WARNING: erroneous pipeline: could not link customsrc0 to mpegtsmux0 - gstreamer

Question

I cannot create a pipeline with gstreamer and I don't know how I can debug it further.
gst-launch-1.0 --gst-debug=GST_CAPS:4 -v -e customsrc num-buffers=1000 ! video/x-h264,width=600,height=600,framerate=1/12,stream-format=byte-stream ! mpegtsmux ! udpsink host=10.92.7.2 port=5000
WARNING: erroneous pipeline: could not link customsrc0 to mpegtsmux0
The capabilities of customsrc and mpegtsmux are matching. But obviously something is missing.
customsrc
Pad Templates:
SRC template: 'src'
Availability: Always
Capabilities:
video/x-h264
width: [ 1, 2147483647 ]
height: [ 1, 2147483647 ]
framerate: [ 1/2147483647, 2147483647/1 ]
stream-format: avc
alignment: au
mpegtsmux
Pad Templates:
SINK template: 'sink_%d'
Availability: On request
Has request_new_pad() function: 0x76beca8c
Capabilities:
video/x-h264
stream-format: byte-stream
alignment: { au, nal }
What else can I do to figure out the mismatch?

Answer 1

Caps are for filtering and defining how the pipeline runs--they don't cause any transformation by themselves. For example, if you have two elements with these caps on their source and sink pads:
video/x-h264
stream-format: byte-stream
alignment: { au, nal }
video/x-h264
stream-format: byte-stream
alignment: { au, nal }
And you place this caps filter between them:
video/x-h264,alignment=nal
You'll cause the pipeline to use nal alignment there. If your elements have these caps on their pads:
video/x-h264
stream-format: avc
alignment: { au, nal }
video/x-h264
stream-format: byte-stream
alignment: { au, nal }
You need to add an element that will convert video/x-h264,stream-format=avc into video/x-h264,stream-format=byte-stream. h264parse will do this because it takes any video/x-h264 content on its sink pad, and outputs whatever stream-format and alignments are needed for its downstream source:
SRC template: 'src'
Availability: Always
Capabilities:
video/x-h264
parsed: true
stream-format: { avc, avc3, byte-stream }
alignment: { au, nal }
SINK template: 'sink'
Availability: Always
Capabilities:
video/x-h264