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Are sequence numbers commonly used in GTP messages on the Iuh interface?

+5 votes

Sequence numbers are required in a handful of message types (handover, etc.), but I need to know if they are usually present even when they are optional in the message definitions. Can you point me to a publically available pcap trace of live traffic?

posted Feb 18, 2014 by anonymous

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AFAIK sequence number are mandatory in GTPv2 may be the reason of making mandatory was because of it was present in all the cases in GTPv1 also.
Sharing the PCAP over a public platform is not feasible.

1 Answer

+1 vote

The sequence number field is used for the reliable delivery of all messages.

It is not mandatory in GTP' and the reason is it has one octet sub field in header named as N-PDU number. which serves the same purpose if SN flag is not set.

In case of GTPv2 sequence number field is mandatory and there is nothing like N-PDU number. It contains from Octet 9 to Octet 11 of header part.

So in case of Iuh interface, it should be present in all messages if it's using GTPv2.

answer Feb 19, 2014 by Hiteshwar Thakur
addition to that Sequence number flag (S): This flag indicates the presence of a meaningful value of the Sequence Number field.
When it is set to '0', the Sequence Number field either is not present or, if present, shall not be interpreted. When
it is set to '1', the Sequence Number field is present, and shall be interpreted, as described below in this section.
For the Echo Request, Echo Response, Error Indication and Supported Extension Headers Notification
messages, the S flag shall be set to '1'. Since the use of Sequence Numbers is optional for G-PDUs, the PGW,
SGW, ePDG and eNodeB should set the flag to '0'. However, when a G-PDU (T-PDU+header) is being relayed
by the Indirect Data Forwarding for Inter RAT HO procedure, then if the received G-PDU has the S flag set to
'1', then the relaying entity shall set S flag to '1' and forward the G-PDU (T-PDU+header). In an End marker
message the S flag shall be set to '0'.
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