID pair is bound to some small set of certificates.
   In order to write data, the writer must prove possession of the
   private key for one of those certificates.  Moreover, all data is
   stored, signed with the same private key that was used to authorize
   the storage.  This set of rules makes questions of authorization and
   data integrity, which have historically been thorny for overlays,
   relatively simple.

13.5.1.  Authorization

   When a node wants to store some value, it MUST first digitally sign
   the value with its own private key.  It then sends a Store request
   that contains both the value and the signature towards the storing
   peer (which is defined by the Resource Name construction algorithm
   for that particular Kind of value).

   When the storing peer receives the request, it MUST determine whether
   the storing node is authorized to store at this Resource-ID/Kind-ID
   pair.  Determining this requires comparing the user's identity to the
   requirements of the access control model (see Section 7.3).  If it
   satisfies those requirements, the user is authorized to write,
   pending quota checks, as described in the next section.

   For example, consider a certificate with the following properties:

          User name: alice@dht.example.com
          Node-ID:   013456789abcdef
          Serial:    1234

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   If Alice wishes to Store a value of the "SIP Location" Kind, the
   Resource Name will be the SIP AOR "sip:alice@dht.example.com".  The
   Resource-ID will be determined by hashing the Resource Name.  Because
   SIP Location uses the USER-NODE-MATCH policy, it first verifies that
   the user name in the certificate hashes to the requested Resource-ID.
   It then verifies that the Node-ID in the certificate matches the
   dictionary key being used for the store.  If both of these checks
   succeed, the Store is authorized.  Note that because the access
   control model is different for different Kinds, the exact set of
   checks will vary.

13.5.2.  Distributed Quota

   Being a peer in an Overlay Instance carries with it the
   responsibility to store data for a given region of the Overlay
   Instance.  However, allowing nodes to store unlimited amounts of data
   would create unacceptable burdens on peers and would also enable
   trivial denial-of-service (DoS) attacks.  RELOAD addresses this issue
   by requiring configurations to define maximum sizes for each Kind of
   stored data.  Attempts to store values exceeding this size MUST be
   rejected.  (If peers are inconsistent about this, then strange
   artifacts will happen when the zone of responsibility shifts and a
   different peer becomes responsible for overlarge data.)  Because each
   Resource-ID/Kind-ID pair is bound to a small set of certificates,
   these size restrictions also create a distributed quota mechanism,
   with the quotas administered by the central configuration server.

   Allowing different Kinds of data to have different size restrictions
   allows new usages the flexibility to define limits that fit their
   needs without requiring all usages to have expansive limits.

13.5.3.  Correctness

   Because each stored value is signed, it is trivial for any retrieving
   node to verify the integrity of the stored value.  More care needs to
   be taken to prevent version rollback attacks.  Rollback attacks on
   storage are prevented by the use of store times and lifetime values
   in each store.  A lifetime represents the latest time at which the
   data is valid and thus limits (although does not completely prevent)
   the ability of the storing node to perform a rollback attack on
   retrievers.  In order to prevent a rollback attack at the time of the
   Store request, it is REQUIRED that storage times be monotonically
   increasing.  Storing peers MUST reject Store requests with storage
   times smaller than or equal to those that they are currently storing.
   In addition, a fetching node which receives a data value with a
   storage time older than the result of the previous fetch knows that a
   rollback has occurred.

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13.5.4.  Residual Attacks

   The mechanisms described here provide a high degree of security, but
   some attacks remain possible.  Most simply, it is possible for
   storing peers to refuse to store a value (i.e., they reject any
   request).  In addition, a storing peer can deny knowledge of values
   which it has previously accepted.  To some extent, these attacks can
   be ameliorated by attempting to store to and retrieve from replicas,
   but a retrieving node does not know whether or not it should try
   this, as there is a cost to doing so.

   The certificate-based authentication scheme prevents a single peer
   from being able to forge data owned by other peers.  Furthermore,
   although a subversive peer can refuse to return data resources for
   which it is responsible, it cannot return forged data, because it
   cannot provide authentication for such registrations.  Therefore,
   parallel searches for redundant registrations can mitigate most of
   the effects of a compromised peer.  The ultimate reliability of such
   an overlay is a statistical question based on the replication factor
   and the percentage of compromised peers.

   In addition, when a Kind is multivalued (e.g., an array data model),
   the storing peer can return only some subset of the values, thus
   biasing its responses.  This can be countered by using single values
   rather than sets, but that makes coordination between multiple
   storing agents much more difficult.  This is a trade-off that must be
   made when designing any usage.

13.6.  Routing Security

   Because the storage security system guarantees (within limits) the
   integrity of the stored data, routing security focuses on stopping
   the attacker from performing a DoS attack that misroutes requests in
   the overlay.  There are a few obvious observations to make about
   this.  First, it is easy to ensure that an attacker is at least a
   valid node in the Overlay Instance.  Second, this is a DoS attack
   only.  Third, if a large percentage of the nodes on the Overlay
   Instance are controlled by the attacker, it is probably impossible to
   perfectly secure against this.

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13.6.1.  Background

   In general, attacks on DHT routing are mounted by the attacker
   arranging to route traffic through one or two nodes that it controls.
   In the Eclipse attack [Eclipse], the attacker tampers with messages
   to and from nodes for which it is on-path with respect to a given
   victim node.  This allows it to pretend to be all the nodes that are
   reachable through it.  In the Sybil attack [Sybil], the attacker
   registers a large number of nodes and is therefore able to capture a
   large amount of the traffic through the DHT.

