OSPF: Redesign LSA checksumming
New LSA checksumming code separates generic Fletcher-16 and OSPF-specific code and avoids back and forth endianity conversions, making it much more readable and also several times faster.
This commit is contained in:
Ondrej Zajicek
parent
30d09eb96e
commit
77edab6409
5 changed files with 226 additions and 133 deletions
196
lib/fletcher16.h
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196
lib/fletcher16.h
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@ -0,0 +1,196 @@
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/*
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* BIRD Library -- Fletcher-16 checksum
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*
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* (c) 2015 Ondrej Zajicek <santiago@crfreenet.org>
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* (c) 2015 CZ.NIC z.s.p.o.
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*
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* Can be freely distributed and used under the terms of the GNU GPL.
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*/
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/**
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* DOC: Fletcher-16 checksum
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*
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* Fletcher-16 checksum is a position-dependent checksum algorithm used for
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* error-detection e.g. in OSPF LSAs.
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*
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* To generate Fletcher-16 checksum, zero the checksum field in data, initialize
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* the context by fletcher16_init(), process the data by fletcher16_update(),
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* compute the checksum value by fletcher16_final() and store it to the checksum
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* field in data by put_u16() (or other means involving htons() conversion).
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*
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* To verify Fletcher-16 checksum, initialize the context by fletcher16_init(),
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* process the data by fletcher16_update(), compute a passing checksum by
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* fletcher16_compute() and check if it is zero.
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*/
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#ifndef _BIRD_FLETCHER16_H_
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#define _BIRD_FLETCHER16_H_
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#include "nest/bird.h"
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struct fletcher16_context
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{
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int c0, c1;
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};
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/**
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* fletcher16_init - initialize Fletcher-16 context
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* @ctx: the context
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*/
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static inline void
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fletcher16_init(struct fletcher16_context *ctx)
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{
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ctx->c0 = ctx->c1 = 0;
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}
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/**
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* fletcher16_update - process data to Fletcher-16 context
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* @ctx: the context
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* @buf: data buffer
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* @len: data length
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*
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* fletcher16_update() reads data from the buffer @buf and updates passing sums
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* in the context @ctx. It may be used multiple times for multiple blocks of
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* checksummed data.
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*/
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static inline void
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fletcher16_update(struct fletcher16_context *ctx, const u8* buf, int len)
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{
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/*
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* The Fletcher-16 sum is essentially a sequence of
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* ctx->c1 += ctx->c0 += *buf++, modulo 255.
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*
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* In the inner loop, we eliminate modulo operation and we do some loop
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* unrolling. MODX is the maximal number of steps that can be done without
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* modulo before overflow, see RFC 1008 for details. We use a bit smaller
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* value to cover for initial steps due to loop unrolling.
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*/
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#define MODX 4096
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int blen, i;
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blen = len % 4;
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len -= blen;
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for (i = 0; i < blen; i++)
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ctx->c1 += ctx->c0 += *buf++;
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do {
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blen = MIN(len, MODX);
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len -= blen;
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for (i = 0; i < blen; i += 4)
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{
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ctx->c1 += ctx->c0 += *buf++;
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ctx->c1 += ctx->c0 += *buf++;
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ctx->c1 += ctx->c0 += *buf++;
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ctx->c1 += ctx->c0 += *buf++;
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}
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ctx->c0 %= 255;
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ctx->c1 %= 255;
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} while (len);
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}
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/**
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* fletcher16_update_n32 - process data to Fletcher-16 context, with endianity adjustment
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* @ctx: the context
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* @buf: data buffer
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* @len: data length
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*
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* fletcher16_update_n32() works like fletcher16_update(), except it applies
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* 32-bit host/network endianity swap to the data before they are processed.
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* I.e., it assumes that the data is a sequence of u32 that must be converted by
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* ntohl() or htonl() before processing. The @buf need not to be aligned, but
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* its length (@len) must be multiple of 4. Note that on big endian systems the
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* host endianity is the same as the network endianity, therefore there is no
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* endianity swap.
