On 28 January 2016 at 10:05, Bill Fischofer <bill.fischo...@linaro.org> wrote:
> Signed-off-by: Bill Fischofer <bill.fischo...@linaro.org> > Reviewed-by Mike Holmes <mike.hol...@linaro.org> > --- > doc/users-guide/users-guide.adoc | 121 > +++++++++++++++++++++++++++++++++++++++ > 1 file changed, 121 insertions(+) > > diff --git a/doc/users-guide/users-guide.adoc > b/doc/users-guide/users-guide.adoc > index b3c08a1..b31e381 100644 > --- a/doc/users-guide/users-guide.adoc > +++ b/doc/users-guide/users-guide.adoc > @@ -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) for 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 examine > +and/or modify this information. > + > +=== Packet Structure and Concepts > +A _packet_ consists of a sequence of octets conforming to an architected > +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 concepts 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 processing 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 platform, 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 > +processing by minimizing the need to examine packet data. This is because > the > +metadata is populated 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 portions > 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 ODP. > + > +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 of the packet and is where the packet's > headroom > +and headers typically reside. Depending on the length of the packet, > +additional segments 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 > +available 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 return 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 > -- > 2.5.0 > > _______________________________________________ > lng-odp mailing list > lng-odp@lists.linaro.org > https://lists.linaro.org/mailman/listinfo/lng-odp > -- Mike Holmes Technical Manager - Linaro Networking Group Linaro.org <http://www.linaro.org/> *│ *Open source software for ARM SoCs "Work should be fun and collborative, the rest follows"
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