Module codec 

GTP codec — GTPv1-C/GTPv1-U (3GPP TS 29.060) and GTPv2-C (3GPP TS 29.274).

Pure: no I/O, no state, no LLM. mod.rs owns the sockets and actions.rs owns the model’s vocabulary; everything that turns bytes into structure and back lives here so the literal-byte tests in tests/server/gtp/codec_test.rs can pin it without a running server.

The two rules this file exists to get right

1. The GTPv1 E/S/PN flags are all-or-nothing. If any of the extension-header, sequence-number or N-PDU-number flags is set, all four optional octets are present (2 sequence, 1 N-PDU, 1 next-extension-header) and each field is only meaningful when its own flag is set. Implementations routinely assume only the flagged field appears, which desynchronises every subsequent byte. See GtpV1Header::optional_present.
2. GTPv1 information elements below type 128 are fixed-length (TV), 128 and above are variable-length (TLV) with a two-octet length. A decoder that assumes TLV throughout misreads a Cause IE as a length prefix. See v1_fixed_ie_len.

GTPv2-C has neither problem: its header length is decided by one flag (T) and every IE is TLIV. It is implemented here because both versions share UDP 2123 and a node has to tell them apart from the first three bits anyway.

Structs

CauseName

A cause the model can name, with its code in each version.

ExtensionHeader

One GTPv1 extension header (TS 29.060 §6.1).

GtpV1Header

GTPv1 header (TS 29.060 §6).

GtpV1Ie

GtpV1Message

A decoded GTPv1 datagram.

GtpV2Header

GTPv2-C header (TS 29.274 §5.1).

GtpV2Ie

GtpV2Message

InnerIp

The decoded header of the user packet a G-PDU carries.

Enums

DecodeError

EncodeError

A value the encoder refuses to put on the wire.

GtpVersion

Which GTP version a datagram announced in its first three bits.

Constants

CAUSES

GTPC_PORT

GTP-C signalling port (TS 29.060 §4.1, TS 29.274 §4.1). Above 1023.

GTPU_PORT

GTP-U user-plane port (TS 29.060 §4.1). Above 1023.

MAX_APN_LABEL_LEN

Longest DNS label an APN may carry, in octets (TS 23.003 §9.1, RFC 1035 §2.3.4).

MAX_GPDU_PAYLOAD_LEN

Largest T-PDU this server will put inside a G-PDU, in octets.

V1_CREATE_PDP_CONTEXT_REQUEST

V1_CREATE_PDP_CONTEXT_RESPONSE

V1_DELETE_PDP_CONTEXT_REQUEST

V1_DELETE_PDP_CONTEXT_RESPONSE

V1_ECHO_REQUEST

V1_ECHO_RESPONSE

V1_END_MARKER

V1_ERROR_INDICATION

V1_G_PDU

V1_IE_ACCESS_POINT_NAME

V1_IE_CAUSE

V1_IE_CHARGING_ID

V1_IE_END_USER_ADDRESS

V1_IE_GSN_ADDRESS

V1_IE_IMEISV

V1_IE_IMSI

V1_IE_MSISDN

V1_IE_MS_TIME_ZONE

V1_IE_NSAPI

V1_IE_PROTOCOL_CONFIG_OPTIONS

V1_IE_QOS_PROFILE

V1_IE_RAT_TYPE

V1_IE_RECOVERY

V1_IE_REORDERING_REQUIRED

V1_IE_SELECTION_MODE

V1_IE_TEARDOWN_IND

V1_IE_TEID_CONTROL_PLANE

V1_IE_TEID_DATA_I

V1_IE_USER_LOCATION_INFO

V1_MAX_SEQUENCE

Largest sequence number a GTPv1 header can carry: the field is 16 bits (TS 29.060 §6).

