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diff --git a/doc/internals/header-tree.txt b/doc/internals/header-tree.txt new file mode 100644 index 0000000..9a97361 --- /dev/null +++ b/doc/internals/header-tree.txt @@ -0,0 +1,124 @@ +2007/03/30 - Header storage in trees + +This documentation describes how to store headers in radix trees, providing +fast access to any known position, while retaining the ability to grow/reduce +any arbitrary header without having to recompute all positions. + +Principle : + We have a radix tree represented in an integer array, which represents the + total number of bytes used by all headers whose position is below it. This + ensures that we can compute any header's position in O(log(N)) where N is + the number of headers. + +Example with N=16 : + + +-----------------------+ + | | + +-----------+ +-----------+ + | | | | + +-----+ +-----+ +-----+ +-----+ + | | | | | | | | + +--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+ + | | | | | | | | | | | | | | | | + + 0 1 2 3 4 5 6 7 8 9 A B C D E F + + To reach header 6, we have to compute hdr[0]+hdr[4]+hdr[6] + + With this method, it becomes easy to grow any header and update the array. + To achieve this, we have to replace one after the other all bits on the + right with one 1 followed by zeroes, and update the position if it's higher + than current position, and stop when it's above number of stored headers. + + For instance, if we want to grow hdr[6], we proceed like this : + + 6 = 0110 (BIN) + + Let's consider the values to update : + + (bit 0) : (0110 & ~0001) | 0001 = 0111 = 7 > 6 => update + (bit 1) : (0110 & ~0011) | 0010 = 0110 = 6 <= 6 => leave it + (bit 2) : (0110 & ~0111) | 0100 = 0100 = 4 <= 6 => leave it + (bit 4) : (0110 & ~1111) | 1000 = 1000 = 8 > 6 => update + (bit 5) : larger than array size, stop. + + +It's easy to walk through the tree too. We only have one iteration per bit +changing from X to the ancestor, and one per bit from the ancestor to Y. +The ancestor is found while walking. To go from X to Y : + + pos = pos(X) + + while (Y != X) { + if (Y > X) { + // walk from Y to ancestor + pos += hdr[Y] + Y &= (Y - 1) + } else { + // walk from X to ancestor + pos -= hdr[X] + X &= (X - 1) + } + } + +However, it is not trivial anymore to linearly walk the tree. We have to move +from a known place to another known place, but a jump to next entry costs the +same as a jump to a random place. + +Other caveats : + - it is not possible to remove a header, it is only possible to empty it. + - it is not possible to insert a header, as that would imply a renumbering. + => this means that a "defrag" function is required. Headers should preferably + be added, then should be stuffed on top of destroyed ones, then only + inserted if absolutely required. + + +When we have this, we can then focus on a 32-bit header descriptor which would +look like this : + +{ + unsigned line_len :13; /* total line length, including CRLF */ + unsigned name_len :6; /* header name length, max 63 chars */ + unsigned sp1 :5; /* max spaces before value : 31 */ + unsigned sp2 :8; /* max spaces after value : 255 */ +} + +Example : + + Connection: close \r\n + <---------+-----+-----+-------------> line_len + <-------->| | | name_len + <-----> | sp1 + <-------------> sp2 +Rem: + - if there are more than 31 spaces before the value, the buffer will have to + be moved before being registered + + - if there are more than 255 spaces after the value, the buffer will have to + be moved before being registered + + - we can use the empty header name as an indicator for a deleted header + + - it would be wise to format a new request before sending lots of random + spaces to the servers. + + - normal clients do not send such crap, so those operations *may* reasonably + be more expensive than the rest provided that other ones are very fast. + +It would be handy to have the following macros : + + hdr_eon(hdr) => end of name + hdr_sov(hdr) => start of value + hdr_eof(hdr) => end of value + hdr_vlen(hdr) => length of value + hdr_hlen(hdr) => total header length + + +A 48-bit encoding would look like this : + + Connection: close \r\n + <---------+------+---+--------------> eoh = 16 bits + <-------->| | | eon = 8 bits + <--------------->| | sov = 8 bits + <---> vlen = 16 bits + |