From 7cd468a3d7dee7d6c92f69a0bb7061ae208ec727 Mon Sep 17 00:00:00 2001 From: Damjan Marion Date: Mon, 19 Dec 2016 23:05:39 +0100 Subject: Reorganize source tree to use single autotools instance Change-Id: I7b51f88292e057c6443b12224486f2d0c9f8ae23 Signed-off-by: Damjan Marion --- src/vnet/map/map_doc.md | 69 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 69 insertions(+) create mode 100644 src/vnet/map/map_doc.md (limited to 'src/vnet/map/map_doc.md') diff --git a/src/vnet/map/map_doc.md b/src/vnet/map/map_doc.md new file mode 100644 index 00000000..17f3c511 --- /dev/null +++ b/src/vnet/map/map_doc.md @@ -0,0 +1,69 @@ +# 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 ] [pool-size ] [buffers ] [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. + + -- cgit 1.2.3-korg