[socket] Support for Payload Protocol Identifier for SCTP sockets in standard socket library

Issueで説明されている標準のエントリーポイント socket.socket(..., IPPROTO_SCTP) と send(bytes, flags) から始め、次に参照されている RFC 9260 の PPID 要件を読みます。標準のソケット API が PPID を受け付けるにはどうすればよいかを判断し、その結果生成される SCTP DATA チャンクに PPID が含まれていることを、言及されている NGAP 値も含めて検証します。Issue ではソースファイルやテストは特定されていません。

Feature or enhancement

Currently is there a way to specify the payload protocol identifier for an SCTP socket created using the socket library?

For example, the SCTP socket object created as sock_amf = socket.socket(socket.AF_INET, socket.SOCK_STREAM, socket.IPPROTO_SCTP) exposes sock_amf.send(bytes, flags) but has no way to mention the payload protocol identifier as mentioned in https://www.rfc-editor.org/rfc/rfc9260.html#section-3.3.1-3.20.1

Payload Protocol Identifier: 32 bits (unsigned integer)
This value represents an application (or upper layer) specified protocol identifier. This value is passed to SCTP by its upper layer and sent to its peer. This identifier is not used by SCTP but can be used by certain network entities, as well as by the peer application, to identify the type of information being carried in this DATA chunk. This field MUST be sent even in fragmented DATA chunks (to make sure it is available for agents in the middle of the network). Note that this field is not touched by an SCTP implementation; the upper layer is responsible for the host to network byte order conversion of this field.
The value 0 indicates that no application identifier is specified by the upper layer for this payload data.

At present the default value of 0 is being set when the data chunk is formed.

Sample packet capture for one such instance.

Pitch

If such an option was there, we could directly implement protocols that require specific ppid with the standard socket library (without having to depend on 3rd party bindings of SCTP C library).
Say for example the NGAP protocol used in 5G which uses

SCTP (IETF RFC 4960 [2]) shall be supported as the transport layer of NG-C signaling bearer. The Payload Protocol
Identifier (ppid) assigned by IANA to be used by SCTP for the application layer protocol NGAP is 60, and 66 for
DTLS over SCTP (IETF RFC 6083 [8])