at RRC we don't store CSI-MeasConfig information so in some scenarios (see #1063) we may not have the full pictures to understand if what RRC received is a valid configuration
we verify at L2 instead after updating CSI-MeasConfig and if it fails we send a message to RRC
Unify SIB container handling by introducing a typed SIB enum and storing
SIB payloads as byte_array_t across common NR types, F1AP helpers, MAC,
and RRC DU setup code.
Changes:
- Include byte_array support in nr_common.h, add nr_sib_type_t (NR_SIB_1–NR_SIB_21),
and change nr_SIBs_t to hold only nr_sib_type_t SIB_type.
- Switch f1ap_sib_msg_t in f1ap_messages_types.h to use a byte_array_t SI_container
instead of raw pointer and length fields.
- Update F1AP encode/decode, equality, copy, and free helpers in
f1ap_interface_management.c to work on SI_container.buf / SI_container.len and
use eq_byte_array, copy_byte_array, and free_byte_array.
- Adapt F1AP tests in f1ap_lib_test.c to build and inspect SIB containers via
SI_container.buf and SI_container.len.
- Replace magic SIB numbers with nr_sib_type_t values in gnb_config.c
(get_sys_info and fill_du_sibs) to validate and configure DU SIBs.
- Rework nr_mac_configure_other_sib in NR_MAC_gNB/config.c to decode SIB2
from a byte_array_t container, fix freeing by releasing the decoded sib2 on
failure, and use NR_SIB_2 / NR_SIB_19 for CU/DU SIB selection.
- Add an add_si_msg helper in rrc_gNB_du.c and refactor the SIB2 branch of
rrc_gNB_process_f1_setup_req to populate cell->SI_msg from encoded local
byte array while iterating SIBs with FOR_EACH_SEQ_ARR.
- all RAN code, CI code, configuration files, dockerfiles, in CSSL v1.0
- all deployment code (openshift, charts, ancillary files like shell
scripts), in MIT
- documentation in CC-BY-4.0
- exceptions might apply and are listed in NOTICE
- there is a new LICENSES folder with all licenses
- CONTRIBUTIONS.md has been updated accordingly
For automated changes based on OAI PL v1.1:
perl -i~ -0pe 's/\/\*.*Licensed to the OpenAirInterface.*openairinterface.org\n#?/\/*\n * SPDX-License-Identifier: LicenseRef-CSSL-1.0\n/s' **/*.{c,h,cpp}
perl -i~ -0pe 's/\/\*.*Licensed to the OpenAirInterface.*openairinterface.org\n#?/\/*\n * SPDX-License-Identifier: LicenseRef-CSSL-1.0\n/s' **/*.ts
perl -i~ -0pe 's/<!--.*Licensed to the OpenAirInterface.*openairinterface.org\n.*-->/<!-- SPDX-License-Identifier: LicenseRef-CSSL-1.0 -->/s' **/*.xml
The rest (cmake, files with missing license, cmake) manually.
Remove the \file directive, as it is always superfluous because the
current file is implicit [1]:
> If the file name is omitted (i.e. the line after \file is left blank)
> then the documentation block that contains the \file command will belong
> to the file it is located in.
Author names and e-mails are not relevant for us: it can always be
inferred from git blame, and is often outdated.
Eurecom code has been contributed and was under OAI PL v1.0/v1.1.
For the cpack package contact: put generic email address that is
independent of an individual and that will remain reachable.
