i would like to implement the support (TODO ) for elided checksum of
compressed udp pakets on the gnrc.
Thanks for offering your help regarding this topic.
So therefore i have some questions:
Is someone already working on it?Is there any reason why this is not
implemented before?Are there some things i should definitely know? ( like
other work/PR's which have to be done first, or something else..)
I think there exist some reasons why this hasn't been implemented yet. While
decompressing packets with elided UDP checksum shouldn't be problematic,
for the compression itself some considerations has to be taken:
1.) RFC 6282, Section 4.3.2  specifies that the upper layer must authorize
the header compression module to elide the checksum. This is only possible
if tunneling (like IPsec Encapsulating Security Payload tunnel mode) or
another message integrity check (such as IPsec Authentication Header) is
present in the UDP payload. Since IPsec has not yet been implemented for
RIOT and to the best of my knowledge also no other tunneling or additional
integrity checking mechanism for UDP payloads exists in GNRC, there is
currently no immediate use case for UDP checksum elision. Also note that
this features is optional.
2.) Since 6LoWPAN and IPHC/NHC operates asynchronously to any upper layer, it
is not trivial to determine for which packets the checksum may be elided.
A node may communicate over UDP to several services or hosts concurrently,
where one connection may use something like IPsec and authorize the NHC
module to elide the checksum, while for another connection this should not
be allowed. I think with the current netapi calls (send and setting of
netopts) this is not easy to implement. Maybe other GNRC knowledgeable
persons such as Martine, Hauke, Cenk, Peter or Johann may have an idea.
RFC 6282 UDP checksum elision is a textbook example of a "cross-layer"
optimization that seems cheap to implement but has non-trivial
In order for an application to indicate that the UDP checksum can be
elided, or for an application to accept UDP datagrams that have had
their UDP checksum elided, this information really needs to be carried
down and up with each single datagram. There is no shortcut that would
allow making these decisions within the adaptation layer.
There actually is a significant use case for UDP checksum elision: DTLS.
A DTLS implementation should indicate that the UDP checksum can be
elided when sending datagrams if they have been integrity-protected by
DTLS -- not so for a few handshake packets. Similarly, a DTLS
implementation should accept datagrams where the UDP checksum has been
elided only if the content of that datagram is then subject to integrity
protection. When initiating the listening, it will need to signal that
it is prepared to accept UDP datagrams with that information attached
and that it is willing to deal with the information.
So to implement this, you have to:
-- implement it in the adaptation layer (RFC6282 implementation);
-- enhance the interfaces so a "checksum can be/has been elided" bit can
be carried around with a datagram;
-- implement that bit in DTLS, and create the necessary signaling;
-- make sure that unsuspecting applications (that don't know about that
bit) never see packets where the bit is set.
a little time has passed since I wrote the last mail about eliding
checksums of UDP packets for the GNRC network stack.
There are two general requirements:
We need to pass the information to elide the checksum of a packet
through the stack. The netapi is not able to pass additional
informations of a packet through the stack.
An application layer protocol that doesn’t allow to receive packets
where checksums have been elided, shouldn’t see any of them.
My proposed solution for the first problem is to insert a new member in
the gnrc_pktsnip_t structure. This member holds the information to
authorize a packet, which is a precondition to elide the UDP checksum.
This way we can carry around that information through the network stack,
for each packet.
My proposal for the second problem is quite similar:
Adding a new member to the gnrc_netreg_entry_t structure with the
information that the corresponding module (or application) is (not)
allowed to receive packets with elided checksums.
In addition one could probably use these newly introduced struct members
for different kinds of information, or just don’t compile it if it’s not
needed. Any ideas on that?
With my proposed solution one does not need to change the netapi but
enhance some functions to handle the additional information.
Looking forward to get some feedback from your side!