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+==================
+ Network Encoding
+==================
+
+This describes the encoding used to serialize data. It doesn't cover specific
+objects/messages but focuses on the base types.
+
+The types are not self documenting in any way. They can not be decoded unless
+you know what they are.
+
+Conventions
+===========
+
+Integers
+--------
+
+The integer types used will be named ``{signed}{size}{endian}``. For example
+``u16le`` is an unsigned 16 bit integer encoded in little endian byte order
+while ``s64be`` is a signed 64 bit integer in big endian. Additionally ``u8``
+and ``s8`` will represent signed and unsigned bytes respectively. Signed
+integers use two's complement encoding.
+
+Complex Types
+-------------
+
+This document will use a c-like syntax for describing structures. The
+structure represents the data that will go over the wire. There will be no
+padding between the elements and the elements will be sent in the order they
+appear. For example::
+
+ struct foo {
+ u8 tag;
+ u32le data;
+ }
+
+When encoding the values ``0x05`` and ``0x12345678`` respectively will appear on
+the wire as ``05 78 56 34 12``.
+
+Variable Arrays
+---------------
+
+Unlike c, length arrays can be used anywhere in structures and will be inline in
+the protocol. Furthermore the length may be described using an earlier item in
+the structure.
+
+::
+
+ struct blob {
+ u32le size;
+ u8 data[size];
+ u32le checksum;
+ }
+
+This structure is encoded as a 32 bit size, followed by ``size`` data bytes,
+then a 32 bit checksum.
+
+Primitive Aliases
+-----------------
+
+These types are just aliases for primitive types.
+
+::
+
+ // From /src/include/types.h
+
+ typedef u32le epoch_t;
+ typedef u32le ceph_seq_t;
+ typedef u64le ceph_tid_t;
+ typedef u64le version_t;
+
+
+Structures
+==========
+
+These are the way structures are encoded. Note that these structures don't
+actually exist in the source but are the way that different types are encoded.
+
+Optional
+--------
+
+Optionals are represented as a presence byte, followed by the item if it exists.
+
+::
+
+ struct ceph_optional<T> {
+ u8 present;
+ T element[present? 1 : 0]; // Only if present is non-zero.
+ }
+
+Optionals are used to encode ``boost::optional``.
+
+Pair
+----
+
+Pairs are simply the first item followed by the second.
+
+::
+
+ struct ceph_pair<A,B> {
+ A a;
+ B b;
+ }
+
+Pairs are used to encode ``std::pair``.
+
+Triple
+------
+
+Triples are simply the tree elements one after another.
+
+::
+
+ struct ceph_triple<A,B,C> {
+ A a;
+ B b;
+ C c;
+ }
+
+Triples are used to encode ``ceph::triple``.
+
+
+List
+----
+
+Lists are represented as an element count followed by that many elements.
+
+::
+
+ struct ceph_list<T> {
+ u32le length;
+ T elements[length];
+ }
+
+.. note::
+ The size of the elements in the list are not necessarily uniform.
+
+Lists are used to encode ``std::list``, ``std::vector``, ``std::deque``,
+``std::set`` and ``ceph::unordered_set``.
+
+Blob
+----
+
+A Blob is simply a list of bytes.
+
+::
+
+ struct ceph_string {
+ ceph_list<u8>;
+ }
+
+ // AKA
+
+ struct ceph_string {
+ u32le size;
+ u8 data[size];
+ }
+
+Blobs are used to encode ``std::string``, ``const char *`` and ``bufferlist``.
+
+.. note::
+ The content of a Blob is arbitrary binary data.
+
+Map
+---
+
+Maps are a list of pairs.
+
+::
+
+ struct ceph_map<K,V> {
+ ceph_list<ceph_pair<K,V>>;
+ }
+
+ // AKA
+
+ struct ceph_map<K,V> {
+ u32le length;
+ ceph_pair<K,V> entries[length];
+ }
+
+Maps are used to encode ``std::map``, ``std::multimap`` and
+``ceph::unordered_map``.
+
+Complex Types
+=============
+
+These aren't hard to find in the source but the common ones are listed here for
+convenience.
+
+utime_t
+-------
+
+::
+
+ // From /src/include/utime.h
+ struct utime_t {
+ u32le tv_sec; // Seconds since epoch.
+ u32le tv_nsec; // Nanoseconds since the last second.
+ }
+
+ceph_entity_name
+----------------
+
+::
+
+ // From /src/include/msgr.h
+ struct ceph_entity_name {
+ u8 type; // CEPH_ENTITY_TYPE_*
+ u64le num;
+ }
+
+ // CEPH_ENTITY_TYPE_* defined in /src/include/msgr.h
+
+.. vi: textwidth=80 noexpandtab