[1] https://www.doxygen.nl/manual/commands.html#cmdfile
- Add Xn Setup Response message type definitions
- Implement encoder and decoder for Xn Setup Response
Xn Setup Response (3GPP TS 38.423v16.2.0 §9.1.3.2)
- Global NG-RAN Node ID (M)
- TAI Support List (M)
- Add equality check and memory management helpers
- Extend XNAP library unit tests to cover Setup Response
- Add initial XnAP library structure under openair2/XNAP/lib
- Implement common XnAP helper utilities (xnap_lib_common)
- Add gNB interface management module for Xn Setup procedures
- Introduce XnAP message type definitions in xnap_messages_types.h
- Integrate XnAP library into build system (CMakeLists.txt)
- Add standalone XnAP library unit test (xnap_lib_test)
- Implement encoder, decoder + test for Xn Setup Request
Xn Setup Request (3GPP TS 38.423v16.2.0 §9.1.3.1)
- Global NG-RAN Node ID (M)
- TAI Support List (M)
- AMF Region Information (M)
- Extend conversions.h with MACRO_BIT_STRING_TO_GNB_ID utility
Co-authored-by: Sreeshma Shiv <sreeshmau@iisc.ac.in>
fix: issue 911 - OAI gNB hangs when AMF connection goes down
Once the AMF is down, based on the AMF UE NGAP ID and RAN UE NGAP ID,
the UE associated with the AMF is found
All the contexts related to the UE are released and the timer is started
in the NGAP to look up for the SCTP association. The AMF-gNB reconnection
is made if an SCTP connection is found, also the registration of multiple
UE is triggered
These actions are based on the reference from 4.2.6 AN release Specs
TS 123 502 V16.7.0 (2021-01)
When the NG-AP signalling connection is lost due to (R)AN or AMF failure,
the AN release is performed locally by the AMF or the (R)AN as described
in the procedure flow below without using or relying on any of the signalling
shown between (R)AN and AMF. The AN release causes all UP connections of the UE
to be deactivated.
closes#911
- Extended fix to support multiple UEs instead of single UE
- Actions now trigger upon AMF reconnection
Per 3GPP TS 123 502 V16.7.0 (2021-01) Section 4.2.6 (AN Release):
When NG-AP signalling connection is lost due to (R)AN or AMF failure,
AN release is performed locally by AMF or (R)AN without relying on
signalling between (R)AN and AMF. This deactivates all UP connections.
RRC now builds F1AP paging messages directly by calling F1AP/direct
callbacks and aligning the CU/DU intergace with the F1AP design in the CU.
Messages are forwarded based on TAI-based cell matching across DUs.
- Remove F1AP_PAGING_IND message def and f1ap_paging_ind_t.
RRC no longer sends ITTI F1AP_PAGING_IND; it builds f1ap_paging_t
and passes it to the DU path.
- rrc_gNB_process_PAGING_IND: new signature (rrc, instance, msg)
with const where appropriate; call in rrc_gnb_task() updated.
- NGAP handler logic: For each TAI, match (PLMN + TAC) to
rrc->configuration; collect matching TAIs. Build one f1ap_paging_t
(5G-S-TMSI via nr_construct_5g_s_tmsi(), identity type
F1AP_PAGING_IDENTITY_CN_UE, optional drx/priority/origin copied
from ngap_paging_ind_t), call rrc_send_paging_to_dus(), then
free_f1ap_paging(). Remove the previous AssertFatal stub, CC loop,
NODE_IS_CU branch, and UE lookup logic: it is now handled by
F1AP/direct CU callbacks.
- rrc_gNB_du: Add rrc_send_paging_to_dus(rrc, tais, n_tai, msg).
Iterate DUs with RB_FOREACH; for each DU, fill msg->cells from
du->cells whose (PLMN + TAC) matches any requested TAI, set
n_cells, then call rrc->mac_rrc.paging_transfer(du->assoc_id,
msg). Skip DUs with no matching cells. Prevents duplicate paging
when multiple TAIs map to the same PLMN.
- Add F1AP_PAGING message type and macro; F1AP defines Paging, not
Paging Indication. CU task now handles F1AP_PAGING instead of
F1AP_PAGING_IND.
- In CU_send_Paging(): take const f1ap_paging_t *; replace inline
ASN.1 encoding with encode_f1ap_paging(); refine memory management;
clean up in the process
Note: F1AP_PAGING_IND definitions left in place and still used in the
RRC callback - to be removed in a follow-up commit.
Add F1AP Paging library and new message type to follow the existing F1AP
library pattern and to support multi-cell paging and both RAN UE (I-RNTI)
and CN UE (5G-S-TMSI) paging identities at the library level per TS 38.473.
* New library (f1ap_paging.c/h, f1ap_lib_includes.h, CMakeLists.txt)
- Implement encode/decode/copy/eq/free for F1AP Paging (procedure 9.2.6).
- Add ASN.1 includes for Paging, PagingIdentity, CN/RAN UE identities.
- Enables unit testing of Paging enc/dec in f1ap_lib_test.c.
* New types in f1ap_messages_types.h (existing f1ap_paging_ind_t unchanged)
- f1ap_paging_t: UE Identity Index, paging identity (union + type enum),
optional paging_drx, paging_priority, paging_origin, and paging_cells[].
