doc: clarify SCMP reality vs. ambition

The SCMP "specification" document described support for SCMP as
mandatory. Furthermore, it mandates the use of SPAO+DRKey authentication
for SCMPs. This does not correspond to what is currently implemented.
SCMP support has been treated as at least semi-optional e.g. in
discussions around the dispatcher-less end host stack.

Regarding the authentication, there is no full implementation of this
neither in router (support in "our" router is experimental/demo-level,
no other router implementation supports it at all) nor in end hosts
(currently, nothing at all). There is not sufficiently clear consensus
around this to pretend that we'll realize this ambition shortly.

Update the documentation specs to reflect this reality.
Move the SCMP authentication description into a design document.

doc/dev/design/index.rst

@@ -42,6 +42,7 @@ Active
    BorderRouter
    router-perf-model
    router-port-dispatch
+   scmp-authentication
 
 .. _design-docs-completed:
 

doc/dev/design/scmp-authentication.rst

@@ -0,0 +1,130 @@
+*******************
+SCMP Authentication
+*******************
+
+- Author(s): Matthias Frei
+- Last updated: 2024-05-23
+- Discussion at: :issue:`3969`
+- Status:
+
+  - On hold. No clear consensus that this is the right approach. See "Discussion" section.
+  - Experimental support in router, added in :issue:`4255`.
+
+
+Abstract
+========
+SCMP error messages can potentially be abused by an attacker to signal spurious network errors, attempting to degrade or deny a victim's use of a service or network path.
+Employ cryptographic validation to check authenticity and authorization of the sender of an SCMP message.
+
+Background
+==========
+
+SCMP error messages can potentially be abused by an attacker to signal spurious network errors.
+Implementations of TCP/IP, face a similar challenge with ICMP error messages; in order to avoid acting on fraudulent errors, they carefully validate ICMP messages using their internal state machine and ignore errors that appear unlikely to happen e.g. after the connection has already been established RFC 5927.
+QUIC goes even further and simply ignores all ICMP errors, except those related to MTU discovery.
+
+This approach can only mitigate the problem.
+Furthermore, this approach is not sufficient in SCION as the applications/transport layer on end hosts are expected to react to events in the network, e.g. path failures, by changing to a different path.
+The occurrence of such events is independent of the transport's internal state machine.
+This aspect does not exist in IP based internet, where the network will transparently adapt the routing (or not) on faults.
+
+To address this, we use :doc:`/cryptography/drkey` and the :doc:`/protocols/authenticator-option` to authenticate SCMP messages.
+
+Proposal
+========
+
+.. _scmp-spao:
+
+SCMP with SCION Packet Authenticator Option
+-------------------------------------------
+
+SCMP messages can be authenticated with a MAC based on a symmetric key established with the :ref:`DRKey infrastructure <drkey>`.
+The MAC is transported in the :ref:`authenticator-option` End-to-End extension header.
+
+The Authenticator MAC algorithm is AES-CMAC (identifier :code:`0`).
+
+SCMP error messages MUST always be authenticated.
+SCMP informational messages CAN optionally be authenticated; a response message
+MUST be authenticated if and only if the corresponding request message was
+authenticated.
+
+All DRKey keys used here are derived with :ref:`protocol identifier <drkey-protocol-identifiers>` :code:`SCMP`, decimal :code:`1`.
+
+SCMP messages from (and to) routers are authenticated with :ref:`AS-host keys <drkey-as-host>`.
+SCMP response messages from a router in AS :math:`D` to a node :math:`H_s` in
+AS :math:`S` are authenticated with the DRKey :math:`K_{D,S:H_s}`.
+SCMP requests (specifically, :ref:`traceroute-request`) processed by a router
+are authenticated with the same key.
+
+SCMP messages between two end-hosts are authenticated with :ref:`host-host keys <drkey-host-host>`.
+An SCMP response message from a node :math:`H_d` in AS :math:`D` to a node
+:math:`H_s` in AS :math:`S` is authenticated with the key
+:math:`K_{D:H_d,S:H_s}`.
+SCMP requests and data packets from :math:`H_s` to :math:`H_d` are
+authenticated with this same key.
+
+For packets addressed to a router directly (specifically for
+:ref:`echo-request` and :ref:`echo-reply`) it is treated like an end-host and
+the corresponding host-host keys are used.
+
+.. note::
+   Recall that :ref:`traceroute-request`\s are *not* addressed to the router.
+   Instead, the router processes the request if its router alert flag is set.
+
+Processing Rules
+----------------
+The processing rules for SCMP messages are extended with the following points:
+
+-  Every SCMP error message MUST be authenticated.
+
+   Every SCMP informational reply message MUST be authenticated if and only if
+   the corresponding request was authenticated.
+
+   .. note::
+      Consequentially, an implementation without support for SCMP
+      authentication MUST never send SCMP error messages and MUST NOT reply to
+      authenticated SCMP informational request messages.
+
+-  When an SCMP message is received, the receiver SHOULD check the
+   authentication header.
+
+   - SCMP error messages without or with an invalid authentication header and
+     SCMP informational messages with an invalid authentication header SHOULD
+     be silently dropped.
+
+   - The receiver checks that the :ref:`DRKey identified by the SPI <spao-spi-drkey>`
+     is appropriate for the SCMP message type and code, as described above in
+     the section :ref:`scmp-spao`.
+
+   - The receiver derives or fetches the relevant key for validation of the MAC.
+
+   - Before checking the authentication, and in particular before fetching a
+     key, the receiver SHOULD check whether the quoted message was possibly
+     recently sent via/to the originator of the error message.
+
+   - The receiver MUST limit the traffic to the control service to fetch keys
+     for verifying the authentication of an SCMP message.
+     At most one packet SHOULD be sent to fetch the key for a received SCMP
+     message. If this fails or is not possible (e.g. because there is no
+     existing TCP session to the control service), the message SHOULD be
+     silently dropped.
+
+Compatibility
+=============
+As SCMP authentication is a new addition, there will be a transition period during which receivers
+may accept SCMP error messages without authentication.
+
+Discussion
+==========
+
+Even though this proposal had been accepted, it later turned out that there is no strong consensus
+to implement this in all routers and end points. For now, this has been put on hold.
+Alternative options that can be considered:
+
+- No authentication, expand the heuristics that end hosts apply to detect fraudulent SCMP messages.
+- Authenticate only :ref:`external-interface-down` and :ref:`internal-connectivity-down` messages.
+  If we authenticate only the information about the link that is down (ISD, AS, interface IDs) and a coarse timestamp, we can reuse the same signed message body for link down notifications for some time period, e.g. a few seconds.
+  Protection against message replay is not a concern for these messages types, as receivers can naturally consider them less and less relevant the longer ago they were created.
+  This allows authenticating these message body with relatively slow asymmetric cryptographic signatures with the CP-PKI, removing the dependency on DRKey.
+  The signature would be part of the SCMP message format for these specific messages and thus no longer requires the SPAO header.
+  Overall, this approach is very similar to the "Path revocation" messages that we historically used to have.