V1_SUPPORTED_EXTENSION_HEADERS_NOTIFICATION

V1_UPDATE_PDP_CONTEXT_REQUEST

V1_UPDATE_PDP_CONTEXT_RESPONSE

V1_VERSION_NOT_SUPPORTED

V2_CREATE_SESSION_REQUEST

V2_CREATE_SESSION_RESPONSE

V2_DELETE_SESSION_REQUEST

V2_DELETE_SESSION_RESPONSE

V2_ECHO_REQUEST

V2_ECHO_RESPONSE

V2_IE_APN

V2_IE_APN_RESTRICTION

V2_IE_BEARER_CONTEXT

V2_IE_CAUSE

V2_IE_EBI

V2_IE_FTEID

V2_IE_IMSI

V2_IE_MSISDN

V2_IE_PAA

V2_IE_PCO

V2_IE_RAT_TYPE

V2_IE_RECOVERY

V2_IE_ULI

V2_MAX_SEQUENCE

Largest sequence number a GTPv2 header can carry: the field is 24 bits (TS 29.274 §5.1).

V2_MODIFY_BEARER_REQUEST

V2_MODIFY_BEARER_RESPONSE

Functions

cause_accepts

Does this numeric cause value grant something?

cause_by_name

Look a cause up by name, tolerating the spellings a model actually produces: "Request accepted", "REQUEST_ACCEPTED", "request-accepted".

cause_names

Every cause name the model may use, for error messages and documentation.

decode_apn

Decode a labelled APN back to dotted form. Returns None if the labels do not tile the buffer exactly, which means it was not an APN.

decode_end_user_address

Decode a GTPv1 End User Address. Returns (pdp_type, address); the address is None for the “give me a dynamic one” form a UE sends in a Create PDP Context Request.

decode_fteid

Decode an F-TEID. Returns (interface_type, teid, address).

decode_inner_ip

Decode the header of an encapsulated user IP packet. None when it is not IP at all.

decode_paa

Decode a GTPv2-C PAA. Returns (pdn_type, address).

decode_tbcd

Decode TBCD back to digits, stopping at the 0xF filler.

encode_apn

Encode an APN as DNS-style labels: one length octet per label, no trailing root label (TS 23.003 §9.1, TS 29.060 §7.7.30).

encode_end_user_address

GTPv1 End User Address (IE 128) for an IETF-allocated PDP address.

encode_fteid

GTPv2-C Fully Qualified TEID (IE 87), TS 29.274 §8.22.

encode_paa

GTPv2-C PDN Address Allocation (IE 79). PDN type 1 = IPv4, 2 = IPv6, 3 = IPv4v6.

encode_pco_dns

Protocol Configuration Options carrying DNS server addresses in the network-to-MS direction (TS 24.008 §10.5.6.3; GTPv1 IE 132, GTPv2 IE 78).

encode_tbcd

Encode decimal digits as TBCD (telephony BCD): two digits per octet, low nibble first, odd counts padded with the filler 0xF. Used for IMSI, MSISDN and IMEI.

encode_v1_ies

Serialise information elements. Fixed types emit no length; 128+ emit a two-octet length.

encode_v2_grouped

Serialise IEs for use as the value of a grouped GTPv2 IE.

find_all_v1

Every IE of a type, in order.

find_v1

First IE of a type, if present.

ip_protocol_name

parse_v1_ies

Parse a GTPv1-C message body into information elements.

parse_v2_grouped

Parse the inner IEs of a grouped GTPv2 IE (Bearer Context and friends).

peek_version

The version bits of a datagram, without decoding anything else.

rat_type_name

Radio access technology name (TS 29.060 §7.7.50 / TS 29.274 §8.17). The two specifications number these differently, so the version has to be passed in.

v1_fixed_ie_len

Length in octets of the value of a GTPv1 fixed-length (TV) information element.

v1_ie_name

Human name for the GTPv1 IEs this server surfaces. Unknown types are reported by number.

v1_message_name

Human name for a GTPv1 message type, for logs and event data.

v2_ie_name

Human name for the GTPv2-C IEs this server surfaces.

v2_message_name

Human name for a GTPv2-C message type.