--- /dev/null
+.. _reassembly:
+
+IP Reassembly
+=============
+
+Some VPP functions need access to whole packet and/or stream
+classification based on L4 headers. Reassembly functionality allows
+both former and latter.
+
+Full reassembly vs shallow (virtual) reassembly
+-----------------------------------------------
+
+There are two kinds of reassembly available in VPP:
+
+1. Full reassembly changes a stream of packet fragments into one
+packet containing all data reassembled with fragment bits cleared
+and fragment header stripped (in case of ip6). Note that resulting
+packet may come out of reassembly as a buffer chain. Because it's
+impractical to parse headers which are split over multiple vnet
+buffers, vnet_buffer_chain_linearize() is called after reassembly so
+that L2/L3/L4 headers can be found in first buffer. Full reassembly
+is costly and shouldn't be used unless necessary. Full reassembly is by
+default enabled for both ipv4 and ipv6 traffic for "forus" traffic
+- that is packets aimed at VPP addresses. This can be disabled via API
+if desired, in which case "forus" fragments are dropped.
+
+2. Shallow (virtual) reassembly allows various classifying and/or
+translating features to work with fragments without having to
+understand fragmentation. It works by extracting L4 data and adding
+them to vnet_buffer for each packet/fragment passing throught SVR
+nodes. This operation is performed for both fragments and regular
+packets, allowing consuming code to treat all packets in same way. SVR
+caches incoming packet fragments (buffers) until first fragment is
+seen. Then it extracts L4 data from that first fragment, fills it for
+any cached fragments and transmits them in the same order as they were
+received. From that point on, any other passing fragments get L4 data
+populated in vnet_buffer based on reassembly context.
+
+Multi-worker behaviour
+^^^^^^^^^^^^^^^^^^^^^^
+
+Both reassembly types deal with fragments arriving on different workers
+via handoff mechanism. All reassembly contexts are stored in pools.
+Bihash mapping 5-tuple key to a value containing pool index and thread
+index is used for lookups. When a lookup finds an existing reasembly on
+a different thread, it hands off the fragment to that thread. If lookup
+fails, a new reassembly context is created and current worker becomes
+owner of that context. Further fragments received on other worker
+threads are then handed off owner worker thread.
+
+Full reassembly also remembers thread index where first fragment (as in
+fragment with fragment offset 0) was seen and uses handoff mechanism to
+send the reassembled packet out on that thread even if pool owner is
+a different thread. This then requires an additional handoff to free
+reassembly context as only pool owner can do that in a thread-safe way.
+
+Limits
+^^^^^^
+
+Because reassembly could be an attack vector, there is a configurable
+limit on the number of concurrent reassemblies and also maximum
+fragments per packet.
+
+Custom applications
+^^^^^^^^^^^^^^^^^^^
+
+Both reassembly features allow to be used by custom applicatind which
+are not part of VPP source tree. Be it patches or 3rd party plugins,
+they can build their own graph paths by using "-custom*" versions of
+nodes. Reassembly then reads next_index and error_next_index for each
+buffer from vnet_buffer, allowing custom application to steer
+both reassembled packets and any packets which are considered an error
+in a way the custom application requires.
+
+Full reassembly
+---------------
+
+Configuration
+^^^^^^^^^^^^^
+
+Configuration is via API (``ip_reassembly_enable_disable``) or CLI:
+
+``set interface reassembly <interface-name> [on|off|ip4|ip6]``
+
+here ``on`` means both ip4 and ip6.
+
+A show command is provided to see reassembly contexts:
+
+For ip4:
+
+``show ip4-full-reassembly [details]``
+
+For ip6:
+
+``show ip6-full-reassembly [details]``
+
+Global full reassembly parameters can be modified using API
+``ip_reassembly_set`` and retrieved using ``ip_reassembly_get``.
