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Currently GTP packets are dropped if the next extension field is set to non-zero value, but this are valid GTP packets. TS 29.281 provides a longer header format, which is defined as struct gtp1_header_long. Such long header format is used if any of the S, PN, E flags is set. This long header is 4 bytes longer than struct gtp1_header, plus variable length (optional) extension headers. The next extension header field is zero is no extension header is provided. The extension header is composed of a length field which includes total number of 4 byte words including the extension header itself (1 byte), payload (variable length) and next type (1 byte). The extension header size and its payload is aligned to 4 bytes. A GTP packet might come with a chain extensions headers, which makes it slightly cumbersome to parse because the extension next header field comes at the end of the extension header, and there is a need to check if this field becomes zero to stop the extension header parser. Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.