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NGAP AllowedNSSAI IE not decoded correctly.

asked 2018-09-12 06:06:48 +0000

updated 2018-09-12 06:35:00 +0000

The AllowedNSSAI IE in NGAP message Initial Context Setup Request (3GPP TS 38413, vf00) doesn't seem to be decoded correctly.

The SD IE in S-NSSAI is optional. So, the optional bit field for presence has to be set/checked while encoding/decoding. However, wireshark seems to expect this IE to be mandatory. What version of the spec is wireshark compliant to?

Below I have the hex for the Initial Context Setup Message with only Allowed-NSSAI IE encoded.

The below hex is decoded (in this the bit field under S-NSSAI is not set)

000E000C000001000040050001010203

The below hex fails to decode (gives malformed packet error)

000E000C000001000040050201010203

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answered 2018-09-12 10:47:53 +0000

Anders gravatar image

updated 2018-09-12 11:28:11 +0000

The ASN1 code of TS 38.413 15.0.0 which is what Wireshark is based on has:

S-NSSAI ::= SEQUENCE {
sST             SST,
sD                  SD,
iE-Extensions       ProtocolExtensionContainer { { S-NSSAI-ExtIEs} }    OPTIONAL,
...

}

e.g SD is NOT OPTIONAL. It is my understanding that this will be changed in the next version of the specification but that is not yet available.

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Asked: 2018-09-12 06:06:48 +0000

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Last updated: Sep 12