- f1ap_paging_cell_item_t, f1ap_paging_identity_type_t, f1ap_paging_identity_t
for multi-cell list and RAN/CN identity discrimination.
- Paging Origin IE is optional per F1AP 8.7.1.2.
Tests: test_f1ap_paging() roundtrip and copy for CN UE (5G-S-TMSI) and
RAN UE (I-RNTI) cases, with optional IEs.
- Refactor ngap_paging_ind_t (openair2/COMMON/ngap_messages_types.h):
- optional paging_drx and paging_priority as pointers
- TAI list as tai_list[] and n_tai
- add optional IEs: Paging Origin, Paging Attempt Information,
UE Radio Capability for Paging.
- Introduce nr_tai_t, ngap_paging_attempt_info_t, ngap_paging_origin_t;
- generate paging enums via FOREACH macros.
- Add NGAP Paging encode/decode/eq/free in openair3/NGAP/ngap_gNB_paging.c
(encode_ng_paging, decode_ng_paging, eq_ng_paging, free_ng_paging).
- Replace inline Paging decode in ngap_gNB_handle_paging() with
decode_ng_paging() and add error handling; add eq_ngap_plmn() in
ngap_common.
- Update rrc_gNB_NGAP.c to use tai_list/n_tai and optional paging_drx
when forwarding paging to F1AP.
- Add ASN.1 includes for Paging in ngap_msg_includes.h; register
ngap_gNB_paging.c in root CMakeLists.txt.
- Add NGAP unit test (openair3/NGAP/tests/ngap_lib_test.c): encode/decode
roundtrip for Paging; tests subdirectory and executable under
ENABLE_TESTS in openair3/NGAP/CMakeLists.txt.
TO_ENUM/TO_TEXT are already used in NAS, and can be adopted in NGAP.
To avoid duplication, the following changes are introduced:
* Move text_info_t, TO_ENUM, TO_TEXT to common/utils/utils.h
* Include utils.h include to ngap_messages_types.h, fgs_nas_utils.h, and fgmm_lib.h
* RRC establishment cause: refactor to FOREACH_RRC_ESTABLISHMENT_CAUSE macro
* Cause Group: refactor to FOREACH_CAUSE_GROUP macro
* Radio Network Cause: refactor to FOREACH_CAUSE_RADIO_NETWORK macro
* Protocol Cause: refactor to FOREACH_CAUSE_PROTOCOL macro
This change is preparatory to the introduction the paging lib with unit
tests, with the goal of reducing the inter-dependency between concurrent
NGAP libs.
There is no such thing as NG Register message, the naming is
inaccurate, however the goal of this commit is only to clarify that
and major refactoring is not yet carried out.
Cleanup Cmake: remove some double compilation
Harmonize some cmake code to avoid double compilation, notably around
polar and smallblock. In develop, we just add their sources to other
targets, whereas now we define STATIC libraries (further changed to
shared library in !3856) to avoid double compilation.
With this command line:
cmake .. -GNinja -DENABLE_TESTS=ON && ninja
shows that before we compile 12631 files, where after it is 12583
there is additional supporting cleanup (move code, remove headers,
types, ...).
there is more harmonization to do, I limited myself to some easy wins.
For instance, we do the same with PDCP/SDAP, and this should be
refactored into STATIC libraries as well.
RRC: Multi-cell handling architecture
This MR implements a foundational multi-cell handling architecture for
RRC by separating DU management from cell management and establishing a
hierarchical cell organization within each DU container. The changes
enable proper multi-cell support where each DU can serve multiple cells,
addressing the previous limitation where DU and cell management were
tightly coupled with the assumption of one cell per DU.
