26. Generic Receive Offload Library

Generic Receive Offload (GRO) is a widely used SW-based offloading technique to reduce per-packet processing overheads. By reassembling small packets into larger ones, GRO enables applications to process fewer large packets directly, thus reducing the number of packets to be processed. To benefit DPDK-based applications, like Open vSwitch, DPDK also provides own GRO implementation. In DPDK, GRO is implemented as a standalone library. Applications explicitly use the GRO library to reassemble packets.

26.1. Overview

In the GRO library, there are many GRO types which are defined by packet types. One GRO type is in charge of process one kind of packets. For example, TCP/IPv4 GRO processes TCP/IPv4 packets.

Each GRO type has a reassembly function, which defines own algorithm and table structure to reassemble packets. We assign input packets to the corresponding GRO functions by MBUF->packet_type.

The GRO library doesn’t check if input packets have correct checksums and doesn’t re-calculate checksums for merged packets. The GRO library assumes the packets are complete (i.e., MF==0 && frag_off==0), when IP fragmentation is possible (i.e., DF==0). Additionally, it complies RFC 6864 to process the IPv4 ID field.

Currently, the GRO library provides GRO supports for TCP/IPv4 packets and VxLAN packets which contain an outer IPv4 header and an inner TCP/IPv4 packet.

26.2. Two Sets of API

For different usage scenarios, the GRO library provides two sets of API. The one is called the lightweight mode API, which enables applications to merge a small number of packets rapidly; the other is called the heavyweight mode API, which provides fine-grained controls to applications and supports to merge a large number of packets.

26.2.1. Lightweight Mode API

The lightweight mode only has one function rte_gro_reassemble_burst(), which process N packets at a time. Using the lightweight mode API to merge packets is very simple. Calling rte_gro_reassemble_burst() is enough. The GROed packets are returned to applications as soon as it finishes.

In rte_gro_reassemble_burst(), table structures of different GRO types are allocated in the stack. This design simplifies applications’ operations. However, limited by the stack size, the maximum number of packets that rte_gro_reassemble_burst() can process in an invocation should be less than or equal to RTE_GRO_MAX_BURST_ITEM_NUM.

26.2.2. Heavyweight Mode API

Compared with the lightweight mode, using the heavyweight mode API is relatively complex. Firstly, applications need to create a GRO context by rte_gro_ctx_create(). rte_gro_ctx_create() allocates tables structures in the heap and stores their pointers in the GRO context. Secondly, applications use rte_gro_reassemble() to merge packets. If input packets have invalid parameters, rte_gro_reassemble() returns them to applications. For example, packets of unsupported GRO types or TCP SYN packets are returned. Otherwise, the input packets are either merged with the existed packets in the tables or inserted into the tables. Finally, applications use rte_gro_timeout_flush() to flush packets from the tables, when they want to get the GROed packets.

Note that all update/lookup operations on the GRO context are not thread safe. So if different processes or threads want to access the same context object simultaneously, some external syncing mechanisms must be used.

26.3. Reassembly Algorithm

The reassembly algorithm is used for reassembling packets. In the GRO library, different GRO types can use different algorithms. In this section, we will introduce an algorithm, which is used by TCP/IPv4 GRO and VxLAN GRO.

26.3.1. Challenges

The reassembly algorithm determines the efficiency of GRO. There are two challenges in the algorithm design:

  • a high cost algorithm/implementation would cause packet dropping in a high speed network.
  • packet reordering makes it hard to merge packets. For example, Linux GRO fails to merge packets when encounters packet reordering.

The above two challenges require our algorithm is:

  • lightweight enough to scale fast networking speed
  • capable of handling packet reordering

In DPDK GRO, we use a key-based algorithm to address the two challenges.

26.3.2. Key-based Reassembly Algorithm

Fig. 26.2 illustrates the procedure of the key-based algorithm. Packets are classified into “flows” by some header fields (we call them as “key”). To process an input packet, the algorithm searches for a matched “flow” (i.e., the same value of key) for the packet first, then checks all packets in the “flow” and tries to find a “neighbor” for it. If find a “neighbor”, merge the two packets together. If can’t find a “neighbor”, store the packet into its “flow”. If can’t find a matched “flow”, insert a new “flow” and store the packet into the “flow”.


Packets in the same “flow” that can’t merge are always caused by packet reordering.

The key-based algorithm has two characters:

  • classifying packets into “flows” to accelerate packet aggregation is simple (address challenge 1).
  • storing out-of-order packets makes it possible to merge later (address challenge 2).

Fig. 26.2 Key-based Reassembly Algorithm

26.4. TCP/IPv4 GRO

The table structure used by TCP/IPv4 GRO contains two arrays: flow array and item array. The flow array keeps flow information, and the item array keeps packet information.

Header fields used to define a TCP/IPv4 flow include:

  • source and destination: Ethernet and IP address, TCP port
  • TCP acknowledge number

TCP/IPv4 packets whose FIN, SYN, RST, URG, PSH, ECE or CWR bit is set won’t be processed.

Header fields deciding if two packets are neighbors include:

  • TCP sequence number
  • IPv4 ID. The IPv4 ID fields of the packets, whose DF bit is 0, should be increased by 1.

26.5. VxLAN GRO

The table structure used by VxLAN GRO, which is in charge of processing VxLAN packets with an outer IPv4 header and inner TCP/IPv4 packet, is similar with that of TCP/IPv4 GRO. Differently, the header fields used to define a VxLAN flow include:

  • outer source and destination: Ethernet and IP address, UDP port
  • VxLAN header (VNI and flag)
  • inner source and destination: Ethernet and IP address, TCP port

Header fields deciding if packets are neighbors include:

  • outer IPv4 ID. The IPv4 ID fields of the packets, whose DF bit in the outer IPv4 header is 0, should be increased by 1.
  • inner TCP sequence number
  • inner IPv4 ID. The IPv4 ID fields of the packets, whose DF bit in the inner IPv4 header is 0, should be increased by 1.


We comply RFC 6864 to process the IPv4 ID field. Specifically, we check IPv4 ID fields for the packets whose DF bit is 0 and ignore IPv4 ID fields for the packets whose DF bit is 1. Additionally, packets which have different value of DF bit can’t be merged.