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*/
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static inline void
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fletcher16_update_n32(struct fletcher16_context *ctx, const u8* buf, int len)
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{
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/* See fletcher16_update() for details */
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int blen, i;
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do {
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blen = MIN(len, MODX);
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len -= blen;
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for (i = 0; i < blen; i += 4)
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{
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#ifdef CPU_BIG_ENDIAN
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ctx->c1 += ctx->c0 += *buf++;
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ctx->c1 += ctx->c0 += *buf++;
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ctx->c1 += ctx->c0 += *buf++;
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ctx->c1 += ctx->c0 += *buf++;
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#else
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ctx->c1 += ctx->c0 += buf[3];
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ctx->c1 += ctx->c0 += buf[2];
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ctx->c1 += ctx->c0 += buf[1];
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ctx->c1 += ctx->c0 += buf[0];
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buf += 4;
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#endif
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}
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ctx->c0 %= 255;
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ctx->c1 %= 255;
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} while (len);
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}
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/**
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* fletcher16_final - compute final Fletcher-16 checksum value
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* @ctx: the context
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* @len: total data length
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* @pos: offset in data where the checksum will be stored
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*
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* fletcher16_final() computes the final checksum value and returns it.
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* The caller is responsible for storing it in the appropriate position.
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* The checksum value depends on @len and @pos, but only their difference
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* (i.e. the offset from the end) is significant.
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*
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* The checksum value is represented as u16, although it is defined as two
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* consecutive bytes. We treat them as one u16 in big endian / network order.
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* I.e., the returned value is in the form that would be returned by get_u16()
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* from the checksum field in the data buffer, therefore the caller should use
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* put_u16() or an explicit host-to-network conversion when storing it to the
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* checksum field in the data buffer.
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*
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* Note that the returned checksum value is always nonzero.
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*/
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static inline u16
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fletcher16_final(struct fletcher16_context *ctx, int len, int pos)
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{
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int x = ((len - pos - 1) * ctx->c0 - ctx->c1) % 255;
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if (x <= 0)
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x += 255;
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int y = 510 - ctx->c0 - x;
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if (y > 255)
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y -= 255;
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return (x << 8) | y;
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}
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/**
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* fletcher16_compute - compute Fletcher-16 sum for verification
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* @ctx: the context
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*
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* fletcher16_compute() returns a passing Fletcher-16 sum for processed data.
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* If the data contains the proper Fletcher-16 checksum value, the returned
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* value is zero.
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*/
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static inline u16
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fletcher16_compute(struct fletcher16_context *ctx)
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{
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return (ctx->c0 << 8) | ctx->c1;
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}
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#endif
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@ -2,14 +2,15 @@
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* BIRD -- OSPF
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*
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* (c) 1999--2004 Ondrej Filip <feela@network.cz>
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* (c) 2009--2014 Ondrej Zajicek <santiago@crfreenet.org>
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* (c) 2009--2014 CZ.NIC z.s.p.o.
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* (c) 2009--2015 Ondrej Zajicek <santiago@crfreenet.org>
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* (c) 2009--2015 CZ.NIC z.s.p.o.
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*
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* Can be freely distributed and used under the terms of the GNU GPL.
