@@ -768,6 +768,127 @@ NOTE: Both ordered and parallel queues improve throughput over atomic queues
due to parallel event processing, but require that the application take
steps to ensure context data synchronization if needed.
+== Packet Processing
+ODP applications are designed to process packets, which are the basic unit of
+data of interest in the data plane. To assist in processing packets, ODP
+provides a set of APIs that enable applications to examine and manipulate
+packet data and metadata. Packets are referenced by an abstract *odp_packet_t*
+handle defined by each implementation.
+
+Packet objects are normally created at ingress when they arrive at a source
+*odp_pktio_t* and are received by an application either directly or (more
+typically) foa a scheduled receive queue. They MAY be implicitly freed when
+they are transmitted to an output *odp_pktio_t* via an associated transmit
+queue, or freed directly via the +odp_packet_free()+ API.
+
+Occasionally an application may originate a packet itself, either _de novo_ or
+by deriving it from an existing packet, and APIs are provided to assist in
+these cases as well. Application-created packets can be recycled back through
+a _loopback interface_ to reparse and reclassify them, or the application can
+do its own parsing as desired.
+
+Various attributes associated with a packet, such as parse results, are
+stored as metadata and APIs are provided to permit applications to examing
+and/or modify this information.
+
+=== Packet Structure and Concepts
+A _packet_ consists of a sequence of octets conforming to an architectued
+format, such as Ethernet, that can be received and transmitted via the ODP
+*pktio* abstraction. Packets of a _length_, which is the number of bytes in
+the packet. Packet data in ODP is referenced via _offsets_ since these reflect
+the logical contents and structure of a packet independent of how particular
+ODP implementations store that data.
+
+These concetps are shown in the following diagram:
+
+.ODP Packet Structure
+image::../images/packet.svg[align="center"]
+
+Packet data consists of zero or more _headers_ followed by 0 or more bytes of
+_payload_, followed by zero or more _trailers_. Shown here are various APIs
+that permit applications to examine and navigate various parts of a packet and
+to manipulate its structure.
+
+To support packet manipulation, predefined _headroom_ and _tailroom_
+areas are logically associated with a packet. Packets can be adjusted by
+_pulling_ and _pushing_ these areas. Typical packet processing might consist
+of stripping headers from a packet via +odp_pull_head()+ calls as part of
+receive procesing and then replacing them with new headers via +odp_push_head()+
+calls as the packet is being prepared for transmit.
+
+=== Packet Segments and Addressing
+ODP platforms use various methods and techniques to store and process packets
+efficiently. These vary considerably from platform to platofrm, so to ensure
+portability across them ODP adopts certain conventions for referencing
+packets.
+
+ODP APIs use a handle of type *odp_packet_t* to refer to packet objects.
+Associated with packets are various bits of system metadata that describe the
+packet. By referring to the metadata, ODP applications accelerate packet
+processin gby minimizing the need to examine packet data. This is because the
+metadata is pupolated by parsing and classification functions that are coupled
+to ingress processing that occur prior to a packet being presented to the
+application via the ODP scheduler.
+
+When an ODP application needs to examine the contents of a packet, it requests
+addressability to it via an API call that makes the packet (or a contiguously
+addressable _segment_ of it) available for coherent access by the application.
+To ensure portability, ODP applications assume that the underlying
+implementation stores packets in _segments_ of implementation-defined
+and managed size. These represent the contiguously addressable portaions of a
+packet that the application may refer to via normal memory accesses. ODP
+provides APIs that allow applications to operate on packet segments in an
+efficient and portable manner as needed. By combining these with the metadata
+provided by packets, ODP applications can operate in a fully
+platform-independent manner while still achieving optimal performance across
+the range of platforms that support DOP.
+
+The use of segments for packet addressing and their relationship to metadata
+is shown in this diagram:
+
+.ODP Packet Segmentation
+image::../images/segment.svg[align="center"]
+
+The packet metadata is set during parsing and identifies the starting offsets
+of the various headers in the packet. The packet itself is physically stored
+as a sequence of segments that area managed by the ODP implementation.
+Segment 0 is the first segment ofthe packet and is where the packet's headroom
+and headers typically reside. Depending on the length of the packet,
+additional segmnets may be part of the packet and contain the remaining packet
+payload and tailroom. The application need not concern itself with segments
+except that when the application requires addressability to a packet it
+understands that addressability is provided on a per-segment basis. So, for
+example, if the application makes a call like +odp_packet_l4_ptr()+ to obtain
+addressability to the packet's Layer 4 header, the returned length from that
+call is the number of bytes from the start of the Layer 4 header that are
+contiguously addressable to the application from the returned pointer address.
+This is because the following byte occupies a different segment and may be
+stored elsewhere. To obtain access to those bytes, the application simply
+requests addressability to that offset and it will be able to address the
+packet bytes that occupy the next segment, etc. Note that the returned
+length for any packet addressability call is always the lesser of the remaining
+packet length or size of its containing segment. So a mapping for segment 2
+in the above figure, for example, would return a length that extends only to
+the end of the packet since the remaining bytes are part of the tailroom
+reserved for the packet and are not usable by the application until made
+avaialble to it by an appropriate API call.
+
+=== Metadata Processing
+As noted, packet metadata is normally set by the parser as part of
+classification that occurs during packet receive processing. It is important
+to note that this metadata may be changed by the application to reflect
+changes in the packet contents and/or structure as part of its processing of
+the packet. While changing this metadata may effect some ODP APIs, changing
+metadata is designed to _document_ application changes to the packet but
+does not in itself _cause_ those changes to be made. For example, if an
+application changes the Layer 3 offset by using the +odp_packet_l3_offset_set()+
+API, the subsequent calls to +odp_packet_l3_ptr()+ will reutrn an address
+starting from that changed offset, changing an attribute like
++odp_packet_has_udp_set()+ will not, by itself, turn a non-UDP packet into
+a valid UDP packet. Applications are expected to exercise appropriate care
+when changing packet metadata to ensure that the resulting metadata changes
+reflect the actual changed packet structure that the application has made.
+
== Cryptographic services
ODP provides support for cryptographic operations required by various security
Signed-off-by: Bill Fischofer <bill.fischofer@linaro.org> --- doc/users-guide/users-guide.adoc | 121 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 121 insertions(+)