doc/manuals/router.rst

@@ -137,7 +137,7 @@ considers the following options.
 
    .. option:: features.experimental_scmp_authentication = <bool> (Default: false)
 
-      Enable the :ref:`DRKey-based authentication of SCMPs <scmp-authentication>` in the
+      Enable the :doc:`DRKey-based authentication of SCMPs </dev/design/scmp-authentication>` in the
       router, which is **experimental** and currently **incomplete**.
 
       When enabled, the router inserts the :ref:`authenticator-option` for SCMP messages.

doc/protocols/scmp.rst

@@ -16,9 +16,13 @@ Introduction
 The SCION Control Message Protocol (SCMP) is analogous to the Internet Control
 Message Protocol (ICMP). It provides functionality for network diagnostics, such
 as ping and traceroute, and error messages that signal packet processing or
-network-layer problems. SCMP is an integral part of SCION, and as such, all
-messages and behavior required by this specification MUST be fully implemented
-by every SCION node.
+network-layer problems.
+SCMP is a helpful tool for network diagnostics and, in the case of
+:ref:`External Interface Down <external-interface-down>` and
+:ref:`Internal Connectivity Down <internal-connectivity-down>` messages, an optimization for end
+hosts to detect network failures more rapidly and fail-over to different paths.
+However, SCION nodes should not strictly rely on the availability of SCMP, as this protocol may not
+be supported by all devices and/or may be subject to rate limiting.
 
 General Format
 ==============
@@ -116,46 +120,6 @@ entire SCMP message, starting with the SCMP message type field, and prepended
 with a "pseudo-header" consisting of the SCION address header and the layer-4
 protocol type as defined in :ref:`pseudo-header-upper-layer-checksum`.
 
-.. _scmp-authentication:
-
-Authentication
---------------
-SCMP messages can be authenticated with a MAC based on a symmetric key
-established with the :ref:`DRKey infrastructure <drkey>`.
-The MAC is transported in the :ref:`authenticator-option` End-to-End extension
-header.
-
-The Authenticator MAC algorithm is AES-CMAC (identifier :code:`0`).
-
-SCMP error messages MUST always be authenticated.
-SCMP informational messages CAN optionally be authenticated; a response message
-MUST be authenticated if and only if the corresponding request message was
-authenticated.
-
-All DRKey keys used here are derived with :ref:`protocol identifier <drkey-protocol-identifiers>` :code:`SCMP`, decimal :code:`1`.
-
-SCMP messages from (and to) routers are authenticated with :ref:`AS-host keys <drkey-as-host>`.
-SCMP response messages from a router in AS :math:`D` to a node :math:`H_s` in
-AS :math:`S` are authenticated with the DRKey :math:`K_{D,S:H_s}`.
-SCMP requests (specifically, :ref:`traceroute-request`) processed by a router
-are authenticated with the same key.
-
-SCMP messages between two end-hosts are authenticated with :ref:`host-host keys <drkey-host-host>`.
-An SCMP response message from a node :math:`H_d` in AS :math:`D` to a node
-:math:`H_s` in AS :math:`S` is authenticated with the key
-:math:`K_{D:H_d,S:H_s}`.
-SCMP requests and data packets from :math:`H_s` to :math:`H_d` are
-authenticated with this same key.
-
-For packets addressed to a router directly (specifically for
-:ref:`echo-request` and :ref:`echo-reply`) it is treated like an end-host and
-the corresponding host-host keys are used.
-
-.. note::
-   Recall that :ref:`traceroute-request`\s are *not* addressed to the router.
-   Instead, the router processes the request if its router alert flag is set.
-
-
 Processing Rules
 ----------------
 
@@ -188,44 +152,6 @@ Implementations MUST respect the following rules when processing SCMP messages:
    #. A packet whose source address does not uniquely identify a single node.
       E.g., an IPv4 or IPv6 multicast address.
 