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*/
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#include "ospf.h"
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#include "lib/fletcher16.h"
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#ifndef CPU_BIG_ENDIAN
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void
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@ -150,144 +151,40 @@ lsa_get_type_domain_(u32 itype, struct ospf_iface *ifa, u32 *otype, u32 *domain)
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}
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/*
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void
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buf_dump(const char *hdr, const byte *buf, int blen)
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lsa_generate_checksum(struct ospf_lsa_header *lsa, const u8 *body)
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{
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char b2[1024];
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char *bp;
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int first = 1;
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int i;
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const char *lhdr = hdr;
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bp = b2;
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for(i = 0; i < blen; i++)
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{
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if ((i > 0) && ((i % 16) == 0))
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{
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*bp = 0;
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log(L_WARN "%s\t%s", lhdr, b2);
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lhdr = "";
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bp = b2;
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}
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bp += snprintf(bp, 1022, "%02x ", buf[i]);
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}
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*bp = 0;
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log(L_WARN "%s\t%s", lhdr, b2);
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}
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*/
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#define MODX 4102 /* larges signed value without overflow */
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/* Fletcher Checksum -- Refer to RFC1008. */
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#define MODX 4102
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#define LSA_CHECKSUM_OFFSET 15
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/* FIXME This is VERY uneficient, I have huge endianity problems */
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void
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lsasum_calculate(struct ospf_lsa_header *h, void *body)
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{
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u16 length = h->length;
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// log(L_WARN "Checksum %R %R %d start (len %d)", h->id, h->rt, h->type, length);
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lsa_hton_hdr(h, h);
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lsa_hton_body1(body, length - sizeof(struct ospf_lsa_header));
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struct fletcher16_context ctx;
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struct ospf_lsa_header hdr;
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u16 len = lsa->length;
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/*
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char buf[1024];
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memcpy(buf, h, sizeof(struct ospf_lsa_header));
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memcpy(buf + sizeof(struct ospf_lsa_header), body, length - sizeof(struct ospf_lsa_header));
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buf_dump("CALC", buf, length);
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* lsa and body are in the host order, we need to compute Fletcher-16 checksum
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* for data in the network order. We also skip the initial age field.
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*/
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(void) lsasum_check(h, body, 1);
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lsa_hton_hdr(lsa, &hdr);
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hdr.checksum = 0;
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// log(L_WARN "Checksum result %4x", h->checksum);
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lsa_ntoh_hdr(h, h);
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lsa_ntoh_body1(body, length - sizeof(struct ospf_lsa_header));
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fletcher16_init(&ctx);
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fletcher16_update(&ctx, (u8 *) &hdr + 2, sizeof(struct ospf_lsa_header) - 2);
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fletcher16_update_n32(&ctx, body, len - sizeof(struct ospf_lsa_header));
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lsa->checksum = fletcher16_final(&ctx, len, OFFSETOF(struct ospf_lsa_header, checksum));
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}
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/*
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* Calculates the Fletcher checksum of an OSPF LSA.
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*
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* If 'update' is non-zero, the checkbytes (X and Y in RFC905) are calculated
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* and the checksum field in the header is updated. The return value is the
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* checksum as placed in the header (in network byte order).
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*
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* If 'update' is zero, only C0 and C1 are calculated and the header is kept
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* intact. The return value is a combination of C0 and C1; if the return value
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* is exactly zero the checksum is considered valid, any non-zero value is
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* invalid.
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*
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* Note that this function expects the input LSA to be in network byte order.
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*/
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u16
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lsasum_check(struct ospf_lsa_header *h, void *body, int update)
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lsa_verify_checksum(const void *lsa_n, int lsa_len)
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{
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u8 *sp, *ep, *p, *q, *b;
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int c0 = 0, c1 = 0;
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int x, y;
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u16 length;
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struct fletcher16_context ctx;
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b = body;
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sp = (char *) h;
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sp += 2; /* Skip Age field */
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length = ntohs(h->length) - 2;
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if (update) h->checksum = 0;
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/* The whole LSA is at lsa_n in net order, we just skip initial age field */
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for (ep = sp + length; sp < ep; sp = q)
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{ /* Actually MODX is very large, do we need the for-cyclus? */
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q = sp + MODX;
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if (q > ep)
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q = ep;
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for (p = sp; p < q; p++)
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{
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/*
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* I count with bytes from header and than from body
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* but if there is no body, it's appended to header
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* (probably checksum in update receiving) and I go on
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* after header
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*/
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if ((b == NULL) || (p < (u8 *) (h + 1)))
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{
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c0 += *p;
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}
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else
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{
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c0 += *(b + (p - (u8 *) (h + 1)));
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fletcher16_init(&ctx);
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fletcher16_update(&ctx, (u8 *) lsa_n + 2, lsa_len - 2);
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return fletcher16_compute(&ctx) == 0;
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}
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c1 += c0;
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}
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c0 %= 255;
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c1 %= 255;
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}
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if (!update) {
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/*
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* When testing the checksum, we don't need to calculate x and y. The
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* checksum passes if c0 and c1 are both 0.