Key Changes:
- Data Structure Separation: Separated for DU containers (DU-specific
info) from cell containers (cell-specific data)
- Global DU and Cell Trees: Cells are now stored in global RB trees in
RRC, similarly to DUs
- Du-specific cell seq_arr: within each DU container (du->cells) is
stored a seq_arr of cells belonging to the DU
- UE Cell Association Framework: Introduced serving_cells array to track
PCell and SCells for each UE with servCellIndex indexing
- Cell Management Library: Extracted cell management functions into
rrc_cell_management.c/h for better testability and maintainability
Other relevant changes:
- F1AP Refactoring: cell list definitions, heap allocation to prevent
stack overflow
- 5G-S-TMSI Helpers: Common helper functions for 3GPP TS 23.003
compliant construction
- Integration Tests: Comprehensive test suite for DU lookup, cell
lookup, and UE cell association
- Cell Duplication Checks: Validation in F1 DU configuration update to
prevent duplicate cells
- Documentation: Updated RRC developer documentation with multi-cell
architecture details
Validated with RFsim in re-establishment and handover scenarios.
openair2/COMMON/rrc_messages_types.h has mostly 4G types. The only 5G
(gNB) RRC type is gNB_RrcConfigurationReq, which we can move to a header
in the corresponding directory (where it is used).
This allows to separate 5G gNB RRC module from the 4G types in
rrc_messages_types.h in the next commit.
This commit fixes two related issues:
1. SDAP Header Mapping Bug (cucp_cuup_handler.c):
The original code incorrectly mapped SDAP header configuration from
internal E1AP representation to ASN.1 enum values. The internal
representation (`bearer_context_sdap_config_t`) uses true=present/false=absent,
while ASN.1 enum (`E1AP_SDAP_Configuration_t`) uses
0=present/1=absent. The fix correctly inverts the mapping:
- Internal true (1) → ASN.1 0 (present)
- Internal false (0) → ASN.1 1 (absent)
The commit also clarifies internal representation in the stack.
2. Test Overhead Adjustment (nr-cuup-load-test.c):
The change in (1) led to a failure in nr_cuup_functional_test
with assertion: "size % 4 == 0" in recv_ng()
- Changed UL packet overhead from 3 bytes to 2 bytes in sender_thread()
- Before the SDAP fix, the test incorrectly accounted for 3 bytes:
* 2 bytes PDCP header (correct)
* 1 byte SDAP header (incorrectly included)
- After the SDAP fix, SDAP headers are correctly set to absent, so
only 2 bytes (PDCP header) are needed
This ensures the test sends packets with the correct header size,
matching what CU-UP expects after the SDAP configuration fix.
The load test does not enable SDAP headers, therefore the original 3-byte
overhead was possibly a workaround for the SDAP header bug.
With the bug fixed, the test correctly uses 2-byte overhead (PDCP header).
This commit refactors the PDU Session Resource Modify Response message
structures to improve naming, documentation, and compliance with 3GPP TS 38.413 9.2.1.6.
Changes in NGAP:
- Rename qos_flow_tobe_modified_t to qos_flow_addmod_response_item_t to better
reflect its purpose as a QoS Flow Add or Modify Response Item IE (as per specs)
- Add mandatory AMF UE NGAP ID field to ngap_pdusession_modify_resp_t structure
- Optimize field types: change nb_of_pdusessions and nb_of_pdusessions_failed
from uint16_t to uint8_t (NGAP_MAX_PDU_SESSION limit) (ref 3GPP TS 38.413, 9.2.1.6)
- Update PDU Session Modify Response encoder to use amf_ue_ngap_id from the
message structure (pdusession_modify_resp_p->amf_ue_ngap_id) instead of
retrieving it from UE context (ue_context_p->amf_ue_ngap_id)
- Add comprehensive comments with 3GPP TS 38.413 section references throughout
Changes in RRC/NGAP:
- Populate amf_ue_ngap_id field in PDU Session Modify Response message from
UE context
- Update type reference to qos_flow_addmod_response_item_t
A DRB To Modify Item contains a Flow Mapping Information,
which overrides previous mapping information. For consistency,
rename the count member to reflect a QoS Flow modification,
since it is not limited to setup only.
- Rename DRB flow mapping counters to `numQosFlowsMod`
for a DRB To Modify List
- Adjust encode/decode/equality + E1AP tests
Add DRB To Remove List IE to PDU Session To Modify list, which is sent
in the Bearer Context Modification request. This allows the CUCP to
inform the CUUP about DRBs to be released in a specific PDU session
during a MODIFY procedure.