-#. Every SCMP error message MUST be authenticated.
-
-   Every SCMP informational reply message MUST be authenticated if and only if
-   the corresponding request was authenticated.
-
-   .. note::
-      Consequentially, an implementation without support for SCMP
-      authentication MUST never send SCMP error messages and MUST NOT reply to
-      authenticated SCMP informational request messages.
-
-#. When an SCMP message is received, the receiver SHOULD check the
-   authentication header.
-
-   - SCMP error messages without or with an invalid authentication header and
-     SCMP informational messages with an invalid authentication header SHOULD
-     be silently dropped.
-
-     .. note::
-        As SCMP authentication is a new addition, there will be a transition period
-        during which receivers may accept SCMP error messages without authentication.
-
-   - The receiver checks that the :ref:`DRKey identified by the SPI <spao-spi-drkey>`
-     is appropriate for the SCMP message type and code, as described above in
-     the :ref:`section Authentication <scmp-authentication>`.
-
-   - The receiver derives or fetches the relevant key for validation of the MAC.
-
-   - Before checking the authentication, and in particular before fetching a
-     key, the receiver SHOULD check whether the quoted message was possibly
-     recently sent via/to the originator of the error message.
-
-   - The receiver MUST limit the traffic to the control service to fetch keys
-     for verifying the authentication of an SCMP message.
-     At most one packet SHOULD be sent to fetch the key for a received SCMP
-     message. If this fails or is not possible (e.g. because there is no
-     existing TCP session to the control service), the message SHOULD be
-     silently dropped.
-
 
 SCMP Error Messages
 ===================
@@ -327,7 +253,7 @@ Packet Too Big
 | MTU          | The Maximum Transmission Unit of the next-hop link.           |
 +--------------+---------------------------------------------------------------+
 
-A **Packet Too Big** message MUST be originated by a router in response to a
+A **Packet Too Big** message SHOULD be originated by a router in response to a
 packet that cannot be forwarded because the packet is larger than the MTU of the
 outgoing link. The MTU value is set to the maximum size a SCION packet can have
 to still fit on the next-hop link, as the sender has no knowledge of the
@@ -457,7 +383,7 @@ External Interface Down
 | Interface ID | The interface ID of the external link with connectivity issue.|
 +--------------+---------------------------------------------------------------+
 
-A **External Interface Down** message MUST be originated by a router in response
+A **External Interface Down** message SHOULD be originated by a router in response
 to a packet that cannot be forwarded because the link to an external AS broken.
 The ISD and AS identifier are set to the ISD-AS of the originating router.
 The interface ID identifies the link of the originating AS that is down.
@@ -509,7 +435,7 @@ Internal Connectivity Down
 | Egress ID    | The interface ID of the egress link.                          |
 +--------------+---------------------------------------------------------------+
 
-A **Internal Connectivity Down** message MUST be originated by a router in
+A **Internal Connectivity Down** message SHOULD be originated by a router in
 response to a packet that cannot be forwarded inside the AS because because the
 connectivity between the ingress and egress routers is broken. The ISD and AS
 identifier are set to the ISD-AS of the originating router. The ingress
@@ -554,7 +480,7 @@ Echo Request
 | Data         | Variable length of arbitrary data                             |
 +--------------+---------------------------------------------------------------+
 
-Every node MUST implement a SCMP Echo responder function that receives Echo
+Every node SHOULD implement a SCMP Echo responder function that receives Echo
 Requests and originates corresponding Echo replies.
 
 .. _echo-reply:
@@ -588,7 +514,7 @@ Echo Reply
 | Data         | The data of the Echo Request                                  |
 +--------------+---------------------------------------------------------------+
 
-Every node MUST implement a SCMP Echo responder function that receives Echo
+Every node SHOULD implement a SCMP Echo responder function that receives Echo
 Requests and originates corresponding Echo replies.
 
 The data received in the SCMP Echo Request message MUST be returned entirely and
@@ -682,7 +608,7 @@ Traceroute Reply
 
 The border router is alerted of the Traceroute Request message through the
 ConsIngress or ConsEgress Router Alert flag in the hop field. When such a packet
-is received, the border router **MUST** reply with a Traceroute Reply message.
+is received, the border router SHOULD reply with a Traceroute Reply message.
 
 The identifier is set to the value of the Traceroute Request message. The ISD
 and AS identifiers are set to the ISD-AS of the originating border router.