npep-187: More protocols support

Signed-off-by: Nadia Pinaeva <[email protected]>

Author: npinaeva

npeps/npep-187-ports-and-protocols.md

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+# NPEP-187: More protocols support
+
+* Issue: [#187](https://github.com/kubernetes-sigs/network-policy-api/issues/187)
+* Status: Provisional
+
+## TLDR
+
+Change existing `ports` selector to allow more protocols in the future.
+
+## Goals
+
+Change existing `ports` selector to allow more protocols in the future.
+
+## Non-Goals
+
+Adding new protocols to the API.
+
+## Introduction
+
+Currently only TCP, UDP, and SCTP protocols are supported, but there are more protocols (ICMP, ICMPv6 are the most popular requests) 
+that may be useful.
+
+## User-Stories/Use-Cases
+
+Story 1: Protocols extension
+
+As a cluster admin I want to be able to use protocols other than TCP, UDP, and SCTP in my ANP.
+For example, I want to only allow ICMP connections to implement health monitoring and deny everything else.
+
+## API
+
+As we only support port-based protocols (TCP, UDP, and SCTP), current protocol matching is expressed as
+```yaml
+ports:
+  - portNumber:
+      protocol: TCP
+      port: 80
+  - portRange:
+      protocol: UDP
+      start: 1
+      end: 100
+```
+To enable more protocols support in the future, we need to change the high-level field name from `port` to `protocol` to be more generic.
+
+In the `protocols` filter design we decided to start from the YAML that we want (i.e. YAML that is easy to write and understand).
+We want to make popular filters (TCP and UDN ports) easy and less popular filters (like ICMP and named ports) possible.
+This brings us to the following design:
+
+```yaml
+protocols:
+  - protocol: TCP
+    port: 8080
+  - protocol: UDP
+    portRange: 
+      start: 8080
+      end: 9090 
+  - namedPort: http
+    # protocol: "" # must be empty string: defaulted / omitempty
+```
+
+with the validation like so:
+- empty `protocols` list is not allowed
+- at least 1 field for each `protocol` element must be set
+- if `namedPort` is set, `protocol` must be unset
+- if `protocol` is TCP/UDP/SCTP, `port: int` or `portRange: {start: int, end: int}` must be set, but not at the same time
+
+Using ICMP as a potential example for a pure protocol match (`protocol: ICMP`) or protocol with extra fields (e.g. ICMP type and code),
+a future extension could look like this:
+
+```yaml
+protocols:
+  # option 1:  A little inconsistent with port match as it has an extra `icmp` field instead of flat parameters
+  - protocol: ICMP
+    icmp:
+      type: 7
+      code: 3
+  # option 2: Flat parameters, may conflict with the future protocol that also have type or code
+  - protocol: ICMP
+    type: 7
+    code: 3
+  # option 3: Same as option 1, but without a discriminator (protocol)
+  - icmp:
+      type: 7
+      code: 3
+```
+
+## Conformance Details
+
+(This section describes the names to be used for the feature or
+features in conformance tests and profiles.
+
+These should be `CamelCase` names that specify the feature as
+precisely as possible, and are particularly important for
+Extended features, since they may be surfaced to users.)
+
+## Alternatives
+
+(List other design alternatives and why we did not go in that
+direction)
+
+## References
+
+(Add any additional document links. Again, we should try to avoid
+too much content not in version control to avoid broken links)