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Recompute L4 checksum on-the-fly for TTag packets

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This commit is contained in:
Srivats P 2023-04-12 15:47:55 +05:30
parent c2967b663d
commit bfdcee2baa
2 changed files with 27 additions and 4 deletions
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2
common/sign.h
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@ -90,6 +90,8 @@ public:
static bool packetGuid(const uchar *pkt, int pktLen, uint *guid);
static bool packetTtagId(const uchar *pkt, int pktLen, uint *ttagId, uint *guid);
// XXX: Any change in kTypeLenXXX or magic value should also be done in
// TxThread/Ttag code as well where hardcoded values are used
static const quint32 kMaxGuid = 0x00ffffff;
static const quint32 kInvalidGuid = UINT_MAX;
static const quint8 kTypeLenTtagPlaceholder = 0x22;

29
server/pcaptxthread.cpp
View File

@ -438,14 +438,35 @@ int PcapTxThread::sendQueueTransmit(pcap_t *p, PacketSequence *seq,
if (seq->ttagPktInterval_
&& ((stats_->pkts % seq->ttagPktInterval_) == 0)) {
ttagPkt = true;
// XXX: write 2xBytes instead of 1xHalf-word to avoid
// potential alignment problem
*(pkt+pktLen-5) = SignProtocol::kTypeLenTtag;
*(pkt+pktLen-6) = ttagId_++;
*(pkt+pktLen-6) = ttagId_;
// Recalc L4 checksum; use incremental checksum as per RFC 1624
// HC' = ~(~HC + ~m + m')
if (seq->ttagL4CksumOffset_) {
quint16 *cksum = reinterpret_cast<quint16*>(
pkt + seq->ttagL4CksumOffset_);
origCksum = *cksum;
*cksum = 0x0000; // FIXME: recalc cksum
origCksum = ntohs(*cksum);
// XXX: SignProtocol trailer
// ... | <guid> | 0x61 | 0x00 | 0x22 | 0x1d10c0da
// ... | <guid> | 0x61 | <TtagId> | 0x23 | 0x1d10c0da
// For odd pkt Length, Ttag spans across 2 half-words
// XXX: Hardcoded values instead of sign protocol constants
// used below for readability
quint32 newCksum = pktLen & 1 ?
quint16(~origCksum) + quint16(~0x221d) + 0x231d
+ quint16(~0x6100) + (0x6100 | ttagId_) :
quint16(~origCksum) + quint16(~0x0022) + (ttagId_ << 8 | 0x23);
while (newCksum > 0xffff)
newCksum = (newCksum & 0xffff) + (newCksum >> 16);
// XXX: For IPv4/UDP, if ~newcksum is 0x0000 we are supposed to
// set the checksum as 0xffff since 0x0000 indicates no cksum
// is present - we choose not to do this to avoid extra cost
*cksum = htons(~newCksum);
}
ttagId_++;
}
if (sync) {
@ -479,7 +500,7 @@ int PcapTxThread::sendQueueTransmit(pcap_t *p, PacketSequence *seq,
if (seq->ttagL4CksumOffset_) {
quint16 *cksum = reinterpret_cast<quint16*>(
pkt + seq->ttagL4CksumOffset_);
*cksum = origCksum;
*cksum = htons(origCksum);
}
}