+
+Defaults
+""""""""
+
+For defaults values, see #defines in
+
+`ip4_full_reass.c <__REPOSITORY_URL__/src/vnet/ip/reass/ip4_full_reass.c>`_
+
+========================================= ==========================================
+#define description
+----------------------------------------- ------------------------------------------
+IP4_REASS_TIMEOUT_DEFAULT_MS timeout in milliseconds
+IP4_REASS_EXPIRE_WALK_INTERVAL_DEFAULT_MS interval between reaping expired sessions
+IP4_REASS_MAX_REASSEMBLIES_DEFAULT maximum number of concurrent reassemblies
+IP4_REASS_MAX_REASSEMBLY_LENGTH_DEFAULT maximum number of fragments per reassembly
+========================================= ==========================================
+
+and
+
+`ip6_full_reass.c <__REPOSITORY_URL__/src/vnet/ip/reass/ip6_full_reass.c>`_
+
+========================================= ==========================================
+#define description
+----------------------------------------- ------------------------------------------
+IP6_REASS_TIMEOUT_DEFAULT_MS timeout in milliseconds
+IP6_REASS_EXPIRE_WALK_INTERVAL_DEFAULT_MS interval between reaping expired sessions
+IP6_REASS_MAX_REASSEMBLIES_DEFAULT maximum number of concurrent reassemblies
+IP6_REASS_MAX_REASSEMBLY_LENGTH_DEFAULT maximum number of fragments per reassembly
+========================================= ==========================================
+
+Finished/expired contexts
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Reassembly contexts are freed either when reassembly is finished - when
+all data has been received or in case of timeout. There is a process
+walking all reassemblies, freeing any expired ones.
+
+Shallow (virtual) reassembly
+----------------------------
+
+Configuration
+^^^^^^^^^^^^^
+
+Configuration is via API (``ip_reassembly_enable_disable``) only as
+there is no value in turning SVR on by hand without a feature consuming
+buffer metadata. SVR is designed to be turned on by a feature requiring
+it in a programmatic way.
+
+A show command is provided to see reassembly contexts:
+
+For ip4:
+
+``show ip4-sv-reassembly [details]``
+
+For ip6:
+
+``show ip6-sv-reassembly [details]``
+
+Global shallow reassembly parameters can be modified using API
+``ip_reassembly_set`` and retrieved using ``ip_reassembly_get``.
+
+Defaults
+""""""""
+
+For defaults values, see #defines in
+
+`ip4_sv_reass.c <__REPOSITORY_URL__/src/vnet/ip/reass/ip4_sv_reass.c>`_
+
+============================================ ==========================================
+#define description
+-------------------------------------------- ------------------------------------------
+IP4_SV_REASS_TIMEOUT_DEFAULT_MS timeout in milliseconds
+IP4_SV_REASS_EXPIRE_WALK_INTERVAL_DEFAULT_MS interval between reaping expired sessions
+IP4_SV_REASS_MAX_REASSEMBLIES_DEFAULT maximum number of concurrent reassemblies
+IP4_SV_REASS_MAX_REASSEMBLY_LENGTH_DEFAULT maximum number of fragments per reassembly
+============================================ ==========================================
+
+and
+
+`ip6_sv_reass.c <__REPOSITORY_URL__/src/vnet/ip/reass/ip6_sv_reass.c>`_
+
+============================================ ==========================================
+#define description
+-------------------------------------------- ------------------------------------------
+IP6_SV_REASS_TIMEOUT_DEFAULT_MS timeout in milliseconds
+IP6_SV_REASS_EXPIRE_WALK_INTERVAL_DEFAULT_MS interval between reaping expired sessions
+IP6_SV_REASS_MAX_REASSEMBLIES_DEFAULT maximum number of concurrent reassemblies
+IP6_SV_REASS_MAX_REASSEMBLY_LENGTH_DEFAULT maximum number of fragments per reassembly
+============================================ ==========================================
+
+Expiring contexts
+^^^^^^^^^^^^^^^^^
+
+There is no way of knowing when a reassembly is finished without
+performing (an almost) full reassembly, so contexts in SVR cannot be
+freed in the same way as in full reassembly. Instead a different
+approach is taken. Least recently used (LRU) list is maintained where
+reassembly contexts are ordered based on last update. The oldest
+context is then freed whenever SVR hits limit on number of concurrent
+reassembly contexts. There is also a process reaping expired sessions
+similar as in full reassembly.
+
+Truncated packets
+^^^^^^^^^^^^^^^^^
+
+When SVR detects that a packet has been truncated in a way where L4
+headers are not available, it will mark it as such in vnet_buffer,
+allowing downstream features to handle such packets as they deem fit.
+
+Fast path/slow path
+^^^^^^^^^^^^^^^^^^^
+
+SVR runs is implemented fast path/slow path way. By default, it assumes
+that any passing traffic doesn't contain fragments, processing buffers
+in a dual-loop. If it sees a fragment, it then jumps to single-loop
+processing.
+
+Feature enabled by other features/reference counting
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+SVR feature is enabled by some other features, like NAT, when those
+features are enabled. For this to work, it implements a reference
+counted API for enabling/disabling SVR.
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