diff options
Diffstat (limited to 'src/vnet/map/map_doc.md')
| -rw-r--r-- | src/vnet/map/map_doc.md | 69 |
1 files changed, 0 insertions, 69 deletions
diff --git a/src/vnet/map/map_doc.md b/src/vnet/map/map_doc.md deleted file mode 100644 index 17f3c51174b..00000000000 --- a/src/vnet/map/map_doc.md +++ /dev/null @@ -1,69 +0,0 @@ -# VPP MAP and Lw4o6 implementation {#map_doc} - -This is a memo intended to contain documentation of the VPP MAP and Lw4o6 implementations. -Everything that is not directly obvious should come here. - - - -## MAP-E Virtual Reassembly - -The MAP-E implementation supports handling of IPv4 fragments as well as IPv4-in-IPv6 inner and outer fragments. This is called virtual reassembly because the fragments are not actually reassembled. Instead, some meta-data are kept about the first fragment and reused for subsequent fragments. - -Fragment caching and handling is not always necessary. It is performed when: -* An IPv4 fragment is received and the destination IPv4 address is shared. -* An IPv6 packet is received with an inner IPv4 fragment, the IPv4 source address is shared, and 'security-check fragments' is on. -* An IPv6 fragment is received. - -There are 3 dedicated nodes: -* ip4-map-reass -* ip6-map-ip4-reass -* ip6-map-ip6-reass - -ip4-map sends all fragments to ip4-map-reass. -ip6-map sends all inner-fragments to ip6-map-ip4-reass. -ip6-map sends all outer-fragments to ip6-map-ip6-reass. - -IPv4 (resp. IPv6) virtual reassembly makes use of a hash table in order to store IPv4 (resp. IPv6) reassembly structures. The hash-key is based on the IPv4-src:IPv4-dst:Frag-ID:Protocol tuple (resp. IPv6-src:IPv6-dst:Frag-ID tuple, as the protocol is IPv4-in-IPv6). Therefore, each packet reassembly makes use of exactly one reassembly structure. When such a structure is allocated, it is timestamped with the current time. Finally, those structures are capable of storing a limited number of buffer indexes. - -An IPv4 (resp. IPv6) reassembly structure can cache up to MAP_IP4_REASS_MAX_FRAGMENTS_PER_REASSEMBLY (resp. MAP_IP6_REASS_MAX_FRAGMENTS_PER_REASSEMBLY) buffers. Buffers are cached until the first fragment is received. - -#### Virtual Reassembly configuration - -IPv4 and IPv6 virtual reassembly support the following configuration: - map params reassembly [ip4 | ip6] [lifetime <lifetime-ms>] [pool-size <pool-size>] [buffers <buffers>] [ht-ratio <ht-ratio>] - -lifetime: - The time in milliseconds a reassembly structure is considered valid. The longer, the more reliable is reassembly, but the more likely it is to exhaust the pool of reassembly structures. IPv4 standard suggests a lifetime of 15 seconds. IPv6 specifies a lifetime of 60 people. Those values are not realistic for high-throughput cases. - -buffers: - The upper limit of buffers that are allowed to be cached. It can be used to protect against fragmentation attacks which would aim to exhaust the global buffers pool. - -pool-size: - The number of reassembly structures that can be allocated. As each structure can store a small fixed number of fragments, it also sets an upper-bound of 'pool-size * MAP_IPX_REASS_MAX_FRAGMENTS_PER_REASSEMBLY' buffers that can be cached in total. - -ht-ratio: - The amount of buckets in the hash-table is pool-size * ht-ratio. - - -Any time pool-size and ht-ratio is modified, the hash-table is destroyed and created again, which means all current state is lost. - - -##### Additional considerations - -Reassembly at high rate is expensive in terms of buffers. There is a trade-off between the lifetime and number of allocated buffers. Reducing the lifetime helps, but at the cost of loosing state for fragments that are wide appart. - -Let: -R be the packet rate at which fragments are received. -F be the number of fragments per packet. - -Assuming the first fragment is always received last. We should have: -buffers > lifetime * R / F * (F - 1) -pool-size > lifetime * R/F - -This is a worst case. Receiving the first fragment earlier helps reducing the number of required buffers. Also, an optimization is implemented (MAP_IP6_REASS_COUNT_BYTES and MAP_IP4_REASS_COUNT_BYTES) which counts the number of transmitted bytes and remembers the total number of bytes which should be transmitted based on the last fragment, and therefore helps reducing 'pool-size'. - -But the formula shows that it is challenging to forward a significant amount of fragmented packets at high rates. For instance, with a lifetime of 1 second, 5Mpps packet rate would require buffering up to 2.5 millions fragments. - -If you want to do that, be prepared to configure a lot of fragments. - - |