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[Malformed Packet: PPP PAP]

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I have encountered a problem about malformed packet in PCO of LTE Attach accept The Data after Password Authentication Protocol (0xc023) in PCO cannot decode successfully.

here is the raw data of attach accept 074202e0060002f44000010064520bc1010912046c69766508766f6461666f6e6503636f6d0501c0a801015d0110300e23920a939640687400407f000000320381272e80802110030100108106c0a801028306c0a80103000d04c0a80102000d04c0a8010300100205dcc0230402010004500bf602f440800100000000011302f44000002305f40000000017e059e04a0302f410640103

Password Authentication Protocol is: c0230402010004 Protocol or Container ID: Password Authentication Protocol (0xc023) Length: 0x04 (4) PPP Password Authentication Protocol Code: Authenticate-Ack (2) Identifier: 1 Length: 4

Please tell me why it have problem. Thanks

Jim Tan's avatar
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Jim Tan
asked 2018-11-14 09:22:16 +0000, updated 2018-11-14 09:22:50 +0000
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Can you share a capture file that contains the issue using a publicly accessible link?

grahamb's avatar grahamb (2018-11-14 10:53:11 +0000) edit
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Hi,

you have several issues:

  • fist of all the MME should not send a PAP protocol id in the Attach Accept, it's only supposed to be sent by the UE and no authenticate-ack is required in LTE as far as I know
  • if I read RFC 1334 correctly, the authenticate_ack should have a msg-length field but it is not present, thus the malformed error
Pascal Quantin's avatar
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Pascal Quantin
answered 2018-11-14 11:45:08 +0000
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Thanks for your reply. after i check RFC 1334. in 2.2. Packet Format, it mention that Data The Data field is zero or more octets. The format of the Data field is determined by the Code field. and in 2.2.1 and 2.2.2, it is impossible that packet's Data field is zero in these three code(request, ack ,Nak). is there any chance that PAP packet's data field is zero?

Thanks.

Jim Tan's avatar Jim Tan (2018-11-15 01:08:42 +0000) edit
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Hi Jim,

Like you I do not see how the data field could be empty with the current specification.

Pascal Quantin's avatar Pascal Quantin (2018-11-15 06:32:46 +0000) edit
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