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

#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

#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

#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

#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.