Changes:
* Add struct definition
* Add enc/dec/eq/cp func
* Add test
- Delete drbs field from gNB_RrcConfigurationReq and its use in RCconfig_NRRRC()
- Remove GNB_CONFIG_STRING_DRBS, help text, index, and GNBPARAMS_CHECK entry from gnb_paramdef.h
- Drop DU-side verification and DRB count logging in gnb_config.c
* Commit applies to Setup Request, CU/DU Configuration Update
* Set standard compliant max number of cells per DU
* Allocate large lists on the heap to prevent stack overflow
due to the large size
* Update relevant free functions
* Also, fix bug in eq_f1ap_cu_configuration_update
Also, improve comments in struct definition.
Implement TS 38.473 transparency for CellGroupConfig forwarding and handle re-establishment on a different DU
This MR implements the following fixes that are relevant in RRC
procedures (e.g. re-establishment, reconfiguration):
1. Transparent CellGroupConfig forwarding
This MR implements TS 38.473 transparency requirements by ensuring that
the CellGroupConfig IE is transparently forwarded from the gNB-DU to the
UE without any re-encoding at the gNB-CU. The CU stores and forwards the
encoded bytes transparently, while the DU is responsible for all
encoding and modifications (e.g., reestablishRLC flags). Motivation
Motivation: The 3GPP TS 38.473 specification mandates that the
CellGroupConfig IE shall be transparently signaled to the UE.
Previously, the CU was re-encoding CellGroupConfig depending on the
scenario, instead of retrieving it from the DU, violating the
transparency requirement. CU may decode for internal use but must not
re-encode for forwarding. Solution
Solution: Core Principle: The CU stores the encoded CellGroupConfig
bytes received from the DU and forwards them directly to the UE without
any re-encoding. The DU handles all encoding and modifications.
Key Changes:
1. Transparent Forwarding in RRC Reconfiguration
- Modified build_RRCReconfiguration_IEs() to use pre-encoded bytes
directly
- Changed parameter from cell_group_config to cgc (encoded byte
array with buffer and size)
2. Encoded Bytes Storage
- Added byte_array_t mcg field to RRC UE structure to store encoded
CellGroupConfig
- Added store_cgc() helper function to save encoded cell group in
the RRC UE Context at gNB (from F1AP messages)
3. DU Encoding Responsibilities
- Add gNB-DU Configuration Query IE support to trigger cgc encoding
to request CellGroupConfig from DU
- DU encodes CellGroupConfig with proper reestablishRLC flags
during re-establishment
- DU removes SRB1 from rlc-BearerToAddModList during
re-establishment (SRB1 re-establishment is done before
RRCReconfiguration)
4. New functions
- Add rrc_detect_reestablishment() function to detect
re-establishment during UE Context Modification
- Extracted rrc_gNB_first_reconfiguration_after_reestablishment()
function to handle first reconfiguration after re-establishment
- Added dump_cgc() and dump_mcg() utility functions for debugging
purposes
5. Re-establishment Flow
- rrc_gNB_process_RRCReestablishmentComplete() requests
CellGroupConfig from DU via gNB_DU_Configuration_Query in UE
Context Modification Request
- DU detects re-establishment and add reestablishRLC IEs
- CU stores encoded CGC from the UE Context Modification Response
and forwards it transparently in subsequent RRC Reconfiguration
Testing
Manual Testing:
- RRC re-establishment procedure completes successfully
- RRC Reconfiguration after re-establishment includes all DU-provided information
Issues fixed
Closes issue #837
2. Trigger UE Context Setup fallback upon reestablishment on DU
different from the original one
As per 38.401 §8.7: if the UE accesses a DU different from the original
one during re-establishment, the CU must trigger UE Context Setup toward
the new DU. This refers to steps 9-10 and steps 9’-10’. Either one or
the other can be executed during re-establishment.
e.g. if this happens during handover, it shall trigger a UE Context
Setup fallback on the CU.
This MR implements the required behavior by:
- Adding a new helper function rrc_handle_f1_ho_reestab_on_target() to
handle F1 handover reestablishment on target DU. The function triggers
UE Context Setup procedure when the UE reestablishes on the target DU
during an ongoing F1 handover.
- Calling this function when detecting that the UE is reestablishing on
the target DU during handover, i.e.: (1) from
handle_rrcReestablishmentComplete(), as per 38.401 §8.7, NOTE on steps
9-10: CU gets UL RRC MESSAGE TRANSFER from a DU different than the
original (2) from rrc_CU_process_ue_modification_required, as per
38.401 §8.7, NOTE on steps 9'-10': CU gets a UE CONTEXT MODIFICATION
REQUIRED from a DU different from the original one.