   Both the Eclipse and Sybil attacks require the attacker to be able to
   exercise control over her Node-IDs.  The Sybil attack requires the
   creation of a large number of peers.  The Eclipse attack requires
   that the attacker be able to impersonate specific peers.  In both
   cases, RELOAD attempts to mitigate these attacks by the use of
   centralized, certificate-based admission control.

13.6.2.  Admissions Control

   Admission to a RELOAD Overlay Instance is controlled by requiring
   that each peer have a certificate containing its Node-ID.  The
   requirement to have a certificate is enforced by using certificate-
   based mutual authentication on each connection.  (Note: the following
   applies only when self-signed certificates are not used.)  Whenever a
   peer connects to another peer, each side automatically checks that
   the other has a suitable certificate.  These Node-IDs MUST be
   randomly assigned by the central enrollment server.  This has two
   benefits:

   o  It allows the enrollment server to limit the number of Node-IDs
      issued to any individual user.

   o  It prevents the attacker from choosing specific Node-IDs.

   The first property allows protection against Sybil attacks (provided
   that the enrollment server uses strict rate-limiting policies).  The
   second property deters but does not completely prevent Eclipse
   attacks.  Because an Eclipse attacker must impersonate peers on the
   other side of the attacker, the attacker must have a certificate for
   suitable Node-IDs, which requires him to repeatedly query the
   enrollment server for new certificates, which will match only by
   chance.  From the attacker's perspective, the difficulty is that if
   the attacker has only a small number of certificates, the region of
   the Overlay Instance he is impersonating appears to be very sparsely
   populated by comparison to the victim's local region.

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13.6.3.  Peer Identification and Authentication

   In general, whenever a peer engages in overlay activity that might
   affect the Routing Table, it must establish its identity.  This
   happens in two ways.  First, whenever a peer establishes a direct
   connection to another peer, it authenticates via certificate-based
   mutual authentication.  All messages between peers are sent over this
   protected channel, and therefore the peers can verify the data origin
   of the last-hop peer for requests and responses without further
   cryptography.

   In some situations, however, it is desirable to be able to establish
   the identity of a peer with whom one is not directly connected.  The
   most natural case is when a peer Updates its state.  At this point,
   other peers may need to update their view of the overlay structure,
   but they need to verify that the Update message came from the actual
   peer rather than from an attacker.  To prevent having a peer accept
   Update messages from an attacker, all overlay routing messages are
   signed by the peer that generated them.

   For messages that impact the topology of the overlay, replay is
   typically prevented by having the information come directly from, or
   be verified by, the nodes that claimed to have generated the update.
   Data storage replay detection is done by signing the time of the node
   that generated the signature on the Store request, thus providing a
   time-based replay protection, but the time synchronization is needed
   only between peers that can write to the same location.

13.6.4.  Protecting the Signaling

   The goal here is to stop an attacker from knowing who is signaling
   what to whom.  An attacker is unlikely to be able to observe the
   activities of a specific individual, given the randomization of IDs
   and routing based on the present peers discussed above.  Furthermore,
   because messages can be routed using only the header information, the
   actual body of the RELOAD message can be encrypted during
   transmission.

   There are two lines of defense here.  The first is the use of TLS or
   DTLS for each communications link between peers.  This provides
   protection against attackers who are not members of the overlay.  The
   second line of defense is to digitally sign each message.  This
   prevents adversarial peers from modifying messages in flight, even if
   they are on the routing path.

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13.6.5.  Routing Loops and DoS Attacks

   Source-routing mechanisms are known to create the possibility for DoS
   amplification, especially by the induction of routing loops
   [RFC5095].  In order to limit amplification, the initial-ttl value in
   the configuration file SHOULD be set to a value slightly larger than
   the longest expected path through the network.  For Chord, experience
   has shown that log(2) of the number of nodes in the network + 5 is a
   safe bound.  Because nodes are required to enforce the initial-ttl as
   the maximum value, an attacker cannot achieve an amplification factor
   greater than initial-ttl, thus limiting the additional capabilities
   provided by source routing.

   In order to prevent the use of loops for targeted implementation
   attacks, implementations SHOULD check the Destination List for
   duplicate entries and discard such records with an
   "Error_Invalid_Message" error.  This does not completely prevent
   loops, but it does require that at least one attacker node be part of
   the loop.

13.6.6.  Residual Attacks

   The routing security mechanisms in RELOAD are designed to contain
   rather than eliminate attacks on routing.  It is still possible for
   an attacker to mount a variety of attacks.  In particular, if an
   attacker is able to take up a position on the overlay routing between
   A and B, it can make it appear as if B does not exist or is
   disconnected.  It can also advertise false network metrics in an
   attempt to reroute traffic.  However, these are primarily DoS
   attacks.

   The certificate-based security scheme secures the namespace, but if
   an individual peer is compromised or if an attacker obtains a
   certificate from the CA, then a number of subversive peers can still
   appear in the overlay.  While these peers cannot falsify responses to
   resource queries, they can respond with error messages, effecting a
   DoS attack on the resource registration.  They can also subvert
   routing to other compromised peers.  To defend against such attacks,
   a resource search must still consist of parallel searches for
   replicated registrations.


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