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package wire
import (
"bytes"
"crypto/rand"
"github.com/lucas-clemente/quic-go/internal/protocol"
"github.com/lucas-clemente/quic-go/internal/utils"
)
// ComposeVersionNegotiation composes a Version Negotiation
func ComposeVersionNegotiation(destConnID, srcConnID protocol.ConnectionID, versions []protocol.VersionNumber) ([]byte, error) {
greasedVersions := protocol.GetGreasedVersions(versions)
expectedLen := 1 /* type byte */ + 4 /* version field */ + 1 /* connection ID length field */ + destConnID.Len() + srcConnID.Len() + len(greasedVersions)*4
buf := bytes.NewBuffer(make([]byte, 0, expectedLen))
r := make([]byte, 1)
_, _ = rand.Read(r) // ignore the error here. It is not critical to have perfect random here.
buf.WriteByte(r[0] | 0xc0)
utils.BigEndian.WriteUint32(buf, 0) // version 0
buf.WriteByte(uint8(destConnID.Len()))
buf.Write(destConnID)
buf.WriteByte(uint8(srcConnID.Len()))
buf.Write(srcConnID)
for _, v := range greasedVersions {
utils.BigEndian.WriteUint32(buf, uint32(v))
}
return buf.Bytes(), nil
}
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