44. PDCP Protocol Processing Library

DPDK provides a library for PDCP protocol processing. The library utilizes other DPDK libraries such as cryptodev, reorder, etc., to provide the application with a transparent and high performant PDCP protocol processing library.

The library abstracts complete PDCP protocol processing conforming to ETSI TS 138 323 V17.1.0 (2022-08)

PDCP would involve the following operations:

  1. Transfer of user plane data
  2. Transfer of control plane data
  3. Header compression
  4. Uplink data compression
  5. Ciphering and integrity protection
../_images/pdcp_functional_overview.svg

Fig. 44.1 PDCP functional overview

PDCP library would abstract the protocol offload features of the cryptodev and would provide a uniform interface and consistent API usage to work with cryptodev irrespective of the protocol offload features supported.

44.1. PDCP entity API

PDCP library provides following control path API that is used to configure various PDCP entities:

  • rte_pdcp_entity_establish()
  • rte_pdcp_entity_suspend()
  • rte_pdcp_entity_release()

A PDCP entity would translate to one rte_cryptodev_sym_session or rte_security_session based on the config. The sessions would be created/destroyed while corresponding PDCP entity operations are performed.

When upper layers request a PDCP entity suspend (rte_pdcp_entity_suspend()), it would result in flushing out of all cached packets and internal state variables are updated as described in 5.1.4.

When upper layers request a PDCP entity release (rte_pdcp_entity_release()), it would result in flushing out of all cached packets and releasing of all memory associated with the entity. It would internally free any crypto/security sessions created. All procedures mentioned in 5.1.3 would be performed.

44.2. PDCP PDU (Protocol Data Unit) API

PDCP PDUs can be categorized as:

  • Control PDU
  • Data PDU

Control PDUs are used for signalling between entities on either end and can be one of the following:

  • PDCP status report
  • ROHC feedback
  • EHC feedback

Control PDUs are not ciphered or authenticated, and so such packets are not submitted to cryptodev for processing.

Data PDUs are regular packets submitted by upper layers for transmission to other end. Such packets would need to be ciphered and authenticated based on the entity configuration.

44.2.1. PDCP packet processing API for control PDU

Control PDUs are used in PDCP as a communication channel between transmitting and receiving entities. When upper layer request for operations such as re-establishment, receiving PDCP entity need to prepare a status report and send it to the other end. The API rte_pdcp_control_pdu_create() allows application to request the same.

44.2.2. PDCP packet processing API for data PDU

PDCP processing is split into 2 parts. One before cryptodev processing (rte_pdcp_pkt_pre_process()) and one after cryptodev processing (rte_pdcp_pkt_post_process()). Since cryptodev dequeue can return crypto operations belonging to multiple entities, rte_pdcp_pkt_crypto_group() is added to help grouping crypto operations belonging to same PDCP entity.

Lib PDCP would allow application to use same API sequence while leveraging protocol offload features enabled by rte_security library.

Lib PDCP would internally change the handles registered for pre_process and post_process based on features enabled in the entity.

Lib PDCP would create the required sessions on the device provided in entity to minimize the application requirements. Also, the rte_crypto_op allocation and free would also be done internally by lib PDCP to allow the library to create crypto ops as required for the input packets. For example, when control PDUs are received, no cryptodev enqueue-dequeue is expected for the same and lib PDCP is expected to handle it differently.

44.3. Supported features

  • 12-bit & 18-bit sequence numbers
  • Uplink & downlink traffic
  • HFN increment
  • IV generation as required per algorithm

44.3.1. Supported ciphering algorithms

  • RTE_CRYPTO_CIPHER_NULL
  • RTE_CRYPTO_CIPHER_AES_CTR
  • RTE_CRYPTO_CIPHER_SNOW3G_UEA2
  • RTE_CRYPTO_CIPHER_ZUC_EEA3

44.3.2. Supported integrity protection algorithms

  • RTE_CRYPTO_AUTH_NULL
  • RTE_CRYPTO_AUTH_AES_CMAC
  • RTE_CRYPTO_AUTH_SNOW3G_UIA2
  • RTE_CRYPTO_AUTH_ZUC_EIA3

44.4. Timers

PDCP utilizes a reception window mechanism to limit the bits of COUNT value transmitted in the packet. It utilizes state variables such as RX_REORD, RX_DELIV to define the window and uses RX_DELIV as the lower pivot point of the window.

RX_DELIV would be updated only when packets are received in-order. Any missing packet would mean RX_DELIV won’t be updated. A timer, t-Reordering, helps PDCP to slide the window if the missing packet is not received in a specified time duration.

While starting and stopping the timer will be done by lib PDCP, application could register its own timer implementation. This is to make sure application can choose between timers such as rte_timer and rte_event based timers. Starting and stopping of timer would happen during pre & post process API.

When the t-Reordering timer expires, application would receive the expiry event. To perform the PDCP handling of the expiry event, rte_pdcp_t_reordering_expiry_handle can be used. Expiry handling would involve sliding the window by updating state variables and passing the expired packets to the application.

struct rte_pdcp_t_reordering {
	/** Timer pointer, to be used in callback functions. */
	void *timer;
	/** Timer arguments, to be used in callback functions. */
	void *args;
	/** Timer start callback handle. */
	rte_pdcp_t_reordering_start_cb_t start;
	/** Timer stop callback handle. */
	rte_pdcp_t_reordering_stop_cb_t stop;
};

44.5. Sample API usage

The rte_pdcp_entity_conf structure is used to pass the configuration parameters for entity creation.