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*/
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return (c0 << 8) | (c1 & 0xff);
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}
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x = (int)((length - LSA_CHECKSUM_OFFSET) * c0 - c1) % 255;
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if (x <= 0)
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x += 255;
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y = 510 - c0 - x;
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if (y > 255)
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y -= 255;
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((u8 *) & h->checksum)[0] = x;
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((u8 *) & h->checksum)[1] = y;
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return h->checksum;
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}
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int
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lsa_comp(struct ospf_lsa_header *l1, struct ospf_lsa_header *l2)
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@ -46,9 +46,9 @@ static inline u32 lsa_get_etype(struct ospf_lsa_header *h, struct ospf_proto *p)
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int lsa_flooding_allowed(u32 type, u32 domain, struct ospf_iface *ifa);
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void lsa_generate_checksum(struct ospf_lsa_header *lsa, const u8 *body);
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u16 lsa_verify_checksum(const void *lsa_n, int lsa_len);
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void lsasum_calculate(struct ospf_lsa_header *header, void *body);
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u16 lsasum_check(struct ospf_lsa_header *h, void *body, int update);
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#define CMP_NEWER 1
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#define CMP_SAME 0
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#define CMP_OLDER -1
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@ -530,8 +530,8 @@ ospf_receive_lsupd(struct ospf_packet *pkt, struct ospf_iface *ifa,
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DBG("Update Type: %04x, Id: %R, Rt: %R, Sn: 0x%08x, Age: %u, Sum: %u\n",
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lsa_type, lsa.id, lsa.rt, lsa.sn, lsa.age, lsa.checksum);
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/* RFC 2328 13. (1) - validate LSA checksum */
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if ((lsa_n->checksum == 0) || (lsasum_check(lsa_n, NULL, 0) != 0))
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/* RFC 2328 13. (1) - verify LSA checksum */
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if ((lsa_n->checksum == 0) || !lsa_verify_checksum(lsa_n, lsa_len))
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SKIP("invalid checksum");
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/* RFC 2328 13. (2) */
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@ -129,7 +129,7 @@ ospf_advance_lsa(struct ospf_proto *p, struct top_hash_entry *en, struct ospf_ls
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en->lsa.age = 0;
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en->init_age = 0;
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en->inst_time = now;
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lsasum_calculate(&en->lsa, en->lsa_body);
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lsa_generate_checksum(&en->lsa, en->lsa_body);
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OSPF_TRACE(D_EVENTS, "Advancing LSA: Type: %04x, Id: %R, Rt: %R, Seq: %08x",
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en->lsa_type, en->lsa.id, en->lsa.rt, en->lsa.sn);
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@ -238,7 +238,7 @@ ospf_do_originate_lsa(struct ospf_proto *p, struct top_hash_entry *en, void *lsa
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en->lsa.age = 0;
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en->init_age = 0;
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en->inst_time = now;
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lsasum_calculate(&en->lsa, en->lsa_body);
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lsa_generate_checksum(&en->lsa, en->lsa_body);
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OSPF_TRACE(D_EVENTS, "Originating LSA: Type: %04x, Id: %R, Rt: %R, Seq: %08x",
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en->lsa_type, en->lsa.id, en->lsa.rt, en->lsa.sn);
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@ -382,7 +382,7 @@ ospf_refresh_lsa(struct ospf_proto *p, struct top_hash_entry *en)
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en->lsa.age = 0;
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en->init_age = 0;
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en->inst_time = now;
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lsasum_calculate(&en->lsa, en->lsa_body);
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lsa_generate_checksum(&en->lsa, en->lsa_body);
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ospf_flood_lsa(p, en, NULL);
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}
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