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hi all, My captured packet of GTP-U ,which makes up with inner ip ,inner udp and outer ip ,outer udp.As we know, the outer ip layer may be fragmented. My question : is the inner ip packet fragmented in some case ?

thanks.

asked 16 Jun '16, 05:26

bigapple119's gravatar image

bigapple119
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edited 16 Jun '16, 15:04

Guy%20Harris's gravatar image

Guy Harris ♦♦
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As stated in 3GPP TS 29.281 version 13.1.0 Release 13

4.2.4  Ingress GTP tunnel (GTPv1-U sending endpoint)
....
Fragmentation policy of the inner datagram is implementation dependent but shall interwork with IETF RFC 791 for inner IPv4 datagrams and IETF RFC 2460 for inner IPv6 packets.

So I would assume the answer to be yes.

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answered 16 Jun '16, 06:25

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Jaap ♦
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Thanks firstly. I have seen the words in 3GPP TS 29.281 version 13.1.0 Release 13.Therefore,I have to consider the fragmented inner ip packet,when i dissect every layer of GTP-U.However,the inner-ip-packet's fragment flag are all not set, which GTP-U packets are displayed in wireshark.

(17 Jun '16, 06:04) bigapple119

While possible, there are specific mechanisms in place to avoid it.

An inner IP MTU value is communicated down to the phone by the network which it is expected to honour, with the catch being that 3GPP accounts for tethered devices by imposing a requirement that the network needs to be able to support 1500 byte MTU for the "PDP PDU" if it becomes necessary (where, when transferred into GTP, fragmentation of the inner IP packet would then become necessary).

The wording of TS 23.060 is:

When the MT and the TE are separated, e.g. a dongle based MS, it is not always possible to set the MTU value by
means of information provided by the network. The network shall have the capability of transferring N-PDUs
containing PDP PDUs, where the PDP PDUs are of 1500 octets, between the MS and GGSN/P-GW.

There, "N-PDU" is a single GTP-transmitted packet between GTP tunnel endpoints, and "PDP PDU" is the end user's IP packet transmitted over the air.

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answered 16 Jun '16, 15:28

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Quadratic
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Yes,i found the words in the TS 23.060 of 2013.09. GTP-U is between nNodeB and S-GW,what you say below is equally applicable to 4G-LTE? Actually,i always think there are some mechanisms avoiding ip-fragment ,because Reassembling twice is inefficient. thanks a lot.

(17 Jun '16, 06:18) bigapple119

It's applicable to GTPv1-U in general, so yes.

At the control level from PGW down to the UE (whether for LTE, untrusted Wifi into the EPC, etc.), there is the concept of PCO options, where inner IP MTU can be communicated, but there are circumstances (such as dongles or tethering) where those mechanisms aren't fool-proof.

(17 Jun '16, 16:31) Quadratic
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question asked: 16 Jun '16, 05:26

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last updated: 17 Jun '16, 16:31

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