Also:
- add helper rrc_fill_f1_ue_context_setup and adopt it in handover
- move DRB list filler fill_drb_to_be_setup into rrc_gNB.c and rename it
rrc_fill_f1_drb_to_setup for consistency
See also TS 38.401 Figure 8.7-1 "RRC connection reestablishment
procedure" for reference
3. DU: Handle missing old UE context in DL RRC Message Transfer
During F1 handover reestablishment, the CU may forward a DL RRC Message
Transfer to the target DU with old_gNB_DU_ue_id included, even though
the target DU does not have the old UE context yet.
The code previously used AssertFatal when the old UE context was not
found, causing a crash: "CU claims we should know UE %04x, but we don't"
According to TS 38.473, the gNB-DU shall release the old gNB-DU UE F1AP ID only:
> if the gNB-DU identifies the UE-associated logical F1-connection If
> there is no matching connection, there is nothing to release.
Fix:
- Check if old UE context exists before accessing it
- If old UE does not exist, log error, clean up any stale F1 UE data,
and return early (do not deliver RRC message since reestablishment
can't be completed)
The goal is to address a recurrent issue in the F1 pipeline, see
jenkins-oai.eurecom.fr/job/RAN-SA-Handover-CN5G/514:
Assertion (oldUE) failed!
In dl_rrc_message_transfer() /oai-ran/openair2/LAYER2/NR_MAC_gNB/mac_rrc_dl_handler.c:956
CU claims we should know UE 35ce, but we don't
This field will be adopted to trigger cgc encoding at DU,
according to the specs (see subsequent commit for details).
Changes:
- Add gNB_DU_Configuration_Query field to f1ap_ue_context_mod_req_t
- Implement encoding/decoding
- Add to cp, eq, and free functions for the new field
- Update tests to include the new field
semi-manual remove of not used #define commands
As i was upset to see all these #define with no code under, i made a
global pass to remove the obvious cases semi-manual remove of not used
bodies
manual script, then a lot of manual review
grep -IRs "#define" ../openair* ../common/* ../executables/ ../nfapi/ ../radio/ | \
fgrep -v '(' | fgrep -v '//' |awk '{print $2}' | egrep -v "#define *$n"| \
while read n ; do
grep -IRs $n ../openair* ../common/* ../executables/ ../nfapi/ ../radio/ | grep -v "#define *$n" | grep -q $n || echo $n;
done
will give a decent list of #define never used. Then, I think we should
keep lists of # define when it comes from standards, even if the code
doesn't use it right now
Add Ethernet PDU session support and make PDU session request configurable
This MR adds support for Ethernet PDU sessions, and makes PDU sessions
configurable: per-PDU session ID, type (IPv4/IPv4v6/IPv6/Ethernet [1]),
DNN, NSSAI.
Concretely, it adds a new configuration file section pdu_sessions below
uicc0 that looks like this if everything is specified (default values
are shown):
uicc0:
[...]
pdu_sessions:
- id: 1
type: "IPv4"
nssai_sst: 1
nssai_sd: 0xffffff
dnn: "oai"
It is possible to list multiple PDU sessions (add new line - id: 2 to
request a second PDU session). It is backwards-compatible to the current
PDU session configuration (uicc0.dnn/nssai_sst/etc), the idea being that
either legacy or new configuration could be used, but (1) if
pdu_sessions exist, it will overwrite the legacy configuration, and (2)
multiple PDU sessions, and type, can only be configured using the new
config. The old parameters --extra-pdu-id and --default-pdu-id are not
accepted anymore. Documentation has been added to the UE tutorial.
It is possible to test an Ethernet PDU session [1]. It is possible to
test the PDU session inside the UE by verifying it has an TAP interface
oaitap_ue1, and ARPing the data network (arping has been added to
oai-nr-ue container for that purpose):
ip addr add 192.168.72.140/26 dev oaitap_ue1 # same subnet as data network!
ip link set dev oaitap_ue1 up
arping -c 20 -I oaitap_ue1 192.168.72.135 # will send ARP request in Ethernet frame to get IP address of data network
[1] See description MR !3769 for more information. On the UE side, only
the type has to be changed to "Ethernet".
Robert Schmidt