Liaison statement
Reply LS on port allocation for the W1 interface

1. Overall Description:
3GPP CT WG4 thanks IESG for their Reply LS port allocation for the W1 interface
(C4-210144).

On Behalf of 3GPP, 3GPP CT WG4 warmly thanks IESG for their action that has
permitted the prompt assignment by IANA of an SCTP port to the W1 interface.

3GPP CT WG4 also thanks IESG for the guidance and clarifications provided in
the Reply LS and these points have been discussed during the last January
meeting. 3GPP CT WG4 would like to share with IESG some information on the
progress of the study on the alternative solutions to port assignment and seek
for additional clarifications on some of the statements given in the Reply LS.

3GPP CT WG4 has further progressed the study in their Technical Report (TR
29.835) and has initiated the evaluation of the solutions that will be
recommended in 3GPP. These solutions include:

•       OAM configuration
•       (m)DNS-based solutions
•       SCTP port multiplexing
•       (D)TLS based solution based ALPN and/or SNI
•       3GPP specific solutions

The latest version of the TR 29.835 (v0.4.0) can be found at:
https://www.3gpp.org/ftp/Specs/archive/29_series/29.835/29835-040.zip

The conclusion of the study is expected for June 2021. Respective guidelines
(TR 29.941) should be finalized by September 2021. 3GPP CT WG4 still welcome
comments from IETF (especially IETF TSV WG) that could be taken into account in
the conclusion of the study.

Regarding the guidance provided by IESG, 3GPP CT WG4 would like further
clarifications on this specific statement:

We like to point to the rules and policies documented in BCP 165 (RFC6335 and
RFC7605) apply to any future request. The one that is most relevant to 3GPP for
any future request is the expectation that assignment of only a single port
number per transport  protocol will be sufficient across all the future
external services, i.e. all 3GPP interfaces that 3GPP defines that use that
transport protocol.

3GPP understands that it could be possible to assign to 3GPP a port per
transport protocol (UDP, TCP, SCTP, DCCP) that will be used for service port
negotiation/discovery for all the future internal interfaces defined by 3GPP,
avoiding the need for a systematic IANA port assignment for interfaces used
only inside the 3GPP system. This possibility will be seriously considered in
the technical report when evaluating the retained solutions for service/port
discovery.

However, besides the assignment of transport protocol port(s) that could be
requested by 3GPP for the deployment of specific service discovery
mechanism(s), it is also the 3GPP understanding that this statement cannot
prohibit 3GPP to request in the future a port assignment for a new service
application for which none of the port assignment alternatives would be
applicable.

The main objective of the ongoing work in 3GPP is obviously to limit as much as
possible the need of assigned ports to identify a 3GPP application.
Nevertheless, there might still be valid cases for new IANA port assignment
request applications from 3GPP as long as the recommendations and policies of
the BCP 165 are carefully followed. A typical example would be the development
of a new roaming interface between mobile operator networks that would span
across multiple domains and intermediate devices such as firewalls or security
gateways filtering services, application that could not be supported by any of
the existing solutions used in roaming. It is clearly understood that a port
assignment request for a valid use case does not imply an automatic assignment
by IANA and the decision to grant this assignment will anyhow depend on the
output of IANA review based on one of the policies described in RFC 5226.

IESG is kindly ask to confirm that the current 3GPP understanding is correct.
If it is not, it is important to clarify why the 3GPP reasoning would be wrong
and to point to specific part(s) of the BCP 165 that could help 3GPP to
understand the misinterpretation.

2. Actions:
To IESG.
ACTION:
        IESG is kindly asked to confirm the 3GPP understanding regarding the
        possible future port assignment request from 3GPP and provide any
        additional information that could help to clarify the issue. IESG is
        kindly invited to provide comments on the latest version of TR.29.835
        (v0.4.0).

3. Date of Next CT4 Meetings:
3GPP TSG CT4#103e               04/2021 E-Meeting
3GPP TSG CT4#104e               05/2021 E-Meeting