Cellular Network Infrastructure » Core Network (CN) » OsmoUPF

For tunmap, we have a netfilter rule that so far matches on UDP traffic with the octets [4..7] matching the GTP TEID.

To avoid non-GTP-U packets to match this rule inadvertently, consider:

(1) add to the rule a match on the local GTP-U port 2152 -- this port number is fixed in the GTP specification.

(2) add matches on distinct GTP-U packet header traits. Some ideas, in pseudocode, from looking at a trace in wireshark:

• GTP v1:

  udp[0] & 0b11100000 == 0b00100000

• protocol = GTP:

  udp[0] & 0b00010000 == 0b00010000

• length:

  uint16(udp[2..3]) == udp.payload_len - 8

(1) probably suffices to eliminate all possible false positives. The GTP port number is fixed, and it is reasonable to assume that all traffic arriving on it is at least intended to be GTP-U.

(2) is probably more of a way to discard invalid packets -- not sure if we want to do that at all, it would probably be better to leave classification of packets to the remote receiver. It is unlikely for an admin to notice sporadic invalid GTP when these packets are silently dropped halfway. Seems much better to get an error log on the receiving side.