The BTP port registry: the real count of C-ITS messages

Ask how many message types cooperative ITS has and the answer comes back as a list of nine acronyms. It is the list this blog has used, and it is the one most tutorials use. It describes the cooperative-safety core well and the standards corpus badly.

There is a more objective census, and it is a single table. A facilities-layer message cannot travel unless the transport layer knows where to deliver it, and the value that carries that information is a BTP port number. Registered ports are the closest thing the corpus has to a membership list.

What the registry is for

The Basic Transport Protocol sits between the facilities layer and GeoNetworking. On transmission it multiplexes messages down; on reception it demultiplexes packets back up to the entity that should handle them. The destination port field is what makes that routing possible.

TS 103 248 fixes the well-known values in one table, and gives everything else the dynamic range: facilities entities without an assigned number use 3 000 to 65 536. A well-known port is therefore a statement that a service is part of the standardised set, allocated centrally so two independent stacks agree without negotiating.

The whole table

Port	Facilities service	Message	Standard
2001	CA	CAM	TS 103 900
2002	DEN	DENM	TS 103 831
2003	RLT	MAPEM	TS 103 301
2004	TLM	SPATEM	TS 103 301
2005	SA	SAEM	TS 102 890-1
2006	IVI	IVIM	TS 103 301
2007	TLC	SREM	TS 103 301
2008	TLC	SSEM	TS 103 301
2009	CP	CPM	TS 103 324
2010	EVCSN POI	EVCSN POI message	TS 101 556-1
2011	TPG	TRM, TCM, VDRM, VDPM, EOFM	TS 101 556-2
2012	Charging	EV-RSR	TS 101 556-3
2013	GPC	RTCMEM	TS 103 301
2014	CTL	CTLM	TS 102 941
2015	CRL	CRLM	TS 102 941
2016	Secured certificate request	EC/AT request	TS 102 941
2017	MCD	MCDM	TS 103 152
2018	VA	VAM	TS 103 300-3
2019	IMZ	IMZM	TS 103 724
2020	PIS	PIM	TS 104 072
2021	P2P_CRL	CRLM	TS 103 601
2022	P2P_DCTL	CTLM	TS 103 601
2023	MRS	MR message	TS 103 759
2024	Peer-to-Peer Full CTL	CTLM	TS 103 601
2025	PAS	POIM-PA	TS 103 916
2026	AVM	MIM, MVM	TS 103 882

Twenty-six ports. Twenty-eight distinct message types once a message that travels on several ports is counted once: two ports carry more than one message, and two messages ride on more than one port.

A port is not a message

The intuitive mapping breaks in both directions, and both cases matter to anyone writing a dispatcher.

Port 2011 carries five, and port 2026 carries two. The tyre-pressure gauge service of TS 101 556-2 puts TRM, TCM, VDRM, VDPM and EOFM on the same number, so the port tells the receiver which service to hand the payload to and the payload identifies itself from there.

And one message type travels on three ports. A certificate trust list message, CTLM, appears at 2014 as the standard distribution defined in TS 102 941, at 2022 as peer-to-peer delta distribution and at 2024 as peer-to-peer full distribution, both from TS 103 601. The revocation list splits the same way across 2015 and 2021. The port encodes the distribution path, so the same content arrives with a different label depending on how it reached you.

The port that changed hands

The registry looks static. It is versioned, and at least one entry has moved.

In V2.4.1, published 2024-07, port 2020 read DS (DSM), allocated for a diagnosis, logging and status service. In V2.5.1, published 2026-01, the same number reads PIS (PIM), the parking information service of TS 104 072. The diagnosis service does not appear anywhere in the newer table, and the parking information service appears nowhere in the older one, which already carried the separate parking availability service on port 2025.

The practical consequence is small in scope and unpleasant in kind. A station implementing the 2024 table and a station implementing the 2026 table are both conformant, and they disagree about what arrives on port 2020. Nothing in the packet resolves the disagreement: the port number is the identifier, and the receiver decides what it means by consulting a table it fixed at build time.

What the census shows

Three things stand out once the whole list is in front of you.

Trust distribution takes seven ports. 2014 through 2016 for trust lists, revocation and certificate requests, then 2021, 2022 and 2024 for the peer-to-peer variants, plus 2023 for misbehaviour reports. A quarter of the registry carries credentials, and complaints about credentials. Road data takes the rest.

The long tail is real infrastructure. Charging reservations, tyre-pressure telemetry, interference management zones, multimedia content dissemination and automated vehicle marshalling from TS 103 882 each hold a number. None of them appears in a message-set tour, and all of them are as standardised as CAM.

The core is nine of twenty-eight. That ratio is the useful thing to carry away. Nine is the right answer to “what does a cooperative-safety deployment emit”, and the wrong answer to “what is a C-ITS message”.

Read it yourself

The registry is TS 103 248 V2.5.1, and the table is short enough to read in a sitting: clause 4 explains the port model and Table 1 holds every allocation. Three of the services in it have been covered here already: the parking message at 2025, the misbehaviour report at 2023 and the infrastructure services that share 2003, 2004, 2006, 2007, 2008 and 2013, mapped in the Release 1 to Release 2 standards map. For the nine of the core, the message set tour remains the place to start.