struct rte_pdcp_entity_conf {
	/** PDCP transform for the entity. */
	struct rte_security_pdcp_xform pdcp_xfrm;
	/** Crypto transform applicable for the entity. */
	struct rte_crypto_sym_xform *crypto_xfrm;
	/** Mempool for crypto symmetric session. */
	struct rte_mempool *sess_mpool;
	/** Crypto op pool. */
	struct rte_mempool *cop_pool;
	/** Mbuf pool to be used for allocating control PDUs.*/
	struct rte_mempool *ctrl_pdu_pool;
	/**
	 * Sequence number value to be used.
	 * 32 bit count value to be used for the first packet
	 * would be derived based on HFN (`rte_security_pdcp_xform.hfn`) and SN.
	 */
	uint32_t sn;
	/** Indicate whether the PDCP entity belongs to Side Link Radio Bearer. */
	bool is_slrb;
	/** Enable security offload on the device specified. */
	bool en_sec_offload;
	/** Device on which security/crypto session need to be created. */
	uint8_t dev_id;
	/**
	 * Reverse direction during IV generation.
	 * Can be used to simulate UE crypto processing.
	 */
	bool reverse_iv_direction;
	/**
	 * Status report required (specified in TS 38.331).
	 *
	 * If PDCP entity is configured to send a PDCP status report,
	 * the upper layer application may request a receiving PDCP entity
	 * to generate a PDCP status report using ``rte_pdcp_control_pdu_create``.
	 * In addition, PDCP status reports may be generated during operations
	 * such as entity re-establishment.
	 */
	bool status_report_required;
	/** Enable out of order delivery. */
	bool out_of_order_delivery;
	/** t-Reordering timer configuration. */
	struct rte_pdcp_t_reordering t_reordering;
};

struct rte_mbuf **out_mb, *pkts[MAX_BURST_SIZE];
struct rte_crypto_op *cop[MAX_BURST_SIZE];
struct rte_pdcp_group grp[MAX_BURST_SIZE];
struct rte_pdcp_entity *pdcp_entity;
int nb_max_out_mb, ret, nb_grp;
uint16_t nb_ops;

/* Create PDCP entity */
pdcp_entity = rte_pdcp_entity_establish(&conf);

/**
 * Allocate buffer for holding mbufs returned during PDCP suspend,
 * release & post-process APIs.
 */

/* Max packets that can be cached in entity + burst size */
nb_max_out_mb = pdcp_entity->max_pkt_cache + MAX_BURST_SIZE;
out_mb = rte_malloc(NULL, nb_max_out_mb * sizeof(uintptr_t), 0);
if (out_mb == NULL) {
        /* Handle error */
}

while (1) {
        /* Receive packet and form mbuf */

        /**
         * Prepare packets for crypto operation.
         * Following operations would be done,
         *
         * Transmitting entity/UL (only data PDUs):
         *  - Perform compression
         *  - Assign sequence number
         *  - Add PDCP header
         *  - Create & prepare crypto_op
         *  - Prepare IV for crypto operation (auth_gen, encrypt)
         *  - Save original PDCP SDU (during PDCP re-establishment,
         *    unconfirmed PDCP SDUs need to be crypto processed again and
         *    transmitted/re-transmitted)
         *
         *  Receiving entity/DL:
         *  - Any control PDUs received would be processed and
         *    appropriate actions taken. If data PDU, continue.
         *  - Determine sequence number (based on HFN & per packet SN)
         *  - Prepare crypto_op
         *  - Prepare IV for crypto operation (decrypt, auth_verify)
         */
        nb_success = rte_pdcp_pkt_pre_process(pdcp_entity, pkts, cop,
                                              nb_rx, &nb_err);
        if (nb_err != 0) {
                /* Handle error packets */
        }

        if ((rte_cryptodev_enqueue_burst(dev_id, qp_id, cop, nb_success)
                        != nb_success) {
                /* Retry for enqueue failure packets */
        }

        ...

        nb_ops = rte_cryptodev_dequeue_burst(dev_id, qp_id, cop,
                                          MAX_BURST_SIZE);
        if (nb_ops == 0)
                continue;

        /**
         * Received a burst of completed crypto ops from cryptodev. It
         * may belong to various entities. Group similar ones together
         * for entity specific post-processing.
         */

        /**
         * Groups similar entities together. Frees crypto op and based
         * on crypto_op status, set mbuf->ol_flags which would be
         * checked in rte_pdcp_pkt_post_process().
         */
        nb_grp = rte_pdcp_pkt_crypto_group(cop, pkts, grp, ret);

        for (i = 0; i != nb_grp; i++) {

                /**
                 * Post process packets after crypto completion.
                 * Following operations would be done,
                 *
                 *  Transmitting entity/UL:
                 *  - Check crypto result
                 *
                 *  Receiving entity/DL:
                 *  - Check crypto operation status
                 *  - Check for duplication (if yes, drop duplicate)
                 *  - Perform decompression
                 *  - Trim PDCP header
                 *  - Hold packet (SDU) for in-order delivery (return
                 *    completed packets as and when sequence is
                 *    completed)
                 *  - If not in sequence, cache the packet and start
                 *    t-Reordering timer. When timer expires, the
                 *    packets need to delivered to upper layers (not
                 *    treated as error packets).
                 */
                nb_success = rte_pdcp_pkt_post_process(grp[i].id.ptr,
                                                       grp[i].m, out_mb,
                                                       grp[i].cnt,
                                                       &nb_err);
                if (nb_err != 0) {
                        /* Handle error packets */
                }

                /* Perform additional operations */

                /**
                 * Transmitting entity/UL
                 * - If duplication is enabled, duplicate PDCP PDUs
                 * - When lower layers confirm reception of a PDCP PDU,
                 *   it should be communicated to PDCP layer so that
                 *   PDCP can drop the corresponding SDU
                 */
        }
}